Iron oxide amended ceramic water purifiers for point-of-use arsenic removal from drinking water

DEPARTMENT OF MECHANICAL ENGINEERING

2009/2010

Iron oxide amended ceramic water purifiers for point-of-use arsenic

removal from drinking water

Sean Rivers

Supervisor: Dr H. Ewing

Word count: 10,529

MEng Mechanical Engineering

i

Acknowledgements

I would like to thank my project supervisor Dr Helen Ewing, for enabling this

project and for providing guidance and advice throughout the duration. I

would also like to thank Engineers Without Borders UK and the members

and friends of Potters for Peace who helped to form the project in the

beginning. In addition I would like to express a special thanks to Prasantha

for showing me round his factory in Sri Lanka. Finally, this project would not

have been possible without input and assistance from James Kelly, Jim

Docherty, Dr John Reglinski, Gerry Johnstone, Fiona Gentles, Keith

Torrance and Prof. Robert Kalin.

ii

Table of Contents

Acknowledgements ________________________________________________________ i

Abstract _________________________________________________________________ iv

LIST OF NOMENCLATURE & ABBREVIATIONS ________________________________ v

List of Figures __________________________________________________________ vii

List of Tables ___________________________________________________________ vii

1. Introduction _________________________________________________________ 1

1.1. Background ______________________________________________________ 1

1.2. Aims ____________________________________________________________ 1

2. Background Research _________________________________________________ 3

2.1. The Problem of Arsenic in Groundwater ________________________________ 3

2.1.1. Summary of the Arsenic Crisis in Bangladesh ________________________ 3

2.1.2. Effects of Arsenic Poisoning _____________________________________ 4

2.1.3. Chemistry of Arsenic ___________________________________________ 6

2.1.4. Mechanisms of Arsenic mobilisation in Groundwater __________________ 6

2.1.5. Arsenic Removal Technologies ___________________________________ 7

2.2. Ceramic Water Purification __________________________________________ 9

2.2.1. Point-of-use water filtration ______________________________________ 9

2.2.2. The filtrón ___________________________________________________ 10

2.2.3. Introduction to ceramics ________________________________________ 12

2.2.4. Ceramic Properties____________________________________________ 12

2.2.5. Ceramic Processing ___________________________________________ 13

2.2.6. Filtrón manufacturing process ___________________________________ 16

2.3. Iron Oxide Amended CWPs _________________________________________ 17

2.3.1. Factors affecting Arsenic sorption ________________________________ 18

2.3.2. Leaching of arsenic from filtróns _________________________________ 18

3. Methodology ________________________________________________________ 19

3.1. Manufacture of Ceramic Specimens __________________________________ 19

3.1.1. Powder Formation ____________________________________________ 20

3.1.2. Mixing ______________________________________________________ 21

3.1.3. Pressing ____________________________________________________ 21

3.1.4. Sintering ____________________________________________________ 22

3.1.5. Collodial Silver application ______________________________________ 24

3.2. Microstructure Analysis ____________________________________________ 25

3.2.1. Pycnometry _________________________________________________ 25

3.2.2. Metallography ________________________________________________ 26

3.3. Experimental Set-up _______________________________________________ 27

3.3.1. Column Apparatus ____________________________________________ 27

3.3.2. Preparation of Arsenic Spiked Water ______________________________ 28

3.4. Analysis ________________________________________________________ 28

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3.4.1. Hydraulic Conductivity _________________________________________ 28

3.4.2. Empty Bed Contact time _______________________________________ 29

3.4.3. Arsenic Removal _____________________________________________ 29

3.4.4. pH _________________________________________________________ 31

4. Results and Observations _____________________________________________ 32

4.1. Manufacture of Ceramic Samples ____________________________________ 32

4.1.1. Pressing ____________________________________________________ 32

4.1.2. Sintering ____________________________________________________ 32

4.2. Microstructure Analysis ____________________________________________ 32

4.2.1. Pycnometry _________________________________________________ 32

4.2.2. Metallurgy ___________________________________________________ 33

4.3. Experimental Set-up _______________________________________________ 37

4.3.1. Apparatus ___________________________________________________ 37

4.3.2. Influent Arsenic Analysis _______________________________________ 38

4.4. Analysis ________________________________________________________ 39

4.4.1. Hydraulic conductivity _________________________________________ 39

4.4.2. Empty Bed Contact Time _______________________________________ 40

4.4.3. Arsenic Removal _____________________________________________ 40

4.4.4. pH _________________________________________________________ 40

5. Discussion _________________________________________________________ 41

5.1. Manufacturing and Processing_______________________________________ 41

5.2. Microstructure Analysis and Hydraulic Conductivity ______________________ 41

5.3. Empty Bed Contact Time ___________________________________________ 43

5.4. Arsenic Removal by iron hydroxide amended ceramic ____________________ 45

5.5. Arsenic Detection Method __________________________________________ 46

6. Conclusions ________________________________________________________ 46

7. Future Work ________________________________________________________ 47

Literature References ____________________________________________________ 48

Appendices 53

iv

Abstract

This research was requested through the research program of Engineers

Without Borders UK by U.S. based not-for-profit organisation Potters for

Peace. This investigation aimed to determine what effect the addition of the

iron (hydr)oxide goethite (α-FeO(OH)) would have in removing arsenic when

included in filtrón ceramic water filtration technology, an already established

an affordable point-of-use water filtration technology in developing countries.

Ceramic specimens were manufactured which represented the commonly

used existing ceramic filters and the new iron oxide amended filters. The

specimen were tested for total porosity and examined under microscope to

determine the distribution of iron oxide additions within the clay

microstructure, and more specifically within the pore surfaces. Arsenic

removal performance was tested by passing arsenic spiked water though the

specimen in a down-flow column test. The hydrostatic head above the

samples was altered to allow arsenic removal to be measured at different

flow rate and empty bed contact times.

Results indicated that the addition of iron oxide increased the porosity of the

ceramic after sintering. Due to the limit of magnification, micrographs of the

ceramic did not give adequate insight into the distribution if iron oxide within

pores, though they did give an insight as to why very low flow rates were

obtained by the specimen during testing. As a result of the very low hydraulic

conductivity of the manufactured specimens it was not possible to obtain

results for arsenic removal by either the control ceramic or iron oxide

amended ceramic.

A discussion is included on perhaps why the samples did not have the

hydraulic conductivity expected. Simple calculations are displayed with

existing scientific data to hypothesise required contact times for arsenic

removal by iron oxide amended filters. The resulting hypothesis is that for

this method of arsenic removal to work, long contact times with the filter is

required which will lower the quantity of water obtained sufficiently that the

device will not be user friendly.

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LIST OF NOMENCLATURE & ABBREVIATIONS

Symbol Description Units

A Filter media surface cross sectional area cm2

As(III) Arsenite

As(V) Arsenate

BGS British Geological Survey

CWP Ceramic water purifier

CWP-Fe Goethite amended ceramic water purifier

EBCT Empty bed contact time mins

Δh Hydrostatic head cm

K Hydraulic conductivity m/h

L Filter media thickness cm

NSF National Sanitation Foundation

P% Percentage total pore volume

POU Point of Use

PfP Potters for Peace

ppm Parts per million

ppb Parts per billion

Q Volume flow rate cm3/h

RDIC Resource Development International - Cambodia

REDOX Reduction-oxidation

RO Reverse osmosis

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T Time mins

TGA Thermal Gravimetric Analyser

V Volume cm3

Vp Pore volume cm3

Vtablet Tablet volume cm3

W Weight g

WHO United Nations World Health Organisation

ρ Density g/cm3

UNICEF United Nations‘ Children Fund

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List of Figures

Figure 1 – Map illustrating Arsenic concentrations in Bangladesh groundwater [9] _________ 3

Figure 2 – Skin legions caused by long-term arsenic consumption [3]. ___________________ 5

Figure 3 – Stacked filtróns []. _______________________________________________________ 11

Figure 4 - Finished filter in container [26].____________________________________________ 11

Figure 5 – Uniaxial press used in filtrón production [28]. _______________________________ 14

Figure 6 – Formation of necks, grain boundaries and pores during sintering [26]. _________ 15

Figure 7 – Creation of interconnected and isolated pores during sintering [26]. ___________ 16

Figure 8 – Vibrating sieving table [26]. ______________________________________________ 20

Figure 9 – Die, plunger and washers [26]. ___________________________________________ 22

Figure 10 – Assembled die and plunger [26]. ________________________________________ 22

Figure 11 – Sintering protocol from RDIC “Ceramic Water Filter Handbook” [24]. _________ 24

Figure 12 –Column apparatus with head control and recirculation of feedwater [26]. ______ 27

Figure 13 – Mounted specimen. C1-C3, left op right respectively [26]. __________________ 28

Figure 14 – Specimen C1 at 50x magnification. ______________________________________ 33

Figure 15 – Specimen C2 at 50x magnification _______________________________________ 34

Figure 16 – Specimen C3 at 50x magnification _______________________________________ 34

Figure 17 – Specimen C1 at 500x magnification. _____________________________________ 35



Figures 20a, b & c – Specimens C1-C3 at 500x magnification, left to right respectively. ___ 37

Figure 21 – Finalised experimental set-up [26]. _______________________________________ 38

Figure 22 – Results of HACH arsenic testing on prepared standard solutions [26]. _______ 39

List of Tables

Table 1 – Scale of Arsenic Poisoning in Bangladesh. __________________________________ 4

Table 2 – Constituents of ceramic specimen. ________________________________________ 19

Table 3 – Pycnometry results. ______________________________________________________ 32

Table 4 – Measured hydraulic conductivity parameters. _______________________________ 39

Table 5 – Comparison of hydraulic conductivity (K) values from various filtrón studies. ___ 40

Table 6 – Empty bed contact times depending on specimen type and hydrostatic head. ___ 40

Iron oxide amended ceramic water purifiers for point-of-use arsenic removal from drinking water

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1. Introduction

1.1. Background

The installation of millions of shallow tube wells in Bangladesh during the

70‘s, 80‘s and 90‘s has been recognised as the greatest mass-poisoning in

history. Due to a well-intentioned international effort to supply the country

with safe drinking water, it is estimated up to 75million people are at risk of

ingesting unsafe levels of cancer-causing arsenic with over 20million already

exposed to varying degrees [1, 2]. This crisis far outweighs the number of

those affected in the catastrophes of Bhopal and Chernobyl [3].

72% of Bangladeshis live in rural areas [4]. For those relying on wells without

centralised treatment, the priority is on identifying the affected wells and

switching to alternate, clean water sources. Where such sources are not

easily available or testing has not been carried out, appropriate intermediate

solutions are required which work successfully on a community level.

This paper presents a study into the adaptation for arsenic removal of the

filtrón, an already existing, affordable and popular method proven to treat

biologically contaminated water in developing countries. In 2009, Brown and

Sobsey demonstrated that the filtrón was more effective at removing viruses

when amended with an iron (hydr)oxide prior to sintering [5]. Iron oxides are

known to bind with inorganic arsenic, although applying this property for

arsenic removal from drinking water has not yet been investigated [6, 7].

1.2. Aims

The objective of this research was to determine whether filtróns amended

with goethite have the potential to provide an appropriate intermediate

solution to removing arsenic from drinking water. For the filtrón to be deemed

fit for the purpose of removing arsenic it must meet the following criteria:

Must be easy to use and maintain.

Must be affordable.

Must Remove all inorganic arsenic (As(III) & As(V)) to below WHO limit of 10 parts per billion (ppb).


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Must have a high absorption capacity (long safe operating life).

Must have a short retention time.

Must not produce toxic waste for disposal.

Must hold its structural integrity in water.

Must not leach arsenic or any other filter materials back into the filtrate or environment upon disposal.

Due to time constraints in this undergraduate project it was not feasible to

carry out a comprehensive test of all criteria. Therefore, it was decided to test

for the most immediate indicators of this technology‘s potential:

Ability to remove inorganic As(V) to within 50ppb and 10ppb..

Minimum empty bed contact time required for removal of As(V) to within 50 and 10ppb.

Distribution of iron oxide within the ceramic pore surface and microstructure.


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2. Background Research

2.1. The Problem of Arsenic in Groundwater

2.1.1. Summary of the Arsenic Crisis in Bangladesh

In the 1970‘s, the overwhelming majority of Bangladeshi citizens relied on

un-safe surface sources of drinking water. It was common to remove water

for drinking, food preparation and hygiene from stagnant ponds and lakes

which led to a range of water-related illnesses such as gastroenteritis,

dysentery and diarrhoea [3]. This countrywide lack of safe water supply

contributed to decreased productivity, lower family incomes, higher child

mortality and poor school attendance (especially among girls).

As part of the United Nations‘ (UN) ―water supply and sanitation decade‖

from 1970-1980, the United Nations Children‘s Fund (UNICEF) and World

Health Organisation (WHO) worked with the Bangladesh Department of

Public Health Engineering (DPHE) to install some 3 million shallow tube

wells in the country. The effort was taken over by many other government

and non-governmental organisations (NGOs) resulting in over 10 million

shallow wells delivering water to over 90% of Bangladeshis [8], In 1997 the

WHO stated that it had surpassed it‘s goal of delivering safe water to over

80% of the country by the year 2000.

Figure 1 – Map illustrating Arsenic concentrations in Bangladesh groundwater [9]


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Cases of arsenicosis (figure 2) were first detected in India in 1983 from

people arriving from West Bengal [10, 8]. Arsenic was first detected in

Bangladeshi groundwater in 1993, though the problem was not fully

acknowledged by international organisations until 1998. Agencies involved in

the shallow well projects did not test water for arsenic as they claimed they

had no reason to suspect it‘s presence in Bangladesh‘s groundwater.

The scale of arsenic poisoning arsenic poisoning known so far is shown in

figure 1 and table 1. The WHO has recommended an upper limit of arsenic in

drinking water of 10µg/l (10ppb) irrespective of arsenic species, in

Bangladesh the national standard is set at 50µg/l (50ppb).

Table 1 – Scale of Arsenic Poisoning in Bangladesh.

Population of Bangladesh 125 million [3]

Total number of tubewells in Bangladesh

11 million [3]

Safe-level of Arsenic in drinking water

50µg/l (Bangladesh national standard) 10 µg/l (WHO standard)

Number of affected districts (out of 64)

59 [1]

Percentage of Bangladesh with contaminated groundwater

85% [1]

Population of regions with contaminated wells

35-77 million [3]

Number of tube wells tested in 1998 by British Geological Survey (BGS) Proportion with Arsenic > 50µg/l Proportion with Arsenic > 300µg/l

2022 [3] 35% [3] 8.4% [3]

2.1.2. Effects of Arsenic Poisoning

Arsenic was used historically as an anaesthetic although it is now regarded

for its more harmful properties as a poison and carcinogen.

Medical Effects of Arsenic Poisoning include but are not limited to the

following:

Legions

Skin cancer

Hypertension and cardiovascular disease

Neurological effects


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Internal cancers

o Bladder

o Kidney

o Lung

Pulmonary disease

Diabetes mellitus

Peripheral vascular disease

There is a latency associated with arsenicosis such that the first symptoms,

being skin legions, are not normally seen for around 10 years after regularly

ingesting elevated levels (more than 10ppb) of arsenic [3]. This latency was

one of the reasons why the arsenic went undetected in the water supply of

Bangladesh and West Bengal for such a long time. Even now that arsenic is

being tested for and detected in wells, the process is a slow one and it will

not be known for years the true number of people who have been affected.

Figure 2 – Skin legions caused by long-term arsenic consumption [3].

Often overlooked by engineers and scientists are the social issues

associated with physical problems. In Bangladesh, paranoia and lack of

education has led people to treat arsenicosis sufferers as lepers, with the

afflicted being ostracized by their relatives and communities. Due to fear of


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―catching‖ arsenicosis,, the healthy population deny the sufferers access to

schools, workplaces and public gatherings. Cases have been reported of

young female sufferers staying unmarried, while affected wives and mothers

are sent back to their families with their children or abandoned altogether [1].

2.1.3. Chemistry of Arsenic

Arsenic is a naturally occurring element constituting less than 1% of the

world‘s surface. Its atomic number is 33 on the periodic table and it has an

atomic weight of 74.9. Inorganic arsenic is more harmful to humans than the

organic form [11]. Inorganic arsenic exists in the environment in four

common valence states, -3, 0 +3, +5, however inorganic arsenic only exists

in groundwater in two of these valence states [12]. Arsenite (As(III)), the

trivalent form, is found under anaerobic conditions and has the ions AsO2 or

AsO3. Arsenate (As(V)) the pentavalent form is found under aerobic

conditions with the ion AsO4. As(III) is more potent and less stable than As

(V). For a time, inorganic arsenic was at the top of the USEPA list of

prioritised pollutants [16].

Due to the complexities of maintaining As(III) in an oxygen free environment

during testing, only As(V) was tested for in this investigation.

2.1.4. Mechanisms of Arsenic mobilisation in Groundwater

The mechanism(s) of arsenic mobilisation in the Ganges basin which

supplies Bangladesh‘s ground and surface water is a contested issue. There

are two generally suggested methods. The core existence of arsenic in

Bangladeshi groundwater is generally accepted to be due to natural arsenic

compounds present in soil sediments. Rock formations containing arsenic

higher up the Ganges suffered erosion over thousands of years, allowing

arsenic to be transported downstream and deposited to low lying floodplains

of Bangladesh in the form of arsenopyrite and attached to iron oxides. The

first mechanism is that under reducing conditions arsenic is converted to

mobile and soluble As(III). Reduction of iron also causes further release of

As(III) into groundwater [13]. The second explanation is that the due to

groundwater abstraction, and a falling water table, arsenopyrite is exposed to

oxygen which releases arsenic when oxidised [1]. The presence of iron,


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arsenic and sulphur reducing bacteria further influence the species of arsenic

and the release of arsenic from sediment compounds.

Arsenic contamination of groundwater is not limited to Bangladesh and west

Bengal. 70 countries around the world including Taiwan, Pakistan, China,

New Zealand, Hungary, USA and Canada suffer from arsenic in groundwater

reserves. It is estimated that as many as 140 million people worldwide have

been exposed to arsenic in drinking water over the WHO limit of 10μg/l [14].

Though the majority of arsenic in the environment is natural, anthropogenic

sources of arsenic include coal production, pesticides, cotton production,

munitions manufacturing, semiconductors, wood preservatives, textiles and

adhesives [15].

2.1.5. Arsenic Removal Technologies

There are many method of removing arsenic technologies. The most

commonly used methods of arsenic removal are described in this section.

Oxidation – Sedimentation

As As(V) is more stable and less mobile than As(III) it makes removal easier

if oxidation is used to transform arsenite to arsenate. Although fresh air is the

obvious choice for oxidation in developing countries, the process is very slow

taking up to weeks [16]. The process can be catalysed however using

bacteria, activated carbon, UV radiation, high temperatures and extreme

acids or alkalis. Potassium permanganate and hypochloride (bleaching

powder) are both commonly available in developing countries though the

former has a longer shelf life.

Sedimentation by leaving water standing in the home is an attractive solution

as it requires little effort or behavioural change. Results are mixed from with

one rapid assessment showing failure to remove arsenic to below 50ppb

[17].

Coagulation – sedimentation – filtration

Some very fine particles are too small to be removed by mechanical filtration

and are held in constant suspension due to their electrostatic charges

repelling one another. Coagulation filtration involves administering a


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flocculant or flocculating agent to bind such particles together into a heavier

―floc‖ which can then be removed by physical means.

Iron/aluminium salts and lime are common flocculating agents for arsenic

removal, although using lime requires pH correction prior to consumption.

Again, oxidation of As(III) to As(V) prior to coagulation increases the removal

efficiency. One disadvantage with this method is the creation of an arsenic

laced ―sludge‖ during sedimentation which must be disposed of, without

leaching arsenic back into the environment and water resources.

Membrane Filtration

Membrane filtration involves using a fine mesh whose pores are too small for

contaminants to physically pass through. The use of synthetic membranes

can successfully remove bacteria, viruses and metal ions. For arsenic

removal a very small pore size is required, hence only high pressure

membrane nanofiltration and reverse osmosis (RO) are effective to a high

degree. Problems with nanofiltration and RO technology in a development

context involve supplying energy for the high pressures required and

protecting the delicate membrane from suspended solids and metals found in

groundwater such as iron and manganese.

Distillation

Distillation uses heat to evaporate water into vapour – leaving contaminants

behind – and subsequently condensing the vapour back into pure water. For

high yields this method requires a lot of heat energy and therefore fuel. A

simple, passive method has been invented which use solar radiation to

evaporate water in a box or ―still‖ with an inclined glass lid. The water then

condenses on the glass and runs off into a collecting gutter. Of all low-cost

technologies for arsenic removal, solar distillation is the only to be accredited

by the National Sanitation Foundation (NSF) for removing 100% of arsenic

including all other heavy metals, salts, bacteria and viruses [18]. The main

downside with solar distillation is the low output per surface area required for

the stills, approx 2-3 litres/m2/day [19].


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Sorptive Filtration

Soprtive filtration uses the electrostatic affinity between arsenic and the

sorptive media to retain arsenic, thus removing it from drinking water.

Adsorption is the process where the adsorbate adheres to the surface of an

adsorbent. Such adsorbent media include iron hydroxides, aluminium,

activated carbon and biomass.

The critical factors for sportive media is the adsorption capacity of the media,

usually measured in milligram of adsorbent per gram of sportive media

(mg/g), and the contact time required to achieve the desired level of

contaminant removal. Removal efficiency is influenced by pH, REDOX

conditions, the presence of other inorganic and organic contaminants which

may competitor for attachment sites, and microbes which can consume or

create oxygen thus affecting REDOX conditions of the adsorbent, adsorbate

and competing compounds such as sulphur.

Sorptive filtration possibly provides the best solution to arsenic removal as it

is quick, does not require the use of coagulants and does not produce a

hazardous sludge. It must be ensured however that arsenic does not leach

from the filter media during use or after disposal due to REDOX conditions or

microbial activity.

2.2. Ceramic Water Purification

2.2.1. Point-of-use water filtration

Point-of-use (POU) is used to describe methods of water purification which

occur immediately prior to consumption rather than at a centralised or

decentralised water treatment plant. In many developing countries where

transmission infrastructure is poorly maintained, or where water is collected

by hand and stored in the home for periods of time, previously treated water

becomes vulnerable to recontamination [20, 21]. POU water treatment

avoids risks of recontamination during transport and storage of water and

has been found to reduce diarrheal diseases by over 70% [22].

POU water filtration can work well in remote rural areas, or informal

settlements with no urban planning, where piped systems are expensive to


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both install and maintain for a range of financial, institutional, technical and

social reasons. Also by making the treatment process visible, POU treatment

also causes communities to consider other health intervention such as

hygiene and sanitation.

POU filters such as porous ceramic water purifiers (CWPs) – a type of

membrane filter – and bio-sand filters can be manufactured by local skilled

people, using local resources, with donor funding being going directly to the

local economy. It however, would be wrong to assume that POU filters are a

perfect solution to the problem. POU treatment required behavioural change

which can be difficult in traditional cultures. Methods such as boiling water

using wood for fuel, whilst POU treatment, contributes to deforestation and

approximately 1.6 million deaths annually due to indoor air pollution [23].

An effective and sustainable POU water treatment system must fulfil a set of

criteria such as recommended by Sobsey et al, 2008 [22]:

Ability to supply required quantity of safe drinking water

Effective for treating a range of influent water qualities

Method is not time intensive, allowing user to pursue other

work/leisure activities

Low purchase and maintenance costs, making treatment

insensitive to income insecurities.

Have spare parts affordable, accessible and easy to replace.

Maintain usage in the long term.

2.2.2. The filtrón

The filtrón, a flowerpot shaped porous CWP (figures 3 & 4) was developed

and by U.S and Nicaragua based not-for-profit organisations Potters for

Peace (PfP), and Resource Development International Cambodia (RDIC)

[24]. Bacteria are removed by physical straining though the ceramic‘s

micropores and contact with the colloidal silver coating – a biocide which

inactivates bacteria. Colloidal silver also inhibits bacterial growth on the filter


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surface. There are currently hundreds of thousands of CWPs of filtrón design

being both manufactured and used in developing countries including

Nicaragua, Cambodia and Sri Lanka. The filtrón has received endorsement

from the UN and WHO [25].

Figure 3 – Stacked filtróns [26]. Figure 4 - Finished filter in container [26].

Some reasons for the success of the filtrón are outlined below:

The design uses locally available materials and skills which

keeps the cost of manufacture and distribution low.

The product resembles a simple terracotta pot, which is

unobtrusive in many cultures.

The device has good impact strength, making it less

susceptible to breakages, and increasing the life span.

The device required minimal effort and infrequent cleaning.

Three main drawbacks with the filtrón are that:

Storage containers are susceptible to re-contamination if not

properly kept clean.

Drinking clean water will not prevent diseases caused by poor

hygiene and sanitation practise.

The filtrón is not currently designed to remove heavy metals

such as Arsenic.


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2.2.3. Introduction to ceramics

Ceramics have been in use for over 10,000 years, making them one of the

oldest engineering materials known to man. The word ceramic comes from

the Greek ―keramos‖ literally meaning ―burnt earth‖. It is assumed that

ceramic processing originated from a simple observation that some types of

mud solidified when left in the sun. Therefore by altering the shape of the

mud in its wet, plastic state, and applying heat though fire, it was possible to

create solid objects of a desired shape with relative ease. This ease of

producing simple components is a particular reason why the technology is

appropriate to communities in developing countries.

2.2.4. Ceramic Properties

Ceramics are an inorganic, non-metallic material group, which are largely

composed of a mixture of metallic and non-metallic compounds. For the clay

in use in this investigation the constituents are mostly silicate and alumina.

Ceramics are formed by the application of heat (and in some cases heat and

pressure). With the exception of amorphous ceramics such as glass, most

ceramics have crystalline structures. Ceramics can be divided into two main

categories depending on their properties and application: traditional and

advanced. Traditional ceramics includes terracotta, earthenware, whiteware,

porcelain and structural ceramics. Advanced ceramics include

semiconductors and technical ceramics used in aerospace applications.

The properties and application of ceramics are largely dependant on their

microstructure. Crystalline solids can exist in two forms: a single crystal, or a

collection of smaller, packed crystals. The latter form is termed

polycrystalline. Single crystals feature a perfect repetition of atoms

throughout the structure which unless produced in a very controlled

atmosphere is difficult to achieve. In polycrystalline ceramics the crystals or

grains are highly varied in size, shape and arrangement. Grains normally

occur in a size range of 1 to 50 microns. Filtróns consist of polycrystalline

ceramic.

Chemical bonds in ceramics are ionic between metals and non-metals, and

covalent between non-metals and metalloids such as Boron, Silicon and


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Arsenic [27]. Unlike metallic bonds, neither ionic nor covalent bonds leave a

free electron, which is why ceramics are typically insulators and also why

dislocations do not slide over one another. The ionic and covalent bonds also

account for the high melting point and high modulus of elasticity of the

material compared to metals. Although the properties of ceramics are

variable, low-tech ceramics are generally hard, brittle and corrosion resistant.

Ceramics tend to have a high, specific melting point.

The interface between grains is a disordered region termed the grain

boundary. Not all grain surfaces are connected however; this is what is

termed porosity, and it is of critical importance in water filtration applications.

2.2.5. Ceramic Processing

Due to the high melting point of ceramics it is not normally feasible to form

products from melt in the same way in which metals are formed. Ceramic

processing normally involves powdering the raw materials, mixing them and

moulding them prior to heating. The method of forming the unfired or green

body has a great effect on the microstructure and the final properties of the

material.

There are many methods of varying accuracy and cost available to produce

powders. The commonly used method in filtrón production is using a hammer

mill to crush constituents into a powder which is then passed through a mesh

screen to sort out particles of a specific size.

When moulding the powder, there are three main options: pressing, casting

and plastic forming. Pressing is the process used in the manufacture of

filtróns. It involves inserting the powdered material into a die and applying

pressure. The pressure causes elastic deformation, plastic deformation and

fracture of the particles, packing the powder densely and reducing empty

pore spaces between the particles.


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Figure 5 – Uniaxial press used in filtrón production [28].

Pressure can either be applied uniaxially by means of a simple plunger or

isostatically by using a flexible mould and applying pressure on all sides by

means of pressurized fluid. In the case of the filtrón, pressure is applied by

the more straightforward plunger method (figure 5).

Once pressure is removed some of the energy stored in the elastically

deformed particles will cause the green body to expand, this is known as

―springback‖. Defects can be caused by different levels of springback within

the green body which can cause shearing between layers within the brittle

body.

Because of the delicate nature of dry green bodies, it is common in ceramic

processing to use a range of additives such as binders, plasticisers and

lubricants to increase the plasticity and strength of the green body during the

forming process.

In filtrón production, water is used as a lubricant. This reduces the friction

between dry particles, allowing the particles to move and form more easily

within the die. This allows higher packing densities to be achieved at lower


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pressures and gives more uniform packing density which decreases the

chance of defects due to springback. Water also gives the green body

strength during handling, making removal and transfer to other processing

stations easier.

Sintering is the final stage in the production of ceramics. This involves

applying heat to the green body which results in compaction of the body,

removal of pore spaces and the formation of strong bonds between particles.

To allow this densification to occur, a mechanism of material transport is

required. This mechanism is the heat in the firing process which allows

diffusion and viscous flow of the ceramic [29].

Sintering occurs in thee stages.

In the initial stage particles rearrange slightly, increasing their contact with

adjacent particles. Where there is sufficient energy at these contact points to

allow material transport, bonds begin to form. These early bonds are called

necks, due to their appearance.

Figure 6 – Formation of necks, grain boundaries and pores during sintering [26].

During the intermediate stage, necks grow larger, making particles less

recognisable as such (figure 6). Particle are now termed grains, with their

‗grain boundaries‘ transecting the necks. Grains move closer together

leading to a decrease in pore size and total pore volume decrease equal to

the volume shrinkage of the component. The intermediate stage is deemed

concluded when pores are isolated from one another and no longer form

Necking

Grain boundaries

Pores

Key:


Sean Rivers 16

continuous channels. Of course it is these channels of interconnected pores

which allow fluid flow in the filtrón. Figure 7 shows how the creation of

interconnected pore channels is aided by burnout material.

Figure 7 – Creation of interconnected and isolated pores during sintering [26].

The final stage of sintering involves further grain growth and isolated pores

being removed by vacancy diffusion. This process depends the rate of grain

growth, as if grain boundaries are formed too quickly, pores may become

trapped with no path to exit.

2.2.6. Filtrón manufacturing process

As with many ‗appropriate‘ technologies the manufacturing process for a

filtrón depends highly on the location, and as such is not an exact science.

As clay powder and burnout materials are often sourced locally and their

chemical composition varies from location to location, there exists a degree

of experimentation and customisation required in each factory‘s

manufacturing process. In the factory visited in Sri Lanka, it took Prasantha

(the factory owner) 9 months of experimentation to finally get the composition

right before any filters were ready for sale.

The manufacturing process detailed below is derived from the RDIC 10 step

production process, on-line manuals from the PfP website and conversation

with a filter factory owner in Matara, Sri Lanka [3, 4, 30].

1. Preparation of raw materials: clay, laterite (optional), rice husks,

water.

2. Mix raw materials

Burnout material

Isolated pores

Water

Clay/ceramic

Key:


Sean Rivers 17

3. Form clay balls for pressing

4. Press clay into pot shape

5. Surface finishing and stamping of pressed filters

6. Dry pressed filters for 2-3 days.

7. Fire filters in a kiln

8. Flow rate testing

9. Application of colloidal silver. 2mg of 3.2% solution to 300ml.

10. Package filter with receptacle, lid, spigot, and information leaflet.

Ready for sale.

2.3. Iron Oxide Amended CWPs

Research by Brown and Sobsey showed that iron oxides provide attachment

sites for virus bacteriophages, removing them from drinking water, in

laboratory tests an 8log (99.999999%) reduction in bacteriophages was

recorded [5].

Goethite (pronounced ―gertide‖) is a ferric hydroxide with the chemical

composition α-FeO(OH). It is a brown mineral formed by the weathering of

iron (eq. 1) and a component in the rich red/brown lateritic soils found in

many tropical regions. [31, 32].

4Fe2++O2+6H2O = 4FeOOH + 8H+ (1)

Lateritic soils are produced where long-term high rainfall dissolves more

soluble compounds in soli leaving behind amounts of insoluble iron and

aluminium, resulting in a hard, rust-coloured soil. During field research in Sri

Lanka, lateritic soil was observed in use in building construction and as a

strength additive in CWP production. During sintering at 260-320°C in an

oxidising environment goethite is transformed to hematite [33, 34, 35].

Dehydration of goethite is shown in equation (2) [36].

2FeOOH -> Fe2O3 + H2O (2)


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As with other iron oxides, Hematite has been demonstrated to be effective at

removing over 80% of As(V) from pH neutral drinking water [37, 7]. Clay is

also shown to absorb arsenic, though not as effectively as iron oxides [46].

2.3.1. Factors affecting Arsenic sorption

Arsenic sorption onto ferric oxides is influenced by a range of abiotic and

biotic factors. The main abiotic factors are contact time, pH, REDOX

conditions and the presence of other adsorbents. Under acidic conditions

As(V) sorption onto iron oxides is more favourable with As(III) sorption being

prevalent under more alkaline conditions [38]. As the proportions of

As(III)/(V) are determined by the oxidation state, it follows that the under

reducing conditions a high pH is preferable, while a low pH is preferable for

aerobic conditions. Competition for sorption surfaces on hematite by sulphur

also influences the efficiency of arsenic removal [39]. This has

consequences for Bangladesh where there is an abundance of pyrite (iron

sulphide) in ground sediments [40]. Dissimilatory reducing bacteria (DRBs)

are also responsible for the reduction of As(V) [41]. Recently bacteria which

oxidise As(III) to As(V) were discovered in California [42].

2.3.2. Leaching of arsenic from filtróns

Desorption or leaching of As(III) and As(V) from filtróns back into supposedly

safe drinking water is of concern. This point was raised by PfP during the

conception of this project, not just for filtróns with the purpose of removing

arsenic but also in areas where arsenic exists in laterite and clay which are

being used for filter production. Leaching of As(III) occurs quicker and to a

greater extent than As(V) possibly due to weaker bonds [43].


Sean Rivers 19

3. Methodology

3.1. Manufacture of Ceramic Specimens

Three different constructions were used in order to asses the effect of the

goethite amended ceramic. C1 was un-modified porous ceramic, C2 was un-

modified porous ceramic coated with colloidal silver, and C3 contained

goethite addition and no colloidal silver coating. C1 and C2 would act as

controls to C3. The constituents of the ceramic specimen are listed in table 2.

Table 2 – Constituents of ceramic specimen.

Specimen Constituents

C1 1/1 clay/sawdust by volume

C2 As for C1, with colloidal silver applied to disc

post-firing

C3 6/1 clay/goethite by mass, 1/1

(clay+goethite)/sawdust by volume.

Specimen type C1 and C2 were used as controls to the goethite amended

ceramic. The reason for having two different controls is that it was unknown

what interactions arsenic may have with the colloidal silver. Though a

solution would be to avoid any problems by just using specimen C1, this

would give an impractical comparison for any added benefits of using

goethite, as filtróns should always coated with colloidal silver. 1/6 was the

largest goethite/clay ratio reported by Brown and Sobsey to be possible

without loss of filter strength [5]. Specimens were disk shaped, 37-39mm

diameter and 10-13mm thick. The thickness of real filtrons vary from 10-

20mm [Error! Bookmark not defined.].

The sample preparation methodology was kept as close to the in field

production method and was as follows:

1) Sieve clay through a 500 µm mesh.

2) Sieve sawdust through a 500 µm mesh.


Sean Rivers 20

3) Combine dry ingredients. 1/6 for oxide/clay by weight and 1/1 for clay

(+ oxide)/sawdust by volume.

4) Mix dry ingredients on roller bed for 10 minutes.

5) Add water, starting at 30% weight of dry ingredients.

6) Mix wet ingredients until the clay is of uniform consistency.

7) Press clay into tablets.

8) Dry samples for 2-3 days at 50 °C.

9) Bake samples in furnace for 24 hours. 860-900 degrees [44]

3.1.1. Powder Formation

From researching various literature there did not appear to be a solid

consensus on the particle size of the ceramic powder prior to sintering [24,

45]. Recommendations varied between a particle size of between 30 and 60

mesh, less than 40 mesh and less than 30 mesh. For this investigation, a

mesh size of 30 (0.5mm aperture) was used as recommended by Prasantha

[30].

Figure 8 – Vibrating sieving table [26].

The clay and sawdust were passed separately through a 30 mesh sieve

mounted on a vibrating sieving table (figure 8). It was found that the first

batch of sawdust obtained from the university workshops contained

aluminium particles which were not removed during sieving. As aluminium is

shown to be an effective sorptive media for arsenic it was crucial that the


Sean Rivers 21

sawdust be aluminium free [46]. Aluminium free sawdust was finally obtained

from Polney sawmill in Dunkeld, Perthshire. To minimise waste, clay which

was too large to pass through the sieve on the first attempt was ground by

hand using a mortar and pestle and sieved again.

3.1.2. Mixing

When mixing the clay and sawdust, a 1/1 volume ratio was used as per the

factory in Sri Lanka. It was unlikely that this 1/1 mix would achieve the

desired porosity and hydraulic conductivity on the first attempt, however due

to setbacks caused by equipment breakdowns there was no time to carry out

an iterative process of testing and multiple hydraulic tests on samples prior to

arsenic sorption tests, so a 1/1 ratio was taken as the standard.

For each batch, 100ml of dry ingredient were sealed in a plastic flask and

placed on a roller bed for 10 minutes to ensure a homogeneous mix.

Afterwards, water was added slowly while manually stirring the mixture in a

Pyrex® dish. Approximately a 1/3 ratio of water/dry matter was required. At

the end of mixing, the dough was removed and folded by hand to ensure

uniform consistency before being weighed and divided into appropriate sized

portions, ready for pressing.

The exact composition of each batch can be seen in appendix I.

3.1.3. Pressing

Samples were pressed in a steel 40mm diameter die (figures 9 & 10) with a

uniaxial applied force of 170N applied in addition to the 2kg weight of the

plunger.

The die was a lined with a plastic shopping bag (as used in field) to prevent

the green body from sticking to the mould. As no information could be found

on the pressure applied to filters in the field, the samples were pressed with

a slowly ramping pressure and inspected at intervals. The samples normally

dried out in the hot workshop and exhibited some cracking at the

circumference during pressing due to the heat and dryness of the workshop.

Any cracking was ―touched-up‖ upon inspection before being re-inserted and

pressed further until the green body achieved 100% packing in the die.


Sean Rivers 22

Figure 9 – Die, plunger and washers [26].

Figure 10 – Assembled die and plunger. Washers are placed inside the die above and below the clay [26].

Once disks were successfully pressed they were dried for 2-3 days at 50°C

in a drying oven, before being transferred to a desiccating chamber while

waiting to be sintered.

3.1.4. Sintering

The sintering protocol for the ceramic specimen was derived from the RDIC

―Ceramic Water Filter Handbook‖ as shown in figure 11 [24].

The equipment used was a Carbolite CWF 1200 furnace. In the first ramp the

clay is heated over 1 hour to one hundred degrees where it is allowed to

dwell for 2 hours. This allows any moisture still present in the ceramic to

evaporate slowly without boiling and damaging the structure of the green

body. Following this dwell, the temperature is ramped steadily up to 890°C.

During this ramp the burn-out material combusts, leaving pore volumes, and

the ceramic begins the sintering process as described in section 2.2.5.

Following the long dwell the temperature is lowered to 400°C gradually, rapid

cooling (quenching) of the specimen can cause thermal shock and lower the

strength of the specimen [47]. Below 400°C sintering is completed and the


Sean Rivers 24

temperature can be decreased as rapidly as the furnace allows without effect

to the microstructure.

Figure 11 – Sintering protocol from RDIC “Ceramic Water Filter Handbook” [24].

3.1.5. Collodial Silver application

Colloidal silver was purchased from Sigma-Aldrich®. In practice colloidal

silver solution is often prepared in a 3.2% concentrate which is then diluted

as required to create a batch of 220mg/l solution for application. As there

was only need to make enough solution for one disc the following method

was adopted:

Measure 50ml of water

Weigh 11mg of colloidal silver

Add colloidal silver to water and stir

Brush solution on tablet surface

The calculation proof is shown in equation (3).

lgwaterofVol

SilverofMassionConcentrat /22.0

50

11

(3)

Following the application of the colloidal silver C2 was dried in an oven at

50°C for one hour.

0

200

400

600

800

1000

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (Hours)

Furnace Protocol


Sean Rivers 25

3.2. Microstructure Analysis

3.2.1. Pycnometry

Pycnometry is a method which uses Archimedes principle to measure the

volume of irregular shaped objects. It was used in order to calculate the total

pore volume of the specimen types.

Archimedes principle states that when an object is placed in water, the

buoyancy force on the object is equal to the weight of mass of water

displaced. When that object is fully submerged in water, it is also true that

the volume of water displaced is equal to the volume of the object. Therefore

by comparing the weight of an object suspended in air and in water, the

buoyancy force on the object can be calculated, hence it follows that the

mass of water displaced can be calculated, which in turn will give the volume

of water displaced – and therefore the bulk volume of the object.

The internal pore volume of the ceramic is calculated by placing the samples

in boiling water, which forces water into the pore volumes and causes air in

the pores to expand and vacate the pores. Upon cooling, any air left in the

pores contracts, creating a vacuum which further draws water into the pores.

By measuring the weight of water gained by the tablets during this process, it

is possible to estimate the volume of the internal pores.

The method for the pore volume investigation is detailed below. The weights

of the tablets in air and water were obtained using Mettler Toledo Excellence

XP/Xs analytical balances. The full method is explained below.

1. Dry tablet was weighed in air

2. The tablet was then weighed in water

3. Tablets were submerged in separate beakers of deionised water

and placed on a hot plate. Once brought to the boil the tablets

were boiled for 10 minutes.

4. The beaker was then removed from the hot plate and allowed to

cool to allow water to be drawn into any pore spaces by

contracting air.


Sean Rivers 26

5. Tablets were then removed from the beakers, dried lightly with a

paper towel and weighed.

The pore volume can be calculated by applying equations (4)-(8).

water

waterair

water

disp

tablet

WWWV

(4)

water

airboiled

water

adsorp

pore

WWWV

(5)

100% tablet

pore

V

VP

(6)

100%

water

waterair

water

airboiled

WW

WW

P

(7)

100%

waterair

airboiled

WW

WWP

(8)

3.2.2. Metallography

In order to examine the microstructure of the ceramic samples and gain an

insight into the distribution of hematite within the pore surfaces, the surface

and profile section of each three specimen types were viewed under a

microscope.

Sections of the ceramic tablet samples were cut using a Struers Accom-5

diamond saw. Using a vacuum impregnation chamber, two samples of each

tablet were mounted in resin such that a surface and cross section of each

could be viewed upon hardening. Once the resin was set, the dies were


Sean Rivers 27

polished and the samples viewed at 50x and 500x using an Olympus GX51

metallurgical microscope.

3.3. Experimental Set-up

3.3.1. Column Apparatus

Figure 12 –Column apparatus with head control and recirculation of feedwater [26].

The test column apparatus (figure 12) consisted of an 800mm tall, 40mm

internal diameter transparent acrylic cylinder with the specimen fixed at the

base. Ceramic specimens were mounted in a frame made from acrylic tubing

(figure 13) and used an Araldite® layer between the mounting and sides of

the specimen to prevent short-circuiting around the ceramic media. Water

was delivered into the top of the cylinder by a peristaltic pump using 4mm

internal diameter Tygon® tubing which drew water from a jerry can. The

hydrostatic head was controlled by an inverted U-bend. This consisted of a

flexible 6mm Tygon® tubing to allow easy adjustment. The inverted U-bend

fed excess water back into the jerry can for recirculation by the peristaltic

pump. Filtrate which passed through the ceramic sample was first collected

in a well created from a regulating well made from a plastic flask and spigot.

Column

Ceramic

Specimen

Inverted

U-bend

Pump

To pump

Collecting

beaker

Regulating

well Reservoir


Sean Rivers 28

This allowed the flow of filtrate into the collection beaker to be controlled

when removing the beaker for analysis.

Figure 13 – Mounted specimen. C1-C3, left op right respectively [26].

A peristaltic pump was used as it prevents the arsenic from coming into

contact with any internal pump parts. This prevents arsenic binding to any

metallic pump internal parts and also means the pump does not require

cleaning after using arsenic.

3.3.2. Preparation of Arsenic Spiked Water

Sodium arsenate dibasic heptahydrate (Na2HAsO4·7H2O) was purchased

from Sigma-Aldrich®. The chemical was of ACS reagent grade and >98%

purity. An influent quality of 300μg/l of inorganic arsenic was chosen as it

was found in 1998 by the BGS that less than 10% of wells investigated in

Bangladesh had arsenic levels exceeding 300μg/l [3]. The arsenic laced

water was prepared by weighing 6.3mg±3 under a fume hood. This was then

added to a container containing 5 litres of deionised water. Full dosing

calculations can be seen in the appendix II.

3.4. Analysis

3.4.1. Hydraulic Conductivity

Hydraulic conductivity is a measure of the conductivity or resistivity of a

porous media to water flowing through it. It is expressed in units of speed.

The hydraulic conductivity is determined by the physical structure of the


Sean Rivers 29

media such as pore size, distribution and total pore volume as well as the

effective diameter and tortuosity of pore channels. In this investigation,

hydraulic conductivity is calculated using Darcy‘s law. Darcy‘s law is used in

filtration theory and groundwater studies and is shown in equation (9).

l

hkAQ

(9)

The hydraulic conductivity of the specimen was obtained by:

Measuring the cross sectional area of the filter media

Measuring the hydrostatic head of water above the filter media

Measuring the volume of filtrate collected over a period of time

Experimental results are substituted into darcy‘s law, rearranged for

hydraulic conductivity as shown in equation (10).

hA

Qlk

(10)

3.4.2. Empty Bed Contact time

The Empty bed contact time (EBCT) is calculated as the time required for a

fluid to pass through a volume of sorptive media. For this study it indicates

the time that water is in contact with the ceramic media. The calculation for

EBCT is shown in equation (11).

Q

VEBCT (11)

3.4.3. Arsenic Removal

To investigate the removal capability of goethite amended ceramic, and the

contact time required for maximum removal, arsenic removal was tested at

different flow rates by altering the hydrostatic head above the filter media. As

shown in Darcy‘s law, the flow rate is directly proportional to the hydrostatic

head. The expectation was to obtain a relationship between contact time and

arsenic removal which would allow optimisation of the filter design to remove

arsenic concentration to within desired standards while maintaining a high


Sean Rivers 30

enough flow rate to enable sufficient quantities of drinking water to be

produced.

Arsenic analysis was performed using a HACH field test kit for arsenic

detection. The field kit is capable of detecting arsenic to below 10ppb which

made it a suitable method for indicating whether or not the apparatus was

successful at reducing arsenic to within the WHO recommended limit. The kit

works by oxidising hydrogen sulphide to sulphate which prevents

interference, the oxidising conditions are then neutralised. Salfamic acid and

powdered zinc produce reducing conditions which convert the inorganic acid

into arsine gas. The arsine gas is directed through the cap of the vessel and

into contact with the test strip. The test strip contains mercuric bromide which

reacts with the arsine gas to produce halogens which discolour the pad on

the test strip. Discolouration occurs from yellow to brown with increasing

concentration of arsenic in the test water. A guide to the quantity of arsenic

present is obtained from comparing the test strip with a colour chart

provided.

The methodology for using the HACH field test kit is as follows:

1. Insert the test strip under the cap holder.

2. Measure 50ml of water into the vessel.

3. Add the contents of reagent 1 (Sodium Phosphate).

4. Add the contents of reagent 2 (Ozone Monopersulfate) and swirl.

5. Wait 3 minutes.

6. Add the contents of reagent 3 (EDTA Tetrasodium Salt) and swirl.

7. Wait 2 minutes.

8. Add the contents of reagent 4 (Sulfamic Acid) and swirl

9. Add the contents of reagent 5 (Zinc) and immediately place the cap on

the beaker.


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10. Swirl and leave for 30 mins (no more than 35 mins), swirling the beaker

twice during this period.

11. Remove the test strip and compare discolouration against the colour

chart.

3.4.4. pH

The pH of both the influent and filtrate was tested with universal indicator

strips which gave a colorimetric indication of pH to within ±0.5.


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4. Results and Observations

4.1. Manufacture of Ceramic Samples

4.1.1. Pressing

The first attempt at pressing samples resulted in destruction of the green

body due to a combination of excessive pressure and a high water content

which increased the plasticity of the green body to such an extend that it was

able to escape around the plungers used in the die. The methodology was

subsequently altered to use less water and apply just as little pressure as

required to obtain an adequately shaped cast from the mould. The method of

lining the mould with a plastic bag was abandoned as it hampered the filling

of corner spaces and the crinkled plastic left an uneven finish on the tablets.

4.1.2. Sintering

The sintering of the ceramic tablets was achieved without any problems.

Upon visual inspection pores appeared clean and free of soot. Both the

control and goethite amended ceramic tablets were strong enough to be

handled.

4.2. Microstructure Analysis

4.2.1. Pycnometry

Pycnometry was only carried out for C1 and C3. It was considered that

results from C1 would be representative of C2 as there was no difference in

ceramic structure between C1 and C2, and it was assumed that the

application of colloidal silver would have a negligible effect on the mass and

volume of the specimen.

Table 3 – Pycnometry results.

Specimen

Dry Weight

(g)

Wet Weight

(g)

Boiled Weight

(g)

Percentage

porosity

C1 21.17343 10.5865 25.974 45%

C3 15.41695 6.38570 20.027 51%

The results in table 3 show that a 1/1 clay:burn-out ration will achieve

approximately a 50% porosity in the fired ceramic.


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4.2.2. Metallurgy

Micrographs are shown in figures 14-19 for all three specimen types at 50x

and 500x. Images are also shown of needle-like formations which were

recorded in the large pore volumes in figures 12a, b & c.

Figure 14 – Specimen C1 at 50x magnification.

Solid 2nd

phase particle Metallic particles

Pore volume


Sean Rivers 34

Figure 15 – Specimen C2 at 50x magnification

Figure 16 – Specimen C3 at 50x magnification

Metallic particles,

Possibly colloidal silver

Possibly hematite particles


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It appears that the samples are consecutively brighter, possibly due to

colloidal silver in figure 15 and the addition of hematite in figure 16. The

contradiction to this observation is that metallic particles should be a bright

white, and the speckles are more of a grey colour, suggesting 2nd phase non-

metallic particulates.


Large pore

Interconnected

pores


Sean Rivers 36



Possibly colloidal silver

Possibly Hematite


Sean Rivers 37

As can be seen in figures 16-18, specimen C3 has more pores >10µm than

C1, however as C2 is also similar no direct conclusion can be drawn as to

whether this is the effect of the goethite. The large white particle in figure 19

may be hematite as no such size of metallic or silica particles were observed

in figures 17 and 18. Further analysis would be required to determine the

nature of the particle. It was also observed that despite colloidal silver being

applied to C2 there did not appear to be any substantial visible difference at

the 500x level.

Figures 20a, b & c – Specimens C1-C3 at 500x magnification, left to right respectively.

Crystalline structures were clearly visible in pores of all samples at the 50x

and 500x levels. It was originally hypothesised that this may be resin due to

inadequate suction during vacuum impregnation. However, another

hypothesis is that it is sap or another hydrocarbon left from the burnout

material. This may further explain the anomaly with the hydraulic

conductivity. The matter is discussed in full in section 5.2.

Ultimately microscopic analysis only gives part of the picture as to the

microstructure. Further methods of obtaining chemical and statistical

information is discussed in section 5.2.

4.3. Experimental Set-up

4.3.1. Apparatus

On preliminary testing of the apparatus it was found that the 6mm tygon®

tubing was too thin to be used for the inverted U bend. The viscosity of water

in such a small diameter tube meant that it was not possible for partial flow at

the top of the U bend and once the U bend began draining excess water, a

siphon effect was created which drained the entire cylinder.


Sean Rivers 38

Figure 21 – Finalised experimental set-up [26].

The obvious solution was to use larger diameter tubing, however large

diameter, flexible, transparent tubing was only available to order in bulk at

considerable expense. Instead, a reservoir was fabricated using a

translucent plastic bottle and discarded length of opaque garden hose pipe.

In practice this method did not work well under the fume hood as it was

difficult to maintain a constant downward gradient in the 100cm of hosepipe

when coiled at low hydrostatic head heights. The final solution was to attach

a copper valve to the 6mm Tygon® tubing (figure 21). As the valve was

metallic and may influence arsenic levels in the apparatus the water was not

re-circulated and was fed straight to a drain in the fume hood.

4.3.2. Influent Arsenic Analysis

It was found when testing the prepared 300ppb arsenic water that the test

strip gave a paler stain than the colour chart. The gloss coated colour chart

also made comparison with the stick difficult. To solve this issue tests were

performed on standard solutions of 30, 150 and 300ppb. The results are

shown in figure 22.

Reservoir

Column

Ceramic

Specimen

Tap Pump

Funnel Collecting

beaker


Sean Rivers 39

Figure 22 – Results of HACH arsenic testing on prepared standard solutions [26].

4.4. Analysis

4.4.1. Hydraulic conductivity

Hydraulic conductivity experiments were originally planned to be carried out

during the arsenic removal experiments to be more efficient than running two

separate tests. However, in preliminary tests using deionised water to test

the integrity of the system, it was found for the first control that the rate of

flow through the sample was significantly less than anticipated, and the small

amount of filtrate collecting in the beaker was evaporating during collection in

the hot laboratory conditions. Under considerable time constraints the

decision was made to cancel the control arsenic experiment and at least

attempt to measure the hydraulic conductivity of the sample at a high head –

this would provide more filtrate and decrease errors caused by evaporation –

before moving on to the arsenic removal testing of the goethite sample. Due

to further complications which were unable to be resolved in time, no flow

rate results were gathered for the goethite sample. The results of the

hydraulic conductivity test for the control sample are given in table 4.

Table 4 – Measured hydraulic conductivity parameters.

Specimen ∆h

(cm)

V

(cm3)

T

(mins)

Q

(cm3/hr)

L

(cm)

A

(cm2)

K

(cm/hr)

C1 70 3.53 62 3.416 1.2 11.9 0.00491


Sean Rivers 40

The hydraulic conductivity of 0.004 cm/h is approximately 40 times lower

than expected for the ceramic with 50/50 burnout/clay ratio. Results from

similar studies on filtrón hydraulic conductivities are shown in table 5.

Table 5 – Comparison of hydraulic conductivity (K) values from various filtrón studies. Units in m/hr.

Rivers Lee [48] Eriksen [49] Lantagne [49] Fahlin [49]

0.00004 0.0016265 0.03 0.004 0.001-0.003

4.4.2. Empty Bed Contact Time

It was only possible to calculate the EBCT for specimen C1 at a head of

70cm. Results are shown in table 6.

Table 6 – Empty bed contact times depending on specimen type and hydrostatic head.

Specimen Δh (cm)

Pore Volume, V (cm3)

Flow rate, Q (cm3/h)

EBCT (hours)

C1 70cm 4.7641185 3.416 1.39

4.4.3. Arsenic Removal

Due to the low flow rate experienced with C1 it was not feasible to carry out

experiments for C1 and C2. In the experiment with C3, sealing problems in

the apparatus caused influent water to leak into the collecting beaker,

contaminating any result. This problem was not overcome, resulting in no

results of any specimen for arsenic removal.

4.4.4. pH

Only the influent water was tested flowing the low flow rates experienced.

The influent water was found to have a pH of 5. This result is good for

arsenic sorption of As(V). pH correction would be required to obtain a neutral

water pH.


Sean Rivers 41

5. Discussion

5.1. Manufacturing and Processing

The clay/goethite ratio suggested by Brown and Sobsey was successful in

producing a suitably strong ceramic for the purposed of this testing. During

cutting with the diamond saw it was noted that cutting fluid (water) caused a

brown residue to be left on anything the ceramic touched while wet. This lack

of integrity in water was a concern. It would be critical to resolve this issue on

application in the field.

During the field research in Sri Lanka, it was noted that coconut oil was used

as a lubricant for extracting the green bodies from the mould. This method is

more environmentally sustainable than the use of plastic bags as is common

practice at present. Currently RDIC do not recommend the use of any oils for

this purpose so this finding may perhaps have a positive impact on the waste

produced during the manufacturing process.

5.2. Microstructure Analysis and Hydraulic Conductivity

The results gained for the microstructure analysis of all specimen types is the

cause of some confusion. As expected, all samples were found to have a

total porosity in the region of 50%±10 with the iron oxide amended ceramic

containing a higher total porosity. However, the results gained from the

hydraulic conductivity – in which the conductivity was found to be

approximately 40x lower than expected – appeared to contradict the findings

of the pycnometry.

As hydraulic conductivity was only calculated for C1 which had a total

porosity of 43%, it is possible that this result is not out of the ordinary. In

practice, burnout/clay ratios can be up to 60/40 rather than 50/50. Lee

showed that a small difference in total pore volume has an exponential effect

on the hydraulic conductivity [48]. It is also possible that by not screening

sawdust particles between two mesh sizes that very fine particles contributed

to the total porosity but resulted in vey small channels for water to flow

though, thus leading to the extra resistance observed.


Sean Rivers 42

Another perhaps stronger hypothesis is that the needle structures observed

in the clay micrographs were not resin, but were the result of tree sap or

similar hydrocarbons present in the burn-out material which did not vaporise

and vacate the ceramic completely during the firing process. This theory

could explain why when exposed to boiling water during pycnometry, pores

could be occupied. It is possible that as the sap melted or became more

pliable with heat during the pycnometry it was possible for water to enter the

pores, however under cold conditions such as in the column tests, the sap

would solidify, preventing water from easily penetrating the pore spaces.

For this hypothesis to be investigated it would be necessary to place an

amount of the burnout material in a furnace while running the protocol shown

in figure 11. If residue was found to be left then the hypothesis is likely true. If

there is no residue left after firing it does not necessarily disprove the theory

as gases could be trapped in the pored and re-crystallise upon cooling.

In determining the composition of hydrocarbons present in the sawdust it

would be possible to use a thermal gravimetric analyser (TGA) to measure

the weight loss of the sawdust over a burn cycle. This can be compared to

data from a differential thermal analyser (DTA) which measures the energy

consumption or emission as a function of temperature. Together the two

profiles can be used to form an idea of the types of hydrocarbons present in

the sawdust. It would also be possible to run multiple TGA tests in

succession and observe whether a decrease in mass is recorded. If all the

burnout material is vaporised during initial sintering then no subsequent

change in mass would be expected by re-firing the sample.

Other methods to ascertain the nature of the structures (including second

phase particles) observed in the micrographs would be to use scanning

electron microscopy (SEM). Energy dispersive x-ray spectroscopy (SEM-

EDX) is a method which can be used to determine the elemental composition

of a compound, however this method is not suited to detecting low numbered

elements such as oxygen and hydrogen, which comprise hydrocarbons.

Although SEM-EDX can give a count of elements in an area, it does not

provide information on how the elements are arranged in compounds.


Sean Rivers 43

Electron backscattered diffraction and x-ray diffraction (EBSD-XRD) is able

to provide crystallographic information on the phase of compounds.

5.3. Empty Bed Contact Time

If the hypothesis regarding the sap is correct then the EBCT would have to

be adjusted for the occlusion of pore volumes which would cause a smaller

empty bed volume and thus a shorter EBCT.

An estimation of possible EBCTs and flow rates required for an arsenic

removing filtrón is calculated below.

As filter hydraulics throughout the depth of the filter are complex [49], this

example calculation will just deal the bottom surface of the filter as it will

experience the highest hydrostatic head and thus the shortest ECBT time of

any part of the filter. This means if the required EBCT is satisfied for the filter

base, then it will be satisfied for all the filter.

Singh reported a contact time of half an hour required for maximum arsenic

removal [6]. As the body is only one seventh hematite then the following

calculation will be carried out for both EBCTs of 0.5 hours and 3.5 hours for

an arsenic removing filtrón.

As the flow rate is directly related to the EBCT, the EBCT is first calculated

for a real filtrón. By knowing the flow rate for a real filtrón (1.5-2l/hr), the flow

rate of an arsenic removing filtrón can be simply calculated by the fractions

of one EBCT over another.

The flow rate for a real filtrón specimen is calculated using the hydraulic

conductivity of 0.001m/hr from table 5 in section 4.4.1, and a filter thickness

of 12mm.

l

hkAQ

(9)

hrcml

hkAQ /8.23

2.1

209.111.0 3


Sean Rivers 44

As EBCT results in section 4.4.2 were considered inaccurate the estimated

EBCT for a real filtrón of 12mm thickness is calculated. The pore volume is

taken from pycnometry measurements in table 6.

Q

VEBCT

(11)

min122.08.23

76.4 hrs

Q

VEBCT

The resulting flow rate for an arsenic removing filtrón can be calculated as a

fraction of equation (11) for each case.

2

1

1

2

2

1

V

V

EBCT

EBCT

Q

Q

(12)

where subscript 1 indicates the arsenic removing filtrón and subscript 2

represents the real filtrón. As the arsenic removing filtrón requires a longer

EBCT it can be assumed that V1/V2 <1. Therefore equation (11) becomes

equation (12).

2

1

21 Q

EBCT

EBCTQ

(12)

So for an arsenic removing filtrón with a 30min contact time,

hlQ /6.05.130

12

For a 3.5hr contact time,

hlQ /086.05.1605.3

12

In summary, flow rates are seen to diminish by at least 60-94% to obtain

arsenic free water by using a filtrón with a 30min contact time for hematite

and depending on hematite coverage within pore surfaces.


Sean Rivers 45

The flow rate could be improved by either using a different sorptive media

requiring a shorter contact time, or allowing a trade of between quality and

quantity of water.

5.4. Arsenic Removal by iron hydroxide amended ceramic

Though this investigation did not provide conclusive results on the ability of

goethite amended ceramic to lower arsenic concentrations it is still of value

to discuss the options possible in the technology. Iron has a chemical affinity

for binding with arsenic and there are many iron hydroxides such as goethite,

hematite, jarosite and lepidocrocite which have been shown to effectively

bind with arsenic. Goethite is generally better at arsenic removal in terms of

capacity and contact time although problems lie in sintering without

converting goethite to hematite. It could be worth looking at ways of sintering

goethite in such a way to prevent a chemical change to hematite. Though it

could be possible to try and achieve this by sintering in a reducing

environment with an argon or nitrogen blanket, this would be difficult to

replicate in a development context.

The question of leaching is of great concern. The very reason for the

groundwater poisoning of Bangladesh is due to the release of arsenic from

ferric compounds under reducing conditions coupled with the presence of

microorganisms. It would be all too easy for filtróns to be improperly cleaned,

or kept under anoxic conditions by being constantly refilled and not aerated,

thus possibly causing dangerous levels of arsenic to be released into

drinking water which people believe to be safe. Leaching can also cause

damage to the environment upon disposal.

The use of oxidising agent and flocculants would have to be either prevented

in the use of an arsenic treating filtrón or perhaps a layer of material (if the

mesh was fine enough) could be used as a barrier to prevent agglomerates

clogging the surface pores. Furthermore, following any laboratory testing

which may prove the viability of arsenic treating filtróns, a small field study

would be necessary to determine whether the technology really is

appropriate outside of controlled laboratory conditions and whether


Sean Rivers 46

beneficiaries are able (and willing) to use and maintain the technology to

ensure it‘s effectiveness.

5.5. Arsenic Detection Method

Although the HACH method was used in this investigation it was not the

preferred method. Inductively Coupled Plasma Optical Emission

Spectroscopy (ICP-OES) or ICP-Mass Spectroscopy (ICP-MS) would have

provided a much more accurate result. The draw back with these methods

compared to the HACH field kit is the expensive equipment required,

stringent sample preparation controls and expertise required to operate the

equipment. It was attempted to use Flameless Atomic Adsorption

Spectroscopy (FAAS) but the detection limit of the equipment was found to

be in the ppb scale meaning the concentrations of arsenic used in this

investigation could not be detected. For detection of arsenic to ppb scale a

carbon tube based FAAS is required which was not available at the time.

6. Conclusions

Though not directly related to arsenic removal, it was found during the field

research that coconut oil can be used as a substitute for plastic bags in the

manufacturing process, providing a more environmentally sustainable to the

problem of removing pressed filters from moulds.

It was confirmed that the addition of goethite into the ceramic at a while

maintaining the volume ratio of burnout to clay+additives led to a greater

porosity than ceramic without goethite. A 1/6 ratio of goethite/clay resulted in

the ceramic porosity to be increased from 45% to 51%.

Microscopic analysis showed the presence of a crystalline formation in the

pore structures, although it is not known what the material is or whether it is

representative of other filters.


Sean Rivers 47

7. Future Work

There were areas where this study was not able obtain results on what may

potentially be an appropriate technology for arsenic removal from drinking

water. As such the author recommends the following future work.

An investigation into the efficacy of arsenic removal by goethite

amended ceramic for As(III) and As(V) at various influent

concentrations and for various contact times.

To obtain Freundlich and Langmuir adsorption isotherm adsorption

expressions for goethite amended ceramic.

To determine the maximum adsorption capacity of goethite amended

ceramic.

To perform a toxic characteristic leaching protocol (TCLP) for goethite

amended ceramic containing adsorbed arsenic (preferably after

capacity breakthrough).

Investigation into the effect of oxic and anoxic operating conditions on

the sorption and leaching characteristics of goethite amended

ceramic.

Methods of preserving goethite during the sintering process.

The use of iron oxides other than hematite in filtrón production.


Sean Rivers 48

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Sean Rivers 49

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Sean Rivers 50

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Sean Rivers 51

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Sean Rivers 53

Appendices

Appendix I - Specimen Batch Information Table A - C1 and C2

Specimen

Number

1 2 3

Clay Weight (g) 30 32 34

Thickness (mm) 11.7-12.66 12.16-12.6 13.34-13.94

Diameter (mm) 38.74 39.24 38.8

Dried Weight (g) 22 23 24

Table B - Goethite amended ceramic specimen

Specimen

Number

1 2 3

Clay Weight (g) 25 27 29

Thickness (mm) ~10 ~11 ~13

Diameter (mm) 18 19 21

Dried Weight (g) 22 23 24


Sean Rivers 54

Appendix II - Arsenic dosing

Atomic Masses for Sodium Arsenate di-basic Heptahydrate

(Na2HAsO4·7H2O)

Table C - Mass calculation for Sodium Arsenate di-basic Heptahydrate

Element Atomic

Mass

Number

of atoms

Total

Mass

Hydrogen

(H)

1 15 15

Oxygen

(O)

16 11 176

Sodium

(Na)

23 2 46

Arsenic

(As)

75 1 75

TOTAL 313

Proportion of mass which is As: 75/313 = 0.239

For a 5l 300ppb As solution is: 0.3mg x 5 = 1.5mg of As

So, the total amount of Sodium Arsenate di-basic Heptahydrate required for

5l of 300ppb As solution is: 1.5/.239 = 6.28mg.

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Iron oxide amended ceramic water purifiers for point-of-use arsenic removal from drinking water

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