21
22
CHAPTER 1
INTRODUCTION
HEAVY METALS: ENVIRONMENTAL THREAT
The pollution of the aquatic environment with heavy metals has become a worldwide
problem during recent years, because they are indestructible and most of them have
toxic effects on organisms (Sen et al., 2011). The term heavy metal is usually referred
to elements having densities greater than 5.0 g/cm-3
commonly adopted as a group
name for the metal and metalloids which are associated with pollution and toxicity
(Lentech, 2004). They are present in trace levels in the earth’s crust, anthropogenic
activities such as industrial processing and use of metals, alloys and metallic
compounds largely adding up their natural background levels and finally find their way
into the human system by two prominent ways. First, through inhalation and
consumption of water, secondly indirect uptake of toxic metals through consumption of
food, ultimately depending on water components of the ecosystem.
Some heavy metals are of great concern because of their toxic effects on plants and
animals and their increasing abundance in the environment. The bio toxic effects refer
to the harmful effects of heavy metals to the body when consumed above the
recommended limits. Although individual metals exhibit specific signs of their toxicity,
the following have been reported as general signs associated with cadmium, lead,
arsenic, mercury, zinc, copper and aluminium poisoning: gastrointestinal (GI)
disorders, diarrhoea, stomatitis, tremor, hemoglobinuria causing a rust–red colour to
stool, ataxia, paralysis, vomiting and convulsion, depression, and pneumonia when
volatile vapours and fumes are inhaled. The nature of effects could be toxic (acute,
chronic or sub-chronic), neurotoxic, carcinogenic, mutagenic or teratogenic (Duruibe et
al., 2007). To avoid health hazards it is essential to remove these toxic heavy metals
from waste water before its disposal.
‘HEAVY METAL POLLUTION: A PRIZE TAG OF MODERN SOCIETY’
23
HEAVY METALS: AN OVERVIEW
Abundant resource of water is available in our country. Unfortunately, rapid
industrialization, fast growth in population and non-judicious use of natural resources
has resulted into many fold increase in water pollution problem. Most of the 14 major
rivers of India are victims of heavy metal water pollution; Ganga and Yamuna ranking
top among them (Singh, 2001; Jain and Sharma, 2001; Kaushik et al., 2003; Sharma,
2003 and Kumar et al., 2005, 2009). Heavy metals (lead, Cadmium, Chromium and
Nickel) being important from local environmental point of view are considered for the
study. Heavy metals have been reviewed thoroughly in the voluminous manner in the
literature. Therefore, important features of target metals like their physical and
chemical properties, environmental sources, environmental concentrations and
toxicity along with permissible limit has been presented in a concise manner.
Figure 1.1: Heavy Metals
24
LEAD
Lead is a main-group element in the carbon group with the symbol Pb (from Latin:
plumbum) and atomic number 82. Lead is a soft, malleable poor metal. Lead accounts
for most of the cases of paediatric heavy metal poisoning. It is a very soft metal and
was used in pipes, drains, and soldering materials for many years. Millions of homes
built before 1940 still contain lead (e.g., in painted surfaces), leading to chronic
exposure from weathering, flaking, chalking, and dust. Every year, industry produces
about 2.5 million tons of lead throughout the world. Most of this lead is used for
batteries. The remainder is used for cable coverings, plumbing, ammunition, and fuel
additives. Other uses are as paint pigments and in PVC plastics, X-ray shielding, crystal
glass production, and pesticides. Target organs are the bones, brain, blood, kidneys, and
thyroid gland.
General Properties
Symbol Pb
Atomic number 82
Group, period, block 14, 6, p
Electronic configuration [Xe] 4f14
5d10
6s2 6p
2
Appearance Metallic grey
Atomic Properties
Crystal structure Face-cantered cubic
Electro negativity 2.33
Atomic radius 0.175 nm
Vander Waal radius 0.202 nm
Covalent radius 0.146 nm
Physical Properties
Characteristics Soft, malleable & poor
metal
Phase Solid
Density 11.34 g cm-3
Chemical Properties
Atomic mass 207.19 g/mol-1
Melting Point 327.460C
Boiling Point 17490C
Electrical resistivity (20 °C) 208 nΩ·m
Lead has been ranked as 2nd in the Environmental Protection
Agency “Top Hazardous Substance Priority List”.
25
L
E
A
D
Environmental Sources Natural
Weathering,
Flaking,
Chalking, and dust.
Anthropogenic Pb batteries Paint Pigments Coatings Pesticides PVC Plastics
Environmental Concentrations
Air
(0.10- 0.30 µg/m3)
Water
(5 µg/L)
Soil
(300 µg/m3 – 1.1 mg/Kg)
Government Standards and Guidelines
The Environmental Protection Agency (EPA) allows 15 parts of cadmium per billion parts of drinking water (5 ppb).
The World Health Organization (WHO) limits 10 parts of cadmium per million parts of food color (15 ppm). The Occupational Safety and Health Administration (OSHA) limits workplace air to 50 micrograms (µg) cadmium per cubic meter (50 µg/m3). .
Health Effects
· Birth defects,
· Mental retardation,
· Autism,
· Psychosis,
· Allergies,
· Dyslexia,
· Hyperactivity,
· Weight loss,
· Shaky hands,
· Muscular weakness,
· Paralysis (beginning in the
forearms)
Source: Agency for Toxic Substances and Disease Registry (ATSDR), 2011
26
CADMIUM
Cadmium was discovered in Germany in 1817 by Friedric Strohmeyer. Cadmium is
odorless and tasteless and chemical analysis is most often required to detect its
presence. Remarkable characteristics of cadmium involve its great resistance to
corrosion, low melting point and excellent electrical conductivity because of which it
plays a critical role in several cutting edge technologies such as solar cells. Cadmium is
one of the few elements that have no constructive purpose in the human body.
General Properties
Symbol Cd
Atomic number 48
Group, period, block 12,5,d
Electronic configuration [Kr] 4d10
5s2
Appearance Silvery grey metallic
Atomic Properties
Crystal structure Hexagonal
Electro negativity 1.69
Atomic radius 0.161 nm
Ionic radius 0.097 nm
Covalent radius 0.148 nm
Physical Properties
Characteristics Malleable and ductile
Phase Solid
Density 8.65 g cm-3
Chemical Properties
Atomic mass 112.41 g/mol-1
Melting Point 3210C
Boiling Point 7670C
Standard Potential -0.402 V
Cadmium has been ranked as 7th in the Environmental
Protection Agency “Top Hazardous Substance Priority List”.
27
Itai-Itai disease
Source: Agency for Toxic Substances and Disease Registry (ATSDR), 2011
C
A
D
M
I
U
M
Environmental Sources Natural
Weathering of rocks Volcanic eruptions Fall out
Anthropogenic Ni-Cd batteries Pigments Coatings Stabilizers Fertilizers
Environmental Concentrations
Air
Rural area : 0.1-5 ng/m3 Urban area : 2-15 ng/m3 Industrial area : 15-50 ng/m3
Water Sea water : ~0.1 mg/L Fresh water : 1-10 mg/L Industrial water : 10-1000 mg/L
Soil : 3 - 750 ppm
Government Standards and Guidelines
The Environmental Protection Agency (EPA) allows 5 parts of cadmium per billion parts of drinking water (5 ppb).
The Food and Drug Administration (FDA) limits 15 parts of cadmium per million parts of food color (15 ppm). The Occupational Safety and Health Administration (OSHA) limits workplace air to 100 micrograms (µg) cadmium per cubic meter (100 µg/m3). .
Health Effects
· Itai-Itai
· Muscle cramps
· Fragile bones
· Lungs and Kidney malfunctioning
· Liver injury
· Sensory disturbance
· Carcinogenicity
· Mutagenicity
· Affect cardiovascular system
· High blood pressure
28
CHROMIUM
Chromium was discovered by French chemist Nicholas Louis Vanquelin in 1797. The
most common oxidation states of Chromium are +2, +3 and +6 with +3 being the most
stable. The oxidation states +4 and +5 are relatively rare. Chromium compounds of +6
oxidation states are powerful oxidizing agents. Chrome metal (Chromium 0) is the
element that makes steel “stainless”.
General Properties
Symbol Cr
Atomic number 24
Group, period, block 6,4,d
Electronic configuration [Ar] 3d4 4s
2
Appearance Silvery metallic
Atomic properties
Crystal structure cubic body centered
Electro negativity 1.6
Atomic radius .166 nm
Ionic radius 0.061 nm
Covalent radius 0.127 nm
Physical properties
Characteristics Lustrous and brittle
Phase Solid
Density 7.19 g cm-3
Chemical properties
Atomic mass 51.99 g/mol-1
Melting Point 19070C
Boiling Point 26720C
Standard Potential -0.402 V
Chromium has been ranked as 17th in the Environmental
Protection Agency “Top Hazardous Substance Priority List”.
29
C
H
R
O
M
I
U
M
Environmental Sources
Natural Weathering of rocks Volcanic eruptions Fall out, Meteorites
Anthropogenic
Chrome plating Dyes and paints As a catalyst Leather tanning Glasswares Magnetic tapes
Environmental Concentrations
Air Electrolysis sector: 500 µg/m3 Urban area : 120 µg/m3 Industrial area : 52 µg/m3
Water
Waste water : 5 mg/L Cr (III) : 0.05 mg/L Cr (VI) Drinking water : 100 µg/L
Soils : 10-1000 ppm
Government Standards and Guidelines
Environmental Protection Agency (EPA) has set a limit of 100 µg chromium (III) and chromium (VI) per liter of drinking water (100 µg/L). The Occupational Safety and Health Administration (OSHA) has set limits of 500 µg water soluble chromium(III) compounds per cubic meter of workplace air (500 µg/m³)
Environmental Toxicity · Carcinogenic in nature
· Kidney and Liver damage
· Lung Cancer
· Respiratory Problems
· Ulcers in nasal septum
· Redness and swelling of skin
· Convulsions
· Skin ulcers
· Nose irritation
Source: Agency for Toxic Substances and Disease Registry (ATSDR), 2011
Lung Cancer
Cancer
30
NICKEL
Nickel is world 24th
most abundant transition metal. The element was discovered
unintentionally in 1751 by Baron Axel Frederick Cronstedt, who extracted it from a
mineral called Niccolite. Nickel can be combined with other elements such as iron,
copper, chromium and zinc to form alloys. These alloys are used to make coins,
jewellery and items such as valves and heat exchangers. Many nickel compounds
dissolve fairly easy in water and have a green colour. The most important oxidation
state of Nickel is +2.
General Properties
Symbol Ni
Atomic number 28
Group, period, block 10,4,d
Electronic configuration [Ar] 3d8 4s
2
Appearance silvery metallic
Atomic properties
Crystal structure Face centered cubic
Electro negativity 1.91
Atomic radius 0.149 nm
Ionic radius 0.069 nm
Covalent radius 0.121 nm
Physical properties
Characteristics Lustrous and hard
Phase Solid
Density 8.9 g cm-3
Chemical properties
Atomic mass 58.69 g/mol-1
Melting Point 14530C
Boiling Point 29130C
Standard Potential -0.25 V
Nickel has been ranked as 57rd in the Environmental
Protection Agency “Top Hazardous Substance Priority List”.
31
Environmental Concentrations
Air
Electrolysis sector: 0.4 mg/m3 Urban area : 17-25 mg/m3 Industrial area : 120-170 mg/m3
Water
Sea water : 0.1-20 mg/L Drinking water : 0.7 mg/L
Soils : 3-1000 ppm
Environmental Sources
Natural Weathering of rocks Volcanic eruptions Fall out, Meteorites
Anthropogenic Roasting plants Smelting plants Electroplating Steel industry Batteries Oil Hydrogenation
N
I
C
K
E
L
Government Standards and Guidelines
The Environmental Protection Agency (EPA) recommends that drinking water should not contain more than 0.7 milligrams of nickel per liter of water (0.7 mg/L). The Occupational Safety and Health Administration (OSHA) limits workplace air to 1 µg of nickel per cubic meter (1 µg/m3).
Environmental Toxicity
§ Dental Prostheses § Acute poisoning § Dermatitis § Asthma § Respiratory cancer § Malignant neoplasm § Lung embolism § Asthma and bronchitis § Heart disorders
Source: Agency for Toxic Substances and Disease Registry (ATSDR), 2011
Dermatitis
32
METAL DECONTAMINATION: TECHNIQUES USED SO FAR
Present age of rapid increase in metal concentration as well as increase in awareness of
the toxicological effects of metals released into environment, a number of studies for
metal recovery and removal for metal solution have been done (Ashraf et al. 2011).
With the enactment of several water legislations and guidelines worldwide (South
Africa Water Act, US Clean Water Act, Australian Water Quality Guidelines, etc)
coupled with the need for environmental sustainability (Goal 7 of the Millennium
Development Goals), several stringent levels of water quality in domestic and industrial
water and wastewater are required. Because heavy metal pollution affects the quality
of drinking water supply and wastewater discharge, great efforts have been made in the
last two decades to reduce pollution sources and remedy polluted water resources. The
commonly used procedures for removing metal ions from aqueous streams include
distillation, ion exchange, reverse osmosis, electro dialysis, precipitation,
coagulation, flocculation and ultra filtration. The basic principle, procedural details
and commercially available instrumentation based on above phenomenon have been
described in brief as below:
DISTILLATION
Distillation is probably the oldest method of
water purification. Water is first heated to
boiling. The water vapour rises to a condenser
where cooling lowers the temperature so the
vapour is condensed, collected and stored. It
removes a broad range of contaminants.
EVAPORATION
Evaporation is the most common and cost effective
technique for recovery of heavy metals. Evaporation can
provide total as well as partial recovery of metals. The
application of atmosphere evaporation is used on a wide
variety of process effluents from industries.
Figure 1.2: Distillation
Figure 1.3: Evaporators
33
CHEMICAL PRECIPITATION
Chemical treatment process prior to biological process is widely applicable in the
treatment of raw wastewaters. Moreover, various existing conventional primary waste
water plants are shifting towards chemically enhanced treatment to improve the quality
of the treated effluent and reduce cost. Chemical
precipitation is incorporated by raising the pH of
waste water by addition of alkaline chemicals, viz.
lime, limestone, caustic soda, soda, ash,
magnesium hydroxide. At alkaline pH most heavy
metals size range between 0.1-100m precipitated as
metal hydroxides or metal carbonates and
separated by gravity clarification or field
separation method.
FLOCCULATION AND COAGULATION
Flocculation is a process which clarifies the water. Clarification is done by causing a
precipitate to form in the water which can be removed using simple physical methods.
For water treatment plants using surface water as the source water, coagulation-
flocculation is the most commonly used
physicochemical process for particle removal
and is an essential pre-treatment process for
sedimentation and filtration [Figure 1.5]. The
three main types of chemical coagulants are
(i) Inorganic electrolyte e.g. Alum, lime,
ferric chloride and ferrous sulfate (ii) Organic
polymers (iii) Synthetic polyelectrolytes with
anionic and cationic functional groups which
are often used for precipitation. On addition
of coagulant, the flocculation occurs and
the size of particle in floc increases by aggregation and settle at the faster rate. Soluble
impurities in water can also be partially removed by coagulation. Because the complete
removal of impurities requires the separation of aggregates from water treatments,
Figure 1.5: Coagulation and Flocculation
Figure 1.4: Precipitation
34
colloidal systems of extremely slow settling velocity are viewed as stabilized systems.
The coagulation process in water treatment includes three sequential steps: (A)
addition of coagulant; (B) destabilization of particles (C) aggregation of destabilized
particles.
ELECTRO COAGULATION
Electro coagulation is based on the formation of the coagulant as the sacrificial anode
corrodes due to an applied current while the simultaneous evolution of hydrogen at the
cathode allow the pollutant removal by floatation. Chemical reaction occurring in the
electro coagulation process shows that the main reaction occurs at the electrodes which
are as follows:
In addition, Al+3
and OH- ions generated at electrode surfaces react in the bulk media
waste water to form aluminium hydroxide [Al(OH)3] flocs which act as adsorbents for
metal ions and eliminate them from the
waste water. The hydroxyl ions which are
produced at the cathode increase the pH in
the electrolyte bulk liquid and may include
co precipitation of metals in the form of
their corresponding hydroxides [Figure 1.6].
This acts synergistically to remove
pollutants from waste water. The efficiency
of electro coagulation is dependent on pH,
current density and metal ion concentration.
ION EXCHANGE
Ion exchange processes in water treatment have
been used primarily for softening. Some of them
are used to deionize, disinfect or scavenge
macromolecules from water. Typical ion
Al Al+3 + 3e-
2H2O + 3e- 3/2 H2 + 3 OH -
Figure 1.7: Ion exchange process
Figure 1.6: Electro Coagulation process
35
exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites,
montmorillonite, clay, soil humus and synthetically produced organic resins. The
synthetic organic resins are frequently used today because their characteristics can be
tailored to specific applications.
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte
solution and a complex [Figure 1.7]. In most cases the term is used to denote the
processes of purification, separation, decontamination of aqueous and other ion-
containing solutions with solid polymeric or mineralic ion exchangers. Ion exchangers
are either cation exchangers that exchange positively charged ions (cations) or anion
exchangers that exchange negatively charged ions (anions). There are also amphoteric
exchangers that are able to exchange both cations and anions simultaneously. However,
the simultaneous exchange of cations and anions can be more efficiently performed in
mixed beds that contain a mixture of anion and cation exchange resins, or passing the
treated solution through several different ion exchange materials.
Ion exchangers can be unselective or have binding preferences for certain ions or
classes of ions, depending on their chemical structure. This can be dependent on the
size of the ions, their charge, or their structure.
ULTRA FILTRATION
Ultra Filtration (UF) is a variety of membrane filtration in which hydrostatic pressure
forces a liquid against a semi permeable
membrane. Suspended solids and solutes of
high molecular weight are retained while
water and low molecular weight solutes pass
through the membrane [Figure 1.8]. This
separation process is used in industry and
research for purifying and concentrating
macromolecular (103-10
6 Da) solutions,
especially protein solutions. Ultra filtration is
not fundamentally different from reverse osmosis
microfiltration or nanofiltration, except in terms of the size of the molecules it retains.
Figure 1.8: Ultra Filtration
36
The UF process is applicable for particles in the molecular range of 0.1-0.01µm. Ultra
Filtration (UF) is a pressure-driven, membrane filtration process that is used to separate
and concentrate macromolecules and colloids from wastewater. A fluid is placed under
pressure on one side of a perforated membrane of a measured pore size. All materials
smaller than the measured pore size pass through the membrane, leaving large
contaminants concentrated on the feed side of the membrane. UF is used as a
pretreatment step to Reverse Osmosis (RO) or as a stand-alone process. The UF process
cannot separate constituents from water as effectively as RO. However, the two
technologies can be used in tandem, with UF removing most of the relatively large
constituents of a process stream before RO application selectively removes water from
the remaining mixture.
REVERSE OSMOSIS
Reverse Osmosis (RO) fills a unique position in the area of water and wastewater
treatment. Reverse osmosis (RO) is the most economical method of removing 90% to
99% of all contaminants. The pore
structure of RO membranes is much tighter
than UF membranes. RO membranes are
capable of rejecting practically all particles,
bacteria and organics >300 Daltons
molecular weight.
Natural osmosis occurs when solutions
with two different concentrations are separated
by a semi-permeable membrane [Figure 1.9]. In water purification systems, hydraulic
pressure is applied to the concentrated solution to counteract the osmotic pressure. Pure
water is driven from the concentrated solution and collected downstream of the
membrane. Because RO membranes are very restrictive, they yield very slow flow
rates. RO also involves an ionic exclusion process. Only solvent is allowed to pass
through the semi-permeable RO membrane, while virtually all ions and dissolved
molecules are retained (including salts and sugars). The semi-permeable membrane
rejects salts (ions) by a charge phenomena action: the greater the charge, the greater the
rejection. Therefore, the membrane rejects nearly all (>99%) strongly ionized
polyvalent ions but only 95% of the weakly ionized mono valent ions like sodium.
Figure 1.9: Reverse Osmosis
37
Table 1.1: List of commercially available Reverse Osmosis membranes
ELECTRO DIALYSIS
Electro Dialysis (ED) is used to transport salt ions from one solution through ion-
exchange membranes to another solution under the influence of an applied electric
potential difference. This is done in a configuration called an electro dialysis cell. The
cell consists of a feed (diluate) compartment and a concentrate (brine) compartment
formed by an anion exchange membrane
and a cation exchange membrane placed
between two electrodes [Figure 1.10]. In
almost all practical electro dialysis
processes, multiple electro dialysis cells
are arranged into a configuration called
an electro dialysis stack, with alternating
anion and cation exchange membranes
forming the multiple electro dialysis
cells. Electro dialysis processes are unique
compared to distillation techniques and other membrane based processes in that
dissolved species are moved away from the feed stream rather than the reverse.
Because the quantity of dissolved species in the feed stream is far less than that of the
fluid, electro dialysis offers the practical advantage of much higher feed recovery in
many applications.
COMMERCIALLY AVAILABLE ACTIVITY COMPANY
REVERSE OSMOSIS MEMBRANES
Cellulose acetate membranes for salt removal Biosour. Inc.,Worcester, U.K
Cellulose tri acetate membranes for bacterial removal Miox Corp., Albuquerque
Figure 1.10: Electro Dialysis
38
EXISTING METAL REMOVAL TECHNOLOGIES: DEMERITS
The conventional processes for removing heavy metals from wastewater include many
processes such as chemical precipitation, flotation, adsorption, ion exchange, and
electrochemical deposition. Chemical precipitation is the most widely used method for
heavy metal removal from inorganic effluent. Adjustment of pH to the basic conditions
(pH 9–11) is the major parameter that significantly improves heavy metal removal by
chemical precipitation. Lime and limestone are the most commonly employed
precipitating agents due to their availability and low-cost in most countries (Mirbagherp
and Hosseini, 2004; Aziz et al., 2008). Lime precipitation can be employed to
effectively treat inorganic effluent with a metal concentration of higher than 1000
mg/L. Other advantages of using lime precipitation include the simplicity of the
process, inexpensive equipment requirement, and convenient and safe operations.
However, chemical precipitation requires a large amount of chemicals to reduce metals
to an acceptable level for discharge. Other drawbacks are its excessive sludge
production that requires further treatment, slow metal precipitation, poor settling, the
aggregation of metal precipitates, and the long-term environmental impacts of sludge
disposal (Aziz et al., 2008). Ion exchange is another method used successfully in the
industry for the removal of heavy metals from effluent. The disadvantage of this
method is that it cannot handle concentrated metal solution as the matrix gets easily
fouled by organics and other solids in the wastewater. Moreover ion exchange is non
selective and is highly sensitive to the pH of the solution. Electrolytic recovery or
electro-winning is one of the many technologies used to remove metals from process
water streams. This process uses electricity to pass a current through an aqueous metal-
bearing solution containing a cathode plate and an insoluble anode. Positively charged
metallic ions cling to the negatively charged cathodes leaving behind a metal deposit
that is strippable and recoverable. A noticeable disadvantage was that corrosion could
become a significant limiting factor, where electrodes would frequently have to be
replaced (Kurniawan et al., 2006).
Although many techniques can be employed for the treatment of wastewater laden with
heavy metals, it is important to note that the selection of the most suitable treatment for
metal contaminated wastewater depends on some basic parameters such as pH, initial
metal concentration, the overall treatment performance compared to other technologies,
39
environmental impact as well as economics parameter such as the capital investment
and operational costs. Finally, technical applicability, plant simplicity and cost
effectiveness are the key factors that play major roles in the selection of the most
suitable treatment system for inorganic effluent. All the factors mentioned above should
be taken into consideration in selecting the most effective and inexpensive treatment in
order to protect the environment.
Table 1.2: Advantages and disadvantages of treatment technologies for the
removal of heavy metals from waste water
Technology Advantages Disadvantages References
Chemical
precipitation
Process simplicity,
Not metal selective,
Inexpensive capital cost
Large amounts of sludge
containing metals
Sludge disposal cost
High maintenance cost
Aderhold et al.,
1996
Ion exchange Metal selective,
Limited pH tolerance
High regeneration
High initial capital cost
High maintenance cost
Aderhold et al.,
1996
Coagulation &
flocculation.
Bacterial inactivation
capability
Good sludge settling and
dewatering characteristic
Chemical consumption
Increased sludge volume
generation
Aderhold et al.,
1996
Flotation Metal selective
Low retention times
Removal of small particles
High initial capital cost
High maintenance and
operation costs
Rubio et al., 2002
Membrane filtration Low solid waste generation
Low chemical consumption
Small space requirement
Possible to be metal
selective
High initial capital cost
High maintenance and
operation costs
Membrane fouling
Limited flow-rates
Mandaeni &
Mansourpanah,
2003
Electrochemical
treatment
No chemical required can
be engineered to tolerate
suspended solids
Moderately metal selective
treat effluent
High initial capital cost
Production of H2 (with
some process)
Filtration process for flocs
Kongsricharoen &
Polprasert, 1995
Kongsricharoen &
Polprasert, 1996
40
BIOSORPTION
The search for new technologies for the removal of toxic metals has directed attention
to biosorption phenomenon which is based on the metal binding capacity of agricultural
wastes (Kumar and Bandyopadhyay, 2006). Biosorption is a relatively new technique
that emerged in the 1980s and gained a considerable amount of attention since it has
shown to be very promising in the removal of contaminants from effluents in an
environmentally friendly manner (Gardea-Torresdey et al., 1996, Volesky 1999).
Unlike typical synthetic ion exchange resins, plant based biomaterials do not require the
use of toxic chemicals in their preparation. Therefore, plant based biomaterial systems
are considered more effective for removal of heavy metals. In recent perspectives,
biomaterials have gained much importance for decontamination of water. Biosorption
To combine technology with environmental safety is one of the key
challenges of the new millennium. There is a global trend of bringing
technology into harmony with natural environment, thus aiming to
achieve the goals of protection of ecosystem from the potentially
deleterious effects of human activity and finally improving its quality. The
challenges of safe and various treating and diagnosing environmental
problems require discovery of newer, more potent, specific, safe and cost
effective synthetic or biomolecules. The magic plants are around and
waiting to be discovered and commercialized. They are now recognized
and accepted as storehouses of infinite and limitless benefits to human
beings. These natural systems are often referred to as ‘Green
Technologies’, as they involve naturally occurring plant materials.
Biosorption is one such important phenomenon, which is based on one of
the twelve principles of Green Chemistry “Use of renewable resources”. It
has garnered a great deal of attention in recent years due to rise in
environmental awareness and consequent severity of legislation regarding
the removal of toxic metal ions from wastewater.
41
Major advantages of biosorption process
involves processes that reduce overall treatment cost through the application of
indigenous agricultural wastes which are particularly attractive as they lessen reliance
on expensive water treatment chemicals, negligible transportation requirements and
offer genuine, local resources as appropriate solutions to tackle local issues of water
quality problems. Regeneration of the biosorbent increases the cost effectiveness of the
process thus warrants its future success.
Biosorption clearly shows that from most perspectives, plants are ideal for
environmental clean up: capital cost is low, ongoing operational costs are minimal,
implementation is easy and non-invasive and public acceptance is high (Veglio and
Belochini, 2001; Volesky, 1999). All this shows that biosorption is a new and vibrant
technology having great potential. To realize this, it will be necessary to understand the
various processes that are involved in it. This may require a multidisciplinary approach
and diverse fields of plant biology.
Æ cost-effectiveness
Æ High efficiency
Æ Minimization of chemical and or biological sludge
Æ Regeneration of biosorbent
Æ Possibility of metal recovery
Æ competitive performance
Æ heavy metal selectivity
(Volesky, 2003)
42
BIOSORPTION: MECHANISTIC ASPECTS
The biosorption process involves a solid phase (biosorbent) and a liquid phase (solvent,
normally water) containing dissolved species to be sorbed (sorbate, metal ions). Due to
higher affinity of the sorbent for the sorbate species, the latter is attracted and bound
thereby different mechanisms. The process continues till equilibrium is established
between the amount of solid bound to the sorbate species and its portion remaining in
the solution. The degree of sorbent affinity for the sorbate determines its distribution
between solid and liquid phases. In Biosorption, the use of non living biomaterials
containing metal binding compounds would have the advantage of not requiring
tremendous care and maintenance as well as being useful in remediating toxic high
levels of contaminants that would otherwise kill live system (Basso et al., 2002).
The complex structure of plant materials and microorganisms implies that there are many
ways for the metal to be taken by the biosorbent. Numerous chemical groups have been
suggested to contribute to biosorption metal binding by either whole organisms or by
molecules. These groups comprises hydroxyl, carbonyl, carboxyl, sulphaydryl, thioether,
sulphonate, amine, amino, imidazole, phosphonate and phosphodiester etc. The importance
of any given group for biosorption of certain metals by plant biomass depends on factors such
as number of sites in the biosorbent material, the accessibility of the sites, the chemical state
of the sites (availability) and affinity between site and metal (Volesky et al., 1999). Various
metal-binding mechanisms have been postulated to be active in biosorption process. Due to
the complexity of the biomaterials used, it is possible that at least some of these mechanisms
are acting simultaneously to varying degrees, depending on the biosorbent and the solution
environment.
CHEMISORPTION
It is the adsorption in which the forces involved are valence forces of the same kind as
those operating in the formation of chemical compounds. Some features, which are
useful in recognizing chemisorption, include:
§ The phenomenon is characterized by chemical specificity.
43
§ Since the adsorbed molecules are linked to the surface by valence bonds, they
will usually occupy certain adsorption sites on the surface and only one layer of
chemisorbed molecules is formed (monolayer adsorption).
§ The energy of chemisorption is of the same order of magnitude as the energy
change in a chemical reaction between a solid and a fluid.
§ Chemisorption is irreversible.
Chemisorption is of four types: ion exchange, complexation, coordination and/or
chelation.
a. ION EXCHANGE
Ion exchange is a reversible chemical reaction wherein an ion in a solution is
exchanged for a similarly charged ion attached to an immobile solid particle. These
solid ion-exchange particles are either naturally occurring inorganic zeolites or
synthetically produced organic resins. Synthetic organic resins are the predominant type
used today because their characteristics can be tailored to specific applications.
Ion exchange reactions are stoichiometric, reversible and as such they are similar to
other solution-phase reactions. For example, in the reaction
NiSO4 + Ca (OH)2 ® Ni(OH)2 + CaSO4
the nickel ions of the nickel sulfate (NiSO4) are exchanged for the calcium ions of the
calcium hydroxide Ca (OH)2 molecule.
b. CHELATION
The word chelation is derived from the Greek word chele, which means claw, and is
defined as the firm binding of a metal ion with an organic molecule (ligand) to form a
ring structure. The resulting ring structure protects the mineral from entering into
unwanted chemical reactions. Examples include the carbonate (CO32–
) and oxalate
(C2O42–
) ions:
44
c. COORDINATION (COMPLEX FORMATION)
A coordination complex is any combination of cations with molecules or anions
containing free pairs of electrons. Bonding may be electrostatic, covalent or a
combination of both; the metal ion is coordinately bonded to organic molecules.
Example of the formation of a coordination compound is:
Cu2+
+ 4H2O ® [Cu (H2O)] 42+
Cu2+
+ 4Cl– ® [CuCl4]
2–
Where coordinate covalent bonds are formed by donation of a pair of electrons from
H2O and Cl– (Lewis bases) to Cu
2+ (Lewis acid).
In general, biosorption of toxic metals and radionuclide is based on non-enzymatic
processes such as adsorption. Adsorption is due to the non-specific binding of ionic
species to polysaccharides and proteins on the cell surface or outside the cell. Bacterial
cell walls and envelopes, and the walls of fungi, yeasts and algae, are efficient metal
biosorbents that bind charged groups. The cell walls of gram-positive bacteria bind
larger quantities of toxic metals and radionuclide than the envelopes of gram-negative
bacteria.
Bacterial sorption of some metals can be described by the linearized Freundlich
adsorption equation:
log S = log K + n log C
Where: S is the amount of metal absorbed in µmol/g, C is the equilibrium solution
concentration in µmol/L, K and n are the Freundlich constants.
Biomass deriving from several industrial fermentations may provide an economical
source of biosorptive materials. Many species have cell walls with high concentrations
of chitin, a polymer of N-acetyl-glucosamine which is an effective biosorbent.
Biosorption uses biomass raw materials that are either abundant (e.g., seaweeds) or
wastes from other industrial operations (e.g., fermentation wastes). The metal-sorbing
performance of certain types of biomass can be more or less selective for heavy metals,
depending on the type of biomass, the mixture in the solution, the type of biomass
preparation, and the chemical-physical environment.
45
It is important to note that the concentration of a specific metal in solution can be
reduced either during the sorption uptake by manipulating the properties of the
biosorbent or upon desorption during the regeneration cycle of the biosorbent.
PHYSIOSORPTION
In physiosorption, physiosorbed molecules are fairly free to move around the sample.
As more molecules are introduced into the system, the adsorbate molecules tend to
form a thin layer that covers the entire adsorbent surface. Some features, which are
useful in recognizing physiosorption, include:
§ The adsorbate molecules are held by comparatively weaker Vander Waal’s
forces, thus resulting into lower activation energy.
§ The process is, however, reversible as the substance adsorbed can be recovered
from the adsorbent easily on lowering the pressure of the system at the same
temperature.
§ Physiosorption may extend beyond a monolayer also, since the physical forces
can operate at any given distances.
§ Physical adsorption is not specific in nature because it involves Vander Waal’s
forces, which exist among the molecules of every two substances.
Physiosorption takes place with the help of Vander Waal’s forces. Knyucak and
Volesky (1987) hypothesized that Uranium, Cadmium, Zinc, Copper and Cobalt
biosorption by certain plant materials takes place through electrostatic attraction
between the metal ions in solution and functional groups present on the cell surface.
46
BIOSORBENTS USED SO FAR
Biosorption promises to fulfill the requirements, which are competitive, effective and
economically viable. Efforts have been made to use different forms of inexpensive plant
materials for the abatement of toxic metals from the aqueous media. Biosorbents, explored so
far in removing toxic metals from water bodies have been listed below:
Table 1.3: List of biosorbents used for metal removal
BIOSORBENT METALS REMOVED REFERENCES
Wood Cu (II), Cr (III), As (III) Clausen, 2000
Fruit peel of Orange Ni (II) Ajmal et al., 2000
Crab shell Pb (II) Park et al., 2001
Cone biomass Cr (VI) Ucun et al., 2002
Portulaca oleracea Cd (II), Cr (II), Pb (II),
As (III), Ni (II)
Vankar and Tiwari, 2002
Petiolar Palm sheath Cd (II), Ni (II), Pb (II), Zn (II) Iqbal et al., 2002
Orange juice residue As (III), As (V) Ghimire et al., 2002
Olive mill solid residue Cd (II), Ni (II), Pb (II), Cr (III) Pagnanelli et al., 2002
Olive pomace Cd (II), Cu (II), Pb (II) Pagnanelli et al., 2003
Lemna Minor Pb (II), Ni (II) Nicholas et al., 2003
Chitosan Cr (VI) Boddu et al., 2003
Cassava waste Cu (II), Zn (II) Horsfall et al., 2003
Fly ash As (III) Nagarnaik et al., 2003
Waste Crab shells Au (II, Cr (VI), Se (II) Niu and Volesky, 2003
Sphagnum peat moss Cd (II), Cu (II), Pb (II), Ni (II) Rosa et al., 2003
Rice husk Cd (II) Ajmal et al., 2003
Water Lettuce As (III), As (V) Basu et al., 2003
Sugarcane Bagasse pith Cd (II) Krishnan and Anirudhan, 2003
Hazelnut shell Cr (VI) Kobya, 2004
Aquatic moss Cd (II), Zn (II) Martins et al., 2004
Polysaccharides Pb (II), Hg (II), Cu (II) Son et al., 2004
Grape stalk wastes Ni (II), Cu (II) Isabel et al., 2004
The major challenge for the biosorption field was to select the most promising
types of biomass from an extremely large pool of readily available and
inexpensive biomaterials.
47
S.natans Au (II), Co (II) Kuyucak et al., 2005
S.cerevisiae Cu (II), Zn (II), Cd (II) Kuyucak et al., 2005
Jute fibers Cu (II), Ni (II), Zn (II) Shukla and Roshan, 2005
Chitosan Cu (II), Cr (III), As (III) Kartal and Imamura, 2005
Lechuguilla Cd (II), Gonzalez et al., 2006
Maize leaf Pb (II) Babarinde et al., 2006
C. baccata Cd (II), Pb (II) Lodeiro et al., 2006
Granular biomass Pb (II), Cd (II), Cu (II), Ni (II) Hawari et al., 2006
Fruits waste Cd (II) Schiewer et al., 2007
Saw dust, rice husk Pb (II) Abdel-Ghani et al., 2007
Saltbush plant Cr (III), Cr (VI), Cd (II) Sawalha et al., 2007
Citrus peels Cu (II) Schiewer et al., 2008
Opuntia Pb (II) Martnez et al., 2008
Sugarcane bagasse and maize cob
Cd (II) Garg U et al., 2008
Cassia fistula Cr (III) and Cr (VI) Abbas et al., 2008
Waste olive cake Zn (II) Fernando et al., 2009
Neem oil cake Cu (II), Cd (II) Rao et al.,2009
Tamarind Bark Fe (II) Prasad et al.,2009
potato peel Fe (II) Prasad et al.,2009
Mango biomass Pb (II), Zn (II), Cu (II), Ni (II) Ashraf et al.,2010
Eichhornia Crassipes Fe (III) , Cu (II),
Zn (II), Pb (II), Cr (III) and Cd (II)
Shama et al., 2010
Rice husk Cr (VI) Wongjunda and Saueprasearsit, 2010
Orange waste Zn (II) Marin et al., 2010
Spirogyra and Cladophora Pb (II) , Cu (II) Lee and Chang, 2011
Natural spider silk Pb (II) , Cu (II) Pelit et al., 2011
Broussonetia papyrifera)
leaf powder
Cu (II), Pb (II), and Cd (II) Nagpal et al., 2011
peanut shell Cu (II), Cr (III) Krowiak et al., 2011
Marine Algae Cu (II), Co (II), Ni (II), Cd (II), Hg
(II), Ag (II), Pb (II)
Elrefaii et al. 2012
Spirogyra hyalina Cd (II), Hg (II), Pb (II), As (III),Co
(II)
Kumar and oomen, 2012
Algae Scenedesmus
quadricauda and
Neochloris pseudoalveolaris
Co (II), Cr (III), Pb (II), Cd (II), Ni
(II), and Mn (II)
Kızılkaya et al. 2012
Water Hyacinth Cu (II), Zn (II) Buasri et al., 2012
Pseudomonas Oleovorans
and Brevundimonas
Vesicularis
Pb (II) Singh and Gadi, 2012
Sargassum wightii and
Caulerpa racemosa
Cr (VI), Cr (III), Pb (II) and Cd (II) Tamilselvan et al., 2012
48
NANOTECHNOLOGY AND WASTE WATER TREATMENT
As part of a major report commissioned by the UK Government from the Royal Society
and the Royal Academy of Engineering in the UK, entitled ‘‘Nanoscience and
nanotechnologies: opportunities and uncertainties’’, the following definitions were
used:
Nanoscience is the study of phenomena and manipulation of materials at atomic,
molecular and macromolecular scales, where properties differ significantly from those
at a larger scale.
OR
Nanotechnologies are the design, characterisation, production and application of
structures, devices and systems by controlling shape and size at nanometre scale.
OR
The creation of functional materials, devices and systems through control of matter on
the nanometre scale (1–100 nm) and exploitation of novel phenomena and properties
(physical, chemical, biological) at that length scale.
Figure 1.11: Comparison of nanoparticles with macro scale particles
Physicist Richard P. Feynman first described the concept of nanoscience in
December, 1959 in a lecture entitled “There's Plenty of Room at the Bottom” to the
American Physical Society and the term nanotechnology was coined in 1974 by
the Japanese researcher Norio Taniguchi to describe precision engineering with
tolerances of a micron or less. In the mid 1980s, Eric Drexler brought
nanotechnology into the public domain with his book Engines of Creation.
49
Recent advances in nano scale science and engineering suggest that many of the current
problems involving water quality could be resolved or greatly ameliorated using
nanosorbents, nanocatalysts, bioactive nanoparticles, nanostructured catalytic
membranes and nano particle enhanced filtration among other products and processes
resulting from the development of nanotechnology. Utilization of specific nanoparticles
either embedded in membranes or on other structural media that can effectively,
inexpensively and rapidly render unusable water potable is being explored at a variety
of institutions. In addition to obvious advantages for industrialized nations, the benefits
for developing countries would also be enormous. Innovative use of nanoparticles for
treatment of industrial wastewater is another potentially useful application. Many
factories generate large amounts of wastewater. Removal of contaminants and recycling
of the purified water would provide significant reductions in cost, time, and labor to
industry and result in improved environmental stewardship. Aquifer and groundwater
remediation are also critical issues, becoming more important as water supplies steadily
decrease and demand continues to increase. Most of the remediation technologies
available today, while effective, very often are costly and time consuming, particularly
pump-and-treat methods. The ability to remove toxic compounds from subsurface and
other environments that are very difficult to access in situ, and doing so rapidly,
efficiently and within reasonable costs is the ultimate goal. Recent selected studies on
the use of nanomaterials as separation and reactive media for water purification.
NANOSORBENTS
Sorbents are widely used as separation media in water purification to remove inorganic
and organic pollutants from contaminated water. Nanoparticles have two key properties
that make them particularly attractive as sorbents. On a mass basis, they have much
larger surface areas than bulk particles. Nanoparticles can also be functionalized with
various chemical groups to increase their affinity towards target compounds. Several
research groups are exploiting the unique properties of nanoparticles to develop high
capacity and selective sorbents for metal ions and anions. Li et al. (2003) have
investigated the sorption of Pb (II), Cu (II) and Cd (II) onto multiwalled carbon
nanotubes (MWCNTs). They reported maximum sorption capacities of 97.08 mg/g for
Pb (II), 24.49 mg/g for Cu (II) and 10.86 mg/g for Cd (II) at room temperature, pH 5.0
and metal ion equilibrium concentration of 10 mg/l. Li et al. (2003) also found that the
50
metal–ion sorption capacities of the MWCNTs were 3–4 times larger than those of
powder activated carbon and granular activated carbon, two commonly used sorbents in
water purification. Qi & Su (2004) have evaluated the sorption of Pb (II) onto chitosan
nanoparticles (40–100 nm) prepared by ionic gelation of chitosan and tripolyphosphate.
The phosphate-functionalized chitosan nanoparticles have a maximum Pb(II) sorption
capacity of 398 mg/g. Peng et al. (2005) have recently developed a novel sorbent with
high surface area (189 m2/g) consisting of cerium oxide supported on carbon nanotubes
(CeO2-CNTs). They showed that the CeO2-CNT particles are effective sorbents for As
(V). Interestingly, Peng et al. (2005) found that the addition (from 0 to 10 mg/l) of two
divalent cations [Ca (II) and Mg (II)] resulted in a substantial increase of the amount of
sorbed As (V) (from 10 to 82 mg/g). Deliyanni et al. (2003) have also synthesized and
characterized a novel As (V) sorbent consisting of akaganeite [b-FeO (OH)]
nanocrystals. In addition, Lazaridis et al. (2005) have shown that nanocrystalline
akaganeite is also an effective sorbent for Cr (VI).
Zeolites are effective sorbents and ion-exchange media for metal ions. NaP1 zeolites
(Na6Al6 Si10O32.12H2O) have a high density of Na ion exchange sites. They can be
inexpensively synthesized by hydrothermal activation of fly ash with low Si/Al ratio at
150 OC in 1.0–2.0 M NaOH solutions (Moreno et al., 2001). NaP1 zeolites have been
evaluated as ion exchange media for the removal of heavy metals from acid mine
wastewaters (Moreno et al., 2001). Alvarez-Ayuso et al. (2003) reported the successful
use of synthetic NaP1 zeolites to remove Cr (III), Ni (II), Zn (II), Cu (II) and Cd (II)
from metal electroplating wastewaters. Self-assembled monolayers on mesoporous
supports (SAMMS) are providing novel opportunities to develop more effective
sorbents for toxic metal ions (Yantasee et al., 2003), anions (Kelly et al., 2001) and
radionuclides (Fryxell et al., 2005; Lin et al., 2005). These sorbents are made via
surfactant templated synthesis of mesoporous ceramics. This produces nano porous
ceramic oxides with very large surface areas (1000 m2/g) and high density of sorption
sites that can be functionalized to increase their selectivity toward target pollutants.
Carbonaceous nanomaterials can serve as high capacity and selective sorbents for
organic solutes in aqueous solutions. Mangun et al. (2001) have synthesized nano
porous activated carbon fibers (ACFs) with an average pore-size of 1.16 nm and
surface areas ranging from 171 to 483 m2/g. They measured the sorption of benzene,
51
toluene, p-xylene and ethyl benzene onto the ACFs at 20 OC. Mangun et al. (2001)
found that the sorption isotherms are adequately described by a Freundlich equation. In
all cases, the ACFs had much higher organic sorption equilibrium constants than
granular activated carbon. Peng et al. (2003) have evaluated the sorption of 1, 2-
dichlorobenzene (DCB) onto CNTs. They found that it takes only 40 min for DCB
sorption onto the CNTs to reach equilibrium with a maximum sorption capacity of 30.8
mg/g. Li et al. (2004) reported that MWCNTs were better sorbents of volatile organic
compounds than carbon black in aqueous solutions. Fugetsu et al. (2004) have
successfully encapsulated MWCNTs inside cross-linked alginate vesicles. The caged
MWCNTs showed high sorption capacity and selectivity for four water soluble dyes
(acridine orange, ethidium bromide, eosin bluish and orange G). Zhao & Nagy (2004)
have synthesized hybrid inorganic–organic nanosorbents by incorporation of sodium
dodecyl sulphate (SDS) into magnesium–aluminum layered double hydroxides (LDHs).
They reported that the SDS functionalized Mg/Al LDHs had higher sorption capacity
for chlorinated alkenes [trichloroethylene (TCE)] in aqueous solutions than organo
clays. Fullerenes can also serve as sorbents for polycyclic aromatic compounds (PAHs)
such as naphthalene (Cheng et al., 2004). Nanocatalysts and redox active nanoparticles
have great potential as water-purification catalysts and redox active media due their
large surface areas and their size and shape dependent optical, electronic and catalytic
properties (Obare & Meyer, 2004). During the last decade, titanium dioxide (TiO2)
nanoparticles have emerged as promising photocatalysts for water purification
(Adesina, 2004). TiO2 nanoparticles are very versatile; they can serve both as oxidative
and reductive catalysts for organic and inorganic pollutants. The removal of total
organic carbon from waters contaminated with organic wastes was greatly enhanced by
the addition of TiO2 nanoparticles in the presence of ultraviolet light as shown by
Chitose et al., (2003). Kabra et al. (2004) have recently reviewed the utilization of
photocatalysts in the treatment of water contaminated by organic and inorganic
pollutants. They documented the successful use of TiO2 nanoparticles to (i) degrade
organic compounds (e.g. chlorinated alkanes and benzenes, dioxins, furans, PCBs, etc.)
and (ii) reduce toxic metal ions [e.g., Cr(VI), Ag(I) and Pt(II)] in aqueous solutions
under UV light. The synthesis of visible light-activated TiO2 nanoparticles has
attracted considerable interest (Asahi et al., 2001; Bae & Choi, 2003; Adesina, 2004;
Obare & Meyer, 2004). One of the most cited studies in the field is that published by
52
Ashasi et al. (2001). They synthesized N-doped TiO2 nanoparticles that were capable
of photo degrading methylene blue under visible light. Bae & Choi (2003) have
synthesized visible light-activated TiO2 nanoparticles based on TiO2 modified by
ruthenium-complex sensitizers and Pt deposits. The Pt/TiO2/RuIIL3 nanoparticles
drastically enhanced the rate of reductive dehalogenation of trichloroacetate and carbon
tetrachloride in aqueous solutions under visible light. Nanoscale zero valent iron (Fe0)
and bimetallic Fe0 particles have emerged as effective redox media for the
detoxification of organic and inorganic pollutants in aqueous solutions. These
nanomaterials (10–100 nm) have larger surface areas and reactivity than bulk Fe0
particles (Schrick et al., 2002; Zhang, 2003; Nurmi et al., 2005). Zhang (2003) has
given an overview of the synthesis, characterization and use of nanoscale Fe0 particle
and Fe0/Pd0, Fe0 /Pt0, Fe0 /Ag0, Fe0/ Ni0 and Fe0/Co0 in environmental remediation.
These nanoparticles can reduce a variety of organic pollutants (e.g., chlorinated alkanes
and alkenes, chlorinated benzenes, pesticides, organic dyes, nitro aromatics, PCBs) and
inorganic anions (e.g., nitrates) in aqueous solutions to less toxic and recalcitrant by-
products. Fe0 and bimetallic Fe0 nanoparticles have also been successfully used to
reduce redox active metal ions such as Cr (VI) to less toxic and mobile Cr (III) species
(Zhang, 2003). The immobilization of metallo porphyrinogens in soil–gel matrices has
also been successfully used to prepare redox and catalytically active nanoparticles for
the reductive dehalogenation of chlorinated organic compounds (PCE, TCE and carbon
tetrachloride) in aqueous solutions (Dror et al., 2005).
NANOMATERIALS IN WATER PURIFICATION: CHALLENGES
Nanomaterials are the drivers of the nanotechnology revolution. However, the
environmental fate and toxicity of a material are critical issues in materials selection
and design for water purification. Not much is known about the environmental fate,
transport and toxicity of nanomaterials (Colvin, 2003; Lecoanet et al., 2004 a,b). No
systematic investigations of the hydrolytic, oxidative, photochemical and biological
stability of nanomaterials (e.g., dendrimers, carbonaceous nanoparticles, metal oxides,
etc) in natural and engineered environmental systems have been published in the peer-
reviewed literature to the best of our knowledge. Recently variety of inorganic nano
structured materials has been widely explored for remediation of metallic ions but
recently found to be associated with toxicity issues (Colvin, 2003; Paul and Richard,
53
2006; Karn et al., 2009). A number of in vitro and in vivo measurements of toxicity and
bio distribution have been carried out (Malik et al., 2000; Jevprasesphant et al., 2003;
Hong et al., 2004) in support of the use of dendrimers as DNA transfection reagents,
metal ion contrast agent carriers for magnetic resonance imaging, targeted drug and
therapeutic agent delivery vehicles and viral inhibitors (Fre´chet & Tomalia, 2001).
These overall studies suggest that non-toxic and biodegradable dendrimers can be
synthesized through a judicious selection of the dendrimer building blocks (e.g., core
and terminal group). The design and synthesis of biocompatible carbon nanotubes
(CNTs) and fullerenes are, on the other hand, much more challenging. Only a few peer
reviewed studies of the toxicity of CNTs and fullerenes have been published (Lam et
al., 2004; Oberdo¨ ster, 2004; Jia et al., 2005). These studies showed that underivatized
CNTs and fullerenes tend to be water insoluble and toxic. However, CNTs and
fullerenes can be functionalized with various functional groups (e.g. hydroxyl,
carboxyl, amines, etc.) to increase their water solubility and biocompatibility in some
cases (Sayes et al., 2004; Bianco et al., 2005). Finally, we would like to point out that
metal-containing particles also exhibit a size dependent toxicity (Chen, 2004). Thus, a
key challenge will be to gain regulatory and public acceptance for using
nanomaterials in water purification because of their unknown toxicity and
environmental impact.
One way to address such issues related to sustainability is to incorporate
renewable materials of miniaturized elements (Sain and Oksman, 2006) of plant
origin. The complexity of organic material represents the achievement of specific
structural order of many length scales and reflects interplay of perfection forming
the novel material under the domain of green nanotechnology. The ability to
control, manipulate and design organic materials on the nano scale by
implementation of green chemistry principles for the production of organic
nanoparticles in the development of technological processes with the use and
reuse of agricultural waste as biosorbent to remediate contaminants
simultaneously avoiding environmental hazardous is a major challenge of 21st
century.
54
Cellulose constitutes the most abundant and renewable polymer resource available
worldwide and comprises of repeating β-D-glucopyranose units covalently linked
through acetal functions between the OH groups of C4 and C1 carbon atoms providing it
hydrophilicity, chirality and reactivity properties. The increasing interest in
nanomaterials of plant origin and their unique properties have led to intensive research
in the area of nano cellulosic materials (Samir et al., 2005; Bondeson et al., 2006;
Visakh and Thomas, 2010). Nano cellulose, due to its nano-size nature, high surface
area and high aspect ratio, becomes an important material with the advantages of being
derived from renewable resources and making it suitable for environmental and health
applications, such as liquid filtration (Ma et al., 2011), biomedical applications (Czaja
et al., 2007), transparent materials for advanced nanocomposites for different purposes
(Lo´pez-Rubio et al. 2007; Shimazaki et al. 2007) etc. However, not much attention
has been paid on the application of nano cellulosic fibers for the remediation of toxic
metals from water bodies.
55
ARTIFICIAL NEURAL NETWORK MODELING
To achieve an optimum management for any control measure, the concept of modeling
for an efficient operation and design should be developed. A high quality representative
model can provide a favorable solution to the process control. It is likely to explain the
real process performance developing a continuous control strategy for such type of
technologies. The nature of the sorption process depends on physical or chemical
characteristics of the adsorbent systems and also on the system conditions. The
adsorption processes irrespective of the adsorbate/adsorbent are usually modeled using
the mechanistic or empirical based kinetic expressions. In addition to that numerous
empirical models have been employed to describe the biosorption equilibrium, namely
Langmuir, Freundlich, Brunauer–Emmett–Teller (BET), Sips, Dubinin–Radushkevich,
Temkin and Toth models. Langmuir and Freundlich equations are the most popular and
widely used models in a large number of studies. Nonetheless, in many cases, these
empirical models fall-short to represent the biosorption phenomena and its in-behind
physical meaning (Febrianto et al., 2009). In addition, predictive conclusions are hardly
drawn from systems operating at different conditions. However, any attempt to make a
relationship between the amount of solute uptake with all the operating variables,
namely, metal concentration, biomass dosage, initial solution pH, volume of solution
treated, and contact time, using the kinetic or mechanistic based models is impossible.
Neural networks are useful when a mathematical relationship is not available to
describe a phenomenon to be modeled. If the property in question can be modeled by
very complex and highly demanding computational techniques, neural networks
provide an alternative approach to obtain accurate numerical values in a
computationally less intensive fashion. Because of reliable, robust and salient
characteristics in capturing the non-linear relationships of variables in complex
systems, application of Artificial Neural Network (ANN) has been successfully
employed in environmental engineering [Shetty and Chellam, 2003; Park et al., 2004]
and bioremediation [Raj et al., 2010; Kardam et al., 2010].
56
NEURAL NETWORKS
A first wave of interest in neural networks (also known as ‘connectionist models’ or
‘parallel distributed processing’) emerged after the introduction of simplified neurons
by McCulloch and Pitts in 1943. These neurons were presented as models of biological
neurons and as conceptual components for circuits that could perform computational
tasks.
BIOLOGICAL NEURON MODEL
The human brain is estimated to have around 10 billion neurons each connected on
average to 10,000 other neurons. An artificial neuron is a computational model inspired
in the biological neurons. Biological neurons receive signals through synapses located
on the dendrites or membrane of the neuron (Figure 1.12). When the signals received
are strong enough (surpass a certain threshold), the neuron is activated and emits a
signal though the axon. This signal might be sent to another synapse, and might activate
other neurons.
Figure 1.12: Biological Neuron a), and Artificial Neuron b).
The complexity of real neurons is highly abstracted when modelling artificial neurons.
These basically consist of inputs (like synapses), which are multiplied by weights
(strength of the respective signals), and then computed by a mathematical function
which determines the activation of the neuron. Another function (which may be the
identity) computes the output of the artificial neuron (sometimes in dependence of a
certain threshold). ANNs combine artificial neurons in order to process information.
57
ARTIFICIAL NEURAL NETWORK
An Artificial Neural Network (ANN) is an information processing paradigm that is
inspired by the way biological nervous systems, such as the brain, process information.
The key element of this paradigm is the novel structure of the information processing
system. It is composed of a large number of highly interconnected processing elements
(neurones) working in unison to solve specific problems. ANNs, like people, learn by
example. An ANN is configured for a specific application, such as pattern recognition
or data classification, through a learning process.
ARTIFICIAL NEURON- BASIC ELEMENTS
Neuron consists of three basic components – weights, threshold and a single activation
function as shown in the figure 1.13.
Figure 1.13: Basic elements of artificial neuron model
WEIGHT FACTORS
The values W1, W2, W3...........Wn are weights to determine the strength of an input
vector X = [X1, X2, X3................... X1,]T. Each input is multiplied by the associated
weight of the neuron connection XT W. The positive weight excites and the negative
weight inhibits the node output.
THRESHOLD ɸ
58
The nodes internal threshold ɸ is the magnitude offset. It affects the activation of the
node output y as:
To generate the final output Y, the sum is passed to non linear filter f called activation
function or transfer function or squash function which releases the output Y.
THRESHOLD FOR A NEURON
In practice, neurons generally do not fire (produce an output) unless their total input
goes above threshold value. The total input of a neuron is the sum of the weighted input
to the neuron minus threshold value. This is then passed through the sigmoid function.
The equation for the transition in a neuron is:
ACTIVATION FUNCTION
An activation function f performs the mathematical operation on the signal output.
Figure 1.14 shows the most common activation functions:
Figure 1.14: Classical activation functions
The activation functions are chosen depending upon the type of problem to be solved
by the network.
59
NEURAL NETWORK ARCHITECTURES
Single Layer Feed Forward Network
The Single Layer Feed Forward Network consists of a single layer of weights,
where the inputs are directly connected to the outputs, via series of weights
(Figure 1.15). The synaptic links carrying weights connect every input to every
output, but not other way. This way it
is considered a network of feed-
forward type. The sum of the product
of the weights and the inputs is
calculated in each neuron node, and if
the value is above some threshold
(typically 0) the neuron fires and takes
the activated value (typically 1); other
it takes the deactivated value (typically
-1).
Figure 1.15: Single Layer Feed Forward Network
Multi Layer Feed Forward Network
As the name suggests, it consists of multiple layers. The architecture of this
class of network, besides having the input and output layers, also have one or
more intermediary layers called hidden layers. The computational units of the
hidden layer are known as hidden
neurons. The hidden layer does
intermediate computation before
directing the input to output layer
(Figure 1.16). A multi layer feed
forward network with l input neurons,
m1 neurons in the first hidden layers,
m2 neurons in the second hidden
layers, and in output neurons in the
output layers is written as (l-m1- m2-n). Figure 1.16: Multi feed- forward Network
Recurrent Network
The recurrent Networks differ from feed-forward architecture. A recurrent
network has at least one feed back as shown in figure. There could be neurons
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with self feedback links; that is the output of a neuron is feedback into itself as
input (Figure 1.17).
Figure 1.17: Recurrent Neural Network
Learning methods in Neural Networks
The learning methods in neural networks are classified into three basic steps:
· Supervised Learning.
· Unsupervised Learning.
· Reinforced Learning.
These three types are classified based on; presence or absence of teacher and the
information provided for the system to learn. These are further categorized,
based on the rules used, as Hebbian, Gradient descent, Competitive and
stochastic learning (Figure 1.18).
Figure 1.18: Hierarchical representation of the learning algorithms
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APPLICATION OF NEURAL NETWORK
Neural Network Application can be grouped in following categories:
Clustering:
A clustering algorithm explores the similarity between patterns and places
similar patterns in a cluster. Best known applications include data compression
and data mining.
Classification/pattern recognition:
The task of pattern recognition is to assign an input pattern (like handwritten
symbol) to one of many classes. This category includes algorithms
implementation such as associative memory.
Function approximation:
The task of function approximation is to find an estimate of the unknown
function subject to noise. Various engineering and scientific disciplines require
function approximation.
Predictive Systems:
The task is to forecast some future values of a time-sequenced data. Prediction
differs from function approximation by considering time factor, System may be
dynamic and may produce different results for the same input data based on
system state (time).
Figure 1.19: Application of Artificial Neural Network in different disciplines of scientific research
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Table 1.4: Optimisation of Biosorption Studies using Artificial Neural Network Modeling
ARTIFICIAL NEURAL NETWORK MODELING:
EQUILIBRIUM & MODELING STUDIES : REFERENCES
Pb, Cu and Cd modeling by ANN using bacterial biomass,
Modeling of Adsorption Isotherm using Neural Networks,
La, Eu and Yb modelization using ANN,
Prediction of two-metal biosorption equlibria using ANN,
Copper biosorption by penicillium cyclopium using ANN,
Predictive Modeling of Competitive biosorption equilibrium,
Prediction of biosorption efficiency for the removal of Cu (II) using ANN,
Cd and Zn biosorption by Sargassum filipendula using ANN,
Adsorption in packed column by macro porous resins using NN,
ANN modeling of Pb (II) adsorption by Antep pistachio shells,
Neural network modeling of Pb2+ removal from waste water.
Artificial neural network modeling of fixed bed biosorption using radial basis
approach
Artificial Neural Network for predicting biosorption of methylene blue by
Spirulina sp.
Thomas and artificial neural network models for the fixed-bed adsorption of
methylene blue by a beach waste Posidonia oceanica (L.)
Modelling the solid–liquid adsorption processes using ANN trained by pseudo
second order kinetics
Comparison of the results of response surface methodology and artificial neural
network for the biosorption of lead using black cumin
Prediction of biosorption of total chromium by Bacillus sp. using artificial neural
network.
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Kumar and Porkodi et al., 2011
Bingöl et al., 2012
Masood et al., 2012