REVIEW OF VARIOUS TECHNIQUES FOR PRECONCENTRATION OF

METAL IONS

2.1.1. Metal ions

According to the importance of metals to the aquatic environments, it may be divided

into three groups i) light metals such as sodium, potassium and calcium which are

normally mobile cations in aqueous solution; ii) transition metals such as zinc, nickel,

copper, cobalt and manganese which may be toxic in high concentrations and iii) heavy

metals and metalloids such as mercury, lead, cadmium, tin, antimony and arsenic which

are generally not required for metabolic activities and are toxic to the cell at quite low

concentrations (Clarck 1993). Studies had been performed on the water sources to

estimate the levels of heavy metals periodically (Kenawy et al. 2000). Heavy metals,

though essential for industrial development, have also been recognized as major

pollutants. The major source of metallic pollutants in aquatic systems is the discharge of

untreated industrial effluents from various industries such as electroplating, mining

operations, dyeing, battery, tanneries, glass, pharmaceutical and chemical manufacturing.

Metals are released continuously into the environment from the natural and

anthropogenic sources such as industrial effluents, atmospheric emission, fossil fuels,

waste stream and urban habitation. Furthermore, these heavy metals are non-

biodegradable and exist for a long time in the environment. They have a tendency to enter

into living tissues. The excessive presence of these metals in living species results in

carcinogenic, mutagenic and other toxic effects on them. Once they are accumulated in

living tissues, they disturb microbial processes and have been reported to be fatal. Thus,

industrial effluents must be treated prior to their discharge into water resources.

Normally, metal ions exist in hydrated form or as complexes associated with anions with

little or no tendency of transformation to polymeric matrix. To convert the metal ion to an

extractable species its charge must be neutralized and some or all water of hydration be

replaced. Thus, the characteristics of metal ions and functional groups and/or donor

atoms, which form complexes with metal ions in solution, are the primary factors in

extraction. Various technologies such as ion exchange, reverse osmosis, electrolytic

removal, reduction, adsorption, precipitation, membrane filtration and flocculation have

been reported for the removal of heavy metals from industrial effluents. But these

technologies have some disadvantages such as high operational and maintenance cost,

expensive equipment, incomplete metal removal, high energy requirement and generation

of toxic residual metal sludge. Disposal of toxic sludge is another major problem in most

methodologies.

In recent years, the significance of trace elements present at the μg/g(ppm), ng/g(ppb) and

pg/g(ppt) levels in geological, biological, environmental and industrial materials has

increasingly been recognized in every field of science and technology. Analytical

chemists continue to search for sample preparation procedures that are faster, easier,

safer, and less expensive, to provide accurate and precise data with reasonable

quantification limits. The direct determination of trace metal ions from natural water is

limited and difficult when its concentration is too low to be determined directly and /or

interference due to matrix cannot be eliminated. A wide variety of modern instrumental

techniques such as optical, electrochemical, nuclear etc. are extensively employed for the

detection and determination of trace elements. Despite the selectivity and sensitivity of

analytical techniques such as flame atomic absorption spectrometry (FAAS),

electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma

optical emission spectrometry (ICP-OES) and inductively coupled plasma mass

spectrometry (ICP-MS) techniques, there is a critical need for the preconcentration and

separation of trace metals prior to their determination, due to their frequent presence at

low concentrations in environmental samples. Accurate analysis of various complex

samples (natural water, tap water, waste water, geological samples and industrial

effluents), especially at trace levels, is one of the most difficult and complicated

analytical task. The rapid development of electronic instrumentation has created powerful

analytical tools for trace elements determination. However some times erroneous results

were obtained due to matrix effect which can be minimized by sorption techniques.

Adsorption offers high efficiency, cost-effectiveness and easy handling. Recovery of the

metals and other adsorbed species is also possible. Preconcentration is a process in which

the ratio of the quantity of a desired trace element to that of the original matrix is

increased. Preconcentration is a technique in which trace determination of metal ions are

carried out from a large volume of aqueous phase. Preconcentration improves the

analytical detection limits, increases the sensitivity by several orders of magnitude,

enhances the accuracy of results and facilitate the calibration. In general, it can be

referred to as the enrichment process which involves separating the minor component

(analyte) from complex matrix or extraction of particular analyte from one phase to other

in which latter one is of less volume than former. Preconcentration and separation

techniques are of great importance owing to the limited sensitivity of modern

instrumental methods for trace analysis and the direct application to actual samples is

frequently difficult and undesirable:

1. When the concentration of the desired trace elements are extremely low.

2. When interfering substances exist in the sample.

3. When the sample is highly toxic, radioactive or expensive to be dissipated in the

environment.

4. When the desired trace elements are not homogeneously distributed in the sample.

5. When the physical and chemical states of the sample are not suitable for the direct

determination.

6. When appropriate calibration standards are not available.

7. When chemical speciation of the desired trace elements is required.

Pre-treatment of an aqueous sample by different sorption technique not only increases the

ion concentration to a detectable level but also eliminates matrix effects. Enrichment is

attained by the use of various preconcentration techniques based on physical, physico-

chemical and chemical principle. Sample preparation processes including separation and

preconcentration have a direct impact on accuracy, precision and detection limits for

many analytical methods [1-7]. The use of chelating sorbents can provide a concentration

factor up to several hundred folds, better separation of interferent ions and high

efficiency. The general trend of modern analytical chemistry is towards the elaboration of

simple, ecologically safe, sensitive, and selective methods for the determination of trace

components combining previous concentration methods and further determination by

physical or physico-chemical methods. Metal quantification at low concentration levels

comprises one of the most considered targets in analytical chemistry. Sample

pretreatment methods, such as separation and /or preconcentration prior to the

determination of metal ions have developed rapidly due to the increasing need for

accurate and precise measurements at extremely low levels of ions in diverse matrices.

Current trends in preconcentration focus on the development of faster, safer and more

environment friendly extraction techniques.

The techniques generally employed in analytical chemistry are liquid-liquid extraction [8-

15], coprecipitation [16-22], ion-exchange resins [23-31], electrothermal deposition [32-

33] and solid-phase extraction [34-53]. Electrochemical deposition used for the

preconcentration of different pollutants by applying the laws of electrolysis in which

cationic species are deposited on the electrode surface. The only disadvantage of this

method is that limitation related to pH control. This control is necessary because in the

acid medium, hydrogen ions are reduced to hydrogen gas on the working electrode

surface. The hydrogen gas generation occurs when more negative potentials are applied.

The reduction of electrode lifetimes is also observed at higher acidity conditions.

Coprecipitation or precipitation is characterized by the formation of insoluble

compounds. The coprecipitation is adopted when direct precipitation can not separate the

desired metallic species due to its low concentration in solution. The coprecipitation

phenomenon can be associated with metal adsorption on the precipitate surface or due to

metal incorporation onto the precipitate structures. Thus, there is a natural limitation

according to this phenomenon, because the metal used for this purpose cannot be

determined. The separation and preconcentration of metal ions and organic pollutants,

after the formation of sparingly water-soluble complex, based on cloud point extraction

have largely been employed in analytical chemistry. Current research in this field has

focused on the development of new surfactant phase separations that surpassed the

limitations associated with non-ionic surfactants [54-56]. Cloud point extraction that is

the temperature-induced phase separation of nonionic surfactants, continues as one of the

leading techniques for the preconcentration of metal ions. But application of cloud point

extraction to the extraction of organic pollutants is less straightforward because of the

coelution problems originated by non-ionic surfactants which are commercially available

as a mixture of homologues and isomers.

The search for alternatives to traditional organic solvents in liquid-liquid extraction has

fostered the use of more environmentally friendly liquids. But according to Hitherto,

liquid-liquid extraction is among the most often used method for the various

preconcentration or separation techniques in view of its simplicity, rapidity, ready

adaptability and easier recovery of analyte, There are, however physical difficulties

associated with the use of solvent extraction for enrichment of large number of samples

and /or requires vigorous agitation to ensure complete partition of the analyte between

two immiscible phases, and this can be achieved only by the application of significant

human or mechanical effort. In addition, there are increasing environmental and cost

pressures to replace, or at the very least reduce, the volume of solvents employed in

analytical procedures. Solid phase extraction continues to be the leading technique for the

extraction of pollutants in aquatic systems; recent developments in this field are mainly

related to the use of new sorbents. Solid phase extraction (SPE) has emerged as a

powerful tool for separation/enrichment of inorganics, organics and biomolecules [57-

58].The basic principle of SPE is the transfer of analytes from aqueous phase to active

sites of adjacent solid phase. Recently, solid-phase extraction technique for

preconcentration of heavy metal ions has become very popular, compared with traditional

solvent extraction techniques and has almost replaced liquid-liquid extraction techniques

because of several advantages:

(1) The fast, simple and direct sample application in very small size (micro liter

volume) without any sample loss.

(2) Higher preconcentration factor.

(3) The ability of combination with different modern analytical techniques.

(4) Time and cost saving.

(5) There is no need of organic solvents which are inflammable, toxic and even some

of them carcinogenic.

(6) Absence of emulsion.

(7) Rapid phase separation.

(8) Stability and re-usability of solid phase.

(9) To isolate analytes from large volumes of sample with minimal or zero

evaporation losses.

The choice of solid-phase extractant is a decisive factor that affects the analytical

sensitivity and selectivity [59]. The main requirement with respect to substances to be

used as solid-phase extractants are as follows:

(1) Possibility of extracting a large number of elements over a wide pH range.

(2) High surface area and high purity.

(3) Good sorption properties including porosity, durability and uniform pore

distribution.

(4) Selectivity for specific analytes.

(5) Fast quantitative sorption and elution.

(6) Regenerability and accessibility.

The substances such as ion-exchange resins [60-62], chelating resins[63-67],

modified silica [68-93], alumina[94], activated carbon[95-102], zeolite[103-105],

chitosan[106-108] and polyurethane foam[109-114] have been used as solid phase

extractant.

Ion-exchange resins even though frequently used for preconcentration of metal

ions, but have the disadvantage of low sensitivity and selectivity, while chelating sorbents

have greater selectivity than ion-exchangers. Slow kinetics, irreversible adsorption of

organics, sensitivity toward many chemical environment, loss of mechanical stability in

modular operation and swelling are the main disadvantages exhibited by polymeric

resins. These problems suggest the use of inorganic supports in place of polymeric resin.

Some of the advantages of inorganic supports are:-

(1) No Swelling

(2) Rapid sorption

(3) Good mechanical stability

(4) Good selectivity

A chelating sorbent essentially consists of two compounds, the chelate forming

functional group and polymeric matrix support. Different polymeric materials used for

chelating group immobilization can be ordered as follows:-

Inorganic: - Silica gel, Alumina, Kieselgur, Controlled pore glass.

Support Natural: - Cellulose, Dextran, Activated carbon

Organic

Synthetic: -Polymeric resins, Fibrous materials,

Foamed plastics

Among the natural organic matrices, cellulose is the most extensively used support for

grafting of suitable functional groups because of its easy availability, low price and high

mechanical strength. Thus, cellulose sorbents with bonded groups of iminodiacetic acid,

8-hydroxyquinoline, mercapto groups, aminoalkyl groups, pyridyl-azoresorcinol have

been frequently used.

In the same way, chelating sorbents with functional groups immobilized by covalent

bonds on silica gel, have been synthesized by chemical transformation of the matrix.

Even though the inorganic supports have high mechanical strength, thermal and chemical

stability, chelating sorbents based on inorganic matrix have a poor degree of

functionalization, reliability, and low sorption capacity. The disadvantages of chelating

sorbents with grafted functional groups determined by synthesis difficulty, such as: low

reversibility of sorption-desorption processes and unsatisfactory kinetic features [115].

1. Immobilization of organofuntional groups on silica gel support, offers pronounced

advantages over other organic/inorganic supports as listed below.

2. Immobilization on the silica results in the great variety of silylating agents allowing

pendant functional groups in the inorganic framework [116].

3. Attachment is easier on silica surface than on organic polymeric supports which

have a high number of cross-linking bonds, requiring hours to reach equilibrium for

surface activation[117].

4. Silica Gel is the first commercially available high specific surface area substrate

with constant composition, enabling easy analysis and interpretation of

results[117].

5. Silica Gel has high mass exchange characteristics, no swelling and great thermal

resistance [118].

But irreversible binding of metal ions and lack of selectivity are the main disadvantages

of silica gel bound ligands in their repeated cyclic use and elution process.

The field of nanometer-sized materials (i.e. nanoparticles) has gained the attention of

scientists and engineers in recent years due to their special properties. Nano-materials,

with a new series of different physical and chemical properties superior to the traditional

materials, is the basis of nanotechnology. Nanoparticles are clusters of atoms or

molecules of metal oxide, ranging in size from 1nm to almost 100nm.One of their most

interesting properties is that a high percentage of the atoms of the nanoparticles is on the

surface. The unsaturated surface atoms can bind with other atoms that possess strong

chemical activity. Consequently, nanometer materials have high adsorption capacity and

can adsorb selective metal ions and has a very high adsorption capacity [119-130].

Nanometer-sized metal oxides, such as Al2O3, TiO2, ZrO2, CeO2, and SiO2 exhibit

intrinsic surface reactivity and high surface areas and can strongly chemisorb several

substances [120-143] and these materials have been proposed and applied for the

preconcentration of trace metals due to their high surface area, high adsorption capacity,

and high chemical activity. Moreover, the preparation of these adsorbents is very simple

and low cost as compared with other commercially available solid-phase extractants. In

recent years, the application of nano-material has made rapid progress in modern

analytical chemistry.In 1996, E.Vassileva studied the adsorption properties of nanometer-

size TiO2 powder for heavy metals. Thereafter, nanometer-size powder materials were

frequently used for the separation and enrichment of trace elements. Additionally, the

coating of complexing reagents onto nanomaterials increase the number of binding sites

and enable to interact with metal ions and changes the binding sites in order to enhance

the uptake of metal ions. Recent advancements suggest that many of issues involving

water quality could be resolved or greatly ameliorated using nanoparticles. Innovative use

of nanoparticles for treatment of industrial wastewater is another potentially useful

application. Many industries generate large amounts of waste water. Removal of

contaminants and recycling of the purified water would provide significant reduction in

cost, time and energy to the industry and result in the improved environmental

stewardship. Aquifer and groundwater remediation are also critical issue, becoming more

important as water supplies steadily decrease and demand continues to increase. Most of

remediation technologies available today, while effective, very often are costly and time

consuming. The ability to remove toxic components from subsurface and other

environments that are very difficult to access in situ. Nanoparticles in analytical

chemistry is the most extensively explored areas of nanotechnology. The objective is to

exploit the excellent properties of nanoparticles to improve analytical methods or to

develop new ones for the analytes or matrices. Nanoparticles have two key properties that

make them particularly attractive sorbent. In addition to the typical advantages of

nanoparticles, their use should lead to improved selectivity, sensitivity, rapidity,

miniaturizability or portability of the analytical system. Nanoparticles can be

incorporated or used in analytical methods either as such or chemically grafted. In the

latter case, nanoparticles can be chemically bonded to a surface or functionalized with

other organic or inorganic compounds in order to increase their sorption capacity.

Chemically unmodified nanoparticles can be used as raw randomized materials or as self

assembled raw materials. Nanoparticles can be used for purposes such as sample

treatment, instrumental separation of analytes, or detection. In combination with the large

variety of nanoparticles available, this provides a wide range of potential applications.

The nanoparticles most widely used in analytical sciences at present include (a) silica

nanoparticles (b) carbon nanoparticles (mainly fullerenes and carbon nanotubes) (c)

metallic nanoparticles (d) supramolecular aggregates. Nanoparticles can also be

functionalized with various chemical groups to increase their affinity towards target

analytes. The unique properties of nanoparticles have been used to develop high capacity

and selective sorbents for metal ions and pollutants. Due to these reasons, the

nanoparticles are synthesized and designed to act as either extractants or reaction media

for pollutants or scaffolds and delivery vehicles for bioactive compounds; thus providing

unprecedented opportunities to develop more efficient and cost effective water

purification processes and systems. Consequently, nanometer material can selectively

adsorb metal ions and have a very high adsorption capacity.

Figure 2. Selected nanomaterials currently being evaluated as functional materials

for water purification.

Nanoparticles play a central role in purification and preconcentration of analytes from

sample matrix. Nanoparticles are used for the preconcentration and separation of

pollutants from the environmental sources. Investigation of the surface chemistry of

highly dispersed metal oxides e.g. TiO2, Al2O3, ZrO2, CeO2 and MnO nanoparticles,

indicate that these materials have very high adsorption capacity and give promising

results when they are used for trace metal analysis of different types of samples. Ferric

hydroxide is used to scavenge a variety of heavy metal contaminants. Last but not the

least advantage of nanoparticles is that they are highly efficient in the preconcentration of

toxic metals and organic pollutants. They can be repeatedly used and matrix effects are

low. There are different types of nanomaterials used for removal of environmental

pollutants.

(1) CNTs present a higher adsorption capacity toward the organic pollutants and

metal ions. Wider a practical applicability of carbon nanotubes may be

hampered by their relatively high unit cost and low processability.

(2) Zero-valent iron nanoparticles have given promising results for the removal of

environmental pollutants but background corrosion of iron particles not only limits

the lifetime of these nanoparticles, but also substantially decrease the reactivity

of these nanoparticles.

(3) Silica, titania, zirconia and iron sulfide nanoparticles give good results for the

preconcentration of metal ions. Among these, silica nanoparticles is a promising

materials as a solid phase extractant because of its large surface area, high

adsorption capacity, low temperature modification, less degree of unsaturation and

low electrophilicity. For example, hydrolysis and condensation rates of titanium

butoxide are much faster than that of tetraethoxysilane. The sequence of reactivity

is expressed as follows:

Zr(OR) 4, Al(OR) 4> Ti(OR) 4> Sn(OR) 4 >> Si(OR)4

Some heavy metal cations are poorly adsorbed on Nanoparticles. To overcome this

problem, physical or chemical modification of surface of these nanoparticles with certain

functional groups containing some donor atoms such as oxygen, nitrogen, sulfur and

phosphorus is necessary. The most often used method is to load a kind of specific

chelating reagent by physical or chemical procedure. The formal method is simple but the

loaded reagent is prone to leaking out from the sorbent, while the chemically bonded

material is more stable and can be used repeatedly. The modification of nanometer sized

materials is usually required in order to prevent a conglomeration of particles and to

improve its consistency in relation to other materials. Also the modification of

nanometer-sized material can improve the selectivity of nanometer–sized materials

toward pollutants. Selectivity of suitable specific functional groups towards metal ions

depends on certain factors such as (1) size of the modifiers (2) activity of loaded group

and also (3) on the basis of the concept of hard-soft acids and bases. Chemisorption of

nanoparticles provides immobility, mechanical stability and water insolubility, thereby

increases the efficiency, sensitivity and selectivity.

2.1.2. Application of nanoparticles for the removal of various pollutants

The selective sorption of certain elements based on the stability of complexes formed

with functional groups of sorbents, has led to the use of these materials for selective

enrichments and separation of inorganic ions from different natural and industrial

sources. According to researchers at the Pacific North laboratory (PNL), a unique

chemically modified nanoporous ceramics can remove contaminants from waste streams

faster and at a significantly lower cost than conventional techniques such as ion-exchange

resins and activated carbon filters. This nanosponge could be used in a wide range of

environmental applications, including drinking-water purification, waste water treatment,

site remediation and waste stabilization [144-150].

Nanotechnology, a revolutionary technology will have a large impact on our life. A core

piece of this technology is the production of nanomaterials which are widely used for

chemical, medical, pharmaceutical and environmental analysis. Granular activated

carbons (GACs) have been accepted as the industry standard for adsorbing contaminants

from water. As a result, they have become ubiquitous in wastewater treatment facilities

and in household for purification of drinking water [151-157]. Scientists have developed

robust filters composed entirely of multiwalled carbon nanotubes, with just benzene,

ferrocene and a simple laboratory apparatus [158-159]. These filters shaped like hollow

cylinders are easy to clean and are reusable. They can remove bacteria and viruses from

water, eliminate heavy hydrocarbons from petroleum, and separate a mixture of benzene

and naphthalene [160-163]. The main drawback of carbon nanotubes is:

1. High unit cost of carbon nanotubes may be hampered by the wider practical

application these.

2. Carbon nanotubes also interact with pollutant, via non-covalent forces (weak

forces), so provides mobility and less stability of the pollutants.

Yue et. al. have used carbon nanoparticles and nanoporous carbon fibers for the

removal of humic acid, benzene, toluene, ethylbenzene, p-xylene, diisopropylmethyl

phosphonate and chloroethylsulfide and other organic compounds. These compounds

have been used for the removal of trace amounts of chlorinated solvents such as

trichloroethylene (TCE), chloroform and atrazine (herbicide)[164-168]. Guodong

et.al. used natural and modified nanomaterial (a surface modified smectite) for the

removal of naphthalene and 17 β-estradiol [169]. Cheng et. al. have illustrated that

C60 fullerene nanoparticles play potential role for the removal of hydrophobic organic

compounds such as naphthalene and 1,2-dichlorobenzene from aquatic systems[170].

Liu et. al. used metal oxides nanoparticles for the adsorption of organic compounds

[171].

Xu et. al. used the bimetallic nanoparticles [Fe0/Ni

0 and Fe

0/Pd

0] for the

reductive transformation of halogenated organic compounds [172]. Cao et. al. used

the metal Fe nanoparticles for the reduction of percholorate, a water-soluble anion

that has been widely used in the manufacturing of explosives and solid propellants for

pyrotechnic devices, rockets and missiles [173]. The Fe metal nanoparticles also

reduce chlorate, chlorite and hypochlorite to chlorine [174].

Kumbhar et al.

synthesized Fe(III)-doped titania nanoparticles and these were used for the

photodegradation of sulforhodamine-B pollutant. Wu et. al. synthesized the cellulose

acetate supported zero-valent iron nanoparticles for the decholorination of

trichloroethylene in water [175]. Some examples are given in Table 1.

2.2. Preconcentration of metal ions using nanoparticles

Recently, it has been found that iron sulfide (FeS) nanoparticles produced by certain

bacteria act as excellent adsorbents for a wide range of metal ions in solution, such as

As(III), Cd(II), Hg(II), Pb(II) [176]. The structures and reactivity of goethite, akaganeite,

hematite, ferrihydrite and schwertmannite nanoparticles (collectively referred as FeOX

nanoparticles) have been discussed by Waychunas et. al.[177]. These are the important

constituents of soil. Goethite nanoparticles are used for the adsorption of As(V), Cu(II),

Hg(II) and Zn(II) [178]. The applicability of maghemite (γ-Fe2O3) nanoparticles for the

selective removal of Cr(VI), Cu(II), Ni(II) from electroplating waste water had been

studied by Jing Hu et al. [179]. Hydrated iron oxide nanoparticles were used for the

selective removal of As(III) and As (V) by Demarco et.al. [180-181]. Ceria nanoparticles

supported on carbon nanotubes (CeO2-CNTs) have been used for the removal of arsenate

from water by Peng et. al. [182]. Allophane and boehmite are the natural nanomaterials

and do not pose much risk either to physical environment or to human health. Allophane

is a nanomaterial of geological origins and a tablespoon of it has a surface area equivalent

to a large playing field. It has been used as a nanoscavenger for removal of Cu(II) from

the environmental samples[183-185]. Boehmite nanoparticles had been used for the

adsorption of arsenic by Anderson et. al. The adsorption behaviour of toxic metal ions

like Cu(II), Cr(III), Mn(II), Ni(II), Zn(II), Cd(II), Mo(VI) and rare earth elements on

TiO2 nanoparticles have been reported in environmental samples[186-195]. The basic

disadvantage of solid sorbents is the lack of selectivity, which result in the interference

with target metal ions. To overcome this problem, physical or chemical modification of

the sorbent surface with some organic compounds, especially chelating ones is required.

Some examples are given in Table 1.

2.3. Physical modification of nanoparticles by using different organic compounds

Immobilization using dithizone, diethyldithiocarbamate, 1-(2-pyridylazo)-2-naphthol and

8-hydroxyquinoline on titania nanoparticles have been reported and these compounds

have been used for pre-enrichment of various toxic metal ions [196-199]. Surface coated

alumina with dithizone has been used for the preconcentration of Pb(II) from drinking

and natural water. Al2O3 nanoparticles immobilized with gallic acid and chromotropic

acid had been used for the preconcentration of Fe(II) and Fe(III) by Xuli Pu et. al. [200].

Al2O3 nanoparticles immobilized chromotropic acid had been used for the

preconcentration of various metals [201]. Some examples are given in Table 2.

Dithizone Diethyldithiocarbamate

1-(2-pyridylazo)-2-naphthol 8-hydroxyquinoline

Gallic acid Chromotropic acid

Chemical modification of nanoparticles using different organic/ inorganic

compounds

Chemical modification is a process that leads to change in chemical characteristics of

surface of nanoparticles. By the modification the adsorption properties are significantly

affected. Chemisorption of chelating molecules on nanoparticle surface provides

immobility, mechanical stability and water insolubility, thereby increases the efficiency,

sensitivity and selectivity of nanoparticles for the analytical application.

Chemical modification of nanoparticles by silylation procedure using different

silylating agents such as 3-aminopropyltriethoxysilane, 3-chloropropyltriethoxysilane and

3-mercaptopropyltriethoxysilane provides immobility, mechanical stability and water

insolubility. Silylation of Al2O3 nanoparticles followed by their chemical modification p-

toluenesulfonylamide and 3-(8-quinolinylazo)-4-hydroxybenzoic acid have been used for

the preconcentration of Cd(II), Cr(VI), Cu(II), Mn(II), Ni(II), Pb(II), Zn(II), La(III),

Y(III), Sc(III), Ag(I), Au(I), Ga(III), In(III), Nb(V) and Pd(II) [202-204].

N-[3-(trimethoxysilyl)propyl]ethylenediammine modified SiO2 nanoparticles have

been used for the preconcentration of some toxic heavy metal ions such as Hg(II), Cu(II),

Zn(II)[205]. Modified silica nanoparticles have also been used for the preconcentration of

drugs and also pesticides. Silylation of silica nanoparticles followed by their chemical

modification using 4-(2-pyridylazo)-resorcinol [206]

and these modified SiO2

nanoparticles have been used for the selective preconcentration of Hg(II). SiO2

nanoparticles also modified with acetylsalicylic acid, p-dimethylaminobenzaldehyde and

5-sulfonylsalicylic acid have been used for the preconcentration of Cr(III), Fe(III) Pb(II)

and Cu(II) [207-210]. Some examples are given in Table 3.

SiO2-3-aminopropyltriethoxysilane nanoparticles

SiO2-3-choloropropyltriethoxysilane nanoparticles

SiO2-3-mercaptopropyltriethoxysilane

O Si

SH

O

O

CH2

CH3

H2C

CH3

CH2CH3

O Si

Cl

O

O

CH2

CH3

H2C

CH3

CH2CH3

O Si

SH

O

O

CH2

CH3

H2C

CH3

CH2CH3

Al2O3-3-mercaptopropyltriethoxysilane nanoparticles

Al2O3-3-aminoropyltriethoxysilane nanoparticles

Al2O3-3-cholropropyltriethoxysilane nanoparticles

O Al

SH

O

O

CH2

CH3

H2C

CH3

CH2CH3

O Al

NH2

O

O

CH2

CH3

H2C

CH3

CH2CH3

O Al

Cl

O

O

CH2

CH3

H2C

CH3

CH2CH3

SiO2-acetylsalicylic acid nanoparticles

SiO2-p-dimethylaminobenzaldehyde

SiO2-4-(2-pyridylazo)-resorcinol nanoparticles

Si

NH

O

O

O

CH3

CH3

Si

O

O

O

CH3

Si

N

O

O

O

CH3

CH3

Si

NCH3 CH3

Si

NH

O

O

O

CH3

CH3

Si

N

N

N

OH OH

Analytical applications of Thioxanthate

Thio compounds play a vital role in analytical chemistry due to highly sensitive color

reaction, stability and selectivity towards various metal ions.Thioxanthates (known as

organotrithio-carbonates) have -CS2 group which makes them more reactive towards

various metals. Thioxanthates have not been much used as analytical reagents although

the related compounds like diethyldithiocarbamates and xanthates have been extensively

used [211-217]. Thioxanthates are referred as organotrithiocarbonates. It has been

reported by Kanekar and coworkers that thioxanthates are weaker ligands than the

corresponding xanthates and stronger ligands than diethyldithiocarbamate. Keeping in

view the reported ligand field strengths of thioxanthates the possibilities of the use of

Thioxanthate as reagent for the spectrophotometric studies has been explored [218].

X> TX> DDC

Complexes of thioxanthates with many metal ions viz. palladium, nickel, cobalt and iron

have been reported in literature. The reagent forms colored complexes with nickel,

palladium, iron, cobalt, bismuth, gold and copper and most of them are fairly stable at

room temperature. Some of the metal ions have been determined spectrophotometrically

after extraction in a suitable solvent [219-221]. The polarographic behaviour of this

reagent has been studied by Rao et al. and conditions have been developed for

amperometric determinations of some metal ions using this reagent [222-224]. Puri et. al.

used thioxanthate for spectrophotometric determination of metal ions[225]. Verma et. al.

had suggested some indicators for the volumetric determination of thioxanthates [227].

2.6. Analytical applications of 8-hydroyquinoline derivatives

8-hydroxyquinoline (oxine) is a versatile reagent and it behaves as a bidentate (N,O)-

univalent ligand to form chelates with several metal ions. Cations with n charge and 2n

coordination number form the so-called "coordination saturated uncharged chelates"

which are insoluble in water, but easily soluble in organic solvents. Owing to its great

ability to form metal complexes, 8-hydroxyquinoline and its derivatives have been the

subject of many studies involving analytical applications. It has extensively been used in

the gravimetric, volumetric, solvent extraction and amperometric determination of metal

ions [227-230].

Table 1

Preconcentration of metal ions by nanoparticles

Nanoparticles

Analytical

method

Analyte

LOD(μg L-1

)

Sample

Reference

TiO2 ICP-AES Cu(II)

Cr(III)

Mn(II)

Ni(II)

0.34

1.14

0.52

1.78

Environmental

and

water

samples

187

TiO2

FAAS

Zn(II)

Cd(II)

1.8

3.0

Environmental

and

water

samples

188

TiO2

GFAAS

Se(IV)

Se(VI)

0.16

0.14

Sediment and

water

samples

189

TiO2 ICP-AES Sm(III)

Ho(III)

Nd(III)

Tm(III)

0.08

0.1

0.1

0.06

Stream

sediments

190

TiO2 ICP-AES Au(III)

Pd(II)

Ag(I)

0.016

0.012

0.006

Geological

samples

191

TiO2 ICP-AES La(III)

Yb(III)

Y(III)

Eu(III)

0.124

0.108

0.108

0.28

Stream

sediments

192

Dy(III) 0.36

TiO2 ICP-AES Cr(III) 0.32 Water

samples

193

MWCNs ICP-OES La(III)

Yb(III)

Eu(III)

Dy(III)

Y(III)

3-57

Water

samples

194

Al2O3 ICP-MS Mn(II)

Zn(II)

Pb(II)

Co(II)

Cd(II)

Cr(III)

V(V)

0.0067

0.078

0.027

0.038

0.0082

0.079

0.015

0.025

ZrO2 ICP-OES Mn(II)

Zn(II)

Cu(II)

Ni(II)

0.012

0.002

0.058

0.007

Water

samples

196

Table 2

Physical modification of nanoparticles using different organic compounds

Nanoparticle

s

Reagent Analytica

l method

Analyt

e

LOD(μ

g L-1

)

Sample Re

f

TiO2 Diethyldithiocarbama

te

ICP-AES Cu(II)

Pb(II)

Zn(II)

Cd(II)

0.41

1.7

0.39

0.52

Natural water 19

6

TiO2 8-hydroxyquinoline ICP-AES Al(III)

Cr(III)

1.96

0.32

Lake water 19

7

TiO2 Dithizone ICP-AES Cr(III)

Pb(II)

0.38

1.72

Food stuffs 19

8

TiO2 1-(2-pyridylazo)-2-

naphthol

ICP-AES Cu(II)

Co(II)

Cr(III)

Y(III)

Bi(III)

Yb(III)

2.8

12.7

3.5

0.5

17.9

0.6

Environment

al

samples

19

9

Al2O3 Gallic acid ICP-MS Fe(II)

Fe(III)

0.48

0.24

Environment

al

samples

20

0

Al2O3 Chromotropic acid ICP-OES Cd(II)

Cr(VI)

Cu(II)

Fe(III)

Mn(II)

Ni(II)

0.14

0.62

0.22

0.54

0.27

0.28

0.53

0.38

Water

samples

20

1

Pb(II)

Zn(II)

Table 3

Preconcentration of metal ions by chemically modified nanoparticles.

Nanoparticles Reagent Analytical

method

Analyte LOD(μg

L-1

)

Sample Ref

Al2O3

3-mercaptotriethoxysilane

ICP-MS

Hg(II)

Cu(II)

Au(III)

Pd(II)

0.00049

0.000066

0.00046

0.00026

Environmental

samples

202

Al2O3 1-phenyl-3-methyl-4-

bonzoil-5-pyrazone

ICP-OES La(III)

Sc(III)

Y(III)

0.39

0.16

0.19

Tea leaves

203

Al2O3 3-(8- 3-(8-quinolinylazo)-

hydroxy

benzoic acid

4-

ICP-OES Ag(I)

Au(I)

Ga(III)

In(III)

Nb(V)

Pd(II)

0.12

0.27

0.19

0.54

0.18

0.44

Geological and

water samples

204

SiO2 Diammine,

Dithiocarbamate

AAS Cu(II) NR Water samples 205

SiO2 4-(2-pyridylazo)-

Resorcinol

CVAAS Hg(II) 0.43 Water samples 206

SiO2 p- toluenesulfonylamide ICP-AES Cr(III) NR Food samples 207

SiO2 Acetylsalicyclic acid ICP-OES Fe(III) 0.49 Biological

samples

208

SiO2 p-dimethylamino

benzaldehyde

ICP-OES

Cr(III)

Fe(III)

Pb(II)

Cu(II)

0.79

0.40

1.79

1.27

Water sample

and biological

samples

209

SiO2 5-5-su 5-sulfonylsalicyclic acid

ICP-OES Fe(III) 0.09 Water sample 210

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