Characterization of Montmorillonite Biocomposite and Its Application for Trace Level Removal of Sb 3 + : Equilibrium and Kinetic Studies

The effluent from various industries is the potential source of water contamination during last few decades. Thus effective methods have been adopted for the removal of toxic heavy metals from industrial effluents that show carcinogenic and mutagenic effects. The present research involves removal of Sb that has been investigated using chitosan-montmorillonite biocomposites. The kinetics and adsorption equilibrium was determined respectively using batch adsorption model, taking into account solution pH, contact time and initial metal ion concentration. The adsorption isotherm parameters were evaluated wherein Freundlich model best represents the experimental data. The highest adsorption capacity of 48.7 mg/g of chitosan-montmorillonite beads was attained from an initial concentration of 100 g/m at 300 K. The equilibrium was achieved during initial phase of contact of 10 minutes only. The biosorbent show comparable high adsorption capacity for Sb and is efficiently functional in broad range of metal ion concentration from 4 mg/m (4 parts per billion) to 100 g/m (100 parts per million) of solution. The adsorption kinetics follows chemical adsorption as the rate determining step. The interaction forces between Sb and adsorbent/s were determined by Fourier Transform Infrared-Attenuated Total Reflectance (FT-IR-ATR). The metal ion desorption and reusability of biosorbent/s up to three cycles was supported by 0.1 M potassium dihydrogen phosphate solution. The Scanning Electron Micrographs and X-Ray Diffractograms of the adsorbents before and after metal interaction were found to correspond to the batch adsorption studies of the metal ion.


Introduction
Antimony is known to be a toxicological and carcinogenic metal.It is ubiquitously introduced to the environment from natural processes that include weathering of rock and soil run-off.
Human activities such as extensive use of lead alloys, battery grids, bearing metal, cable sheathing, plumber's solder, pewter, ammunition, sheet and pipe also add to the concentration of antimony in the environment.The total consumption of antimony in various industrial products is 1,00,000 tonnes per year worldwide.Among the most important uses of antimony in non-metal products are textiles, paints and lacquers, rubber compounds, ceramic enamels, glass and pottery abrasives, and certain types of matches [SbCl 3 ] (Ramesh, Hasegawa, Maki, Ueda, 2007).The metal, when discharged in wastewater represent a serious threat to human population.The concentrations of antimony in groundwater and surface water are found to be in the range of 0.1-0.2mg/m 3 Hence the discharge of antimony into aquatic bodies and sources of drinking water has begun to be strictly controlled (Khan, Rasul, Munir, Habibuddowla, Alauddin, Newaz, & Hussam, 2000).
The bioavailability and toxicological effects of antimony depend on its chemical form and oxidation state (Haron, Rahim, Abdullah, Hussein, & Kassim, 2008).The two common inorganic forms of antimony in natural waters are, Sb 5+ , [Sb (OH)] 6− and Sb 3+ , [Sb(OH) 3 ].The toxicity of trivalent form of antimony, Sb 3+ is ten times more than pentavalent form, Sb 5+ (Mohanty, Majumder, & Mohanty, 2006).Its potential and proven carcinogenicity, immunotoxicity, genotoxicity, and reproductive toxicity makes the compounds of antimony to be considered as pollutants of priority interest by the United States Environmental Protection Agency (USEPA), and the European Union (EU).The World Health Organization (WHO) guidelines limit the concentration of antimony as 0.005 g/L (5 mg/m 3 ) in drinking water (Monvisade and Siriphannon, 2009;Haque, Morrison, Jorge, & Torresday, 2008;Casariego, Souza, Cerqueira, Teixeira, Cruz, Dıaz, & Vicente, 2008).The increasing study on presence and behaviour of antimony in environment reflects the essential need to its remedy to be worked upon.The chronic toxicity of metal and the ecological demand for efficient removal of Sb 3+ from parts per billion level in water has led us to make research for its remediation.
Adsorption and ion exchange, coagulation, flocculation and sedimentation, precipitation and microfiltration are the various conventional techniques reported for the removal of Sb 3+ from aqueous solution (Rana, Halim, Waliul Hoque, Hasan, & Hossain, 2009;Jong & Stefan, 2008).Adsorption is observed to develop as a front line of defence for the remediation of antimony pollution.It has suitability for batch and continuous processes, effectiveness, ease of operation.It leads to little sludge generation and is economical and versatile in nature.The limited adsorption studies of antimony on natural sorbent have been reported till date, to the best of our knowledge.The adsorption capacity of Fe, Mn and Al hydroxides for Sb 3+ indicates a decrease in the sequence from MnOOH > Al(OH) 3 > FeOOH (Antonio & Cestari, 2004;Nimrod & Mishael, 2010).
The present remediation methodology has not yet been reported for the removal of Sb 3+ , (SbO 3 ) 3-and to the best of our knowledge, the lowest detection limit of 1 g/m 3 has been reported by Xi, He & Lin (2011) using bentonite.
The maximum adsorption efficiency is reported to be attained within contact time of 24 hours.The objective of research is to develop a potent biosorbent that meets the ecological demands and is effective also at very low concentration, up to parts per billion of Sb 3+ in water.Chitosan has been selected for the investigations since it is hydrophilic, biocompatible, biodegradable, and possess anti-bacterial property (Choudhari & Mahadevappa, 2009).At pH <5.5, chitosan forms gel that restricts its use as an adsorbent for Sb 3+ from acidic industrial effluents.Hence, a natural occurring clay montmorillonite (Mt) has been used to treat chitosan to form composites.The treated chitosan is acid resistant and also provide higher surface area and stronger mechanical properties than pure chitosan.The adsorption of Sb 3+ using synthesized biocomposites is likely to prevent widespread metal toxicity in nature.

Solubility
In water 55 g/L at 20 ˚C

Colour White
The modification of physical structure and chemical properties of chitosan using montmorillonite results in maximization of its adsorption capacity.Thus three chitosan treatments were investigated as well as the effect of pH, contact time and initial concentration on metal ion removal was studied.The feasibility of regeneration of sorbent after adsorption of Sb 3+ was investigated using 0.1 M and 1 M sodium hydroxide, 0.1 M KH 2 PO 4 .A potential benefit of the study is the identification of a low cost, biocompatible, Sb 3+ sensitive efficient sorbent for the removal of metal ions from aqueous solution.The work is a stepping stone to provide economical, easy and point-of-use systems for small communities and water utilities.

Materials and Methods
The clay mineral, Montmorillonite-KSF (Mt) was used in this study.The 2:1 layer mineral was obtained from Sigma Aldrich Pvt. Ltd. (Germany).Sodium hydroxide and hydrochloric acid were obtained from Qualingens (India).Leuco crystal violet (C 25 H 31 N 3 ) was obtained from Sigma Aldrich Pvt. Ltd.The stock solution of 250 g/m 3 Leuco crystal violet (LCV) was prepared in acidic medium in double distilled water.The volume of the solution was made up to 1000 ml in a standard volumetric flask.The solution was stable for several hours when kept in an amber flask away from the direct sunlight.Chitosan (Chi) was obtained from Everest Biotech.
Potassium antimonyl tartrate taken with 99% assay (Table 1), was obtained from Merck (India).Ortho-phosphoric acid was obtained from Merck (India).Sb 3+ stock solution (1000 g/m 3 ) was prepared by dissolving 0.2668 g of potassium antimonyl tartrate in double distilled water.The volume of the solution was made up to 1 L in a standard volumetric flask.The AR grade chemicals/reagents and double distilled water was used for the experiment.

Chi-Mt Biocomposites
The biocomposite of Chi-Mt was synthesized by modifying the procedure reported previously by Jong and Stefan (2008).Chitosan was dissolved in 1 % (v/v) acetic acid under constant stirring for 4 hours to obtain chitosan solution.Mt was sieved, washed and dried at 80 ºC.The pH of chitosan solution was adjusted to ~ 5 using NaOH solution before adding to Mt suspension to avoid any structural variation of clay.The solution was then slowly added to the Mt suspension (2 %) at 323 K.The mixture was stirred for 24 hours and residue was finally washed with double distilled water until free from acetate ions.The residue thus obtained was dried at 323 K and labelled as Chi-Mt.

Beads of Chi and Chi-Mt
Chi and Chi-Mt beads were synthesized using 200 mg of chitosan flakes.The Chi flakes were allowed to stand overnight in 100 mL of 1 %( v/v) acetic acid.The resulting solution was added to a bath containing 0.5 M NaOH solution through a clinical syringe to form hydrogel beads.The chitosan hydrogel beads were allowed to stand in alkaline solution for 1 hour and then filtered and washed with double distilled water until the solution pH was neutral.The beads thus obtained were labelled as Chib.
Chi-Mt beads were synthesized using 200 mg of chitosan flakes and 300 mg of Mt.The mixture was allowed to stand overnight in 12 mL of 1 % (v/v) acetic acid.The resulting solution was added to a bath containing 0.5 M NaOH solution through a clinical syringe to form hydrogel beads.The Chi-Mt hydrogel beads were allowed to stand in alkaline solution for 1 hour and then filtered and washed with double distilled water until the solution pH was neutral and labelled as Chi-Mtb.

Spectrophotometric Analysis
Quantitative estimation of Sb 3+ was performed by UV-VIS spectrophotometric method using LCV as reported in the literature (Wong, Szeto, Cheung, & McKay, 2004).A rectilinear calibration graph (absorbance vs. concentration) of crystal violet (CV) was obtained at 592 nm by measuring the absorbance of solution over a known concentration range of Sb 3+ as represented by the equation: The batch adsorption studies were carried out as a function of pH, contact time and metal ion concentration.0.1 g of the respective sorbent was exposed to 50 ml of [SbO 3 ] 3-solution.The concentration of Sb 3+ in the supernatant was estimated spectrophotometrically.The percentage of Sb 3+ adsorbed onto the biosorbent was calculated using the equation 1.
Amount of Sb 3+ adsorbed (q e ) was calculated from the relationship where C i represents initial Sb 3+ concentration (mg/L) and C e the final Sb 3+ concentration in the solution after equilibrium was attained (mg/L), V is the volume of the Sb 3+ solution (mL) and m is the mass of the adsorbent (mg) used.

Removal of Sb 3+ as a Function of pH
The important factor that affects removal of Sb 3+ is pH of metal ion solution.The effect of pH on removal of Sb 3+ on Chi and its biocomposites was investigated in the pH range of 2-8, using adsorbent doze of 2 g/L, at 20 ˚C.The behaviour of Sb 3+ towards chitosan based adsorbents could be explained by pK a of chitosan that is 5.6.Thus, Chib was restricted from removing Sb 3+ from the solution by charge neutralization.The adsorption of Sb 3+ using Chib may be a function of hydrogen bond formation or Van der Waals force between pH 4-9, thereby supporting the application of Chib and Chi-Mtb for removal of Sb 3+ in broad range of pH.
The Chib were found to be highly unstable in acidic range and thus no considerable data could be obtained at pH less than 4. The removal of Sb 3+ was observed to be a pH independent phenomenon beyond pH range of 5 for Chib and during pH range of 5 -9 for Chi-Mt.The adsorption of metal ion upon Chib decreases from 96.4 % at pH 6 to 95.8 % at pH 7 (figure 2A).The adsorption of metal ion increases from 79 % at pH 2 to 95.5 % at pH 7 upon Chi-Mt.Due to insignificant difference in Sb 3+ removal, pH 5-6 appeared to be the optimal range for efficient adsorption of Sb 3+ .No significant decrease in Sb 3+ adsorption was observed until the pH >9 under the experimental conditions.At pH <4, the amount of Sb 3+ removed from solution is found to decrease.The free amine groups probably present on the external surface as well as in the interlayer of Chi-Mt were responsible for the adsorption of specie from water.The neutral and negatively charged Sb 3+ ions present at pH >9 are attracted to the positively charged surface of the adsorbent thereby resulting in sufficient Sb 3+ adsorption.
At pH > 6, decrease in adsorption of Sb 3+ is due to competition for the sorption sites between hydroxyl ions and hydroxylated complexes of Sb 3+ .The adsorption maxima at around pH 6 can be explained by the fact that the difference between the energy released upon adsorption and energy required to dissociate the acid is at a maximum.

Removal of Sb 3+ as a Function of Contact Time
The effect of contact time on the amount of Sb 3+ removed from aqueous solution was investigated during 1-120 minutes as shown in figure 2A.The Chi was observed to show an increase in Sb 3+ removal from 93% during the initial contact time of 10 minutes to 96% at 60 minutes.An increase in removal of metal ion from 90.5% during initial 10 minutes to 97.5% at 60 minutes was observed using Chi-Mtb.
Chi-Mt shows an increase in adsorption of Sb 3+ from 91.6% during 10 minutes to 95.5% during 60 minutes.A steep rise in Sb 3+ removal was observed during the initial contact time of 1-10 minutes as shown in figure 2 due to the availability of a large number of adsorption sites on the adsorbent surface.
The equilibrium was investigated by exposing the Sb 3+ ions for contact time of 10,20,30,40,50,60,70,80,90,100, 120 minutes respectively.The constant removal of metal ion was observed within 70 minutes.To investigate the change in removal capacity of adsorbents the batch studies were extended up to 120 minutes.

Equilibrium Studies of Sb 3+ Adsorption
The adsorption capacity of biosorbents increased with an increase in initial metal ion concentration.The maximum removal of 97.5 % observed from 100 g/m 3 aqueous Sb 3+ concentration using Chi-Mtb (figure 2B) after which no further increase in adsorption was observed due to nearly complete coverage of the sorption sites of biosorbents at high initial concentration of Sb 3+ .At lower metal ion concentrations, the ratio of initial number of Sb 3+ ions to the available adsorption sites is low, whereas at higher concentrations, the number of available adsorption sites becomes lower, and subsequently the removal of Sb 3+ ions depend on the initial concentration.
The Chib shows an increase in adsorptive removal of metal ion from 87 % to 96.4 % on increase in initial metal ion concentration from 10-100 g/m 3 .The removal of Sb 3+ from an initial concentration of 0.1 g/m 3 was observed to decrease to 40%.The Chi-Mtb shows an increase in adsorption of Sb 3+ from 91 % to 97.5 % with an increase in concentration of metal ion from 10-100 g/m 3 .It shows an adsorption of 50.3 % from an initial concentration of 0.1 g/m 3 .
The increase in adsorption of Sb 3+ within the given concentration range indicates a heterogeneous system wherein adsorption is not restricted to monolayer formation.The adsorption capacity of Sb 3+ using Chi-Mtb was observed to be higher than Chib as well as the pristine Mt as reported by Anjum and Datta (2012).The Chi-Mt shows an increase in adsorption from 87.6 % to 95.5 % with an increase in concentration of metal ion from 10 -100 g/m 3 (figure 2B).The biosorbent has also been found to possess the efficiency to remove almost 75 % of Sb 3+ from an initial concentration of 0.1 g/m 3 .The trace level removal of Sb 3+ from 0.008 g/m 3 shows 42 % removal using Chib, 34 % removal using Chi-Mt, and 46 % removal using Chi-Mtb respectively.The zetametry (+30 mV for Chi-Mt and +10 mV for Mt) suggest electrostatic attraction between the positively charged adsorbent and negatively charged adsorbate.The increased in adsorption potential of Chi-Mtb may be attributed to increase in surface area of spherical beads and incorporation of the characteristics of Mt and chitosan as well.
The Langmuir isotherm did not show a good linear fit as can be seen from isotherm parameters in Table 1.This implies that Sb 3+ adsorption is not monolayer and independent adsorbent sites are occupied by every metal ion only during initial phase of adsorption.Maximum adsorption capacity calculated using this model was observed to be 11.9 mg/g, and 1.77 mg/g using Chi-Mt and Chi-Mtb was respectively.This capacity is much less in magnitude when compared with the Freundlich isotherm model.0.98 0.98 0.99 The Freundlich adsorption isotherm shows no apparent plateau for the range of concentrations studied indicating the effectiveness of biosorbents for the adsorption of metal ion.The experimental data shows a satisfactory fit to Freundlich isotherm (better than the Langmuir isotherm fit) particularly for adsorption on Chi-Mtb followed by Chi-Mt and somewhat less so on Chib as can be seen from table 2. The Freundlich isotherm constant depicts higher sorption affinity of metal ion for Chi-Mtb (n=1.67)followed by Chi-Mt (n=1.25).The least affinity for Sb 3+ is shown by Chib (n=1).This agrees well with the batch extraction studies that shows maximum adsorption efficiency of the order Chi-Mtb > Chi-Mt.The adsorption efficiency of Chi-Mt (50.3 %) was observed to be 25 % more than Chi-Mtb (75 %) from initial concentration of as low as 0.1 g/m 3 .Thus, Sb 3+ adsorption was observed to follow the order Chi-Mtb > Chi-Mt > Chib.The maximum adsorption capacities calculated from Langmuir isotherm was found to be in the order Chi-Mt > Chi-Mtb > Chib as shown in table 2. However the adsorption capacity of biosorbents obtained from Langmuir isotherm was appreciably less.The isotherm shows a poor regression coefficient (R 2 = 0.67, 0.67 and 0.70 respectively for Chi-Mtb, Chib, and Chi-Mt).The goodness of the fit of experimental data measured by the determination coefficients, R 2 shows Freundlich isotherm as the best model for adsorption of Sb 3+ using biosorbents.

X-ray D
The X-ray (1.544 Å) diffractogr  22.2 Å was related to the intercalation of chitosan bilayers as reported by Monvisade and Siriphannon (2009).
The interaction of Mt with Sb 3+ ions made a shift of d (001) diffraction peak towards lower 2theta values from 6•10° to 5•38° indicating the expansion of basal spacing to 17 Å (an increase of 4Åwith respect to Mt).This could be attributed to the movement of metal ion into the interlayer gallery of Mt thereby causing an increase in the basal spacing.This is due to the presence of large number of NH 3 + sites in the network of the biopolymer that accommodates an appreciable number of the metal ions that makes the basal spacing to increase.The disappearance of the chitosan peak at 18° has been found to disappear after Sb 3+ adsorption.
The 2theta value was found to decrease from 5.02° to 2.89° on interaction of Chi-Mt with Sb 3+ ions.This corresponds to an increase in basal spacing to 31.2Å.The d ( 001) diffraction peak at 6.70° was found to shift to 5.06° and an increase in basal spacing to 21.2Å as shown in figure 2D.Thus, the observed decease in diffraction d (001) peak value points towards an intercalation of Sb 3+ into the interlayer gallery of Chi-Mt that contributes to the extraction efficiency of the adsorbents for Sb 3+ .As evident from the diffractograms the basal spacing depends on the amount of the available moiety intercalated and varies from one adsorbent to another.Thus, the order of intercalation corresponds well with the adsorption efficiency of the adsorbents as obtained by the batch adsorption studies.The intercalation of Sb 3+ in Mt was possibly due to the availability of freely accessible intergallery space.Therefore it can be deduced that some Sb 3+ ions are expected to move into the interlayer gallery which in conjunction with adsorption on the other sites consequently result in greater Sb 3+ adsorption capacity and also corresponds to the spectrophotometric studies.

Fourier Transform-Infra Red-Attenuated Total Reflectance Analysis
The FT-IR-ATR spectrum of Mt after Sb 3+ interaction at optimized pH and contact time shows the presence of small peak at around 1454 cm -1 and 866 cm -1 respectively that corresponds to the stretching vibrations of Sb-O bond.The Si-O stretching vibration (1047 cm -1 ) was found to shift to lower wavenumber at 1038 cm -1 .The O-H bending vibration (1641 cm -1 ) was found to shift to higher wavenumber at 1656 cm -1 that reflect the interaction of Sb 3+ with the hydroxyl group of Mt.The O-H stretching vibration (3600 cm -1 ) was found to shift to lower wavenumber at 3582 cm -1 respectively that reflect the presence of hydrogen bonded O-H group possibly with the adsorbate present in the solution (Sb 3+ ).
The N-H bending vibration of protonated amino groups (1632 cm -1 ) in Chib was observed to be shifted to a higher wavenumber at 1638 cm -1 after interaction with Sb 3+ suggesting ionic attraction between R′-NH 3 + and (SbO 2 ) - group as well as possible hydrogen bonding between antimony and -OH group of chitosan.The C-O-C stretching vibration in Chib (1148 cm -1 and 1033 cm -1 ) shifted to lower wavenumber at 1071 cm -1 and 1029 cm -1 respectively after interaction with Sb 3+ due to formation of bidentate inner complex of Sb 3+ with oxygen of C-O-C groups (adjacent to C1 and C5) of chitosan.However, a new stretching vibration at 1360 cm -1 has been observed possibly due to presence of Sb-O bond (figure3).
The N-H bending vibration frequency 1543 cm -1 (NH 2 vibration of protonated amino groups) of Chi-Mtb after interaction of Sb 3+ with the N-H bending vibration frequency 1543 cm -1 (NH 2 vibration of protonated amino groups) (SbO 2 ) -was found to disappear suggesting the ionic attraction between R′-NH 3 + and (SbO 2 ) -group.The shift in the frequency of stretching vibration of Si-O and C-O-C (overlapping band at 1023 cm -1 ) in Chi-Mtb to higher frequency (1037 cm -1 ) after interaction with Sb 3+ and has been interpreted as due the formation of bidentate inner complex of Sb 3+ with oxygen of C-O-C and Si-O groups.The 911 cm -1 band assigned to the Al-O stretching vibration of the biosorbent is not observed after interaction with Sb 3+ , suggesting a likely shift in the vibrational frequency beyond 650 cm -1 .
After the interaction of Chi-Mt with Sb 3+ the overlapping Si-O and C-O-C stretching vibration (1023 cm -1 ) have been found to shift to higher frequency at 1055 cm -1 and has been interpreted as due to the formation of a chemical bond (inner sphere complex formation by Sb 3+ and oxygen of the ring (C-O-C) and oxygen on the surface of the clay.The overlapping bands of O-H and N-H stretching vibration have been observed to shift to a higher frequency (3625 cm -1 to 3630 cm -1 ) after interaction with Sb 3+ .The shift in Al-O stretching vibration was observed to a lower wavenumber at 622 cm -1 after Sb 3+ interaction that suggest chemical interaction between Sb 3+ and Al-O group at the edges of clay particles.The N-H bending vibration frequency (1630 cm -1 ) of protonated amino groups (at C2') has been found to shift to a higher wavenumber at 1642 cm -1 after interaction of Sb 3+ .After interaction of Chi-Mt with metal ion a new peak was observed at 885 cm -1 and 728 cm -1 that signifies the possibility of presence of Sb-O bond.
The reduction in peak wavenumber, a new peak as a result of splitting of the initial peak is indicative of chemical bonding or an inner-sphere complex.The basic role in the formation of Sb 3+ -chitosan compound has been played by

Scanning Electron Microscopy
The biosorbents after interaction with Sb 3+ show an appreciable change in morphology of the adsorbents as shown in figure 4. Since the more porous adsorbents provides increased surface area for efficient extraction of Sb 3+ thus, after the metal ion adsorption the reduced porosity of the adsorbents further proves the successful metal treatment that provides a less porous morphology to the natural and synthesized biosorbents.The scanning micrographs reveals smooth, homogenous and densely-packed morphology of the matrices that may be attributed to the presence of adsorbed species onto the porous structure, probably to the bound Sb 3+ .The transmission electron micrograph shows diameter of shows that diameter of Chi-Mt after Sb 3+ ranges from 10-20.3 nm respectively.

Kinetics of Sb 3+ Adsorption
The mechanism of Sb 3+ removal using the three biosorbents and the potential rate controlling step has been subjected to extensive research.A steep rise in rate of metal ion removal was observed during initial contact time of 10 minutes after which the rate slowed down as the equilibrium is approached.
The comparison of best fit sorption mechanism of Sb 3+ on Chi-Mtb, Chi-Mt, and Chib were made with 100 g/m 3 of Sb 3+ using 2 g/L of adsorbent dose.The overall calculation for the rate of adsorption of Sb 3+ was estimated and the mechanism of adsorption was determined by the rate determining step.Lagergren's pseudo-first order model for the adsorptive extraction of Sb 3+ has been investigated.The rate equation for the liquid/solid system based on solid capacity 15 has the linear form of equation 3.
This kinetic model assumes that the rate of change of solute uptake is directly proportional to the difference in saturation concentration and the amount of solute uptake.The linear fit correlation coefficient (R 2 ) of log (q 1 − q t ) against t for Sb 3+ adsorption on Chi-Mtb, Chi-Mt, and Chib as shown in figure 5 were found to be 0.78, 0.98 and 0.87 respectively.The results indicate that Sb 3+ adsorption does not totally agree for the strict first order adsorption mechanism and thus deviates the pseudo-first-order reaction model slightly particularly on Chi-Mt and Chib.Thus, physisorption is said to be accompanied with another model related to pore diffusion.
Ho and Mckay pseudo-second order kinetic model is based on the sorption capacity of the solid phase and is represented by equation 4.
This model states that chemisorption may be the rate limiting step of the sorption system involving valence forces through sharing or exchange of electrons between sorbent and sorbate.
The model shows the linear correlation coefficient (R 2 ) values to be 0.98 for Chib, 0.99 for Chi-Mtb and 0.99 for Chi-Mt (figure 5).Chi-Mtb and Chi-Mt show favourable adsorption coefficient indicating that the adsorptive extraction of Sb 3+ involves chemical bond formation with the respective biosorbents favourably following the pseudo second order model as also supported by the FT-IR-ATR studies.
A possibility of transport of Sb 3+ into the interlayer space and pores of biosorbent/s has been observed due to attiring on batch process as also supported by the experimental coefficient correlation of pseudo first order reaction model.This possibility was investigated in terms of Weber and Morris Intraparticle diffusion model that depicts a graphical relationship between the amount of Sb 3+ adsorbed and square root of time as depicted by equation 5.   ln (q e -q t )= 1• q t = 7•95 t 0•5 t/q t = 0•0106 ln (q e -q t )= 0• q t = 0•623 t 0ˑ5 t/q t = 0•0132+ ln (q e -q t )= 0• q t = 0•44 t 0ˑ5 t/q t = 0•0137+ ln (q e -q t )= 0• q t = 0•566 t 0• t/q t = -0•0027 nts consecutive fiv M NaOH durin

Conclusions
The maximum extent of Sb 3+ adsorption depends on pH and is highest in the pH range of available drinking water due to the speciation of Sb 3+ .A quantitative prediction of Sb 3+ adsorption to natural materials is however, a challenging task mainly due to its inherent complexity, particularly with respect to surface heterogeneity.In this study the adsorption mechanism was examined.It was observed that a variety of other processes, in particular interactions/and redox transformation of Sb 3+ with natural organic matter contribute to overall sorption in nature.The information obtained from the results here should provide essential first step in efficient extraction of the most toxic arsenic specie from aquatic systems.
The adsorptive extraction efficiency of the biocomposites investigated at very low concentration of metal ion was found to be dynamically sensitive up to four parts per billion of Sb 3+ .The biocomposites have been found to be sensitive to very dilute solution of metal ion concentration up to 0.008 g/m 3 .The Chi-Mtb show promising behaviour for the removal of trace levels of Sb 3+ from water.The analyte undergoes instantaneous adsorption onto the surfaces that is responsible for adsorptive extraction during initial phase of contact.The surface complexation of Sb 3+ with the adsorbent further enhances the adsorption capacity of the analyte onto the adsorbents during later phase of extraction.This phenomenon was observed to be well supported by the kinetic studies.The higher adsorptive extraction on Chi-Mtb may be explained by the high total surface area resulting from spherical shape of beads and the increased porous nature due to containing interlinked channels.
The batch extraction results have been well supported by FT-IR-ATR analysis of the adsorbents after Sb 3+ adsorption that indicate surface complexation between -OH, -NH sites and Sb 3+ respectively rather than solid phase precipitation that may add to a large amount of sludge generation.
Thus, the integration of Chitosan with Montmorillonite has proved to enhance the extraction capacity of the pure Chitosan as an adsorbent for the removal of toxic metal from water.

Figure 1 .
Figure 1.Schematic representation of synthesis of Chitosan Mt beads, Chitosan beads Figure 2. solution, [ K; C-Freu A decrease favourably adsorption knowledge water.
Figure 5.A

Table 1 .
Characteristic properties of Sb 3+ compound taken for studies (Potassium antimonyl tartrate)

table 3
An increase in d (001) spacing of Mt is observed that is due to the movement of chitosan into the interlayer gallery causing an increase of 9.2 Å with respect to Mt.The broad reflection around 2θ value of 6.8° (d 001 12.96 Å) as shown in figure2was assumed to be due to monolayer of chitosan in the interlayer gallery of Mt whereas d (001)