Sorption Capacity of Toxic Heavy Metal Cr(VI) Ion on Bentonite Clay from Aqueous Solution by Kinetic and Thermodynamic Studies

The present study removal of toxic heavy metal Cr(VI) ion on bentonite clay from aqueous solution under different experimental conditions and also study sorption capacity are effect different pH, concentration, temperature and contact time of shaking. My study focused on use of bentonite clay as a best adsorbent for the removal of toxic heavy metal Cr(VI) ions from aqueous solution. Initial metal ions concentration increases 50 to 250mgL-1 the %removal Cr(VI) ion decreases from 66.64% to 53.94% but amount of sorption increases from 0.833 to 3.371mgg-1, The particle size of bentonite clay increase from 45 to 150 μ by amount of adsorption of Cr(VI) decreasing from1.589 mgg-1 to 1.153mgg-1, pH increasing from 2.0 to 10.0 the amount of sorption decrease from 2.018 to 1.161mgg-1, temperature increasing from 303K to 313K adsorption is also increases from 1.589 to 1.822mgg-1. Kinetic study for value of rate constant k1 and k2 is favour concentration increases the amounts of the metals sorbent qcal also increases. Q and kf sorption capacity related to Langmuir and Freundlich adsorption isotherm constant. Value of both is favour sorption is temperature dependent. Thermodynamic study the value of ΔG0 increase -1.402 to -2.655it is best evidence temperature increases from 303K to 323K the adsorption of Cr(VI) ion increases because value of ΔG0 is negative favour the adsorption. My investigation best evidence the value of kinetic, equilibrium and thermodynamic parameter also favours the Sorption of Cr(VI) ion on bentonite clay from aqueous solution.


INTROdUCTION
Chromium is a fundamental supplement for plants and creature digestion. Chromium species is basically Cr 3+ and Cr 6+ distinctive two oxidation states in aqueous solution. Cr(III) is considered as a fundamental minor component for the support of successful glucose, lipid and protein digestion in warm blooded creatures 1 . On the other hand, Cr(VI) which is present as either dichromate (Cr 2 O 7 2-) in acidic environment or chromate (CrO 4 2-) in alkaline environments can be toxic for biological systems and carcinogenic for both humans and animals 2 . Therefore the Poisonousness and natural movement of the Chromium relies upon the aggregate sum as well as on its compound form 3 . Chromium in its hexavalent structure being harmful is brought into the water bodies from various mechanical cycles like tanning, electroplating, metal preparing, paint producing, steel creation and agrarian runoff 4 . As per USEPA the greatest reasonable cutoff points in versatile and squander water for chromium is 0.05 and 1.0 ppm respectively 5 . Adsorption techniques are viably and financially among different techniques to eliminate toxic heavy metals from aqueous solution 6 . The project of the study to effect of sorption depend of metal ion concentration on bentonite clay at other parameter like pH (2), temperature (303K) and particle size (45μ) are constant. Initial metal ions concentration increases 50 to 250 mgL -1 the %removal Cr(VI) ion decreases from 66.64% to 53.94% but amount of sorption increases from 0.833 to 3.371mgg -1 The particle size of bentonite clay increase from 45 to 150μ by amount of adsorption of Cr(VI) decreasing from1.589mgg -1 to 1.153mgg -1 , pH increasing from 2.0 to 10.0 the amount of sorption decrease from 2.018 to 1.161mgg -1 , temperature increasing from 303K to 313K adsorption is also increases from 1.589 to 1.822mgg -1 . Parameter calculated kinetic equation and equilibrium adsorption isotherm. Kinetic study for value of rate constant k 1 and k 2 is favour concentration increases the amounts of the metals sorbent q cal also increases. Q and k f sorption capacity related to Langmuir and Freundlich adsorption isotherm constant. Value of both is favour sorption is temperature dependent. The thermodynamic investigate adsorption capacity of bentonite clay to removal of Cr(VI) metal Ion. In order of determine the thermodynamic feasibility 2 and the thermal effect of the adsorption. Table 3 the value of (ΔG 0 ), (ΔS 0 ) and (ΔH 0 ) will be calculated for study at different temperature from 303K to 313K. The values ΔG 0 are negative the favour sorption process is spontaneous, ΔH o Value positive indicate adsorption nature is endothermic and ΔS 0 value positive indicates affinity of the bentonite clay for the Cr(VI) ions.
In the present study adsorption capacity of bentonite clay for the removal of Cr(VI) ions has been analysed at different pH, concentration, temperature and contact time of shaking was examined. To the best of the knowledge of authors, bentonite has not been used before for the removal of Cr(VI) ion using present parameters.

MATERIALS ANd METHOdS
All chemical used our investigation of  Figure 1 represents percentage removal of Cr(VI) ion versus initial Cr(VI) metal ion concentration. Graph study increase from 50mgL -1 to 250mgL -1 metal ion concentration the % removal Cr(VI) ion decrease from 66.64% to 53.94%. Because adsorption possesses a "limited number of active sites and theses active sites become saturated at certain concentration" 7 Figure 2 represents increase metal ion concentration the amount of adsorbent at q e mgg -1 increase as increase from 50 mg/L to 250 mg/L metal ion concentration the amount of adsorbent increase from 0.833mgg -1 to 3.371mgg -1 . "Because due to the fact that mass transfers resistance of Cr(VI) ion between aqueous and solid phase is possibly overcome by the initial metal ion concentration" 8 .

Effect of pH
Adsorption is highly depending on pH of the solution. Cr(VI) is highly adsorption in high acidic medium. It is proved by this observation the adsorption of Cr(VI) metal ion decreases with increases pH of the system. Every single metallic ions of degree of hydrolysis depend on pH of the solution. Fig. 4 evidences pH increasing from 2.0 to 10.0 the amount of adsorption decrease from 2.018 to 1.161mgg -1 because decrease pH negative Cr(VI) ion absorption is easily 2 . Figure 5 It is proved by this observation the adsorption of Cr(VI) metal ion increases with increases temperature of the system. Mobility of metal ion increases as temperature is increase. Along these lines probability of diffusion of Cr(VI) metal ions in pores of clay increases. Pore size likewise increases with temperature. Diffusion forms is endothermic in nature should increases with rise in temperature. It watched information is accordance with this. Along these lines it very well may be presumed that rise of temperature supports' the sorption. The increasing temperature from 303K to 313K amount of adsorption is also increases from 1.589 to 1.822mgg -1 at 303K, 313K and 323K are increase temperature with respect indicate the amount of adsorption increase 2,10 . Effect of particle size Figure 6 the particle size of bentonite clay on adsorption of Cr(VI) ion is affected. It proofed from figures and tables the size of sorbent particle increases from 45μ to 150μ as the amount of adsorption of Cr(VI) decreases from 1.589mgg -1 to 1.153mgg -1 because size of bentonite particle increases its surface area decreases consequently the number of active sites decreases as a result sorption decreases.

Kinetic studies
To explain the adsorption kinetics, pseudofirst-order kinetic equation will be applied to the experimental data the linearized form of the well known Lagergren equation for adsorption kinetic at different concentration is as below:-Where q e are the amounts of the metals sorbent at equilibrium and qt are the amounts of the metals sorbent time "t" respectively and k 1 is the rate constant of sorption can be determined experimentally by plotting graph log (q e -q t ) versus t. The pseudo-second-order kinetic is given as "Where k 2 is the rate constant of the pseudo second order equation can be calculate experimentally by graph plotting of t/q t versus t" 5 .  Table 1 give the kinetic model result high correlation coefficient obtained for the pseudo second order kinetic all the studies concentration in the comparison pseudo second order kinetic model are better explained by first order kinetic model. Experimental data q e (cal) value better q e (exp.) and K 2 constant with increase in metal ion concentration. The explanation behind  this might be plausibility of lower rivalry for surface dynamic locales of adsorbent at lower concentration. As the metal particle concentration increase surface active sites increase with decreases the rate.

Equilibrium studies Langmuir adsorption isotherm modal
Experimental equilibrium data obtained will be discussed using Langmuir adsorption isotherm modal accepts sorption on a "homogeneous surface by monolayer sorption without collaboration between adsorbed molecule the linear type of Langmuir isotherm which is as underneath" 13 Where C e (ppm) is equilibrium concentration of Cr(VI) ion, Q and b are constant indicating the sorption capacity and energy respectively. Q and b calculate slope and intercept of the straight line graph plotting between C e /q e Vs C e .

The Freundlich isotherm model
For sorption on heterogeneous surface this model is used. It's linearized from is represent as "Where k f and n is freundlich constant related to sorption capacity and intensity of the sorbent" 14 the value of k f and n calculate slope and intercept of the straight line graph plotting between log q e verses log C e is linear. It indicates the applicable of the above model. The present Fig 9-10 and Table 2 the high correlation coefficients R 2 values are shown by isotherm Frendlich equation than Langmuir equation. It may be concluded that the experimental data fits in Langmuir isotherm but Frendlich isotherm data fits better for bentonite clay. Besides as 0<Q it indicate that adsorption process is favourable normally increases in temperature adsorption capacity Q increase which indicate high adsorption it has been observed. Similarly as temperature increases adsorption intensity b also increases. Various data regarding Q and n have been Table 2.

Thermodynamic study
In order of determine the thermal effect and thermodynamic feasibility of the adsorption 2 . The Gibb's free energy (ΔG 0 ), the entropy (ΔS 0 ) and the enthalpy (ΔH 0 ) will be calculated for study at different temperature from 303K to 313K using following equation Were Kc is the equilibrium constant. ΔG 0 has been calculated from the above equation. Plot graph between logKc verses 1/T gives strait line. The value of ΔH 0 and ΔS 0 by various graphs slope and intercept of this line. The value of ΔH 0 and ΔS 0 all value listed Table 3. The process is endothermic and exothermic nature is concluded by the sign of ΔH. If process is endothermic the value of ΔH positive but sing of ΔH is negative it show the process is exothermic in nature. The ΔG Gibb's free energy change is important parameters. If value of ΔG is negative the processer is more effective at higher temperature because increases in temperature the negative value of ΔG decreases, the increases in temperature.

Activation energy
Using modified Arrhenius type equation the value of activation energy and sticking probability have been determined from the experimental data related to surface coverage θ as follows Where the term have their typical significance the sticking probability Sᶿ is a function of the adsorbate/adsorbent system under consideration depending on temperature and should satisfy the condition 0 < Sθ << 1. The value of Ea and Sθ can be determined from the plot of graph log (1-θ) versus 1/T gives straight line. Slope and intercept of this straight line gives the value of E a and Sθ. Sorption studies at different initial concentration of sorbate show that the amount adsorbed increases but percentage amount adsorbed decreases with the increase of initial concentration. Adsorption is highly depending on pH of the solution. Cr(VI) is highly adsorption in high acidic medium. It is proved by this observation the adsorption of Cr(VI) metal ion decreases with increases pH of the system. Normally particle size increases value of qe decrease. Temperature has stamped impact on adsorption the rate as well as percentage adsorption increases with the increase of temperature The calculated data proved that the value of ΔG is negative show that process is spontaneous. The processer is endothermic nature because value of ΔH is positive. The affinity of the adsorbent towards the adsorbate species the value of ΔS is positive. They also supported the positive value of E a adsorption process is endothermic nature. The adsorption process I mixed process physical and chemical adsorption it is proved by the positive value of activation energy. That the probability to stick on surface of clay is very High because Sθ<<1. From the above observation and discussion it may be concluded that bentonite clay are best adsorbent under specific condition.