Studies On Adsorption Of Cd (Ⅱ) From Aqueous Solution By Jingyuan Attapulgite

For the excellent adsorption efficiency, low cost and convenient regeneration, attapulgite，which is crystal hydrated magnesium aluminum silicate with a high surface area and a greatperformance in the adsorption of many pollutants, has been obtaining increasing scientificattention in the application on the treatment of polluted aqueous solution by heavy metals.The attapulgite used in this study is from Jingyuan in Gansu province. Adsorption ofcadmium ions by natural attapulgite （NA）, purified attapulgite （PA） and attapulgite underacid-thermal modification （AH-A） are conducted by batch experiments, and the property ofthe adsorption of cadmium ions onto attapulgite under different time, initial concentration andtemperature factor is investigated, and the adsorption behaviors is describes from the aspect ofadsorption thermodynamics, kinetics. The experimental result indicatesthe following:（1） Adsorption efficiency and absorption capacity of Cd（II） onto three kind ofattapulgites increase rapidly within first20min, as time extended continually, the adsorptionefficiency and absorption capacity of Cd（II） onto attapulgites growth slowly, and adsorptionreach balance after180-240min. Adsorption capacity of Cd（II） onto attapulgite increase astemperature and initial concentration rise clearly, but adsorption efficiency decrease asincrearing initial concentration.（2） The kinetic study shows that the adsorption of Cd（II） onto three kind of attapulgitesdoes not comply with the pseudo-first-order process and the diffusion model in the wholeadsorption time but fit the pseudo-second order process very well, which indicateschemisorption is the rate-controlling step in the whole adsorption process; Meanwhile, theadorption data in first60min could be well represented by elovich equation and two-constantsequation, which suggests chemisorptions on highly heterogeneous adsorbents and aheterogeneous diffusion process at the beginning of the adsorption. The increase kineticsparameters of k₂、h0、qe₂indicates higher temperature enhance the adsorption rate, and thedecrease of k₂is due to the resistance for adsorption under higher initial concentration.（3） The adsorption kinetics of Cd（II） onto three kind of attapulgites comprises three steps,which is concertedly manifest by the multiple linear regression of kinetics data, solute transferof Cd（II） to the attapulgite particle surface （film diffusion） and the adsorption is mainlydominated by outer-surface adsorption or ions exchange, transfer from the attapulgite surfaceto the intraparticle active sites （particle diffusion）, and retention on the active sites viasorption, surface complexation or further ions exchange. The adsorption processes arecontrolled by the combination of film diffusion （physical process） and intra-particle diffusion,film diffusion controls the initial20min, and intra-particle diffusion becomes the rate-determing step as the adsorption processed, and interior surface adsorption dominates theadsorption rate as the approaching of adsorption equilibrium.（4） The adsorption experimental data are found to be well fitted to the Langmuir,Freundlich、D-R and Temkin isotherm models, as the Langmuir model gives higher fittingthan other models, which implied themonolayer adsorption and heterogeneous surfaceconditions coexist under experimental conditions studied, and the adsorption of Cd（II） ontothree attapulgites is complex involving more than one mechanism. The mean free energy （Es）from D-R model （9.901-12.127kJ/mol for NA,11.323-13.868kJ/mol for PA,12.127-15.076kJ/mol for AH-A） inferred that the ion exchange reaction maybe play an important role in theadsorption process. The saturated adsorption capacity of Cd（II） are12.690-16.420mg/g forNA,28.653-32.258mg/g for PA, and39.063-46.083mg/g, respectively. The isotherm modelparameters of b、Kf、n、qm, which increase as temperature rises, concertedly manifest that theadsorption is favorable and elevated temperature benefits the adsorption.（5） The positive values of enthalpy （△H⁰）, entropy （△S⁰） and the nagetive values ofGibbs free energy （△G⁰） calculated from adsorption data indicated that the adsorption ofCd（Ⅱ） onto attapulgite was a feasible, endothermic and spontaneous process in nature andagain confirmed the physical adsorption and chemical adsorption simultaneously controlledthe adsorption process. The increase of△H⁰and decrease of△G⁰with increasingtemperature indicates more efficient adsorption at higher temperature, and the increase of△S⁰with increasing temperature suggest some structure changes. The adsorption activatedenergy （Ea） obtained for the three kind of attapulgites are24.834,25.484, and15.487kJ/mol,respectively.