Study On Adsorption Properties Of Cadmium By Different Shells

At present,the heavy metal pollution has been widely concerned at home and abroad.As a typical heavy metal pollutant,cadmium(Cd)has the strong biological toxicity.Adding the adsorbent material to the cadmium(Cd)contaminated water and soil is one of the effective measures to repair the cadmium(Cd)pollution.Shells are the by-product of aquatic products,with rich resources,low cost,non-pollution and other characteristics.In this study,effects of natural and thermal modified scallop shells and razor shells on the adsorption characteristics of cadmium(Cd)in water and the cadmium(Cd)formations in soil were studied by the adsorption experiments and the laboratory incubation experiments.The main conclusions are as follows:The tested shells has a strong adsorption ability of Cd2+ in water.With the dose of shells increased,the adsorption capacity of Cd2+ on shells in water decreased rapidly and then reached smoothly.With the initial pH(1?6)of Cd2+ solution increased,the adsorption capacity increased gradually;with the initial concentration of Cd2+ solution increased,the adsorption capacity increased gradually.During the initial period of 0?2h,the adsorption capacity increased rapidly;and in the late period of 2?72h,adsorption tended to be constant.For the 400 ? scallop shell(400ss),natural scallop shell(ss)and 400 ? razor clam shell(400rcs),the Freundlich isothermal adsorption model could better fit the adsorption experimental data than the Langmuir isotherm model,0.944<R2<0.993;for the natural razor shell(rcs),the adsorption process were found to be well fitted to the Freundlich and Langmuir isotherm models,R2>0.966.In the adsorption process of Cd2+ on shells,the temperature was helpful for the adsorption,indicating the adsorption process is the endothermic process.The kinetic study showed that the adsorption of Cd2+ onto shells fit the three kinetics models(pseudo-first-order kinetic model,pseudo-second-order kinetic model and intra-particle diffusion kinetics model),but the pseudo-second-order kinetic model was found to be most consistent,R2 reached 0.999.For different sources of shells,the adsorption effect of rcs was much higher than 400ss,ss and 400rcs,the adsorption capacity reached 485.32mg/g.Adding different shells treatments in the soil had a certain contribution to the increase of the soil pH.After incubation for 50 days,ssl?ss5,rcsl?rcs5,400ss1?400ss5 and 400rcs1?400rcs5(the shell addition dose was 1%?5%of the dry soil weight)were higher than the CK 0.99?1.09,1.03?1.12,1.00?1.05 and 1.03?1.10 units;compared with the CK,there were significant differences(p<0.05).At the same time,the adding proportion of shells had significant differences(p<0.05).Under the different incubation period,adding different shells could decrease the soil available cadmium(Cd)contents in different degrees,and the performance of the reduction was more obvious with the increase in the adding proportion.After 50 days of incubation,the soil available cadmium(Cd)contents of the ss5,rcs5,400ss5 and 400rcs5 are 5.46,5.37,5.51 and 5.51 mg/kg,respectively.With the extension of incubation or the increase of shells addition proportion,the exchangeable Cd transformed to the carbonates,Fe-Mn oxides,organics and residual Cd further.The effects of different shells on the forms of Cd in soil was different.The effect of rcs on cadmium(Cd)contaminated soil was better than 400rcs,and ss was better than 400ss.It was not necessary to roast two kinds of natural shells to 400 ? when repairing cadmium(Cd)contaminated soil.After 50 days,the results showed that rcs5 had the best effect on the remediation of cadmium(Cd)contaminated soil.Compared with CK,the exchangeable Cd decreased by 28.51%,and the carbonates,Fe-Mn oxides,organics and residual Cd increased by 50.19%,16.08%,29.74%and 32.68%,respectively.