Effect Of Surface Modified Activated Carbon On Adsorption And Stabilization Of Lead In Soil

Heavy metal pollution has gradually drawn a wide public attention due to its characteristics of a variety of sources and large contaminated area.Compared with other methods for remediation of heavy metal-containing soil,stabilization/solidification has been widely used to stabilize/solidify heavy metals in sediment,sludge,industrial waste and soil owing to its advantages of cost-effectiveness,simple operation,and wide ly application.However,people are still look ing for the heavy metal stabilizers which are high efficient,cost-effective and friendly to environment.Activated carbon,which has excellent adsorption performance characteristics and stable chemical properties,being friendly to environment,has been widely used to adsorb heavy metals in gas phase systems and liquid phase systems,while there are few applications of activated carbon in solid system.The adsorption performance of heavy metals on activated carbon is mainly depended on the pore structures and surface properties which can provide the active sites for the chemical adsorption of heavy metals.In this paper,the surface modification of activated carbon was carried out to enhance the surface chemical properties for the remediation of Pb-contaminated soil.Moreover,the stab ilization mechanism of Pb on activated carbon was discussed.Commercial activated carbon(marked with AC)was modified by means of nitric acid oxidation,ammonia reduction and sulfur modification,respectively,to acquire functional groups containing O,N and S(marked with OAC,NAC and SAC,respectively).The physical and chemical properties of activated carbons were characterized by Boehm titration,elemental analysis,SSA-PSD,FT-IR,XPS and p Hpzc.The results showed that compared with AC,the specific surface area of OAC was increased by 27.06%,with average pore size almost unchanged,and the content of oxygen functional groups of O AC increased including-COOH and-OH,especially the 27.60% increase of carboxyl group(-COOH).The specific surface area of NAC was increased by 20.21%,with the average pore size was almost unchanged,and functional groups as pyridine oxide,pyridine and pyrrole on NAC were introduced.The specific surface area of SAC was decreased by 34.60%,with the average pore size almost unchanged,and the oxygen content increased,the sulfur-containing groups of-SO3 and S=O,O=S=O were acquired.To analyze the stabilization effect of lead by modified activated carbons,BCR sequential extraction method was employed to evaluate the morphological changes of lead in soil.It was found that the ability of four kinds of activated carbons to stabilize lead in soil was as OAC > SAC > AC > NAC.With 1% OAC added to the Pb-containing soil,the residual content of lead in soil increased from 11.74% to 49.42%,and the amount of lead in exchangeable form decreased from 75.27% to 25.33% in soil.Dynamic leaching of the Pb-containing soil with 1% addition of activated carbons was carried out.The results indicated that the four kinds of activated carbons can reduce the concentration of lead in leaching solution to different degrees,and the ability of four kinds of activated carbons to stabilize lead in soil was as OAC > SAC > AC > NAC.In order to investigate the bioavailability of lead in soil with OAC and biochar modified by ammonia reduction(NBC),a pot experiment was conducted with pepper as an indicator plant.The measured content of lead in different pepper organs implied that more than 90% lead was accumulated in the root of pepper.The total content of lead in pepper from the Pb-containing soil decreased with addition of OAC and N BC,indicating that OAC and N BC can reduce the mobility and bioavailability of lead in soil.Langmuir,D-R,Freundlich and Temkin models can be used to describe the adsorption isotherm of Pb(II)on OAC?SAC?AC and NAC,and pseudo second order kinetic model and elovich model(only for high temperature)can be applied to describe the adsorption kinetics characteristics of Pb(II)on four kinds of activated carbons.The results revealed that the adsorption process of Pb(II)on four kinds of activated carbons included physical adsorption and chemical adsorption.Combining X-ray photoelectron spectroscopy(XPS)to analyze the surface properties of OAC,SAC,AC and N AC before and after adsorbing Pb(II)showed that the functional groups(such as-COOH,sulfone,and pyrrole)introduced by the modification of activated carbons were involved in the adsorption process of Pb(II)in solution,which strengthen the effect of chemical adsorption.