Effect Of Fe-modified Biochar On The Immobilization And Bioavailability Of Typical Heavy Metals In Soils

In recent years,with the rapid development of industrial and agricultural production,unreasonable and relatively backward waste disposal methods have led to the pollution of more and more agricultural farming soils in China.Heavy metal(loid)s in soil have become one of the most harmful and difficult environmental problems for human beings due to their high mobility and biotoxicity.Among them,As,Cd and Pb are the common pollutants in agricultural soils.As a new type of green soil amendment,due to porous structure and abundant surface functional group,biochar can adsorb and fix heavy metal(loid)s,reduce the bioavailability and environmental risks of pollutions,and improve soil quality after being applied to soil.The existence of iron(Fe)oxides could reduce the mobility of meta(liod)(especially of As),but an exclusive application of iron oxides may result in the soil hardening and acidification.Therefore,biochar derived from Platanus orientalis branches and its Fe-modified biochar were used for the batch equilibration adsorption test,which was used to determine the kinetic adsorption and isothermal adsorption behavior of As(V)in soils after biochars applied,and explore the adsorption mechanism;meanwhile,a pot experiment was conducted using Rice(Oryza sativa L.).As an indicator plant to evaluate the effect of raw and iron(Fe)-modified biochars on the bioavailability and transportation of As,Cd and Pb in paddy soils and plant growth with different water managements(continuously flooded and alternately wet and dry),in order to provide theoretical basis for the application of biochar in remediation of As-Cd-Pb co-contamination soil.The main results are as following1.The method of modifying the biochar by FeCl3 to support the iron on the surface of the biochar was feasible,and iron content of the modified biochar was significantly(P<0.05)increased compared with the raw biochar.Fe-modified biochar had a lower pH and carbon content,whereas the ash content and electrical conductivity was increased than raw biochar.Fe-modified biochar contained a richer elemental composition and more types of mineral elements.In addition,the cross section of Fe-modifled biochar became honeycomb-shaped,and the number of function groups on surface was reduced2.Application of raw biochar and Fe-modified biochar enhanced the adsorption capacity of As(V)by soil,and the increase of saturated adsorption capacity of As(V)was higher after Fe-modified biochar applied to soil than raw biochar applied.Langmuir isotherm adsorption model and pseudo-second order kinetics model were more suitable for describing the adsorption process of As(V)in each treatment3.Application of raw biochar significantly(P<0.05)increased,whereas the Fe-modified biochar noticeably decreased soil pH.Both biochars addition increased the organic carbon content of soil,and the raw biochar had a more profound impact.The concentrations of soil available As,Cd and Pb and the accumulation of each pollutant in rice grains significantly(P<0.05)decreased after biochar amendment,the application of Fe-modified biochar had better effect in reducing the migration ability and bioavailability of As and Pb in soil while the raw biochar was more conductive to reducing the content of soil available Cd and the accumulation in rice grain,and the decreasing was larger in the alternately wet and dry treatments than that in the continuously flooded treatments.Soil catalase and urease activities were enhanced for raw biochar treatment,but were decreased for Fe-modified biochar treatment.Besides,the application of both biochars could increase rice yield.In conclusion,application of biochar could enhance soil adsorption of As,Cd and Pb and reduce their bioavailability,thus reducing their stress on plants and promoting plant growth.Among them,the raw biochar was more suitable for the repair of Cd contamination soil,while the Fe-modified biochar was more suitable for As and Pb contamination soil.In addition,Besides,the alternately of wet and dry treatment contributed to the reduction of available As,Cd and Pb in soil and their accumulations in rice,therefore it could be a more appropriate management option for paddy rice irrigation in As,Cd and Pb contaminated soil.