Simultaneous Immobilization And Mechanisms Of Arsenic And Lead In Soils By Sulfur/Iron Modified Biochar

Heavy metal contamination in soil has become a worldwide environmental problem.Especially,arsenic（As）and lead（Pb）pollution are serious and poses a severe threat to soil ecology safety and human health.As and Pb existed in the form of anions and cations respectively,and their quite different geochemical behavior characteristics are still considered as a great challenge with respect to the simultaneous immobilization of As and Pb.Therefore,it is of great significance to design a green,efficient and economical new functional material for simultaneous immobilization of As and Pb.With low cost and stable chemical properties,biochar has a great potential in environmental pollution control,such as improving soil quality,and carbon sequestration.In this dissertation,a sulfide Fe3O4 decorated biochar composite（S/Fe-BC）was synthesized via modified biochar（BC）from the root powder of long-root Eichhornia crassipes by loading Fe3O4 and sulfurization,which realized the efficient immobilization of As and Pb.In addition,advanced characterization techniques were used to reveal the simultaneous immobilization mechanisms of As and Pb.Moreover,estensive laboratory culture experiments and numerical simulation were used to explore the long-term stability performance and internal mechanisms of As and Pb immobilized by S/Fe-BC.Finally,ecotoxicological experiments were also carried out to reveal the potential effects and mechanisms of S/Fe-BC on soil ecology.The research results can provide a scientific basis for the application of S/Fe-BC in the remediation of multi-metal contaminated soil,and provide an effective way for the resource utilization of long-root Eichhornia crassipes.The major conclusions are as follows:（1）The effects of Fe₃O₄ loading and sulfurization modification on the properties of biochar were clarified.The properties of Fe3O4 decorated biochar（Fe-BC）were different from S/Fe-BC prepared by one-step simultaneous loading and sulfurization.Fe-BC presented a good crystallinity along with the attachment of lot of spherical agglomerated Fe3O4 nanoparticles to its surface.S/Fe-BC was amorphous,and more finely dispersed FeS,iron polysulfide,Fe3O4 particles were attached to its surface.These nanoparticles were loaded by BC mainly through pore filling and interaction with its functional groups such as Fe-O bond was formed by the single tooth chelation and double tooth coordination of Fe3+/Fe2+ with surface functional groups.Compared with Fe-BC,sulfurization enriched the surface functional groups（-CH,C-O）,pore structures（micropore and mesopore）,and increased the specific surface area（an increase of 61.24 m2/g）.Due to these outstanding characteristics,S/Fe-BC showed great adsorption potential for As and Pb.In addition,S/Fe-BC with different ratios of S and Fe had the different reactivity to As and Pb.When the ratio of Fe was higher than S,the adsorption of As was favorable;while when the ratio of S was higher than Fe,which was beneficial to the adsorption of Pb.（2）The stability and simultaneous immobilization mechanisms of S/Fe-BC for As and Pb were revealed.The application of S/Fe-BC in As and Pb-contaminated soil was studied.The results showed that S/Fe-BC exhibited a high adsorption capacity and a wide pH applicability for adsorption of As and Pb,which was better than that of Fe-BC and BC.When the dosage was 2%（wt%）,the immobilization efficiency of S/Fe-BC to As and Pb was 99.9%and 90.1%,respectively.In the pH range of 4.0～8.0,its immobilization efficiency for both metals was above 75%.When 1%（wt%）S/Fe-BC was applied to the actual contaminated site soil,the immobilization efficiency of As and Pb was above 90%,which verified the practical application potential of S/Fe-BC.Soil organic acids and soil physical and chemical parameters had a great influence on the immobilization of As and Pb.Among them,soil organic matter and amorphous iron oxide were the key factors affecting adsorption.These two parameters were negatively correlated with the immobilization efficiency of As and Pb（p<0.05;p<0.01）.The co-existence of Pb promoted the adsorption of S/Fe-BC for As,and the synergistic immobilization was related to the ternary surface complexes formation of Fe Oxide-As（Ⅲ）-Pb（Ⅱ）and co-precipitation of PbFeAsO4（OH）.The antagonism was related to the complexation of As and Pb in competition with hydroxyl group.FT-IR,XRD,and XPS characterization revealed the simultaneous immobilization mechanisms of As and Pb,including adsorption（electrostatic attraction,complexation,and ion exchange）,precipitation/co-precipitation,and redox of As.Sulfurization significantly improved the immobilization efficiency,mainly because sulfurization increased the specific surface area,enhanced the ion exchange effect of Pb2+,promoted the precipitation of Pb（PbS）and the complexation of As（Fe-As-S）,as well as promoted the oxidation of As（Ⅲ）to As（Ⅴ）by activating more hydroxyl radicals（·OH）.（3）The long-term stability performance and internal mechanisms of As and Pb immobilization by S/Fe-BC were revealed.The immobilization effects of S/Fe-BC on As and Pb were evaluated by soil culture,soil column experiments,simulated environmental changes,and numerical simulation.After 28 days of S/Fe-BC immobilization,the bioavailability,bioaccessibility,and TCLP leaching toxicity of As decreased by 62.0%,57.6%,and 85.7%,respectively,while the contents of Pb by these three extracted methods were decreased by 82.1%,56.1%,and 60.2%,respectively.The leaching amounts of As and Pb were reduced by 99.9%and 90.2%,respectively after S/Fe-BC treated.The retention of As and Pb in the upper layer（0～4 cm）of the soil column increased by 41.2%and 10.3%,respectively.After 30 cycles of dry-wet and freeze-thaw alternation,the immobilization efficiency of As and Pb by S/Fe-BC was kept at 69.2%～99.7%.The prediction of site-scale modeling showed that the concentrations of As and Pb in the shallow groundwater at 100 cm after applying S/Fe-BC could still meet the requirements of the Class Ⅲ in Groundwater Quality Standard（GB 14848-2017）of China after 16 and 40 years of natural rainfall leaching.The overall effective immobilization time of S/Fe-BC for As and Pb increased by 46.9%and 51.9%to that of Fe-BC,respectively.The above results indicated the long-term stability performance of S/Fe-BC on As and Pb immobilization.Moreover,the internal mechanisms are as follows:Firstly,S/Fe-BC changed the adsorption behaviors of As and Pb in soil,and the adsorption isotherm changed from Langmuir to Freundlich model suggesting that S/Fe-BC promoted the chemical adsorption process of As and Pb;Secondly,S/Fe-BC promoted the transformation of As and Pb from the highly active exchangeable and carbonate-bound fractions to the stable residual fraction;Thirdly,S/Fe-BC exhibited stable structure and performance,and can maintain its functional group characteristics and high adsorption activity during the immobilization process.（4）The potential ecological effects and mechanisms of S/Fe-BC immobilization of As and Pb were revealed.By adding materials to uncontaminated and As,Pb-contaminated soil,the toxicity effect of materials and the possible influence on soil ecology in the immobilization process were investigated.In uncontaminated soil,the inhibition of soil enzyme activity,the microbial community,and the induction of oxidative stress of earthworms by S/Fe-BC were stronger than that of BC and Fe-BC,but much lower than that of As and Pb pollution.The disturbances of S/Fe-BC to soil organisms was greater than that of the other two materials because of its smaller and dispersed nanoparticles,larger specific surface area,more element doping,Fe release during immobilization,and incomplete lattice structure.However,in the polluted soil,S/Fe-BC was beneficial to the restoration of microbial community diversity（Shannon and Simpson’s index increased by 7.4%and 9.4%,respectively）,and alleviated the oxidative damage caused by As and Pb to earthworms（ROS accumulation decreased by 27.2%,and the activities of SOD and CAT enzyme were significant（p<0.05）inhibited）.Principal component analysis,redundancy analysis,and Person correlation analysis showed that S/Fe-BC could regulate microbial activity by affecting soil enzyme activity and organic carbon content,and alleviated antioxidant damage of earthworms by reducing the accumulations of As and Pb.The concentrations of As and Pb in earthworm were decreased by 83.0%and 49.6%,respectively.In conclusion,although S/Fe-BC had some adverse effects on soil ecology,it can be considered an effective environmentally friendly adsorption material for the immobilization of As and Pb-contaminated soil.