Remediation Of Cadmium And Arsenic Co-contaminated Soil By Iron-modified Biochar:Effect And Mechanism

Heavy metals and metalloids in the soil pose a serious threat to human health and food safety.The high toxicity of cadmium（Cd）and arsenic（As）and their risks to humans and plants are quite concerning.The simultaneous remediation of Cd and As in co-contaminated soils poses numerous challenges due to their opposite behaviors.Thus,there is an urgent need to reduce both Cd and As mobility and plant uptake in co-contaminated soil.Biochar is a cost-effective amendment for soil remediation because it can immobilize toxic metals.However,some studies demonstrated that biochar could decrease Cd mobility but increase As mobility in multiple cation and anion co-contaminated soils.Therefore,the main objective of the present study was to modify biochar to improve its ability for both Cd and As immobilization.In the present study,modification of biochar using different types of irons,such as iron oxyhydroxide（goethite）,sulfide-iron oxyhydroxide（sulfide-goethite）,and iron oxide（Fe₂O₃）,was used to remediate Cd and As in Cd-As co-contaminated soil.The effects of raw biochar（BC）,goethite modified biochar（GBC）,sulfide-goethite modified biochar（S-GBC）,and iron oxide（Fe₂O₃）modified biochar（FBC）on Cd and As mobility,bioaccessibility,fractionations in the soil,plant growth and physiological responses,and metal uptake by plants were evaluated in a combination of incubation and greenhouse experiments.The main results were summarized as follows:（1）An incubation experiment for 90 days was conducted to study the effect of BC,goethite（G）,goethite combined biochar（BC+G）,and goethite modified biochar（GBC）at a 1% application rate on Cd and As immobilization in co-contaminated soil.The sequential extraction procedure,the European Community Bureau of Reference（BCR）,the toxicity characteristics leaching procedure（TCLP）,and extraction with Ca Cl2 and Na HCO3 were carried out.The results showed that BC effectively immobilized Cd but significantly increased As mobility,whereas G immobilized As more than Cd.The reduction of Cd mobility in the soil was in the following order: GBC > BC+G > BC > G > unamended soil（CK）,while the reduction of As mobility was as follows: GBC > BC+G > G > CK > BC.The GBC was the optimum amendment within the immobilization efficiency for Ca Cl2-Cd（57.03%）and TCLP-As（61.11%）compared to CK.The GBC amendment showed some interactions between BC and G,which played an essential role in the simultaneous immobilization of Cd and As.In conclusion,the GBC showed great benefit as an efficient environmental amendment for Cd and As remediation in co-contaminated paddy soil.（2）Next,a pot experiment was conducted to study the effects of biochar（BC）,goethite（G）,and goethite modified biochar（GBC）at a 1% application rate on Chinese cabbage growth and the uptake and immobilization of Cd and As in co-contaminated paddy soil.The GBC composites were prepared with initial mass ratios of Fe/BC of 1:1 and 2:1 and labeled as GBC1 and GBC2,respectively.The results showed that GBC amendments significantly improved Chinese cabbage growth,increased chlorophyll content,leaf relative water content,and gas exchange attributes compared to BC and G applied singly.The BC decreased Cd uptake by Chinese cabbage shoots and roots while increasing As uptake.The GBC1 amendment reduced Cd and As uptake by Chinese cabbage shoots（47.27% and 35.20%）and roots（36.80% and 41.12%）,respectively,and reduced Cd and As bioaccessibility by 40.09% and 39.35%,respectively,compared to unamended soil.Overall,these results indicated that GBC application could simultaneously immobilize both Cd and As and reduce their uptake by Chinese cabbage in Cd and As co-contaminated soil to mitigate food security risks.（3）The previous experiment found that GBC amendments effectively reduced Cd and As uptake by Chinese cabbage.Thus,in this experiment,the same soil was used to grow maize（Zea mays L.）following Chinese cabbage without any additional amendments to assess the residual impacts of BC,G,and GBC amendments on Cd and As mobility,uptake by maize,and their impacts on maize growth.The results revealed that GBC amendments enhanced maize growth,chlorophyll content,leaf relative water content,and gas exchange parameters（photosynthetic rate,transpiration rate,stomatal conductance,and intracellular CO2 concentration）compared to raw BC and G.Moreover,the GBC1 amendment lowered Cd and As uptake by maize shoots（29.48% and 61.56%）and roots（33.07% and 37.32%）,respectively,compared to unamended soil（CK）.After maize harvesting,the reduction of Cd mobility and bioaccessibility occurred in the following order: GBC1 > GBC2 > BC >G > CK,while the reduction of As mobility and bioaccessibility occurred as follows: GBC1 >GBC2 > G > CK > BC.In general,the GBC has a significant residual impact on maize growth and Cd and As uptake in co-contaminated soil.（4）An incubation experiment was conducted to explore the efficiency of biochar（BC）,goethite modified biochar（GBC）,and sulfide-goethite modified biochar（S-GBC）on Cd and As immobilization in co-contaminated soil.The GBC amendments were prepared using four initial mass ratios of Fe/BC（0.25,0.5,1,2）and labeled as GBC0.25,GBC0.5,GBC1,and GBC2,respectively.In addition,each GBC amendment was further modified with a Na2 S solution to prepare S-GBC amendments as S-GBC0.25,S-GBC0.5,S-GBC1,and SGBC2,respectively.Cadmium and arsenic co-contaminated soil was incubated for 60 days using the different amendments at a 1% application rate.The results showed that raw BC decreased Cd mobility but increased As mobility.GBC and S-GBC amendments effectively immobilized both Cd and As compared to the raw BC.The S-GBC amendments could immobilize Cd and As better than the GBC amendments.The GBC1 and S-GBC1 decreased TCLP-Cd by 27.36% and 35.90%,respectively,and Na HCO3-As by 25.51 and35.70%,respectively,compared to CK.Additionally,between the different amendments,three application rates（0.5,1,and 1.5%）of BC,GBC1,and S-GBC1 were used to examine their influence on Cd and As immobilization after 60 days of soil incubation.The results demonstrated that rising GBC and S-GBC application rates from 0.5 to 1.5% significantly reduced the mobility of both Cd and As.The S-GBC amendments could immobilize Cd and As and decrease their ecological risk assessments better than the BC and GBC amendments.In conclusion,several functional group bonds,the formation of Fes,a favorable specific surface area,and the porous structure of S-GBC could increase the simultaneous immobilization of Cd and As through complexation,co-precipitation,and adsorption.（5）In addition to the modification of biochar using goethite and goethite-sulfide as a type of iron oxyhydroxide,the modification of biochar using Fe₂O₃ as a type of iron oxide was also examined to evaluate Cd and As immobilization in co-contaminated soil.Incubation and greenhouse experiments were conducted to test the efficiency of biochar（BC）,Fe₂O₃ combined biochar（BC+F）,and iron oxide modified biochar（FBC）at a 1%application rate on the immobilization and uptake of Cd and As by wheat（Triticum aestivum L.）.The BCR,TCLP,simple bioaccessibility extraction test（SBET）,and extraction with Na HCO3 were carried out.After 60 days of incubation,the reduction of Cd mobility and bioaccessibility was in the following order: FBC > BC+F > BC > CK;however,the reduction of As mobility and bioaccessibility was as follows: FBC > BC+F >CK > BC.Additionally,the FBC showed the highest increase in wheat growth compared to other amendments.FBC exhibited the greatest decrease in Cd and As uptake by wheat and their translocation factors from roots to grain.The FBC decreased Cd and As in wheat grains by 37.40% and 36.20%,respectively,compared to unamended soil.Overall,FBC was better than raw biochar on immobilization and decreased plant uptake of Cd and As,probably due to the presence of maghemite（γ-Fe₂O₃）on a biochar surface,which may aid Cd and As immobilization via electrostatic complexation and specific adsorption.In summary,raw biochar lowered Cd mobility and plant uptake but increased As mobility and plant uptake.All iron-modified biochars decreased Cd and As mobility,bioaccessibility,and uptake by various plants,along with improved plant growth and physiological parameters.All modified biochars share similar characteristics,such as a high specific surface area,different functional groups,including Fe-O,and a porous structure compared to raw biochar;these characteristics can enhance the immobilization of Cd and As via different mechanisms like co-precipitation,surface complexations,and physical adsorption;however,each type of iron modified biochar has some remarkable/different properties.The FBC has strong magnetic properties compared to other types,which helped immobilize Cd and As.In addition,the GBC was deemed more effective than the FBC for immobilizing Cd and As in the co-contaminated soils due to the high number of hydroxyl groups and the generally higher specific surface area.GBC has a significant direct and residual impact on Chinese cabbage and maize growth and Cd and As uptake in cocontaminated soil.The sulfidation process for GBC to obtain S-GBC improved the immobilization efficiency of Cd and As compared to GBC.Thus,among all the amendments,S-GBC is the potential material for Cd and As co-contaminated soil remediation,which combines the benefits of goethite,sulfur,and biochar.