Simultaneous Passivation Mechanism Of Antimony And Arsenic By Iron-modified Biochar Gel In Soil-rice System

China produces the largest amount of antimony（Sb）worldwide,contributing to 90%of the world’s total Sb production.The mining and smelting activities in antimony mining areas led to the release of antimony-containing pollutants into soils,threatening food security and human health.Thus,it is urgent to remediate and manage agricultural soils contaminated with Sb.Previous studies have shown that the application of iron-modified biochars（FeBCs）is considered an effective strategy remediate and manage soils contaminated with antimony and arsenic（Sb/As）.However,the efficiency of FeBCs for the passivation of heavy metals is usually low,and its effects on antimony/arsenic passivation and the underlying mechanisms are poorly understood.Therefore,in this study,different agricultural and forestry wastes（such as sugarcane bagasse and rice husk）were used as feedstocks for the production of raw biochars and then iron-based materials such as ferric chloride（FeCl₃）and ferrous sulfate（FeSO₄）were loaded on biochars to obtain various FeBCs.The adsorption capability of FeBCs on Sb was investigated through batch adsorption experiments.An incubation experiment was conducted to study the impacts of FeBCs on the passivation of Sb/As,as well as the relevant mechanisms in a co-contaminated soil.Thereafter,a novel iron-modified biochar gel（FeBG）was synthesized using low-cost graphene oxide as crosslinking agent.The adsorption capability and mechanisms of FeBG on trivalent antimony were explored via batch adsorption experiments.The effects of FeBG on the mobilization and transformation of Sb/As were tested through an incubation experiment.In addition,a rice pot experiment was carried out to elucidate the transformation and translocation of Sb/As in the soil-rice system,as well as the control（CK）and regulation mechanisms of the absorption of both elements by rice plants.The main results of this study are as follows:（1）The adsorption and oxidation behaviors of Sb（Ⅲ）by FeBC500（Pyrolyzed at 500℃）were assessed through adsorption experiment.Additionally,the density functional theory calculation（DFT）was conducted to further analyze the role of the iron active substances in the adsorption of Sb（Ⅲ）by the iron-modified biochar.DFT results showed that the Fe₃O₄in FeBC500 could significantly increase the adsorption capacity of surface functional groups.For instance,the C=O increased by 83%,as compared to that in the raw biochar,indicating a significant synergistic effect between Fe₃O₄and C=O.The electron accepting and donator capacities exhibited that the surface oxygen-containing functional groups and iron-active substances in FeBC500 simultaneously promoted the adsorption and oxidation of Sb（Ⅲ）.（2）The incubation experiment showed that FeBCs could simultaneously reduce the bioavailability of As（4.81%-10.53%）and Sb（9.76-21.73%）.In addition,the ability of FeBCs on Sb passivation is greatly affected by the pyrolysis temperatures.FeBCs prepared at 500℃ inhibited a higher Sb passivation ability than that produced at 650℃.Compared to CK,the available As and Sb decreased by 8.84%and 21.73%,respectively.After the application of FeBCs,the cation exchange and the content of amorphous iron in the Sb/As co-contaminated soil increased by 5.30%-13.54%and 8.990%-31.42%,respectively.In addition,the application of FeBC500 significantly altered soil enzyme activities.In which,the activities of neutral phosphatase and sucrase decreased by 25.95%and 15.03%,respectively;however,the activities ofβ-glucoglycase,cellodiglycosidase,β-N-acetylglucosamine,and urease increased by 21.77%,15.06%,183.07%,and 397.90%,respectively.（3）The adsorption experiments showed that the adsorption capacity of FeBG on Sb（Ⅲ）increased from 28.6 to 113.1 mg g^-1,which was approximately two times that of FeBC Batch adsorption experiments and the spectral characterization results showed that electrostatic effect,pore filling,surface complexation andπ-πcoordination were the predominant mechanisms of Sb（Ⅲ）onto FeBG.In addition,X-ray photoelectronic spectroscopy（XPS）analysis indicated that FeBG could oxidize 78.5%of the adsorbed Sb（Ⅲ）to Sb（V）,which is significantly higher than the oxidation capacity of pristine biochar（oxidized 49.3%of the adsorbed Sb（Ⅲ）to Sb（V））.The enhancement of the oxidation ability of Sb（Ⅲ）by FeBG may be related to the relatively high electron transport ability of graphene and the three-dimensional network structure of the biochar gel.The above results show that FeBG might be an effective antimony passivator,and has a positive prospect in the remediation of soils contaminated with Sb/As.（4）Apart from the high-efficient antimony adsorption of FeBG,the effects of FeBG on soil physicochemical properties,nutrient characteristics,Sb/As bioavailability,and soil enzyme activity were investigated through soil culture experiments.The results showed that FeBG significantly reduced soil pH（0.80 units）,whereas increased soil organic carbon content（18.39%）,soil conductivity（4 times）,and soil cation exchange（12.64%）.Furthermore,the concentrations of available potassium,alkaline hydrolyzable,and bioavailable Sb/As in soil increased by 50.91%,12.64%,9.89%and 16.44%,respectively,after the application of FeBG.In addition,the application of FeBG caused a decrease in the concentration of non-specifically adsorbed Sb/As by 16.51%and 18.26%,respectively.In conclusion,FeBG is a novel soil amendment that could simultaneously alleviate Sb/As pollution,and improve soil fertility and microbial activity in soil.（5）The effects of biochars（BCs）,iron-modified biochars（FeBCs）and iron-modified biochar gel（Pyrolyzed at 500℃,FeBG）on rice planting in Sb/As contaminated soil were studied.The results showed that the modified biochars could significantly reduce the content of Sb and As in various organs of rice,and also increase the rice yield.Compared to CK,the application of FeBG increased the biomass of panicle,roots,stems and leaves of rice by44.52%,71.12%and 51.09%,respectively.Meanwhile,the grain weight of rice increased by13.03%.Furthermore,FeBG treatment reduced the concentrations of Sb and As in rice grain by 6.25%and 33.33%,respectively,due to the increase in soil nutrient levels and the concentration of Fe in the soil,thereby affecting the transformation of Sb/As and promoting the growth of rice.（6）The effects of FeBG on soil nutrients,Sb/As fractionation,enzyme activity and bacterial community structure were systematically studied on rice planting.The application of FeBG increased the concentrations of available N,P,and K,as well as the content of crystalline iron-manganese oxide bound and residual Sb/As,whereas decreased the specifically bound Sb/As.In addition,the application of FeBG increased the activities of urease,β-glucoglycase,cellodiglycosidase andβ-N-acetylglucosamine,whereas decreased the activities of catalase activity,and significantly（P<0.05）reduced the activity of superoxide dismutase（SOD）in rice roots,therefore producing an increase of CAT activity in rice roots from 1.18 umol min^-1g^-1to 3.69 umol min^-1g^-1.Compared to CK,FeBG increased the relative abundance of Proteobacteria phylum and Curvularia vulgaris by 28.56%and 13.89%,respectively.The increase of relative abundance indicated that it could improve the relative abundance of some microorganisms and inhibit other microorganisms to some extent.Therefore,the immobilization mechanism of Sb/As in contaminated soil-rice system near antimony mining area by FeBG could involve changing soil physicochemical properties and soil enzyme activities,adjusting soil microbial community structure,which has an important influence on the regulation of the morphology and transport of Sb/As in soil.