Migration And Transformation Mechanism And Process Regulation Of Arsenic And Antimony In Paddy Soil-rice System

The mining and smelting activities of ores often produce massive industrial�three wastes�containing heavy metals(metalloids)concomitantly,which would enter the surrounding water bodies along with surface runoffs and atmospheric dusts,deposit in the cultivated lands,and eventually be uptaken and accumulated by crops to impair the health and safety of the masses.The pollution of arsenic(As)/antimony(Sb)in the rice fields around the world's largest antimony mine(Xikuangshan)has been serious due to the long-term mining activities;meanwhile,the accumulations of As/Sb in rice grains around the mining area have been excess,which has threatened the local food security;and thus it is urgent to adopt efficient soil remediation schemes.However,the traditional alkaline remediation strategies are not suitable for the remediation of As and Sb,due to they existing as anions.Therefore,it is essential to study the biogeochemical behaviors of As/Sb in paddy soils,and accordingly to develop the comprehensive remedying strategeis for simultaneous passivation of As and Sb pollutes in soils.To this end,1)the dynamic processes and influencing factors of As and Sb migration,transformation and accumulation in soil-rice system were investigated via pot experiments over a whole lifecycle of rice.The experimental results demonstrated that the key components for synchronously suppresseing the mobility of As/Sb are iron-bearing minerals,and As/Sb would be gradually exposed to improve their bioavailabilities with the reductive dissolution of Fe(?)-bearing minerals in the flooded environments.Besides,As and Sb share the similar oxidation-reduction potentials,so they would be biochemically reduced simultaneously by functional microorganisms in anoxic soil.Variously,the As(V)is more easily to be immobilized by soil iron oxide than reductive As(?),while Sb(?)is more likely to be fixed by soil iron oxide minerals than Sb(?);accordingly,the concentrations of dissolved As increased continuously,while that of soluble Sb increased first and then decreased..Moreover,in this alkaline condition,the soil sulfides indicated the strong fixation actions on Sb and few effects on As.Therefrom,under the flooded condition,the geochemical behaviors and bioavailabilities of As and Sb in the paddy soils showed both similarities and differences,which are the internal factors to be considered in the remediation of As/Sb co-contaminated paddy fields.2)Given that the iron minerals of soils are the key factors for passivating the availabilities of both As and Sb,the addition of iron-bearing agents is generally considered as an effective method to remedy As/Sb contaminated soils.For screening suitable iron remediation agents,the regulating effects and processes of three main forms of iron agents,zero valent iron(ZVI),ferrous iron(?)and trivalent iron(Fe(?))on As/Sb in paddy soils were investigated by pot experiments.The results indicated that such three iron agents all lessened the bioavailabilities of As and Sb in paddy soils during the entire lifecycle of rice;moreover,their passivating abilities on As followed order of Fe(?)?(close to)Fe(?)>ZVI,while that abilities on Sb followed order of Fe(?)>Fe(?)>ZVI.To reveal the mechanisms,we found although all three forms of irons can be rapidly transformed into iron bearing minerals in the soils,but the mobility of Fe(?)in soil is stronger than Fe(?)and much stronger than ZVI;and the stronger mobility of iron means more dispersed newly-generated iron minerals and the more iron plaques on the root surfaces.Besides,Fe(?)and Fe(?)could reduce the p H of soil,while ZVI just can elevate it.Compared with Fe(?),Fe(?)can promote the formation of iron sulfides and Sb(?),further enhancing its ability to passivate Sb;while Fe(?)can lead to higher As(?)contents than Fe(?),finally causing the passivation effects of Fe(?)for As close to that of Fe(?).The results of this research part can provide a theoretical basis for the application and optimization of different forms of iron-containing remediation agents in As/Sb contaminated soils.3)Considering the characteristics of zero valent iron powder(ZVI),such as low cost,wide sources,low toxicity but lack of passivation ability,the widely-applied soil remediation agent biochar(BC)was selected to be utilized in combination with ZVI for promoting the formation and dispersion of iron minerals and improving the overall passivation ability of ZVI+BC combination for As/Sb.The results reflected that both ZVI and the ZVI+BC combination had passivated the bioavaibilities of As and Sb synchronously,while BC just obviously passivated Sb but showed no significant effects on As.The descents of As/Sb in rice grain caused by the ZVI+BC combination were greater than the total descents of As/Sb led by single ZVI and BC,displaying that the ZVI+BC combination demonstrated some synergistic effects in remediation of the As/Sb co-contaminated soils.Furthermore,it had been found that the ZVI+BC combination could not only promote the formation and dispersion of iron minerals which own strong passivation effects on As/Sb pollutants,but also promote the formations of sulfide/Sb(?)in the soils and iron plaques on the root surfaces.In conclusion,the ZVI+BC combination owns the synergistic effects on the passivation of both As and Sb in paddy soils,which should provide a new insights for the safe utilization of moderately As/Sb-contaminated paddy fields.4)For further improving the passivation ability of Fe(?)and reduce its dissolution loss,the widely-utilized nitrogen nitrate(NO₃^-)was selected to combine with Fe(?)to investigate the the regulating effects and processes of NO₃^-or/and Fe(?)on the bioavailability of As/Sb in paddy soils.The results indicated that Fe(?)or/and NO₃^-all significantly synchronously reduced the contents of As and Sb in rice grains,and the effect from the Fe(?)+NO₃^-combination was the strongest.The Fe(?)+NO₃^-combination showed the synergistic effects on passivating As in soil;but for passivation of Sb,this combination demonstrated no synergistic effects but partial superposition of passivating effects from Fe(?)and NO₃^-.To investigate the mechanisms,the Fe(?)+NO₃^-combination can simultaneously increase soil iron oxides minerals and induce the oxidation of As(?),and thus has showed the synergy in the passivation of As in paddy soils.Although the Fe(?)+NO₃^-combination is beneficial to the formation and maintenance of iron minerals,it can also induce microbial nitrate reduction coupled with Sb(?)/sulfide oxidation,which has some adverse effects on the fixation of Sb,thereby resulting in no synergistic effect on the passivation of Sb.In brief,the Fe(?)+NO₃^-combination can achieve higher passivation effects than single Fe(?)or NO₃^-treatment,which could provide a new insight for the comprehensive treatment of As/Sb contaminated paddy fields,and should be also a promising remediation strategy.