Mechanisms Of Arsenic Immobilization And Release By Iron/Aluminum Mineral Transformation In Metal Mine Soil

The soil in metal mine areas is always severely contaminated with arsenic（As）.The occurrence and distribution of As in mine soil significant affected by soil minerals.However,the co-transformation and migration mechanism of As with soil iron or aluminum minerals in the mine area are unclear.This study investigated the spatial distribution of heavy metals and the host mineral characteristics of As in soil in an arsenic mine.By exploring the coupled relationship between soil minerals and the forms of As,the spatial differentiation pattern of As pollution in mine soil has been revealed.Furthermore,we investigated the mineral phase transformation mechanism of the main As host mineral（Al-substituted ferrihydrite,AlFh）and the behavior of As fixation and release during mineral transformation under different conditions in the mine area using laboratory simulation experiments.The main research findings are as follows:（1）The soils in the arsenic mine area were primarily contaminated with As,Hg,Sb,and Tl,originating mainly from mining and smelting activities and the mineralization of high-arsenic geological bodies.Metal contamination was concentrated in surrounding pollution sources.The average As,Hg,Sb,and Tl contents in the mine soil were 394,1.79,6.47,and 1.68 mg/kg,respectively,exceeding the local soil background values by 23.18,52.70,4.52,and 2.43 times.As,Hg,and Sb were mainly distributed within a 500 m radius of the pollution sources.The primary sources of As、Hg、Sb、Tl pollution in mine soils were the primary halos of mineral deposits and mining-related activities.Industrial sources mainly contributed to soil Pb pollution,while fertilizer production and usage were responsible for soil Cd.These findings indicate that the distribution of As,Hg,Sb,and other pollutants in mine soil is primarily related to mining and smelting activities,as well as the distribution of mineral deposits.The surrounding the pollution sources should be the focal area for heavy metal pollution control in the mine soil.（2）The environmental reactivity of As in the soil of mining and smelting areas was significantly higher than that in high-arsenic geological mineralization zones.The dissociation and dissolution of iron-aluminum minerals and surrounding rock ores（such as carbonatite）in the soil had a notable influence on the environmental reactivity of As.In the mining and smelting areas,soil As primarily existed in weakly adsorbed and aluminum oxide-bound forms.The soil in high-arsenic geological mineralization zones contained mainly the residual form of As（>50%）.However,even in this form,the non-residual As content was as high as 1635 mg/kg,indicating a significant potential ecological risk.The soil As in the geological mineralization and beneficiation zones was primarily associated with iron（hydro）oxide minerals（79.23%and 93.10%,respectively）.In the smelting and mining areas,the host minerals for As in the soil were iron-rich aluminosilicate minerals（85.07%）and surrounding rock ores,such as carbonate minerals（calcite）（40.59%）.The higher degree of mineral dissociation and weathering dissolution of host minerals for As and changes in soil properties(such as pH and CO₃^2-)contributed to the higher proportion of reactive forms of As in smelting and mining area soils.Therefore,priority control and remediation of soils should be given to the upper layers of soil in mining areas and other mining-related activity areas,as well as in surrounding rock ores,to prevent and mitigate potential risks associated with As in mine soil.（3）The soil pH in mining areas significantly affected the transformation of AlFh and the adsorption and retention of As in mine soil.This study indicates that the differential dissolution of AlFh during the transformation process promoted the dissolution and reprecipitation of AlFh under acidic and alkaline conditions.In acidic conditions,structural Fe（4.73 mg/L）is preferentially dissolved from AlFh,whereas structural Al（1.25 mg/L）is preferentially dissolved in alkaline conditions.Under neutral conditions,the transformation of AlFh mainly occurred through directional aggregation,resulting in the formation of Al-substituted goethite with a lower degree of transformation.Following the AlFh transformation,the adsorbed As contents gradually decreased under all pH conditions,whereas the non-extractable As contents increased.Under alkaline and acidic conditions,the proportion of non-extractable As in the transformation products of Fh and AlFh increased by 14.02-19.72%and12.27-16.28%,respectively.Under neutral conditions,it only increased by 12-13.02%.However,the dissolved As content in the system was lower（0.02-0.14 mg/L）,indicating that neutral soil conditions favor the adsorption of As by AlFh,while alkaline conditions can further promote the transformation and immobilization of aluminous hydroxides.Therefore,regulating the pH of soil in mining areas can,to some extent,alter the speciation of As and reduce the environmental activity of As in acidic mining soil.（4）The presence of carbonate significantly influenced the transformation of AlFh to Al-substituted goethite in the mine soil,altering the retention and release of soil As.Density functional theory and charge sharing theory calculations revealed that carbonate adsorption on the surface of AlFh increased the surface energy gap(E_gap),overall hardness（η）,and the strength of the Fe-O bond in≡Fe-OCO₂.Simultaneously,carbonate hydrolysis caused an increase in the pH of the system.Together,these factors collectively decreased t he chemical stability of AlFh and enhanced the reactivity of the system,which promoted the dissolution and reprecipitation of As-bearing AlFh as Al-substituted goethite.Following Al-substituted goethite forms,10.45-17.74%of the adsorbed As was converted to non-extractable forms,facilitating the fixation of As in the system.However,with an increase in the C/Fe ratio in the system,the dissolved As increased 16.33-22.98%,indicating that the high CO₃^2-concentrations promoted the release of As during the transformation of AlFh.Furthermore,under the same C/Fe ratio conditions,the degree of AlFh transformation,dissolved As（0.03-5.08 mg/L）concentration,and proportion of non-extractable As（15.60-17.74%）were higher when AlFh first adsorbed CO₃^2-and then reacted with As（pre-carbonation）than when AlFh first adsorbed As and then reacted with CO₃^2-（pre-adsorption）.The research results demonstrated that high concentrations of carbonate in the surface soil in mining areas would increase the dissolution,release,and migration risks of As during the transformation of AlFh.（5）In anaerobic systems,the transformation of AlFh and the reduction and release of As were higher compared to microaerobic systems.Carbonate increased the reducible potential on the mineral surface of AlFh and induced the formation of precursor nuclei,which promoted the transformation of AlFh and the reduction and release of As under controlled oxygen conditions.In both microaerobic and anaerobic systems,the electron transfer process between the structural Fe（Ⅲ）of AlFh and Fe（Ⅱ）generated highly reactive mineral precursors（≡Fe（Ⅲ）-O-Fe（Ⅱ））,which were subsequently transformed into Al-substituted goethite.Carbonate enhanced the reducible potential on the mineral surface and increased the system’s pH values,which intensified the formation of≡Fe（Ⅲ）-O-Fe（Ⅱ）by enhancing the electron transfer between Fe（Ⅱ）and AlFh.This promoted AlFh transformation and As retention in microaerobic and anaerobic systems.However,in microaerobic systems,O₂ accelerates the rapid oxidation of partial Fe（Ⅱ）,leading to lower degrees of AlFh transformation and As retention.The XPS etching and HRTEM results indicated that during the mineral phase transition,As was incorporated into the secondary mineral lattice through encapsulation or doping.In microaerobic systems,non-extractable As accounted for 26.35-28.08%of the solid-phase As,while in anaerobic systems,it accounted for 30.80-37.40%.Along with the increase in the C/Fe ratio,the dissolved As increased in microaerobic and anaerobic systems,and the proportion of As（Ⅲ）increasing from 0 to 51.61%in anaerobic systems.This indicates that carbonate promotes the reduction and release of As during the transformation of AlFh in microaerobic and anaerobic systems.These results suggest that crystalline Fe/Al minerals and non-extractable As account for a higher proportion in the lower layers of mine soil.However,carbonate exacerbates the risk of As reduction and release in the soil profile.The results of this study provide preliminary insights into the spatial variations of total As content and forms in mine soils influenced by soil minerals.This reveals that the effects of pH and carbonate caused by weathering and dissolution of ore-bearing rocks on the transformation of the primary host mineral for soil As（AlFh）,as well as the retention and release mechanisms of soil As during the mineral transformation process.This research provides a scientific theoretical basis for the prevention,control,and remediation of soil As pollution in mine areas.73 Figures,21 Tables,301 References...