Coupling Mechanism Of Ferrous Oxidation And As/Sb Transport/transformation In Karst AMD

Acid mine drainage（AMD）is characterized by low pH,high concentrations of dissolved Fe（Ⅱ）and toxic heavy metals.Due to the strong acidity and toxic effects,AMD has been a worldwide environmental problem for a long time.The carbonate rock is widely distributed around the karst area.The reaction of carbonate rock with AMD may facilitate the quick oxidation of Fe（Ⅱ）by O₂ via increase of the pH of AMD,which could produce reactive oxidants such as hydroxyl radical（·OH）and iron（hydr）oxides.The reactive oxidants and iron（hydr）oxides may further affect the transformation and transfer of heavy metals such as arsenic（As）and antimony（Sb）,and change their distribution in AMD.However,these geochemical processes are poorly understood at present.This research firstly investigate the effects of pH,Ca²⁺and CO₃^2-on·OH production which generated from Fe（Ⅱ）oxygenation in the process of the reaction of calcium carbonate（CaCO₃）with AMD.In order to explore the environmental influences of·OH,the arsenite（As（Ⅲ））and p-aminobenzenesulfonamide were added to the same reaction system,respectively.The results showed that·OH was produced from oxygenation of simulated AMD(8.93 mmol/L Fe²⁺,pH 3)in the presence of CaCO₃.With the increase in CaCO₃ dosages from 0.67 to 2.78 g/L,Fe（Ⅱ）oxidation rate increased accordingly,but the maximum concentration of cumulative·OH appeared at the CaCO₃ dosage of 1.39 g/L,being 59.3μM within 24 h.The production of·OH was mainly attributed to the rise of AMD pH due to dissolution of CaCO₃,rendering the appearance of adsorbed Fe（Ⅱ）on the newly formed lepidocrocite and ferrihydrite and complexed Fe²⁺by carbonate（i.e.,siderite）.Oxygenation of these Fe（Ⅱ）species was accountable for the production of·OH.An appropriate pH（i.e.,5⁶）was required for the moderate rate of Fe（Ⅱ）oxidation,corresponding to the maximum production of·OH.The·OH produced from AMD oxygenation could concurrently oxidize the contaminants of As（Ⅲ）and p-aminobenzenesulfonamide.Based on the above research,this part further studied the transformation and transfer of As（Ⅲ）and Sb（Ⅲ）during AMD oxygenation induced by the dissolution of carbonate.A series of dynamic simulation experiments were conducted to investigate the spatial and temporal distribution of As and Sb in AMD and sediment in presence of limestone.The experimental results showed that low flow rates and high pH of AMD facilitated Fe（Ⅱ）oxygenation to produce reactive oxidants（e.g.O₂·-,·OH and Fe（IV））and iron（hydr）oxides which could accelerate oxidation and sedimentation of As（Ⅲ）and Sb（Ⅲ）.The initial Fe（Ⅱ）concentration（100⁵00 mg/L）had negligible influence on oxidation and sediment of As（Ⅲ）and Sb（Ⅲ）.With increase of time,the differences of concentrations of As（Ⅲ）or total As,Sb（Ⅲ）or total Sb in AMD located in different distances were decreasing,and their concentrations were approaching the concentrations in AMD at outlet.Both of reactive oxidants and iron（hydr）oxides produced from Fe（Ⅱ）oxygenation could oxidize As（Ⅲ）and Sb（Ⅲ）.As and Sb which from AMD coexisted with iron（hydr）oxides in sediment.The total As contents in precipitates increased with the longer migration distance of AMD and the amount of As was positive correlated to produced Fe（hydr）oxides.On the contrary,Sb was equally distributed in the precipitates and its deposit wasn’t linked with produced Fe（hydr）oxides.Based on the sequential extraction experiments for As and Sb in precipitates,As and Sb were mainly composed of As/Sb co-precipitated with amorphous iron（hydr）oxides in precipitates,then strongly adsorbed species and ionically bound species.Notably,the most of As/Sb in above three species were As（Ⅴ）/Sb（Ⅴ）.In order to verify the simulations experiments results,this research have done a field study in Jiaole village,Xingren country of Guizhou province where high-arsenic AMD polluted and kast landform was widely distributed.The experimental results showed that total As and As（Ⅲ）concentrations decreased along with the AMD migration.The total As concentrations positive correlated with Fe（Ⅱ）concentrations in AMD（R²=0.941）,especially at the outlet of AMD.The results of controlled experiments showed that As（Ⅲ）was mainly oxidized by microorganism,and the oxidation rate of As（Ⅲ）was very slow without another environmental media.In the initial stage of AMD formation,As（Ⅲ）in AMD was oxidized and deposited by reactive oxidants and iron（hydr）oxides produced from the reaction of soil which contained carbonate with AMD,and the oxidation rate of As（Ⅲ）was much quicker than that without addition of soil.In the late stage of AMD formation,As（Ⅲ）was oxidized mainly by microorganism stuck to sediment which was mainly composed of iron（hydr）oxides,and the oxidation rate of As（Ⅲ）was also quicker than that without of sediment.So,both of soil and sediment could facilitate As（Ⅲ）oxidation during the different period of AMD development.The total As concentrations decreased with the longer migration distance of AMD.Because of the oxidation of biotic and abiotic factors,the most of As was As（Ⅴ）in sediment.A part of Fe（Ⅲ）minerals and As（Ⅴ）were reduced in sediment containing high content of organic matter.This process would increase the environmental risk of As in sediment.The results of sequential extraction experiments for As in sediments showed that most of As was coprecipitated with amorphous iron（hydr）oxides in sediment,especially at outlet of AMD.The proportion of As coprecipitated with amorphous iron（hydr）oxides decreased,the diversity of species and proportions of another As species increased with the longer migration distance of AMD.A part of As coprecipitated with very amorphous iron（hydr）oxides had transformed into As coprecipitated with amorphous iron（hydr）oxides and strongly adsorbed with time increased.These species were stabilized in sediment during the transformation of As species.