Mechanisms Of The Oxidation And Reactive Oxygen Species Formation Behaviour Of Sulfide Minerals And Its Environmental Impact

Pyrite and arsenopyrite are typical sulfide minerals,and the impact of their oxidation on environment is in two sides:serious environmental pollution caused by produced acid waste water and harmful ions,and the elimination/transforamtion effect of generated reactive oxygen species（ROS）on pollutants.Recentlty,more and more attention has been paid on the latter.The related research has been constrained to abiotic condition or monomineralic system.In natural environment,pyrite usually interacts with microorganisms or arsenopyrite.There is limited data on how microorganisms or coexisted minerals mediate sulfide mineral oxidation,ROS production and and contaminants conversion.This work is to explore the mechanism of ROS formation induced by A.ferrooxidans-mediated pyrite oxidation and its influence on organic contaminant degradation,and the effect of pyrite/activated carbon-arsenopyrite coexistence on their oxidation,ROS production behavior and As species transformation for a comprehensive and in-depth understanding of basic principle of sulfide ore oxidation and the formulation of pollution control strategies.The removal efficiencies of 2,4-DCP in the biosystems of A.ferrooxidans-mediated pyrite oxidation were 93.66%and 43.77%,which were increased by 13.79%and 50.93%compared with those in the abiotic systems under anaerobic and aerobic conditions,respectively.The improving effect of A.ferrooxidans on 2,4-DCP removal was mainly due to the continuous ROS increase.A.ferrooxidans can promote ROS production in the biosystem through three ways.First,A.ferrooxidans accelerated the renewal of pyrite surface,leading to the exposure of Fe（Ⅲ）and Fe（Ⅱ）sites for water deprotonation and O₂reduction to efficiently produce ROS.Second,A.ferrooxidans was also able to oxidize sulfite to form sulfate anion radicals.Additionally,A.ferrooxidans was to produce O₂^·-,H₂O₂and HO˙.Moreover,A.ferrooxidans-pyrite interaction could give rise to sustainable ROS production.2,4-DCP was degraded to CO₂and H₂O by ROS via the processes of dechlorination,chlorine substitution,dehydrogenation,C-C bond breakage and mineralization.Notably,in the case of anoxic systems,better 2,4-DCP elimination also occurred in biological groups,reminding us the underestimated environmental effect of A.ferrooxidans-minerals interaction.HO˙,O₂^·-and H₂O₂ were produced in the systems of pyrite-mediated arsenopyrite oxidation and single arsenopyrite.HO˙was the main reactive species for As（Ⅲ）oxidation.Under aerobic and anaerobic conditions,the concentrations of As（T）,As（V）and cumulative HO˙in the pyrite-arsenopyrite system were higher than those in the single arsenopyrite system.Due to the galvanic effect,pyrite promoted the oxidation of arsenopyrite and O₂reduction to produce HO˙,thus enhacing the oxidative dissolution of arsenopyrite and the oxidation of As（Ⅲ）to As（V）.With the increase of pH from 3 to 7,the final concentration of As（T）increased from 168.74 mg/L to 198.34mg/L,the final concentration of As（V）decreased from 95.66 mg/L to 80.27 mg/L and cumulative HO˙decreased from 3.76 m M to 3.13 m M in the system of pyrite-mediated arsenopyrite oxidation under aerobic condition.With increasing pH from 3 to 7,the final As release rate increased,but As（V）formation rate decreased.It might be attributed to more iron（hydr）oxides produced at higher pH condition.Iron（hydr）oxides facilitated the electron transfer between pyrite and arsenopyrite with enhancing arsenopyrite oxidation.And O₂reduction to produce ROS was also enhanced.However,the higher reactivity of H₂O₂ decomposition by iron（hydr）oxides decreased the production of HO˙with increasing pH and thus resulted in the decreased rate of As（V）formation.The concentrations of As（T）,As（V）and cumulative HO˙in the aerobic system of pyrite-mediated arsenopyrite oxidation were higher than those in the anaerobic system.In the aerobic system,O₂intensified the galvanic interaction with consequently enhcaing the arsenopyrite oxidation,HO˙formation and the transformation of As（Ⅲ）to As（V）.The concentrations of As（T）,As（V）and cumulative HO˙were lower in the systems of SOP-arsenopyrite than those in the the systems of pyrite-arsenopyrite.The iron（hydrogen）oxide was formed on the SOP.The electrochemical analysis of SOP-arsenopyrite further confirms the role of iron（hydrogen）oxides in the pyrite-mediated arsenopyrite oxidation system in promoting the oxidative dissolution of arsenopyrite,inhibiting HO˙formation and affecting the conversion from As（Ⅲ）to As（V）.However,compared with the pyrite-arsenopyrite system,more iron（hydrogen）oxides was produced in the SOP-arsenopyrite system,which was more conducive to the adsorption of As.This resulted in the lower final concentration of As（T）in SOP-arsenopyrite system.ROS generated by the pyrite or SOP mediated arsenopyrite oxidation contributed to the conversion of As（Ⅲ）to As（V）and reduced the toxicity of As,but the presence of pyrite or SOP promoted the release of As with causing As contamination.Therefore,it is necessary to explore the fixed method of As.Compared with the system of single arsenopyrite,the final concentrations of As（T）in the system of activated carbon（AC）-mediated arsenopyrite oxidation were significantly lower.With the increase of AC loadings,As（T）decreased sharply.With the addition of 0.05 g and 0.15 g AC,the final concentrations of As（T）fell by 6.90%and 47.34%,respectively.AC-mediated arsenopyrite oxidation is to promote As immobilization.AC can accelerate the oxidation of As（Ⅲ）and Fe（Ⅱ）to Fe（Ⅲ）and As（V）,and consequenltly catalyzed the formation of iron arsenate by directly activating O₂ through its persistent free radicals（PFRs）,oxygen-containing active groups and graphite crystal structure,or by mediating electron transfer between arsenopyrite and O₂ to promote ROS production.Moreover,AC directly adsorbed As during the oxidation of arsenopyrite,and promoted the growth of iron arsenate as"crystal seed"to lower the concentrations of As（T）in the AC-arsenopyrite system.These findings may help to enhance our knowledge of the mechanism of ROS generation induced by pyrite biooxidation and its environmental effect.Meanwhile,these findings may deepen our understanding of the coupling behavior of arsenopyrite with other sulfide minerals or redox active substances and its influence on the geochemical cycling of arsenic.And It also provides a strong theoretical basis for controlling arsenic pollution from the source.