Arsenic Release Process From Arsenopyrite (Photo) Chemical Oxidation And The Formation And Action Mechanism Of Reactive Oxygen Species

The pollution of farmland,rivers and groundwater by arsenic（As）poses a serious threat to the environment and human health.The major sources of As pollution include industrial and agricultural production,rock weathering and the mining and smelting of arsenic-containing minerals.High content of As is commonly found in sulfide minerals in the form of associated minerals.Arsenic-containing pyrite,realgar and arsenopyrite are major sulfide minerals,and arsenopyrite is the most common arsenic-containing sulfide minerals.The As content in arsenopyrite can reach up to 45 wt.%.Therefore,the mechanisms of the oxidative dissolution of arsenopyrite and the release of As are a hotspot of research in the field of soil science,mineralogy and environmental science.Most of the current researches ignore the interaction process between arsenopyrite and dissolved As,especially the photochemical oxidation process of arsenopyrite driven by solar irradiation and the influence of environmental factors.Natural arsenopyrite was used as the research object in this work.The process of As released from the oxidative dissolution of arsenopyrite under different environmental conditions,and the mechanism of the photochemical oxidation of arsenopyrite were studied.The formation of reactive oxygen species（ROS）and its mechanism affecting the oxidative dissolution process of arsenopyrite had been clarified.The specific research content included:the interaction process between arsenopyrite and additional As（Ⅲ）;the effect of ultraviolet（UV）irradiation on the oxidative dissolution process of arsenopyrite under acidic conditions;the influence mechanism of SO₄^2–and low-molecular-weight organic acids（citric acid）on the process of As release from arsenopyrite.The main experiments and results are listed as follows.1.The interaction process between arsenopyrite and additional As（Ⅲ）was investigated.The effects of initial additional As（Ⅲ）concentration,dissolved oxygen and pH on the oxidative dissolution process of arsenopyrite and the fate of As in the system were further studied.The results showed that arsenopyrite can promote the oxidation of additional As（Ⅲ）,and the concentration of As（V）produced in the reaction system increases with increasing initial additional As（Ⅲ）concentration.During this process,the reactive oxygen species OH^·and H₂O₂ produced on arsenopyrite surface are the main oxidants for the rapid oxidation of As（Ⅲ）to As（V）.The oxidation rate of As（Ⅲ）gradually decreased with increasing p H from 3.0 to 7.0.As（Ⅲ）also affected the oxidative dissolution process of arsenopyrite.The oxidation rate of arsenopyrite increased with increasing initial additional As（Ⅲ）concentration at pH 3.0.However,under the conditions of pH 5.0 and7.0,additional As（Ⅲ）could inhibit the oxidative dissolution of arsenopyrite.Dissolved oxygen could greatly promote the production of ROS on the surface of arsenopyrite,thereby accelerating the oxidation of As（Ⅲ）and arsenopyrite.2.The oxidative dissolution process of arsenopyrite in acidic solution under UV irradiation was investigated.The effects of waveband of incident light,oxygen partial pressure and p H on the dissolution and release of As were further studied.The results showed that UV irradiation can promote the oxidative dissolution of arsenopyrite and the release of As.As（V）,Fe（Ⅱ）and SO₄^2-were the major forms of the released As,Fe and S,respectively.ROS including O₂^·-/HO₂^·,OH^·and H₂O₂ were generated through the oxidation of As（Ⅲ）by oxygen under far-UV irradiation,photo-Fenton reaction of Fe（II）/Fe OH²⁺and conversion of OH^·/O₂^·-,which play important roles in arsenopyrite oxidation.Oxygen accelerated the formation of ROS,which facilitated the oxidative dissolution of arsenopyrite and the release of As.At initial pH 3.0,the released As mainly existed in the form of ions in the solution.With initial p H increasing to 5.0,the formation of ferric arsenate precipitate and goethite contributed to the adsorption and fixation of released As.3.The mechanism of SO₄^2-affecting the oxidative dissolution of arsenopyrite and the release of As under UV irradiation was studied.The effects of SO₄^2-concentration,p H and dissolved oxygen on the oxidative dissolution of arsenopyrite were further investigated.The results showed that SO₄^2-can promote the oxidative dissolution of arsenopyrite under UV irradiation,and the formation of amorphous schwertmannite played an important role in this process.The photochemical oxidation rate of arsenopyrite increased first and then decreased with increasing SO₄^2-concentration.In the range of pH2.0–4.0,the photochemical oxidation rate of arsenopyrite increased gradually with increasing p H due to the inhibition of Fe SO₄⁺on the photo-Fenton reaction(Fe（Ⅱ）/FeOH²⁺).Under dark reaction conditions,SO₄^2-had no significant effect on the oxidation rate of arsenopyrite.In the range of pH 2.0–4.0,the oxidative dissolution rate of arsenopyrite gradually increased with increasing pH.Under UV irradiation and dark reaction conditions,dissolved oxygen was the main electron acceptor in the oxidation and dissolution process of arsenopyrite,which affected the oxidative dissolution process of arsenopyrite.4.The As release from arsenopyrite was studied in the presence of citrate under UV irradiation,and the effects of dissolved oxygen and citrate concentrations and p H on As release rate were also investigated.The results indicated that As release from the oxidative dissolution of arsenopyrite is affected by the complexation between citrate and dissolved iron ions.Under dark conditions in air atmosphere,dissolved oxygen,Fe（Ⅲ）-citrate and the active intermediate product O₂^·-facilitated the release of As at p H7.0,and the As release rate increased first and then decreased with increasing p H from 5.0to 9.0.Under UV irradiation in air atmosphere at p H 7.0,the ROS including O₂^·-and OH^·generated by Fe（Ⅲ）-citrate through the photo-Fenton reaction accelerated the As release and oxidation.However,Fe（Ⅲ）-citrate photolysis led to the rapid flocculation and precipitation of dissolved iron ions,inhibiting the further oxidation of arsenopyrite.With increasing p H from 5.0 to 9.0,the As release rate gradually decreased under UV irradiation.The increases in the concentrations of citrate and dissolved oxygen promoted the formation of Fe（Ⅲ）-citrate and ROS in the reaction system under both UV irradiation and dark conditions.This work initially clarifies the influence of several environmental factors on the oxidation and dissolution behavior of arsenopyrite and the migration and transformation process of As,and provides a clearer understanding of the geochemical behavior of As in sulfide mining areas and surrounding areas,and enriches As geochemical cycling.