The Role Of Defect In Arsenic Oxidation By Fe(Ⅱ)-mediated Goethite

Arsenic pollution in farmland is one of the most important environmental problems in China.Arsenic mainly exists as the form of As（Ⅲ）and As（V）,among which As（Ⅲ）is more toxic and mobile than As（V）.Currently,most studies focused on the influence of minerals on arsenic adsorption,but there are few studies focusing on the process of arsenic transformation on the surface of minerals and the structure effect on it.On the one hand,goethite is the main iron oxide in the farmland.Because of the complexity of geological conditions,there are goethite with different degree of defect,which has a great effect on the reactivity of minerals.On the other hand,Fe（Ⅱ）is reduced and released during the alternating process of dry and wet farmland.When Fe（Ⅱ）is adsorbed on the surface of iron oxides,it exchanges atoms with the structure Fe（Ⅲ）.At the same time,it improves the whole reactivity of iron oxide.As the degree of goethite defect increase,more Fe（Ⅱ）transfer electrons are to iron oxide.Therefore,hydrothermal goethite and copper doped goethite was synthesized to diminish and increase defect,respectively.By simulating arsenic transformation at the surface of minerals in farmland,the influence and the mechanism of defect in arsenic transformation by Fe（Ⅱ）-mediated goethite were studied.The main findings are as follows:（1）Under anoxic condition,As（Ⅲ）was oxidized by Fe（Ⅱ）-activated goethite.With the increase of goethite defects,the content of generated As（V）increased from3.7%to 10.1%.Utilizing iron stable isotope to trace the electron transferring during the reaction,the results show that the Fe（Ⅱ）added to the reaction system is oxidized on the surface of the goethite,and the electrons are transferred to the Fe（Ⅲ）of the goethite.The new forming Fe（Ⅲ）may fill the iron vacancies on the surface of the goethite and participate in the recrystallization of the goethite.Compared with the structure Fe（Ⅲ）of goethite,the newly formed Fe（Ⅲ）may be more reactive.When As（Ⅲ）enters the Fe（Ⅱ）-goethite reaction system,As（Ⅲ）reacts with the active iron species formed on the surface of the goethite and transfers electrons to the structure Fe（Ⅲ）of the goethite.As the initial concentration of Fe（Ⅱ）increased from 1 mg/L to16 mg/L,the content of generated As（V）rose from 1.8%to 10.7%;as the dosage of goethite increased from 0.5 g/L to 1.5 g/L,the content of generated As（V）rose from7.8%to 13.1%.The initial concentration of Fe（Ⅱ）and the dosage of goethite affect the oxidation of As（Ⅲ）possibly by regulating the generated reactive iron species and the content of oxygen vacancies iron vacancies involved in the reaction.（2）Under aerobic conditions,goethite can significantly promote oxidation of As（Ⅲ）catalyzed by adsorbed Fe（Ⅱ）.With the increase of the goethite defects,the initial concentration of Fe（Ⅱ）and the dosage of goethite,the content of generated As（V）in the reaction system with adsorbed Fe（Ⅱ）increases.The generated As（V）is mainly exist in the solid phase.As（Ⅲ）oxidation has nothing to do with Cu O.On the one hand,Fe（Ⅱ）adsorbed on the surface of goethite may transfer electrons to oxygen adsorbed on oxygen vacancies and generate superoxide radicals,or react with dissolved oxygen to generate other reactive oxygen species,which thereby oxidize As（Ⅲ）.On the other hand,the oxygen vacancies and structure Cu（Ⅱ）on goethite may further promote As（Ⅲ）oxidation by promoting the decomposition of H₂O₂and the Fe（Ⅱ）cycle.This studies have found that·O₂^-and H₂O₂are the main reactive oxygen species for As（Ⅲ）oxidation.The defect content of goethite,the initial concentration of Fe（Ⅱ）,and the dosage of goethite may controlled As（Ⅲ）oxidation by regulating the content of oxygen vacancies and iron vacancies involved in the reaction,the content of adsorbed Fe（Ⅱ）involved in activating oxygen,and the Fe（Ⅱ）cycle in the reaction.This study is of great significance for understanding the transformation of As（Ⅲ）at the iron mineral interface during farmland flooding and drying.