Arsenate sequestration by secondary minerals from chemodenitrification of Fe(II) and nitrite: pH Effect and mechanistic insight

The abiotic reactions of Fe(II) oxidation and nitrite (NO) reduction (chemodenitrification) are a vital geochemical processes at the interface of anoxic sediments and water. Chemodenitrification leads to the formation of secondary iron (Fe)-bearing minerals, which can influence the dynamic behavior of arsenate (As(V)). However, the coupled mechanism and pH effect of chemodenitrification and As(V) sequestration are not well understood. Here, immobilization of As(V) was investigated at pH 5.5, 6.5, and 8.0 during chemodenitrification. Kinetics of 9 mM Fe(II) oxidation and 3 mM nitrite reduction increased with increasing pH but decreased in the presence of 0.5 mM As(V). The results of X-ray diffraction and linear combination fitting of the Fe K-edge EXAFS spectra together showed that high pH and low As(V) were favorable for the formation of secondary crystalline Fe oxides, such as lath-like lepidocrocite, rod-like goethite, and nano-magnetite. Despite efficient removal, the retention of As(V) species on the newly-formed secondary Fe minerals varied substantially at different pH values. Microscopic and spectroscopic analyses at nano- and molecular-scales revealed that As(V) was mainly adsorbed onto poorly crystalline Fe minerals at pH 5.5, while it was partially incorporated into the lattice structure of crystalline Fe minerals through isomorphic substitution at pH 6.5 and 8.0. The amount of As(V) sequestration was positively correlated with newly-formed crystalline Fe minerals. A model was established by integrating Fe mineral transformation and nonlinear As(V) binding to various Fe minerals, which could accurately predict distribution of adsorbed As(V). The pH increase during chemodenitrification promoted immobilization of As(V) in newly-formed crystalline Fe minerals. The findings on pH-dependent chemodenitrification with As have significant implications for understanding the long-term solubility and migration of As(V), and could provide a potential strategy for As immobilization in sediments.