Synergetic effects of Mn(II) production and site availability on arsenite oxidation and arsenate adsorption on birnessite in the presence of low molecular weight organic acids

Manganese oxides and organic acids are key factors affecting arsenic mobility, but As(III) oxidation and adsorption in the coexistence of birnessite and low molecular weight organic acids (LMWOAs) are poorly understood. Herein, As(III) immobilization by birnessite was investigated with/without LMWOAs (including tartaric (TA), malate (MA), and succinic acids (SA) with two, one and zero hydroxyl groups, respectively). In the low-As(III) system with less Mn(II) production, LMWOAs generally inhibited As(III) oxidation. The slower decrease in As(III) concentration in TA-amended batches resulted from stronger bonding interaction between TA and edge sites, evidenced by higher removal of TA than MA and SA in solutions and the higher proportion of shifted C-OH component in solids. In high-As(III) systems with abundant Mn(II) production, higher concentrations of dissolved Mn and Mn(III) in LMWOA-amended batches than in LMWOA-free batches revealed that LMWOA-induced complexing dissolution caused the release of adsorbed Mn(II), which was conducive to As(III) oxidation and As(V) adsorption onto the edge sites. The lowest concentrations of dissolved Mn and Mn(III) in TAamended batches indicated that the hydroxyl group constrained complexing dissolution. This study reveals that concentrations of produced Mn(II) determined the roles of LMWOAs in As(III) behavior and highlights the impacts of the hydroxyl group on arsenic mobility.

Automated summary

Manganese oxides and organic acids are important factors affecting arsenic mobility. Low concentrations of arsenic are generally inhibited from oxidation by organic acids. However, high concentrations of arsenic can lead to manganese dissolution, thereby facilitating arsenic oxidation and adsorption.