Abiotic anoxic reduction of AsO4 adsorbed Mg(II)–Al(III)/Fe(III)–CO3/SO4 Layered Double Hydroxides: Implications of As release and phase transformations

Abstract

In this work, the stability of arsenate adsorbed Mg(II)–Al(III)/Fe(III)–CO3/SO4 Layered Double Hydroxides (LDHs) under three abiotic anoxic reductive conditions that may be encountered in Tailings Management Facilities (TMFs) were evaluated. At pH 8 and 10 mM Fe(II) (aq), the formation of Fe(III)-oxy/hydroxides (FeOHs) occurred for all 5 LDHs but the Fe-based LDHs precipitated the greatest amounts. All LDHs released <0.3 mg/L total As(aq) and the reacted solid surface remained as As(V) except for MgFeSO4. For reactions at pH 8 and 0.5 mM Fe(II) (aq), precipitation of FeOHs occurred but preferential formation to magnetite (for MgFeSO4) and 2-line ferrihydrite (for MgFeCO3) transpired. The highest total As(aq) release followed the order: MgAlSO4 > MgAlFeCO3SO4 > MgAlCO3 > MgFeCO3/MgFeSO4. The solid surfaces remained as As(V) except for MgAlSO4, MgAlCO3, and MgAlFeCO3SO4 LDHs where 10–25% As(III) formed. Finally, at pH 10 and 0.5 mM Fe(II) (aq), the formation of FeOHs occurred to various degrees but a significant amount of CaCO3(s) precipitated. A 2-stage release and re-adsorption mechanism of total As(aq) occurred following the order: MgAlFeCO3SO4 > MgAlSO4 > MgFeCO3 > MgAlCO3 > MgFeSO4. A significant portion of the solid surfaces (30–90%) was found as As(III) for all reacted LDHs. This work provides a guideline for the environmental behavior of As(V) adsorbed LDHs where relevant underwater cover TMF abiotic reducing conditions may exist.