The influence of sediment redox chemistry on chemically active forms of arsenic, cadmium, chromium, and zinc in estuarine sediment

Abstract

Kinetics and chemical fractionation procedures were used to quantify the effect of the sediment redox (Eh) condition on the behavior of As, Cd, Cr, and Zn in the bottom sediment collected from a Louisiana coastal site receiving produced water discharge. Sediment samples were incubated in microcosms in which Eh-pH conditions were controlled. Sediment was sequentially extracted to determine metals in various chemical fractions (water soluble, exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to insoluble organic and sulfides) and the chemical inactive fraction (mineral residue). Under oxidizing conditions, As, Zn, and Cr behavior was governed by redox chemistry of Fe(III) and Mn(IV) oxides. Cd transformations were controlled by both Fe(III) and Mn(IV) oxides and carbonates. Under a reducing condition, the behaviors of Zn and Cr were controlled primarily by insoluble large molecular humic material and sulfides; the behavior of Cd was controlled by carbonates. When sediment redox potential increased, the affinity between Fe(III) and Mn(IV) oxides and As, Cd, Cr, and Zn increased. When sediment redox potential decreased, the affinity between carbonates and Cd and Zn increased; the affinity between insoluble sulfides, large molecular humic matter and As, Cd, Cr, and Zn increased; the soluble Cd and Zn decreased; the soluble As and Cr remained constant. Results suggest reducing sediment conditions would reduce Cd and Zn toxicity. Under reducing or anaerobic conditions, the solibilization rate constants (mg kg-1 d-1) for As, Cr, Cd, and Zn bound to Fe(III) and Mn(IV) oxides were -0.88, -0.32, -0.01, and -6.5, respectively; the rate constants (mg kg-3 d-3) for dissolved Cd and Zn were -0.09 and -1.78, respectively.