THE INFLUENCE OF SEDIMENT REDOX CHEMISTRY ON CHEMICALLY ACTIVE FORMS OF ARSENIC, CADMIUM, CHROMIUM, AND ZINC IN ESTUARINE SEDIMENT

Kinetics and chemical fractionation procedures were used to quantify t he effect of the sediment redox (Eh) condition on the behavior of As, Cd, Cr, and Zn in the bottom sediment collected from a Louisiana coast al site receiving produced water discharge. Sediment samples were incu bated in microcosms in which Eh-pH conditions were controlled. Sedimen t was sequentially extracted to determine metals in various chemical f ractions (water soluble, exchangeable, bound to carbonates, bound to i ron and manganese oxides, bound to insoluble organic and sulfides) and the chemical inactive fraction (mineral residue). Under oxidizing con ditions, 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, th e behaviors of Zn and Cr were controlled primarily by insoluble large molecular humic material and sulfides; the behavior of Cd was controll ed by carbonates. When sediment redox potential increased, the affinit y between Fe(III) and Mn(IV) oxides and As, Cd, Cr, acid Zn increased. When sediment redox potential decreased, the affinity between carbona tes and Cd and Zn increased; the affinity between insoluble sulfides, large molecular humic matter and As, Cd, Cr, and Zn increased; the sol uble Cd and Zn decreased; the soluble As and Cr remained constant. Res ults suggest reducing sediment conditions would reduce Cd and Zn toxic ity. Under reducing or anaerobic conditions, the solibilization rate c onstants (mg kg(-1) d(-1)) for As, Cr, Cd, acid Zn bound to Fe(III) an d Mn(IV) oxides were -0.88, -0.32, -0.01, and -6.5, respectively; the rate constants (mg kg(-1) d(-1)) for dissolved Cd and Zn were -0.09 an d -1.78, respectively. (C) 1997 Elsevier Science Ltd.