Estuarine trace metal distributions in Galveston Bay: importance of colloidal forms in the speciation of the dissolved phase

Abstract

Abstract Cross-flow ultrafiltration studies were carried out in the Trinity river estuary of Galveston Bay in July, 1993, May, 1994 and July 1995 to study the phase speciation for a number of metals (e.g., Cd, Cu, Co, Fe, Ni, Pb, Zn) in estuarine waters. Both conservative and non-conservative estuarine mixing profiles were observed for all trace metals investigated. All metals showed significant colloidal (≥1 kDa) fractions (45±9% for Cd, 55±4% for Cu, 19±6% for Co, 36±6% for Ni, 64±9% for Pb, 91±5% for Zn, and 79±11% for Fe). Colloidal metal concentrations (except, at times, Fe and Pb) correlated significantly with colloidal organic carbon concentrations (≥1 kDa), suggesting that colloidal metals resulted from metal–organic complexation. Iron was preferentially associated with high molecular weight (≥10 kDa) colloids, while Cu, Ni and Pb were associated mostly with low molecular weight (1 kDa–10 kDa) colloidal material. Molecular weight and size distribution studies of organic carbon and trace metals showed two competing processes: (1) coagulation and colloidal pumping, and (2) production of lower molecular weight species, possibly mediated through photochemical or microbiological reactions. Values of partition coefficients between colloids and true solution (Kc) were considerably higher than those between particles and true solution (Kp) for all trace metals except Fe, indicating a high complexation capacity and binding intensity of colloidal macro-molecular organic matter.