Distribution and partition behavior of heavy metals between dissolved and acid-soluble fractions along a salinity gradient in the Changjiang Estuary, eastern China

The concentrations of dissolved (< 0.22 gm) heavy metals were determined across a salinity gradient from 0.15 parts per thousand to 19 parts per thousand in surface waters of the Changjiang Estuary. Analyses were by atomic absorption spectrometry following preconcentration by extraction with ammonium pyrrolidine dithiocarbamate (APDC)/diethylammonium diethyldithiocarbamate (DDDC) into Freon-TF (1,1,2-trichloro-2,2,1-trifluoroethane) and back-extraction by concentrated nitric acid. The concentrations of acid-soluble heavy metals extracted from suspended matter using diluted hydrochloric acid were also measured to explore the water/ particle interactions that occur in the estuary. The results shows that, in the Changjiang Estuary, the rapid removal of dissolved Fe occurs at very low salinities (below 1.5 parts per thousand) due to the salt-induced coagulation of river-borne colloids. Dissolved Fe shows a slight concentration peak at 10 parts per thousand salinity because of the resuspension of sediments and/or deflocculation of coagulated particles in that forceful mixing region. Dissolved Zn also shows removal behavior in the low salinity region (< 2 parts per thousand) and nonconservative excess at intermediate and high salinities during the Changjiang estuarine mixing, implying that flocculation at low salinities and deflocculation and/or desorption processes at high salinities occurred separately for dissolved Zn. Dissolved Cu and Ni have similar distributions at the Changjiang Estuary. Their concentrations increase at low salinities and then decrease at middle to high salinities, showing nonconservative excess behavior owing to desorption reactions. The concentrations of Co and Ph decrease continually with increasing salinity, a nonconservative characteristic of trace metals during estuarine mixing. The removals of both Ph and Co are relatively intense at low salinities compared to intermediate and high salinities. Sorption onto suspended matter is the major mechanism for the distributions of dissolved Co and Ph in the Changjiang Estuary. The same nonconservative excess behavior of dissolved Cd reported in other estuaries or in mixing experiments is also observed during the water mixing in the Changjiang Estuary. The concentration of dissolved Cd increases linearly with increasing salinity within the study area. Acid-soluble heavy metals appear to have similar distributions in the Changjiang Estuary. All of them increase in concentration across almost the full salinity range. The suspended matter-normalized concentrations of acid-soluble heavy metals are negatively correlated with the amount of suspended matter, implying that the distribution of acid-soluble heavy metals in the Changjiang Estuary is mainly controlled by the concentration of suspended matter and salinity. In the Changjiang estuarine waters, the proportion of heavy metals in the acid-soluble phase relative to the total concentration (dissolved plus acid-soluble) decreases in the following sequence: Fe>Co>Zn>Pb>Ni>Cu>Cd. The fraction in the acid-soluble phase ranges from 99% to 4%, indicating considerable diversity of particle reactivity among these heavy metals. (C) 2003 Elsevier B.V. All rights reserved.