Speciation of trace metals in natural waters

The speciation of trace metals in natural waters has been extensively studied owing to their potential effects on geochemical, biological and environmental processes. Chemical speciation modeling provides a useful approach to interpret and predict the speciation of trace metals in natural waters. This dissertation focuses on examining the ionic interactions between trace metals and inorganic ligands in natural waters using ionic interaction model. The models include ion pairing and specific interactions of metals and ligands. The models can be used to examine the interactions of ions as a function of ionic strength and temperature. The models require reliable stability constants (beta *n) for the formation of metal complexes over a wide range of temperature (0 to 50°C) and ionic strength (0 to 6 m). At the present the knowledge on effect of ionic strength and temperature on the stability constants is limited. This study examines the interaction of the trace metals (Mn2+, YREE and Pb2+) with inorganic ligands (CO32-, F and Cl-) over a wide range of temperature and ionic strength. These studies include: (1) Solubility of Rhodochrosite (MnCO3) in NaCl solutions (Journal of Solution Chemistry, 2003, 32 , 405--416) (2) Effects of temperature and ionic strength on the stabilities of the first and second fluoride complexes of yttrium and the rare earth elements (Geochim. Cosmochim. Acta, 2004, 68, 4301--4308) (3) Stability constants for the formation of lead chloride complexes as a function of temperature and ionic strength (Geochim. Cosmochim. Acta, in press); Different techniques (solubility measurements, cation exchange on resins, and spectrophotometry) were used to determine the stability constants of the metal complexes as a function of temperature (0 to 50°C) and ionic strength (0 to 6 m). The temperature effect has been examined using the Van't Hoff equation to determine the enthalpies for the formation of the metal complexes. The measurements as a function of ionic strength have been analyzed using ion pairing and the Pitzer specific interaction models. The resulting Pitzer coefficients and stability constants of the metal complexes can be used in ionic interaction models to better understand metal speciation in natural waters over a wide range of temperature and ionic strength.