Laserfiche WebLink
potential, solution speciation) or may sorb on to suspended matter. Gundersen and Steinnes (2003) <br />reported that <10% of zinc was sorbed on particles or colloids in river water from two rivers with average <br />pHs of 3.1 and 5.1 (rivers with mining activity near Roes, Norway), whereas 21% of zinc occurred in <br />sorbed form in six pH neutral rivers. <br />In the aquatic environment, zinc partitions to sediments or suspended solids in surface waters through <br />sorption onto hydrous iron and manganese oxides, clay minerals, and organic material. Reservoirs <br />located downstream from lead -zinc mining and milling areas were found to contain higher concentrations <br />of zinc than reservoirs in other areas, and the zinc was more highly concentrated in reservoir bottom <br />sediments than in the surrounding soils (Pita and Hyne 1975). In addition, the zinc content in sediment <br />closely correlated with the depth, organic content, and clay content of the sediments. Phosphates and iron <br />hydroxides affect the transfer of metals (including zinc) from river water to the sediments, according to a <br />study by Houba et al. (1983). In this study, zinc was bound predominantly to carbonate and amorphous <br />matter (iron, aluminum, and manganese hydroxides). In addition, mobile components of naturally <br />occurring organic matter contributed to the increase in the metal hydroxide -bound fraction. <br />The transport of zinc in the aquatic environment is controlled by anion species. In natural waters, <br />complexing agents, such as humic acid, can bind zinc. The stability of zinc complexes depends on the pH <br />of the water and the nature of the complex. The dissociation of the complex may determine the amount of <br />free zinc ions in solution. Zinc -humic acid complexes may be 50% dissociated at pH 5.5 and the <br />dissociation rate may be higher as the pH decreases (Guy and Chakrabarti 1976). Therefore, as the pH of <br />the water decreases, the concentration of zinc ions in the water phase increases at the same rate as that of <br />the release of zinc from the sediment. The magnesium found in the silicate minerals of igneous rocks is <br />often replaced with the divalent zinc ion; consequently, weathering of this zinc- containing bedrock gives <br />rise to Zn +2 in solution. The hydrated cation is the dominant form when the pH is S9 (EPA 1979d). <br />The tendency of zinc to be sorbed is affected not only by the nature and concentration of the sorbent but <br />also by pH and salinity (EPA 1979d). Zinc tends to sorb more readily at a high pH (pH >7) than at a low <br />pH (EPA 1979d). Desorption of zinc from sediments occurs as salinity increases (Helz et al. 1975), <br />apparently because of displacement of the adsorbed zinc ions by alkali and alkaline earth cations, which <br />are abundant in brackish and saline waters (EPA 1979d). In column leaching tests with sediment <br />collected from the banks of the Rhone River, the presence of dissolved organic matter and pH was found <br />to be the factors controlling the adsorption and mobility of zinc (Bourg and Darmendrail 1992). <br />A small fraction of zinc will exist in the aquatic phase as soluble inorganic zinc compounds (e.g., zinc <br />chloride, zinc sulfate). Soluble inorganic zinc compounds hydrolyze in solution, forming zinc hydroxide <br />precipitates. Hydrolysis may lower pH, but the buffering action present in most natural water prevents a <br />significant alteration in pH. The precipitation of zinc hydroxide and zinc carbonate was studied by <br />Patterson et al. (1977), who found that zinc hydroxide precipitates faster than zinc carbonate. Zinc <br />carbonate is soluble in pure water at 25 °C at concentrations of _.107 mg zinc /L. The hydroxide is soluble <br />only at concentrations of _<0.2 mg zinc /L. As a result, some of the inorganic forms of zinc that are <br />expected to be present in water are basic carbonate (Zn2[0HJ2CO3), hydroxide (Zn[0HJ2), and silicate <br />(Zn2SiO4) (Florence 1980; NAS 1977). When the pH is _>8, most of these compounds will precipitate; <br />however, as the pH decreases, more and more of these compounds will dissolve and remain in the water <br />phase (EPA 1979d). <br />Revenue Mine August 2012 T -5 <br />