Geochemical signatures and processes in a stream contaminated by heavy mineral processing near Ipoh city, Malaysia

摘要

The environmental impacts of a heavy mineral processing involving rare earth elements (REEs) in the Kinta River and its tributaries near Ipoh city in Malaysia is investigated using geochemical and isotopic signatures and geochemical modeling. Analysis of water, sediment, suspended particulate matter, and rock samples reveals that stream waters close to the heavy mineral processing plant site are anthropogenically influenced, as indicated by slightly low pH (4.98-6.24) and elevated concentrations of Fe (up to 129 mg/L), Cl- (1540 mg/L), total REEs (1580 mu g/L), U (17.2 mu g/L), and Th (0.33 mu g/L). REE patterns and Pb isotope ratios of waters and sediments from a tributary are similar to those of tailings pond seepage and sediments, suggesting that not only these elements but also U and Th are derived from heavy mineral processing. The delta Fe-56 values of water in the tributary decrease downstream, accompanied by a decrease in Fe concentration, which is likely due to isotope fractionation via oxidative precipitation of Fe hydroxides. The concentrations of REEs, U, and Th decrease downstream, accompanied by an increase in pH, with different removal factors (Th > U > REEs). The influence of contamination is confined to within similar to 1.5 km downstream of the source. Tamm's acid oxalate extraction (to dissolve only low crystalline phases) showed that Th and Al concentrations were lower while U and Fe concentrations were higher in downstream suspended particulate samples, suggesting that Th and U may be removed by different mechanisms from stream water. The results of geochemical modeling suggest that Th precipitated as a solid phase, such as an intermediate between ThO2 and Th(OH)(4), with increasing stream water pH. In contrast, U may have been removed by adsorption onto Fe hydroxides, as supported by surface complexation modeling. The geochemical signatures and processes discussed in this study may be applicable for environmental assessment of REE processing. (C) 2017 Elsevier Ltd. All rights reserved.