Speciation of heavy metals in the surface waters of a former tin mining catchment

This study was conducted to investigate the chemical speciation of dissolved and particulate elements (Pb, Zn, Cu, Cr, As and Sn) in the mining wastewater. Speciation patterns of dissolved elements were estimated by adsorptive stripping voltammeter while particulate elements were analysed using a newly developed sequential extraction leaching procedure. The procedure has been operationally defined among five host fractions, namely exchangeable, carbonate, reducible, organic bound and residual. A total of six elements (Pb, Zn, Cu, Cr, As and Sn) were analysed in 30 samples at 10 locations (P1-P10) representing three subsequent samples at the same location to obtain the average value from ex-tin mining catchment. The results showed that the heavy metal pollution in P4 and P8 was more severe than in other sampling sites, especially Sn and Pb pollution. In the water samples at P4 and P8, both the total contents and the most dangerous non-residual fractions of Sn and Pb were extremely high. More than 90% of the total concentrations of As and Cr existed in the residual fraction. Cu and Zn mainly (more than 60%) occurred in the residual fraction. However, Pb and Sn were predominantly present in the non-residual fractions of the surface water. For all the six dissolved elements, the less labile species formed the predominant fraction in their speciation patterns. We conclude that the speciation patterns of particulate elements show that most of the Pb, Zn, Cu, Cr, As and Sn were found in the reducible fraction whereas Pb and Sn were mainly associated with the organic fraction.