Effects of humic acid amendment on the mobility of heavy metals (Co, Cu, Cr, Mn, Ni, Pb, and Zn) in gold mine tailings in Thailand

The purpose of this study was to investigate the possibility of effects of humic acid on the fate and mobility of seven heavy metals (Co, Cr, Cu, Mn, Ni, Pb, and Zn) in gold mine tailings. Ten samples were taken from the tailing storage facilities of the Chatree (Akara) Gold Mine in the Phichit province, Thailand. The heavy metal fractionation in the samples was determined using a modified three-step BCR sequential extraction with an additional microwave-assisted aqua regia digestion for the residual fraction. The amount of metal in the exchangeable fraction was ranked: Mn > Co > Ni similar to Cu > Zn > Pb > Cr, while that in the reducible fraction was Pb > Mn > Ni > Co > Cr > Cu > Zn, giving that for the mobilizable (bioavailable) metal ions of Mn > Pb > Ni > Co > Cu > Zn > Cr. The addition of humic acid (5-10 % (w/w)) to the tailings inhibited the bioavailability of Cu and Pb in a dose- and weakly time-dependent manner over the 30-day assay period, suggesting that these metal ions bind more strongly to the oxidizable fraction. Humic acid dramatically decreased the mobility factor (MF) of Cu and Pb and slightly decreased that of Co, Cr, and Zn but slightly increased that for Mn and Ni in a dose-dependent manner.