Effect of pH on the release of heavy metals from stone coal waste rocks

Static leaching experiments were performed to detect the heavy metals release rule of stone coal waste rocks at different solution pH levels. Results showed that solution pH exerted a significant influence on mineral dissolution. As solution pH decreased, the dissolved quantities of minerals increased gradually. With leaching time progressing, the dissolved quantities of Cr and V at each experimental pH level increased almost all the way, while As and Cd dissolved quickly at first and then the dissolution decreased. The dissolved quantities of major elements in the stone coal waste rocks were also detected.Ca and Mg dissolution increased rapidly in the beginning and then decreased slowly; however, the dissolved quantities of Al and Fe were relatively low. The maximum dissolved quantities at a solution pH of 2 were observed in the following order: Ca (2695.65 mg/kg) > Mg (510.92 mg/kg) > Al (23.64 mg/kg) > Fe (11.55 mg/kg) > V (6861 μg/kg) > Cr (1005.35 μg/kg) > Cd (751.71 μg/kg) > As (323.66 μg/kg). It can be seen that the dissolution of the elements is not correlated with their total content in the stone coal waste rocks, but is related with their content in easily soluble fraction (F1). The stone coal waste rocks were found to exhibit a relatively high neutralizing power, in that, in the range of initial solution pH at 3-9, final leachate pH stabilized at 8.3-8.5. This neutralizing ability was conducive to precipitate newly dissolved heavy metals, thus was favorable for reducing the waste rocks environmental toxicity.