Abstract

Mine water pollution in South Africa is known to originate mainly from abandoned and operating coal and gold mines, primarily as acid mine drainage (AMD). Two types of treatment are commonly used to treat mine water: active treatment and passive treatment. Active treatment requires high maintenance for aeration, power generation for pumping and the addition of chemicals to treat the water. Passive treatment methods are typically relatively inexpensive to operate, do not require the addition of chemicals for treatment, and are usually low maintenance systems. An experimental passive treatment Vertical Flow Reactor (VFR) has been installed at an abandoned coal mine site near Carolina, Mpumalanga, South Africa (Fanie Nel Discharge). The site is located approximately 8.6 km from the Boesmanspruit reservoir, which was previously believed to be contaminated by coal mine water discharges from the surrounding area. A VFR was installed with the objective of remediating the acidic mine water and investigating its ability to operate under relatively low pH conditions. This was achieved by utilising an intermediate bulk container (IBC) and a gravel bed, with raw mine water flowing vertically through the container by gravity. The objective was to remove Fe (≈ 230 mg/L) by an autocatalytic process with ochre precipitation, to aerate the water, and to study the chemistry of the mine water, which has an electrical conductivity of 2.1 mS/cm. The pH of the discharged mine water is approximately 3, and the redox potential approximately 560 mV, indicating that ferrous iron predominates. Consequently, the anticipated Fe removal rate of 65–75% was not achieved, and the mine water could not be treated as intended. A second VRF was installed, and the influent water to this VRF was treated by an additional open limestone drain. This raised the pH by buffering and increased the Fe removal rate from 35% to 69%. This result was unexpected when compared to the Cwm Rheidol VRF in Wales, which treated mining-influenced water (MIW) with a similar chemical composition. Nevertheless, the results demonstrate the necessity of either the presence of crystallisation nuclei of iron oxyhydrates in low pH mine water or, alternatively, higher pH values that support the formation of these crystallisation nuclei for a VFR to function effectively. In the case of the Fanie Nel Discharge, neither of these conditions were present, resulting in the trial’s outcomes falling short of initial expectations. Consequently, a full-scale VFR was not installed at the site.