Remediation of AMD through bentonitic clay adsorption

Abstract

The discharge of acid mine drainage (AMD) from several abandoned mines in South-Africa has become a serious concern and environmental hazard. Since 2002 AMD has been decanting from the Number 8 shaft of the abandoned Harmony gold mine near Krugersdorp at 15-20 million litres per day. This acidic water, which contains high concentrations of metal ions in solution, has since contaminated a number of water sources in the surrounding area, including the Tweelopiespruit and several dams in the Krugersdorp game reserve. The general method for treating AMD is by neutralising its acidity by the addition of a base compound such as lime or limestone. This method has proven to be rather ineffective, however, as iron ions present in the water can cause it to re-acidify once it has been released back into the environment. The use of lime and limestone to increase the pH of AMD also has an inherent drawback as it can lead to the formation of gypsum scaling which causes blockages of pipes and equipment, incurring additional costs for AMD processes. Bentonite clay has received a lot of attention lately for its ability to adsorb a wide range of ions from solution, a trait which can be used to remove contaminants from AMD, and several AMD treatment methods and patents have been created based on this characteristic. An added advantage is that bentonite is readily available for large scale AMD treatment processes based on the number and output of bentonite mines in South Africa. A study was undertaken to determine what effect the pH level of AMD has on the ability of bentonite to remove contaminants from it. Experiments were done where AMD samples collected from the Harmony AMD treatment site were treated with bentonite clay after the pH thereof had been altered by the addition of burnt dolomite powder, calcium hydroxide or magnesium hydroxide. The theory that burnt dolomite may be able to prevent the formation of gypsum was also investigated. It was found that increased pH levels improved the removal efficiency of contaminants such as Fe, S and Al, though treatment at pH 7 is not advised as the shift from acidic to basic causes adsorption and precipitation to become erratic. Burnt dolomite powder was also found to be incapable of prompting gypsum precipitation as the magnesium content thereof precipitated too rapidly. The addition of pure magnesium hydroxide to the AMD did however decrease both Ca and SO42- concentrations, indicating that it was successful in precipitating an amount of gyspum. It is recommended that narrower pH ranges should be tested in future to further study the effects of pH on AMD treatment. A method to increase bentonite’s adsorption capacity per weight unit by the removal of the clay’s silica content should also be investigated. Polymers created by mixing bentonite clay with compounds such as Ca(OH)2 and CaCO3, as discussed by Ntwampe et al. (2015) also deserves further study as it shows promise for AMD treatment.