MechChem Africa

By Michael O Daramola

Water is connected to almost everything on earth. In South Africa, the words ‘mining’ and ‘water’ lead one to think about acid mine drainage (AMD). This month we feature an opinion piece by Professor Michael Daramola on membrane technology to treat AMD. The views expressed in this article are his own, informed by experience, literature review and other expert opinion pieces.

SAIChE News Water Management Book coverA major sanitation and water pollution challenge associated with the mining industry is acid mine drainage (AMD), which forms when sulphide rocks are exposed to air and water for prolonged periods. The formation of acid mine drainage is a natural process, but reactions are caused by exposing sulphide-containing rocks to the environment through mining operations and are often catalysed by bacterial activity. The natural process of AMD formation takes close to 15 years in the absence of bacteria, for ferric iron to produce acid, but the presence of bacteria shortens this reaction time.

Typical characteristics of AMD are very low pH and high concentrations of metals and sulphates. If left untreated, AMD has the significant negative environmental impact of mineralisation of affected areas and acidification of receiving ground and surface waters. The solubility of transition metals is greater in low pH media, hence AMD carries with it high concentrations of metals such as Al, As, and Mg and other transition metals such as Cu, Zn, Pb, Co, Mn and Cd, depending on the host rock. It enters the aquatic environment uncontrollably, posing a threat to humans, domesticated animals and the ecosystem.

Conventional treatment processes: active and passive treatment

Active treatment involves neutralising the acidity with alkaline substances such as lime to trigger precipitation of metal hydroxide, which is easily be removed by sedimentation. Furthermore, ion exchange technology – which explores the advantage of oppositely charged pollutants and employs solid resins to remove cations and anions from solutions – has also been proposed as a treatment method. A high metal ion uptake capability of this resin makes ion exchange an attractive technology, but it is a preferred technology for low metal ion concentrations and becomes very expensive when dealing with high concentrations of metal ions in solutions.

In addition, the resins need to be regenerated when exhausted by chemicals and this regeneration can produce secondary pollution and elevate operational costs. On the other hand, the passive treatment of AMD relies on biological, geochemical and gravitation processes in natural or constructed wetland ecosystems. Furthermore, conventional methods can only achieve partial treatment, and they also have the disadvantages of producing sludge, requiring high-energy consumption and frequent maintenance.

Therefore, growing global demand for clean water and increasing environmental concerns, warrant the need to search for more sustainable and environmentally friendly technologies for metal ion removal from mining wastewater. There is a great need for water recycling and efficiency of water recycling will depend strongly, amongst other factors, on the performance of existing water treatment techniques and processes to provide potable and clean water for the use of the human race without posing environmental hazards.

Potable water coupled with good sanitation should be affordable by all, which is the sixth of the sustainable development goals (SDG): Clean water and sanitation for all.

Membrane technology

In the search for alternative technologies, membrane technology has proven to be a promising option. Membrane technology, due to its easy operation, inexpensiveness, high separation efficiency and low energy consumption, has emerged as a promising substitute to conventional methods for AMD treatment.

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