Use of microbes for cost reduction of metal removal from metals and mining industry waste streams

Acid-rock drainage (ARD) - also known as acid-mine drainage (AMD) - results from the exposure of sulfide minerals, particularly pyritic and pyrrhotitic minerals, to atmospheric oxygen and water. AMD directly impacts tens of thousands of kilometers of streams, lakes and estuaries throughout the world. The impacted water bodies tend to have elevated concentrations of metals in the water column or sediments, and are also stressed by significant inputs of hydrogen ions. There are several conventional treatment technologies available. The most common is chemical precipitation using lime or other basic substances. These systems produce large volumes of wet sludge that often require drying facilities to concentrate the metal hydroxide sludge. Wetland treatment systems have also been used for several decades to treat AMD. Recent developments and improvements have resulted in construction of bioreactors that have a smaller footprint, and treat the metals and acidity more effectively. Many studies have demonstrated that the primary removal mechanisms for the metals are sulphate-reducing bacteria (SRB). These microbes facilitate the conversion of sulphate to sulphide. The sulphides react with metals to precipitate them as metal sulfides, many of which are stable in the anaerobic conditions of the treatment system. Plants have been shown to remove metals by uptake or oxidative precipitation near the roots. Plants seem to account for only a small percentage of the metal removal capacity of the wetland treatment systems. Adsorption of metals to the organic substrates of the treatment systems can result in metal removal, but adsorption capacity is saturated in short periods of time. High oxygen, low pH waters often enter the treatment systems. The SRB are obligate anaerobes which prefer conditions between pH 5 and 8. Thus, the input water characteristics could impact the efficiency and life expectancy of the treatment systems. The most important characteristic of input waters seems to be pH. Low oxygen of the influent waters did not enhance treatment capabilities. Low pH waters do reduce the capacity of the treatment systems to treat metals effectively. Oxyanions such as chromate and arsenate can be removed using the wetland treatment system (passive bioreactor) technology. Arsenic is removed as an arsenic sulfide compound and chromate is reduced to Cr(III) and precipitated as a hydroxide. The passive bioreactor - wetland treatment system - offers a less expensive alternative to the conventional chemical precipitation technologies. There still are problems of system hydraulics and useful life to be addressed. (c) 2005 Published by Elsevier Ltd.