The presence of sulphides in blasted rock may lead to the onset of ARD. ARD occurs when sulphide minerals react with oxygen (or other oxidizing agents) and water to form sulphuric acid which can then leach other metals from waste materials to produce an acidic, metal rich drainage.
The following generalized chemical reactions describe the generation of ARD due to oxidation of pyrite (FeS2) and the resulting formation of sulphate (SO42-), iron hydroxide (Fe(OH)3(s)) and acidity (H+).
1. FeS2(s) + 15/4O2(g) + 7/2H2O(l) > Fe(OH)3(s) + 2SO42-(aq) + 4H+(aq) (at pH >5)
2. FeS2 + 14Fe3+ + 8H2O ↔ 15Fe2+ + 2SO42- + 16H+ (at pH <3)
3. Fe2+ + 1/4O2 + H+ ↔ Fe3+ + 1/2H2O (at pH <3)
Aerobic oxidation of pyrite (reaction 1) produces four moles of acidity for each mole of pyrite. At lower pH (<pH3) ferric iron can act as an oxidising agent resulting in the generation of four times as much acidity i.e. sixteen moles of acidity per mole of pyrite oxidized (reaction 2).
The severity of ARD at any given site will depend upon the balance between the amount and type of sulphides exposed to water and air and the ability of waste materials to neutralize acidity. The acid neutralization capacity of mined rock is primarily dependent upon the presence of readily reactive carbonate minerals, and to a lesser extent upon the presence of less reactive acid consuming minerals such as aluminosilicates. ARD chemistry evolves over time in three main stages: i) dissolution of reactive primary minerals (e.g. pyrite), ii) dissolution of precipitated secondary minerals (e.g. goethite), and iii) dissolution of relatively unreactive primary and secondary minerals (e.g. quartz).
Once started, ARD can be present at a site for decades to hundreds of years. Significant metal leaching can occur independently of acidic conditions for elements that are mobile under near-neutral pH. Elements present in sulphide phases such as arsenic, antimony, selenium, nickel, and zinc can therefore leach from waste materials at concentrations of concern even when ARD is absent. Waste rock particle size plays an important role in ARD / ML processes with the small sub-2mm sized fraction typically exhibiting increased reactivity in previously unweathered waste rock due to its large surface area to volume ratio.
Acid (and alkaline) rock drainage (ARD) is the primary source of mining-related pollution, and is always a significant factor in the mine permitting process. With upwards of 10,000 abandoned mine sites located in Canada alone, the potential liability for cleanup may run into the billions of dollars, with the Provincial/Federal Governments on the hook for cleanup costs for the majority of these sites. Multiply this by the number of legacy mine site worldwide and you can appreciate just how serious an issue ARD is.
Not just restricted to mining, ARD can be an issue with any construction project where bedrock is blasted and exposed to water and air. In North America examples include road construction projects in British Columbia, Canada ( ), Pennsylvania, USA ( ), Airport development in Halifax, Nova Scotia, Canada ( )