Environmental consequences of sulfide and arsenic oxidation

3 Environmental consequences of sulfide and arsenic oxidation 3.7.3.1 Formation of mine drainage 

Not all mine drainage or natural runoff from rock outcrops are acidic, even when extensive sulfide oxidation is present. Synthetic cyanide solutions, which are often used to extract gold and other metals from ores, can greatly increase the alkalinity of mining wastes and neutralize sulfuric acid (Craw et al., 1999). In other cases, the sulfuric acid is effectively neutralized by alkaline soils, limestones, dolostones, marbles, shams, or other carbonate-rich rocks (Pfeifer et al., 2004, 219 Razo et al., 2004 Lee, Lee and Lee, 2001, 491 Mendoza et al., 2006). Reactions between calcium carbonate and sulfuric acid may precipitate gypsum (CaS04 2H20). 

Malaysia (Williams, 2001), Thailand (Williams, 2001), Philippines (Williams, 2001), Brazil (Williams, 2001 Borba, Figueiredo and Matschullat, 2003 Matschullat et al., 2000), Hungary (Odor et al., 1998), Tanzania (Bowell et al., 1995), Zimbabwe (Williams, 2001), and Switzerland (Pfeifer et al., 2004). Williams (2001) is an exceptionally broad survey of arsenic in mine drainage samples from 34 sites in Asia, Africa, and South America. The arsenic concentrations of the waters ranged from 0.005 to 72 mgL-1. Like most other mine drainage results, the majority of samples in Williams (2001) contained less than 5 mgL-1 of arsenic. The arsenic was also well oxidized. As(V) represented more than 50 % of the dissolved inorganic arsenic at 29 of the 34 sites and at least 18 of the sites had more than 95 % of their dissolved arsenic as As(V) (Williams, 2001, 269-270). The pH values of the samples ranged from 0.52 to 10. Only 3 of the 10 sites with arsenic concentrations above 1 mgL-1 had pH values below 4 (Williams, 2001, 269-270). 

3 Long-term environmental impacts of mining wastes 

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