Controls and Rates of Acid Production in Commercial-Scale Sulfur Blocks

Acidic drainage (pH 0.4—1.0) from oxidizing elemental sulfur (S°) blocks is an environmental concern in regions where S° is stockpiled. In this study, the locations, controls, and rates of H 2 SO 4 produaion in commercial-scale S 0 blocks (-1―2 x 10 6 m 3 ) in northern Alberta, Canada, were estimated. In situ modeling of O 2 concentrations ([O 2 ]) suggest that 70 to >97% of the annual H 2 SO 4 production occurs in the upper 1 m of the blocks where temperatures increase to >15°C during the summer. Laboratory experiments show that S 0 oxidation rates are sensitive to temperature (Q 10 = 4.3) and dependent on the activity of autotrophic S 0 ―oxidizing microbes. The annual efflux of SO 4 in drainage water from a S 0 block (5.5 x 10 5 kg) was within the estimated range of SO 4 production within the block (2.7 x 10 5 to 1.2 x 10 6 kg), suggesting that H 2 SO 4 production and removal rates were approximately equal during the study period. The low mean relative humidity within the block (68%; SD = 17%; n = 21) was attributed to osmotic suction from elevated H 2 SO 4 concentrations and suggests a mean in situ pH of approximately ―2.1. The low pH of drainage waters was attributed to the mixing of fresh infiltrating water and low-pH in situ water. Heat generation during S 0 oxidation was an important factor in maintaining elevated temperatures (mean, 11.1°C) within the block. The implications of this research are relevant globally because construction methods and the physical properties of S 0 blocks are similar worldwide.