Institute of Water Security and Science
Flocculates and precipitates from coal mine drainage (CMD) are an integral part of the geochemical setting in mining-impacted streams. The precipitates may be very small, amorphous, and, depending on their chemistry, play a role in the storage and release of alkalinity. To evaluate the potential impact of CMD-solids on alkalinity, we conducted various tests using model and natural solids. Laboratory measurements of alkalinity are operationally defined by a titration to a fixed pH. When the solid-solution mixtures were tested using the standard alkalinity measurement method, no alkalinity could be detected; however, a possible reason for that was that the standard method is too rapid for the kinetics of solid dissolution and therefore only accounts for dissolved alkalinity. To test this hypothesis, we conducted a series of “batch-alkalinity” measurements where the pH was measured after the mixture had equilibrated and shaken for 10 minutes to 48 hours. A proof-of-concept test using calcium carbonate confirmed that the batch approach to measuring total alkalinity was comparable to the standard method for dissolved alkalinity. Using this batch method, we demonstrated that, given time, CMD-solids can contribute alkalinity to the solid-water system. The method was applied to ferrihydrite as a model mineral as well as to dried CMD-sediments collected from a field site. Both the ferrihydrite and the field sediments generated measurable alkalinity in the batch experiments but not using the standard method. These experiments demonstrate that mine-drainage solids can contribute to alkalinity and play a role in the moderation of pH in these systems.
Lei, L and DJ Vesper. 2021. Quantifying the Contribution of Solid Precipitates to the Total Alkalinity in a Coal-Mine Discharge. IWSS Spring Workshop Series (Virtual).