USE OF A BATCH-ALKALINITY TEST TO DETERMINE THE CONTRIBUTION OF MINE-DRAINAGE SEDIMENTS TO NET ALKALINITY

Water discharged from coal mines is often chemically unstable and may rapidly precipitate metal hydroxides when in contact with the atmosphere. This process is often observed downstream from discharges as colored mineral precipitates, sediment accumulation, and turbid water. If entrained in water samples, the particles can contribute to net, unfiltered alkalinity. To quantify this non-carbonate alkalinity, mixtures of sediments and deionized water were titrated using a standard method to measure the alkalinity, but there was no measurable contribution from the sediments. In parallel, a series of “batch alkalinity” experiments were conducted in which the sediment-water mixtures were allowed to react for longer to allow the mixtures more time to reach equilibrium. The batch titrations were created by creating mixtures with a fixed solid:water ratio and then adding stepwise additions of acid.

The standard and batch titration approaches were first applied to a carbonate solution to demonstrate that the two methods were consistent. The batch titration was then applied to mixtures of water with laboratory-grade ferrihydrite and with a bulk sediment sample collected from a mine discharge channel. The bulk sample was collected, air-dried, and sieved prior to use. The final pHs of the mixtures, after 48 hours of shaking, were graphed relative to the amount of acid added to create a titration curve. The alkalinity was quantified using a gran titration with 2-pH endpoints: 4.2 and 3.9. For both solids, the use of the batch titration resulted in approximately an order-of-magnitude increase in alkalinity (from ca. 0.5 meq/L to 5-6 meq/L). This work demonstrates that if alkalinity is defined relative to the standard method, then the sediments are not contributors to the net outcome. However, if more time is provided to allow sediments to equilibrate in a batch approach, then the resulting net alkalinity is measurably higher. These data indicate that sediments associated with mine waters play a role in the pH regulation of the system.