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Controlling the Rate of Acid Loss from Strip Mine Spoil¹

ABSTRACT

The objective was to identify and compare processes that potentially dominate acid production and loss at the spoil particle scale. This information is presently lacking, but is critical for correctly hypothesizing and testing the effects of alternative management (reclamation) on ground water quality. The effectiveness of selected diffusion and kinetic processes for controlling the rate of acid product loss from the spoil particle were compared. Initially, the acid production rate in unweathered fresh spoil exposed to 20% O₂ appears controlled by the pyrite oxidation rate. However, the control of acid product loss rates from a partially weathered spoil particle decreased in the order: acid product diffusion > O₂ diffusion > pyrite oxidation.

The acid product contributing zone of the assumed-cylindrical spoil particle was calculated using experimental and published data. For the acid products, i.e., common SO₄ salts, the acid product contributing zone was 0.022-cm depth/0.25-cm particle radius or 17% of total particle volume. For O₂, the zone was 0.06-cm depth/0.25-cm particle radius or 42% of total particle volume. Under these experimental conditions, the remaining particle volume was essentially noncontributing. Based on these data, increasing the particle radius substantially reduced the contributing volume.

Experimental results showed that acid-product generation over the long term averaged 0.16 mg SO₄/g pyrite per hour for the air-incubated system and 0.018 mg SO₄/g pyrite per hour for the O₂-free (N₂) incubated system.

Key Words: coal • pyrite • oxidation • diffusion • acid production • salt production • Pennsylvania • ground water

¹ Contribution from the USDA-SEA-AR in cooperation with the Pennsylvania Agric. Exp. Stn., Pennsylvania State Univ., University Park, Penn. Supported by the EPA-SEA-AR, Interagency Agreement Funds: EPA-IAG-D5-E763.

² Soil Scientist, Soil Scientist, and Mathematican, respectively, USDA-SEA-AR, Northeast Watershed Res. Center, 110 Res. Bldg. A, University Park, PA 16802.

Received for publication December 1, 1979.