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Effects of Nutrients on the Formation of Acidic Mine Drainage

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This project evaluates the effects of water saturation and of different additives, including calcareous compounds, inorganic nutrients, and sewage sludge, on the production of acid and the transport of metals from pyritic, coaly shale. The shale, which was a source of acidic drainage at an active coal mine, was subjected to leaching in a series of laboratory experiments to investigate biogeochemical interactions and water- quality variations associated with the use of calcareous additives and sewage sludge for coal-mine reclamation.

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In general, rates of pyrite oxidation, indicated by transport of dissolved SO⁴ and Fe, were greater under biologically active conditions than under bacteriostatic conditions and were greatest under variably saturated, aerobic conditions. Under discontinuously saturated, aerobic (moist) conditions, effluent pH was 2; under continuously saturated, stagnant (wet) conditions, without and with CaCO₃, pH was 3 and greater than 5, respectively. Under wet conditions, oxidation of pyrite was minimized and Fe-Mn oxide and carbonate minerals decomposed. However, under moist conditions, active oxidation of pyrite released acid, SO₄^2- and Fe³⁺, and the acid aggressively decomposed kaolinite, Fe-Mn oxides, and carbonate minerals, releasing Al³⁺, Mn²⁺, and other metals. Rates of pyrite oxidation and transport of SO₄^2- and metals were reduced by the addition of CaCO₃; production of alkalinity was significant only if CaCO₃ was maintained under wet conditions. The measured pyrite oxidation rates were comparable with those reported for other laboratory tests, but were 10 to 100 times greater than rates estimated for field tests, owing to the high surface areas of small particles (<1-cm diameter) used in laboratory tests.

Positive linear correlations between number of iron-oxidizing bacteria and concentrations of total NH₃, PO₄, SO₄, metals, and acidity in leachate suggest that (1) nitrogen- and/or phosphorus-rich materials added to pyritic shale can encourage the growth of iron-oxidizing bacteria and the oxidation of pyrite, or (2) nitrogen and phosphorus solubility and transport are enhanced under acidic conditions. Because of extreme acidification under variably saturated conditions (pH 1.5-2), mineral sources of nutrients rapidly decomposed producing soluble species, NH₄⁺ and H₃PO₄^o. Nevertheless, additions of sewage sludge and small concentrations of NO3- (0.1 and 1 mmol/L) increased the rate of pyrite oxidation by 30 percent or more, as indicated by significantly higher transport of acidity, SO₄, and Fe during 12 to 18 weeks of leaching relative to untreated controls. However, the addition of 10 mmol/L of NO₃^-, 1 mmol/L of NH₄⁺, H₂PO₄^-, or K⁺, or equivalent amounts of solid N-P-K fertilizer did not increase pyrite oxidation rates. Results for 10 mmol/L of NO₃^- are consistent with reports that indicate high NO₃^- concentrations can be toxic to iron-oxidizing bacteria. For the tests with addition of 1 mmol/L of NO₃^-, decreases in NO₃^- and correlative increases in NH₄⁺ and SO₄^2- in the leachate indicated that pyrite oxidation coupled with reduction of NO₃^- to NH₄⁺ could have occurred in variably saturated columns, but was not significant in continuously saturated columns relative to controls. Microbial catalysis of the NO₃-reduction and pyrite-oxidation reactions was suggested because high concentrations of NO₃^- (=10mmol/L) were inhibitory, producing decreased transport of SO₄^2-, whereas low concentrations of NO₃^- (<1 mmol/L) produced greater rates of SO₄^2- production.

Preliminary results of this project were published in an abstract, "Effects of water saturation and microbial activity on acid production and metals transport from pyritic shale" (Cravotta, C.A. III, and Bird, P.H., 1995, EOS, v. 76, no. 17, p. S149). The complete results, including experimental data and detailed discussion, were reported in "Municipal sludge use in coal-mine reclamation and potential effects on the formation of acidic mine drainage" (Charles A. Cravotta III, 1996, Unpublished Ph.D. thesis, Department of Geosciences, Pennsylvania State University, University Park, Pa 16802).

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