July 1, 2004 | General

Reader's Q & A

BioCycle July 2004, Vol. 45, No. 7, p. 14

Q: “Can the addition of compost/digestate as a mine spoil cover stabilize pH and redox and so prevent oxidation of pyrites? What is the hard evidence for redox and pH effects of compost on the spoil?” That question comes from a BioCycle reader with an environmental technology company in the United Kingdom.
A: The following responses to the question about utilizing compost/digestate as a mine spoil cover were received from researchers across North America. Our thanks to Richard Stehouwer of Penn State University; Sally Brown of the University of Washington; Charles Henry of the University of Washington; Bo Ge and Daryl McCartney of the University of Alberta for their comments and analysis:
Richard Stehouwer, Environmental Soil Science, Penn State University
I have conducted a number of experiments using compost as an amendment for mine spoils. The added organic matter helps to buffer pH, but does not increase pH much unless the compost itself is quite alkaline. Actual pH change would depend on the acidity of the spoil material and alkalinity of the compost. The organic matter in compost definitely decreases phytotoxicity – presumably by complexing Aluminum (Al) and Iron (Fe) and thereby decreasing activity of free.
Al and Fe in soil solution. In a couple of my experiments, just adding compost to very acidic mine spoils (pH 2.5 — 3.5) has allowed plant growth to occur even though pH increases only by about 1 pH unit. I do not think compost will do anything directly to inhibit pyrite oxidation, unless it were to inhibit activity of the sulfur and iron oxidizing bacteria. I have not seen experiments that document that. Conceivably mixing a high BOD organic material with pyritic material might do so by creating anaerobic conditions. (I can imagine a lot of problems with trying to put something like that into practice.) Indirectly, if compost creates a vigorous microbial and plant root community in the surface, oxidation of pyritic material deeper in the profile would be inhibited because the biological activity in the surface layer will consume oxygen and decrease redox potential of water that percolates down.
Sally Brown, University of Washington, Forest Resources, Soils Laboratory, Ecosystem Sciences Division
Charles Henry, University of Washington, Interdisciplinary Arts & Sciences
In general, there is a fair amount of buffering capacity in compost. Digestate – if applied at a significant (greater than agronomic) quantity – will lower the pH through nitrification. In our research at Pack Forest in Washington State, the pH was dropped down to the mid 3s, eventually rising back to the mid 4s. If there is significant pyritic material in a mine spoil, it is imperative that a liming material (in addition to the compost/biosolids) be added, so that over time, it counteracts the effect of the pyrite oxidation.
There may also be a delay with compost as the bacteria that can oxidize the pyrite to form sulfuric acid will have stiff competition from other bacteria. But over time, the material will oxidize and the pH will drop. Compost can delay this, but compost and lime are a much better remedial option.
Bo Ge and Daryl McCartney, Department of Civil & Environmental Engineering, University of Alberta
Mine spoil is comprised of particles in the range 1 mm — 50 mm (and sometimes greater). Spoil heaps are usually loose with large particles falling to the toe (lower edge) of the heap, and finer particles remaining in the upper layers. Due to the greater potential for oxidative dissolution of sulphide minerals such as pyrites in the comparatively shallow, open structures of these deposits, mine spoil is a significant source of acidic mine waters. The quality of waters arising from spoil heaps is often worse than that from deep mines (The Piramid Consortium, 2003).
Mature compost is in the neutral pH range of 6.5 — 7.5 and can be used as a soil conditioner. The addition of compost can raise or lower pH of soil to this range. Compost also helps retain water in sandy soils. The addition of compost/digestate as a mine spoil cover can improve water content in the top soil and development of a vegetative cover across the site. The revegetation of a mine spoil heap provides a suitable soil quality, structure and texture, which will support plant growth in the long-term (ADTI, 1998). Therefore, a good surface cover which inhibits the oxidation of pyrites forms.
The dry surface cover is one of the methods of passive prevention of acid mine drainage. For pyrite-rich mine waste, its basic principle is to minimize the oxygen supply in order to alter redox environment by inhibiting electron acceptors and reduce the weathering of pyrite (ADTI, 1998). As a result, the oxidation of pyrite and production of acid are prevented.
Figure 1 is a Eh-pH (redox potential) diagram. According to Evangelou (1998), Eh-pH values falling within the boundaries of Fe(OH)3 represents acid mine drainage-producing conditions. A particular site represented by such Eh-pH values would be under strong oxidative conditions, and most iron released would likely precipitate as Fe(OH)3. When Eh-pH data fall within the boundaries of FeS2, pyrite would be under stabilizing conditions.
In addition, formation of Fe3+ – organic complexes limits oxidation of pyrite by Fe3+, and specific adsorption of organic materials on the pyrite surface prevents Fe3+, DO, or oxidizing microbes from reaching the pyrite surface (Evangelou, 1998).
According to Acid Drainage Technology Initiative (ADTI, 1998), in 1977, digested and dewatered municipal biosolids were applied to an abandoned surface mine at the rate of 184 Mg/ha (85 tons/ac) on a 0.4 ha plot in Pennsylvania. Data were collected for a 5-year period and the site was reevaluated after 12 years. The results showed that vegetation could be established and that the groundwater quality improved. The state of Pennsylvania has used this technology to revegetate over 2,000 ha (5,000 ac) of mine land.
ADTI. 1998. Handbook of technologies for avoidance and remediation of acid mine drainage. Acid Drainage Technology Initiative. The National Mine Land Reclamation Center, West Virginia University, Morgantown, West Virginia
Evangelou, V. P. 1998. Environmental soil and water chemistry: Principles and application. John Wiley & Sons, Inc., New York, p252.
Geochemistry — GSC 300. n. d. Eh-pH Diagrams I. Retrieved June 24, from Geochem/Eh-pH%20Diagrams%20I.ppt.
The Piramid Consortium. 2003. Engineering guidelines for the passive remediation of acidic and/or metalliferous mine drainage and similar wastewater. University of Newcastle Upon Tyne, U.K. 166pp.

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