During acid mine drainage (AMD) treatment by alkaline reagent neutralisation, Ni and Zn are partially removed via sorption to Fe and Al hydroxide precipitates. This research evaluated the effect of surface area of precipitates, formed by neutralisation of AMD using three alkalinity reagents (NaOH, Ca(OH)2, and CaCO3), on the sorption of Ni and Zn. The BET surface area of the precipitates formed by neutralisation of AMD with NaOH (173.7？m2？g？1) and Ca(OH)2 (168.2？m2？g？1) was an order of magnitude greater than that produced by CaCO3 neutralisation (16.7？m2？g？1). At pH 6.5, the residual Ni concentration was 0.32 and 0.41？mg L？1 for NaOH and Ca(OH)2 neutralised AMD, respectively, resulting in up to 60% lower Ni concentrations than achieved by CaCO3 neutralisation which had no effect on Ni removal. The residual Zn concentration was even more dependent on precipitate surface area for NaOH and Ca(OH)2 neutralised AMD (0.33 and 1.02？mg？L？1), which was up to 85% lower than the CaCO3 neutralised AMD (2.20？mg？L？1). These results suggest that the surface area of precipitated flocs and the selection of neutralising reagent critically affect the sorption of Ni and Zn during AMD neutralisation. 1. Introduction Acid mine drainage (AMD) is one of the major environmental impacts of coal mines that disturb pyritic overburden on the West Coast of the South Island of New Zealand [1]. The formation of acidity associated with AMD is illustrated by As part of AMD treatment, acidity neutralisation to pH 6-7 by alkaline reagent decreases the solubility of Fe and Al, resulting in Fe and Al hydroxide precipitation and removal by sedimentation [2, 3]. However, potentially ecotoxic metals (e.g., Ni, Zn, Cu, Pb, and Cd) require a further increase in pH (to 8 or above) to precipitate as metal hydroxides [4–6]. Incomplete removal and resultant discharge of residual metals may affect downstream freshwater fish assemblages [7]. Previous studies have reported removal of Ni and Zn during AMD neutralisation, where these metals were removed to varying degrees via coprecipitation and sorption onto Fe and Al hydroxide precipitates [8–11]. Davies et al. [10] showed a negative correlation between dissolved Zn concentration and Zn incorporation into neutralised AMD floc. Batch experiments showed that, at pH 7 and for ratios of Zn to sorbent of 1？:？10 to 1？:？100, between 70% and 90% of Zn was removed from AMD due to sorption to precipitated metal oxides [12, 13]. A similar trend was shown for Ni removal (at a similar sorbent to metal concentration ratio) where 66% of Ni was adsorbed to hydrous
References
[1]
J. Pope, N. Newman, D. Craw, D. Trumm, and R. Rait, “Factors that influence coal mine drainage chemistry West Coast, South Island, New Zealand,” New Zealand Journal of Geology and Geophysics, vol. 53, no. 2-3, pp. 115–128, 2010.
[2]
D. B. Johnson and K. B. Hallberg, “Acid mine drainage remediation options: a review,” Science of the Total Environment, vol. 338, no. 1-2, pp. 3–14, 2005.
[3]
D. Trumm, “Selection of active and passive treatment systems for AMD—flow charts for New Zealand conditions,” New Zealand Journal of Geology and Geophysics, vol. 53, no. 2-3, pp. 195–210, 2010.
[4]
F. Morel, Principles of Aquatic Chemistry, John Wiley & Sons, 1983.
[5]
M. Kelly, Mining and the Freshwater Environment, Elsevier Applied Science, Durham, UK, 1988.
[6]
P. L. Younger, S. A. Banwart, and R. S. Hedin, Mine Water: Hydrology, Pollution, Remediation, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002.
[7]
H. S. Greig, D. K. Niyogi, K. L. Hogsden, P. G. Jellyman, and J. S. Harding, “Heavy metals: confounding factors in the response of New Zealand freshwater fish assemblages to natural and anthropogenic acidity,” Science of the Total Environment, vol. 408, no. 16, pp. 3240–3250, 2010.
[8]
G. Lee, J. M. Bigham, and G. Faure, “Removal of trace metals by coprecipitation with Fe, Al and Mn from natural waters contaminated with acid mine drainage in the Ducktown Mining District, Tennessee,” Applied Geochemistry, vol. 17, no. 5, pp. 569–581, 2002.
[9]
B. G. Lottermoser, Mine Wastes: Characterization, Treatment and Environmental Impacts, Springer, Cairns, Australia, 2003.
[10]
H. Davies, P. Weber, P. Lindsay, D. Craw, B. Peake, and J. Pope, “Geochemical changes during neutralisation of acid mine drainage in a dynamic mountain stream, New Zealand,” Applied Geochemistry, vol. 26, no. 12, pp. 2121–2133, 2011.
[11]
M. Cui, M. Jang, S.-H. Cho, J. Khim, and F. S. Cannon, “A continuous pilot-scale system using coal-mine drainage sludge to treat acid mine drainage contaminated with high concentrations of Pb, Zn, and other heavy metals,” Journal of Hazardous Materials, vol. 215-216, pp. 122–128, 2012.
[12]
M. F. Schultz, M. M. Benjamin, and J. F. Ferguson, “Adsorption and desorption of metals on ferrihydrite: reversibility of the reaction and sorption properties of the regenerated solid,” Environmental Science and Technology, vol. 21, no. 9, pp. 863–869, 1987.
[13]
J. G. Webster, P. J. Swedlund, and K. S. Webster, “Trace metal adsorption onto an acid mine drainage iron(III) oxy hydroxy sulfate,” Environmental Science and Technology, vol. 32, no. 10, pp. 1361–1368, 1998.
[14]
Y. Xu, L. Axe, T. Boonfueng, T. A. Tyson, P. Trivedi, and K. Pandya, “Ni(II) complexation to amorphous hydrous ferric oxide: an X-ray absorption spectroscopy study,” Journal of Colloid and Interface Science, vol. 314, no. 1, pp. 10–17, 2007.
[15]
E. L. Sj？berg and D. T. Rickard, “Calcite dissolution kinetics: surface speciation and the origin of the variable pH dependence,” Chemical Geology, vol. 42, no. 1–4, pp. 119–136, 1984.
[16]
M. Alkattan, E. H. Oelkers, J.-L. Dandurand, and J. Schott, “An experimental study of calcite and limestone dissolution rates as a function of pH from -1 to 3 and temperature from 25 to 80°C,” Chemical Geology, vol. 151, no. 1–4, pp. 199–214, 1998.
[17]
American Public Health Association (APHA), Standard Methods for the Examination of Water & Wastewater, Washington, DC, USA, 2005.
[18]
C. A. McCauley, A. D. O'Sullivan, P. A. Weber, and D. Trumm, “Variability of stockton coal mine drainage chemistry and its treatment potential with biogeochemical reactors,” New Zealand Journal of Geology and Geophysics, vol. 53, no. 2-3, pp. 211–226, 2010.
[19]
W. E. Olds, D. C. W. Tsang, and P. Weber, “Acid mine drainage treatment assisted by lignite-derived humic substances,” Water, Air, & Soil Pollution, vol. 224, pp. 1521–1533, 2013.
[20]
P. R. Adler and P. L. Sibrell, “Sequestration of phosphorus by acid mine drainage floc,” Journal of Environmental Quality, vol. 32, no. 3, pp. 1122–1129, 2003.
[21]
R. J. Crawford, I. H. Harding, and D. E. Mainwaring, “Adsorption and coprecipitation of single heavy metal ions onto the hydrated oxides of iron and chromium,” Langmuir, vol. 9, no. 11, pp. 3050–3056, 1993.
[22]
J. G. Webster, “Chemical processes affecting trace metal transport in the Waihou River and estuary, New Zealand,” New Zealand Journal of Marine and Freshwater Research, vol. 29, no. 4, pp. 539–553, 1995.
[23]
V. L. Snoeyink and D. Jenkins, Water Chemistry, John Wiley & Sons, 1980.
