Journal of Mining

Volume 2013 (2013), Article ID 698031, 5 pages

http://dx.doi.org/10.1155/2013/698031

Research Article

Nickel and Zinc Removal from Acid Mine Drainage: Roles of Sludge Surface Area and Neutralising Agents

William E. Olds,1,2 Daniel C. W. Tsang,2,3 Paul A. Weber,¹ and Chris G. Weisener⁴

¹Solid Energy New Zealand Ltd, Private Bag 1303, Christchurch 8024, New Zealand

²Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch 8140, New Zealand

³Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

⁴Great Lakes Institute for Environmental Science, University of Windsor, ON, Canada N9B 3P4

Received 28 June 2013; Accepted 26 September 2013

Academic Editor: Yong Sik Ok

Copyright © 2013 William E. Olds et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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 iron oxide at pH 7 [14].

The purpose of this paper is to investigate the importance of the surface area of floc formed during AMD neutralisation by three industrial alkaline reagents (NaOH, Ca(OH)₂, and CaCO₃) for Ni and Zn removal from AMD. As sorption is a solid-liquid interface process, we hypothesise that the sorption capacity of an AMD neutralised floc is proportional to its surface area.

2. Experimental Methods

2.1. Sample Collection

The AMD used in these trials was collected from a fully oxidised stream at the Stockton Coal Mine on the West Coast of the South Island of New Zealand, and stored in 20-L plastic drums. The sample was stored for two weeks to allow full settlement of TSS before the supernatant was carefully decanted off for lab trials. The AMD had a pH of 2.6 (determined in the lab) and an acidity of 815 mg L⁻¹ CaCO₃ equivalent, as determined by titration to pH 7.

2.2. AMD Neutralisation

The AMD neutralisation pathway was determined by titrating AMD with 5-mL increments of 0.1 M NaOH under constant mixing in a 1-L beaker. The acidity (in mg L⁻¹ CaCO₃ equivalent) was determined based on the amount of NaOH required to neutralise the AMD to pH 7.

The AMD was neutralised with Ca(OH)₂ and CaCO₃ in addition to NaOH. A 1 M NaOH stock solution was made using laboratory-grade pelletised NaOH; 1 M stock slurries of 74 g L⁻¹ Ca(OH)₂ (laboratory-grade, >99.5% purity), and 100 g L⁻¹ CaCO₃ (ultrafine limestone from Murchison Lime Works, >99% purity) were used and homogenised using a magnetic stirrer throughout dosing. Stock solutions/slurries of alkaline reagents were used to allow volumetric dosing of AMD (up to 18 mL of slurry per litre of AMD), simulating the slurry dosing conditions in the field at the Stockton Coal Mine.

Jar stirrers were used for neutralisation of the AMD in 1-L beakers, with different mixing regimes for different alkaline reagents. The NaOH neutralised samples were dosed with NaOH followed by a 1-min rapid mixing phase (100 rpm) and a 25-min flocculation phase (20 rpm), as neutralisation by NaOH was instantaneous. By contrast, the Ca(OH)₂ and CaCO₃ neutralised samples were dosed with slurry followed by a 60-min rapid mixing phase (100 rpm). In order to determine residual metal concentrations in the circum-neutral pH range, a range of alkaline reagent doses were used to neutralise AMD to between pH 6 and 7.5 for NaOH and Ca(OH)₂, and between pH 6 and 8 for CaCO₃. Neutralisation by Ca(OH)₂ was essentially instantaneous, while a continued increase in pH (to a maximum pH of 8) was recorded for neutralisation using CaCO₃ up to seven days after dosing. The slow dissolution kinetics of CaCO₃ was also documented in earlier studies [15, 16]. In total, 21 Ca(OH)₂, 15 NaOH, and 7 CaCO₃ neutralised samples were prepared.

At the end of the flocculation/mixing period, the beakers of neutralised AMD were allowed to settle for 2 hours. After settling, a 10 mL supernatant sample was drawn off using a syringe from 10 mm below the water surface, to avoid sampling of any floc remaining in the water column. The sample was filtered (0.45 μm) into a plastic test tube and acidified (pH < 2) using 68% nitric acid before storage in a fridge at 4°C. The major cations (Na, Mg, Al, Mn, and Fe) and trace elements (Ni, Cu, and Zn) were determined using an Agilent 7500cx (California, USA) Inductively Coupled Plasma Mass Spectrometry (ICP-MS) instrument, under standard hot plasma. For quality control, every tenth sample was duplicated and every twentieth was spiked, after which three blanks were run before further analysis. The instrument detection limits were determined using the three-sigma method (multiplying the standard deviation of the metal concentrations of blank samples by three). The experimental detection limits were 100 times the instrument detection limits (as samples were diluted 100x before analysis), corresponding to 0.007 mg L⁻¹ and 0.014 mg L⁻¹ for Ni and Zn, respectively. The Ca concentration of the AMD was determined using the EDTA titrimetric method described in the APHA standard methods textbook [17]. The sulphate concentration was determined by HACH spectrophotometer using Method 8051.

2.3. AMD Precipitate Analysis

After the 10 mL supernatant sample was collected, the remaining supernatant was carefully decanted, leaving only the sludge layer. The sludge from beakers of NaOH, Ca(OH)₂, and CaCO₃ neutralised AMD to pH 6.5 was then poured into separate plastic containers and allowed to dewater by gravity overnight before freeze-drying. The three floc samples (NaOH, Ca(OH)₂, and CaCO₃ neutralised AMD) were analysed for BET surface area at the University of Windsor using a Quantachrome Instruments Nova 1200e Surface Area and Pore Size Analyzer (Florida, USA). Each sample was degassed under vacuum at room temperature for 24 hours prior to nitrogen gas adsorption analysis at liquid nitrogen temperatures.

The precipitate chemical composition was analysed by borate fusion X-ray fluorescence spectrometry (XRF) at CRL Energy Ltd. Solid samples for XRF analysis were dried at 110°C and finely ground in a ring mill. Boric acid backed discs (40 mm diameter by 1–10 mm thick) of ground material were prepared under high pressure and analysed under vacuum. The multi-element XRF scanning procedure (Semiquant) was calibrated using Siemens standard samples containing known concentrations (ISO 17025).

3. Results and Discussion

3.1. AMD Composition and Fe/Al Precipitation

The concentrations of major cations, trace elements, and anions in the AMD are shown on Table 1, which are typical and comparable with previous studies [18, 19].

Table 1: Concentration (mg L⁻¹) of major cations, trace elements, and anions in the acid mine drainage.

Major cations
Trace elements
Anion

Na
Mg
Al
Mn
Fe
Ca
Ni
Cu
Zn

13.1
20.7
86.0
2.6
45.9
37.7
0.75
0.91
4.42
1100

Figure 1 shows the neutralisation pathway of AMD determined by NaOH titration. Two buffer zones were clearly visible at pH ~3.3 (Fe) and pH ~4.5 (Al), which corresponded to the hydrolysis of Fe and Al that resulted in the formation of metal hydroxide precipitates and the release of H⁺ (see (2)). These precipitates provided the solid phase for sorption of Ni and Zn. All AMD in this trial were neutralised to the pH 6–8 range, resulting in removal of 98.9%, and 99.9% of Fe and Al, respectively:

Figure 1: Neutralisation pathway of AMD (determined by 0.1 M NaOH titration).

3.2. AMD Precipitate Properties

The flocs formed by NaOH and Ca(OH)₂ neutralisation of AMD had a surface area an order of magnitude (173.7 and 168.2 m2  g⁻¹, resp.) higher than the CaCO₃ neutralised floc (16.7 m2  g⁻¹). The surface areas of NaOH and Ca(OH)₂ neutralised AMD floc are similar to amorphous Fe and Al hydroxide precipitates formed by other treatment methods (85.4 to 135.4 m2  g⁻¹, [11, 20]). The difference in floc structures between the alkaline reagents was visible by inspection (Figure 2). Flocs formed by NaOH and Ca(OH)₂ neutralisation were fluffy and amorphous in appearance, visually suggesting a low density and a high surface area, contrary to the floc formed by CaCO₃ neutralisation which had a granular appearance as it contained alot of unreacted CaCO₃ particulates (Table 2).

Table 2: Results of major oxide by XRF for Ca(OH)₂ and CaCO₃ neutralised AMD precipitates.

wt.%
Alkaline reagent

Ca(OH)₂
CaCO₃

SiO₂
15.4
3.5

Al₂O₃
30.4
15.2

Fe₂O₃
21.1
9.4

CaO
1.5
33.7

SO₃
4.9
3.6

Na₂O
0.3
0.1

LOI
29.1
34.0

Figure 2: Fluffy Ca(OH)₂ floc compared to dense CaCO₃ floc.

The XRF analysis of the sludge samples (Table 2) showed that 21 wt.% and 30 wt.% of the Ca(OH)₂ neutralised AMD sludge was Fe and Al oxides, respectively, which were recognised as the active sites for metal sorption [8, 10, 11]. Only 9 wt.% and 15 wt.% of the CaCO₃ neutralised AMD sludge were Fe and Al oxides, respectively, due to significant unreacted CaCO₃ bulking out the sludge. Relatively high loss-on-ignition (LOI) values were reported, probably reflecting the amount of CaCO₃ and physisorbed water. The NaOH neutralised sludge was assumed to be compositionally similar to the Ca(OH)₂ sludge as the neutralisation efficiencies for both NaOH and Ca(OH)₂ were close to 100%.

3.3. Residual Metal Concentration

3.3.1. Ni Sorption

Figure 3 shows that Ni concentration was lower at a given pH for NaOH and Ca(OH)₂ neutralised AMD compared to CaCO₃ neutralised AMD. For example, at pH 6.5, Ni concentration was 0.32 and 0.41 mg L⁻¹ for NaOH and Ca(OH)₂ neutralised AMD, respectively. This was up to 60% lower than the CaCO₃ neutralised Ni concentration of 0.8 mg L⁻¹ at the same pH. A relatively linear trend between pH and Ni concentration was shown for all three alkaline reagents in Figure 3. This was probably because the potential of Ni sorption and coprecipitation increased proportionally with increasing surface deprotonation of the AMD sludge at pH above the point of zero charge [6, 21, 22]. Over the range of pH 6 to 8, Ni concentration after CaCO₃ neutralisation was approximately 0.4 mg L⁻¹ higher than that of the other two alkaline reagents. Since the surface area of CaCO₃ neutralised floc was an order of magnitude less per unit mass, this suggests that the greater surface area of flocs produced by NaOH and Ca(OH)₂ neutralised AMD increased the removal of Ni.

Figure 3: Acid mine drainage Ni concentration (initial = 0.8 mg L⁻¹) and concentration after neutralisation by NaOH, Ca(OH)₂, and CaCO₃.

3.3.2. Zn Sorption

Figure 4 shows that the sorption of Zn (initial concentration of 4.42 mg L⁻¹) was even more dependent than Ni on neutralisation reagent. For example, at pH 6.5, Zn concentration was 0.33 and 1.02 mg L⁻¹ in the NaOH and Ca(OH)₂ neutralised AMD. This was 85% and 54% lower (for NaOH and Ca(OH)₂, resp.) than Zn concentration of 2.20 mg L⁻¹ in the CaCO₃ neutralised AMD at the same pH. The clustering of data points at pH 6 suggests that about half of Zn removal occurred irrespective of the neutralisation reagent used. Between pH 6 and 6.5, the data diverged with a residual Zn concentration hierarchy of NaOH neutralised < Ca(OH)₂ neutralised < CaCO₃ neutralised. This may correspond to the saturation of the limited sorption sites on CaCO₃ neutralised precipitates, while the greater surface area of NaOH and Ca(OH)₂ precipitates allowed Zn sorption and coprecipitation to continue [8, 21].

Figure 4: Acid mine drainage Zn concentration (initial = 4.42 mg L⁻¹) and concentration after neutralisation by NaOH, Ca(OH)₂, and CaCO₃.

3.3.3. Calcium Competition

The difference in Zn removal from NaOH and Ca(OH)₂ neutralised AMD at a given pH may also result from the effect of background cations. Neutralisation of AMD using NaOH and Ca(OH)₂ was found to result in the release of significant quantities of Na and Ca (~400 mg L⁻¹), respectively. Dissolved Ca complexation with hydroxyl groups was potentially significant given the elevated Ca concentrations after neutralisation [23]. Thus, competition for binding sites on AMD precipitates from Ca may be responsible for the reduction in Zn removal when comparing NaOH and Ca(OH)₂ neutralisation systems (Figure 4). In the case of CaCO₃ neutralisation, the combination of low precipitate surface area and Ca competition for sorption sites led to significantly lower sorption of Ni and Zn.

There was a minor difference in NaOH and Ca(OH)₂ neutralised AMD Ni concentrations, which suggests Ca competition does not affect Ni removal. This may be due to the relatively low Ni concentration in AMD, and the relatively low tendency for Ni to adsorb onto the precipitates rendering Ca competition for binding sites negligible.

4. Conclusions

The removal of Ni and Zn by sorption onto AMD precipitates is influenced by the surface area of floc formed during neutralisation. Neutralisation of AMD by NaOH and Ca(OH)₂ produces large fluffy floc, with surface areas, an order of magnitude greater than floc formed by CaCO₃ neutralisation. As a result, significantly more Ni and Zn were sorbed and coprecipitated on the NaOH and Ca(OH)₂ floc. Moreover, competition for sorption sites from the Ca released during Ca(OH)₂ dissociation appears to reduce the removal of Zn when compared to NaOH neutralised AMD. These results account for the lower Ni and Zn concentrations in water discharged from treatment systems using NaOH or Ca(OH)₂ as opposed to CaCO₃, which is an important consideration when selecting alkaline reagents for AMD treatment.

Acknowledgment

The authors thank the Foundation for Research Science and Technology Fund for the financial support of this study under contract SENZ0901.

References

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.

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.

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.

F. Morel, Principles of Aquatic Chemistry, John Wiley & Sons, 1983.

M. Kelly, Mining and the Freshwater Environment, Elsevier Applied Science, Durham, UK, 1988.

P. L. Younger, S. A. Banwart, and R. S. Hedin, Mine Water: Hydrology, Pollution, Remediation, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002.

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.

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.

B. G. Lottermoser, Mine Wastes: Characterization, Treatment and Environmental Impacts, Springer, Cairns, Australia, 2003.

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.

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.

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.

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.

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.

E. L. Sjöberg 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.

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.

American Public Health Association (APHA), Standard Methods for the Examination of Water & Wastewater, Washington, DC, USA, 2005.

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.

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.

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.

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.

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.

V. L. Snoeyink and D. Jenkins, Water Chemistry, John Wiley & Sons, 1980.