Effect of neutralization on pH and heavy metals content of acid mine drainages
Alkaline materials are used for treatment of acidic water, including acid mine drainage (AMO), primarily to increase its pH. The efficacy of this treatment, however, depends on several factors. In this study, the significance of dissolved metals, in particular Fe3+ and Al3 +, in neutralization effic...
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| Format: | Thesis |
| Language: | English English |
| Published: |
2017
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| Subjects: | |
| Online Access: | https://eprints.ums.edu.my/id/eprint/38502/1/24%20PAGES.pdf https://eprints.ums.edu.my/id/eprint/38502/2/FULLTEXT.pdf |
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| Summary: | Alkaline materials are used for treatment of acidic water, including acid mine drainage (AMO), primarily to increase its pH. The efficacy of this treatment, however, depends on several factors. In this study, the significance of dissolved metals, in particular Fe3+ and Al3 +, in neutralization efficiency was the main focus of investigation. In the case of AMO, the dissolved metals can also be represented by the parameter total acidity. Batch neutralization experiments using two alkaline materials, namely NaOH and calcareous sandstone, were carried out involving acidic aqueous solutions, without and with the presence of Fe3 + and/or Al3+ at concentration range 2mg/L-100 mg/L, as well as seven AMO samples collected from Mamut, Ranau. The physico-chemical characteristics of these AMO samples were analyzed according to APHA Standard Methods. The neutralization process was also studied at different dosage of calcareous sandstone and under the condition where this material is repeatedly exposed to acidic aqueous solutions in the presence of Fe3 + and/or Al3 + as well as to selected AMD samples. The principal parameter monitored and measured in these experiments was solution pH. Additionally, the effect of neutralization using both alkaline materials on dissolved metals concentration of selected AMO samples was also investigated. The final concentration of Fe, Al, Cu, Zn and Mn in solution was determined using ICP-OES. The results showed that the AMDs from Mamut, Ranau have varying pH (2. 77- 3.36), TDS (741-1485 mg/L), EC (1489-2975 μS/cm), total acidity (316-807 mg CaCO3/L), sulfate (649-2148 mg/L), and concentration of Fe (0.34-8.93 mg/L), Al (35.59-111.24 mg/L), Mn (6.82-30.88 mg/L), Cu (3.60-26.99 mg/L) and Zn (1.77- 9.10 mg/L). The total acidity of the samples was positively correlated (R2 =0.968) with dissolved Al concentration. During neutralization of aqueous acidic solutions with NaOH, increasing concentration of Fe3+ and/or Al3 + in solution resulted in increasing amount of base required to increase the pH to 7.0. At the same metal concentration (mg/L), the presence of Al3 + requires about 2X more alkalinity for this purpose compared with Fe3+ . When calcareous sandstone was used as the alkaline material, the presence of Al3 + in solution slowed down the rate of increase in pH particularly at high concentration. Such effect was not evident in the presence of Fe3+ . When AMO samples were used, the amount of alkalinity (NaOH) required to increase the pH to 7 .0 increased with increase in dissolved Al concentration as well as total acidity of the AMO. When calcareous sandstone was used as the alkaline material, the rate of increase in pH was slow and vary between AMDs. The pH attained after 8 hours neutralization is dependent (R2 =0.9415) on the total acidity whereby the value decreased with increase in total acidity of AMO. The effectiveness of calcareous sandstone in the neutralization process increased with dosage in the order 1.0 g > 0.5 g > 0.1 g, while at a fixed dosage (0.5g) decreased rapidly during successive 24h neutralization cycles when involving aqueous acidic solution with high concentration of Al3 + as well as AMO samples with high total acidity. Meanwhile, neutralization of AMO using NaOH resulted in effective removal of heavy metal from solution. This removal was pH dependent and occurred sequentially starting from Fe (pH~4.0) followed by Al (pH~5.0), Cu (pH~7.0), Zn (pH~8.0) and Mn (pH~lO.0). This removal also produced precipitates of which the amount increased with increased in pH attained as well as with increase in total acidity of AMD. By contrast, the effectiveness of neutralization using calcareous sandstone in removing heavy metals from solution decreased with increased in total acidity of AMD. Overall, the significant presence of dissolved metals in particular at high concentration of Fe3+ and Al3+ can reduce the effectiveness of neutralization treatment, either using alkaline solution (NaOH) or alkaline generating material (calcareous sandstone), in increasing pH. Consequently, this can reduce the effectiveness of the treatment in removing heavy metals from AMD. Therefore, besides the initial pH, another parameter that need to be considered during treatment of AMD by alkaline materials is the total acidity of the AMD. |
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