Available online at ScienceDirect. Procedia Environmental Sciences 30 (2015 )

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1 Available online at ScienceDirect Procedia Environmental Sciences 30 (2015 ) International Conference on Environmental Forensics 2015 (ienforce2015) Passive treatment of acid mine drainage using mixed substrates: batch experiments Siti Nurjaliah Muhammad a, Faradiella Mohd Kusin a,b, *, Mohd Syakirin Md Zahar a, Normala Halimoon a and Ferdaus Mohamat Yusuf a,b a Department of Environmental Sciences, Faculty of Environmental Studies, Universiti Putra Malaysia, UPM Serdang, Malaysia b Environmental Forensics Research Centre, Faculty of Environmental Studies, Universiti Putra Malaysia, UPM Serdang, Malaysia Abstract Treatment of acid mine drainage (AMD) has been tested in batch experiments to evaluate the efficiency of both single and mixed substrates for mine-impacted waters. Synthetic mine water was used in all experiments. The single treatment was performed using limestone (LS), activated sludge (AS), and spent mushroom compost (SMC). In addition, different ratios of treatment media combined as mixed substrates were also tested for AMD in an anoxic condition. Physico-chemical parameters and heavy metals (Mn, Fe, Cu, Pb, Zn) were analyzed throughout 120h. Hence, SMC showed great contribution in the process of AMD treatment in this study. The results showed that the mixed substrates were effective in removing heavy metals and sulfate i.e. uncrushed limestone mixed with AS, SMC and woodchips at 12 hours contact time with 88.15% of removal efficiency The Authors. Published by Elsevier B.V B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of organizing committee of Environmental Forensics Research Centre, Faculty of Environmental Studies, Peer-review Universiti Putra under Malaysia. responsibility of organizing committee of Environmental Forensics Research Centre, Faculty of Environmental Studies, Universiti Putra Malaysia. Keywords: Acid mine drainage (AMD); passive treatment; batch tests; heavy metals; spent mushroom compost (SMC) 1. Introduction Acid mine drainage (AMD) can be characterized by an acidic solution that contains high concentration of heavy metals high level of sulfate, and high amount of suspended solids [1]. Acid mine drainage (AMD) is formed when sulfide minerals in rocks are exposed to oxidizing conditions that produce acidity and sulfate. The weathering reactions are accountable for releasing heavy metals into both groundwater and surface water [2]. Acid mine drainage (AMD) is a worldwide problem that can lead to ecological destruction in watersheds and the contamination of human water sources by sulfuric acid and heavy metals such as arsenic, copper, and lead. Once acid-generating rock is crushed and exposed to oxygen and the surface environment, acid generation is very difficult to contain or stop, and can continue for tens or thousands of years until the available sulfide minerals are exhausted [3]. There are various methods for treating mine-impacted waters. One of the methods is passive treatment which is an economical and low maintenance technology for AMD treatment [4,5]. Bacterial sulfate reduction (BSR) has been a common mechanism for * Corresponding author. Tel.: ; fax: address: faradiella@upm.edu.my The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of organizing committee of Environmental Forensics Research Centre, Faculty of Environmental Studies, Universiti Putra Malaysia. doi: /j.proenv

2 158 Siti Nurjaliah Muhammad et al. / Procedia Environmental Sciences 30 ( 2015 ) AMD treatment of acidic, sulfate and metal-containing mine water. Various treatment media have been used in conjunction with the treatment that incorporates the use of sulfate reducing bacteria (SRB). Furthermore, it is necessary to understand the needs of the bacteria used in the treatment for the selection of substrates that best complement the treatment mechanisms. Most SRBs need almost-neutral ph, an appropriate nutrient source, a solid matrix on which they can develop and survive in critical conditions. A suitable treatment medium for the so-called bioreactor treatment is a medium with large pore spaces, low surface area, and a small void volume. The characteristics of the medium are preferred because it reduces the possibility of clogging of the bioreactor [6]. Sulfate-reducing bacteria (SRB) may use a wide range of substrates as electron donors and carbon sources, which oxidize incompletely (to acetate) or thoroughly (to CO 2 ). These substrates are generally organic compounds composed of activated sludge, wood chips, farm manure, sawdust, mushroom compost, and other agricultural wastes [7]. This study aims to evaluate the efficiency of each treatment medium and the potential use of mixed substrates as treatment media for AMD. 2. Methodology 2.1 Preparation of synthetic AMD Synthetic mine water was prepared in acidic condition (ph 3.06) with high concentration of sulfate (1600 mg/l) and heavy metals such as manganese, Mn (22.4 mg/l), iron, Fe (3.816 mg/l), copper, Cu (3.773 mg/l), lead, Pb (1.687 mg/l), and zinc, Zn (0.876 mg/l). The synthetic mine water solution was prepared using analytical grade salts which is dissolved in distilled water and the ph was controlled using concentrated HCl [3,6].The characteristics of synthetic AMD prepared were within the range of Mamut former mining ponds [8] and were compared with the Ministry of Health (MOH) [9], Malaysia standard limit of untreated raw water [10]. 2.2 Batch experiments In batch experiments, the treatment media has been single-tested to know the capability of each medium to treat the AMD. The batch tests were performed by using different treatment media such as crushed limestone (CLS) and uncrushed limestone (LS) for alkalinity generation [11,12] activated sludge (AS) as the source of bacteria, and spent mushroom compost (SMC) as an electron donor to feed the SRB in synthetic mine water [6]. In addition, wood chips (WC) were used to increase the permeability of media in the bioreactor [11]. Moreover, different ratios of all treatment media combined as mixed substrates were also tested. The best proportion mix of substrates would be selected at the end of experiment. An amount of 450g of the treatment media were single-tested in different 2000ml beakers and were added by 1500ml of AMD. The proportions were divided for two different types of limestone for instance, crushed limestone with the size of 1-2cm 3 and uncrushed limestone with the size of 4-5cm [12]. For ratio 1, the treatment media that have been combined were 50% limestone, 20% SMC, 20% AS, and 10% woodchips; whereas 40% limestone, 30% SMC, 20% AS, and 10% wood chips were mixed for ratio 2. The experimental set-up was in anoxic condition in order to treat the contaminated water that contains high concentrations of heavy metals and sulphate [12,13,14]. Parameters measured were ph, temperature, conductivity, redox potential (Eh), total dissolved solids (TDS), total organic carbon (TOC), alkalinity, and heavy metals (Zn, Pb, Cu, Fe, Mn); measured at the start of experiment, 1h, 7h, 12h, 24, 72h, and 120h [6,12,15]. 3. Results and discussion 3.1 Heavy metals removal Generally, Mn removal is considered as a slow process [16]. Manganese removal by using single treatment medium (SM) showed slightly different results for both types of limestone; LS showed its potential to reduce manganese to 6.20 mg/l at ph 7.35 compared to 6.58 mg/l by CLS at ph 7.26 at 120h. Manganese is known among the toughest elements to be removed from acid mine drainage. This is due to its property of high solubility in both neutral and acidic condition [17]. Meanwhile, AS showed the highest reduction in the end of experiment of single medium treatment with 0.50 mg/l. Based on Fig.1 (a), Mn removal for all ratios of mixed substrates (MS) are not stable until they reach 7h retention time and gradually drop until 120h. All mixed substrates show satisfactory reduction while LSR1 has the highest reduction from 22.4 mg/l to 0.56 mg/l with ph 7.07 at 120h. It is generally known that to remove manganese by means of oxidation, the ph must be above 9 or 10 which is in highly alkaline state [18]. Therefore, further removal of Mn may be observed if greater ph level during the batch test can be achieved. It is also known that Mn removal can be resulted through adsorption processes. In addition, the reduction of Mn is usually associated with the reduction of Fe. Referring to Fig.1 (b), the concentration of Fe is not stable at the start of experiment and it drops at 7h for all media in the range of mg/l. Then it slightly increase but it still shows some reduction from the initial concentration of AMD. LSR1 showed steady decrease in Fe throughout 120h even though there was interference between Mn and Fe. The interference might be due to the removal of Mn and Fe that require the

3 Siti Nurjaliah Muhammad et al. / Procedia Environmental Sciences 30 ( 2015 ) same materials which are oxygen and hydroxide. Fe will react efficiently with oxygen even though oxygen is not strictly necessary for its removal and Fe will deprive the oxygen from Mn [16]. Single medium treatment showed that limestone also played an important role as it can help in removing dissolved iron [19]. Even though Fe reduction by limestone shows a slower process than the other two media (i.e. SMC and AS), Fe sharply drop for crushed limestone at 7h from 4.42 mg/l to 0.62 mg/l with neutral ph and gradually decrease until the end of experiment. On the other hand, the increment of the Fe in all mixed substrates at 72h (Fig.1(b)) show Fe remobilization after 24h since ferrous iron, Fe(II) is soluble in water regardless of ph level [20]. Despite increment at 72h, Fe drops at 120h for all mixed substrates in the range of 0.84 to 1.32 mg/l except for LSR2 which increase up to 2.50 mg/l. Therefore, the treatment of Fe could be best achieved around 12-24h retention time. Cu in single media treatment showed some reductions at the start of experiment from original concentration of Cu in synthetic AMD. All single media continued to decrease Cu except for SMC at 1h. Obviously, SMC increased the concentration of all heavy metals except for Mn at 1h. This might be due to organic matter contained in SMC which has unstable behaviour at early stage of the test. The total organic carbon (TOC) content became 1529 mg/l at 0h, increased from initial concentration of mg/l. The SMC released organic carbon faster than other media. The TOC concentration for SMC at 7h dropped to 1185 mg/l whilst it showed the increment of copper concentration in SMC jar test. The concentration of Cu fell drastically in single media (i.e. LS, CLS, SMC, and AS) but it showed gradual decrease in AS. Even though SMC and AS showed steady reduction of Cu at 12h onwards, Cu was still high in both types of limestone and slowly decreased at 24h with 2.14 mg/l and 2.35 mg/l for CLS and LS respectively. Further, they started to drop at 72h with 0.41 mg/l for CLS and 0.13 mg/l for LS. Naturally, limestone acts as alkalinity generator but it can help the removal of heavy metals from AMD in small amounts indirectly. In the mixed substrates (MS) treatment as shown in Fig.1 (c), all the MS show reduction of Cu at 0h, however Cu in CLSR1 and LSR2 goes up at 1h which is between 1.64 to 2.18 mg/l but still below the initial concentration of mg/l. Then, the Cu in all the MS decrease sharply at 7h and gradually decrease until 120h. The concentration of Cu is very low until they reach 24h retention time. Despite this, the Cu concentration, still exceeds the standard limit allowed by the Ministry of Health (MOH), Malaysia [9] which is 1 mg/l. Meanwhile, all SM showed reduction in Pb at 7h with the range of to mg/l except for AS with mg/l. The SM showed that the suitable retention time was 24h as the value of Cu for AS went down to below detection limits. However, the reduction of Pb in MS (Fig.1 (d)) show unstable pattern but as they reach 12h contact time, the concentration almost reach below MOH permissible limit which is 0.1mg/L. After 12h of contact time, majority of the MS show Pb below detection limits. Henceforth, Zn is known as an ecologically toxic metal from abandoned mines [21], and it is present in the synthetic mine water at very low concentration which is already below the standard limit of MOH, i.e. lower than 5mg/L. Nevertheless, the removal of Zn can still occur until it reaches 24h for all treatment media (Fig.1 (e)). Apparently, the concentration of Zn was reduced as early as 0h for both single media and mixed substrates. Based on MOH [9] drinking water quality standard, the maximum concentration of Zn that is suitable for drinking water is about 3 mg/l. 3.2 Treatment performance In the single medium treatment, it was found that activated sludge was effective in removing most of the heavy metals with 97.98% of removal efficiency. Basically, SMC which was effective in sulfate removal also has the best alkalinity generation that has been discovered in this study with 95% sulphate removal efficiency. Limestone can help in reducing Mn and Fe because the slower reaction rate with lime caused by the dissolution tends to make Mn or Fe oxidized efficiently. Furthermore, mixed substrate that has the best performance is uncrushed limestone with ratio 2 (LSR2). Despite small reduction in Fe, LSR2 can reduce most of the heavy metals and excellent in reducing sulfate. As discussed earlier, the Fe has slower reduction due to the interference with Mn. Even though LSR1 has the greatest reduction in all heavy metals, it could not achieve half of the removal percentage for sulfate. Thus, the combination of all treatment media in this study can help reduce the heavy metals and sulfate in acid mine drainage at appropriate proportion but also constrained by some constituents behaviour in the water [22,23,24].

4 160 Siti Nurjaliah Muhammad et al. / Procedia Environmental Sciences 30 ( 2015 ) Fig. 1. Removal of heavy metals for each element (a) Manganese (b) Iron, (c) Copper, (d) Lead and (e) Zinc using mixed substrates (MS) in synthetic AMD shows significant reduction in the period of 120h 4. Conclusion In a conclusion, the batch test that aims to evaluate the efficiency of each treatment medium has found that spent mushroom compost (SMC) has the best potential for both heavy metals and sulfate removal with overall removal efficiency of 89.98%. Hence, after all the treatment media have been mixed, the SMC which was good in reducing sulfate helps the overall treatment in mixed substrates. In fact, activated sludge also showed its potential for heavy metal removal but not in sulfate removal with 97.98% and 43.75% of removal efficiency respectively, which made up the AS to fall below SMC with only 88.94% for overall removal efficiency. However, AS also contributes in the process of AMD treatment. The potential use of mixed substrates as

5 Siti Nurjaliah Muhammad et al. / Procedia Environmental Sciences 30 ( 2015 ) treatment media for AMD has been shown by uncrushed limestone ratio 2 (LSR2) i.e. less than 5cm in size with overall removal efficiency of 88.15%. Despite small reduction in Fe, LSR2 can reduce most of the heavy metals and excellent in reducing sulfate. As discussed earlier, Fe has slower reduction rate possibly due to the interference with Mn in the water. The mixed substrates had been tested for various parameters to ensure the overall reduction in AMD. This experiment has been done in anoxic condition, which allows limited oxygen to be present. Selected substrates and retention time that have been identified in this study will be used in the following column experiment for better evaluation of the results. Acknowledgements The authors gratefully acknowledge the technical assistance by the Department of Mineral and Geology, Perak and Imerys Minerals Malaysia Sdn. Bhd. 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