Acid mine drainage treatment by integrated submerged membrane distillation–sorption system
Graphical abstract
Introduction
The formation of acid mine drainage (AMD) is a natural process attributed to the oxidation of sulfide minerals such as pyrites (Kalin et al., 2006; Mosley et al., 2018). Active and abandoned mines intensifies the formation of AMD due to open pits, mining waste rock, structures and tailings that are exposed to water, air and bacterial activity (Kalin et al., 2006; Mosley et al., 2018; Tolonen et al., 2014). AMD is characterized by low pH and high concentration of sulfate, as well as high concentrations of heavy metal activity (Kalin et al., 2006; Mosley et al., 2018; Tolonen et al., 2014). Nearby water streams are susceptible to AMD infiltration, resulting in discoloration of streams, decrease in pH and accumulation of heavy metals. In Australia, there are a significantly high amount of abandoned mines (more than 50,000 mines) compared to actively operating mines (around 380 mines) (Parbhakar-Fox et al., 2014; Unger et al., 2012). An estimated total land area of 215,000 km2around coastlines and inlands in Australiacontain acid sulfate soils attributed to AMD (Fitzpatrick et al., 2009). The long-term impact of AMD contaminant on aquatic organisms, plant growth and human health is a significant concern, which necessitates AMD treatment (Mosley et al., 2018).
Conventionally, AMD is treated by using alkaline neutralizing chemicals such as caustic soda or limestone, to elevate the pH and precipitate metals (Tolonen et al., 2014). Although efficient, precipitation results in large volumes of sludge containing heavy metals that require safe disposal (Marcello et al., 2008). Various other active and passive remediation approaches such as bioremediation, wetlands, adsorption, phytoremediation are also used to treat AMD (Zhang, 2011; Vasquez et al., 2016; Crane and Sapsford, 2018). In this regard, the uptake of heavy metals by low-cost sorbents are especially promising as a cost effective treatment method for AMD.
In Australia, naturally occurring zeolites are available in large quantities at relatively low cost (Santiago et al., 2016). A significant advantage of zeolite is its tendency to adsorb cations. The ion exchange affinity of natural and synthetic zeolites for metal extraction from wastewater solution including acid mine drainage has been described by previous studies (Motsi et al., 2009; Rios et al., 2008; Wingenfelder et al., 2005). Castle Mountain, Australiaproduces a natural clinoptilolites (An et al., 2011). The uptake of heavy metals from AMD by Australian natural clinoptilolites may offer a low cost treatment option for AMD. In this regard, a number of approaches are used to enhance the sorption capacity of natural zeolite such as heat treatment, surface and chemical modification (Motsi et al., 2009; Taffarel and Rubio, 2010; Turner et al., 2000). Motsi et al. (2009) reported on the enhanced heavy metal removal of natural zeolite upon microwave and furnace heat treatment. Although there is a tradeoff of energy requirement with heat treatment, it is still advantageous given that it requires no additional chemicals and complex processes. Heat treatment at appropriate temperature ranges improves adsorption capacity of zeolite attributed to factors such as enhanced pore volume and channel vacancy. According to Margeta et al. (2013), around 10–25% of the entire zeolite mass constitutes of water in the framework of the pore cages and channels, and therefore, reducing water contents increases the surface area for zeolite adsorption.
Compared to the conventional approach of treat and discharge, more focus is now being placed on achieving water reuse for AMD treatment. Therefore, membrane processes are becoming favourable AMD treatment options. This is especially reflected by the increase in the implementation of membrane treatment processes such as reverse osmosis (RO) and nanofiltration (NF) at actual mining sites (Aguiar et al., 2016; Ambiado et al., 2017). Although NF and RO do meet good water reuse standards, membrane fouling and low recovery rate remain challenges. In view of this, recent studies are exploring the potential of alternative membrane processes such as electrodialysis, and forward osmosis for AMD treatment. For instance, Martí-Calatayud et al. (2014)reported on the promising capacity of electrodialysis for treating AMD but inorganic membrane precipitation by metals such as iron was a significant drawback. Similarly, Vital et al. (2018)explored the feasibility of using forward osmosis with NaCl as a draw solution for treating AMD. Although FO was able to achieve more than 98% rejection of ions, the phenomenon of reverse salt flux and dilution of draw solution were major limitations.
Alternatively, membrane distillation (MD), a thermal based membrane process, has shown promising potential for treating acid based wastewater from metal pickling industry (Tomaszewska et al., 2001), and concentrating various types of acid including sulfuric acid from AMD (Kesieme et al., 2012; Tomaszewska and Mientka, 2009). The suitability of MD for concentrating acid is attributed to its capacity to achieve high rejection of non-volatile compounds with up to 90% water recovery ratio, producing good quality fresh water by using vapor pressure difference as its driving force. Additionally, MD requires minimal electrical energy requirement compared to pressure operated systems such as RO and NF while the low thermal requirement (40–80 °C) can be met by alternative thermal sources such as solar or waste heat (Khayet, 2013). MD offers a promising potential for achieving near zero liquid discharge for small scale treatment such as AMD (Naidu et al., 2014; Plattner et al., 2017).
Direct contact MD (DCMD) is the most studied MD configuration due to its simplicity (Naidu et al., 2017a, Naidu et al., 2017b). A number of operating approaches has been considered to improve the performance of DCMD. One such approach is using submerged DCMD, in which the membrane module is submerged directly into the feed solution tank (Choi et al., 2017). This configuration enables to achieve a compact system with reduced heat losses, attributed to the elimination of feed recirculation and reheating. Another promising aspect of submerged DCMD is its flexibility to be used as an integrated single system such as a submerged membrane–sorption system (Naidu et al., 2017a, 2018). The application of submerged DCMD as an integrated single system for heavy metal removal while simultaneously producing fresh water and concentrating AMD has yet to be explored.
The focus of this study is to evaluate the performance of (i) Australia's natural and modified (heat treated) zeolite for heavy metal removal from AMD (ii) submerged DCMD for producing water for reuse from AMD and (iii) integrated submerged DCMD –sorption system for simultaneously removing heavy metals and producing water for reuse from AMD.
Section snippets
Acid mine drainage solution
A model acid mine drainage (AMD) solution was prepared based on AMD characteristics from actual mining sites reported by previous studies (Caraballo et al., 2009; Contreras et al., 2015) (Table 1). The model solution was prepared by dissolving analytical grade CaSO4, MgSO4·(3H2O), NaOH, FeO(OH), Fe(SO4)·7H2O, ZnSO4·7H2O, CuSO4·5H2O, Al2(SO4)3·18H2O and Ni(NO3)2·6H2O (Sigma-Aldrich and Thermo Fisher Scientific) in Milli-Q water. The pH of the solution was adjusted using concentrated H2SO4(10 M).
Performance of natural and modified (heat treated) zeolite
The sorption capacity of natural and modified (heat treated) zeolite was tested for heavy metal removal from AMD. Higher heavy metal removal was achieved with heat treated zeolite compared to natural untreated zeolite (Table 3). The removal of heavy metals increased from 1 to12% with natural zeolite by up to 30–38% upon heating (500 °C). Heating may have removed water on the surface as well as internal channels of the natural zeolite, resulting in vacant channels which enhances heavy metal
Conclusion
An integrated submerged DCMD – zeolite sorption system for simultaneous removal of heavy metals and fresh water production from AMD was evaluated in this study. The results showed:
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A simple heat treatment was effective to increase the performance of natural zeolite for heavy metal removal from AMD solution. Heat treatment of natural zeolite at 500 °C enhanced heavy metal removal by 26–30%.
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The removal affinity for heavy metal was in the order of Fe > Al > Zn > Cu > Ni. The maximum sorption
Acknowledgements
The authors acknowledge the support received for this study fromUniversity of Technology Sydney’s (UTS) Early Career Researcher Fund and Centre for Technology in Water and Wastewater UTS Early Career Researcher program and by Basic Science Research Program Fund through the National Research Foundation of Korea (NFR) funded by the Ministry of Education (2017R1A6A3A04004335).
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