Currently, acid mine drainage (AMD) is considered to be the most serious environmental problem associated with the metal and coal industry (Zhang, 2011). During the mining process, metallic minerals are exposed to microorganisms, oxygen and water, resulting in oxidation of associated sulfur elements and leaching of heavy metals (Hughes et al., 2013). After being carried by rainfall, AMD with SO42−, H+ and various heavy metals as the main ions is formed. AMD carrying heavy metals flows into the surrounding environment, causing not only environmental acidification and heavy metal contamination but also threatening human health through the food chain (He and Chen, 2014). Therefore, the removal of heavy metals from AMD has been a major problem in recent years.
At present, there are many treatment methods for AMD, including adsorption (Dlamini et al, 2019), membrane filtration (Ricci et al, 2015), advanced oxidation processes (Munyengabe et al., 2020), and chemical reactions (Kefeni et al., 2017). Nevertheless, these methods are extremely resource intensive with high costs or defective in treating AMD with high concentration of ions, which limits their large-scale application (Shi et al., 2019). Moreover, some improved physicochemical methods aiming at reducing the cost have made progress, while secondary toxic chemical sludge generated by these technologies may be the culprit leading to further environmental problems ( Iqbal et al., 2009; Rawat et al., 2014; Masindi et al., 2018).
Consequently, as a low-cost and environmentally friendly technology, biological methods, including plant technology and microbial technology, are extremely promising (Shi et al., 2020). Compared with traditional methods, biological methods also show an advantage in remediating heavy metal pollution in large water basins with concentrations of heavy metals below 100 mg/L (Aryal and Liakopoulou-Kyriakides, 2015). In addition, some microorganisms in the contaminated soil have strong reduction and fixation of heavy metals as well as exhibit tolerance (Fei et al., 2022), which has promising applications for heavy metal remediation in water. For microbial remediation, extracellular polymeric substances (EPS) secreted by species-rich biofilms can adsorb heavy metals and reduce toxic effects, while the porous structure of biofilms can also promote heavy metal absorption (Liu et al., 2018). Emerging technologies such as microbial fuel cell (MFC) have been applied to treat AMD, in which anaerobic microorganisms separate metals from liquids by oxidizing metal ions and promoting their formation as precipitates (Cheng et al., 2007). MFC can not only remove heavy metals but also use microorganisms to generate electricity, which has broad development potential in clean production. However, MFC is more suitable for the removal of heavy metals in small volume (generally less than 1L) of soil and water bodies. There are a lot of restrictions when applied to magnified water bodies such as the reactor structure, membrane as well as buffering capacity (Pandit et al., 2020). AMD treatment is also often achieved by constructing a continuous alkalinity production (SAPS) reactor, which, in addition to the carbon source layer required for microbial growth, needs to be filled with an alkalinity layer to raise the pH of the reactor effluent (Jung et al., 2014). In fact, anaerobic bacteria play a key role in both MFCs and SAPS in terms of treating AMD due to their purification effect on multiple heavy metals (Lu et al., 2011; Le Pape et al., 2017; Yan et al., 2018). However, as an anaerobic species, SRB grow under harsh conditions, which is not practical in industrial applications. Microorganisms growing under aerobic conditions grow faster than anaerobic microorganisms. Consequently, if biosorption is the main removal route, aerobic microorganisms have greater advantages for more tolerant growth conditions and faster growth rates.
Indigenous bacteria generally showed higher resistance to pollutants for their adapted enzymatic reactions (Ighalo et al., 2022), therefore, bioremediation strains are easier to obtain in areas that have been contaminated for a long time ( Wen et al., 2018; Gallardo-Rodríguez et al., 2019). Mixed microorganism culture has many advantages over single culture, such as increasing the number of substrates available and enhancing heavy metal resistance (Cui et al., 2017; Kang et al., 2016; Liu et al., 2018). Wen et al. used three screened copper-resistant bacteria to construct a flora and inoculated it into sludge to enhance the removal of heavy metals. In this system, bacteria formed a stable, balanced, and drug-resistant flora, and the copper removal rate reached 98.0 ± 0.3% (Hughes et al., 2013). It can be seen that constructing flora is beneficial for microorganisms in practical applications. Moreover, heavy metals are likely to be more toxic to free individual microorganisms, so it is a common method to attach microorganisms to the carrier, which can also prevent the loss of microorganisms and ensure their retention in the reactor before adapting to the environment (Wen et al., 2018).
In this study, a microbial-plant coupled reactor was constructed to remediate AMD based on strains with purification ability for Cu2+ and Zn2+ which were screened from soil contaminated by AMD. In addition, the mechanisms of four carbon sources affecting the treatment effect of the reactor and the reinforcement between microorganisms and plants were further investigated, aiming at providing an economical and suitable treatment method for field application in mitigating heavy metal contamination of mining areas.