1
Passive Remediation of Acid Mine Drainage Using Phytoremediation: Role of Substrate, Plants, and External Factors in Inorganic Contaminants Removal

Nguegang Beauclair1,2*, Vhahangwele, Masindi1,3, Titus Alfred Makudali Msagati4 and Tekere Memory1

1Department of Environmental Science, College of Agriculture and Environmental Sciences (CAES), University of South Africa (UNISA), Florida, South Africa

2Department of Chemical Sciences, Faculty of Science, University of Johannesburg (UJ), Auckland Park, Johannesburg, South Africa

3Magalies Water (MW), Scientific Services (SS), Research & Development (R&D) Division, Brits, South Africa

4Institute of Nanotechnology and Water Sustainability, College of Science, Engineering and Technology (CSET), University of South Africa (UNISA), Florida, South Africa

Abstract

Acid mine drainage (AMD) and its associated toxicological effects degrade the environment and its suitability to support life. Active and passive biological technologies currently applied for AMD treatment are not environmentally friendly and unsuitable for long-term treatment thereby owing to expense. Free water system constructed wetland (FWS-CW) equipped with Vetiveria zizanioides and compost soil as substrate for the treatment of AMD was explored. The experiments were performed in three replicates over a period of 30 days using real AMD. The water quality was monitored at 24-hour intervals, and the average result of a 5-day period was recorded. Results revealed a tolerance index of 1.02 for Vetiveria zizanioides, the decrease of TDS and EC from 3880 to 1400 mg/L and from 5 to 2 mS/cm, respectively, and the rise of pH from 2.6 to 3.1 and a net removal of pollutants in the following order: Fe (90%) > Zn (73%) > (67%) > Mn (58%) > Cu (34%) > Al (31%) > Ni (12%). Furthermore, partitioning of contaminants revealed that metals were removed in the order: substrate = plant = external factors of which, the substrate contributed 77.23% (Mn), 72.01% (Al), 69.91% (Zn), 66.51% (Ni), 60% (Cu), and 56.56% (Fe). The plant contributed Fe (40.42%) > Cu (36.66%) > Ni (30.09) > Zn (27.89%) > Al (22.11%) > Mn (20.58%), and the external factor contributed 5.88% (Al), 3.4% (Ni), 3.34% (Cu), 3.02% (Fe), 2.19% (Mn), and 2.2% (Zn). The translocation assessment revealed that Al, Fe, and Ni were mainly localized in the roots, whereas Cu, Mn, and Zn showed greater translocation to shoot. The XRF, XRD, FTIR, and SEM-EDS analyses revealed the presence of pollutants in sediment and Vetiveria zizanioides roots. The PH REdox EQuilibrium (in C language) (PHREEQC) geochemical model confirm that metals existed as divalent and trivalent in solution. This experiment revealed that this synergetic approach between substrate, plants and external factors can significantly improve the quality of AMD; however, a polishing technology is required to ameliorate the quality of product water to prescribed environmental discharge standards, specifications, and guidelines.

Keywords: Acid mine drainage, treatment, free water surface flow constructed wetland, phytoremediation, substrate, external factors, Vetiveria zizanioides

1.1 Introduction

Mining exploration and minerals exploitation are amongst the most important revenue-generating industries in countries with advanced mining industries [1, 2]. Despite their immense socio-economic benefits, such as job creation and boosting the GDP of any given country, mining exploitation has been associated with significant amount of environmental pollution [3, 4]. This has been endorsed due to its ability to generate toxic and hazardous by-products, such as acidic and metalliferous mine drainages. Specifically, acid mine drainage (AMD) is a commonly known legacy of almost all mining industries worldwide and mostly, from gold and coal mines. Acid mine drainage is generated following the contact of sulphide bearing minerals (pyrite, arsenopyrite) with water and oxygen as illustrated in Equation 1.1 [5, 6].

The product effluent commonly known as acid mine drainage (AMD) or acid rock drainage (ARD) is characterized by low pH (pH = 2 - 3.5), high electrical conductivity (EC) and total dissolved solid (TDS), notable amounts of sulphate ions and toxic heavy metals such as Al, Fe, Mn. Zn, Pb, and trace level of metal, such as Cu and Ni, including radionuclides [7]. Due to its composition and chemistry thereof, environmental problems associated with AMD are diverse and vary in their degree of severity and toxicity [8, 9]. Areas predominated by mining activities experience AMD contamination and this leads to the decline in myriads of ecological benefits [10, 11]. Globally, the environmental problems associated with mining industries are very significant since actives and abandoned mines remains non-point source of pollution thus rendering the rehabilitation more complex [4].

In South Africa and other regions alike, the issue of AMD is very significant, and the available water resources are under serious threat of pollution from mining industries and afield [11]. According to Masindi [12], around 340 ML of mine water is produced daily in the Witwatersrand basin leading to an impairment of ground and surface water due to high level of toxic pollutants that extremely exceed the set standards and targets. Ecological custodians require that this wastewater stream be treated prior releasing it to different environmental compartments [13]. In light of that, various technologies have been developed for the treatment of AMD [4, 14]. They include active and passive technologies. Active treatment methods rely on precipitation, adsorption, ion exchange, filtration, and freeze crystallization [4, 6]. Passive treatment methods include biogeochemical processes which primarily rely on aerobic and anaerobic processes facilitated by lime drains, reactive lime barriers, biological processes, and phytoremediation [3, 14]. Due to the challenges of active treatment process that include cost effectiveness, generation of heterogeneous, and complex secondary sludge containing toxic and hazardous chemical species, frequent energy and chemicals requirement further compound their unfriendly ecological status [15, 16]. Passive AMD treatment processes have received lot of attention simply because of their zero energy requirement, and ecological footprints [17, 18]. Following challenges of natural wetlands and the potential of certain macrophytes to accumulate pollutants from polluted sites, researchers and environmental engineers are investigating emerging passive treatment systems, such as constructed wetland and phytoremediation as long-term solution toward AMD management [17, 18].

Employment of constructed wetlands as low cost and environmentally friendly technology for the treatment of AMD has gained lot of attention. This technology uses macrophytes which play a key role in chemical species attenuation and metals removal [19, 20]. Substrate media which play a huge role in inorganic contaminants removal in constructed wetland since substrate retain metals through sedimentation process thereby enhancing their accumulation by plant [21]. Furthermore, the study of Nguegang et al. [22] has revealed that minor fraction of inorganics contaminants in constructed is removed by external factors. Recently, a wide range of literature has been focusing on the use of Vetiveria zizanioides for the treatment of wastewater and attenuation of toxic chemical species [23-25]. As such, the use of Free Water Surface Constructed Wetlands (FWS-CW) with Vetiveria zizanioides can be successful in treating AMD. Vetiveria zizanioides is a perennial bunchgrass of the family of Panacea, it is a non-invasive perennial grass that is fast-growing with a large biomass and extensive root system [17, 18, 25]. Vetiveria zizanioides is a hydrophyte plant (growth normally in both aquatic and non-aquatic environment), tolerant to acidic conditions, and is able to accumulate various types of metals [25, 26]. Following the potential of natural wetland in wastewater treatment, the role of phytoremediation in removal of heavy metals and the ability of Vetiveria zizanioides to tolerate very acidic conditions and accumulate various types of metals, this study was set out to investigate an integrated approach using a combination of FWS-CW and phytoremediation with Vetiveria zizanioides for AMD treatment. Kiiskila et al. [24] explored the treatment of metalliferous mine water by a floating CW planted with Vetiveria zizanioides with promising results. However, CWs are not effective to treat very acidic mine water, limited by climatic conditions and plants species ability for continuous metals accumulation. However, the use of compost soil as wetland substrate has never been examined, and this will be the first study in design and execution to determine the partitioning of toxic chemicals from AMD to substrate, plants, and external factors. This will enable us to understand the role and effect of each component of the system in the attenuation of toxic chemical species from AMD.

1.2 Materials and Methods

1.2.1 Samples Collection and Characterization

Acid mine drainage was collected from Sibanye Gold mine in Krugersdorp, Gauteng, South Africa....