Passive treatment of acid mine drainage using an integration of neutralisation and a constructed aerobic wetland

Abstract

Water pollution and environmental degradation caused by acid mine drainage (AMD) is an ever more pronounced matter in environmental pollution. This is imputed to the nature and magnitude of its effects on the environment and its resources. Acid mine drainage is generated from the hydro-geo-chemical oxidation of sulphides bearing minerals such as arsenopyrite, marcasite. Acid mine drainage is prevalently rich in Aluminum, iron, Manganese and sulphate with traces of toxic and hazardous chemical components such as Copper, Nickel, Zinc, Lead, Cobalt, Arsenic, Chrome, and nuclides. As a legal requirement, these components must be removed from acid mine drainage before they could contaminate the environment. A number of passive and active treatment technologies have been developed to curtail the impacts of acid mine drainage. However, they have their own drawbacks and advantages. In light of that, technologies have been migrating towards an integrated approach. This study was therefore motivated with the aim of using an integration of neutralisation and staged hybrid constructed wetland to treat acid mine drainage from active and disused mines. It integrated neutralisation upstream and staged hybrid constructed wetland downstream to polish the water. To achieve this, two batches of experiments were conducted: the first batch consisted to assess the performance of three different types of constructed wetland operating individually for the treatment of acid mine drainage while the second batch assessed a hybrid approach integrating neutralisation of acid mine drainage using cryptocrystalline magnesite and bioremediation using staged hybrid constructed wetland. The present study consisted of seven chapters including: (1) Introduction and background information, (2) Literature review, (3) materials and methods, (4) passive remediation of acid mine drainage using phytoremediation: partitioning of inorganics contaminants between plant, substrate and external factors, (5) the treatment of acid mine drainage using vertically flowing wetland: insight into fate of chemicals species, (6) the assessment of the performance of subsurface horizontally flowing constructed-wetland for treatment of acid mine drainage, (7) The treatment of acid mine drainage using a combination of cryptocrystalline magnesite and a staged hybrid constructed wetland equipped with Vetiveria zizanioides. In the first part the study, the root of AMD were identified and described, the scope and extent to which previous studies have investigated the problems, the existing gaps and attempts to address them. In the second part, the origins and formation process of AMD were explored and explained as well as its associated environmental, socio economic and public health impact. This second part also explored the existing technologies for AMD treatment, their limitation as well as description of constructed wetland and properties of some plants used as wetland macrophytes. The third part of the study described succinctly how study was conducted, study design, samples collection and analysis; it also outlined ethical consideration, risks and limitation of the study as well as wetland maintenance plan. In the fourth part, the passive remediation of AMD using free water surface constructed wetland equipped with Vetiveria zizanioides was assessed with focus on partitioning of inorganic contaminant between plant, substrate and external factors. Water quality parameters were monitored by chemical analysis of samples every 24 hours for 30 days retention time. The average result of five days period was recorded. The results showed net removal of pollutants with Fe registering the highest removal percentage of 90%, followed by Zn (73%), SO42- (67%), Mn (58%), Cu (34%), Al (31%) and Ni (12%) with huge quantity of metals and sulphate retained in the substrate. The total dissolved solid (TDS) decreased from 3880 to 1400 mg/l and electrical conductivity (EC) decreased from 5 to 2 mS/cm while pH rose from 2.6 to 3.1. The finding further revealed that Vetiveria zizanioides was tolerant in AMD water while metals removed were partitioned in the following 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). Overall the Free Water Surface-Constructed Wetland equipped with Vetiveria zizanioides improved the quality of AMD but did not meet the water quality standard required by Department of Water and Sanitation (DWS) and Department of Environmental Affairs (DEA) for effluents discharge. In the fifth part, AMD was treated using subsurface vertically flowing wetland equipped with Vetiveria zizanioides with focus on the fate of chemical species. The experiment lasted for 30 days and leachates parameters were monitored by chemical analysis of samples every 24 hours. The results revealed a net reduction of sulphate from 3137 mg/L to 1406 mg/L (55.18%), net removal of metals with removal efficiency ranged in the order: Fe (71.25%) > Zn (70.40%) > Al (68.93%)> Mn (62%) > SO42─(55.18%) > Ni (35%) > Cu (18.83%). The results further revealed that Vetiveria zizanioides was tolerant to acid mine water with tolerant index of 1.031. Substrate played a huge role in metals removal with contribution faction ranged from 65.12% for nickel to 81.39% for zinc whereas the accumulation of heavy metals by Vetiveria zizanioides accounted only for small faction (10.46% to 30.57%) and external factors contribution accounted for a minor faction for Cu (0.36%), Al (1.13%) and Fe (1.3%) and small faction for Mn (8.1%), Zn (8.12%) and Ni (24.26%) of the overall heavy metals removal by the wetland. The vertically flowing wetland planted with Vetiveria zizanioides slightly improved the quality of mine water. In the sixth part of the study, AMD was treated using subsurface horizontal flowing wetland equipped with Vetiveria zizanioides. Leachates parameters were monitored daily and the average result of five days period was recorded for 30 days retention time. The results showed a slight increase of pH by 1.4 and net reduction of EC, TDS and SO42─ at 47.20%, 46% and 33.04% respectively. There was a net removal of metals with Zn registering the highest removal percentage of 77.75%, followed by Fe (75.36%), Mn (67.48%), Al (55.05%), Ni (44.01%) and Cu (11.36%). This system significantly improved the quality of AMD. However, the water quality guidelines as set by the Department of Environmental Affairs and Department of Water and Sanitation were not met. In the seventh part, the potential of hybrid technology in treating AMD was assessed. Feed AMD was treated with magnesite at the ration of 1:100 (one gram of magnesite and 100 ml of AMD water). The contact of magnesite with AMD water for an optimal time of one hour led to an increase in pH from 2.6 to 9.8 and a net reduction of EC, TDS, metals and sulphate. The product water was further treated by a bioremediation process using staged hybrid constructed wetland and the finding revealed that pH was further increased from 9.8 to 10.4 within 30 days retention time, TDS was reduced from 1552 to 780.3, EC was reduced from 2.1 to 0.7 mS/cm, sulphate was reduced from 1354.8 to 608.7 mg/L; Al was reduced from 12.64 to 0.71 mg/L; Cu was reduced from 0.84 to 0.11 mg/L; Fe was reduced from 17.05 to 0.66 mg/L; Mn was reduced from 1.48 to 0.28 mg/L; Ni was reduced from 0.47 to 0.09 mg/L and Zn was reduced from 0.68 to 0.14 mg/L. Overall, the removal efficiency of hybrid technology (neutralisation and bioremediation) was 86%, 79.88%, 80.59%, 99.5%, 97.38%, 99.8%, 99.24%, 97.7% and 98.36% for EC, TDS, sulphate, Al, Cu, Fe, Mn, Ni and Zn respectively and a pH increment of 8.8. The hybrid technology (neutralisation and staged hybrid constructed wetland) removed contaminants to below South African legal requirement for effluent discharge and water use. As such it can be concluded that neutralisation with magnesite and bioremediation using staged hybrid wetland planted with Vetiveria zizanioides has a synergetic potential in AMD treatment.