Passive remediation of acid mine drainage using phytoremediation: Role of substrate, plants, and external factors in inorganic contaminants removal

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%) >-SO⁴₂ (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.