Valorisation of acid mine drainage : recovery of valuable minerals and their application for wastewater remediation

Abstract

The increasing pollution of surface water resulting from diverse activities within the catchment areas presents challenges and pressures for downstream water treatment facilities. More specifically, the existence of heavy metals, organic constituents, sulphates and nutrients such as phosphates and nitrates has become a matter of significant concern for water treatment facilities due to strict regulatory standards mandating the elimination of these pollutants before water can be used for consumption. The search for an efficient, effective, and economic technology to remediate wastewater before it is distributed for human consumption has become a global priority. This research project aims to address the dual challenge of environmental remediation and resource recovery by focusing on the valorisation of acid mine drainage (AMD). Specifically, this study seeks to recover valuable minerals from AMD and explore their application for wastewater remediation, including the removal of arsenate (As(V)), hexavalent chromium (Cr(VI)), Congo Red dye (CR), sulphates (SO4), phosphates (PO4), ammonia (NH3) and nitrates (NO3) from aqueous systems. Polycationic metals were recovered from authentic acidic coal mine drainage through selective precipitation using NaOH. Subsequently, the synthesised polycationic metals were tested as a potential adsorbent for As(V), Cr(VI), CR dye, sulphates, ammonia, phosphates and nitrates. Characterisation of the synthesised polycationic metals was done using different tools, including FTIR, XRF, SEM-EDS and EDX before and after adsorption of contaminants to determine the physicochemical properties. Optimal conditions for the removal of As(V), Cr(VI) and CR dye, as well as for municipal wastewater (MWW) contaminants via adsorption were obtained. Optimum conditions for As(V) removal were observed to be 150 ppm of As(V), Solid: Liquid ratio – 1g: 250 mL, agitation time of 60 minutes, and ambient pH and temperature. For Cr(VI) removal, optimum conditions were observed to be 50 mg/L initial Cr(VI), Solid: Liquid ratio – 3g: 100 mL, initial pH = 3, 180 minutes equilibration time and temperature of 35 ºC. CR dye was effectively removed at 100 mg/L CR; 0.5 g PDFe/Al in 500 mL CR dye aqueous solution; 20 minutes agitation time; pH = 3 – 8; and temperature of 35 ºC. For removal of contaminants from MWW, i.e. NO3, PO4, SO4 and NH3, optimum conditions were discovered to be 2g feed dosage in 100 mL, 90 minutes contact time and 35°C temperature. Under optimised conditions, the adsorbent showed >99%, >95%, >99%, >99.9, >99.7, >99 and >96% removal of As(V), Cr(VI), CR dye, PO4, NH3, NO3 and SO4, respectively. Adsorption kinetics followed pseudo-second-order for CR dye adsorption, thereby confirming chemisorption, while As(V) and Cr(VI) followed Weber and Morris intraparticle diffusion. However, a fusion of physisorption and adsorption limited by mass transfer, which includes inner-sphere complexation takes place on the adsorbent surface, accompanied by the simultaneous release of H+ ions for As(V) removal. For the removal of contaminants from MWW, pseudo-first-order kinetic model was followed by PO4, NO3 and SO4 adsorption to signify physisorption, while NH3 followed pseudo-second-order. Adsorption isotherms followed Two-surface Langmuir model for As(V), CR dye, NO3 and SO4 which shows that the adsorption process involved heterogenous surfaces with two well-defined types of adsorption sites, whereas Cr(VI), PO4 and NH3 followed Freundlich thus showing multilayer adsorption. The results of the sludge characterisation indicated that polycationic metals are predominantly constituted of iron (Fe) and aluminium (Al), as recovered from authentic acid mine drainage. The evaluation of the recovered sludge elucidated the fate of the contaminants under study, specifically arsenic, chromium, Congo Red dye, phosphate, ammonium, nitrates, and sulphates from different wastewater streams, thereby demonstrating the effectiveness of the adsorption mechanism. The application of the recovered and synthesised polycationic metals provides a dual advantage for the remediation of AMD and wastewater, thus offering a sustainable and economically viable strategy for environmental management and circular economy. Additional research is essential to assess continuous flow processes, scale-up technology, and the practical implementation of polycationic metals in real-world water remediation context.