Acid mine drainage (AMD) is generally managed via active or passive treatment strategies. Between the two, passive treatment is explored as a more sustainable alternative, especially for abandoned and legacy mines due to their low energy, manpower and material requirements. Recent studies of the authors have explored the use of limestone and waste materials like low-grade ores (LGO), fly ash (FA), and concrete wastes (CW) for AMD treatment and found that although these materials generated alkalinity individually, they could only partially remove sulfate (SO₄ ^2-) and some heavy metals. To address this limitation, a mixed media approach using these four materials is proposed to neutralize the pH of AMD and maximize metal and SO₄ ^2- removal. A total of twenty (20) mixtures of the four materials were identified based on the response surface methodology (RSM) experimental design. Laboratory-scale experiments using simulated AMD were performed to assess the performance of each mixture by monitoring the pH, oxidation-reduction potential (Eh), electrical conductivity (EC), and concentrations of metals and SO₄ ^2-. Based on the results, three optimized mixed media compositions were identified. Overall, simulated AMD passively treated by the optimized mixed media compositions met the Philippine effluent standards except for SO₄ ^2-.

The global accumulation of phosphogypsum (PG), with annual generation exceeding 175 Mt/year, presents significant environmental challenges. While studies have demonstrated PG's potential as a filler material in geopolymer composite, comprehensive environmental impact assessments of such valorization approaches remain limited. This study presents the first comparative life cycle assessment (LCA) of acid- and alkali-activated PG-CFA geopolymers in the context of sustainable industrial waste management. Geopolymer technology can eliminate the need for traditional landfilling of PG in coastal areas and, therefore, reduce their negative environmental impacts. LCA was conducted to assess the impacts of repurposing 1kg functional unit of PG as geopolymer precursors coupled with acid- and alkali-based activators compared to the current disposal practices of these solid wastes. The inventory was modeled after a phosphoric acid plant using the wet dihydrate process, a coal-fired power plant, and a laboratory-scale coal fly ash-phosphogypsum geopolymer (FAPG) synthesis upscaled for industrial application. The most number of environmental benefits was observed for acid FAPG particularly via reductions in CO₂-eq emissions by 40%, 90% in energy consumption, and 36% in mineral resource extraction. Alkali FAPG excelled in water acidification and scarcity by 60% and it could outperform acid FAPG environmentally via sensitivity analysis under a similar formulation blend. Further research can focus on optimizing FAPG formulation, finding alternatives for the acid and alkali activators, and reviewing industrial standards for widespread FAPG applications. These results imply the potential of integrating FAPG manufacturing in PG- and CFA-generating industries to emulate a circular economy.

Acid mine drainage (AMD) is an environmental concern that needs to be addressed by some mining industries because of its high concentrations of metals and acidity that destroy affected ecosystems. Its formation typically persists beyond the operating life of a mine site. Its management is even more challenging for sites that are abandoned without rehabilitation. In this study, a legacy copper–gold mine located in Sto. Niño, Tublay, Benguet, Philippines, generating a copper- and manganese-rich AMD (Cu, maximum 17.2 mg/L; Mn, maximum 2.90 mg/L) at pH 4.59 (minimum) was investigated. With its remote location inhabited by the indigenous people local community (IPLC), a novel limestone-based hybrid passive treatment system that combines a limestone leach bed (LLB) and a controlled modular packed bed reactor (CMPB) has been developed from the laboratory and successfully deployed in the field while investigating the effective hydraulic retention time (HRT), particle size, and redox conditions (oxic and anoxic) in removing Cu and Mn and increasing pH. Laboratory-scale and pilot-scale systems using simulated and actual AMD, respectively, revealed that a 15 h HRT and both oxic and anoxic conditions were effective in treating the AMD. Considering these results and unsteady conditions of the stream in the legacy mine, a hybrid multi-stage limestone leach bed and packed bed were deployed having variable particle size (5 mm to 100 mm) and HRT. Regular monitoring of the system showed the effective removal of Cu (88.5%) and Mn (66.83%) as well as the increase of pH (6.26), addressing the threat of AMD in the area. Improvement of the lifespan of the system needs to be addressed, as issues of Cu-armoring were observed, resulting in reduced performance over time. Nonetheless, the study presents a novel technique in implementing passive treatment systems beyond the typical treatment trains reported in the literature.

This study developed a system intended to treat a copper-rich acid mine drainage (AMD) in a legacy mine in Sto. Niño, Tublay, Benguet, Philippines. Based on previous sampling campaigns in the area, Cu and Mn concentrations and pH exceed the local effluent standards. Using local limestone as the media for treatment of AMD, several laboratory experiments using synthetic AMD were conducted to identify the effective particle size and hydraulic retention time. Consequently, the parameters identified were then deployed into a pilot-scale system investigating the effectiveness of an oxic and anoxic system for the actual AMD. Conducting the analysis, it has revealed that a hybrid of oxic and anoxic system with variable particle size and hydraulic retention time may be deployed considering the unsteady condition of the stream in the legacy mine. In Consideration of the topography of the mine site, a hybrid multi-stage limestone leach bed and packed bed has been deployed. With regular monitoring of the system, it showed an effective removal of Cu and Mn and increase pH of the stream. This observation thus addresses the threat of AMD in the area. However, improvement in terms of the lifespan of the system needs to be demonstrated as common issues of armoring from high concentration of Cu is observed resulting in the reduction of treatment performance over time.

Phosphogypsum (PG) is a waste by-product of the production of phosphoric acid from phosphate ores. Despite it being utilized already for building materials, most of the PG is left in stack and stored in large areas. Valorization of this industrial waste is being explored, including its use for carbon sequestration through indirect mineral carbonation. This process involves leaching a metal, usually Ca or Mg, from mineral wastes followed by the reaction of Caor Mg-rich leachate with CO2 to obtain stable and slightly soluble carbonates. PG samples were collected from a fertilizer production plant in the Philippines and was characterized through SEM-EDS, XRF, XRD, FTIR, and TGA. Philippine PG is mainly composed of Ca which may be leached out for use in mineral carbonation. Two leaching agents were investigated, namely NaCl and HCl. Using a response surface methodology, different factors such as leaching agent concentrations, contact time, temperature, and liquid-to-solid ratio were considered to optimize calcium extraction efficiency. Measuring the concentration of Ca in the leachate via atomic absorption spectroscopy (AAS), HCl is identified to have the highest extraction efficiency.

Acid mine drainage (AMD)—the strongly acidic and highly polluted effluents from mine sites—are generally managed via active or passive treatment. Active treatment strategies are effective but requires continuous input of energy, chemicals and manpower making them unsustainable in the long term. Because of this, passive treatment is explored as a more sustainable alternative especially for abandoned and legacy mines. Recent studies of the authors have explored the use limestone and waste materials like low-grade ores (LGO), fly ash (FA), and concrete wastes for AMD treatment and found that although these materials generated alkalinity individually, they could only partially remove sulfate (SO₄²⁻) and some heavy metals. To address this limitation, a mixed media approach using these four materials is proposed to neutralize the pH of AMD and maximize heavy metals (Cu, Fe, Mn, Ni, and Al) and SO₄²⁻ removal. A total of twenty (20) mixtures of the four materials were identified based on the response surface methodology (RSM) experimental design. Laboratory-scale experiments using simulated AMD were performed to assess the performance of each mixture by monitoring the pH, oxidation-reduction potential (Eh), electrical conductivity (EC), metal concentrations, and SO₄²⁻ concentration. Based on the results, three optimized mixed media compositions were identified in wt%: (i) 43% LGO, 40% limestone, 17% CW; (ii) 44% LGO, 51% limestone, 6% CW; and (iii) 89% limestone, 11% LGO. Overall, simulated AMD passively treated by the optimized mixed media compositions met the Philippine effluent standards except for SO₄²⁻. Simulated AMD treated by the optimized mixed media achieved pH values of < 9 and removal efficiencies for Cu, Fe, Mn, Ni, and Al of about 99%, 99%, 98%, 70%, and 96%, respectively.