Abstract
Acid Mine Drainage (AMD) is one of the most significant environmental challenges associated with mining activities, particularly in mineral-rich countries like South Africa. AMD results from the oxidation of sulphide minerals, leading to highly acidic water contaminated with toxic heavy metals such as iron, lead, copper, and nickel. If left untreated, AMD poses severe risks to water quality, aquatic life, soil health, and human well-being, making effective remediation strategies essential. Traditional AMD treatment methods, such as chemical precipitation and lime neutralization, are often costly and generate secondary waste, necessitating the exploration of sustainable and cost-effective alternatives.
In this study, sewage sludge collected from the Polokwane Wastewater Treatment Plant was used to produce activated carbon (AC) for the treatment of acid mine drainage (AMD) obtained from Sibanye Stillwater in Johannesburg. The sludge was oven-dried, sieved, and pyrolyzed at 450 °C under an argon atmosphere to produce biochar, which was then chemically activated using phosphoric acid at varying ratios, with a 1:1 impregnation ratio found to be optimal. The AC was characterized using techniques such as FTIR, SEM-EDS, BET, TGA, XRD, and XRF to determine its structural, thermal, and chemical properties. Batch adsorption experiments were conducted to evaluate the effects of adsorbent dosage, pH, temperature, and contact time on the removal efficiency of metal ions, using ICP-OES for metal analysis. Optimal adsorption conditions were determined, and performance comparisons were made between sludge-derived and commercial AC.
The sludge was dried, sieved, and pyrolyzed at 450 °C under an argon atmosphere to produce biochar, which was then chemically activated using 4 M phosphoric acid, with a 1:1 impregnation ratio found to be optimal. Characterization of the AC revealed that the ash content increased from 6.06% in raw sludge to 83.39% in AC, indicating the concentration of inorganic residues after pyrolysis. Moisture content also rose slightly from 0.23% to 4.19%, while volatile matter increased from 6.32% to 9.87%, suggesting partial retention of organic compounds or functional groups. In the ultimate analysis, carbon content decreased slightly from 5.03% in raw sludge to 4.39% in AC, while nitrogen and hydrogen contents were also slightly reduced. These changes reflect the transformation of organic matter during activation and suggest that the resulting AC, while structurally suitable for adsorption, may be limited by its high ash and low carbon content. BET surface area of 10, 1164 m²/g, indicating a well-developed porous structure suitable for adsorption. SEM analysis showed a rough, porous
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surface morphology, while FTIR spectra confirmed the presence of functional groups such as –OH, C–O, and C=C that enhance metal binding.
XRD and XRF analyses identified mineral phases like iron and aluminium oxide, which contribute to additional adsorption sites. TGA demonstrated good thermal stability of the material up to 400 °C. Batch adsorption tests showed that the highest removal efficiencies were achieved at pH 4, with optimal conditions being 6 g of adsorbent dosage, 35 °C temperature, and 45 minutes of contact time. Under these conditions, the sludge-derived AC removed over 90% of Pb²⁺, 80% of Cu²⁺, and 80% of Zn²⁺, outperforming commercial AC in most cases. The findings demonstrate that sludge-derived activated carbon exhibits significant adsorption capacity for heavy metals, with removal efficiencies comparable to those of commercial AC in certain conditions. The adsorption kinetics followed the pseudo-second-order model, suggesting that chemisorption is dominant in the metal ion uptake process. Furthermore, the study highlights the economic and environmental benefits of utilizing sewage sludge for AC production, offering a dual-purpose solution for both wastewater treatment and sludge management.
This research contributes to the growing body of knowledge on low-cost, sustainable adsorbents for AMD remediation. It provides a practical approach for mitigating mining-related water pollution while promoting circular economy principles. Future studies should explore the regeneration and reusability of sludge-derived AC and its potential application in large-scale AMD treatment systems.