Abstract:
Acid mine drainage (AMD) is a highly acidic and biorecalcitrant wastewater matrix that is rich in dissolved metals and wreaks havoc into receiving environments due to its toxic nature that poses mutagenic, teratogenic, and carcinogenic effects to living organisms on exposure. This is mainly triggered by embodied chemical species and their severe toxicological effects. Stringent regulatory frameworks require acid mine drainage to be contained and treated before it could be discharged to the receiving biosphere. In recent decades, there has been a shift in paradigm, where chemical species enshrined in wastewater streams present a different avenue of resource mining and could be recovered for beneficiation, especially when pragmatically and technically viewed under circular economy, waste beneficiation, and waste valorisation concepts. This study seeks to pursue such a quest as inspired by waste beneficiation and valorisation. Specifically, acid mine drainage is rich in Fe and sulphate contents due to the weathering of pyrite because of the parent mineral. The high iron (Fe(III)), i.e., post oxidation, content in AMD presents an opportunity for recovery and use for iron salt production hence the gross focus of this novel study.
This study focused on selective and fractional recovery of Fe(III) from real coal mine AMD, using magnesium oxide nanoparticles (MgO-NPs), and subsequently employ it for green production of ferric chloride (FeCl3), a typical coagulant used in water and wastewater treatment. The goals of this innovative study were achieved by using a batch experimental method and protocol to optimize the conditions that are favourable for the removal of pollutants from rivers and wastewater. Specifically, the one-factor-at-a-time (OFAAT) were duly enacted, and they comprise the effect of contact time (mins.), effect of dosage (mL), and effect of mixing speed (rpm). Real water samples collected from the river was used for optimisation purposes. The quality of raw and final water was analysed in the SANAS/IEC/ISO 17025 accredited laboratory using state-of-the-art facilities such as photo spectrometers, ICP-MS, ICP-OES, Gallery plus discreet, and multi-parameter devises. Standard methods were also used to draw other technical conclusions. The composition of the recovered sludge from real AMD was characterised using X-ray fluorescence (XRF). Experiments were performed in triplicate and results were reported as mean values, with the cognizant of SDV of ≤5 for quality, repeatability, and trustworthiness of the obtained results. The efficacy of the synthetized ferric chloride was compared with the performance of commercial ferric chloride. Furthermore, its robustness and up-scalability were assessed using river water and municipal wastewater.
From the obtained results, optimum conditions were observed to be 0.2 mL/L (v/v ratio), 100 rpm of mixing speed, and 5 min of equilibration time. Under those conditions, the percentage removal efficacies for contaminants registered the following sequence: 99.8 ≥ 99.8 ≥ 99.7 ≥ 99.5 ≥ 99.2 ≥ 98.2 ≥ 95.3 ≥ 92.9 ≥ 81.1 ≥ 67.6 ≥ 52 ≥ 50 ≥ 48.8 ≥ 22.4 ≥ 11.8 ≥ 10.1 ≥ 10 ≥ 9.7 ≥ 6.7 ≥ 4.3% for Al ≥ ammonia ≥ turbidity ≥ Fe ≥ Cr ≥ Ni ≥ COD ≥ Mn ≥ Cu ≥ nitrite ≥ nitrate ≥ colour ≥ Ca ≥ alkalinity ≥ Mg ≥ K ≥ Na ≥ sulfate ≥ EC ≥ pH, respectively. Similar results were obtained when the same raw water was treated with commercially available FeCl3, which is already used by the water and wastewater industry in South Africa. The results also highlighted that the AMD-synthesized FeCl3 did not contain toxic heavy metals in its matrix, while the treated river water itself was within South African specifications for drinking water quality (SANS 241:2015). Interestingly, the efficacies of both the AMD-synthesized and commercially available FeCl3 were assessed for water and wastewater treatment and were found comparable, and similar performance was observed with municipal wastewater. This highlights that AMD-synthesized FeCl3 can be safely employed by the water and wastewater industry in South Africa and further afield and this will, although to a certain extent, address price and availability concerns surrounding commercial flocculants and coagulants. Furthermore, through AMD beneficiation, i.e., Fe(III) recovery, and partial acidity correction, water reclamation opportunities from AMD can be further pursued. Overall, resource recovery from waste and their reuse for treating other wastes can introduce sustainable paradigms and explicitly promote the United Nations (UN) sustainable development goals (SDGs). Future studies should look into pilot trials and mass production of ferric chloride using Fe(III) recovered from acid mine drainage, and this will ensure sustainability in the management of mine water and preserve the use of virgin materials for the production of ferric chloride.
