Evans Martin Nkhalambayausi Chirwa

This study investigated the bioreduction of metal ions from real gold mine wastewater collected from a small- scale mine. An indigenous, mining-adapted bacterial mixed culture comprising Stenotrophomonas maltophilia, Klebsiella pneumoniae, and Acinetobacter bereziniae and Bacillus cereus, isolated from gold mine wastewater and tailings was used. The consortium, previously demonstrated to reduce synthetic Au (III) and cyanide ions, was applied to real mine wastewater under batch experimental conditions. Key physicochemical parameters including oxidation-reduction potential (ORP), pH, Cu (II), were investigated for their influence on gold reduction efficiency in 24 hours. Maximum Au (III) bioreduction of 97.8% was achieved alongside 56.4 % removal of copper ions and almost complete cyanide degradation.,Untargeted LC-MS-based metabolomic profiling of the consortium identified 22 (positive ionization) and 10 (negative ionization) putative molecular feature categories. NADPH-cycling folate intermediates, glutathione-related molecules, peptides, and organic acids were the primary metabolites driving Au (III) reduction through hydride-transfer, thiol-mediated electron donation, and carbonyl oxidation mechanisms, respectively. The recovered precipitate was characterized using transmission and scanning electron microscopy (TEM and SEM), (FTIR) X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). XPS Au4f deconvolution indicated successful bioreduction, revealing a dominant metallic Au⁰ component at ~83.8–84.0 eV and a secondary component at ~85.0–86.0 eV assigned to organically-complexed gold. The nanoparticles were stabilised within a biomolecular corona arising from coordinative bonding between surface gold atoms and bacterial biomolecules. These findings establish a systematic link between specific bacterial metabolites and the biogenic synthesis of stable gold nanoparticles from complex mine wastewater.

Recently, an attempt to develop an alternative method for the treatment of acid mine drainage of the three Witwatersrand basins in South Africa resulted in the recovery of calcium carbonate, the aragonite polymorph. The recovered mineral displayed different and unconventional morphologies without the use of organic additives. In this paper, an in situ Raman Spectroscopy experiments were conducted to gain insights on the formation of the mineral from the acid mine drainage. The objective was to understand the mechanism of the mineral formation for maximum value extraction from the mineral. The results indicated that mineral formation was via the formation of prenucleation clusters followed by aragonite formation. The initial pH of the solutions, sodium, magnesium and sulfate ion concentrations had a significant impact on the reaction kinetics as well as the size, shape and morphologies of the recovered particles. The Avrami model parameter, dimensionality of growth, were in agreement with the observed morphologies of the particles. However, the Biphasic Dose Response model presented the best fit of the uncalibrated Raman Spectroscopy data, presenting the first adaptation of the model to explain the precipitation of aragonite.

The escalating discharge of synthetic dyes, such as brilliant green (BG), into aquatic ecosystems poses severe environmental and health risks due to their toxicity, carcinogenicity, and bioaccumulation potential. This study reports on the synthesis of black titanium dioxide (B-TiO2) via chemical reduction of commercial Degussa using NaBH4 at 300 °C under N2 atmosphere, aiming to enhance visible-light photocatalytic activity for BG degradation. Comprehensive characterization via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), Brunauer-Emmett-Teller (BET) analysis, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) confirmed the introduction of Ti³⁺ defects and oxygen vacancies, resulting in a reduced bandgap (~1.5 eV), extended visible-light absorption, increased surface area, and improved charge separation. Photocatalytic evaluations under visible-light irradiation (λ > 240 nm) exhibited optimal degradation of BG (10 mg/L) at a catalyst load of 1 g/L and pH 11, achieving complete mineralization in 30 min adhering to pseudo-first-order kinetics (k = 0.062 min⁻¹). Radical scavenging experiments identified superoxide radicals (O2⁻•) as the primary reactive species, with minor contributions from hydroxyl radicals (•OH) and holes (h⁺). The catalyst demonstrated remarkable reusability, maintaining 100% efficiency over four cycles. These findings underscore B-TiO2 as a robust, solar-driven photocatalyst for sustainable wastewater remediation, in alignment with UN’s Sustainable Development Goals for clean water and health.

Engineering can be made simple and more impactful by observing and understanding how organisms in nature solve eminent problems. For example, scientists around the world have observed green plants thriving without organic food inputs using the complex photosynthesis process to kick-start a self-sustaining biochemical food chain. In this study, two biological analogues for advanced water treatment, i.e., visible-light photocatalysis using porphyrin-Bi12O17Cl2 and BiOIO3 compounds and interfacial solar desalination by a by Reduced Graphene Oxide-Black TiO2 (rGO-Black TiO2) were investigated. For the visible-light photocatalytic process for dye decolouration, a porphyrin@Bi12O17Cl2 system was applied to successfully degrade Rhodamine B dye in batch experiments, achieving up to 79% degradation within 240 minutes. These results show that more advances and more efficient engineered systems can be achieved by observing nature and how these systems have survived over billions of years. The rGO-Black TiO2 system achieved increases in temperature for 25oC to 60oC in less than 5 seconds. Based on these observations, the research group from the Water Utilisation Group at the University of Pretoria have studied and developed fundamental processes for the degradation and remediation of unwanted compounds such as disinfection byproducts (DBPs), volatile organic compounds (VOCs) and pharmaceutical products from water.

The increasing presence of pharmaceuticals, particularly antiretroviral drugs (ARVs), in wastewater has raised concerns regarding their environmental and health impacts. Photocatalysis, driven by advanced photocatalysts, such as coloured TiO₂, ZnO, and composites with carbon-based materials, has shown promise as an effective method for degrading these pollutants. Despite significant laboratory-scale success, challenges remain in scaling this technology for real-world applications, particularly in terms of photocatalyst stability, the formation of toxic degradation by-products, and economic feasibility. This paper explores the current state of photocatalytic degradation for ARVDs, emphasizing the need for further research into degradation pathways, the development of more efficient and cost-effective photocatalysts, and the integration of photocatalysis into hybrid treatment systems. The future of photocatalysis in wastewater treatment hinges on improving scalability, reactor design, and hybrid systems that combine photocatalysis with traditional treatment methods to ensure comprehensive pollutant removal. Innovations in catalyst design and reactor optimization are essential for advancing photocatalysis as a viable solution for large-scale wastewater treatment.

Recently, there has been a growing interest in solar‐driven interfacial desalination technology, which focuses on the localization of heat to the air‐water interface. In this study, 3D nitrogen‐doped reduced graphene oxide (3D N@rGO) photothermal material is synthesized with a facile one‐step hydrothermal process. The material exhibited richer porosity, high hydrophilicity for efficient water channeling, and all‐directional solar absorption potential. The 3D N@rGO solar absorber attained up to ≈55 °C surface temperature rise and showed ≈134% photothermal conversion efficiency with 1.94 kg m⁻² h⁻¹ net freshwater generation rate under 1 sun solar illumination, owing to efficient latent heat recycle. On a high salinity desalination study performed using 10 and 20 wt.% salinity levels, the photothermal material showed 1.66 and 1.31 kg m⁻² h⁻¹ evaporation rates respectively. It sustained stable long‐term desalination performance without visible salt accumulation on the surface up to a salinity level of 10 wt.%. In a three‐day outdoor test carried out utilizing simulated seawater with a 3.5 wt.% NaCl solution, the 3D evaporator demonstrated an average freshwater production rate of 2.61 kg m⁻² h⁻¹, during the test the solar power density reached up to 1.1 kW m⁻². The 3D solar absorber exhibited a promising potential for large‐scale seawater desalination in water‐scarce regions worldwide.