Nhamo Chaukura

Biochar-based water treatment systems as a potential low-cost and sustainable technology for clean water provision

Approximately 600 million people lack access to safe drinking water, hence achieving Sustainable Development Goal 6 (Ensure availability and sustainable management of water and sanitation for all by 2030) calls for rapid translation of recent research into practical and frugal solutions within the remaining 13 years. Biochars, with excellent capacity to remove several contaminants from aqueous solutions, constitute an untapped technology for drinking water treatment. Biochar water treatment has several potential merits compared to existing low-cost methods (i.e., sand filtration, boiling, solar disinfection, chlorination): (1) biochar is a low-cost and renewable adsorbent made using readily available biomaterials and skills, making it appropriate for low-income communities; (2) existing methods predominantly remove pathogens, but biochars remove chemical, biological and physical contaminants; (3) biochars maintain organoleptic properties of water, while existing methods generate carcinogenic by-products (e.g., chlorination) and/or increase concentrations of chemical contaminants (e.g., boiling). Biochars have co-benefits including provision of clean energy for household heating and cooking, and soil application of spent biochar improves soil quality and crop yields. Integrating biochar into the water and sanitation system transforms linear material flows into looped material cycles, consistent with terra preta sanitation. Lack of design information on biochar water treatment, and environmental and public health risks constrain the biochar technology. Seven hypotheses for future research are highlighted under three themes: (1) design and optimization of biochar water treatment; (2) ecotoxicology and human health risks associated with contaminant transfer along the biochar-soil-food-human pathway, and (3) life cycle analyses of carbon and energy footprints of biochar water treatment systems.

Organic contaminants in African aquatic systems: Current knowledge, health risks, and future research directions

Organic contaminants (OCs) are increasingly being reported in African aquatic systems, yet a critical evaluation of the literature is still lacking. The objectives of this review were to: (1) identify hotspot reservoirs, transfer pathways and ecological and human risks of OCs, (2) identify potential interventions to minimize the health risks, and (3) highlight knowledge gaps and research constraints. OCs widely reported in aquatic systems include pesticides, pharmaceuticals, plasticizers, solvents, endocrine disrupting compounds, and antimicrobial resistance genes, originating from applications in crop protection, veterinary and animal husbandry, human sanitation and hygiene, human vector and disease control. Potential hotspot reservoirs of OCs include wastewaters, on-site sanitation systems, leachates from non-engineered landfills and contaminated recharge of shallow groundwater systems. OCs could be transferred into humans via drinking of contaminated water, consumption of contaminated crops and aquatic foods, and to a lesser extent, inhalation and dermal contact. Ecological effects including intersex, estrogenicity, and acute and chronic toxicity occur in avian and aquatic species. Although the evidence base of human ecotoxicological effects of OC remains weak, pesticides have been reported in human milk, serum and sperms, pointing to potential chronic and acute toxicity and endocrine disruption. The prevalence of antimicrobials and their resistance genes could in turn lead to antimicrobial resistance in humans. The lack of OC monitoring in drinking water, coupled with over-reliance on untreated drinking water vulnerable to OC contamination predisposes humans to OC health risks. Appropriate water treatment methods, were identified, and a conceptual framework developed to minimize the ecological and human health risks. Future research directions on OC hotspot reservoirs, environmental behaviour and fate, ecotoxicology, epidemiology and interventions to minimize health risks are highlighted. However, lack of advanced analytical facilities in most African countries and other developing regions will continue to constrain OC research for now and in the foreseeable future.

Sources, behaviour, and environmental and human health risks of high-technology rare earth elements as emerging contaminants

Recent studies show that high-technology rare earth elements (REEs) of anthropogenic origin occur in the environment including in aquatic systems, suggesting REEs are contaminants of emerging concern. However, compared to organic contaminants, there is a lack of comprehensive reviews on the anthropogenic sources, environmental behaviour, and public and ecological health risks of REEs. The current review aims to: (1) identify anthropogenic sources, transfer mechanisms, and environmental behaviour of REEs; (2) highlight the human and ecological health risks of REEs and propose mitigation measures; and (3) identify knowledge gaps and future research directions. Out of the 17 REEs, La, Gd, Ce and Eu are the most studied. The main sources of anthropogenic REE include; medical facilities, petroleum refining, mining and technology industries, fertilizers, livestock feeds, and electronic wastes and recycling plants. REEs are mobilized and transported in the environment by hydrological and wind-driven processes. Ecotoxicological effects include reduced plant growth, function and nutritional quality, genotoxicity and neurotoxicity in animals, trophic bioaccumulation, chronic and acute toxicities in soil organisms. Human exposure to REEs occurs via ingestion of contaminated water and food, inhalation, and direct intake during medical administration. REEs have been detected in human hair, nails, and biofluids. In humans, REEs cause nephrogenic systemic fibrosis and severe damage to nephrological systems associated with Gd-based contrast agents, dysfunctional neurological disorder, fibrotic tissue injury, oxidative stress, pneumoconiosis, cytotoxicity, anti-testicular effects, and male sterility. Barring REEs in medical devices, epidemiological evidence directly linking REEs in the environment to human health conditions remains weak. To minimize health risks, a conceptual framework and possible mitigation measures are highlighted. Future research is needed to better understand sources, environmental behaviour, ecotoxicology, and human epidemiology. Moreover, research on REEs in developing regions, including Africa, is needed given prevailing conditions predisposing humans to health risks (e.g., untreated drinking water).

Comparative Adsorption of Zn2+ from Aqueous Solution Using Hydroxylated and Sulphonated Biochars Derived from Pulp and Paper Sludge

Thermally robust hydroxylated biochar (HBC) and sulphonated biochar (SBC) were synthesised from paper and pulp sludge (PPS) and used for the adsorption of Zn2+ from synthetic wastewater through batch experiments. FTIR analyses proved successful incorporation of the hydroxyl and sulphonic functional groups in HBC and SBC, respectively. The effects of initial solution pH, initial Zn2+ concentration, solution temperature and equilibrium contact time were investigated. The removal efficiency of Zn2+ increased with increase in both solution temperature and initial Zn2+ concentration. Adsorption of Zn2+ was greatest at pH 3. HBC and SBC removed 38–99% and 68–90% of Zn2+ from solution, respectively. Zn2+ adsorption on SBC followed both Langmuir (R2 = 0.994) and Freundlich isotherm models (R2 = 0.999), while adsorption on HBC followed the Freundlich model (R2 = 0.989). Zn2+ adsorption on both biosorbents followed pseudo-second-order kinetics (R2 = 0.994–0.999). The increase in enthalpy of adsorption indicated the adsorption process was endothermic and a decrease in Gibbs free energy signified the spontaneity of adsorption. Positive entropy change values imply that the adsorbed Zn2+ ions are randomly distributed over the adsorbent surface. The research demonstrated that although their adsorption mechanisms had salient differences, HBC and SBC can effectively remove Zn2+ from wastewater. Development of HBC and SBC from PPS provides potential low-cost biosorbents for water and wastewater, while simultaneously minimising the environmental and public health risks associated with current disposal practices of PPS.

Removal of Trace Metals from Acid Mine Drainage Using a Sequential Combination of Coal Ash-Based Adsorbents and Phytoremediation by Bunchgrass (Vetiver [ Vetiveria zizanioides L])

Potentially scalable low-cost treatment methods for acid mine drainage (AMD) are very limited. We used a sequential combination of adsorption and phytoremediation by bunchgrass (Vetiver [Vetiveria zizanioides L]) in a semi-batch system to remove Zn, Mn, Ni, and Cu from AMD. The objectives were: (1) to compare the removal of these metals by raw and NaOH-activated coal ash (NaOH-CA); and (2) to determine the effect of sequencing adsorption and phytoextraction on metal removal. The NaOH-CA adsorbed significantly more metals than raw coal ash (RCA) in both batch and semi-batch fixed column experiments, demonstrating the effectiveness of NaOH hydrothermal activation, which forms zeolites. Adsorption by NaOH-CA removed 59.1, 95.7, 67.6, and 77.9% of the Zn, Mn, Ni, and Cu, respectively, compared to 50.6, 95.1, 30.2, and 60.5% for the RCA. Metal removal by phytoremediation was generally less than that by adsorption, accounting for between 3.4 and 54.6% for both adsorbents. Phytoremediation following adsorption by NaOH-CA removed 89.2–99.9% of the metals compared to 70.8–98.5% when phytoremediation followed adsorption by RCA. Overall, relatively high metal removal efficiencies were attained, considering the acidic conditions (pH <4), at hydraulic residence times of 1 to 5 days. Using coal ash to treat AMD is potentially a low-cost and environmentally friendly option for minimizing the adverse public health and environmental risks associated with both wastes.ZusammenfassungEs gibt nur sehr wenige potenziell skalierbare Behandlungsmethoden für saure Grubenwässer (AMD) mit niedrigen Kosten. Wir untersuchten eine sequenzielle Kombination aus Adsorption und Pflanzenbehandlung mit Bunch-Gras (Vetiver [Vetiveria zizanioides L]) in einem Semi-Batch-Versuch, um Zn, Mn, Ni und Cu aus saurem Grubenwasser zu entfernen. Ziele der Untersuchung waren: (1) die Entfernung dieser Metalle durch Rohasche (RKA) und Na-OH-aktivierte Kohleasche (NaOH-KA) zu vergleichen und (2) den Effekt von sequenzieller Adsorption und Pflanzenbehandlung auf die Metallentfernung zu bestimmen. NaOH-KA adsorbierte signifikant mehr Metalle als RKA sowohl in Batch-Versuchen als auch in Semi-Batch-Säulen-Versuchen. Das demonstrierte die Effektivität der hydrothermalen NaOH-Aktivierung, bei der Zeolite gebildet werden. Die Adsorption mit NaOH-KA entfernte 59,1%, 95,7%, 67,6% und 77,9% des Zn, Mn, Ni bzw. Cu, verglichen zu 50,6%, 95,1%, 30,2% und 60,5% durch RKA. Die Metallentfernung durch Pflanzenbehandlung war generell geringer als durch Adsorption, zwischen 3,4% und 54,6% für beide Adsorbenzien. Pflanzenbehandlung nach Adsorption durch NaOH-KA entfernte 89,2-99,9% der Metalle im Vergleich zu 70,8-98,5% durch Pflanzenbehandlung nach Adsorption mit RKA. Insgesamt wurden in Anbetracht der sauren Bedingungen (pH<4) relativ hohe Metallentfernungsraten erreicht bei hydraulischen Aufenthaltszeiten von 1 bis 5 Tagen. Die Nutzung von Kohleaschen ist eine potenzielle Niedrig-Kosten- und umweltfreundliche Möglichkeit, um die schädlichen Gesundheits- und Umweltrisiken beider Abfälle zu minimieren.ResumenLos métodos para el tratamiento de drenaje ácido de minas (AMD) potencialmente escalables son muy limitados. Hemos usado una combinación secuencial de adsorción y fitorremediación por hierba (Vetiver [Vetiveria zizanioides L]) en un sistema semi-batch para remover Zn, Mn, Ni y Cu desde AMD. Los objetivos fueron: (1) comparar la remoción de estos metales con la de cenizas de carbón crudo (RCA) y cenizas de carbón activadas con NaOH (NaOH-CA); y (2) determinar el efecto de adsorción secuencial y fitoextracción sobre la remoción de metales. NaOH-CA adsorbió metales de modo más significativo que RCA tanto en batch como en experimentos semi-batch en columnas, demostrando la efectividad de la activación hidrotermal con NaOH, que forma zeolitas. La adsorción por NaOH-CA removió 59,1, 95,7, 67,6 y 77,9% de Zn, Mn, Ni y Cu, respectivamente, comparado con 50,6, 95,1, 30,2 y 60,5% para RCA. La remoción de metales por fitorremediación fue generalmente menor que la obtenida por adsorción (entre 3,4 y 54,6% para ambos adsorbentes). La fitorremediación siguiendo la adsorción por NaOH-CA removió 89,2-99,9% de los metales comparados con 70,8-98,5% cuando la fitorremediación fue seguida por adsorción por RCA. En forma global se obtuvieron relativamente altas tasas de remoción de metales considerando la acidez (pH<4), a tiempos de residencia de 1 a 5 días. El uso de cenizas de carbón para tratar AMD es potencialmente una opción de bajo costo y ambientalmente amigable para minimizar los riesgos ambientales y sobre la salud humana que están asociados a ambos residuos.顺次粉煤灰吸附和丛生禾草(香根草)植物修复法去除酸性矿山废水痕量金属具有潜在可扩展能力的低成本酸性矿山废水(AMD)处理方法并不多。本文顺次利用吸附和丛生禾草(香根草)生物修复方法去除酸性矿山废水锌、锰、镍和铜。研究目标:(1) 比较“生”粉煤灰(RCA)与NaOH活化粉煤灰(NaOH-CA)的金属去除效果;(2) 评价吸附与生物修复顺次处理方案的去除效果。NaOH活化粉煤灰(NaOH-CA)的批次和半批次柱试验吸附效果都明显比“生”粉煤灰(RCA)好,显示出NaOH的热液活性和沸石生成作用。NaOH活化粉煤灰的锌、锰、镍和铜去除率分别为59.1%、95.7%、67.6%和77.9%,“生”活化粉煤灰(RCA)去除率分别为50.6%、95.1%、30.2%和60.5%。NaOH-CA吸附处理后的顺次丛生禾草(香根草)生物修复将金属去除率升至89.2~99.9%,而RCA吸附处理后的丛生禾草(香根草)生物修复将金属去除率提高至70.8-98.5%。总体上,在酸性条件(pH<4)和水力驻留时间1~5天的条件下,两种顺次处理方法都可获得更高的痕量金属去除率。粉煤灰酸性矿山废水处理是一种有潜力、低成本、环境友好型处理方法,能够同时减小粉煤灰和废性矿山废水的健康和环境风险。

Preparation and Characterization of Polymer-Grafted Montmorillonite-Lignocellulose Nanocomposites by In Situ Intercalative Polymerization

Lignocellulose-clay nanocomposites were synthesized using an in situ intercalative polymerization method at 60°C and a pressure of 1 atm. The ratio of the montmorillonite clay to the lignocellulose ranged from 1 : 9 to 1 : 1 (MMT clay to lignocelluloses, wt%). The adsorbent materials were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). FTIR results showed that the polymers were covalently attached to the nanoclay and the lignocellulose in the nanocomposites. Both TEM and XRD analysis showed that the morphology of the materials ranged from phase-separated to intercalated nanocomposite adsorbents. Improved thermal stability, attributable to the presence of nanoclay, was observed for all the nanocomposites. The nanocomposite materials prepared can potentially be used as adsorbents for the removal of pollutants in water treatment and purification.

Lignin and Chitosan-Based Materials for Dye and Metal Ion Remediation in Aqueous Systems

In view of dwindling fresh water sources, water pollution due to dyes and toxic metals is cause for concern. The increase in industrial activity around the world results in the emission of dyes and toxic metals into the aquatic environment and exerts pressure on water treatment plants. The removal of these contaminants is problematic because they can be available in very low concentrations, and water treatment plants are not designed to remove them effectively. A number of approaches including coagulation, precipitation, membrane filtration, and activated carbon adsorption, have been used for the remediation of contaminated water, but these methods are generally limited by high cost and poor selectivity. Lignin- and chitosan-based nanocomposites are potentially useful for these applications because they have minimal environmental footprints, are cost effective, and are compatible with a wide range of materials in composites. Laboratory scale experiments carried out to evaluate these materials have shown that the composites of these materials have remarkable dye and heavy metal (HM) removal capacities, thus making the technology accessible and potentially manageable at a large scale. Using Web of Science, Scopus, Sciencedirect, Springer, and Google Scholar, we evaluated literature on (1) the prevalence and environmental and health impact of pollution due to dye- and metal-laden effluents, (2) available remediation technologies, (3) the synthetic pathways for different chitosan-based nanocomposites, and (4) the potential of chitosan-based nanocomposites for dye and HM removal. There has been a gradual increase in the research of the use of lignin/chitosan-based adsorbent, showing the rapid interest and potential in the materials.

Chitosan-lignin-titania nanocomposites for the removal of brilliant black dye from aqueous solution

A nanoadsorbent was synthesized from kraft lignin derived from paper and pulp black liquor, chitosan, and titania (TiO2) and used to remove Brilliant Black dye (BB) from aqueous solution. Transmission electron microscopy measurements confirmed the material was nanoscale and BET studies showed a pore width of 11.36 nm with a BET surface area (SBET) of 10.75 m2/g. The presence of NH, O and TiO functional groups was confirmed by ATR-FTIR, and thermogravimetric analysis indicated the nanoadsorbent was thermally stable up to 300 °C. Scanning electron microscopy showed that lignin had larger particles with well-defined edges, while the surface morphology of chitosan showed non-uniform, short fibrous microstructures. The diffraction patterns of the nanocomposite showed a polycrystalline anatase phase and selected area electron diffraction analysis showed the nanocomposite has small spots making up a ring, indicating the nanoparticles has a crystalline structure. The effects of contact time, solution pH, adsorbent dosage, and initial dye concentration on the adsorption of BB were investigated. The batch adsorption data obeyed the Freundlich isotherm (r2 = 0.91), and the monolayer adsorption capacities calculated using the linear Langmuir isotherm was 15.8 mg/g at 25 °C. The adsorption kinetic data were described by the pseudo-second order kinetic model (r2 = 0.93).