Efficient Decaffeination with Recyclable Magnetic Microporous Carbon from Renewable Sources: Kinetics and Isotherm Analysis.

Rapid global urbanization and population growth have ignited an alarming surge in emerging contaminants in water bodies, posing health risks, even at trace concentrations. To address this challenge, novel water treatment and reuse technologies are required as current treatment systems are associated with high costs and energy requirements. These drawbacks provide additional incentives for the application of cost-effective and sustainable biomass-derived activated carbon, which possesses high surface area and low toxicity. Herein, we synthesized microporous activated carbon (MAC) and its magnetic derivative (m-MAC) from tannic acid to decaffeinate contaminated aqueous solutions. Detailed characterization using SEM, BET, and PXRD revealed a very high surface area (>1800 m2/g) and a highly porous, amorphous, heterogeneous sponge-like structure. Physicochemical and thermal analyses using XPS, TGA, and EDS confirmed thermal stability, unique surface moieties, and homogeneous elemental distribution. High absorption performance (>96 %) and adsorption capacity (287 and 394 mg/g) were recorded for m-MAC and MAC, respectively. Mechanistic studies showed that the sorption of caffeine is in tandem with multilayer and chemisorptive mechanisms, considering the models' correlation and error coefficients. π-π stacking and hydrogen bonding were among the interactions that could facilitate MAC-Caffeine and m-MAC-Caffeine bonding interactions. Regeneration and reusability experiments revealed adsorption efficiency ranging from 90.5-98.4 % for MAC and 88.6-93.7 % for m-MAC for five cycles. Our findings suggest that MAC and its magnetic derivative are effective for caffeine removal, and potentially other organic contaminants with the possibility of developing commercially viable and cost-effective water polishing tools.

Advances in the design and use of carbon dots for analytical and biomedical applications.

Carbon dots (CDs) have garnered significant interest for their potential use in multiple applications due to their size, fluorescent properties, high photostability, low toxicity and biocompatibility. CDs can be tailored for specific needs, as they can be synthesized with diverse precursors and techniques for functionalization. Since the applications of CDs are rapidly expanding, this review highlights recent developments in this burgeoning field. Specifically, we describe advances in CD synthesis tailored for applications that include pH and temperature sensing, biochemical analysis, and bioimaging. We also discuss various challenges and practical solutions that will drive CD-based research forward. Challenges include the lack of standardized synthesis and purification methods for CDs, the lack of clarity regarding their mechanism of action, and procedural flaws in their applications. In conclusion, we provide recommendations for collaboration among disciplines to bridge existing knowledge gaps and address the key challenges required for CDs to be fully commercialized.

Humic substances derived from unconventional resources: extraction, properties, environmental impacts, and prospects.

Humic substances comprise up to 70% of the total organic matter in soils, between 50 and 80% of the dissolved organic matter in water, and about 25% of dissolved organic matter in groundwater. Elucidation of the complex structure and properties of humic substances requires advanced analytical tools; however, they are of fundamental importance in medicine, agriculture, technology, and the environment, at large. Although they are naturally occurring, significant efforts are now being directed into their extraction owing to their relevance in improving soil properties and other environmental applications. In the present review, the different fractions of humic substances were elucidated, underlying the mechanisms by which they function in soils. Furthermore, the extraction processes of humic substances from various feedstock were illustrated, with the alkali extraction technique being the most widely used. In addition, the functional group and elemental composition of humic substances were discussed. The similarities and/or variations in the properties of humic substances as influenced by the source and origin of feedstock were highlighted. Finally, the environmental impacts of humic substances were discussed while highlighting prospects of humic acid production. This review offers enormous potential in identifying these knowledge gaps while recommending the need for inter- and multidisciplinary studies in making extensive efforts toward the sustainable production of humic substances.

Hexadecylamine functionalised graphene quantum dots as suitable nano-adsorbents for phenanthrene removal from aqueous solution.

In this study, three novel hexadecylamine graphene quantum dots (hexadecyl-GQDs) with varying moieties on the surface were synthesised and characterised to examine the effect of surface functionalisation on their adsorption efficiency. Introducing hydrophobic moieties via in situ generation of an amide bond by coupling hydrophilic carboxylic acid-functionalised GQDs with hexadecylamine improved the adsorption capacity of GQDs. The AFM and Raman spectroscopy results revealed the formation of multilayered GQDs with an average diameter of 10.0, 9.8, and 7.0 nm for C16-GQDs, C16-NGQDs and C16-SNGQDs, respectively. According to the sum of squared error (SSE) values obtained from fitting phenanthrene adsorption experimental data to two- and three-parameter models, the Sips hybrid isotherm model best described the adsorption of sorbate to the doped hexadecyl-GQDs (0.55 ≥ SSE ≤ 0.72), with R 2 values >0.9. Furthermore, doping of the GQDs favoured a multilayer adsorption mechanism over monolayer adsorption, enhancing adsorption capacities (K F & K d). Hexadecylamine sulphur and nitrogen co-doped GQDs (C16-SNGQDs) gave the highest maximum adsorption capacity (q m) of ∼1377 mg g-1. Pristine GQDs interact with phenanthrene endothermically, while the adsorption of phenanthrene onto modified GQDs is spontaneously exothermic. The adsorption of phenanthrene is most affected by the presence of sulfate and phosphate anions and is best at ambient temperature and acidic pH. Co-doping with nitrogen and sulphur groups and modification of hydrophilic to hydrophobic form enhanced the physicochemical and adsorption performance of C16-SNGQDs, making them potentially suitable as materials for the extraction of hydrophobic organic pollutants from water.

A bibliometric analysis of pre- and post-Stockholm Convention research publications on the Dirty Dozen Chemicals (DDCs) in the African environment.

Persistent organic pollutants (POPs) are toxic chemicals that stay in the environment for a long time. To address the toxicity issues, global nations, including 53 African countries, ratified the Stockholm Convention to minimize or eliminate the production of 12 POPs known as the "Dirty Dozen". However, these Dirty Dozen Chemicals (DDCs) still exist in significant concentration in the African environment, prompting numerous research to investigate the level of their occurrences. Here, we conducted a bibliometric analysis to examine the publication trends in DDCs-related research in Africa using articles published between 1949 and 2021 from the Web of Science and Scopus databases. A total of 884 articles were published within the survey period, with a publication/author and author/publication ratio of 0.36 and 2.76, respectively. South Africa ranked first in terms of number of publications (n = 133, 15.05%), and total citations (n = 3115), followed by Egypt (n = 117), Nigeria (n = 77), USA (n = 40), and Ghana (n = 38). Research collaboration was relatively high (collaboration index = 2.88). The insignificant difference between the theoretical and observed Lotka's distribution indicates Lotka's law does not fit the DDC literature. An annual growth rate of 0.57% implies that a substantial increase of articles in years to come is not expected. More research programs should be established in other African countries to measure up to South Africa's supremacy. This is critical in order to provide a basis for effective compliance to the Stockholm Convention on POPs in Africa.

Adsorptive and photocatalytic remediation of hazardous organic chemical pollutants in aqueous medium: A review.

The provision of clean water is still a major challenge in developing parts of the world, as emphasized by the United Nation Sustainable Development Goals (SDG 6), and has remained a subject of extensive research globally. Advancements in science and industry have resulted in a massive surge in the amount of industrial chemicals produced within the last few decades. Persistent and emerging organic pollutants are detected in aquatic environments, and conventional wastewater treatment plants have ineffectively handled these trace, bioaccumulative and toxic compounds. Therefore, we have conducted an extensive bibliometric analysis of different materials utilized to combat organic pollutants via adsorption and photocatalysis. The classes of pollutants, material synthesis, mechanisms of interaction, merits, and challenges were comprehensively discussed. The paper highlights the advantages of various materials used in the removal of hazardous pollutants from wastewater with activated carbon having the highest adsorption capacity. Dyes, pharmaceuticals, endocrine-disrupting chemicals, pesticides and other recalcitrant organic pollutants have been successfully removed at high degradation efficiencies through the photocatalytic process. The photocatalytic degradation and adsorption processes were compared by considering factors such as cost, efficiency, ease of application and reusability. This review will be good resource material for water treatment professionals/scientists, who may be interested in adsorptive and photocatalytic remediation of organic chemicals pollutants.

Assessment of reusable graphene wool adsorbent for the simultaneous removal of selected 2-6 ringed polycyclic aromatic hydrocarbons from aqueous solution.

The United States Environmental Protection Agency categorized polycyclic aromatic hydrocarbons (PAHs) as hazardous to humans upon acute and/or chronic exposure. This study investigated the simultaneous adsorption of several PAHs onto graphene wool (GW), thereby providing holistic insights into the competitive adsorption of PAHs onto graphene-based materials. SEM, TEM and FTIR provided evidence for the adsorption of PAHs and successful regeneration of the adsorbent accompanied by distinct morphological changes. Isotherm experiments revealed that adsorption of PAHs was significantly influenced by hydrophobic interactions between the sorbate and hydrophobic surface of GW. The Freundlich multilayer isotherm model best fit the experimental data obtained for both multi-component PAH and single-solute experiments as indicated by the Error Sum of Squares (SSE) obtained from nonlinear regression analysis. Experiments revealed that competitive adsorption had a limiting effect on the overall adsorption capacity as qmax and Kd were higher in single-solute than multi-component PAH experiments. The results suggest that partition distribution coefficients (Kd) between the solid-liquid interphase played a significant role in the overall adsorption and a positive correlation between Kd and LogKow of PAHs was established in single-solute experiments. Sorption-desorption experiments revealed that PAHs were adsorbed with a maximum removal efficiency of 100% at an optimum GW dosage of 2 g/L. Adsorption thermodynamics revealed that PAH adsorption onto GW is spontaneous and endothermic. The adsorbent was regenerated and reused for up to six times and its efficiency remained fairly constant.

Antiretroviral Drugs in African Surface Waters: Prevalence, Analysis, and Potential Remediation.

The sources, ecotoxicological impact, and potential remediation strategies of antiretroviral drugs (ARVDs) as emerging contaminants in surface waters are reviewed based on recent literature. The occurrence of ARVDs in water bodies raises concern because many communities in Africa depend on rivers for water resources. Southern Africa is a potential hotspot regarding ARVD contamination due to relatively high therapeutic application and detection thereof in water bodies. Efavirenz and nevirapine are the most persistent in effluents and are prevalent in surface water based on environmental concentrations. Whereas the highest concentration of efavirenz reported in Kenya was 12.4 µg L-1 , concentrations as high as 119 and 140 µg L-1 have been reported in Zambia and South Africa, respectively. Concentrations of ARVDs ranging from 670 to 34 000 ng L-1 (influents) and 540 to 34 000 ng L-1 (effluents) were determined in wastewater treatment plants in South Africa, compared with Europe, where reported concentrations range from less than limit of detection (LOD) to 32 ng L-1 (influents) and less than LOD to 22 ng L-1 (effluents). The present African-based review suggests the need for comprehensive toxicological and risk assessment of these emerging pollutants in Africa, with the intent of averting environmental hazards and the development of sustainable remediation strategies. Environ Toxicol Chem 2022;41:247-262. © 2021 SETAC.

Crude oil exploration in Africa: socio-economic implications, environmental impacts, and mitigation strategies.

Crude oil exploration is a source of significant revenue in Africa via trade and investment since its discovery in the mid-19th Century. Crude oil has bolstered the continent's economy and improved the wellbeing of the citizenry. Historically, Africa has suffered from conflicts due to uneven redistribution of crude oil revenue and severe environmental pollution. Advancements in geophysical survey techniques, such as magnetic and gravity methods, to seismic methods, have made the commercial exploration of crude oil possible for some other countries in Africa apart from Nigeria, Angola, Algeria, Libya, and Egypt. The occurrence of organic-rich, oil-prone Type I, II, and mixed II/III kerogens in sedimentary basins and entrapment within reservoir rocks with intrinsic petrophysical properties are majorly responsible for the large deposits of hydrocarbon in Africa. The unethical practices by some multinational oil corporations have resulted in social movements against them by host communities and human rights groups. The unscrupulous diversion of public funds, award of oil blocks, and production rights to certain individuals have impaired economic growth in Africa. The over-dependence on crude oil revenues has caused the economic recession in oil-producing countries due to plummeting oil prices and global pandemic. Most host communities of crude oil deposits suffer from a lack of infrastructure, arable soils, clean water, and their functioning capabilities are violated by crude oil exploratory activities, without adequate compensations and remedial actions taken by oil companies and the government. Thus, this review examines crude oil exploration in Africa and provides insight into the environmental and socio-economic implications of crude oil exploration in Africa. Furthermore, this report highlights some recommendations that may ensure ethical and sustainable practices toward minimizing negative impacts and improving the quality of life in affected communities.

Advances in water treatment technologies for removal of polycyclic aromatic hydrocarbons: Existing concepts, emerging trends, and future prospects.

In the last two decades, environmental experts have focused on the development of several biological, chemical, physical, and thermal methods/technologies for remediation of PAH-polluted water. Some of the findings have been applied to field-scale treatment, while others have remained as prototypes and semi-pilot studies. Existing treatment options include extraction, chemical oxidation, bioremediation, photocatalytic degradation, and adsorption (employing adsorbents such as biomass derivatives, geosorbents, zeolites, mesoporous silica, polymers, nanocomposites, and graphene-based materials). Electrokinetic remediation, advanced phytoremediation, green nanoremediation, enhanced remediation using biocatalysts, and integrated approaches are still at the developmental stage and hold great potential. Water is an essential component of the ecosystem and highly susceptible to PAH contamination due to crude oil exploration and spillage, and improper municipal and industrial waste management, yet comprehensive reviews on PAH remediation are only available for contaminated soils, despite the several treatment methods developed for the remediation of PAH-polluted water. This review seeks to provide a comprehensive overview of existing and emerging methods/technologies, in order to bridge information gaps toward ensuring a green and sustainable remedial approach for PAH-contaminated aqueous systems. PRACTITIONER POINTS: Comprehensive review of existing and emerging technologies for remediation of PAH-polluted water. Factors influencing efficiency of various methods, challenges and merits were discussed. Green nano-adsorbents, nano-oxidants and bio/phytoremediation are desirous for ecofriendly and economical PAH remediation. Adoption of an integrated approach for the efficient and sustainable remediation of PAH-contaminated water is recommended.

Optimization of the sorption of selected polycyclic aromatic hydrocarbons by regenerable graphene wool.

A novel graphene wool (GW) material was used as adsorbent for the removal of phenanthrene (PHEN) and pyrene (PYR) from aqueous solution. Adsorption kinetics, adsorption isotherms, thermodynamics of adsorption and effect of pH, ionic strength, and temperature on the adsorption of PHEN and PYR onto GW were comprehensively investigated. Isothermal and kinetic experimental data were fitted to Langmuir, Freundlich, Temkin, Sips and Dubinin-Radushkevich models, as well as pseudo-first-order and pseudo-second-order kinetic models. The adsorption kinetic data best fit the pseudo-second-order kinetic model for PHEN and PYR sorption with R2 value >0.999, whilst the Sips model best fit isotherm data. Kinetic data revealed that 24 hr of contact between adsorbent and polycyclic aromatic hydrocarbons (PAHs) was sufficient for maximum adsorption, where the Langmuir maximum adsorption capacity of GW for PHEN and PYR was 5 and 20 mg g-1 and the optimum removal efficiency was 99.9% and 99.1%, respectively. Thermodynamic experiments revealed that adsorption processes were endothermic and spontaneous. Desorption experiments indicated that irreversible sorption occurred with a hysteresis index greater that zero for both PAHs. The high adsorption capacity and potential reusability of GW makes it a very attractive material for removal of hydrophobic organic micro-pollutants from water.