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Wastewater pollution is an existing and serious environmental issue. One of the main wastewater pollution sources is from the dyestuff industries and its discharge poses a serious threat to the ecosystem due to its adverse effects on human and environmental health. Conventional wastewater treatments are limited in terms of effectively treating such non-biodegradable contaminants; thus, this work aimed to investigate the degradation of the commercial dye methylene blue (MB) via advanced oxidation processes (AOPs) using carbon dots as Fenton-like catalysts. Iron and nitrogen-containing carbon dots (Fe, N-CDs) were synthesised through a hydrothermal reaction and utilised as catalysts in the degradation of MB. Multiple characterisation techniques were employed to describe the dots properties including FTIR, XPS, TEM, ζ-potential, PL and UV-Vis analyses. Characterisation revealed the synthesis of 7 nm diameter, spherical negatively charged dots rich in iron and nitrogen surface functionalities. Methylene blue was degraded under mild conditions using an innovative green remediation technology which removed nearly 100 % of the dye (20 mg L-1) under optimised conditions (at 50 °C, pH 8, H2O2 147 mM, and a catalyst concentration of 0.5 mg mL-1) in only 60 minutes. The active mechanism of the Fe, N-CDs was deduced suggesting the participation of free radicals such as hydroxyl radicals, singlet oxygens and holes. Importantly, the effluents toxicity assessed using Microtox® confirmed the generation of a non-toxic effluent. This investigation showed that using Fe, N-CDs was an effective way to degrade wastewater pollution whilst significantly minimising its toxicity.

Hydrogels offer a unique medium for sensing applications as they can draw upon the benefits of both solid and liquid sensing platforms. Incorporation of functionalised fluorescent nanoparticles within the hydrogel matrix results in a three-dimensional nanocomposite capable of interacting with water-soluble analytes, facilitating quantitative sensing applications. Here, we report the solvothermal synthesis of a novel Polyvinylpyrrolidone assembled hydrogel stabilised with zinc oxide quantum dots which does not require additional organic crosslinkers. Functionalization of these hydrogel with novel, brightly photoluminescent 2-amino-2-methyl-1-propanol quantum carbon dots (quantum yield of 62.5%) results in a composite capable of specific interaction with hexavalent chromium. Quantitative fluorescence quenching measurements of the hydrogel composite in the presence of hexavalent chromium shows a limit of detection of 1.2 μM Cr6+ which is below maximum allowable concentrations for drinking water. The hydrogel composite is cheap to manufacture and can be injected into 96 well plates for high throughput analysis of environmental water samples. These results are encouraging for the development of hydrogels and polymetric films to be used as novel fluorescent sensing platforms for environmental diagnostic applications.

The continued discharge of synthetic dyes into aqueous ecosystems from industrial effluent sources represents a critical issue for both human and environmental health. Although there exist various treatments for the treatment of dye-containing wastewater, a low-cost, green, effective, and ecofriendly approach still remains to be developed. In this work, carbon nanodots were synthesised using prickly pear and urea as green precursors and tested as catalysts in the Fenton-like reaction for the catalytic decolourization of cationic dyes. Approximately 90% of a mix of three common cationic dyes (crystal violet, basic fuchsin and methyl green) at 5 × 10−5 M were removed from water at 20 °C and pH 7.5 in the presence of H2O2 at 0.5% after only 60 minutes. The carbon nanodots utilised in this study were characterized by FTIR, XPS, zeta potential, UV-Vis and TEM. They exhibited negative surface charge, excellent aqueous solubility, and nitrogen rich surface functionalities which were involved in the transference of electrons, participating in the decomposition of H2O2, and therefore generated hydroxyls, superoxide ions and singlet oxygens as detected using the free radical quenching technique. This work provides an insight into exploiting an ecofriendly non-toxic metal-free catalyst using a low cost and efficient approach that showed promising decolourization results, without the use of external energy, at a neutral pH solution that could be potentially applied in catalysis of a range of environmental pollutants.

Graphical abstract, Solid or liquid platforms have been traditionally employed for measuring the fluorescent properties of quantum carbon dots (QCD). Hydrogels possess both liquid and solid properties which allow them to overcome several shortfalls of both solid and liquid sensing platforms. Hydrogels offer a three dimensional platform which can house nanoparticles with different attributes (i.e. fluorescent QCD’s) and prevents their aggregation. Here, we incorporate QCD’s (made from the hydrothermal treatment of 1-naphthylamine and citric acid) into the matrix of a zinc oxide hydrogel. This nanocomposite was shown to have hexavalent chromium (Cr6+) specific fluorescence quenching properties. Detailed fluorescence analysis of the hydrogel with Cr6+ was conducted in a high throughput manner by loading the hydrogel into wells of a black 96-well plate. Fluorescence quenching of the hydrogel-QCD-nanocomposites in the presence of dilutions of Cr6+ was measured using a fluorescence spectrophotometer and showed incremental fluorescence decreases with increasing Cr6+ concentration. Furthermore, this was quantitatively confirmed by Stern–Volmer plots showing a linear quenching trend (R2 = 0.9975) when comparing fluorescence intensities against increasing Cr6+ concentrations (0.234–1.875 μM). This technology can be applied for routine water quality testing in agricultural, natural and potable water sources for the early detection of heavy metal pollutants.

Water polluted with dyes has become a serious global concern during the twenty-first century, especially for developing countries. Such types of environmental contaminant pose a severe threat to biodiversity, ecosystems, and human health globally; therefore, its treatment is an utmost requirement. Advanced technologies including the use of nanomaterials represent a promising water treatment technology with high efficiencies, low production costs, and green synthesis. Among the nanomaterials, carbon dots, as a new class of carbon-based nanoparticles, have attracted attention due to their unique features and advantages over other nanomaterials, which include high water solubility, easy fabrication and surface functionalisation, excellent electron-donating ability, and low toxicity. Such properties make carbon dots potential nanocatalysts for the Fenton-like degradation of environmental pollutants in water. Although recent studies show that carbon dots can successfully catalyse the degradation of dyes, there are still limited and controversial studies on the ecotoxicity and fate of these nanoparticles in the environment. In this review, the authors aim to summarise the recent research advances in water remediation by technologies using carbon dots, discuss important properties and factors for optimised catalytic remediation, and provide critical analysis of ecotoxicity issues and the environmental fate of these nanoparticles.

Understanding the changes in microbial community structure and its impact during the storage of diesel and biodiesel is critical to prevent damage to the system and to ensure a good quality product to consumers. In this study, the storage of pure diesel (B0), diesel and biodiesel blends (B5, B10, and B50), and pure biodiesel (B100) was simulated. During the simulation, and after contamination with inoculum, microbial growth was monitored through biomass formation. Fungi and bacteria were quantified by qPCR. The effect of contamination on the biodegradation of fatty acid methyl esters (FAMEs) and n-alkanes was assessed by GC/qMS. After 60 days of incubation, B100 and B50 had the highest biomass production at the oil-water interface. Microorganism abundance was estimated by the amplification of 16S rRNA and 18S rRNA genes using qPCR. The fungal population formed at the oil-water interface was larger than the bacterial population, in all treatments. The abundance of bacteria with the alkB gene found in the blends was one or two orders of magnitude higher than that of the total bacteria estimated. The highest biodegradation rates of the C16:0, C18:0, C18:1 cis and C18:2 FAMEs (25.5, 17.5%, 34.5%, and 27.7%, respectively) occurred in the B5 blend. The highest n-alkane degradation occurred in the B50 and B10 blends. The n-alkanes C 20 , C 18 , C 15 , C 14 , and C 13 had 72.1%, 57.1%, 18.5%, 21.8%, and 19.6% degradation in B50, respectively. The n-alkanes C19, C12, and C10 had 61.3%, 6.9%, and 22.97% degradation in B10, respectively. The results indicate that knowledge about the microbial contamination of fuels is important for a correct management and to reduce the associated economic losses.

Biodiesel’s susceptibility to oxidation and its hygroscopic characteristics can cause chemical oxidation and microbial growth during storage, leading to fuel biodeterioration. Although a wide variety of commercial antioxidants have been used to prevent biodiesel oxidation, little is known about the effects of these agents on microbial community. The minimum inhibitory concentration (MIC) of tert -butylhidroquinone (TBHQ) and butylated hydroxytoluene (BHT) against microorganisms isolated from fuel storage tanks was investigated, and only TBHQ presented antimicrobial activity. In laboratory scale, microcosm experiments with soybean biodiesel and different TBHQ concentrations (0; 50; 100; 200; 300; and 600 ppm) incubated for 45 days at 30 °C were performed. The microbial biomass formed at the oil-water interfaces was weighted, and DNA was isolated for DGGE analysis of 16S gene. The quality of commercial biodiesel was investigated by determining oxidation stability, acid number, viscosity, water content, and ester degradation. The results showed higher oxidation stability for the samples containing 100, 300, and 600 ppm TBHQ. The highest biomass formation occurred at TBHQ concentrations of 0, 300, and 600 ppm. The highest microbial diversity was observed in both the commercial samples and those containing 100 and 200 ppm TBHQ. The addition of antioxidant to the microcosms neither inhibited nor enhanced biomass production. An increase in viscosity, acid number, and water content was observed, thus the biodiesel was out of specification for commercialization. Degradation of C16, C18, C18:1, C18:2, and C18:3 esters was not observed in all samples.

Due to their renewable and sustainable nature, biodiesel blends boost studies predicting their stability during storage. Besides chemical degradation, biodiesel is more susceptible to biodegradation due to its raw composition. The aim of this work was to evaluate the deteriogenic potential (growth and degradation) of Pseudallescheria boydii and Meyerozyma guilliermondii in degrading pure diesel (B0), pure biodiesel (B100), and a B10 blend in mineral medium during storage. The biodeterioration susceptibility at different fuel ratios and in BH minimal mineral medium were evaluated. The biomass measurements of P. boydii during 45 days indicated higher biomass production in the B10 blend. The growth curve of M. guilliermondii showed similar growth in B10 and B100. Although there was no significant production of biosurfactant, lipase production was detected in the tributyrin agar medium of both microorganisms. The main compounds identified in the aqueous phase by GC-MS were alcohols, esters, acids, sulfur, ketones, and phenols. The results showed that P. boydii grew at the expense of fuels, degrading biodiesel esters, and diesel hydrocarbons. M. guilliermondii grew in B100 and B10; however, degradation was not detected.

Leaks and spillages during the extraction, transport and storage of petroleum and its derivatives may result in environmental contamination. Biodiesel is an alternative energy source that can contribute to a reduction in environmental pollution. The aim of the present work was to evaluate biodegradation of diesel, biodiesel, and a 20% biodiesel-diesel mixture in oxisols from southern Brazil, using two bioremediation strategies: natural attenuation and bioaugmentation/biostimulation. Fuel biodegradation was monitored over 60 days by dehydrogenase activity, CO 2 evolution and gas chromatography. The bacterial inoculum employed for bioaugmentation/biostimulation consisted of Bacillus megaterium , Bacillus pumilus , Pseudomonas aeruginosa , and Stenotrophomonas maltophilia and PCR-DGGE using 16S RNAr primers showed that some members of this consortium survived in the soil after 60 days. The biodegradation of pure biodiesel was higher for bioaugmentation/biostimulation than for natural attenuation, suggesting that the addition of the microbial consortium, together with adjustment of the macronutrient ratio, increased biodiesel degradation. The results of dehydrogenase and respiratory activity, together with GC analysis, suggested that the presence of biodiesel may, by stimulating general microbial degradative metabolism, increase the biodegradation of petroleum diesel. The microbial community was altered by both treatments, with natural attenuation producing a lower diversity index than the amended soil. The bioaugmentation/biostimulation strategy was showed to have a high potential for cleaning up soils contaminated with diesel and biodiesel blends.

The aim of this study was to evaluate the antimicrobial activity of bio-oil (obtained by pyrolysis of biomass: soybean oil, eucalyptus sawdust and coffee grounds) added to the blend B10 (diesel and biodiesel). The bio-oil is compatible to diesel oil and it contains promising compounds that can exhibit antimicrobial activity during fuel storage. We evaluated the antimicrobial activity of bio-oil added to diesel oil B10 by determining the minimum inhibitory concentration at different concentrations (0-10%) using oil-deteriorating microorganisms for 10 days at 30 °C. Bio-oil was separated from the aqueous phase by solid phase extraction using ultrapure water and sodium hydroxide. The aqueous phase containing the solubilized compounds of bio-oil was characterized by comprehensive two‑dimensional gas chromatography with time‑of‑flight mass spectrometric detection. Minimum inhibitory concentration in the range of 0.25% to 4% was observed for the tested inocula. The chromatographic analysis of both extracts allowed the identification of several oxygenated compounds, which the majority of the analytes consisted of phenolic compounds, followed by ketones.

Accidents caused by leaks and/or spills on soils need to be addressed. Natural attenuation, biostimulation and bioaugmentation can be useful bioremediation strategies for decontamination processes in soils of diesel/biodiesel mixtures. The purpose of this study was to evaluate the degradation rate of the different fuels (B0, B20 and B100) in an ultisol under natural attenuation and biostimulation/bioaugmentation during 60 days of incubation in a controlled microcosm simulating a surface spill over soil. The degradation of different diesel/biodiesel mixtures was monitored for up to 60 days by dehydrogenase activity, respirometry by CO2 release, the most probable number of heterotrophic and degrading microorganism and gas chromatography. The bacterial inoculum employed for biostimulation/bioaugmentation strategy consisted of Bacillus megaterium, Bacillus pumilus, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. The two bioremediation strategies have showed great degradation rates. The natural attenuation was effective for B0 and B20 treatments. The addition of the bacterial consortium and macronutrients contributed to the increased degradation of pure biodiesel in relation to natural attenuation, with higher rates for CO2 release, enzymatic and degrading activity. It is suggested that the bacterial consortium has proven effective for presenting significant values for such parameters until the end of the 60-day incubation period.