Mn/Ti-doped carbon xerogel for efficient catalysis of microcystin-LR degradation in the water surface discharge plasma reactor

Comprehensive strategies for microcystin degradation: A review of the physical, chemical, and biological methods and genetic engineering

Addressing the threat of harmful cyanobacterial blooms (CyanoHABs) and their associated microcystins (MCs) is crucial for global drinking water safety. In this review, we comprehensively analyze and compares the physical, chemical, and biological methods and genetic engineering for MCs degradation in aquatic environments. Physical methods, such as UV treatments and photocatalytic reactions, have a high efficiency in breaking down MCs, with the potential for further enhancement in performance and reduction of hazardous byproducts. Chemical treatments using chlorine dioxide and potassium permanganate can reduce MC levels but require careful dosage management to avoid toxic by-products and protect aquatic ecosystems. Biological methods, including microbial degradation and phytoremediation techniques, show promise for the biodegradation of MCs, offering reduced environmental impact and increased sustainability. Genetic engineering, such as immobilization of microcystinase A (MlrA) in Escherichia coli and its expression in Synechocystis sp., has proven effective in decomposing MCs such as MC-LR. However, challenges related to specific environmental conditions such as temperature variations, pH levels, presence of other contaminants, nutrient availability, oxygen levels, and light exposure, as well as scalability of biological systems, necessitate further exploration. We provide a comprehensive evaluation of MCs degradation techniques, delving into their practicality, assessing the environmental impacts, and scrutinizing their efficiency to offer crucial insights into the multifaceted nature of these methods in various environmental contexts. The integration of various methodologies to enhance degradation efficiency is vital in the field of water safety, underscoring the need for ongoing innovation.

Physico-chemical treatments for the removal of cyanotoxins from drinking water: Current challenges and future trends

Cyanobacteria are highly prevalent blue-green algae that grow in stagnant and nutrient-rich water bodies. Environmental conditions, such as eutrophication and human activities, increased the cyanobacterial blooms in freshwater resources worldwide. The excessive bloom formation has also resulted in an alarming surge of cyanobacterial toxins. Prolonged exposure to cyanotoxins is a potential threat to natural ecosystems, animal and human health by the spoilage of the quality of bathing and drinking water. Various molecular and analytical methods have been proposed to monitor their occurrence and understand their global distribution. Moreover, different physical, chemical, and biological approaches have been employed to control cyanobacterial blooms and their toxins to mitigate their occurrence. Numerous strategies have been engaged in drinking water treatment plants (DWTPs). However, the degree of treatment varies greatly and is primarily determined by the source, water properties, and operating parameters such as temperature, pH, and cyanotoxin variants and levels. A comprehensive compilation of methods, from traditional approaches to more advanced oxidation processes (AOPs), are presented for the removal of intracellular and extracellular cyanotoxins. This review discusses the effectiveness of various physicochemical operations and their limitations in a DWTP, for the removal of various cyanotoxins. These operations span from simple to advanced treatment levels with varying degrees of effectiveness and differing costs of implementation. Furthermore, mitigation measures applied in other toxin systems have been considered as alternative strategies.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Overview of toxic cyanobacteria and cyanotoxins in Ibero-American freshwaters: Challenges for risk management and opportunities for removal by advanced technologies

The increasing occurrence of cyanobacterial blooms worldwide represents an important threat for both the environment and public health. In this context, the development of risk analysis and management tools as well as sustainable and cost-effective treatment processes is essential. The research project TALGENTOX, funded by the Ibero-American Science and Technology Program for Development (CYTED-2019), aims to address this ambitious challenge in countries with different environmental and social conditions within the Ibero-American context. It is based on a multidisciplinary approach that combines ecology, water management and technology fields, and includes research groups from Chile, Colombia, Mexico, Peru and Spain. In this review, the occurrence of toxic cyanobacteria and cyanotoxins in freshwaters from these countries are summarized. The presence of cyanotoxins has been confirmed in all countries but the information is still scarce and further monitoring is required. In this regard, remote sensing or metagenomics are good alternatives at reasonable cost. The risk management of freshwaters from those countries considering the most frequent uses (consumption and recreation) has been also evaluated. Only Spain and Peru include cyanotoxins in its drinking water legislation (only MC-LR) and thus, there is a need for regulatory improvements. The development of preventive strategies like diminishing nutrient loads to aquatic systems is also required. In the same line, corrective measures are urgently needed especially in drinking waters. Advanced Oxidation Processes (AOPs) have the potential to play a major role in this scenario as they are effective for the elimination of most cyanotoxins classes. The research on the field of AOPs is herein summarized considering the cost-effectiveness, environmental character and technical applicability of such technologies. Fenton-based processes and photocatalysis using solar irradiation or LED light represent very promising alternatives given their high cost-efficiency. Further research should focus on developing stable long-term operation systems, addressing their scale-up.

Electrochemical treatment of humic acid using particle electrodes ensembled by ordered mesoporous carbon

In order to degrade the macromolecular pollutant of humic acid, the powder ordered mesoporous carbon (POMC, average pore diameter 4.29 nm) was first applied for preparing the granular OMC (GOMC, Φ × H = 4 × 3-6 mm) as electrodes in a continuous three-dimensional (3D) electrochemical system. The POMC was synthesized by hard-templating method and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size distribution, N₂ adsorption/desorption technology, and Fourier-transform infrared (FT-IR). The effects of electrochemical degradation parameters, such as current and hydraulic retention time (HRT), were investigated, and the degradation mechanism of HA was explored as well. The results indicated that the degradation efficiency of HA, chemical oxygen demand (COD), and total organic carbon (TOC) reached 95.3, 86.2, and 62.7%, respectively, under initial HA of 100 mg/L, current of 0.2 A, and HRT of 130 min. The detection of electron paramagnetic resonance (EPR) showed that plenty of ˙OH was generated on GOMC electrodes, which made the 3D system more effective than the conventional two-dimensional (2D) system. The cyclic voltammetry curves indicated that the reactions of HA on the OMC materials surface included both direct oxidation and direct reduction.

Enhanced removal of humic acid from micro-polluted source water in a surface discharge plasma system coupled with activated carbon

Surface discharge plasma (SDP) combined with activated carbon (AC) was employed to eliminate dissolved organic matter from micro-polluted source water, with humic acid (HA) as the model pollutant. Synergistic effect on HA removal was observed in the SDP-AC system; HA removal efficiency reached 60.9% within 5-min treatment in the SDP-AC system with 5.0 g AC addition, whereas 16.7 and 17.4% of HA were removed in sole SDP system and AC adsorption, respectively. Scanning electron microscope and Boehm titration analysis showed that chemical reactions between active species and functional groups of AC occurred. The existence of isopropanol or benzoquinone exhibited inhibitive effects on HA removal in the SDP system, while these inhibitive effects were weakened in the SDP-AC system. The influences of AC on ozone equivalent concentration and H₂O₂ concentration were evaluated, and there were approximately 39 and 20% decline in ozone equivalent concentration and H₂O₂ concentration within 6-min treatment in the SDP-AC system, respectively, compared with those in the sole SDP system. Dissolved organic carbon, specific ultraviolet absorbance, and UV absorption ratios analysis demonstrated that the SDP treatment destroyed the chromophoric groups, double bonds, and aromatic structure of HA molecules, and these destructive actions were strengthened by AC.

Process optimization for microcystin-LR degradation by Response Surface Methodology and mechanism analysis in gas-liquid hybrid discharge system

A gas-liquid hybrid discharge system was applied to microcystin-LR (MC-LR) degradation. MC-LR degradation was completed after 1 min under a pulsed high voltage of 16 kV, gas-liquid interface gap of 10 mm and oxygen flow rate of 160 L/h. The Box-Behnken Design was proposed in Response Surface Methodology to evaluate the influence of pulsed high voltage, electrode distance and oxygen flow rate on MC-LR removal efficiency. Multiple regression analysis, focused on multivariable factors, was employed and a reduced cubic model was developed. The ANOVA analysis shows that the model is significant and the model prediction on MC-LR removal was also validated with experimental data. The optimum conditions for the process are obtained at pulsed voltage of 16 kV, gas-liquid interface gap of 10 mm and oxygen flow rate of 120 L/h with ta removal efficiency of MC-LR of 96.6%. The addition of catalysts (TiO₂ or Fe²⁺) in the gas-liquid hybrid discharge system was found to enhance the removal of MC-LR. The intermediates of MC-LR degradation were analyzed by liquid chromatography/mass spectrometry. The degradation pathway proposed envisaged the oxidation of hydroxyl radicals and ozone, and attack of high-energy electrons on the unsaturated double bonds of Adda and Mdha, with MC-LR finally decomposing into small molecular products.

Research on dye wastewater decoloration by pulse discharge plasma combined with charcoal derived from spent tea leaves

Pulsed discharge plasma (PDP) combined with charcoal (PDP-charcoal) was employed to treat dye wastewater, with methyl orange (MO) as the model pollutant. The charcoal was prepared using spent tea leaves and was characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, and Boehm titration to investigate the adsorption and catalytic characteristics before and after adsorption and PDP treatment. The prepared charcoal exhibited a high MO adsorption capacity, and the adsorption process followed the pseudo-second-order kinetic model and the Freundlich model. The MO decoloration efficiency reached 69.8 % within 7.5 min of treatment in the PDP-charcoal system, whereas values of 29.2 and 25.9 % were achieved in individual PDP and charcoal systems, respectively. The addition of n-butanol and H2PO4 (-) presented inhibitive effects on MO decoloration in the PDP system. However, these effects were much weaker in the PDP-charcoal system. In addition, the effects of charcoal on O3 and H2O2 formation were evaluated, and the results showed that both the O3 and H2O2 concentrations decreased in the presence of charcoal. The MO decomposition intermediates were analyzed using UV-Vis spectrometry and GC-MS. 1,4-Benzoquinone, 4-nitrophenol, 4-hydroxyaniline, and N,N'-dimethylaniline were detected. A possible pathway for MO decomposition in this system was proposed.