Solid surface photochemistry of montmorillonite: mechanisms for the arsenite oxidation under UV-A irradiation

Arsenic adsorbent derived from the ferromanganese slag

Arsenic-contaminated groundwater has a severe negative impact on the health of living beings. Groundwater majorly contains arsenite (As(III)) as well as arsenate (As(V)). Among these two, the arsenite species are more carcinogenic, mobile, and lethal. Hence, it is more difficult to remove by conventional water treatment methods. Ferromanganese slag, waste generated from steel industries, has been utilized in this study for the development of arsenic adsorbent. A chemical treatment method is applied to the ferromanganese slag to prepare efficient arsenic adsorbent, and it is easy to scale up. An adsorbent with the capacity for simultaneous oxidation of As(III) and adsorption of total arsenic species can be efficient for arsenic decontamination. X-ray photoelectron spectroscopy and X-ray absorption near edge spectra techniques prove the As(III) oxidation capability of the developed material is about 70 ± 5% based on initial As(III) concentration. The adsorbent not only oxidizes the As(III) species but also adsorbs both the arsenic species. The Langmuir isotherm model estimates the maximum adsorption capacities at the equilibrium concentration of 10 μg/L are 1.010 ± 0.004 mg/g and 1.614 ± 0.006 mg/g for As(III) and As(V), respectively. The rate of adsorption of As(III) was higher compared to the As(V), which was confirmed by the pseudo-second-order kinetic model. Therefore, the treated water quality meets the World Health Organization and Indian drinking water standards.

UV photochemical hydride generation using ZnO nanoparticles for arsenic speciation in waters, sediments, and soils samples

The environmental disasters that occurred due to the leakage of mining waste in Mariana-MG (2015) and Brumadinho-MG (2019), located in Brazil, attracted the attention of the scientific community. This designated efforts to investigate the environmental consequences of toxic waste in the affected ecosystem. Therefore, a simple, easily executed and accessible method was presented for arsenic speciation [As(III), As(V), and DMA]. Using an atomic absorption spectrometer coupled to the hydride generation system, the heterogeneous photocatalysis was applied in the reduction of As(V) and DMA to As(III). After the optimization, a calibration curve was constructed, with LODs equivalent to 3.20 μg L^-1 As(III), 3.86 μg L^-1 As(V), and 6.68 μg L^-1 DMA. When applying the method for quantification in environmental samples, a concentration of up to 103.1 ± 9.4 μg L^-1 As(V) was determined for surface water samples. The soil samples, 84.1 ± 3.6 μg L^-1 As(III) and 112.4 ± 9.9 μg L^-1 As(V) were quantified, proving the contamination of the ecosystems impacted by the environmental disasters. We proceeded the study through an addition/recovery method with samples of water, soil, and sediments (collected from impacted environments). Recovery values were equivalent to 99.0% for As(III), 93.8% for As(V), and 99.2% for DMA. Graphical abstract Photocatalytic reduction mechanism of As(V) and DMA to As(III) by heterogeneous photocatalysis.

Investigating photo-driven arsenics' behavior and their glucose metabolite toxicity by the typical metallic oxides in ambient PM2.5

It is essential and challenged to understand the atmospheric arsenic pollution because it is much more complicated than in water and top-soil. Herein the different behavior of arsenic species firstly were discovered within the ambient PM_2.5 collected during daytime and nighttime, winter and summer. The diurnal variation of arsenic species in PMs is significantly correlated with the presence of metallic oxides, specifically, ferrous, titanium and zinc oxides, which might play a key role in the process of the photo-oxidation of As(III) to As(V) with the meteorological parameters and regional factors excluded. Subsequently, the photo conversion of arsenite was detected on metal-loaded glass-fiber filters under visible light. The photo-generated superoxide radical was found to be predominantly responsible for the oxidation of As(III). In order to reveal toxicity differences induced by oxidation As(III), HepG2 cells were exposed to various arsenic mixture solution. We found that the antioxidant enzyme activities suppressed with increasing the As(III)/As(V) ratio in total, followed by the accumulation of intracellular ROS level. The glucose consumption and glycogen content also displayed an obvious reduction in insulin-stimulated cells. Compared to the expression levels of IRS-1, AKT and GLUT4, GLUT2 might be more vulnerable to arsenic exposure and lead to the abnormalities of glucose metabolism in HepG2 cells. Taken together, these findings clarify that the health risk posed by inhalation exposure to As-pollution air might be alleviated owing to the photo-driven conversion in presence of metal oxides.

Photochemical emission and fixation of NOX gases in soils

Gaseous nitrogen oxides (NOx), which result from the combustion of fossil fuels, volcanic eruptions, forest fires, and biological reactions in soils, not only affect air quality and the atmospheric concentration of ozone, but also contribute to global warming and acid rain. Soil NOx emissions have been largely ascribed to soil microbiological processes; but there is no proof of abiotic catalytic activity affecting soil NO emissions. We provide evidence of gas exchange in soils involving emissions of NOx by photochemical reactions, and their counterpart fixation through photocatalytic reactions under UV-visible irradiation. The catalytic activity promoting NOx capture as nitrate varied widely amongst different soil types, from low in quartzitic sandy soils to high in iron oxide and TiO₂ rich soils. Clay soils with significant amounts of smectite also exhibited high rates of NOx sequestration and fixed amounts of N comparable to that of NO (nitric oxide) losses through biotic reactions. In these soils, a flux of 100 µg N_NO m^-2 h^-1, as usually found in most ecosystems, could be reduced by these photochemical reactions by more than 60%. This mechanism of N fixation provides new insight into the nitrogen cycle and may inspire alternative strategies to reduce NO emissions from soils.

Efficient photocatalytic oxidation of arsenite from contaminated water by Fe2O3-Mn2O3 nanocomposite under UVA radiation and process optimization with experimental design

The efficiency of photocatalytic oxidation process in arsenite (As(III)) removal from contaminated water by a new Fe₂O₃-Mn₂O₃ nanocomposite under UV_A radiation was investigated. The effect of nanocomposite dosage, pH and initial As(III) concentration on the photocatalytic oxidation of As(III) were studied by experimental design. The synthesized nanocomposite had a uniform and spherical morphological structure and contained 49.83% of Fe₂O₃ and 29.36% of Mn₂O₃. Based on the experimental design model, in photocatalytic oxidation process, the effect of pH was higher than other parameters. At nanocomposite concentrations of more than 12 mg L^-1, pH 4 to 6 and oxidation time of 30 min, photocatalytic oxidation efficiency was more than 95% for initial As(III) concentration of less than 500 μg L^-1. By decreasing pH and increasing the nanocomposite concentration, the photocatalytic oxidation efficiency was increased. Furthermore, by increasing the oxidation time from 10 to 240 min, in addition to oxidation of As(III) to arsenate (As(V)), the residual As(V) was adsorbed on the Fe₂O₃-Mn₂O₃ nanocomposite and total As concentration was decreased. Therefore, Fe₂O₃-Mn₂O₃ nanocomposite as a bimetal oxide, at low doses and short time, can enhance and improve the efficiency of the photocatalytic oxidation and adsorption of As(III) from contaminated water resources. Furthermore, the energy and material costs of the UV_A/Fe₂O₃-Mn₂O₃ system for photocatalytic oxidation of 1  mg L^-1 As(III) in the 1 L laboratory scale reactor was 0.0051 €.

Oxidation of organic contaminant in a self-driven electro/natural maghemite/peroxydisulfate system: Efficiency and mechanism

Electro-assisted iron-mediated persulfate (PS) activation process has been successfully employed to oxidize organic contaminant. However, a majority of iron-based catalysts used for PS activation was synthesized through complicated or demanding procedures and may have potential risks on environment during the preparation process. Herein, natural maghemite (NM) which is abundant on the earth was employed to activate peroxydisulfate (PDS) in an electrolytic cell. The voltage was provided by microbial fuel cell (MFC) instead of external power as reported in the previous studies, so as to establish a self-driven electro/natural maghemite/PDS system (MFC/NM/PDS) for the oxidation of acid orange 7 (AO7). The results showed that above 90% removal efficiency of AO7 was achieved in a wide range of pH (3.0-9.0) after 100min reaction. Singlet oxygen was identified for the first time during PDS activation and surface bound sulfate radicals served as the dominant active species responsible for AO7 oxidation. The underlying mechanism of AO7 elimination in the MFC/NM/PDS system was elucidated through quenching tests, electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) techniques. The variation of TOC and cytotoxicity to Escherichia coli was explored. The intermediate products formed were identified using LC-TOF-MS technique and a possible pathway of AO7 degradation was proposed.