Photooxidation Of Arsenite Research Articles

Unveiling the effects of dissolved organic matter (DOM) extracted from coastal algae and river on the photooxidation of arsenite

The coastal environment is an important ecosystem connecting land and sea, and arsenite (As(III)) in coastal seawater can seriously affect human health through the food chain. However, the effects of dissolved organic matter (DOM) extracted from coastal algae and rivers on As(III) photooxidation remain unclear. Results show that coastal algal DOM (CA-DOM) is significantly more effective than Suwannee River natural organic matter (SRNOM) in photooxidation of As(III), with a rate 8.3 times higher after correcting for light screening effects. CA-DOM accelerates As(III) photooxidation mainly through the 3DOM⁎ pathway, contributing 78.7 % to the process, whereas 3NOM⁎ contributes only 37.2 % for SRNOM. CA-DOM consists primarily of low-excited tyrosine and tryptophan-like protein substances, whereas SRNOM consists of humic and fulvic acid-like substances. Thus, CA-DOM exhibits a higher steady-state concentration of 3DOM⁎, and the 3DOM⁎ reacts much faster with As(III) than the 3NOM⁎. The increase in CA-DOM concentration can significantly accelerate the photooxidation of As(III), whereas the effect of SRNOM concentration is negligible. Increased salinity can accelerate As(III) photooxidation for all types of DOM. Our results provide new insights into the role of DOM from different sources in the photooxidation of As(III) in the natural environment or engineering applications.

Photo-oxidation of arsenite in acidic waters containing Suwannee River fulvic acid: roles of 3SRFA* and hydroxyl radical.

The photo-oxidation of arsenite (As(III)) in solution containing Suwannee River fulvic acid (SRFA) under the ultraviolet A (UVA) irradiation (λmax = 365 nm) was studied. In a solution containing 100.0 μg·L-1 As(III) and 10.0 mg·L-1 SRFA at pH 3.0, SRFA induced As(III) photo-oxidation by producing the triplet excited state of SRFA (3SRFA*) and hydroxyl radical(HO˙). Approximately 82% of As(III) oxidation was attributed to HO˙ which depended strongly on HO2˙/O2˙-. The remaining 18% of As(III) oxidation was attributed to the direct reaction between As(III) and 3SRFA*. The photo-oxidation of As(III) was significantly affected by solution pH. Excess SRFA inhibited As(III) photo-oxidation. The addition of a low concentration of ferric ions retarded the photo-oxidation of As(III) due to the poor photo-activity of Fe(III)-SRFA complexes. In contrast, the addition of ferric ions at high concentration greatly accelerated As(III) photo-oxidation because of the high photo-activity of Fe(III)-OH complexes. The fractions of SRFA with different molecular weight showed different oxidizing capacities under UV irradiation which was possibly related to the different contents of phenolic OH groups. The findings have important environmental implications for the photo-transformation behavior of As(III) in natural surface waters containing dissolved organic matter, especially acidic waters.

Effect of copper(II) species on the photooxidation of arsenite under UV-C irradiation

The effect of Cu(II) on the photooxidation of arsenite (As(III)) to arsenate (As(V)) under UV-C (254 nm) irradiation and the role of Cu(II) in the As(III) oxidation processes were mechanistically investigated. The presence of Cu(II) enhanced the oxidation of As(III), and the positive effect of Cu(II) increased with increasing Cu(II) concentration and pH. However, other metal ions, such as Al(III), Cr(III), Ni(II), and Mn(II), had little effect on the photooxidation kinetics of As(III). The UV absorption spectra of the As(III) and Cu(II) solutions indicated that not only As(III), but also Cu(II), can absorb 254 nm light. The absorption of 254 nm light by Cu(II) can induce electron transfer from Cu(II) to oxygen, which accelerates the production of hydroxyl radicals (OH) as a primary oxidant for As(III) oxidation and generates Cu(III) as another primary oxidant. The photooxidation of As(III) in the absence of Cu(II) was limited under anoxic conditions, but was observed in the presence of Cu(II). This result suggests another role of Cu(II) as an alternative electron acceptor, which enhances the production of OH and Cu(III) by mediating electron transfer from As(III) (or Cu(II)) to oxygen. Therefore, both OH and Cu(III) are involved in the oxidation of As(III) to As(V) as primary oxidants in the presence of Cu(II). The photooxidation of As(III) with Cu(II) was not completely inhibited in the presence of excess tert-butyl alcohol (TBA, an OH scavenger), in contrast with the case without Cu(II). This result confirms the generation of Cu(III) and its involvement in the As(III) oxidation processes in the presence of Cu(II). The present results imply that Cu(II) can act as both a UV-C photosensitizer and an electron acceptor in photochemical water treatment processes under UV-C irradiation and significantly affect the kinetics of pollutant oxidation.

Hydroxyl-functionalized TiO2@SiO2@Ni/nZVI nanocomposites fabrication, characterization and enhanced simultaneous visible light photocatalytic oxidation and adsorption of arsenite

In this study, salicylic acid-TiO2@SiO2@Ni/nZVI (SA-NFST) nanocomposites with rich hydroxyl groups were synthesized using in situ surface modified, co-precipitation, and sol-gel methods. A variety of techniques, including scanning electron microscopy (SEM), X-ray powder diffraction (XRD), vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), the point of zero charge (PZC), UV–vis diffuse reflectance spectra (UV–vis), infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), were used to characterize the properties of the nanoparticles.The results showed that the SA-NFST exhibited high visible light-harvesting efficiency, easily magnetic separation, rich hydroxyl groups and hydroxyl radicals, and larger surface area. The adsorption and photo-oxidation of arsenite by the new materials were explored in different experimental conditions, including pH, initial arsenite concentration, catalyst dosage, contact time, and coexisting anions (PO43−, SiO32−, CO32−, and SO42−).The maximum adsorption capacities (qm) of As(III) onto SA-NFST calculated from Langmuir adsorption isotherm were 73.9 and 83.6 mg/g without and with visible light irradiation, respectively. >99.8% of As(III) (5.0 mg/L) was removed in the presence of 0.25 g/L SA-NFST nanoparticles (NPs) under 1.5 h visible light illumination. The residual concentration of As(III) (9.0 µg/L) was less than 10 µg/L, which is below the maximum allowable values in drinking water. As(III) removal may be due to the synergic combined effects of surface adsorption, photocatalytic oxidation under visible light illumination.

Photocatalytic Oxidation of Arsenite Using Goethite and UV LED

Arsenic (As) has been considered as the most toxic one among various hazardous materials and As contamination can be caused naturally and anthropogenically. Major forms of arsenic in groundwater are arsenite [(As(III)] and/or arsenate [(As(V)], depending on redox condition: arsenite and arsenate are predominant in reduced and oxidized environments, respectively. Because arsenite is much more toxic and mobile than arsenate, there have been a number of studies on the reduction of its toxicity through oxidation of As(III) to As(V). This study was initiated to develop photocatalytic oxidation process for treatment of groundwater contaminated with arsenite. The performance of two types of light sources (UV lamp and UV LED) was compared and the feasibility of goethite as a photocatalyst was evaluated. The highest removal efficiency of the process was achieved at a goethite dose of 0.05 g/L. Based on the comparison of oxidation efficiencies of arsenite between two light sources, the apparent performance of UV LED was inferior to that of UV lamp. However, when the results were appraised on the basis of their emitting UV irradiation, the higher performance was achieved by UV LED than by UV lamp. This study demonstrates that environmentally friendly process of goethite-catalytic photo-oxidation without any addition of foreign catalyst is feasible for the reduction of arsenite in groundwater containing naturally-occurring goethite. In addition, this study confirms that UV LED can be used in the photo-oxidation of arsenite as an alternative light source of UV lamp to remedy the drawbacks of UV lamp, such as long stabilization time, high electrical power consumption, short lifespan, and high heat output requiring large cooling facilities. Key words: Arsenite [As(III)], Arsenate [As(V)], Photocatalytic Oxidation, UV Lamp, UV LED, Goethite

Photooxidation of Arsenite under 254 nm Irradiation with a Quantum Yield Higher than Unity

Arsenite (As(III)) in water was demonstrated to be efficiently oxidized to arsenate (As(V)) under 254 nm UV irradiation without needing any chemical reagents. Although the molar absorption coefficient of As(III) at 254 nm is very low (2.49 ± 0.1 M(-1)cm(-1)), the photooxidation proceeded with a quantum yield over 1.0, which implies a chain of propagating oxidation cycles. The rate of As(III) photooxidation was highly enhanced in the presence of dissolved oxygen, which can be ascribed to its dual role as an electron acceptor of photoexcited As(III) and a precursor of oxidizing radicals. The in situ production of H2O2 was observed during the photooxidation of As(III) and its subsequent photolysis under UV irradiation produced OH radicals. The addition of tert-butyl alcohol as OH radical scavenger significantly reduced (but not completely inhibited) the oxidation rate, which indicates that OH radicals as well as superoxide serve as an oxidant of As(III). Superoxide, H2O2, and OH radicals were all in situ generated from the irradiated solution of As(III) in the presence of dissolved O2 and their subsequent reactions with As(III) induce the regeneration of some oxidants, which makes the overall quantum yield higher than 1. The homogeneous photolysis of arsenite under 254 nm irradiation can be also proposed as a new method of generating OH radicals.

Natural montmorillonite induced photooxidation of As(III) in aqueous suspensions: Roles and sources of hydroxyl and hydroperoxyl/superoxide radicals

Photooxidation of arsenite(As(III)) in a suspension of natural montmorillonite under the irradiation of metal halide lamp (λ≥313nm)has been investigated. The results showed that the natural montmorillonite induced the photooxidation of As(III) by generating hydroxyl radicals (HO) and hydroperoxyl/superoxide radicals (HO2/O2−). HO which was responsible for the As(III) photooxidation. Approximately 38% of HO was generated by the photolysis of ferric ions, and the formation of the remaining 62% was strongly dependent on the HO2/O2−. The presence of free ironions (Fe2+ and Fe3+), made significant contributions to the photogeneration of these reactive oxygen species (ROS). The photooxidation of As(III) in natural montmorillonite suspensions was greatly influenced by the pH values. The photooxidation of As(III) by natural montmorillonite followed the Langmuir–Hinshelwood equation. In addition, the photooxidation of As(III) could be enhanced by the addition of humic acid. This work demonstrates that photooxidation may be an important environmental process for the oxidation of As(III) and may be a way to remove As(III) from acidic surface water containing iron-bearing clay minerals.

Photooxidation of arsenite by natural goethite in suspended solution

Iron and arsenic have been found to coexist in a water environment and the fate of arsenite in the aquatic system is influenced by iron. Goethite is a form of iron hydroxide, which is commonly found in sediments. In previous studies, we have used iron complexes to degrade organic pollutants. Results have shown that some organic pollutants could be totally degraded by iron complexes and our work indicated that iron might cause conversion of arsenic when irradiated. This work attempts to investigate the conversion of arsenite [As(III)] using natural goethite, as the iron source, to quantify the effect of various factors on photooxidation. We also consider the possible mechanism for photooxidation of As(III) using a suspension of natural goethite. The As(III) concentration variation under illumination was compared with the one in the dark to quantify the contribution of light to As(III) oxidation to As(V) in goethite suspended solution. The experiments under N(2) and air atmosphere confirmed the participation of dissolved oxygen. The photooxidation efficiency of As(III) under different conditions was compared to determine the effect of different environmental factors such as pH value, goethite concentration, and humic acid concentration on the photooxidation reaction. In the solution containing 100 μg L(-1) arsenite and 0.1 g L(-1) suspended goethite at pH 3.0, nearly 80 % of As(III) was photooxidized after irradiation by a 250-W metal halogen lamp (λ ≥ 313 nm) after 6 h. The effects of initial pH and goethite concentration and humic acid concentration were all examined. The results show that the greatest efficiency of photooxidation of As(III) was at pH 3.0. The extent of photooxidation decreased with increasing goethite concentration and fell sharply in the presence of humic acid under the conditions in this work. Although about 80 % of As(III) was photooxidized after irradiation by a 250-W halogen lamp at pH 3.0 in the presence of goethite suspension, photooxidation was also affected by factors such as pH, concentration of goethite, and presence of humic acid. The scavenger experiments showed that the HO• radical and photogenerated hole are the predominant oxidants in this system responsible for 87.1 % oxidation of As(III), while HO (2)(•) /O(2)(•-) is responsible for 12.9 % oxidation of As(III).