Evaluation of the photooxidation efficiency of As(III) applying the UVC/oxalate technique

In this study, the photooxidation capacity of UVC/Oxalate (Ox) was evaluated using As(III) as a typical pollutant. The results show that the direct oxidation amount of As(III) induced by UVC in water was negligible, but the presence of Ox remarkably accelerated the oxidation rate of As(III). Under UVC irradiation, 50 μM As(III) can be completely oxidized to As(V) in the case of Ox concentration above 300 μM within 60 min. As(III) oxidation was found greatly related with the photodecomposition of Ox. Much more Ox can be mineralized in more acidic solution. At the same time, the photooxidation of As(III) was significantly favored at decreased initial pH from 8.0 to 3.0. In this reaction system, the role of oxygen was indispensable for Ox photodecomposition and As(III) photooxidation, which can be ascribed to its special roles as a precursor of reactive superoxide and an electron acceptor. In oxygen-present atmosphere, the in situ production of H₂O₂ was detected during the photolysis of Ox and its photolysis product, i.e., OH primarily contributed to the oxidation of As(III). However, the photodecomposition of Ox and photooxidation of As(III) were significantly inhibited in the anaerobic environment. In general, the homogeneous photolysis of Ox in many commonly practiced UVC oxidation processes can be also proposed as a supplementary method of generating highly oxiditive species in aerobic condition.

Near-UV photooxidation of As(III) by iron species in the presence of fulvic acid

Photooxidation of As(III) in ternary As(III) - Fe(III) - Fulvic acid system at pH 4 was investigated by optical spectroscopy, steady-state photolysis (365 nm) and atomic-emission spectrometry with inductively coupled plasma techniques. It was found that at all values of [FA]/[Fe] ratio the main photoactive species is OH radical formed by photolysis of Fe(III) hydroxocomplexes. Addition of fulvic acid leads to mainly negative effect on As(III) photooxidation due to the following reasons: (i) slow dark reduction of photoactive Fe(III) species with formation of scattering particles and photoinert Fe(II) species; (ii) formation of photoreductive Fe(III)-FA complexes incapable to oxidize As(III), (iii) competition of both FA and Fe(III)-FA complexes for UVA quanta with FeOH²⁺ complex and for OH radicals with As(III). Aging of ternary system is also very important parameter leading to one order decrease of quantum yields of both Fe(II) formation and As(III) photooxidation.

Photooxidation of arsenic(III) to arsenic(V) on the surface of kaolinite clay

As one of the most toxic heavy metals, the oxidation of inorganic arsenic has drawn great attention among environmental scientists. However, little has been reported on the solar photochemical behavior of arsenic species on top-soil. In the present work, the influencing factors (pH, relative humidity (RH), humic acid (HA), trisodium citrate, and additional iron ions) and the contributions of reactive oxygen species (ROS, mainly HO and HO2/O2(-)) to photooxidation of As(III) to As(V) on kaolinite surfaces under UV irradiation (λ=365nm) were investigated. Results showed that lower pH facilitated photooxidation, and the photooxidation efficiency increased with the increase of RH and trisodium citrate. Promotion or inhibition of As(III) photooxidation by HA was observed at low or high dosages, respectively. Additional iron ions greatly promoted the photooxidation, but excessive amounts of Fe(2+) competed with As(III) for oxidation by ROS. Experiments on scavengers indicated that the HO radical was the predominant oxidant in this system. Experiments on actual soil surfaces proved the occurrence of As(III) photooxidation in real topsoil. This work demonstrates that the photooxidation process of As(III) on the soil surface should be taken into account when studying the fate of arsenic in natural soil newly polluted with acidic wastewater containing As(III).

Arsenic removal from water employing a combined system: photooxidation and adsorption

A combined system employing photochemical oxidation (UV/H2O2) and adsorption for arsenic removal from water was designed and evaluated. In this work, a bench-scale photochemical annular reactor was developed being connected alternately to a pair of adsorption columns filled with titanium dioxide (TiO2) and granular ferric hydroxide (GFH). The experiences were performed by varying the relation of As concentration (As (III)/As (V) weight ratio) at constant hydrogen peroxide concentration and incident radiation. Experimental oxidation results were compared with theoretical predictions using an intrinsic kinetic model previously obtained. In addition, the effectiveness of the process was evaluated using a groundwater sample. The mathematical model of the entire system was developed. It could be used as an effective tool for the design and prediction of the behaviour of these types of systems. The combined technology is efficient and promising for arsenic removal to small and medium scale.

Effects of γ-ray irradiation on optical absorption and laser damage performance of KDP crystals containing arsenic impurities

The effects of γ-irradiation on potassium dihydrogen phosphate crystals containing arsenic impurities are investigated with different optical diagnostics, including UV-VIS absorption spectroscopy, photo-thermal common-path interferometer and photoluminescence spectroscopy. The optical absorption spectra indicate that a new broad absorption band near 260 nm appears after γ-irradiation. It is found that the intensity of absorption band increases with the increasing irradiation dose and arsenic impurity concentration. The simulation of radiation defects show that this absorption is assigned to the formation of AsO₄⁴⁻ centers due to arsenic ions substituting for phosphorus ions. Laser-induced damage threshold test is conducted by using 355 nm nanosecond laser pulses. The correlations between arsenic impurity concentration and laser induced damage threshold are presented. The results indicate that the damage performance of the material decreases with the increasing arsenic impurity concentration. Possible mechanisms of the irradiation-induced defects formation under γ-irradiation of KDP crystals are discussed.

Superconducting properties of (Ba-K)Fe2As2 single crystals disordered with fast neutron irradiation

Resistivity ρ(T), Hall coefficient RH(T), superconducting transition temperature Tc and slopes of the upper critical field dHc2/dT were studied in (Ba1-xKx)Fe2As2 (x = 0.218, 0.356, 0.531) single crystals irradiated with fast neutrons. It is found that dTc/dρSC-the rate of decreasing Tc as a function of the ρSC (ρSC is the resistivity at T = Tc)-linearly increases with concentration x. Slow changes in the Hall coefficient RH, as well as the quadratic electronic contribution to the resistivity, show that there are no substantial changes in the topology of the Fermi surface caused by irradiation. The slopes of the upper critical field dHc2/dT in ab and c directions as a function of ρSC determined by Hall measurements show a reasonable agreement with a model that suggests constancy of the band parameters.

Characterization and detoxification of a mature landfill leachate using a combined coagulation-flocculation/photo Fenton treatment

The aim of the present work was to characterize and treat a mature landfill leachate using a coagulation/flocculation process followed by a photo-Fenton oxidation treatment. The leachate was obtained from a landfill in Tetlama, Morelos (Mexico) during the drought season and was characterized in terms of its major pollutants. Considerable levels of chemical oxygen demand (COD), total carbon (TC) and NH4+ were identified, as well as high concentrations of Hg, Pb, and As. Other heavy metals such as Ni, Co, Zn, Cd, and Mn were detected at trace levels. The lethal concentration (LC50) of the leachate, evaluated on Artemia salina, was 12,161±11 mg/L of COD, demonstrating an antagonistic interaction among the leachate's components. The treatment of this effluent consisted of a coagulation-flocculation process using an optimal dose of FeCl3 · 6H2O of 300 mg/L. The supernatant was treated using a photo-Fenton process mediated with FeCl2 · 4H2O and H2O2 in a compound parabolic concentrator (CPC) photo-reactor operating in batch mode using an R ratio (R=[H2O2]/[Fe2+]) of 114. The global removal efficiencies after treatment were 56% for the COD, 95% for TC, and 64% for NH4+. The removal efficiencies for As, Hg, and Pb were 46%, 9%, and 85%, respectively.

TiO₂-photocatalytic reduction of pentavalent and trivalent arsenic: production of elemental arsenic and arsine

Heterogeneous photocatalytic reduction of As(V) and As(III) at different concentrations over TiO(2) under UV light in deoxygenated aqueous suspensions is described. For the first time, As(0) was unambiguously identified together with arsine (AsH(3)) as reaction products. As(V) reduction requires the presence of an electron donor (methanol in the present case) and takes place through the hydroxymethyl radical formed from methanol oxidation by holes or hydroxyl radicals. On the contrary, As(III) reduction takes place through direct reduction by the TiO(2)-conduction band electrons. Detailed mechanisms for the photocatalytic processes are proposed. Although reduction to solid As(0) is convenient for purposes of As removal from water as a deposit on TiO(2), attention must be paid to formation of AsH(3), one of the most toxic forms of As, and strategies for AsH(3) treatment should be considered.

Effects of light regime, temperature, and plant age on uptake of arsenic by Spartina pectinata and Carex stricta

We report here on efforts to show that a combination of native wetland plant species might perform better than a monoculture in wetlands designed for arsenic remediation by supplementing weaknesses. Carex stricta and Spartina pectinata were used in hydroponic experiments. (i) Arsenic uptake was first assessed at two ages via exposure to control or arsenic-laden solutions (0 or 1.5 mg As L(-1) as Na2HAsO4) for two weeks. Age had no significant effect on arsenic concentrations in roots, but translocation factors were greater in older plants of C. stricta and S. pectinata (0.45 and 0.07, respectively) than in younger plants (0.10 and 0.01, respectively). (ii) Seasonal effects were assessed by determining uptake kinetics for both species in conditions representative of spring temperatures (15/5 degrees C) and light regimes (1050 micromol m(-2) s(-1), 13 h day(-1)) and summer temperatures (28/17 degrees C) and light regimes (1300 micromol m(-2) s(-1), 15 h day(-1)). Both species had comparable rates of arsenic uptake into roots in summer conditions (44.0 and 46.5 mg As kg(-1) dry wt. h(-1) in C. stricta and S. pectinata, respectively), but C. stricta had a higher maximum net influx rate in spring conditions (24.5 versus 10.4 mg As kg(-1) dry wt. h(-1)).

Randomized placebo-controlled trial of 2,3-dimercaptosuccinic acid in therapy of chronic arsenicosis due to drinking arsenic-contaminated subsoil water

INTRODUCTION

Chronic arsenic toxicity producing various clinical manifestations is currently epidemic in West Bengal, India, Bangladesh, and other regions of the world. Animal studies have indicated that 2,3-dimercaptosuccinic acid can be used as an oral chelating agent. A prospective, double-blind, randomized controlled trial was carried out to evaluate the efficacy and safety of 2,3-dimercaptosuccinic acid for chronic arsenicosis due to drinking arsenic-contaminated (> or = 50 micrograms/L) subsoil water in West Bengal.

METHOD

Twenty-one consecutive patients with chronic arsenicosis were individually randomized (random number; assignment made by individual not evaluating patients) into 2 groups: 11 patients (10 male, age 25.5 +/- 8 years) received 2,3-dimercaptosuccinic acid 1400 mg/d (1000 mg/m2) in the first week and 1050 mg/d (750 mg/m2) during the next 2 weeks with a repeat course 3 weeks later. The other 10 patients (all male, age 32.2 +/- 9.7 years) were given placebo capsules for the same schedule. The clinical features were evaluated by an objective scoring system before and after treatment. Routine investigations including liver function tests, arsenic concentrations in urine, hair, and nails, and skin biopsy evaluations were also completed.

RESULTS

Though there was improvement in the clinical score of 2,3-dimercaptosuccinic acid-treated patients, similar improvement was observed in the placebo-treated group. There were no statistical differences in the clinical scores between the 2 groups at the beginning and at the end of treatment. Similarly, no differences were found for the other investigated parameters.

CONCLUSION

Under the conditions of this study, 2,3-dimercaptosuccinic acid was not effective in producing any clinical or biochemical benefit or any histopathological improvement of skin lesions in patients with chronic arsenicosis.