Unexpected Formation of Higher Polythionates in the Oxidation of Thiosulfate by Hypochlorous Acid in a Slightly Acidic Medium

Electro-Fenton treatment of contaminated mine water to decrease thiosalts toxicity to Daphnia magna

Treatment of organic contaminants using the electro-Fenton (EF) process is efficient but generates toxic by-products. The aim of the present study was to assess the residual toxicity associated to the treatment of real mine effluents using EF and to perform a preliminary techno-economic analysis to compare the costs of different techniques. Two mine effluents from northern Quebec with different concentrations of thiosalts (ME_low and ME_high) were tested for acute toxicity to Daphnia magna, before and after EF treatment. The higher toxicity of untreated ME_low compared to ME_high, despite its lower thiosalts content (58 vs 199 mg/L), suggests the presence of an unidentified toxic species, which was removed during EF treatment, or that higher thiosalts concentrations mitigate the toxicity of other toxicants. EF treatment of ME_high, initially non-acutely toxic (50% mortality), resulted in the elimination of D. magna mortality. A preliminary techno-economic analysis conducted for northern Quebec vs the rest of Canada and the USA showed that energy consumption was the main contributor (52-95%) to the total operating costs. Electricity-related costs nearly doubled (55%) for northern Quebec relative to the rest of Canada. These findings provide new insights for the potential application of the EF for the treatment of thiosalts in mine water, for operations in central jurisdictions and in remote northern areas.

Electro-Fenton beyond the Degradation of Organics: Treatment of Thiosalts in Contaminated Mine Water

Electro-Fenton (EF) is an emerging technology with well-known outstanding oxidation power; yet, its application to the treatment of inorganic contaminants has been largely disregarded. Thiosalts are contaminants of emerging concern in mine water, responsible for delayed acidity in natural waterways. In this study, EF was used to treat thiosalts in synthetic and real mine water. Thiosulfate (S₂O₃^2-) solutions were first used to optimize the main parameters affecting the process, namely, the current density (2.08-6.25 mA cm^-2), temperature (4 vs 20 °C), and S₂O₃^2- concentration (0.25-2 g L^-1). S₂O₃^2- was almost completely removed in 2 h of treatment at 6.25 mA cm^-2, while temperature played no important role in the process efficiency. The optimal conditions were then applied to treat a real sample of contaminated mine water, resulting in complete S₂O₃^2- and S₄O₆^2- oxidation to SO₄^2- in 90 min at 6.25 mA cm^-2 (95% removal in only 60 min). The reaction mechanisms were investigated in detail based on the quantification of the main degradation byproducts. This study opens new possibilities for EF application to the treatment of thiosalt-contaminated mine water and other oxidizable inorganic-impacted wastewaters.

Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl+ Versus O Atom Transfer

Sulfur-containing compounds are known to be susceptible to oxidation by aqueous HOCl, but the factors affecting the rates of these reactions are not well-established. Here we report on the kinetics of oxidation of thiosulfate, thiourea, thioglycolate, (methylthio)acetate, tetrathionate, dithiodiglycolate, and dithiodipropionate at 25 °C and 0.4 M ionic strength. These reactions obey the general rate law -d[OCl^-]/dt = (k_OCl^-[OCl^-] + k_HOCl[HOCl])[substrate] with some exceptions: tetrathionate and the two disulfides undergo rate-limiting hydrolysis at high pH, and dithiodiglycolate has an additional term in the rate law that is second order in [substrate]. The reactions of HOCl are believed to have a Cl⁺ transfer mechanism, and in the case of thiosulfate the rate of hydrolysis of the ClS₂O₃^- intermediate was determined. In the case of thiourea evidence was obtained for thiourea monoxide as a long-lived product. It is shown that sulfite and species with terminal sulfur atoms have k_HOCl values in the vicinity of 1 × 10⁹ M^-1 s^-1, while SCN^- and thioethers react somewhat more slowly; tetrathionate, trithionate, and disulfides react much more slowly. Comparison of the rate constants with those for oxidation of these sulfur substrates by H₂O₂ and [Pt(CN)₄Cl₂]^2- shows that HOCl reacts a few orders of magnitude more rapidly than [Pt(CN)₄Cl₂]^2- and ∼9 orders of magnitude more rapidly than H₂O₂. Many of the k_HOCl values are leveled by the high electrophilicity of HOCl. It is proposed that the k_OCl^- values correspond to oxygen-atom transfer mechanisms, as supported by LFERS (linear free energy relationships) relating these rate constants to those for reactions of H₂O₂ and [Pt(CN)₄Cl₂]^2-.

Construction of a multipurpose photochemical reactor with on-line spectrophotometric detection

A versatile photoreactor was constructed from commercially available parts, which is capable of recording high quality kinetic traces in homogeneous and heterogeneous photoreactions and also easily adaptable to flow-through operation.

High performance liquid chromatography study on the kinetics and mechanism of chlorite-thiosulfate reaction in slightly alkaline medium

Chlorite-thiosulfate reaction was studied by high performance liquid chromatography under slightly alkaline solution by monitoring the concentration of thiosulfate and tetrathionate simultaneously during the course of reaction. It is demonstrated that various polythionates are formed and the composition of polythionates mainly depends on pH. Initial rate studies have revealed that the formal kinetic order of hydrogen ion is unambiguously unity but that of chlorite ion is significantly larger, while that of thiosulfate is lower than one. A 10-step kinetic model is proposed with seven fitted and three fixed parameters by simultaneous evaluation of kinetic data taking all the important characteristics of the measured kinetic curves into account. It is also enlightened that higher formal kinetic order of chlorite and lower kinetic order of thiosulfate than unity is an inherent feature of the system and may explain the systematic deviation between the measured and the calculated data encountered in previous studies.

Kinetics and Mechanism of the Decomposition of Tetrathionate Ion in Alkaline Medium

The kinetics of the alkaline decomposition of tetrathionate has been studied spectrophotometrically in the pH range of 9.2-12.2 using both phosphate/hydrogen phosphate and carbonate/hydrogen carbonate as buffer systems and by adjusting the ionic strength to I=0.5 M with sodium perchlorate at T=25.0+/-0.1 degrees C. Matrix rank analysis of the spectra recorded between 265 and 330 nm shows the presence of three independent absorbing species. Besides tetrathionate, thiosulfate and trithionate are identified as absorbing products of the decomposition, but sulfite and a trace amount of sulfate are also formed during the alkaline degradation process. With pentathionate, sulfoxylic acid (S(OH)2), S2O3OH-, and S3O3OH- as key intermediates, a 10-step kinetic model is proposed with six fitted kinetic parameters to take all the important characteristics of the experimental curves into account. On the basis of the stoichiometric measurements and the model proposed, it is also enlightened that the product distribution of the reaction continuously varies with pH; thus the kinetic traces cannot be evaluated by assuming a single stoichiometry in contrast to recent studies. Buffer dependence of the decomposition is also discussed.