Magnetically assembled electrodes based on Pt, RuO2-IrO2-TiO2 and Sb-SnO2 for electrochemical oxidation of wastewater featured by fluctuant Cl- concentration

Magnetically assembled electrode (MAE) flexibly attracts magnetic particles (auxiliary electrodes, AEs) on a main electrode (ME) by the magnetic force, where the role of ME is always ignored. In this study, Ti/Pt, Ti/RuO₂-IrO₂-TiO₂ and Ti/Sb-SnO₂ were selected as the ME for comparison in treating synthetic wastewater (acid red G or phenol) with variable Cl^- content. The effects of ME type, loading amount of Fe₃O₄/Sb-SnO₂ AEs, and Cl^- concentration were investigated, followed by varied electrochemical characterizations. Results show that AEs played a vital role in electrode activity and selectivity, and MEs also exerted an unignorable influence on the performance of the MAEs. Among the three MEs, Ti/RuO₂-IrO₂-TiO₂ has the best OER/CER ability, activating more extra active sites with same AEs loading amount, leading to higher organics degradation efficiency under chlorine-free condition. However, this MAE is featured by the noticeable accumulation of intermediate products under chlorine-free condition even if 0.3 g·cm^-2 of AEs are loaded. All electrodes' performances were enhanced in the presence of Cl^-. With high concentration chloride (0.5 M NaCl), the accumulation of intermediate products was reduced significantly, especially on Ti/RuO₂-IrO₂-TiO₂ based MAE, and no chlorinated compound was identified. Finally, the structure-activity relationships of these MAEs were proposed.

Effects of residual disinfectants on the redox speciation of lead(ii)/(iv) minerals in drinking water distribution systems

This study investigated the reaction kinetics on the oxidative transformation of lead(ii) minerals by free chlorine (HOCl) and free bromine (HOBr) in drinking water distribution systems. According to chemical equilibrium predictions, lead(ii) carbonate minerals, cerussite PbCO3(s) and hydrocerussite Pb3(CO3)2(OH)2(s), and lead(ii) phosphate mineral, chloropyromorphite Pb5(PO4)3Cl(s) are formed in drinking water distribution systems in the absence and presence of phosphate, respectively. X-ray absorption near edge spectroscopy (XANES) data showed that at pH 7 and a 10 mM alkalinity, the majority of cerussite and hydrocerussite was oxidized to lead(iv) mineral PbO2(s) within 120 minutes of reaction with chlorine (3 : 1 Cl2 : Pb(ii) molar ratio). In contrast, very little oxidation of chloropyromorphite occurred. Under similar conditions, oxidation of lead(ii) carbonate and phosphate minerals by HOBr exhibited a reaction kinetics that was orders of magnitude faster than by HOCl. Their end oxidation products were identified as mainly plattnerite β-PbO2(s) and trace amounts of scrutinyite α-PbO2(s) based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. A kinetic model was established based on the solid-phase experimental data. The model predicted that in real drinking water distribution systems, it takes 0.6-1.2 years to completely oxidize Pb(ii) minerals in the surface layer of corrosion scales to PbO2(s) by HOCl without phosphate, but only 0.1-0.2 years in the presence of bromide (Br-) due the catalytic effects of HOBr generation. The model also predicts that the addition of phosphate will significantly inhibit Pb(ii) mineral oxidation by HOCl, but only be modestly effective in the presence of Br-. This study provides insightful understanding on the effect of residual disinfectant on the oxidation of lead corrosion scales and strategies to prevent lead release from drinking water distribution systems.

Ferrate(VI) pretreatment of water containing natural organic matter, bromide, and iodide: A potential strategy to control soluble lead release from PbO2(s)

Lead dioxide (PbO₂(s)) is a corrosion product of lead-containing plumbing materials in water distribution pipelines. The presence of reductants in water could cause the release of soluble lead (mainly Pb(II)) from PbO₂(s). Lead in drinking water is detrimental to public health. This paper presents the first application of ferrate (Fe^VIO₄^2-, Fe(VI)) to decreasing the generation of soluble lead in water containing PbO₂(s) and common reducing constituents (e.g., natural organic matter (NOM), iodide (I^-), and bromide (Br^-)) at different pH conditions (i.e., 6.0, 7.0, and 8.0). The released soluble lead from PbO₂(s) was found to be dominantly controlled by NOM in water, via the redox dissolution of PbO₂(s) and the reduction of PbO₂(s) by reducing moieties of NOM. The feasibility of both processes increased when pH decreased. The I^- and Br^- in water played minor roles in generating soluble lead. Fe(VI) reacted with reducing functional groups of NOM, as determined by ¹³C nuclear magnetic resonance spectroscopy. Water pretreatment with Fe(VI) inhibited the reaction of NOM with PbO₂(s) and therefore, caused lower soluble lead concentrations compared to water samples without Fe(VI) treatment. This study indicates that Fe(VI) pretreatment is a potential approach to controlling soluble lead in drinking water.

Hexavalent Chromium Release in Drinking Water Distribution Systems: New Insights into Zerovalent Chromium in Iron Corrosion Scales

Upon cast iron corrosion in contact with residual disinfectants, drinking water distribution systems have become potential geogenic sources for hexavalent chromium Cr(VI) release. This study investigated mechanisms of Cr(VI) release from cast iron corrosion scales. The oxidation of the corrosion scales by residual disinfectant chlorine released Cr(VI) and exhibited a three-phase kinetics behavior: an initial 2 h fast reaction phase, a subsequent 2-to-12 h transitional phase, and a final 7-day slow reaction phase approximately 2 orders of magnitude slower than the initial phase. X-ray absorption spectroscopy analysis discovered that zerovalent Cr(0) coexisted with trivalent Cr(III) solids in the corrosion scales. Electrochemical corrosion analyses strongly suggested that Cr(0) in the corrosion scales originated from Cr(0) in the cast iron alloy. Cr(0) exhibited a much higher reactivity than Cr(III) in the formation of Cr(VI) by chlorine. The presence of bromide in drinking water significantly accelerated Cr(VI) release because of its catalytic effect. Meanwhile, chlorine consumption was mainly attributed to the oxidation of organic matter and ferrous iron. Findings from this study point to a previously unknown but important pathway of Cr(VI) formation in drinking water, that is, direct oxidation of Cr(0) by chlorine, and suggest new strategies to control Cr(VI) in drinking water by inhibiting Cr(0) reactivity.

Differential Cytotoxicity of Haloaromatic Disinfection Byproducts and Lead Co-exposures against Human Intestinal and Neuronal Cells

Disinfecting drinking water with chlorine inadvertently generates disinfection byproducts (DBPs) which can cause potential adverse health effects to humans. Haloaromatic DBPs are a group of emerging DBPs recently identified, suspected to be substantially more toxic than haloaliphatic DBPs but have not been extensively studied. Simultaneously, service pipelines made of lead materials are widely used in water distribution systems and become a source of dissolved lead (Pb) in tap water. In this study, we investigated the cytotoxicity of nine haloaromatic DBPs and lead ion (Pb²⁺), both separately as well as in combination, to human epithelial colorectal adenocarcinoma (Caco-2) and neuroblastoma (SH-SY5Y) cells. Results show that the cytotoxicity of the DBPs against Caco-2 cells followed the descending rank order of 2,4,6-triiodophenol ≅ 2,5-dibromohydroquinone > 2,4,6-tribromophenol > 3,5-dibromo-4-hydroxybenzaldehyde ≅ 2,4,6-trichlorophenol > 4-chlorophenol ≅ 3,5-dibromo-4-hydroxybenzoic acid > 2,6-dichlorophenol >5-chlorosalicylic acid, and the cytotoxicity of the DBPs against SH-SY5Y cells followed a similar rank order, 2,4,6-triiodophenol ≅ 2,5-dibromohydroquinone > 2,4,6-tribromophenol > 3,5-dibromo-4-hydroxybenzaldehyde ≅ 2,4,6-trichlorophenol > 4-chlorophenol > 3,5-dibromo-4-hydroxybenzoic acid > 2,6-dichlorophenol ≅ 5-chlorosalicylic acid. Lead in water did not change the toxicity of 3,5-dibromo-4-hydroxybenzoic acid (to either cell-type) or the toxicity of 4-chlorophenol (to the neuronal cell-type); but Pb²⁺ exhibited different degrees of synergistic effects with other tested DBPs. The synergism resulted in different rank orders of cytotoxicity against both intestinal and neuronal cells. These data indicate that future prioritization and regulation of emerging haloaromatic DBPs in drinking water should be considered in terms of their own toxicity and combinatorial effects with lead in water.