Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water

Bromate-induced oxidation of carbamazepine and toxicity assessment of transformation products in the freezing-sunlight process: Effects of trivalent chromium

Bromate (BrO3-)-induced pharmaceutical and personal care products (PPCPs) oxidation is enhanced in freezing systems. Reduced forms of metals are widely present, often coexisting with various contaminants. However, their effects on the interaction of PPCPs with BrO3- in ice in cold regions may have been overlooked. Herein we investigated the effects of representative reducing metal Cr(III) on the interaction between the representative PPCP carbamazepine (CBZ) and BrO3- in the freezing system. Our findings demonstrated that the degradation rate constants of CBZ by BrO3- and Cr(III) were 29.4%-60.3% lower than those by BrO3- in ice, revealing the inhibition of Cr(III) on CBZ degradation by BrO3- in ice. In BrO3-/freezing/sunlight system, BrO3- contributed 62.8% to CBZ degradation. In BrO3-/Cr(III)/freezing/sunlight system, Cr(III) promoted the generation of hydroxyl radical (·OH), leading to 51.0% contribution of ·OH to CBZ degradation. Oxidants were consumed by Cr(III) to form Cr(VI) rather than reacting with CBZ, thereby decreasing CBZ degradation by BrO3- in ice. Due to sunlight-induced Cr(VI) reduction in ice, only 0.3% of Cr(III) was converted to Cr(VI) in BrO3-/Cr(III)/freezing/sunlight system. BrO3--induced CBZ degradation rate in ice decreased in order of Fe(II), Cr(III), and Mn(II), which was due to the different reducing capabilities. An effective reduction in comprehensive toxicity of systems followed the freezing-sunlight process, even in the presence of Cr(III). This work sheds new light on the environmental behaviors and fate of PPCPs, brominated disinfection by-products, and reducing metals during seasonal freezing.

The self-boosting ultrafast removal of Cr(VI) and organic dye in textile wastewater through sulfite-induced redox processes

The treatment of textile wastewater containing harmful metal ions poses a significant challenge in industrial applications due to its environmental impact. In this study, the use of sulfite for treating simulated dye wastewater containing New Coccine (NC) and Cr(VI) was investigated. The removal of NC was influenced by the redox reaction between Cr(VI) and sulfite, demonstrating a strong self-boosting effect of Cr(VI) on NC removal. Remarkable NC decoloration (95%) and Cr(VI) reduction (90%) were achieved within 1 min, highlighting the effectiveness of the treatment. Quenching experiments and electron paramagnetic resonance (EPR) technology confirmed that singlet oxygen (1O2) was the main oxidative agent for organic dye removal and SO4•-, •OH and Cr(V) were also identified as key contributors to NC degradation. The Cr(VI)/sulfite system exhibited higher efficiency in degrading azo dyes, such as NC and Congo Red (CR), compared to non-azo dyes like Methylene Blue (MB). This superiority may be attributed to the action of Cr(V) on azo groups. Additionally, the COD removal experiments were conducted on the actual dye wastewater, showing the excellent performance of the Cr(VI)/Sulfite system in treating industrial textile wastewater. This approach presents a promising strategy for effective "waste control by waste", offering great potential for addressing challenges related to dye wastewater treatment and environmental pollution control in practical industrial scenarios.

Adsorption of Cr(VI) and Cr(III) on layered pipe scales and the effects of disinfectants in drinking water distribution systems

Pipe scales in drinking water distribution systems (DWDS) potentially adsorb chromium (Cr). Meanwhile, the fate of Cr in pipe scales and water could be influenced by the disinfectants used in DWDS since they might influence the valence state of Cr. Therefore, the adsorption of Cr (Cr(VI) and Cr(III)) on pipe scales, the transformation between different valence states, and the effects of disinfectants present in DWDS are important research topics for improving tap water quality but have not yet been sufficiently investigated. This study investigated the properties of layered pipe scales and conducted adsorption kinetic experiments in single and binary Cr(VI) and Cr(III) systems, as well as experiments related to the oxidation and adsorption of Cr(III) under the influence of decaying disinfectants. According to the results, pipe scales exhibited distinct layered structures with varying mechanisms for the adsorption of Cr(VI) and Cr(III). Cr(VI) was adsorbed through surface complexation on the surface and porous core layers, while redox reactions predominantly occurred on the shell-like layer. Furthermore, Cr(III) was adsorbed via surface precipitation on the three-layer pipe scales. Importantly, disinfectants promoted the transformation of Cr(III) to the less readily released Cr(VI) in pipe scales, reducing the Cr exposure risk from the pipe scale phase. Pipe scales also decreased the Cr(VI) concentration in water (almost 0 mg/L), enhancing the safety of DWDS. This study provides theoretical guidance on the safe operation of DWDS.

Chronic low-dose chromium VI exposure induces oxidative stress and apoptosis with altered expressions of DNA repair genes and promoter hypermethylation in the liver of Swiss albino mice

The present investigation dealt with harmful effects of hexavalent chromium (Cr [VI]) on liver of Swiss albino mice. This variant exhibited cytotoxicity, mutagenicity, and carcinogenicity. Our study focused on elucidating the hepatotoxic effects of chronic low-dose exposure to Cr (VI) (2, 5, and 10 ppm) administered via drinking water for 4 and 8 months. The observed elevation in SGPT, ALP, and SGOT and increased oxidative stress markers unequivocally confirmed the severe disruption of liver homeostasis at these low treatment doses. Noteworthy alterations in histoarchitecture, body weight, and water intake provided further evidences of the harmful effects of Cr (VI). Production of reactive oxygen species (ROS) during metabolism led to DNA damages. Immunohistochemistry and qRT-PCR analyses revealed that chronic low-dose exposure of Cr (VI) induced apoptosis in liver tissue. Our study exhibited alterations in the expression pattern of DNA repair genes (Rad51, Mutyh, Mlh1, and Ogg1), coupled with promoter hypermethylation of Mutyh and Rad51, leading to transcriptional inhibition. Our findings underscored the potential of low-dose Cr (VI) exposure on hepatotoxicity by the intricate interplay between apoptosis induction and epigenetic alterations of DNA repair genes.

Chromium immobilization and release by pipe scales in drinking water distribution systems: The impact of anions

Due to its high toxicity, the release of chromium (Cr) by pipe scales poses a serious risk to drinking water quality and human health. This study looked into how Cr immobilized and released by pipe scales. SEM, XRD, and XPS were applied to evaluate the physicochemical characterization of pipe scales. To identify times of immobilization and release and the proper scale to water ratio, the behaviors of Cr were examined in stagnation experiments. Afterward, the common anions in drinking water were designed as nine concentration gradients to explore their species and concentrations impacts on the immobilization and release process of Cr. It is worth mentioning that the pipe scales were classified into block pipe scales, lumpy pipe scales, and powder pipe scales in this experiment. The types of pipe scales were rarely considered as an influencing factor. Results revealed that in contrast to powder pipe scales, block pipe scales and lumpy pipe scales exhibited extremely comparable trends. Specifically, in terms of accumulation capacity, the order from largest to smallest was powder pipe scales, lumpy pipe scales, and block pipe scales. However, the potential of Cr release from block pipe scales was the highest, indicating a high danger of heavy metal re-release in actual circumstances. Findings from this study discovered the turning points of chloride and sulfate concentrations associated with the pipe scales species in the anion-promoted release. These results provide insight into the relationship between pipe scales and Cr in drinking water distribution systems (DWDS).

Inhibition of Hexavalent Chromium Release from Drinking Water Distribution Systems: Effects of Water Chemistry-Based Corrosion Control Strategies

In drinking water distribution systems, the oxidation of zerovalent chromium, Cr(0), in iron corrosion scales by chlorine residual disinfectant is the dominant reaction to form carcinogenic hexavalent chromium, Cr(VI). This study investigates inhibitive corrosion control strategies through adjustments of chemical water parameters (i.e., pH, silicate, phosphate, calcium, and alkalinity) on Cr(VI) formation through oxidation of Cr(0)(s) by free chlorine under drinking water conditions. The results show that an increase in pH, silicate, alkalinity, and calcium suppressed Cr(VI) formation that was mainly attributed to in situ surface precipitation of new Cr(III) solids on the surface of Cr(0)(s), including Cr(OH)3(s), Cr2(SiO3)3(s), CrPO4(s), Cr2(CO3)3(s), and Cr10Ca(CO3)16(s). The Cr(III) surface precipitates were much less reactive with chlorine than Cr(0)(s) and suppressed the Cr redox reactivity. The concentration of surface Cr(III) solids was inversely correlated with the rate constant of Cr(VI) formation. Adding phosphate either promoted or inhibited the Cr(VI) formation, depending on the phosphate concentration. This study provides fundamental insight into the Cr(VI) formation mechanisms via Cr(0) oxidation by chlorine and the importance of surface precipitation of Cr(III) solids with different corrosion control strategies and suggests that increasing the pH/alkalinity and addition of phosphate or silicate can be effective control strategies to minimize Cr(VI) formation.

Adsorption of Chromium and Nickel Ions on Commercial Activated Carbon-An Analysis of Adsorption Kinetics and Statics

The adsorption of nickel Ni(II) and chromium Cr(III) ions on the commercial activated carbons WG-12, F-300 and ROW 08, which differ in their pore structure and the chemical nature of their surfaces, were analyzed. The nickel ions Ni2+ were best adsorbed on the WG-12 activated carbon, which had the largest number of carboxyl and lactone groups on the surface of the activated carbons, and the largest specific surface area. Chromium, occurring in solutions with pH = 6 in the form of Cr(OH)2+ and Cr(OH)2+ cations, was best adsorbed on the ROW 08 Supra activated carbon, which is characterized by the highest values of water extract. The precipitation of chromium hydroxide in the pores of the activated carbon was the mechanism responsible for the high adsorption of Cr(III) on this carbon. For the other sorbents, the amount of carboxyl and lactone groups determined the amount of Cr(III) and Ni(II) adsorption. The adsorption kinetics results were described with PFO, PSO, Elovich and intraparticle diffusion models. The highest correlation coefficients for both the Cr(III) and Ni(II) ions were obtained using the PSO model. Among the seven adsorption isotherm models, very high R2 values were obtained for the Toth, Temkin, Langmuir and Jovanovic models. The Cr(III) ions were removed in slightly larger quantities than the Ni(II) ions. The capacities of the monolayer qm (calculated from the Langmuir isotherm) ranged from 55.85 to 63.48 mg/g for the Cr(III), and from 40.29 to 51.70 mg/g for the Ni(II) ions (pH = 6). The adsorption efficiency of Cr(III) and Ni(II) cations from natural waters with different degrees of mineralization (spring, weakly and moderately mineralized) was only a few percent lower than that from deionized water.

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1-x(OH)3

Cr(VI) rebound is the primary risk associated with the reduction remediation of Cr(VI)-contaminated soil. The potential impact of sulfites, which can be produced by microbial activities or originate from sulfur-containing remediation agents, on the Cr(VI) rebound in the vadose zone has been overlooked. When sulfites are present, the stability of CrxFe1-x(OH)3 is compromised and significantly inferior to that of Cr(OH)3, as demonstrated in this paper. First, Fe acts as a catalyst for the conversion of adsorbed sulfite to SO4·-, which subsequently triggers the oxidation of Cr(III) and results in the rebound of Cr(VI). The heterogeneous catalysis by Fe on the surface of CrxFe1-x(OH)3 plays a predominant role, contributing to 78% of the actual oxidation of Cr(III) among all employed catalytic processes. The presence of ambient Cl- can exacerbate the rebound effect of Cr(VI) by promoting the generation of HOCl. Furthermore, a portion of released Cr(VI) was reduced to Cr(III) by dissolved sulfite in the presence of dissolved Fe as a catalyst, thereby increasing the dissolution and migration risk associated with CrxFe1-x(OH)3. Hence, the presence of sulfites results in a significant increase in the Cr(VI) rebound and Cr(III) release from CrxFe1-x(OH)3. This challenges the conventional understanding of the stability of CrxFe1-x(OH)3.

Chromium removal from chromium gypsum through microwave hydrothermal crystal phase regulation

Chromium gypsum (CG) is a common hazardous waste formed in chromium salt or electroplating industries. The trapped or lattice-doped CrO42- in gypsum crystals are difficult to be reduced or removed by traditional methods, which will be re-oxidized or slowly released during long-term hypaethral storage. In this study, microwave hydrothermal treatment was applied to remove chromium in CG. Under optimal conditions (solid-liquid ratio of 1:5, 0.1 M sulfuric acid as liquid media, and 110 °C), over 99% of the chromium in CG can be removed within 10 min. XRD spectra indicated that 59.8% gypsum was transformed to from dihydrate gypsum to hemihydrate gypsum. The toxicity leaching test shows that chromium in CG is 377.0 mg/L before detoxification and 0.55 mg/L after detoxification, which proves that chromium in CG lattice can be efficiently removed. This work enables to significantly advance the dehydration phase transformation process of gypsum and release the heavy metal impurities within it more quickly and provides new possibilities to treat similar solid waste containing gypsum or minerals with hydration water.

New Insights into the Role of Natural Organic Matter in Fe-Cr Coprecipitation: Importance of Molecular Selectivity

Coprecipitation of Fe/Cr hydroxides with natural organic matter (NOM) is an important pathway for Cr immobilization. However, the role of NOM in coprecipitation is still controversial due to its molecular heterogeneity and diversity. This study focused on the molecular selectivity of NOM toward Fe/Cr coprecipitates to uncover the fate of Cr via Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). The results showed that the significant effects of Suwannee River NOM (SRNOM) on Cr immobilization and stability of the Fe/Cr coprecipitates did not merely depend on the adsorption of SRNOM on Fe/Cr hydroxides. FT-ICR-MS spectra suggested that two pathways of molecular selectivity of SRNOM in the coprecipitation affected Cr immobilization. Polycyclic aromatics and polyphenolic compounds in SRNOM preferentially adsorbed on the Fe/Cr hydroxide nanoparticles, which provided extra binding sites and promoted the aggregation. Notably, some specific compounds (i.e., polyphenolic compounds and highly unsaturated phenolic compounds), less unsaturated and more oxygenated than those adsorbed on Fe/Cr hydroxide nanoparticles, were preferentially incorporated into the insoluble Cr-organic complexes in the coprecipitates. Kendrick mass defect analysis revealed that the insoluble Cr-organic complexes contained fewer carbonylated homologous compounds. More importantly, the spatial distribution of insoluble Cr-organic complexes was strongly related to Cr immobilization and stability of the Fe/Cr-NOM coprecipitates. The molecular information of the Fe/Cr-NOM coprecipitates would be beneficial for a better understanding of the transport and fate of Cr and exploration of the related remediation strategy.

Hierarchical S-Scheme Heterostructure of CdIn2S4@UiO-66-NH2 toward Synchronously Boosting Photocatalytic Removal of Cr(VI) and Tetracycline

Developing highly efficient photocatalysts toward synchronously removing heavy metals and organic pollutants is still a serious challenge. Herein, we depict hierarchical S-scheme heterostructured photocatalysts prepared via in situ anchoring UiO-66-NH₂ nanoparticles onto the CdIn₂S₄ porous microsphere structures assembled with numerous nanosheets. In the mixed system of Cr(VI) and tetracycline (TC), the optimal photocatalyst (CIS@U66N-30) shows remarkable photocatalytic activities toward the synchronous removal of Cr(VI) (97.26%) and TC (close to 100% of) under visible-light irradiation for 60 min, being the best removal rates among those of the reported photocatalysts, and sustains the outstanding stability and reusability. Its reaction rate constants of Cr(VI) reduction and TC degradation are about 2.06 and 1.58 folds that in the single Cr(VI) and TC systems, respectively. The enhanced photocatalytic activities of CIS@U66N-30 mainly result from the following synergism: (1) its hierarchical structure offers abundant active sites, and the S-scheme migration mechanism of charge carriers in the heterostructure accelerates the separation and migration of the useful photoinduced electrons and holes with the high redox capability; (2) Cr(VI) and TC can serve as the electron scavenger for TC oxidation degradation and the hole and ^•OH scavenger for Cr(VI) reduction, respectively, further enhancing the separation and utilization efficiency of photoinduced electrons and holes. Besides, the possible TC degradation pathway and plausible S-scheme photocatalytic mechanism over CIS@U66N-30 for the concurrent elimination of Cr(VI) and TC are proposed.

Electro-peroxone enables efficient Cr removal and recovery from Cr(III) complexes and inhibits intermediate Cr(VI) generation in wastewater: Performance and mechanism

Available oxidation processes for removing Cr(III) complexes from water/wastewater usually encounter the formation of highly toxic Cr(VI) and the generation of Cr enriched waste sludge, posing challenges on the subsequent disposal. Herein, we achieve efficient removal of Cr(III)-organic complexes and simultaneous recovery of Cr from wastewater with enhanced curtailment of intermediate Cr(VI), by using an electrochemically driven peroxone (i.e., electro-peroxone) process with activated carbon fiber (ACF) electrodes. For Cr(III)-EDTA, electro-peroxone could remove ∼90% total Cr from 11.50 mg/L to 1.20 mg/L and ∼80% total organic carbon, with a strong curtailment of Cr(VI) to less than 0.2 mg/L. Additionally, the process could obtain a complete recovery of the removable Cr, of which 78.3% are enriched at ACF cathode as amorphous Cr(OH)₃ deposits and the remaining 21.7% are adsorbed at the anode, thus avoiding the generation of Cr laden sludge. Mechanism studies show the electro-generated H₂O₂ reacts with O₃ to generate abundant HO· for decomplexation, which sequentially oxidizes Cr(III) to Cr(VI), and degrades the released EDTA via stepwise decarboxylated process, as confirmed by HPLC analysis. Multiple pathways including electro-reduction, H₂O₂ reduction and electro-adsorption synergistically curtail and immobilize the formed intermediate Cr(VI). ACF characterizations and continuous 5-cycle experiments substantiate the excellent reusability of the ACF electrodes. Moreover, this process exhibits satisfactory effectiveness to Cr(III) complexed with other ligands (e.g., citrate and oxalate), and complexed Cr(III) in the real electroplating wastewater. We believe this study would provide an efficient and eco-friendly alternative for Cr(III) complexes removal from wastewater.

New insights into the capture performance and mechanism of hazardous metals Cr3+ and Cd2+ onto an effective layered double hydroxide based material

The phosphonate functionalized layered double hydroxide constructed through intercalation reaction, and efficiently applied to capture toxicant metal ions. The characterization results indicated that the functionalized composite with many functional groups has adsorption potential to heavy metals. The strong chelation of the phosphonate groups with heavy metal ions proved it an excellent adsorbent leading to a maximum adsorption capacity of 156.95 mg/g (Cr³⁺) and 198.34 mg/g (Cd²⁺) separately. The data of kinetics and isotherm revealed that the chelating adsorption was dominated by chemisorption and monolayer interaction. Notably, the spent adsorbent presented satisfactory reusability after six cycles. Furthermore, the Forcite simulation with the CLAYFF-CVFF force field implied that the critical mechanism for modifiers and the surface sites of the interlayer is electrostatic interaction. Our in-depth exploration in terms of the weak interactions not only demonstrated the strength and nature but also provided a novel way to intuitively capture the type of interactions that occurred around interesting regions. In the end, we made detailed investigations on the chelation mechanism, and the covalent nature played a leading role in the binding interaction. This work provides a valuable strategy for researchers to design novel materials in practice.

Bimetallic FeNi nanoparticles immobilized by biomass-derived hierarchically porous carbon for efficient removal of Cr(VI) from aqueous solution

Nano zero-valent iron (nZVI) is an effective material for Cr(VI) treatment, however excessive agglomeration and surface oxidation limit its application. Herein, straw derived hierarchically porous carbon supported FeNi bimetallic nanoparticles (FeNi@HPC) was prepared for effective removal of Cr(VI) from water. FeNi nanoparticles were successfully loaded onto HPC with good dispersibility, and HPC caused an increase in specific surface area of FeNi nanoparticles. FeNi@HPC exhibited a significantly enhanced removal efficiency for Cr(VI) in comparison to Fe@HPC and FeNi NPs. The Ni doping content was further optimized, and the best Ni content in bimetallic NPs was estimated as 10 wt%. The conditions optimal for the activity of FeNi@HPC were assessed, and the highest removal efficiency equivalent to 30 mg L^-1 of Cr(VI) was achieved at pH= 4.0 in 360 min with a dosage of 0.5 g L^-1. Higher temperatures favored the removal of Cr(VI) and FeNi@HPC manifested the lowest activation energy as compared to Fe@HPC and FeNi NPs. The action mechanisms of FeNi@HPC presumably involved electron transfer from Fe⁰, Fe²⁺and atomic hydrogen. This work not only provide a cost-effective and available HPC material to stabilize nZVI but also revealed that using FeNi@HPC is a promising approach for the remediation of water pollution.

Kinetics of Thallium(I) Oxidation by Free Chlorine in Bromide-Containing Waters: Insights into the Reactivity with Bromine Species

The oxidation of thallium [Tl(I)] to Tl(III) by chlorine (HOCl) is an important process changing its removal performance in water treatment. However, the role of bromide (Br^-), a common constituent in natural water, in the oxidation behavior of Tl(I) during chlorination remains unknown. Our results demonstrated that Br^- was cycled and acted as a catalyst to enhance the kinetics of Tl(I) oxidation by HOCl over the pH range of 5.0-9.5. Different Tl(I) species (i.e., Tl⁺ and TlOH(aq)) and reactive bromine species (i.e., HOBr/BrO^-, BrCl, Br₂O, and BrOCl) were kinetically relevant to the enhanced oxidation of Tl(I). The oxidation by free bromine species became the dominant pathway even at a low Br^- level of 50 μg/L for a chlorine dose of 2 mg of Cl₂/L. It was found that the reactions of Tl⁺/BrCl, Tl⁺/BrOCl, and TlOH(aq)/HOBr dominated the kinetics of Tl(I) oxidation at pH < 6.0, pH 6.0-8.0, and pH > 8.0, respectively. The species-specific rate constants for Tl⁺ reacting with individual bromine species were determined and decreased in the order: BrCl > Br₂ > BrOCl > Br₂O > HOBr. Overall, the presented results refine our knowledge regarding the species-specific reactivity of TI(I) with bromine species and will be useful for further prediction of thallium mobility in chlorinated waters containing bromide.

Coprecipitation of Fe/Cr Hydroxides with Organics: Roles of Organic Properties in Composition and Stability of the Coprecipitates

Iron hydroxides are important scavengers for dissolved chromium (Cr) via coprecipitation processes; however, the influences of organic matter (OM) on Cr sequestration in Fe/Cr-OM ternary systems and the stability of the coprecipitates are not well understood. Here, Fe/Cr-OM coprecipitation was conducted at pH 3, and Cr hydroxide was undersaturated. Acetic acid (HAc), poly(acrylic acid) (PAA), and Suwannee River natural organic matter (SRNOM) were selected as model OMs, which showed different complexation capabilities with Fe/Cr ions and Fe/Cr hydroxide particles. HAc had no significant effect on the coprecipitation, as the monodentate carboxyl ligand in HAc did not favor complexation with dissolved Fe/Cr ions or Fe/Cr hydroxide nanoparticles. Contrarily, PAA and SRNOM with polydentate carboxyl ligand had strong complexation with Fe/Cr ions and Fe/Cr hydroxide nanoparticles, leading to significant amounts of PAA/SRNOM sequestered in the coprecipitates, which caused the structural disorder and fast aggregation of the coprecipitates. In comparison with that of PAA, preferential complexation of Cr ions with SRNOM resulted in higher Cr/Fe ratios in the coprecipitates. This study advances the fundamental understanding of Fe/Cr-OM coprecipitation and mechanisms controlling the composition and stability of the coprecipitates, which is essential for successful Cr remediation and removal in both natural and engineered settings.

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.

Cr(VI) Formation via Oxyhalide-Induced Oxidative Dissolution of Chromium Oxide/Hydroxide in Aqueous and Frozen Solution

The oxidative dissolution of Cr(III) species (Cr₂O₃ and Cr(OH)₃) by oxyhalide species, which produces hexavalent chromium (Cr(VI)), was studied in aqueous and frozen solution. The oxyhalide-induced oxidation of Cr(III) in frozen solution showed a different trend from that in aqueous solution. Cr(VI) production was higher in frozen than aqueous solution with hypochlorous acid (HOCl) and bromate (BrO₃^-) but suppressed in frozen solution with hypobromous acid (HOBr) and periodate (IO₄^-). In particular, bromate markedly enhanced Cr(VI) production in frozen solution, whereas it had a negligible activity in aqueous solution. On the contrary, periodate produced Cr(VI) significantly in aqueous solution but greatly suppressed it in frozen solution. Bromate was found to be much more concentrated in the ice grain boundary than periodate according to both chemical and Raman spectral analyses. The oxidative transformation of Cr(III) to Cr(VI) was accompanied by the concurrent and stoichiometric reduction of oxyhalide species. Dissolved O₂ had little effect on the oxidative dissolution, but dissolved organic matter retarded the oxidation of Cr₂O₃ in both aqueous and frozen conditions. This study proposes that the oxyhalide-induced oxidation of Cr(III) (particularly by bromate) in frozen conditions might have a significant effect on the generation of Cr(VI) in the frozen environment.

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.

Reduced NOM triggered rapid Cr(VI) reduction and formation of NOM-Cr(III) colloids in anoxic environments

Natural organic matter (NOM) can influence the toxicity and speciation of chromium (Cr) in subsurface through redox reactions and complexation. Under anoxic conditions, NOM can be reduced by microorganisms or geochemical reductants, and the reduced NOM (NOM_red) represents a large reservoir of organic matter observed in anoxic sediments and water. While the current body of work has established the kinetic of Cr(VI) reduction by oxidized NOM (NOM_ox) under oxic conditions, much less is known about the rates and mechanisms of Cr(VI) reduction triggered by NOM_red under anoxic conditions and the colloidal properties of the reaction products. This study provided new information regarding the NOM_red-mediated Cr(VI) reduction and colloidal stability of reduced Cr(III) particles over a wide range of environmentally relevant anoxic conditions. We show that under dark anoxic conditions reduced humic acid (HA_red) moieties (e.g., quinone) can quickly reduce Cr(VI) to Cr(III), and the reduced Cr(III) can subsequently complex with carboxyl groups of HA leading to the formation of stable HA-Cr(III) colloids. Rates of Cr(VI) reduction by HA_red are 3-4 orders of magnitude higher than those by oxidized HA (HA_ox) due primarily to the higher reducing capacity of HA_red. The stable HA-Cr(III) colloids are formed across a range of HA concentrations (8-150 mg C/L) and pH conditions (6-10) with hydrodynamic diameter in the range of 210-240 nm. Aberration-corrected scanning transmission electron microscopy (Cs-STEM) and X-ray photoelectron spectroscopy (XPS) confirmed that the particles are composed of HA-Cr(III). The high colloidal stability of HA-Cr(III) particles could be attributed to the enhanced electrosteric stabilization effect from free and adsorbed HA, which decreased particle aggregation. However, the presence of divalent cations (Ca²⁺ and Mg²⁺) promoted particle aggregation at pH 6. These new findings are valuable for our fundamental understanding of the fate and transport of Cr in organic-rich anoxic environments, which also have substantial implications for the development and optimization of subsurface Cr sequestration technology.

Hexavalent chromium in drinking water: Chemistry, challenges and future outlook on Sn(II)- and photocatalyst-based treatment

Chromium (Cr) typically exists in either trivalent and hexavalent oxidation states in drinking water, i.e., Cr(III) and Cr(VI), with Cr(VI) of particular concern in recent years due to its high toxicity and new regulatory standards. This Account presented a critical analysis of the sources and occurrence of Cr(VI) in drinking water in the United States, analyzed the equilibrium chemistry of Cr(VI) species, summarized important redox reaction relevant to the fate of Cr(VI) in drinking water, and critically reviewed emerging Cr(VI) treatment technologies. There is a wide occurrence of Cr(VI) in US source drinking water, with a strong dependence on groundwater sources, mainly due to naturally weathering of chromium-containing aquifers. Challenges regarding traditional Cr(VI) treatment include chemical cost, generation of secondary waste and inadvertent re-generation of Cr(VI) after treatment. To overcome these challenges, reductive Cr(VI) treatment technologies based on the application of stannous tin or electron-releasing titanium dioxide photocatalyst hold extreme promise in the future. To moving forward in the right direction, three key questions need further exploration for the technology implementation, including effective management of residual waste, minimizing the risks of Cr(VI) re-occurrence downstream of drinking water treatment plant, and promote the socioeconomic drivers for Cr(VI) control in the future.

Efficient reduction of Cr(VI) and immobilization of Cr driven by an iron-air fuel cell: Reaction mechanisms and electricity generation

The iron-air fuel cell (IAFC) has been successfully employed for the oxidative removal of many pollutants, but its feasibility for reductive immobilization of Cr(VI) is still unknown. Herein, we developed an IAFC system consisting of an iron anode and an activated carbon-PTFE based air-cathode, and evaluated its performance for Cr(VI) removal and power generation. In this reaction system, cathodic reduction and Fe(II) reduction both contributed to the reductive removal of Cr(VI). It was found that the decrease of solution pH from 6.0 to 3.0 promoted the removal of Cr(VI) due to the enhanced yield of Fe(II) ions and cathodic reduction, accompanying the increased power generation from 1040 mW m^-2 to 2880 mW m^-2. Besides, the Cr(VI) removal and power generation could be also promoted by elevating Na₂SO₄ concentration from 0.01 M to 0.1 M. In the IAFC process, Cr(VI) was initially reduced to less soluble ionic Cr(III) homogeneously and heterogeneously and then Cr(III) was immobilized by adsorption and/or co-precipitation with the fresh Fe(III) (oxy)hydroxides. Generally, this study is of great interest for the engineering community to design the environmentally benign and cost-effective strategy for the treatment of wastewater in remote areas, where the electricity is not easily available.

Enhanced Transformation of Cr(VI) by Heterocyclic-N within Nitrogen-Doped Biochar: Impact of Surface Modulatory Persistent Free Radicals (PFRs)

Redox processes mediated by biochar(BC) enhanced the transformation of Cr(VI), which is largely dependent on the presence of PFRs as electron donors. Natural or artificial dopants in BC's could regulate inherent carbon configuration and PFRs. Until recently, the modulation of PFRs and transformation of Cr(VI) in BC by nonmetal-heterocyclic dopants was barely studied. In this study, changes in PFRs introduced by various nitrogen-dopants within BC are presented and the capacity for Cr(VI) transformation without light was investigated. It was found N-dopants were effectively embedded in carbon lattices through activated-Maillard reaction thus altering their charge and PFRs. Transformation of Cr(VI) in N doped biochar relied on mediated direct reduction by surface modulatory PFRs. The kinetic rate of transformation of Cr(VI) was increased 1.4-5 fold in N-BCs compared to nondoped BCs. Theortical calculation suggested a deficiency in surface electrons induced Lewis acid-base bonding which could acted as a bridge for electron transfer. Results of PCA and orbital energy indicated a colinear relationship between PFRs and pyrrolic N, as well as its dual-mode transformation of Cr(VI). This study provides an improved understanding of how N-doped BC contributes to the evolution of PFRs and their corresponding impacts on the transformation of Cr(VI) in environments.

Novel synthesis of Ni/Fe layered double hydroxides using urea and glycerol and their enhanced adsorption behavior for Cr(VI) removal

Novel modified Ni/Fe layered double hydroxides with different morphology of spherical - like shape were fabricated via using urea as a ligand and glycerol (Ni/Fe LDH/GL) with Ni:Fe molar ratios of 2:1 by the simplest co -precipitation method. Also, for comparison purposes, Ni/Fe LDH was synthesized to be used as a control one. A suggested interpretation for the morphology change was also given. The materials were characterized by X-ray diffraction (XRD), The Fourier transform infrared (FT - IR) spectroscopy, field emission scanning electron microscopy (FESEM), EDX for elemental analysis, high resolution transmission electron microscopy (HRTEM), Brunauer, Emmett, and Teller (BET) equation, particle size distributions and Zeta potential measurements. In addition, the synthesized materials were used as adsorbents for removal of potassium dichromate from aqueous solutions under various experimental conditions. The adsorption of Cr (VI) was strongly pH dependant and the pH_PZC was studied. Kinetic studies were evaluated through different models including, pseudo first and second orders, mixed 1, 2 orders, intra particle diffusion and Avrami models. For adsorption isotherms, two-parameter models (Langmuir, Freundlich and Temkin) and three parameter models (Sips, Langmuir-Freundlich and Tooth) were investigated showing maximum adsorption capacity of 50.43 mg/g and 136.05 mg/g for Ni/Fe LDH and Ni/Fe LDH/GL, respectively. Also, the effect of temperature was investigated at (23, 35, 45, 55 °C) and the thermodynamic parameters (∆H°, ∆S° and ∆G°) were calculated showing exothermic and spontaneous adsorption process. The effect of coexisting anions (Cl^-, SO₄^2- and HPO₄^2-) and humic acid at different concentrations on the removal efficiency of dichromate ions was investigated. Chemical stability and recyclability of these adsorbents were also studied. The intermolecular hydrogen bonds formation between dichromate ion, urea, glycerol, LDH was explored by Monte Carlo simulation This study suggested that the modified Ni/Fe LDH/GL materials were promising nanoadsorbents for efficient potassium dichromate removal.

Multi-functional photocatalytic fuel cell for simultaneous removal of organic pollutant and chromium (VI) accompanied with electricity production

It is of significant importance to realize the efficient wastewater treatment and energy recovery. This study presents a multi-functional photocatalytic fuel cell (PFC), which could reductively treat Cr(VI) contaminant and oxidatively degrade organic pollutant simultaneously along with electricity production in an economical strategy. TiO₂ nanotube arrays (TNA) and graphite were used as photoanode and cathode in two separated chambers, respectively. The optimized PFC with open circuit voltage of 1.06 V, maximum power density of 1 W m^-2 and short circuit current density of 3.7 A m^-2 can be obtained by increasing Cr(VI) concentration and decreasing pH values in catholyte. Under optimized PFC conditions, more photogenerated electrons will be transferred to cathode for Cr(VI) reduction, and accelerating electron-hole separation in the photoanode, then facilitating the oxidation of organic pollutants on anode. More than 96.8% removal efficiency for 6.8 mM Cr(VI) with a cathodic efficiency of 95.1% can be achieved within 6 h. Methylene blue (MB), an organic model pollutant, is totally decolorized on photoanode, which is significantly improved compare to photocatalysis (61.5% removal efficiency). The stable cycle operation of this economical PFC has obtained owing to the stable and low cost materials of both photoanode and cathode. This work may provide an efficient and economical method to simultaneously remove two types of pollutants with electricity harvested in one cell.

Kinetic and mechanistic aspects of selenite oxidation by chlorine, bromine, monochloramine, ozone, permanganate, and hydrogen peroxide

Selenium (mainly in the forms of selenite (Se(IV)) and selenate (Se(VI)) is a regulated drinking water contaminant, but there is little information on the kinetics and mechanisms of Se(IV) oxidation during water treatment. Species-specific and apparent second-order rate constants for the oxidation of Se(IV) at pH 7.0 were determined in buffered solutions and they decrease in the order bromine (5.8 ± 0.3 × 10³ M^-1 s^-1) > ozone (O₃, 513.4 ± 10.0 M^-1 s^-1) > chlorine (61.0 ± 3.6 M^-1 s^-1) > permanganate (2.1 ± 0.1 M^-1 s^-1), monochloramine (NH₂Cl, (1.3 ± 0.1) × 10^-3 M^-1 s^-1), and hydrogen peroxide (H₂O₂, (2.3 ± 0.1) × 10^-5 M^-1 s^-1). The reaction stoichiometries for the reactions of Se(IV) with bromine, O₃, chlorine, NH₂Cl, and H₂O₂ are 1:1. For Mn(VII), the stoichiometries varied with pH and were 5:2, 3:2, and 1:2 for acidic, neutral, and alkaline conditions, respectively. Based on the reaction orders and stoichiometries, the corresponding Se(IV) oxidation mechanisms for various oxidants are discussed. The role of bromide for Se(IV) oxidation was also investigated during chlorination and ozonation of Se(IV)-containing water. During chlorination, bromide-catalysis enhances the rate of the oxidation of Se(IV) to Se(VI) from 50% to nearly 90% with bromide concentrations of 50 μg L^-1 and 200 μg L^-1, respectively, at pH 7.0 and a chlorine dose of 2.0 mg L^-1 (within 15 min). During ozonation, bromide had no effect on Se(IV) oxidation. Based on the determined second order rate constants, the oxidation of Se(IV) by chlorine and ozone were successfully predicted in a natural water by a kinetic model. The second order rate constants for the same oxidants were also investigated and/or evaluated for other related anions, such as arsenite (As(III)) and sulfite (S(IV)). They decreased in the order S(IV) > As(III) > Se(IV).

Effects of oxalic acid on Cr(VI) reduction by phenols in ice

Since Cr(VI) is highly toxic, the environmental reduction of Cr(VI) to Cr(III) has attracted significant attention. Oxalic acid, a primary component of dissolved organic matter (DOM), is widely distributed throughout the natural environment but the reduction of Cr(VI) by oxalic acid is insignificant at the low concentrations present in the environment; however, the reduction of Cr(VI) is accelerated significantly in ice. In terms of combined pollution, Cr(VI) can coexist with other organic pollutants in the environment but the impact of organic pollutants on the reduction of Cr(VI), changes to the organic pollutants themselves, and the role of oxalic acid in these reactions are unknown. In this study, we investigated redox reactions between Cr(VI) and phenolic compounds in ice (- 15 °C) in the presence of oxalic acid and compared these to room temperature redox reactions in aqueous solutions (20 °C). While these redox reactions were negligible in aqueous solution, they were significantly accelerated in ice under acidic conditions, which was primarily attributed to the freeze concentration effect. Oxalic acid has two functions in these redox reactions; the first is to provide the H⁺ required for the reaction and the second is to serve as a reducing agent. When oxalic acid and phenolic pollutants coexist, Cr(VI) preferentially reacts with the phenolic compounds. Phenol, 4-chlorophenol (4-CP), and 2,4-dichlorophenol (2,4-DCP) were each demonstrated to reduce Cr(VI) in ice, but the reaction rate and overall reactivity of these three phenolic compounds are different.

Biofilm-Templated Heteroatom-Doped Carbon-Palladium Nanocomposite Catalyst for Hexavalent Chromium Reduction

In this study, we report an interdisciplinary and novel strategy toward biofilm engineering for the development of a biofilm-templated heteroatom-doped catalytic system through bioreduction and biofilm matrix-facilitated immobilization of the in situ-formed catalytic nanoparticles followed by controlled pyrolysis. We showed that (i) even under room temperature and bulk aerobic conditions, Shewanella oneidensis MR-1 biofilms reduced Pd(II) to form Pd(0) nanocrystals (∼10 to 20 nm) that were immobilized in the biofilm matrix and in cellular membranes, (ii) the MR-1 biofilms with the immobilized Pd(0) nanocrystals exhibited nanocatalytic activity, (iii) exposure to Pd(II) greatly increased the rate of cell detachment from the biofilm and posed a risk of biofilm dispersal, (iv) controlled pyrolysis (carbonization) of the biofilm led to the formation of a stable heteroatom-doped carbon-palladium (C-Pd) nanocomposite catalyst, and (v) the biofilm-templated C-Pd nanocomposite catalyst exhibited a high Cr(VI) reduction activity and maintained a high reduction rate over multiple catalytic cycles. Considering that bacteria are capable of synthesizing a wide range of metal and metalloid nanoparticles, the biofilm-templated approach for the fabrication of the catalytic C-Pd nanocomposite we have demonstrated here should prove to be widely applicable for the production of different nanocomposites that are of importance to various environmental applications.

Chromium isotopes tracking the resurgence of hexavalent chromium contamination in a past-contaminated area in the Friuli Venezia Giulia Region, northern Italy

The origin of a resurgent hexavalent chromium contamination in groundwater from a phreatic aquifer in the Friuli Venezia Giulia Region plain was investigated by chromium isotopic systematics. The area underwent a severe Cr(VI) contamination by industrial effluents in 1997, when Cr(VI) concentration in groundwater reached 4500 µg/L. In subsequent years the contamination naturally attenuated, totally disappearing in 2003. A renewal of water contamination was observed in 2008, Cr(VI) reaching 1560 µg/L. The δ⁵³Cr value in groundwater and extracts from sediments was measured in 2009-2011, and it ranges between -3.21 and +0.21‰ and between -4.71 and +1.26‰, respectively. Due to the lack of geogenic Cr-sources, these data are interpreted as evidence of the subsequent oxidation through Mn-oxides of the Cr(III) hosted in the aquifer and originated by the reduction of the original industrial chromates. Cr(III) is characterized by negative δ⁵³Cr, starting from the δ⁵³Cr value around zero of Cr(VI) in industrial effluents. Oxidation liberates soluble Cr(VI) which is transported by groundwater and permeated soils. The complex Cr-isotopic vs. concentration distribution reflects both the new Cr(VI) reduction and dilution processes in the aquifer system. From an environmental point of view, the data raise concerns regarding the potential impact of past Cr(VI)-contamination.

Preparation, characterization and performance of an electrospun carbon nanofiber mat applied in hexavalent chromium removal from aqueous solution

Hexavalent chromium, Cr(VI), a highly toxic oxyanion known as a carcinogen and mutagen, is an issue of concern due to its adverse impact on human health. Therefore, development of effective technologies and/or materials for Cr(VI) removal from water has been of great interest for researchers. In this study, an electrospun carbon nanofiber (CNF) mat was prepared via electrospinning polyacrylonitrile (PAN), followed by thermal pre-oxidation and carbonization. Scanning electron microscopy (SEM) observation showed that the fiber diameter of the CNF with carbonization temperature of 950°C (CNF₉₅₀) was about 266 nm. Potentiometric titration analysis demonstrated that the point of zero charge pH (pH_pzc) of CNF₉₅₀ was around 7.93. CNF₉₅₀ demonstrated high adsorption capacity and fast adsorption kinetics for Cr(VI) at pH < 3. Langmuir isotherm calculations showed that the maximum adsorption capacity of Cr(VI) on CNF₉₅₀ was 118.8 mg/g at pH 2. The adsorption isotherm of Cr(VI) on CNF₉₅₀ was well described by the Redlich-Peterson model, revealing that Cr(VI) adsorption was the result of a combination of monolayer and multilayer adsorption, depending on the initial Cr(VI) concentration. Solution pH greatly affected Cr(VI) adsorption onto CNF₉₅₀ due to the electrostatic interaction, and the adsorption capacity was relatively high when pH was below 3. X-ray photoelectron spectroscopy (XPS) analysis revealed that the removal of Cr(VI) might be the result of a combination of redox reaction and electrostatic adsorption. The adsorption-saturated CNF₉₅₀ could be regenerated by NaOH solution. This study extends the potential applicability of electrospun CNF mats for Cr(VI)-contaminated water purification.

Water Decontamination from Cr(III)-Organic Complexes Based on Pyrite/H2O2: Performance, Mechanism, and Validation

Fenton reaction is a widely used pretreatment technology to degrade toxic metal-organic complexes. However, its efficiency is greatly compromised for Cr(III)-organic complexes due to accumulation of more toxic Cr(VI) and pH dependence. Herein, we proposed a combined pyrite/H₂O₂-precipitation process to efficiently remove Cr(III) (initially at 10.4 mg Cr/L) complexed by various ligands (citrate, EDTA, oxalate, and tartrate). Negligible Cr(VI) and <0.3 mg/L Cr were detected in the effluent treated by pyrite/H₂O₂-precipitation over a wide pH range of 3-9. In contrast, > 0.5 mg/L Cr(VI) and >5 mg/L Cr remained after treatment by the ZVI/H₂O₂-precipitaion process at pH₀ > 5. As for the mechanisms, pyrite/H₂O₂ produced a considerable amount of aqueous Fe(II) to initiate Fenton reaction, concurrently releasing massive H⁺ to keep the reaction pH at ∼3.0 irrespective of the initial pHs. The generated •OH radicals oxidized Cr(III) into Cr(VI) and thereby releasing the organic ligands for further mineralization. The generated Cr(VI) was in situ reduced back to Cr(III) by aqueous Fe(II) and FeS₂. Subsequently, all the free metal ions including Cr(III), Fe(III), and Fe(II) were removed via precipitation. Kinetic modeling of the pyrite/H₂O₂ process involving 17 reactions was performed to verify the proposed mechanism. Additionally, the effectiveness of the combined process was further validated by its satisfactory performance in treating authentic tannery wastewater.

Oxidation of Cr(III)-Fe(III) Mixed-Phase Hydroxides by Chlorine: Implications on the Control of Hexavalent Chromium in Drinking Water

The occurrence of chromium (Cr) as an inorganic contaminant in drinking water is widely reported. One source of Cr is its accumulation in iron-containing corrosion scales of drinking water distribution systems as Cr(III)-Fe(III) hydroxide, that is, Fe _xCr_{(1- x)}(OH)_3(s), where x represents the Fe(III) molar content and typically varies between 0.25 and 0.75. This study investigated the kinetics of inadvertent hexavalent chromium Cr(VI) formation via the oxidation of Fe _xCr_{(1- x)}(OH)_3(s) by chlorine as a residual disinfectant in drinking water, and examined the impacts of Fe(III) content and drinking water chemical parameters including pH, bromide and bicarbonate on the rate of Cr(VI) formation. Data showed that an increase in Fe(III) molar content resulted in a significant decrease in the stoichiometric Cr(VI) yield and the rate of Cr(VI) formation, mainly due to chlorine decay induced by Fe(III) surface sites. An increase in bicarbonate enhanced the rate of Cr(VI) formation, likely due to the formation of Fe(III)-carbonato surface complexes that slowed down the scavenging reaction with chlorine. The presence of bromide significantly accelerated the oxidation of Fe _xCr_{(1- x)}(OH)_3(s) by chlorine, resulting from the catalytic effect of bromide acting as an electron shuttle. A higher solution pH between 6 and 8.5 slowed down the oxidation of Cr(III) by chlorine. These findings suggested that the oxidative conversion of chromium-containing iron corrosion products in drinking water distribution systems can lead to the occurrence of Cr(VI) at the tap, and the abundance of iron, and a careful control of pH, bicarbonate and bromide levels can assist the control of Cr(VI) formation.

Thermodynamic properties of an emerging chemical disinfectant, peracetic acid

Peracetic acid (PAA or CH₃COOOH) is an emerging disinfectant with a low potential to form carcinogenic disinfection by-products (DBPs). Basic thermodynamic properties of PAA are, however, absent or inconsistently reported in the literature. This review aimed to summarize important thermodynamic properties of PAA, including standard Gibbs energy of formation and oxidation-reduction (redox) potential. The standard Gibbs energies of formation of CH₃COOOH_(aq), CH₃COOOH_(g), CH₃COOOH_(l), and CH₃COOO_(aq)^- are -299.41kJ·mol^-1, -283.02kJ·mol^-1, -276.10kJ·mol^-1, and -252.60kJ·mol^-1, respectively. The standard redox potentials of PAA are 1.748V and 1.005V vs. standard hydrogen electrode (SHE) at pH 0 and pH 14, respectively. Under biochemical standard state conditions (pH 7, 25°C, 101,325Pa), PAA has a redox potential of 1.385V vs. SHE, higher than many disinfectants. Finally, the environmental implications of the thermodynamic properties of PAA were systematically discussed. Those properties can be used to predict the physicochemical and biological behavior of aquatic systems exposed to PAA.

Effect of alkaline precipitation on Cr species of Cr(III)-bearing complexes typically used in the tannery industry

Various organic compounds extensively used in the leather industry could influence the performance of alkaline precipitation with Cr(III). This study focused on two typical Cr(III)-bearing complexes (Cr(III)-collagen and Cr(III)-citrate) ubiquitous in tannery effluent yet with distinct treatment efficiencies, as Cr(III) was much more difficult to remove in the Cr(III)-citrate solution. Comprehensive analytical methods were employed to explore the intrinsic mechanism. It was found that a lower removal efficiency towards Cr(III) was significantly associated with higher oligomers. The molecular size of the Cr(III)-citrate complex continued to increase with rising pH, making it larger overall than Cr(III)-collagen species. The growing oligomer moiety of dissolved Cr(III)-complex species could persist in the stronger basic pH range, leading to the large amount of residual Cr(III) in the Cr(III)-citrate system. Combining this result with potentiometric titration and X-ray photoelectron spectroscopy data, it was believed that the polymeric species other than monomers facilitated resisting the attack from hydroxide ions, and the postulated Cr(III)-citrate species towards higher oligomers were discovered. Beyond that, both charge neutralization and sweeping effects were presented among the gradually emerging flocs in the Cr(III)-collagen system together with the electric double layer compression effect derived from salinity, thus resulting in a larger floc size and higher Cr(III) removal efficiency in saline solutions.

Characterization of CMC–LDH beads and their application in the removal of Cr(vi) from aqueous solution

In this study, CMC–LDH beads were prepared and characterized using SEM, FTIR and TG analysis. The beads were applied for the removal of Cr(vi) from aqueous solution. The effects of adsorbent dosage, initial pH and initial concentration of Cr(vi) solution on Cr(vi) uptake were investigated in detail. Moreover, adsorption isotherms and adsorption kinetic models were employed to analyze the adsorption process, and a preliminary study of the reusability of the adsorbent was performed. The experimental results showed that the CMC–LDH beads could remove Cr(vi) from aqueous solution efficiently. When the initial concentration of the Cr(vi) solution was 100 mg L⁻¹ and the adsorbent dosage was 12 g L⁻¹, the removal efficiency of Cr(vi) reached 96.2%. After the CMC–LDH beads were reused 10 times, the removal efficiency of Cr(vi) still remained at 89.6%.

CMC–LDH beads were prepared, characterized and applied for the removal of heavy metal ions in this study.

Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III) system: Negligible Cr(VI) accumulation and mechanism

Most available processes are incapable of removing Cr(III)-organic complexes from water due to their high solubility, extremely slow decomplexation rate, and possible formation of more toxic Cr(VI) during oxidation. Herein, we proposed a new combined process, i.e., UV/Fe(III) followed by alkaline precipitation (namely UV/Fe(III)+OH), to achieve highly efficient and environmentally benign removal of Cr(III)-organic complexes from water. The combined process could remove Cr(III)-citrate from 10.4 mg Cr/L to 0.36 mg Cr/L and ∼60% total organic carbon as well. More attractively, negligible Cr(VI) (<0.06 mg/L) was formed during the process. In the viewpoint of mechanism, the added Fe(III) generates ·OH radicals to transform Cr(III) into Cr(VI) and simultaneously released the citrate ligand to form Fe(III)-citrate simultaneously. Then, the photolysis of Fe(III)-citrate under UV irradiation involved the citrate degradation and the production of massive Fe(II) species, which in turn transformed the formed Cr(VI) back to Cr(III). The free metal ions, including Cr(III), Fe(II) and Fe(III) were removed by the subsequent alkaline precipitation. Also, the combined process is applicable to other Cr(III) complexes with EDTA, tartrate, oxalate, acetate. The applicability of the combined process was further demonstrated by treating two real tanning effluents, resulting in the residual Cr(III) below 1.5 mg/L (the discharge standard of China) and negligible formation of Cr(VI) (<0.004 mg/L) as well. In general, the combined process has a great potential for efficient removal of Cr(III) complexes from contaminated waters.

Mechanistic Aspects of the Formation of Adsorbable Organic Bromine during Chlorination of Bromide-containing Synthetic Waters

During chlorination of bromide-containing waters, a significant formation of brominated disinfection byproducts is expected. This is of concern because Br-DBPs are generally more toxic than their chlorinated analogues. In this study, synthetic water samples containing dissolved organic matter (DOM) extracts and bromide were treated under various disinfection scenarios to elucidate the mechanisms of Br-DBP formation. The total concentration of Br-DBPs was measured as adsorbable organic bromine (AOBr). A portion of the bromine (HOBr) was found to react with DOM via electrophilic substitution (≤40%), forming AOBr, and the remaining HOBr reacted with DOM via electron transfer with a reduction of HOBr to bromide (≥60%). During chlorination, the released bromide is reoxidized (recycled) by chlorine to HOBr, leading to further electrophilic substitution of unaltered DOM sites and chlorinated DOM moieties. This leads to an almost complete bromine incorporation to DOM (≥87%). The type of DOM (3.06 ≤ SUVA₂₅₄ ≤ 4.85) is not affecting this process, as long as the bromine-reactive DOM sites are in excess and a sufficient chlorine exposure is achieved. When most reactive sites were consumed by chlorine, Cl-substituted functional groups (Cl-DOM) are reacting with HOBr by direct bromination leading to Br-Cl-DOM and by bromine substitution of chlorine leading to Br-DOM. The latter finding was supported by hexachlorobenzene as a model compound from which bromoform was formed during HOBr treatment. To better understand the experimental findings, a conceptual kinetic model allowing to assess the contribution of each AOBr pathway was developed. A simulation of distribution system conditions with a disinfectant residual of 1 mgC₂ L^-1 showed complete conversion of Br^- to AOBr, with about 10% of the AOBr formed through chlorine substitution by bromine.

Bromide-assisted catalytic oxidation of lead(ii) solids by chlorine in drinking water distribution systems

This study investigated the effect of bromide on the oxidation of three lead(ii) solids by chlorine – a redox process critical to the control of lead release in drinking water distribution systems.

Photocatalytic removal of hexavalent chromium by newly designed and highly reductive TiO2 nanocrystals

Hexavalent chromium Cr(VI), a highly toxic oxyanion, widely occurs in drinking water supplies. This study designed and synthesized a new type of highly reductive TiO₂ nanocrystals for photochemical Cr(VI) removal, via the thermal hydrolysis of TiCl₄ in the presence of diethylene glycol (DEG). Surface analyses and hydroxyl radical measurements suggested that DEG was chemically bonded on TiO₂ surface that resulted in an internal hole-scavenging effect and a high electron-releasing capacity, making it advantageous to conventional TiO₂ materials. Upon UV irradiation, the synthesized TiO₂ photocatalyst exhibited fast Cr(VI) reduction kinetics in diverse water chemical conditions. Fast elimination of Cr(VI) was achieved on a time scale of seconds in drinking water matrices. The removal of Cr(VI) by reductive TiO₂ exhibited a three-stage kinetic behavior: an initial fast-reaction phase, a lag phase resulting from surface precipitation of Cr(OH)_3(s), and a final reaction phase due to surface regeneration from oxidation-reduction induced ripening process. The lag phase disappeared in acidic conditions that prevented the formation of Cr(OH)_3(s). The catalyst exhibited extremely high electron-releasing capacity that can be reused for multiple cycles of Cr(VI) removal in drinking water treatment.

On-site separation of Cr(vi) and Cr(iii) in natural waters by parallel cartridge ion-exchange columns

Development of a simple, fast, portable, and solvent-free method for on-site separation of Cr( = 6 \* ROMAN vi) and Cr( = 3 \* ROMAN iii) in natural waters.

Dynamics of Chromium(VI) Removal from Drinking Water by Iron Electrocoagulation

The potential for new U.S. regulations for Cr(VI) in drinking water have spurred strong interests in improving technologies for Cr(VI) removal. This study examined iron electrocoagulation for Cr(VI) removal at conditions directly relevant to drinking water treatment. Cr(VI) is chemically reduced to less soluble Cr(III) species by the Fe(II) produced from an iron anode, and XANES spectra indicate that the Cr is entirely Cr(III) in solid-phases produced in electrocoagulation. The dynamics of Cr(VI) removal in electrocoagulation at pH 6 and pH 8 at both oxic and anoxic conditions can be described by a new model that incorporates Fe(II) release from the anode and heterogeneous and homogeneous reduction of Cr(VI) by Fe(II). Heterogeneous Cr(VI) reduction by adsorbed Fe(II) was critical to interpreting Cr(VI) removal at pH 6, and the Fe- and Cr-containing EC product was found to catalyze the redox reaction. Dissolved oxygen (DO) did not observably inhibit Cr(VI) removal because Fe(II) reacts with DO more slowly than it does with Cr(VI), and Cr(VI) removal was faster at higher pH. Even in the presence of common groundwater solutes, iron electrocoagulation lowered Cr(VI) concentrations to levels well below California's 10 μg/L.

Au-Pd Bimetallic Nanoparticle Electrodes for Direct Electroreduction of Hexavalent Chromium Complexes

This paper reports a simple, low-cost, and effective electrochemical technique for sensing and reducing CrVI based on a Au-Pd bimetallic nanoparticle (BNP)-decorated indium tin oxide (ITO) conducting glass electrode. It was observed that the Au-Pd BNP-decorated ITO electrode could significantly boost the electrochemical reduction of CrVI when compared with either Au nanoparticle- or Pd nanoparticle-decorated ITO electrodes. These BNP-decorated electrodes exhibited a wide linear concentration range of 0.001–100 μM, a very low detection limit (signal-to-noise ratio = 3) of 0.3 nM, and a high sensitivity of 1.701 μA μM–1. From electrochemical impedance spectroscopy, it was revealed that this significant improvement was mainly due to the reduction in the charge-transfer resistance, which leads to faster free exchange of the reaction intermediates. The proposed Au-Pd BNP electrode also demonstrated excellent stability, selectivity, repeatability, and reproducibility.