Formation of Halogenated Byproducts upon Water Treatment with Peracetic Acid.

Peracetic acid has quickly gained ground in water treatment over the last decade. Specifically, its disinfection efficacy toward a wide spectrum of microorganisms in wastewater is accompanied by the simplicity of its handling and use. Moreover, peracetic acid represents a promising option to achieve disinfection while reducing the concentration of typical chlorination byproducts in the final effluent. However, its chemical behavior is still amply debated. In this study, the reactivity of peracetic acid in the presence of halides, namely, chloride and bromide, was investigated in both synthetic waters and in a real contaminated water. While previous studies focused on the ability of this disinfectant to form halogenated byproducts in the presence of dissolved organic matter and halides, this work indicates that peracetic acid also contributes itself as a primary source in the formation of these potentially carcinogenic compounds. Specifically, this study suggests that 1.5 mM peracetic acid may form around 1-10 µg/L of bromoform when bromide is present. Bromoform formation reaches a maximum at near neutral pH, which is highly relevant for wastewater management.

Optimization of physico-chemical and membrane filtration processes to remove high molecular weight polymers from produced water in enhanced oil recovery operations.

Polymer flooding is an enhanced oil recovery technique to extract the large portion of leftover subsurface oil following conventional extraction methods. In the flooding process, a long-chain polymer, such as partially hydrolyzed polyacrylamide (HPAM), is added to the displacing fluid to increase the mobility and extraction of the oil phase. Nevertheless, the challenge of managing produced water from polymer flooding operations is high because residual HPAM results in significantly high viscosity and organic content in the stream. Commonly used methods for produced water treatment, such as gravity settling and flotation, cannot be applied to obtain a purified stream efficiently, while innovative techniques are not yet feasible in practical operations. In this work, a simple method of polymer precipitation prompted by divalent ions is evaluated, optimized, and compared to membrane ultrafiltration. The physico-chemical properties of the HPAM are investigated and polymer precipitation tests are conducted by varying the main operational parameters, including pH, salinity, temperature, calcium and/or magnesium concentration, and polymer concentration. Response surface developed by central composite design method is used to optimize the process and identify the correct dosage of divalent cations coagulants and pH, the two main factors promoting HPAM separation. The removal of HPAM is well-described and maximized (>85%) by the model, which is also validated on three synthetic samples representing real wastewaters from polymer flooding applications. Optimized ultrafiltration, using ceramic membranes with surface pore size of 15 kDa, also shows the ability to remove HPAM effectively from water, but the precipitation method seems to be more versatile and easier to apply. The two processes, precipitation and ultrafiltration, may potentially be used in sequence as they complement each other in several ways.

Supramolecular Recognition Allows Remote, Site-Selective C-H Oxidation of Methylenic Sites in Linear Amines.

Site-selective C-H functionalization of aliphatic alkyl chains is a longstanding challenge in oxidation catalysis, given the comparable relative reactivity of the different methylenes. A supramolecular, bioinspired approach is described to address this challenge. A Mn complex able to catalyze C(sp3 )-H hydroxylation with H2 O2 is equipped with 18-benzocrown-6 ether receptors that bind ammonium substrates via hydrogen bonding. Reversible pre-association of protonated primary aliphatic amines with the crown ether selectively exposes remote positions (C8 and C9) to the oxidizing unit, resulting in a site-selective oxidation. Remarkably, such control of selectivity retains its efficiency for a whole series of linear amines, overriding the intrinsic reactivity of C-H bonds, no matter the chain length.

Assessment of new hydrogen peroxide activators in water and comparison of their active species toward contaminants of emerging concern.

Advanced oxidation processes are the most efficient tool to thwart the overaccumulation of harmful organic compounds in the environment. In this direction bioinspired metal complexes may be a viable solution for oxidative degradations in water. However, their synthesis is often elaborated and their scalability consequently low. This study presents alternative easy-to-synthesize bioinspired metal complexes to promote degradations in water. The metals employed were iron and manganese ions, hence cheap and highly accessible ions. The complexes were tested toward Phenol, Estrone, Triclosan, Oxybenzone, Diclofenac, Carbamazepine, Erythromycin, Aspartame, Acesulfame K, Anisole and 2,4-Dinitrotoluene. The reaction favoured electron-rich compounds reaching a removal efficiency of over 90%. The central ion plays a crucial role. Specifically, Mn(II) induces a non-radical pathway while iron ions a predominant radical one (⋅OH is predominant). The iron systems resulted more versatile toward contaminants, while the manganese ones showed a higher turn-over number, hence higher catalytic behaviour.

Acids from fruits generate photoactive Fe-complexes, enhancing solar disinfection of water (SODIS): A systematic study of the novel "fruto-Fenton" process, effective over a wide pH range (4 - 9).

This study aimed to enhance solar disinfection (SODIS) by the photo-Fenton process, operated at natural pH, through the re-utilization of fruit wastes. For this purpose, pure organic acids present in fruits and alimentary wastes were tested and compared with synthetic complexing agents. Owing to solar light, complexes between iron and artificial or natural chelators can be regenerated through ligand-to-metal charge transfer (LMCT) during disinfection. The target complexes were photoactive under solar light, and the Fe:Ligand ratios for ex situ prepared iron complexes were assessed, achieving a balance between iron solubilization and competition with bacteria as a target for oxidizing species. In addition, waste extracts containing natural acidic ligands were an excellent raw material for our disinfection enhancement purposes. Indeed, lemon and orange juice or their peel infusions turned out to be more efficient than commercially available organic acids, leading to complete inactivation in less than 1 h by this novel "fruto-Fenton" process, i.e. in the presence of a fruit-derived ligand, Fe(II) and H2O2. Finally, its application in Lake Leman water and in situ complex generation led to effective bacterial inactivation, even in mildly alkaline surface waters. This work proposes interesting SODIS and fruit-mediated photo-Fenton enhancements for bacterial inactivation in resource-poor contexts and/or under the prism of circular economy.

Evaluation of the effectiveness, safety, and feasibility of 9 potential biocides to disinfect acidic landfill leachate from algae and bacteria.

This study evaluates 9 biocides as disinfectants against microbiological contaminants, specifically, microalgae and E. coli, while assessing their safety and environmental impact. Specifically, the biocide effectiveness and corresponding generation of halogenated compounds is assessed in a real contaminated groundwater receiving acidic leachate from a phosphogypsum landfill. Oxidizing agents are investigated, namely, hypochlorite, peracetic acid, hydrogen peroxide, chlorine dioxide, and persulfate, together with electrophilic biocides, namely, 2,2-dibromo-2-cyanoacetamide and (chloro-) methylisothiazolinone. In addition, a novel disinfection approach is assessed by applying reducing agents, namely, sulfite and metabisulfite. The disinfection mechanism and the formation of halogenated compounds are discussed on the basis of the mode of action and of the molecular structure of each biocide. Overall, the results show that an optimal dosage of the biocides exists to minimize the formation of harmful compounds in water while maximizing disinfection, especially for hypochlorite and peracetic acid. This dosage was between 0.03 mM and 0.15 mM depending on the biocide. The safety of electrophilic biocides is found to be associated to their molecular structure rather than their mode of action. Hydrogen peroxide, MIT, and metabisulfite are the most promising disinfectants in the contaminated groundwater matrix of interest since no halogenated by-products are detected upon successful disinfection, while they are able to completely inactivate bacteria and remove over the 80% of microalgae in the selected matrix. In particular, metabisulfite represents a highly promising biocide, owing to its low environmental and health impacts, as well as economic feasibility (estimated reagent cost ~0.002 € per cubic meter of treated water).

Evaluation of Fenton and modified Fenton oxidation coupled with membrane distillation for produced water treatment: Benefits, challenges, and effluent toxicity.

Membrane distillation is a promising technology to desalinate hypersaline produced waters. However, the organic content can foul and wet the membrane, while some fractions may pass into the distillate and impair its quality. In this study, the applicability of the traditional Fenton process was investigated and preliminarily optimized as a pre-treatment of a synthetic hypersaline produced water for the following step of membrane distillation. The Fenton process was also compared to a modified Fenton system, whereby safe iron ligands, i.e., ethylenediamine-N,N'-disuccinate and citrate, were used to overcome practical limitations of the traditional reaction. The oxidation pre-treatments achieved up to 55% removal of the dissolved organic carbon and almost complete degradation of the low molecular weight toxic organic contaminants. The pre-treatment steps did not improve the productivity of the membrane distillation process, but they allowed for obtaining a final effluent with significantly higher quality in terms of organic content and reduced Vibrio fischeri inhibition, with half maximal effective concentration (EC50) values up to 25 times those measured for the raw produced water. The addition of iron ligands during the oxidation step simplified the process, but resulted in an effluent of slightly lower quality in terms of toxicity compared to the use of traditional Fenton.

Fe-chitosan complexes for oxidative degradation of emerging contaminants in water: Structure, activity, and reaction mechanism.

Versatile and ecofriendly methods to perform oxidations at near-neutral pH are of crucial importance for processes aimed at purifying water. Chitosan, a deacetylated form of chitin, is a promising starting material owing to its biocompatibility and ability to form stable films and complexes with metals. Here, we report a novel chitosan-based organometallic complex that was tested both as homogeneous and heterogeneous catalyst in the degradation of contaminants of emerging concern in water. The stoichiometry of the complex was experimentally verified with different metals, namely, Cu(II), Fe(III), Fe(II), Co(II), Pd(II), and Mn(II), and we identified the chitosan-Fe(III) complex as the most efficient catalyst. This complex effectively degraded phenol, triclosan, and 3-chlorophenol in the presence of hydrogen peroxide. A putative ferryl-mediated reaction mechanism is proposed based on experimental data, density functional theory calculations, and kinetic modeling. Finally, a film of the chitosan-Fe(III) complex was synthesized and proven a promising supported heterogeneous catalyst for water purification.

Natural iron ligands promote a metal-based oxidation mechanism for the Fenton reaction in water environments.

The Fenton reaction is an effective advanced oxidation process occurring in nature and applied in engineering processes toward the degradation of harmful substances, including contaminants of emerging concern. The traditional Fenton application can be remarkably improved by using iron complexes with organic ligands, which allow for the degradation of contaminants at near-neutral pH and for the reduction of sludge production. This work discusses the mechanisms involved both in the classic Fenton process and in the presence of ligands that coordinate iron. Cyclohexane was selected as mechanistic probe, by following the formation of the relevant products, namely, cyclohexanol (A) and cyclohexanone (K). As expected, the classic Fenton process was associated with an A/K ratio of approximately 1, evidence of a dominant free radical behavior. Significantly, the presence of widely common natural and synthetic carboxyl ligands selectively produced mostly the alcoholic species in the first oxidation step. A ferryl-based mechanism was thus preferred when iron complexes were formed. Common iron ligands are here proven to direct the reaction pathway towards a selective metal-based catalysis. Such a system may be more easily engineered than a free radical-based one to safely remove hazardous contaminants from water and minimize the production of harmful intermediates.

Metabisulfite as an Unconventional Reagent for Green Oxidation of Emerging Contaminants Using an Iron-Based Catalyst.

In this work, contaminants of emerging concern were catalytically degraded in the homogeneous phase with the use of unconventional green reagents. Three reagents, namely, sulfite, metabisulfite, and persulfate, were tested and compared with conventional hydrogen peroxide in the degradation process activated by Fe-TAML. The latter is a biodegradable, homogeneous tetra-amido macrocyclic ligand catalyst containing iron(III). Metabisulfite showed the highest efficiency among the three tested reagents, and its reactivity was similar to that of H2O2. However, metabisulfite is a safer and cleaner reagent compared to H2O2. A comprehensive study of the activity of metabisulfite with Fe-TAML was carried out toward the oxidative degradation of eight contaminants of emerging concern. The catalytic process was tested at different pH values (7, 9, and 11). Metabisulfite showed the highest activity at pH 11, completely degrading some of the tested micropollutants, but in several cases, the system was active at pH 9 as well. In particular, metabisulfite showed the best efficiency toward phenolic compounds. A preliminary study on the reaction mechanism and the nature of the active species in the Fe-TAML/metabisulfite system was also conducted, highlighting that a high-valent iron-oxo species might be involved in the degradation pathways.

Desalination of Produced Water by Membrane Distillation: Effect of the Feed Components and of a Pre-treatment by Fenton Oxidation.

The treatment of produced waters (by-products of oil and gas extraction) with the innovative process of membrane distillation is challenging, because these highly saline streams contain high concentrations of organic compounds and hydrocarbons that cause membrane wetting and impairment of performance. To design the most compact treatment scheme and with the aim of obtaining an easier management of produced water for reuse purposes, Fenton oxidation is here investigated as a feed pre-treatment that may produce an effluent easily handled by membrane distillation. In high-recovery membrane distillation tests, we systematically investigate the detrimental effects of individual contaminants in a synthetic produced water mimicking the composition of a real sample. The recovery rate depends strongly on the initial salinity, which eventually causes scaling and pore blocking. Surfactants are found to be mainly responsible for membrane wetting, but volatile and hydrophobic organics also spoil the quality of the product water. A Fenton oxidation pre-treatment is thus performed to degrade the target organics, with the aim of enhancing the effectiveness of the following membrane distillation and to improve the quality of the final product. The combined oxidation-membrane distillation scheme has both advantages and limitations, which need to be carefully evaluated and further investigated.