Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination

Revisiting iodide species transformation in peracetic acid oxidation: unexpected role of radicals in micropollutants decontamination and iodate formation

Peracetic acid (PAA) is an alternative disinfectant for saline wastewaters, and hypohalous acids are typically regarded as the reactive species for oxidation and disinfection. However, new results herein strongly suggest that reactive radicals instead of HOI primarily contributed to decontamination during PAA treatment of iodine-containing wastewater. The presence of I- could greatly accelerate the micropollutants (e.g., sulfamethoxazole (SMX)) transformation by PAA. Chemical probes experiments and electron paramagnetic resonance analysis demonstrate acetylperoxyl radical rather than reactive iodine species primarily responsible for SMX degradation. The kinetic model was developed to further distinguish and quantify the contribution of radicals and iodine species, as well as to elucidate the transformation pathways of iodine species. Density functional theory calculations indicated that the nucleophilic attack of I- on the peroxide bond of PAA could form unstable O-I bond, with the transition state energy barrier for radical generation lower than that for HOI formation. The transformation of iodine species was regulated by acetylperoxyl radical to generate nontoxic IO3-, greatly alleviating the iodinated DBPs formation in saline wastewaters. This work provides mechanistic insights in radical-regulated iodine species transformation during PAA oxidation, paving the way for the development of viable and eco-friendly technology for iodide containing water treatment.

Bacterial dealkylation of benzalkonium chlorides in wastewater produces benzyldimethylamine, a potent N-nitrosodimethylamine precursor

N-nitrosodimethylamine (NDMA) is a carcinogenic disinfection byproduct that forms during chloramine disinfection of municipal wastewater effluents which are increasingly used to augment drinking water supplies due to growing water scarcity. Knowledge of wastewater NDMA precursors is limited and the known pool of NDMA precursors has not closed the mass balance between precursor loading, precursor NDMA yield, and formed NDMA. Benzalkonium chlorides (BACs) are the most prevalent quaternary ammonium surfactants and have antimicrobial properties. The extensive utilization of BACs in household, commercial and industrial products has resulted in their detection in wastewater at elevated concentrations. We report the formation of a potent NDMA precursor, benzyldimethylamine (BDMA) from the biodegradation of BACs during activated sludge treatment. BDMA formation and NDMA formation potential (FP) were functions of BAC and mixed liquor suspended solids concentration at circumneutral pH, and the microbial community source. Sustained exposure to microorganisms reduced NDMA FP through successive dealkylation of BDMA to less potent precursors. BAC alkyl chain length (C8 - C16) had little impact on NDMA FP and BDMA formation because chain cleavage occurred at the C-N bond. Wastewater effluents collected from three facilities contained BDMA from 15 to 106 ng/L, accounting for an estimated 4 to 38 % of the NDMA precursor pool.

Iodine revisited: If and how inorganic iodine species can be measured reliably and what cause their conversions in water?

This study revisited a list of inorganic iodine species on their detections and conversions under different water conditions. Several surprising results were found, e.g., UV-vis spectrophotometry is the only reliable method for I3- and I2 determinations with coexisting I-/IO3-/IO4-, while alkaline eluent of IC and LC columns can convert them into I- completely; IO4- can be converted into IO3- completely in IC columns and partly in LC columns; a small portion of IO3- was reduced to I- in LC columns. To avoid errors, a method for detecting multiple coexisting iodine species is suggested as follows: firstly, detecting I3- and I2 via UV-vis spectrophotometry; then, analyzing IO4- (> 0.2 mg/L) through LC; and lastly, obtaining I- and IO3- concentrations by deducting I- and IO3- measured by IC from the signals derived from I3-/I2/IO4-. As for stability, I- or IO3- alone is stable, but mixing them up generates I2 or H2OI+ under acidic conditions. Although IO4- is stable within pH 4.0-8.0, it becomes H5IO6/H3IO62- in strongly acidic/alkaline solutions. Increasing pH accelerates the conversions of I3- and I2 into I- under basic conditions, whereas dissolved oxygen and dosage exert little effect. Additionally, spiking ICl into water produces I2 and IO3- rather than HIO.

Overlooked Iodo-Disinfection Byproduct Formation When Cooking Pasta with Iodized Table Salt

Iodized table salt provides iodide that is essential for health. However, during cooking, we found that chloramine residuals in tap water can react with iodide in table salt and organic matter in pasta to form iodinated disinfection byproducts (I-DBPs). While naturally occurring iodide in source waters is known to react with chloramine and dissolved organic carbon (e.g., humic acid) during the treatment of drinking water, this is the first study to investigate I-DBP formation from cooking real food with iodized table salt and chloraminated tap water. Matrix effects from the pasta posed an analytical challenge, necessitating the development of a new method for sensitive and reproducible measurements. The optimized method utilized sample cleanup with Captiva EMR-Lipid sorbent, extraction with ethyl acetate, standard addition calibration, and analysis using gas chromatography (GC)-mass spectrometry (MS)/MS. Using this method, seven I-DBPs, including six iodo-trihalomethanes (I-THMs) and iodoacetonitrile, were detected when iodized table salt was used to cook pasta, while no I-DBPs were formed with Kosher or Himalayan salts. Total I-THM levels of 11.1 ng/g in pasta combined with cooking water were measured, with triiodomethane and chlorodiiodomethane dominant, at 6.7 and 1.3 ng/g, respectively. Calculated cytotoxicity and genotoxicity of I-THMs for the pasta with cooking water were 126- and 18-fold, respectively, compared to the corresponding chloraminated tap water. However, when the cooked pasta was separated (strained) from the pasta water, chlorodiiodomethane was the dominant I-THM, and lower levels of total I-THMs (retaining 30% of the I-THMs) and calculated toxicity were observed. This study highlights an overlooked source of exposure to toxic I-DBPs. At the same time, the formation of I-DBPs can be avoided by boiling the pasta without a lid and adding iodized salt after cooking.

Iodide sources in the aquatic environment and its fate during oxidative water treatment - A critical review

Iodine is a naturally-occurring halogen in natural waters generally present in concentrations between 0.5 and 100 µg L^-1. During oxidative drinking water treatment, iodine-containing disinfection by-products (I-DBPs) can be formed. The formation of I-DBPs was mostly associated to taste and odor issues in the produced tap water but has become a potential health problem more recently due to the generally more toxic character of I-DBPs compared to their chlorinated and brominated analogues. This paper is a systematic and critical review on the reactivity of iodide and on the most common intermediate reactive iodine species HOI. The first step of oxidation of I^- to HOI is rapid for most oxidants (apparent second-order rate constant, k_app > 10³ M^-1s^-1 at pH 7). The reactivity of hypoiodous acid with inorganic and organic compounds appears to be intermediate between chlorine and bromine. The life times of HOI during oxidative treatment determines the extent of the formation of I-DBPs. Based on this assessment, chloramine, chlorine dioxide and permanganate are of the highest concern when treating iodide-containing waters. The conditions for the formation of iodo-organic compounds are also critically reviewed. From an evaluation of I-DBPs in more than 650 drinking waters, it can be concluded that one third show low levels of I-THMs (<1 µg L^-1), and 18% exhibit concentrations > 10 µg L^-1. The most frequently detected I-THM is CHCl₂I followed by CHBrClI. More polar I-DBPs, iodoacetic acid in particular, have been reviewed as well. Finally, the transformation of iodide to iodate, a safe iodine-derived end-product, has been proposed to mitigate the formation of I-DBPs in drinking water processes. For this purpose a pre-oxidation step with either ozone or ferrate(VI) to completely oxidize iodide to iodate is an efficient process. Activated carbon has also been shown to be efficient in reducing I-DBPs during drinking water oxidation.

Formation Mechanism of Iodinated Aromatic Disinfection Byproducts: Acid Catalysis with H2OI. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2022;56:1791-1800. [PMID: 35061374 DOI: 10.1021/acs.est.1c05484]

Iodinated aromatic disinfection byproducts (I-DBPs) are a group of nonregulated but highly toxic DBPs. The formation of I-DBPs is attributed mainly to HOI because it is the most abundant reactive iodine species in chloraminated water. In this study, we used computational modeling of thermodynamics to examine the mechanism of iodination of aromatic contaminants, e.g., dipeptides and phenols. Computational prediction of the energy barriers of the formation of iodinated tyrosylglycine (I-Tyr-Gly) (66.9 kcal mol^-1) and hydroxylated Tyr-Gly (OH-Tyr-Gly) (46.0 kcal mol^-1) via iodination with HOI favors the formation of OH-Tyr-Gly over I-Tyr-Gly. Unexpectedly, mass spectrometry experiments detected I-Tyr-Gly but not OH-Tyr-Gly, suggesting that I-Tyr-Gly formation cannot be attributed to HOI alone. To clarify this result, we examined the thermodynamic role of the most reactive iodine species H₂OI⁺ in the formation of aromatic I-DBPs under chloramination. Computational modeling of thermodynamic results shows that the formation of a loosely bonded complex of aromatic compounds with H₂OI⁺ is the key step to initiate the iodination process. When H₂OI⁺ serves as an acid catalyst and an iodinating agent, with HOI or H₂O acting as a proton acceptor, the energy barrier of I-DBP formation was significantly lower (10.8-13.1 kcal mol^-1). Therefore, even with its low concentration, H₂OI⁺ can be involved in the formation of I-DBPs. These results provide insight into the mechanisms of aromatic I-DBP formation and important information for guiding research toward controlling I-DBPs in drinking water.

Freeze-Thaw Cycle-Enhanced Transformation of Iodide to Organoiodine Compounds in the Presence of Natural Organic Matter and Fe(III)

The formation of organoiodine compounds (OICs) is of great interest in the natural iodine cycle as well as water treatment processes. Herein, we report a pathway of OIC formation that reactive iodine (RI) and OICs are produced from iodide oxidation in the presence of Fe(III) and natural organic matter (NOM) in frozen solution, whereas their production is insignificant in aqueous solution. Moreover, thawing the frozen solution induces the further production of OICs. A total of 352 OICs are detected by Fourier transform ion cyclotron resonance mass spectrometry in the freeze-thaw cycled reactions of Fe(III)/I^-/humic acid solution, which are five times as many as OICs in aqueous reactions. Using model organic compounds instead of NOM, aromatic compounds (e.g., phenol, aniline, o-cresol, and guaiacol) induce higher OIC formation yields (10.4-18.6%) in the freeze-thaw Fe(III)/I^- system than those in aqueous (1.1-2.1%) or frozen (2.7-7.6%) solutions. In the frozen solution, the formation of RI is enhanced, but its further reaction with NOM is hindered. Therefore, the freeze-thaw cycle in which RI is formed in the frozen media and the resulting RI is consumed by reaction with NOM in the subsequently thawed solution is more efficient in producing OICs than the continuous reaction in frozen solution.

Preferential Halogenation of Algal Organic Matter by Iodine over Chlorine and Bromine: Formation of Disinfection Byproducts and Correlation with Toxicity of Disinfected Waters

The increasing occurrence of harmful algal blooms (HABs) in surface waters may increase the input of algal organic matter (AOM) in drinking water. The formation of halogenated disinfection byproducts (DBPs) during combined chlorination and chloramination of AOM and natural organic matter (NOM) in the presence of bromide and iodide and haloform formation during halogenation of model compounds were studied. Results indicated that haloform/halogen consumption ratios of halogens reacting with amino acids (representing proteins present in AOM) follow the order iodine > bromine > chlorine, with ratios for iodine generally 1-2 orders of magnitude greater than those for chlorine (0.19-2.83 vs 0.01-0.16%). This indicates that iodine is a better halogenating agent than chlorine and bromine. In contrast, chlorine or bromine shows higher ratios for phenols (representing the phenolic structure of humic substances present in NOM). Consistent with these observations, chloramination of AOM extracted from Microcystis aeruginosa in the presence of iodide produced 3 times greater iodinated trihalomethanes than those from Suwannee River NOM isolate. Cytotoxicity and genotoxicity of disinfected algal-impacted waters evaluated by Chinese hamster ovary cell bioassays both follow the order chloramination > prechlorination-chloramination > chlorination. This trend is in contrast to additive toxicity calculations based on the concentrations of measured DBPs since some toxic iodinated DBPs were not identified and quantified, suggesting the necessity of experimentally analyzing the toxicity of disinfected waters. During seasonal HAB events, disinfection practices warrant optimization for iodide-enriched waters to reduce the toxicity of finished waters.

The identification of halogenated disinfection by-products in tap water using liquid chromatography-high resolution mass spectrometry

In this paper, a comprehensive method for the identification of the unknown halogenated DBPs (X-DBPs, X = Cl, Br, and I) in the tap water of Wuhan, China via liquid chromatography-high resolution mass spectrometry (LC-HRMS) was developed. 123 X-DBPs were identified through the stepwise procedure, 94 of them were newly identified, and 3 of them were confirmed by standards. Most X-DBPs were aliphatic compounds and highly unsaturated and phenolic compounds, some X-DBPs contained multiple halogen atoms and rich in carboxyl groups, such as C₂H₂O₂BrCl, C₂H₂O₂Br₂, and C₂H₂O₂ClI. It was worth noting that the concentration of some X-DBPs had the same trend with time. Most Cl-DBPs remained stable and I-DBPs were detected occasionally by monitoring the change of concentration of these X-DPBs with the time during three consecutive months. The results demonstrate that the proposed method could provide valuable molecular formula and structure information on unknown multiple halogenated DBPs, or be used for the identification of other multiple halogenated organic compounds in different media.