Moieties Of Natural Organic Matter Research Articles

Unraveling Multiple Pathways of Electron Donation from Phenolic Moieties in Natural Organic Matter.

Natural organic matter (NOM) exhibits a distinctive electron-donating capacity (EDC) that serves a pivotal role in the redox reactions of contaminants and minerals through the transformation of electron-donating phenolic moieties. However, the ambiguity of the molecular transformation pathways (MTPs) that engender the EDC during NOM oxidation remains a significant issue. Here, MTPs that contribute to EDC were investigated by identifying the oxidized products of phenolic model compounds and NOM samples in direct or mediated electrochemical oxidation (DEO or MEO, respectively) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). It was found that the oxidation of newly formed phenolic-OH (ArOH) and the oxidative coupling reaction of the phenoxy radical are the main MTPs that directly contribute to EDC, in addition to the transformation of hydroquinones to quinones. Notably, the oxidative coupling reaction of ArOH contributed at least 22-42% to the EDC. Ferulic acid-like structures can also directly contribute to EDC by incorporating H2O into their acrylic substituents. Furthermore, the opening of C rings can indirectly attenuate the EDC through structural alterations in the electron-donating process of NOM. Decarboxylation can either weaken or enhance the EDC depending on the structure of the phenolic moieties in NOM. These findings suggest that the EDC of NOM is a comprehensive result of multiple NOM MTPs, involving not only ArOH oxidation but also the addition of H2O to olefinic bonds and bond-breaking reactions. Our work provides molecular evidence that aids in the comprehension of the multiple EDC-associated transformation pathways of NOM.

Enhanced Oxidation of Antibiotics by Ferrate Mediated with Natural Organic Matter: Role of Phenolic Moieties.

The increasing presence of antibiotics in water sources threatens public health and ecosystems. Various treatments have been previously applied to degrade antibiotics, yet their efficiency is commonly hindered by the presence of natural organic matter (NOM) in water. On the contrary, we show here that nine types of NOM and NOM model compounds improved the removal of trimethoprim and sulfamethoxazole by ferrate(VI) (FeVIO42-, Fe(VI)) under mild alkaline conditions. This is probably associated with the presence of phenolic moieties in NOMs, as suggested by first-order kinetics using NOM, phenol, and hydroquinone. Electron paramagnetic resonance reveals that NOM radicals are generated within milliseconds in the Fe(VI)-NOM system via single-electron transfer from NOM to Fe(VI) with the formation of Fe(V). The dominance of the Fe(V) reaction with antibiotics resulted in their enhanced removal despite concurrent reactions between Fe(V) and NOM moieties, the radicals, and water. Kinetic modeling considering Fe(V) explains the enhanced kinetics of antibiotics abatement at low phenol concentrations. Experiments with humic and fulvic acids of lake and river waters show similar results, thus supporting the enhanced abatement of antibiotics in real water situations.

Transformation of amino acids and formation of nitrophenolic byproducts in sulfate radical oxidation processes

In this study, the transformation of five amino acids (AAs), i.e., glycine (GLY), alanine (ALA), serine (SER), aspartic acid (ASP), and methionine (MET), in a heat activated peroxydisulfate (PDS) oxidation process was investigated. Experimental data showed that the nitrogen in the AA molecules was oxidized to NH4+ and nitrate (NO3-) sequentially. However, in the presence of natural organic matter (NOM), nitrophenolic byproducts including 4-nitrophenol, 2,4-dinitrophenol, 5-nitrosalicylic acid, 3,5-dinitrosalicylic acid were formed. The nitrogen dioxide radical (NO2•) generated during the transformation of NH4+ to NO3- was presumed to be the key nitrating agent. It reacted with phenolic moieties in NOM and was transformed to nitrophenolic byproducts. Among the selected AAs, SER showed the highest nitrophenolic byproducts formation potential. A total yield of 0.258 μM was observed at the condition of 0.1 mM SER, 5 mg/L (as TOC) NOM, 2 mM PDS, and pH 7.0. The formation from GLY and ALA was lowest, only 0.009 μM detected under the same conditions. The nitrophenolic byproducts formation potential of the AAs was positively related to their reactivity toward SO4•- and can be explained by the local nucleophilicity index (Nk). These findings underline the potential risks in the application of SO4•- based oxidation technology.

1H-13C heteronuclear single quantum coherence NMR evidence for iodination of natural organic matter influencing organo-iodine mobility in the environment

The complex biogeochemical behavior of iodine (I) isotopes and their interaction with natural organic matter (NOM) pose a challenge for transport models. Here, we present results from iodination experiments with humic acid (HA) and fulvic acid (FA) using 1H-13C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy. Even though not a quantitative approach, 1H-13C HSQC NMR corroborated that iodination of NOM occurs primarily through aromatic electrophilic substitution of proton by I, and also revealed how iodination chemically alters HA and FA in a manner that potentially affects the mobility of iodinated NOM in the environment. Three types of iodination experiments were conducted with HA and FA: a) non-enzymatic iodination by IO3− (pH 3) and I− (pH 4 and 7), b) addition of lactoperoxidase to promote I−-iodination in the presence of the co-substrate, H2O2 (pH 7), and c) addition of laccase for facilitating I−-iodination in the presence of O2, with or without a mediator (pH 4). When mediators or H2O2 were present, extracellular oxidases and peroxidases enhanced I− incorporation into NOM by between 54% and 3400%. Iodination of HA, which was less than that of FA, enhanced HA's stability (inferred from increases in aliphatic compounds, decreases in carbohydrate moieties, and thus increased molecular hydrophobicity) yet reduced HA's tendency to incorporate more iodine. As such, HA is expected to act more as a sink for iodine in the environment. In contrast, iodination of FA appeared to generate additional iodine binding sites, which resulted in greater iodine uptake capability and enhanced mobility (inferred from decreases in aliphatic compounds, increases in carbohydrates, and thus decreases in molecular hydrophobicity). These results indicate that certain NOM moieties may enhance while others may inhibit radioiodine mobility in the aqueous environment.

Covalent binding with model quinone compounds unveils the environmental fate of the insensitive munitions reduced product 2,4-diaminoanisole (DAAN) under anoxic conditions

2,4-Dinitroanisole (DNAN) is an insensitive munitions compound expected to replace 2,4,6-trinitrotoluene (TNT). The product of DNAN’s reduction in the environment is 2,4-diaminoanisole (DAAN), a toxic and carcinogenic aromatic amine. DAAN is known to become irreversibly incorporated into soil natural organic matter (NOM) after DNAN’s reduction. Herein, we investigate the reactions between DAAN and NOM under anoxic conditions, using 1,4-benzoquinone (BQ) and methoxybenzoquinone (MBQ) as model humic moieties of NOM. A new method stopped the fast reactions between DAAN and quinones, capturing the fleeting intermediates. We observed that DAAN incorporation into NOM (represented by BQ and MBQ models) is quinone-dependent and occurs via Michael addition, imine (Schiff-base) formation, and azo bond formation. After dimers are formed, incorporation reactions continue, resulting in trimers and tetramers. After 20 days, 56.4% of dissolved organic carbon from a mixture of DAAN (1 mM) and MBQ (3 mM) had precipitated, indicating an extensive polymerization, with DAAN becoming incorporated into high-molecular-weight humic-like compounds. The present work suggests a new approach for DNAN environmental remediation, in which DNAN anaerobic transformation can be coupled to the formation of non-extractable bound DAAN residues in soil organic matter. This process does not require aerobic conditions nor a specific catalyst.

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

Lead dioxide (PbO2(s)) is a corrosion product of lead-containing plumbing materials in water distribution pipelines. The presence of reductants in water could cause the release of soluble lead (mainly Pb(II)) from PbO2(s). Lead in drinking water is detrimental to public health. This paper presents the first application of ferrate (FeVIO42−, Fe(VI)) to decreasing the generation of soluble lead in water containing PbO2(s) and common reducing constituents (e.g., natural organic matter (NOM), iodide (I−), and bromide (Br−)) at different pH conditions (i.e., 6.0, 7.0, and 8.0). The released soluble lead from PbO2(s) was found to be dominantly controlled by NOM in water, via the redox dissolution of PbO2(s) and the reduction of PbO2(s) by reducing moieties of NOM. The feasibility of both processes increased when pH decreased. The I− and Br− in water played minor roles in generating soluble lead. Fe(VI) reacted with reducing functional groups of NOM, as determined by 13C nuclear magnetic resonance spectroscopy. Water pretreatment with Fe(VI) inhibited the reaction of NOM with PbO2(s) and therefore, caused lower soluble lead concentrations compared to water samples without Fe(VI) treatment. This study indicates that Fe(VI) pretreatment is a potential approach to controlling soluble lead in drinking water.

Fate of natural organic matter and oxidation/disinfection by-products formation at a full-scale drinking water treatment plant

ABSTRACT This paper investigates the fate of natural organic matter (NOM) during the full-scale drinking water treatment plant supplied by Danube river bank filtration. After the recent reconstruction of the plant, special attention was devoted to the effects of ozone dose and granulated activated carbon (GAC) filtration on the formation and behaviour of oxidation by-products (carbonyl compounds and bromate), as well as carbonaceous and nitrogenous chlorination by-products. For the oxidation of aromatic NOM moieties that absorb light at UV254, a lower ozone dose (1.0 g O3/m3) is sufficient, whereas to achieve a measurable reduction (about 20%) of total organic carbon, an ozone dose of 1.5 g O3/m3 is required. The content of carbonyl compounds in the water after ozonation increases relative to the content before oxidation treatment, and is up to 12 times higher in the case of aldehydes and up to 2 times higher in the case of carboxylic acids. Seasonal variations, including changes in temperature and the amount of precipitation, were also shown to affect the content of organic matter in the raw water, with slight effects on the quality of the treated water. In the winter, the organic matter content is slightly higher, meaning their transformation products aldehydes and carboxylic acids, are also higher during the winter than the summer.

Formation of Nitrophenolic Byproducts during Heat-Activated Peroxydisulfate Oxidation in the Presence of Natural Organic Matter and Nitrite.

Sulfate radical (SO4•-)-based advanced oxidation is a viable in situ remediation technology for degrading organic contaminants in the subsurface. In this study, we demonstrated that SO4•- could induce the activation of nitrite, an anion commonly present in the subsurface environment, leading to the formation of nitrophenolic byproducts. Fourier-transform infrared spectroscope and 15N nuclear magnetic resonance analysis revealed that the inorganic nitrite was incorporated into natural organic matter (NOM) to form organic nitrogen upon SO4•- oxidation. Nitrophenolic byproducts, including 2-hydroxy-5-nitrobenzoic acid, 4-nitrophenol, and 2,4-dinitrophenol, were identified using high-resolution mass spectrometry in combination with a 15N labeling technique. Formation of nitrated byproducts was ascribed to the scavenging of SO4•- by nitrite, which not only generated the nitrating agent NO2• but also inhibited the degradation of organic compounds, making them more available to the reactions with NO2•. The phenolic moieties in NOM served as the main reactive sites for NO2• attack. The nitration begins with H abstraction on the phenoxy oxygen, followed by the addition of another NO2• to its ortho or para site. Decarboxylation followed by NO2• addition can also generate nitrophenolic byproducts. To the best of our knowledge, this is the first study reporting the nitration of NOM and formation of toxic nitrophenolic byproducts during SO4•--based oxidation. It sheds light on the potential risks of this technology in subsurface remediation practices.

ClO2 pre-oxidation changes the yields and formation pathways of chloroform and chloral hydrate from phenolic precursors during chlorination

Phenolic moieties in natural organic matter (NOM) are important precursors of disinfection by-products (DBPs). In this study, the formation of chloral hydrate from chlorination of seventeen phenolic compounds, including mono-, di- and tri-hydroxybenzenes, were evaluated and the role of chlorine dioxide (ClO2) pre-oxidation on its formation pathways was explored. Chloroform, was also evaluated for comparison. Chlorination of resorcinol exhibited the highest chloral hydrate yield (2.83 ± 0.13%) followed by chlorination of 2,4,6-trichlorophenol (0.61 ± 0.03%). The median of chloral hydrate yields from the tested phenolic compounds was 0.22%. ClO2 pre-oxidation reduced the yields of chloroform from phenol derivatives by 37–97%, except 4-methoxyphenol, catechol and 2,3-dihydroxyphenol. On the contrary, ClO2 pre-oxidation of di- and tri-hydroxybenzenes tended to increase chloral hydrate yields in post-chlorination. Mixed results (both increases and decreases) were observed in chloral hydrate formation from chlorination of mono-hydroxybenzenes after ClO2 pre-oxidation. The changes of their formation were dependent on ClO2 pre-oxidation time and dosages. Identification of transformation products suggested that phenolic compounds were mainly converted to unsaturated carbonyl structures by ClO2. Chlorine substituted benzoquinones and cyclopent-4-ene-1,3-diones were important transformation products after a series of ring open, decarboxylation, hydrolysis and chlorine substitution reactions. The changes in the formation yields of chloral hydrate and chloroform were governed by the difference in initial phenolic precursors and the transformation products after ClO2 pre-oxidation. ClO2 pre-oxidation in water treatment can effectively reduce chloroform formation but may have a risk of increasing chloral hydrate formation.

Reduction of Plutonium(VI) to (V) by Hydroxamate Compounds at Environmentally Relevant pH.

Natural organic matter is known to influence the mobility of plutonium (Pu) in the environment via complexation and reduction mechanisms. Hydroxamate siderophores have been specifically implicated due to their strong association with Pu. Hydroxamate siderophores can also break down into di and monohydroxamates and may influence the Pu oxidation state, and thereby its mobility. In this study we explored the reactions of Pu(VI) and Pu(V) with a monohydroxamate compound (acetohydroxamic acid, AHA) and a trihydroxamate siderophore desferrioxamine B (DFOB) at an environmentally relevant pH (5.5-8.2). Pu(VI) was instantaneously reduced to Pu(V) upon reaction with AHA. The presence of hydroxylamine was not observed at these pHs; however, AHA was consumed during the reaction. This suggests that the reduction of Pu(VI) to Pu(V) by AHA is facilitated by a direct one electron transfer. Importantly, further reduction to Pu(IV) or Pu(III) was not observed, even with excess AHA. We believe that further reduction of Pu(V) did not occur because Pu(V) does not form a strong complex with hydroxamate compounds at a circum-neutral pH. Experiments performed using desferrioxamine B (DFOB) yielded similar results. Broadly, this suggests that Pu(V) reduction to Pu(IV) in the presence of natural organic matter is not facilitated by hydroxamate functional groups and that other natural organic matter moieties likely play a more prominent role.

Effects of natural organic matter with different properties on levofloxacin adsorption to goethite: Experiments and modeling

Adsorption of levofloxacin (LEV) to goethite in the pH range of 3–10, and in the absence or presence of natural organic matter (NOM) represented by nine types of humic acid (HA) and fulvic acid (FA), was studied using batch experiments. The adsorption of LEV to goethite was weak and showed a maximum around pH 5.8. Adding NOM to goethite strongly increased LEV adsorption to goethite, but hardly affected its pH dependency. The adsorption envelopes were well fitted to a linear additive model, in which LEV adsorption to goethite was simulated with the Charge Distribution Multi-Site Complexation (CD-MUSIC) model, and LEV adsorption to NOM was simulated with the Langmuir model. The fitted affinity constants (log K) for LEV adsorption to NOM were significantly and positively correlated with the SUVA (specific ultraviolet absorbance at 280 nm) values of NOM, and negatively correlated with E2/E3 (absorbance ratio at 250 nm and 365 nm) values, carboxyl contents, and the polarity of NOM. The results indicated that aromatic moieties of NOM play a key role in the interactions between LEV and NOM, and hydrophobic interactions and π-π interactions were the major mechanisms for LEV adsorption to NOM, whereas H-bond or surface complexation might not play an important role. Results show that both the concentrations and properties of NOM have a significant effect on the distribution or treatment of antibiotics in soils and waters, which will eventually affect the influence of antibiotics on microorganisms in the environmental systems.

Use of differential absorbance to estimate concentrations of chlorinated disinfection by-product in drinking water: Critical review and research needs

ABSTRACTDifferential UV absorbance (ΔA) has been proposed as a method for estimating disinfection by-products (DBP) concentrations in drinking water at a lower cost and at higher frequencies than the sampling programs required by regulations. Various equations developed in laboratory settings between individual DBP concentrations and ΔA272 have been reported in the literature. Although the form of the DBP–ΔA relationships (linear or exponential) is shared by most published studies, the fitting parameters vary and must be calibrated for each site. This is usually explained by differences in the nature of the natural organic matter (NOM) reacting with free chlorine. This literature review suggests that future research should first focus on improving our understanding of the reaction between NOM moieties and chlorine, and its impact on UV absorbance spectra, and second on overcoming the challenges of applying ΔA for estimating DBPs in full-scale facilities.

Reaction of bromine and chlorine with phenolic compounds and natural organic matter extracts – Electrophilic aromatic substitution and oxidation

Phenolic compounds are known structural moieties of natural organic matter (NOM), and their reactivity is a key parameter for understanding the reactivity of NOM and the disinfection by-product formation during oxidative water treatment. In this study, species-specific and/or apparent second order rate constants and mechanisms for the reactions of bromine and chlorine have been determined for various phenolic compounds (phenol, resorcinol, catechol, hydroquinone, phloroglucinol, bisphenol A, p-hydroxybenzoic acid, gallic acid, hesperetin and tannic acid) and flavone. The reactivity of bromine with phenolic compounds is very high, with apparent second order rate constants at pH 7 in the range of 104 to 107 M−1 s−1. The highest value was recorded for the reaction between HOBr and the fully deprotonated resorcinol (k = 2.1 × 109 M−1 s−1). The reactivity of phenolic compounds is enhanced by the activating character of the phenolic substituents, e.g. further hydroxyl groups. With the data set from this study, the ratio between the species-specific rate constants for the reactions of chlorine versus bromine with phenolic compounds was confirmed to be about 3000.Phenolic compounds react with bromine or chlorine either by oxidation (electron transfer, ET) or electrophilic aromatic substitution (EAS) processes. The dominant process mainly depends on the relative position of the hydroxyl substituents and the possibility of quinone formation. While phenol, p-hydroxybenzoic acid and bisphenol A undergo EAS, hydroquinone, catechol, gallic acid and tannic acid, with hydroxyl substituents in ortho or para positions, react with bromine by ET leading to quantitative formation of the corresponding quinones. Some compounds (e.g. phloroglucinol) show both partial oxidation and partial electrophilic aromatic substitution and the ratio observed for the pathways depends on the pH. For the reaction of six NOM extracts with bromine, electrophilic aromatic substitution accounted for only 20% of the reaction, and for one NOM extract (Pony Lake fulvic acid) it accounted for <10%. This shows that for natural organic matter samples, oxidation (ET) is far more important than bromine incorporation (EAS).

Natural organic matter in urban aerosols: Comparison between water and alkaline soluble components using excitation–emission matrix fluorescence spectroscopy and multiway data analysis

Understanding the complexity of Natural Organic Matter (NOM) in atmospheric aerosols has remained an important goal for the atmospheric research community. This work employs a Parallel Factor Model (PARAFAC) with Alternating Least Squares (ALS) algorithm to decompose and further compare sets of excitation-emission matrices fluorescence spectra of Water-soluble and Alkaline-soluble Organic Matter (WSOM and ASOM, respectively), sequentially extracted from urban aerosols collected during different seasons. The PARAFAC-ALS modelling identified three components in both WSOM and ASOM, whose maximum intensities follow a clear seasonal trend and which are likely to represent the dominant fluorescent moieties in NOM from urban aerosols. The PARAFAC-ALS modelling also indicated differences between the colder and warmer seasons in the fluorescence map of one WSOM component, which contrast with the results obtained for the ASOM, where the fluorescence signatures were found to be constant along the seasons, suggesting that the ASOM may have an in situ origin.

Analysis of aquatic‐phase natural organic matter by optimized LDI‐MS method

The composition and physiochemical properties of aquatic-phase natural organic matter (NOM) are most important problems for both environmental studies and water industry. Laser desorption/ionization (LDI) mass spectrometry facilitated successful examinations of NOM, as humic and fulvic acids in NOM are readily ionized by the nitrogen laser. In this study, hydrophobic NOMs (HPO NOMs) from river, reservoir and waste water were characterized by this technique. The effect of analytical variables like concentration, solvent composition and laser energy was investigated. The exact masses of small molecular NOM moieties in the range of 200-1200 m/z were determined in reflectron mode. In addition, spectra of post-source-decay experiments in this range showed that some compounds from different natural NOMs had the same fragmental ions. In the large mass range of 1200-15,000 Da, macromolecules and their aggregates were found in HPO NOMs from natural waters. Highly humic HPO exhibited mass peaks larger than 8000 Da. On the other hand, the waste water and reservoir water mainly had relatively smaller molecules of about 2000 Da. The LDI-MS measurements indicated that highly humic river waters were able to form large aggregates and membrane foulants, while the HPO NOMs from waste water and reservoir water were unlikely to form large aggregates.

Mechanistic insights into the formation of chloroform from natural organic matter using stable carbon isotope analysis

Chloroform can be naturally formed in terrestrial environments (e.g. forest soils, peatland) by chlorination of natural organic matter (NOM). Recently, it was demonstrated that natural and anthropogenic chloroform have a distinctly different carbon isotope signature that makes it possible to identify its origin in soil and groundwater. In order to evaluate the contribution of different functional groups to chloroform production and factors controlling the isotopic composition of chloroform, carbon isotope trends during chlorination of model compounds, soil organic matter (SOM) and humic acids were evaluated, and apparent kinetic isotope effects (AKIEs) quantified. Phenol and propanone were selected as model compounds representing common functional groups in NOM. Chlorination was induced by hypochlorous acid to mimic natural chlorination. The pH ranged between 4 and 8 to cover typical soil conditions. For each model compound and pH, different AKIEs were observed. For phenol, the AKIE was normal at pH 4 (1.0156±0.0012) and inverse at pH 8 (0.9900±0.0007). For 2-propoanol, an opposite pH dependence was observed with an inverse AKIE at pH 4 (0.9935±0.0007) and a normal AKIE at pH 8 (1.0189±0.0016). The variations of the AKIE values suggest that the rate-limiting step of the reaction is either the re-hybridization of the carbon atom involved in chloroform formation or the hydrolysis of trichloroacetyl intermediates depending on the nature of functional group and pH. The chloroform formation from humic acid and SOM gives rise to small isotope variations. A comparison of the isotopic trends of chloroform formed from humic acid and SOM with those found for the model compounds suggest that opposed AKIE associated with the chlorination of phenolic and ketone moieties of NOM partly compensate each other during chlorination of NOM indicating that different types of functional groups contribute to chloroform formation.

Copper Redox Transformation and Complexation by Reduced and Oxidized Soil Humic Acid. 1. X-ray Absorption Spectroscopy Study

Natural organic matter (NOM) exerts strong influence on copper speciation and bioavailability in soils and aquatic systems. In redox-dynamic environments, electron transfer reactions between copper and redox-active moieties of NOM may trigger Cu(I) and Cu(0) formation. To date, little is known about Cu-NOM redox interactions and Cu(I) binding to NOM. Here, we present X-ray absorption spectroscopy results on copper redox transformations upon addition of Cu(II) or Cu(I) to untreated and electrochemically reduced soil humic acid (HA) under oxic and anoxic conditions. Both untreated and reduced HA mediated copper redox transformations. Under anoxic conditions, Cu(II) and Cu(I) added to reduced HA were primarily complexed and thereby stabilized as Cu(I)-HA at low loadings, whereas high copper loadings resulted in the additional formation of Cu(0) nanoparticles (16-64% of total copper). Cu(I) bound to HA was predominantly 2-fold coordinated and to a lower extent 3- to 4-fold coordinated, with a contribution of at least one nitrogen and/or sulfur ligand group. Under oxic conditions, Cu(II)-HA complexes prevailed, but smaller fractions of copper were also stabilized as Cu(I)-HA in a 3- to 4-fold coordination. Our results show that Cu-HA redox interactions are strongly affected by binding of Cu(II) and Cu(I) to HA and that HA contributes to the stabilization of Cu(I) against disproportionation.

Complexes of the antimicrobial ciprofloxacin with soil, peat, and aquatic humic substances

Natural organic matter (NOM) is implicated in the binding of antibiotics by particles in soils and waters. The authors' previous computational study revealed structural rearrangement of both hydrophilic and hydrophobic moieties of NOM to favor H-bonding and other intermolecular interactions, as well as both competition with ion-exchange reactions and bridging interactions by NOM-bound divalent cations. The importance of these interactions was investigated using fluorescence-quenching spectroscopy to study the adsorption of ciprofloxacin (Cipro), a fluoroquinolone antibiotic, on 4 reference humic substances (HSs): Elliott soil humic acid (HA), Pahokee peat HA, and Suwannee river HA and fulvic acid. A simple affinity spectrum HS model was developed to characterize the cation-exchange capacity and the amount of H-bond donor moieties as a function of pH. The adsorption results stress the influence of both pH conditions and the type of HS: both soil HA and peat HA exhibited up to 3 times higher sorption capacity than the aquatic HS at pH ≥ 6, normalizing to the aromatic C content accounted for the differences among the terrestrial HS, and increasing the concentration of divalent cations led to a decrease in adsorption on aquatic HA but not on soil HA. In addition, the pH-dependent speciation models of the Cipro-HS complexes illustrate an increase in complexation due to an increase in deprotonation of HS ligands with increasing pH and, at circumneutral and alkaline pH, enhanced complexation of zwitterionic Cipro only in the presence of soil HA and peat HA. The findings of the present study imply that, in addition to electrostatic interactions, van der Waals interactions as facilitated by aromatic structures and H-bond donating moieties in terrestrial HS may facilitate a favorable binding environment. Environ Toxicol Chem 2013;32:1467-1478. © 2013 SETAC.

Assessing the Effect of Humic Acid Redox State on Organic Pollutant Sorption by Combined Electrochemical Reduction and Sorption Experiments

Natural Organic Matter (NOM) is a major sorbent for organic pollutants in soils and sediments. While sorption under oxic conditions has been well investigated, possible changes in the sorption capacity of a given NOM induced by reduction have not yet been studied. Reduction of quinones to hydroquinones, the major redox active moieties in NOM, increases the number of H-donor moieties and thus may affect sorption. This work compares the sorption of four nonionic organic pollutants of different polarities (naphthalene, acetophenone, quinoline, and 2-naphthol), and of the organocation paraquat to unreduced and electrochemically reduced Leonardite Humic Acid (LHA). The redox states of reduced and unreduced LHA in all sorption experiments were stable, as demonstrated by a spectrophotometric 2,6-dichlorophenol indophenol reduction assay. The sorption isotherms of the nonionic pollutants were highly linear, while paraquat sorption was strongly concentration dependent. LHA reduction did not result in significant changes in the sorption of all tested compounds, not even of the cationic paraquat at pH 7, 9, and 11. This work provides the first evidence that changes in NOM redox state do not largely affect organic pollutant sorption, suggesting that current sorption models are applicable both to unreduced and to reduced soil and sediment NOM.

Assessing the Reactivity of Free Chlorine Constituents Cl2, Cl2O, and HOCl Toward Aromatic Ethers

Cl(2) and Cl(2)O are highly reactive electrophiles capable of influencing rates of disinfection byproduct (DBP) precursor chlorination in solutions of free available chlorine (FAC). The current work examines how organic compound structure influences susceptibility toward chlorination by Cl(2) and Cl(2)O relative to the more abundant (but less reactive) electrophile HOCl. Chlorination rates and products were determined for three aromatic ethers, whose reactivities with FAC increased in the order: 3-methylanisole <1,3-dimethoxybenzene <1,3,5-trimethoxybenzene. Varying solution conditions (pH, [FAC], [Cl(-)]) permitted quantification of regiospecific second-order rate constants for formation of each product by Cl(2), Cl(2)O, and HOCl. Our results indicate that as the reactivity of methoxybenzenes decreases, the importance of Cl(2) and Cl(2)O (relative to HOCl) increases. Accordingly, Cl(2) and Cl(2)O are likely to play important roles in generating DBPs that originate from natural organic matter (NOM) constituents of somewhat moderate reactivity. As [Cl(2)] is proportional to [Cl(-)] and [Cl(2)O] is proportional to [HOCl](2), ramifications for DBP control measures may differ significantly for these precursors compared to more reactive NOM moieties likely to react predominantly with HOCl. In particular, the role of chloride as a chlorination catalyst challenges its traditional classification as an "inert" electrolyte in water treatment processes.