Natural Organic Matter Model Compounds Research Articles

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.

Revealing the effect of anion exchange resin conditioning on the pH and natural organic matter model compounds removal mechanisms

Natural organic matter (NOM) negatively affects water quality and can cause serious issues during water treatment processes. Anion exchange resins (AER) have already proven their effectiveness to remove a major part of this NOM. However, due to its complexity, understanding and improving NOM removal remains challenging. This study aimed to provide in-depth insights in NOM removal with macroporous styrenic weak (WBA), strong (SBA) and combined weak/strong basic AER. Batch experiments were performed with resins in two industrially relevant counter ion forms: OH- and free base form for SBA and WBA resins respectively, using NaOH conditioning, and Cl- form resins using NaCl and HCl conditioning for SBA and WBA resins respectively. Synthetic water containing model compounds for different NOM fractions was used, focussing on the biopolymer fraction which is typically poorly removed by classical ion exchange processes and is a known disinfection by-product precursor in drinking water. The applied resin conditioning procedure influenced the equilibrium pH and consequently NOM removal efficiency and mechanisms. When ion exchange is the dominant driving force for NOM removal, the Cl- form proved to be most efficient. For uncharged NOM containing hydroxyl groups, SBA resins in the OH- form are preferable due to H-bonding. Adsorption of aromatic NOM through π-π interactions with the polystyrene backbone was not affected by the resin conditioning. This work clearly demonstrates that not only the AER type, but also the conditioning procedure and the corresponding counter ion form play a key role in the design of successful NOM removal processes.

Effect of dissociation constant (pKa) of natural organic matter on photo-generation of reactive oxygen species (ROS)

Natural organic matter (NOM) contains a large number of aromatic ketones and quinones, which can produce reactive oxygen species (ROS), such as 1O2 and HO·, through photosensitization. These ROS play important roles in the attenuation of organic pollutants in sunlit surface water. The NOM components fractionated with XAD-8 resin at different pH influence the photo-generation of ROS, which may be related to the degree of dissociation of functional groups. In this study, probe chemicals were used to investigate the effect of dissociation degree (pKa) of NOM model compounds on ROS production under 254 nm UV irradiation. Experimental results showed that six model compounds with similar molecular weights and structures but different pKa values have different ROS production abilities in a pH 7.0 reaction system. The ability for DHB2,5 (2.97), DHB3,4 (4.48), and GUA (9.99) to photo-generate 1O2 increased with increasing dissociation. The ability for DHB3,4 and GUA to photo-generate HO· followed the same trend. The correlation between NOM dissociation constant and ROS generation is preliminary established, which explains the influence of functional groups. Our results may provide some basic information for understanding the correlation between NOM structural features and ROS generation.

Effect of Natural Organic Matter Model Compounds on the Structure Memory Effect of Different Layered Double Hydroxides

The structural transformation of layered double hydroxides (LDHs) plays an important role in a variety of natural environmental behaviors. As one of the important ways of structure transformation i...

Formation of brominated trihalomethanes during chlorination or ozonation of natural organic matter extracts and model compounds in saline water

Oxidation experiments (chlorine, ozone and bromine) were carried out with synthetic saline waters containing natural organic matter (NOM) extracts and model compounds to evaluate the potential of these surrogates to mimic the formation of brominated trihalomethanes (Br-THMs) in natural saline waters. Synthetic saline water with Pony Lake fulvic acid (PLFA) showed comparable results to natural brackish and sea water for Br-THMs formation during chlorination and ozonation for typical ballast water treatment conditions ([Cl2]0 ≥ 5 mg/L or [O3]0 ≥ 3 mg/L). The molar CHBr3 yield in synthetic saline waters is higher for chlorination than for ozonation, since ozone reacts slower with bromide and faster with THM precursors. For bromination, the molar yields of CHBr3 for the NOM model compounds phenol, resorcinol, 3-oxopentanedioic acid and hydroquinone are 28, 62, 91 and 11%, respectively. CHBr3 formation is low during chlorination or ozonation of resorcinol-containing synthetic saline waters due to the faster reaction of resorcinol with these oxidants compared to the bromine formation from bromide. Oxidation experiments with mixtures of hydroquinone and phenol (or resorcinol) were conducted to mimic various functional groups of NOM reacting with Cl2 (or O3) in saline water. With increasing hydroquinone concentrations, the CHBr3 formation increases during both chlorination and ozonation of the mixtures, except for chlorination of the mixture of hydroquinone and resorcinol. The formation of THMs during chlorination of the mixture of hydroquinone and resorcinol is similar to that of resorcinol alone due to the much faster reaction of HOX with resorcinol compared to hydroquinone. In general, PLFA seems to be a reasonable DOM surrogate to simulate CHBr3 formation for realistic ballast water treatment. During chlorination, CHBr3 formations from phenol- and PLFA-containing synthetic brackish waters are comparable, for similar phenol contents.

Coexistence of adsorption and coagulation processes of both arsenate and NOM from contaminated groundwater by nanocrystallined Mg/Al layered double hydroxides

In this study, nanocrystallined Mg/Al layered double hydroxides (LDH-CO3) and chloridion intercalated nanocrystallined Mg/Al LDHs (LDH-Cl) were synthesized and used for simultaneous removal of arsenic and natural organic matter (NOM) from contaminated groundwater. Humic acid (HA) was selected as a model compound of NOM. The maximum adsorption capacities of arsenate (As(V)) on LDH-CO3 and LDH-Cl are 44.66 and 88.30 mg/g, respectively, and those of HA on LDH-CO3 and LDH-Cl are 53.16 and 269.24 mg/g, respectively. It was found that more than 98% of arsenic and 94% of NOM were eliminated by LDH-Cl from both arsenic and NOM-rich groundwater, which is used as drinking water in Togtoh County, Inner Mongolia, China. The arsenic concentration declined from 231 to 4 μg/L, which meets the drinking water standard. The adsorption mechanisms were determined by using X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and extended X-ray absorption fine structure spectroscopy techniques (EXAFS). The results showed that the removal of HA was mainly via surface complexation as well as coagulation at the surface of LDHs, while the adsorption of As(V) was mainly via ion-exchange process. The presence of HA exhibited little inhibiting effect on As(V) adsorption by occupying partial binding sites on LDH surfaces. Nevertheless, it could not affect the ion-exchange process of As(V) with the interlayer anions of LDHs. The removal of As(V) and HA can be carried out independently due to the different adsorption mechanisms. By integrating the experimental results, it is clear that LDH-Cl can be potentially used as a cost-effective material for the purification of both arsenic and NOM contaminated groundwater.

Significant decrease of THMs generated during chlorination of river water by previous photo-Fenton treatment at near neutral pH

► THMs generation decrease by reducing NOM concentration prior to the chlorination step. ► Photo-Fenton process at neutral pH is effective system for the NOM degradation. ► Solar photo-Fenton process can be used to obtain at low cost drinking water safe. An important issue in chlorine drinking water disinfection is the generation from natural organic matter (NOM) of disinfection by-products (DBPs), especially trihalomethanes (THMs). The THMs generation can be limited by reducing the levels of NOM prior to the chlorination step. Herein, photo-Fenton system under solar simulator degraded humic acid solutions (as NOM model compounds), as well as NOM contained in river water (Pance River, Cali-Colombia), and dramatically reduced THMs formation during the subsequent chlorination step. In both cases the initial pH of treated water was near to 7. In no pre-treated river water 100–160 μg/L of THMs are formed during chlorination while values of 20–60 μg/L were reached in water previously treated for 4 h by neutral photo-Fenton under solar simulator. The decrease in the generated THMs/total organic carbon ratio suggests that mainly the NOM fraction sensitive to THMs formation was photo-degraded. Therefore, the neutral photo-Fenton reagent could be implemented as a technological option for NOM removal for limiting THMs formation during a subsequent chlorination step.

Effects of natural organic matter model compounds on the transformation of carbon tetrachloride by chloride green rust

Interest has grown in the use of reactive minerals for natural and engineered transformation of ground water contaminants. This study investigated how the structural properties of 10 model compounds representing natural organic matter (NOM) influenced their adsorption to chloride green rust (GR-Cl), and how this adsorption affected rate constants for transformation of carbon tetrachloride (CT) by GR-Cl. The affinity of benzoic acid, phthalic acid, trimesic acid, pyromellitic acid, and mellitic acid for the GR-Cl surface generally increased in the order of increasing number of carboxylic acid functional groups, increasing acidity of these functional groups, and increasing charge density. For NOM model compounds that had phenolic functional groups ( p-hydroxybenzoic acid, α-resorcylic acid, and caffeic acid), the affinity for the GR-Cl surface was greatest for caffeic acid, which had two adjacent phenolic functional groups. Some NOM model compounds had experimentally determined Langmuir maximum adsorption capacities ( q max−Langmuir) greater than those calculated based on external surface area measurements and the size of the NOM model compound, suggesting adsorption to internal as well as external sites at the GR-Cl surface for these compounds. Rate constants for CT transformation by GR-Cl generally decreased as the affinity of the NOM model compounds (estimated by Langmuir K values) increased, but there was no statistically significant correlation between Langmuir parameters (i.e., K and q max−Langmuir) and rate constants, perhaps due to significant adsorption of some NOM model compounds to sites that were not accessible to CT, such as interlayer sites. Unlike the other NOM model compounds, caffeic acid, which adsorbed to a significant extent to the GR-Cl surface, increased the rate constant for CT transformation. The influence of NOM on rate constants for CT transformation by green rusts should be considered in ground water remediation planning.

The role of NOM fouling for the retention of estradiol and ibuprofen during ultrafiltration

The impact of membrane fouling by natural organic matter (NOM) on the behavior of micropollutants during ultrafiltration (UF) was investigated. Batch experiments with radioactively labeled estradiol and ibuprofen in the presence of NOM model compounds (alginate, Nordic aquatic and Aldrich humic acid) were performed using a hydrophilic and a hydrophobic membrane. The results indicate that the impact of the NOM fractions studied on micropollutant retention correlated with the fouling mechanism of the individual NOM fractions. NOM substances of high molecular weight such as alginate and Aldrich HA that foul the membrane by pore blocking and cake/gel formation had a greater impact than the Nordic aquatic humic acid of lower molecular weight. The effect of cake formation was attributed partly to micropollutant-NOM partitioning and subsequent NOM retention and partly to the effect of the fouling layer itself acting as a kind of second membrane. Fouling by NOM cake/gel formation led to a significantly increased retention of estradiol, whereas the impact of fouling on ibuprofen retention was negligible due to significantly lower log K oc values of ibuprofen compared to estradiol. Moreover, NOM adsorption on the membrane can lower the adsorption of hydrophobic micropollutants. Membrane adsorption of estradiol was in the order of mg/m 2 and was largely reversible with caustic solutions. The results of this study may prove useful for predicting the mass flow of micropollutants in UF applications.

Degradation of DBPs' precursors in river water before and after slow sand filtration by photo-Fenton process at pH 5 in a solar CPC reactor

The generation of disinfection by-products during water treatment can be controlled by reducing the levels of precursor species prior to the chlorination step. The Natural Organic Matter (NOM) is the principal organic precursor and conventional removal of pollutants such as coagulation, flocculation and filtration do not guarantee the total NOM removal. In this study the degradation of NOM model compounds (dihydroxy-benzene) as well as the removal of NOM from river water via photo-Fenton process in a CPC solar photo-reactor is presented. The effect of solar activated photo-Fenton reagent at pH 5.0 before and after a slow sand filtration (SSF) in waters containing natural iron species is investigated and the details reported. The results showed that the total transformation of dihydroxy-benzene compounds along a mineralization higher than 80% was obtained. The mineralization of the organic compounds dissolved in natural water was higher than in Milli-Q water, suggesting that the aqueous organic and inorganic components (metals, humic acids and photoactive species) positively affect the photocatalytic process. When 1.0 mg/L of Fe 3+ is added to the system, the photo-Fenton degradation was improved. Therefore the photo-Fenton reagent could be an interesting complement to other processes for NOM removal. Comparing the response of two rivers as media for the organic compounds degradation it was observed that the NOM photo-degradation rate depends of the water composition.

ATR-FTIR investigation of the role of phenolic groups in the interaction of some NOM model compounds with aluminum hydroxide

The role of phenolic groups in the interaction of natural organic matter (NOM) with metal hydroxides was investigated with ATR-FTIR spectroscopy and adsorption tests by employing a series of dihydroxybenzoic acids (DHBAs) as the NOM surrogates and aluminum hydroxide as the adsorbent. All DHBAs examined in this study were found to be adsorbed on aluminum hydroxide by forming inner-sphere complexes. Carboxylic groups governed the complexation of DHBAs with aluminum hydroxide at low pH or in cases when the two hydroxyl groups were not adjacent to each other and neither of them was ortho to the carboxylic group. The involvement of the phenolic groups, ortho to another phenolic group or ortho to the carboxylic groups, in the complexation increased with increasing pH as the deprotonation of phenolic groups was easier at higher pH. The presence of phenolic groups increased the electron density of the carboxylic groups and facilitated the inner-sphere complexation of the carboxylic groups with metal hydroxide. The correlation between the p K a values and the amount of organic acid adsorbed on the aluminum hydroxide revealed that the adsorption of DHBAs at acidic pH was largely dependent on the surface chelate formation rather than on the electronic effect.

Effects of Natural Organic Matter, Anthropogenic Surfactants, and Model Quinones on the Reduction of Contaminants by Zero-Valent Iron

Recent studies of contaminant reduction by zero-valent iron metal (Fe 0) have highlighted the role of iron oxides at the metal–water interface and the effect that sorption has at the oxide–water interface on contaminant reduction kinetics. The results suggest that a variety of organic surface-active substances might enhance or inhibit contaminant degradation, depending on the degree to which they promote solubilization, sorption, and/or reaction. Of particular interest is the effect of natural organic matter (NOM), because of its ubiquitous presence in natural waters and amphiphilic properties; anthropogenic surfactants, because of their use in groundwater remediation; and certain quinones, which represent the redox-active functional groups associated with NOM. In this study, no well-defined effects of these substances were found on the reduction of nitro benzene by Fe 0, but the reduction of carbon tetrachloride and trichloroethylene (TCE) was inhibited by NOM. Results with carbon tetrachloride showed that the inhibitory effect of humic acids was greater than fulvic acids, but that several quinonoid NOM model compounds (juglone, lawsone, and anthraquinone disulfonate) increased the rate of reduction by Fe 0. Isotherms for adsorption of TCE and NOM onto Fe 0 showed evidence of competition for surface sites.