Adsorption of natural organic matter to air-water interfaces during transport through unsaturated porous media

To better understand how interactions with the air phase influence the movement of natural organic matter (NOM) through the vadose zone, we measured the transport of soil-humic acid (SHA) through laboratory columns packed with partially saturated sand. Our results demonstrate that sorptive reactions at air-water interfaces reduce SHA mobility and that the affinity of SHA for the air phase increases as the porewater pH declines from 8 to 3.9. SHA desorption from air-water interfaces is negligible for conditions of constant pH, but release of bound SHA occurs in response to perturbations in porewater pH. We analyzed the effluent samples collected from our laboratory columns using high-performance size-exclusion chromatography. The results of this analysis demonstrate that the SHA did not fractionate appreciably during transport through the columns, suggesting that the various components of the SHA pool (as distinguished on the basis of molecular weight) express an equal affinity for the air-water interfaces over the range of pH conditions tested. A mathematical model incorporating irreversible, second-order rate laws to simulate adsorption at air-water and solid-water interfaces closely describes the SHA breakthrough data. The mass-transfer parameters that govern this model vary in a discernible fashion with changes in porewater pH, and the parameter trends are consistent with published theories for SHA adsorption.     

Influence of natural organic matter on As transport and retention

Natural organic matter (NOM) can affect the behavior of arsenic within surface and subsurface environments. We used batch and column experiments to determine the effect of peat humic acids (PHA), groundwater fulvic acids (GFA), and a soil organic matter (SOM) extract on As sorption/transport in ferrihydrite-coated sand columns. A reactive transport model was used to quantitatively interpret the transport of As in flow-through column (breakthrough) experiments. We found that As(III) breakthrough was faster than As(V) by up to 18% (with OM) and 14% (without OM). The most rapid breakthrough occurred in systems containing SOM and GFA. Dialysis and ultrafiltration of samples from breakthrough experiments showed that in OM-containing systems, As was transported mostly as free (noncomplexed) dissolved As but also as ternary As-Fe-OM colloids and dissolved complexes. In OM-free systems, As was transported in colloidal form or as a free ion. During desorption, more As(III) desorbed (23-37%) than As(V) (10-16%), and SOM resulted in the highest and OM-free systems the lowest amount of desorption. Overall, our experiments reveal that (i) NOM can enhance transport/mobilization of As, (ii) different fractions of NOM are capable of As mobilization, and (iii) freshly extracted SOM (from a forest soil) had greater impact on As transport than purified GFA/PHA.     

Impact of natural organic matter on the physicochemical properties of aqueous C60 nanoparticles

Existing toxicity data indicate that industrial-scale production of C60 fullerene poses a potential threat to the environment. Evaluating the environmental impact of C60 requires careful characterization of its physicochemical properties in the natural aqueous environment. Our study aims to determine the effects of aquatic natural organic matter (NOM) on the physicochemical properties of aqueous C60 nanoparticles, nC60. Stable nC60 suspensions were formed using three different solvent exchange protocols. They were thoroughly characterized for particle size, morphology, and electrophoretic mobility in the absence or presence of two model NOM components, Suwannee River humic acid and fulvic acid. NOM caused disaggregation of nC60 crystals and aggregates under typical solution conditions of natural water, leading to significant changes in particle size and morphology. Such effect increased with increasing NOM concentration. The changes in nC060 size and morphology strongly depended on the nC60 formation pathway. Results from this study indicate that NOM may play a critical role in the transport and toxicity of C60 in the natural aqueous environment.     

Impact of natural organic matter on floc size and structure effects in membrane filtration

Hematite (10 mg of Fe/L) floc-humic acid assemblages have been formed at pH 4 either by first aggregating hematite particles with salt (100 mM KCl) and then adding humic acid (salt-particle-organic or SPO assemblages) or by suspending the hematite particles in humic acid solutions and then adding salt to induce aggregation (organic-particle-salt or OPS assemblages). The behavior of these assemblages upon deposition on microfiltration (MF) membranes has then been investigated. In the OPS case, the fractal dimension (dF) of the assemblages formed varied dramatically depending upon the extent of charge neutralization by added fulvic acid with dF values typical of diffusion-limited cluster aggregates at low (0.1-0.2 mg/L) humic acid concentrations and dF values typical of reaction-limited cluster aggregates either in the absence of humic acid or concentrations greater than 0.4-0.6 mg/L. In the SPO case, dF values on the order of 2.1 were initially observed and were found to decrease to around 1.8-1.9 for humic acid concentrations greater than 0.6-0.8 mg/L. OPS assemblages with low fractal dimensions were found to be highly compressible once deposited on MF membranes with significantly higher specific cake resistances than was the case for SPO assemblages at transmembrane pressures of 50 kPa and above. These results highlight the importance of both the choice of coagulant (e.g., preformed vs formed in situ) and the transmembrane pressure to which a membrane filtration process might be allowed to rise prior to removal of the fouling layer.     

Modeling metal binding to soils: the role of natural organic matter

The use of mechanistically based models to simulate the solution concentrations of heavy metals in soils is complicated by the presence of different sorbents that may bind metals. In this study, the binding of Zn, Pb, Cu, and Cd by 14 different Swedish soil samples was investigated. For 10 of the soils, it was found that the Stockholm Humic Model (SHM) was able to describe the acid-base characteristics, when using the concentrations of "active" humic substances and Al as fitting parameters. Two additional soils could be modeled when ion exchange to clay was also considered, using a component additivity approach. For dissolved Zn, Cd, Ca, and Mg reasonable model fits were produced when the metal-humic complexation parameters were identical for the 12 soils modeled. However, poor fits were obtained for Pb and Cu in Aquept B horizons. In two of the soil suspensions, the Lund A and Romfartuna Bhs, the calculated speciation agreed well with results obtained by using cation-exchange membranes. The results suggest that organic matter is an important sorbent for metals in many surface horizons of soils in temperate and boreal climates, and the necessity of properly accounting for the competition from Al in simulations of dissolved metal concentrations is stressed.     

Prediction of the sorption of organic compounds into soil organic matter from molecular structure

A new model to estimate the soil-water partition coefficient of non-ionic organic compounds normalized to soil organic carbon, Koc, from the two-dimensional molecular structure is presented. Literature data of log Koc for 571 organic chemicals were fitted to 29 parameters with a squared correlation coefficient r2 of 0.852 and a standard error of 0.469 log units. The application domain includes the atom types C, H, N, O, P, S, F, Cl, and Br in various important compound classes. The multilinear model contains the variables molecular weight, bond connectivity, molecular E-state, an indicator for nonpolar and weakly polar compounds, and 24 fragment corrections representing polar groups. The prediction capability is evaluated through an initial two-step development using an 80%:20% split of the data into training and prediction, cross-validation, permutation, and application to three external data sets. The discussion includes separate analyses for subsets of H-bond donors and acceptors as well as for nonpolar and weakly polar compounds. Comparison with existing models including linear solvation energy relationships illustrates the superiority of the new model.     

Influence of the composition of natural organic matter on Pb bioavailability to microalgae

The current work examines the effects of model allochtonous (humic substances) and autochtonous (microbial polysaccharides) natural organic matter (NOM) on Pb speciation and bioaccumulation. The results demonstrated that polysaccharides, in particular alginic acid, had complexing properties and effects on Pb bioaccumulation by the green alga Chlorella kesslerii that were similar to equivalent complexing capacity of humic substances. Pb uptake decreased in the presence of humic, alginic, and polygalacturonic acids with respect to noncomplexed Pb, but accumulated Pb was higher than predicted from measured Pb2+ concentrations or from previous results obtained in the presence of simple synthetic ligands. An improved fit between experimental observations and Pb speciation was obtained by taking into account the formation of a ternary complex at the algal surface. The contribution of the ternary complexes to Pb bioaccumulation was dependent on the relative binding constants of the Pb to the NOM and to the binding sites on the biological surface. In the presence of the humic acid, a decreased surface charge and increased membrane permeability were considered to be of secondary importance to explain the observation of increased Pb uptake with respect to that predicted on the basis of [Pb2+]. The environmental implications of the results are discussed with respect to the development of site-specific water quality criteria.     

Fractionation of UV and VUV pretreated natural organic matter from drinking water

Recent studies have examined the potential of ultraviolet (UV, 254 nm) and vacuum ultraviolet (VUV, 185 nm + 254 nm) irradiation as either a pretreatment for a biological process or as a sole treatment for the removal of natural organic matter as dissolved organic carbon from drinking water. To understand the potential of UV and VUV irradiation followed by subsequent biological treatment, treated water was fractionated into four components: very hydrophobic acid (VHA), slightly hydrophobic acid (SHA), hydrophilic charged (CHA), and hydrophilic neutral (NEU). The VHA fraction was found to be very susceptible to both UV and VUV irradiation, and the fragmentation products of the high molecular weight VHA and SHA molecules contributed to the CHA and NEU fractions to form a pool of biodegradable, non-UV-absorbing, low molecular weight moieties. The NEU fraction was the most difficult to remove, as most of the components in this fraction were refractory to both the biological and photo-oxidative processes. Therefore, enhanced removal of the NEU fraction is required to increase the effectiveness and potential of the treatment process.     

How the natural organic matter to coagulant ratio impacts on floc structural properties

Periods of elevated natural organic matter (NOM) loadings at water treatment works (WTW) can lead to operational problems as a result of deterioration in floc structural quality. This study used a range of diagnostic tools to evaluate floc structure with increasing organic fraction in the floc. It was observed that when the organic fraction in the floc went significantly over a mass ratio of 1 mg of DOC to 1 mg of Fe (coagulant), the floc size, settling velocity, fractal dimension, and strength were seen to decrease even when the NOM removed during coagulation remained high. A model was proposed to explain these changes that was dependent upon the adsorption of NOM on primary particle surfaces. The operational significance of these results suggests that for the coagulant under investigation the correct coagulant dose must be applied to give good floc structure.     

Impact of natural organic matter on monochloramine reduction by granular activated carbon: the role of porosity and electrostatic surface properties

Steady-state monochloramine reduction in fixed-bed reactors (FBRs) was quantified on five types of granular activated carbon (GAC) using two background waters-one natural source water (LAW) containing 2.5-3.5 mg/L organic carbon and one synthetic organic-free water (NW). While more monochloramine was reduced at steady-state using NW compared to LAW for each GAC and empty-bed contact time studied, the differences in removal varied considerably among the GACs tested. Physical characterization of the GACs suggested that the degree of interference caused by natural organic matter (NOM) increased with increasing GAC surface area contained within pores greater than 2 nm in width. Acid/base and electrostatic properties of the GACs were not found to be significant in terms of NOM uptake, which indicated that size exclusion effects of the GAC pores overwhelmed the impact of the GAC surface chemistry. Therefore, selection of GAC to limit the impact of NOM on monochloramine reduction in FBRs should be based on pore size distribution alone, with the impact of NOM decreasing with decreasing mesoporosity and macroporosity.     

Production of hydrated electrons from photoionization of dissolved organic matter in natural waters

Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is electron ejection to produce hydrated electrons (e-aq). The efficiency of this process has been studied in both fresh water and seawater samples with both steady-state scavenger (S-SS) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of e-aq for the same samples using the two methods differ by as much as a factor of 100, necessitating a closer re-examination of how the process is measured. We developed a highly sensitive multipass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, we measured the AQY of e-aq from Laurentian fulvic acid as 1.3 x 10(-4), and set the upper limit for other CDOM samples at 6 x 10(-5), bringing the LFP results into agreement with those from S-SS methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments, but well above those likely to occur at the earth's surface. This multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.     

Reduction of disinfection byproduct formation by molecular reconfiguration of the fulvic constituents of natural background organic matter

Experiments were performed to assess the effects of treating the fulvic acid fractions of background natural organic matter (NOM) by catalyst-induced oxidative coupling reactions. Changes in the molecular characteristics of the fulvic acids and related disinfection byproduct formation potentials of these important NOM constituents were investigated. The coupling reactions were induced by addition of horseradish peroxidase (HRP) and hydrogen peroxide to aqueous solutions of the fulvic acids (FAs) in semicontinuous flow reactors. Subsequent removal of organic matter by ultrafiltration was found to be markedly enhanced for FA solutions subjected to oxidative coupling treatment. Uniform formation condition tests further indicated that the disinfection byproducts formed upon chlorination of FAs treated via induced oxidative coupling were reduced significantly on a unit carbon basis relative to those formed upon chlorination of their untreated counterparts. Spectroscopic examinations revealed thatthe FA molecules were effectively reconfigured in the oxidative coupling reactions. Substantial conversion of aromatic hydroxyl groups into ether-bonded moieties is evident, and a loss of primary amine groups, probably through conversion into secondary or tertiary amines, was also observed. These conversions apparently result in cross-linking of the natural FA moieties to form stable species of larger sizes, thus rendering them more readily removable by ultrafiltration and less reactive with chlorine. The results of the study may be interpreted as indicating that catalytically induced oxidative coupling reactions of the type conducted in this work can be combined with ultrafiltration to provide an effective scheme for removal of disinfection byproduct precursors.     

Nature and abundance of organic radicals in natural organic matter: effect of pH and irradiation

Dissolved natural organic matter (NOM) plays an essential role in freshwater geochemical and biochemical processes. A major property, its redox behavior, can be attributed to the chinone building blocks, which can form stable radicals. However, electron paramagnetic resonance (EPR) data indicating free radicals on solid NOM are sparse. Here we present EPR spectra of 23 NOM from European surface waters isolated by reverse osmosis. The organic radical concentrations of NOM ranged from 5 x 10(15) to 1.84 x 10(17) spins g(-1), and g values ranged from 2.0031 to 2.0045. Number and type of organic radicals in solid NOM are significantly influenced by the pH of raw water. EPR experiments indicate the presence of semiquinone-type radicals in coexistence with carbon-centered "aromatic" radicals, with the semiquinone-type radicals dominating at alkaline pH. Basically these processes are reversible. Organic radical concentrations in NOM adjusted to pH 6.5 before freeze-drying correlate with iron and aluminum contents. UV- and VIS-irradiation of solid NOM can lead to more than a 10-fold increase of the concentration of organic radicals. These radicals were long-lived and had the same g value as the original radical. Similar effects were not observed with isolated humic and fulvic acids, demonstrating the limited reflection of environmental properties of organic carbon by the classical isolation procedure.     

The impact of UV/H2O2 advanced oxidation on molecular size distribution of chromophoric natural organic matter

The impact of hydroxyl radical (*OH) on the molecular weight distribution of natural organic matter (NOM) was investigated. *OH was generated via the photolysis of hydrogen peroxide (H2O2) by ultraviolet (UV) radiation of 254 nm, also known as UV/ H2O2 advanced oxidation (AO). Additionally, the impact of combined membrane and UV/H2O2 treatment on the molecular weight distribution of NOM was studied. High performance size exclusion chromatography (HPSEC) was used to determine the apparent molecular weight (AMW) distribution of chromophoric NOM (CNOM). Prior to AO, 33% of the CNOM in the water had AMW greater than 1400 Da. Meanwhile, lower AMW CNOM made up smaller amounts of the CNOM, with CNOM of AMW less than 450 Da making up 5% of the total. Under the AO conditions typically applied in drinking water treatment applications, NOM was not mineralized but was partially oxidized resulting in significant reduction in aromaticity. *OH preferentially reacted with higher AMW CNOM and the fragmentation of high AMW CNOM led to the formation of smaller AMW CNOM. Ultrafiltration removed all CNOM greater than 1400 Da AMW and a large portion of other high AMW fractions. In the absence of high AMW CNOM, *OH reacted more readily with all AMW fractions leading to a reduction in concentration of most AMW fractions. Whereas *OH reacted nonspecifically with all AMW fractions, the reaction rate between *OH and CNOM was observed to be dependent on molecular size.     

Effects of natural organic matter and ionic species on membrane surface charge

The surface charges of clean and natural organic matter (NOM)-adsorbed membrane surfaces of two different types of membranes (a UF and a NF membrane composed of the same material but having different pore sizes) were investigated. Concentrated NOM and its fractionated constituents were used as adsorbate and interacting macromolecules nearthe membrane surface. The zeta potential and the acidity of membranes were measured using electrophoresis and potentiometric titration methods, respectively, from the perspective of charge characterization, along with demonstration of ionic strength effects. The membrane surface was also characterized with attenuated total refractive Fourier transform infrared spectra to determine intrinsic functional groups and those changes before and after NOM adsorption. As a comparative study for the electrokinetic property of membrane, the zeta potentials for both examined polymeric membranes were determined by the electrophoresis and the streaming potential measurement methods as functions of ionic strength and the pH of measuring solution. Selectivity tests were performed to decide the relative importance of charge valence of cation in terms of the surface charge of membrane. It was demonstrated that divalent cations (Ca2+, Mg2+) increase zeta potentials relatively compared to monovalent cations (Na+, K+) because divalent cations have a greater potential in approaching membrane surfaces (i.e., inside the Stern layer). Thus, divalent cations can provide a greater double layer compaction and, when near the shear plane (available for both the zeta potential measurement methods), exist to a lesser extent than monovalent cations.