Natural Organic Matter Complexes Research Articles

Minimizing residual aluminum concentration in treated water by tailoring properties of polyaluminum coagulants

Aluminum coagulants are widely used in water treatment plants to remove turbidity and dissolved substances. However, because high aluminum concentrations in treated water are associated with increased turbidity and because aluminum exerts undeniable human health effects, its concentration should be controlled in water treatment plants, especially in plants that use aluminum coagulants. In this study, the effect of polyaluminum chloride (PACl) coagulant characteristics on dissolved residual aluminum concentrations after coagulation and filtration was investigated. The dissolved residual aluminum concentrations at a given coagulation pH differed among the PACls tested. Very-high-basicity PACl yielded low dissolved residual aluminum concentrations and higher natural organic matter (NOM) removal. The low residual aluminum concentrations were related to the low content of monomeric aluminum (Ala) in the PACl. Polymeric (Alb)/colloidal (Alc) ratio in PACl did not greatly influence residual aluminum concentration. The presence of sulfate in PACl contributed to lower residual aluminum concentration only when coagulation was performed at around pH 6.5 or lower. At a wide pH range (6.5–8.5), residual aluminum concentrations <0.02 mg/L were attained by tailoring PACl properties (Ala percentage ≤0.5%, basicity ≥85%). The dissolved residual aluminum concentrations did not increase with increasing the dosage of high-basicity PACl, but did increase with increasing the dosage of normal-basicity PACl. We inferred that increasing the basicity of PACl afforded lower dissolved residual aluminum concentrations partly because the high-basicity PACls could have a small percentage of Ala, which tends to form soluble aluminum–NOM complexes with molecular weights of 100 kDa–0.45 μm.

Molecular models of natural organic matter and its colloidal aggregation in aqueous solutions: Challenges and opportunities for computer simulations

Natural organic matter (NOM) is ubiquitous in soil and groundwater, and its aqueous complexation with various inorganic and organic species can strongly affect the speciation, solubility, and toxicity of many elements in the environment. Despite significant geochemical, environmental, and industrial interest, the molecular-scale mechanisms of the physical and chemical processes involving NOM are not yet fully understood. Recent molecular dynamics (MD) simulations using relatively simple models of NOM fragments are used here to illustrate the challenges and opportunities for the application of computational molecular modeling techniques to the structural, dynamic, and energetic characterization of metal–NOM complexation and colloidal aggregation in aqueous solutions. The predictions from large-scale MD simulations are in good qualitative agreement with available experimental observations, but also point out the need for simulations at much larger time- and length-scales with more complex NOM models in order to fully capture the diversity of molecular processes involving NOM.

Interactions of Pb and Cd mixtures in the presence or absence of natural organic matter with the fish gill

Metal gill binding and toxicity can be modeled using the concentration addition model, in which the toxic unit (TU) concept is used to determine if constituent metals are acting in a strictly additive, less than, or greater than additive fashion. To test this hypothesis, rainbow trout (Oncorhynchus mykiss) were exposed to a matrix of Pb plus Cd mixtures (nominal concentrations=0.75, 1.5, 2.25, 3.0μmolL−1), in the presence or absence of mainly terrigenous (allochthonous; 10mg CL−1) natural organic matter (NOM), and metal–gill binding, and toxicity was quantified. Based on its greater affinity for metal–gill binding sites, Cd–gill binding was expected to exceed Pb–gill binding during metal mixture exposure, but this only occurred at the lowest metal concentrations (0.75μmolL−1); at higher concentrations Pb–gill binding was greater than Cd–gill binding. These unexpected observations were because Pb and Cd likely bind to different populations of high affinity, low capacity binding sites on the gill, which was borne out in subsequent attempts to mathematically model metal–gill interactions during metal-mixture exposure. The presence of an additional low affinity, high capacity population of Pb–gill binding sites also contributed to higher Pb–gill accumulation. Metal–gill interactions were complicated by NOM, which exacerbated toxicity during Cd-only exposure despite lowering Cd–gill accumulation. NOM also promoted Cd–gill binding in the presence of low-moderate concentrations of Pb (0.75 and 1.50μmolL−1). We suggest that direct interactions of Cd–NOM complexes with the gill, and increases in Cd bioavailability due to Pb outcompeting Cd for NOM–metal binding sites due to its greater affinity for such ligands, accounted for greater Cd–gill binding and toxicity. We conclude that interactions of Pb and Cd with the gill cannot be predicted using the concentration addition model, and that NOM is not universally protective against metal–gill binding and toxicity when fish are exposed to metal mixtures.

Complexes with aquatic organic matter suppress hydrolysis and precipitation of Fe(III)

The fate of iron (Fe) in freshwaters is largely influenced by chemical interactions with natural organic matter (NOM). However, the knowledge about the molecular structure of the Fe species formed in association with aquatic NOM is still limited. In this work we have studied the interactions between Fe(III) and aquatic organic matter from the Suwannee River at a range of pH values and Fe concentrations (6489–55,659μgg−1 dry weight, pH 3.0–6.8) by means of extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopy. Our results showed that Fe(III) formed stable complexes with aquatic NOM, and that carboxylates were the predominant functional group involved in these interactions. At low Fe concentrations (6489–11,953μgg−1; pH 3.0–6.7) mononuclear Fe(III)–NOM complexes dominated the speciation while at higher pH values and Fe concentrations (~23,000μgg−1 pH 5.0–6.2 and 55,659μgg−1 pH 3.1–6.7) we detected mixtures of mononuclear Fe(III)–NOM complexes and a polymeric Fe(III) (hydr)oxide phase. The structure of the latter was indicated to be ferrihydrite-like. It was evident that the strong complexation between Fe and aquatic NOM suppressed the hydrolysis and polymerization of Fe(III), and it is likely that presence of NOM favored the formation of presumably small ferrihydrite particles with an inherent large reactivity due to high surface area per unit mass. Furthermore, our IR data showed that the formation of hydrolyzed Fe(III) was accompanied with structural changes of the NOM material, which implied that these interactions potentially also could affect the reactivity of NOM. Thus, our aggregative results highlight the importance of Fe(III)–NOM interactions for the geochemistry of Fe and NOM as well as for other elements (e.g. P and As), that is known to be strongly associated with Fe(III), in freshwater systems.

Impact of NOM character on copper adsorption by trace ferric hydroxide from iron corrosion in water supply system

The fate of trace concentrations of cupric sulfate (<0.4mg/L) dosed into chloraminated distribution systems to inhibit nitrification has been shown in this study to be controlled by at least two crucial factors: the character of natural organic matter (NOM) and iron hydroxide corrosion products present at low concentrations (<2mg/L). This research quantified the removal of Cu(II) ions added into waters containing trace Fe(OH)3 flocs and the effect of NOM of different character on this removal. The dominant dissolved copper species in NOM-containing waters were found to be Cu(II)–NOM complexes. Both intramolecular chelation and intermolecular complexation can occur, with the latter occurring preferentially and resulting in the aggregation of smaller organic molecules to form larger molecules. The presence of ferric hydroxide flocs when Cu(II) ions were added into NOM-containing waters was shown to result in removal of Cu(II) ions, presumably as Cu(II)–NOM complexes. This removal was through adsorption processes obeying Freundlich isotherms, although the presence of larger NOM molecules and heterogeneous copper species (e.g. Cu(OH)2(s) and CuO(s)) appeared to shield smaller Cu(II)–NOM complexes from adsorption to some extent. For the strategy of inhibition of nitrification in distribution systems by the addition of Cu(II) ions, complexation of Cu(II) ions by NOM and adsorption of Cu(II)–NOM complexes by ferric hydroxide flocs released from pipe walls pose significant operational challenges to maintaining the concentration of Cu(II) ions through the distribution system.

Long term case study of MIEX pre-treatment in drinking water; understanding NOM removal

Removal of natural organic matter (NOM) is a key requirement to improve drinking water quality. This study compared the removal of NOM with, and without, the patented magnetic ion exchange process for removal of dissolved organic carbon (MIEX DOC) as a pre-treatment to microfiltration or conventional coagulation treatment over a 2 year period. A range of techniques were used to characterise the NOM of the raw and treated waters. MIEX pre-treatment produced water with lower concentration of dissolved organic carbon (DOC) and lower specific UV absorbance (SUVA). The processes incorporating MIEX also produced more consistent water quality and were less affected by changes in the concentration and character of the raw water DOC. The very hydrophobic acid fraction (VHA) was the dominant NOM component in the raw water and was best removed by MIEX pre-treatment, regardless of the raw water VHA concentration. MIEX pre-treatment also produced water with lower weight average apparent molecular weight (AMW) and with the greatest reduction in complexity and range of NOM. A strong correlation was found between the VHA content and weight average AMW confirming that the VHA fraction was a major component of the NOM for both the raw water and treated waters.

Ecological consequences of bacterioplankton lifestyles: changes in concepts are needed

In recent years, microbial ecology has developed from a peripheral discipline into a central field of microbiology. This change in state and perception is mainly driven by a rapid development of methods applied in the manifold fields related to microbial ecology. In biogeochemistry, for example, the use of high-resolution techniques such as FT-ICR-MS (Fourier transform ion cyclotron mass spectroscopy) has uncovered an enormous diversity and complexity of natural organic matter produced or degraded microbially either in dissolved or particulate forms. On the other hand, the introduction of high-throughput sequencing methods, such as 454 pyrosequencing, in combination with advances in bioinformatics allows for studying the bacterial diversity in natural samples circumventing cultivation dependent approaches. These new molecular tools enable in depth studies on single-cell genomes, distinct populations or even metacommunities. In combination with metatranscriptome and proteome studies it is for the first time possible to simultaneously unravel the structure and function of complex communities in situ. These technique-derived findings have, on the one hand, dramatically increased our knowledge on the vast diversity and complexity of bacterial habitats and, on the other hand, on phylogentic diversity and physiological responses of natural bacterial communities to their environment. However, until now microbial ecology is lacking an ecologically relevant species definition and useful tools for the identification of ecologically coherent taxa. Studies on intra- and interspecies interactions even with higher organisms demonstrate that bacteria can rapidly adapt to temporal and spatial changes in their environment. Aquatic bacteria have optimized and dramatically expanded their living space by efficient exploitation of organic matter point sources such as particles/aggregates and higher organisms. Although it is evident that particles/aggregates and organisms such as phytoplankton are 'hotspots' for microbial growth and transformation processes, it has not affected sampling strategies of aquatic microbial ecologists, who often focus solely on the free-living bacterial fractions and a priori exclude higher organisms by non-representative water sampling. Therefore, aquatic microbial ecologists have largely overlooked the fact that many aquatic bacteria may possess a complex lifestyle and frequently alternate between a free-living and a surface-associated stage. Here, I propose that modern concepts in aquatic microbial ecology should take into account the high chemical diversity and spatio-temporal variability of the bacterial environment. Interactions of aquatic bacteria with surfaces including living organisms are the key to understanding their physiological adaptations and population dynamics, as well as their contribution to biogeochemical cycles. New sampling strategies and theoretical concepts are needed in aquatic microbial ecology to access the whole spectrum of bacterial lifestyles and their ecological and evolutionary consequences.

Influence of natural organic matter (NOM) quality on Cu–gill binding in the rainbow trout ( Oncorhynchus mykiss)

Natural organic matter (NOM) in aquatic environments reduces metal toxicity to fish by forming metal–NOM complexes, which reduce metal bioavailability, metal–gill binding and toxicity. However, differences in the chemical composition of different types of NOM (quality) could also affect metal–NOM binding and toxicity. We predicted that Cu–gill binding would vary in trout exposed to Cu in the presence of NOM of different qualities. NOM was collected from three sources: Luther Marsh (terrigenous ∼ allochthonous), Bannister Lake (nominally autochthonous), and from a local sewage treatment plant (designated Preston effluent). Excitation–emission matrix spectroscopy (EEMS) revealed that terrigenous Luther Marsh NOM was primarily humic acid-like material (74%), whereas Bannister Lake and Preston sewage effluent NOM had lower humic acid-like material but greater fulvic acid-like material (30% and 50%, respectively). The specific absorption coefficient (SAC) of Luther Marsh NOM was also much higher (SAC = 37.8), consistent with its darker color, compared to more autochthonous, lightly coloured Bannister Lake (SAC = 12.4) NOM, and Preston effluent NOM (SAC = 9.2). At low-moderate waterborne Cu (0–2,000 nmol L −1), all NOM isolates reduced Cu–gill accumulation by 70–90%. Surprisingly, there were no measurable differences in Cu–gill binding amongst the three NOM treatments when fish were exposed to Cu in the low-moderate range. Only at higher Cu (>2,000 nmol L −1) were differences observed, where terrigenous Luther Marsh and Preston effluent NOM reduced Cu–gill binding by 40–50% more than the more autochthonous Bannister Lake NOM. Although Cu–gill binding estimates using the HydroQual BLM showed similar trends, the BLM consistently underestimated Cu–gill binding. We conclude that differences in Cu toxicity at lower-moderate Cu concentrations in the presence of different types of NOM are not necessarily related to measurable differences in Cu–gill accumulation. Rather, we suggest that differences in Cu toxicity reported in the presence of different types of NOM might be explained by direct actions of NOM on the gills, which are quality dependent.

Total Mass Difference Statistics Algorithm: A New Approach to Identification of High-Mass Building Blocks in Electrospray Ionization Fourier Transform Ion Cyclotron Mass Spectrometry Data of Natural Organic Matter

The ultrahigh-resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrum of natural organic matter (NOM) contains several thousand peaks with dozens of molecules matching the same nominal mass. Such a complexity poses a significant challenge for automatic data interpretation, in which the most difficult task is molecular formula assignment, especially in the case of heavy and/or multielement ions. In this study, a new universal algorithm for automatic treatment of FTICR mass spectra of NOM and humic substances based on total mass difference statistics (TMDS) has been developed and implemented. The algorithm enables a blind search for unknown building blocks (instead of a priori known ones) by revealing repetitive patterns present in spectra. In this respect, it differs from all previously developed approaches. This algorithm was implemented in designing FIRAN-software for fully automated analysis of mass data with high peak density. The specific feature of FIRAN is its ability to assign formulas to heavy and/or multielement molecules using "virtual elements" approach. To verify the approach, it was used for processing mass spectra of sodium polystyrene sulfonate (PSS, M(w) = 2200 Da) and polymethacrylate (PMA, M(w) = 3290 Da) which produce heavy multielement and multiply-charged ions. Application of TMDS identified unambiguously monomers present in the polymers consistent with their structure: C(8)H(7)SO(3)Na for PSS and C(4)H(6)O(2) for PMA. It also allowed unambiguous formula assignment to all multiply-charged peaks including the heaviest peak in PMA spectrum at mass 4025.6625 with charge state 6- (mass bias -0.33 ppm). Application of the TMDS-algorithm to processing data on the Suwannee River FA has proven its unique capacities in analysis of spectra with high peak density: it has not only identified the known small building blocks in the structure of FA such as CH(2), H(2), C(2)H(2)O, O but the heavier unit at 154.027 amu. The latter was identified for the first time and assigned a formula C(7)H(6)O(4) consistent with the structure of dihydroxyl-benzoic acids. The presence of these compounds in the structure of FA has so far been numerically suggested but never proven directly. It was concluded that application of the TMDS-algorithm opens new horizons in unfolding molecular complexity of NOM and other natural products.

Kinetic Controls on the Complexation between Mercury and Dissolved Organic Matter in a Contaminated Environment

The interaction of mercury (Hg) with dissolved natural organic matter (NOM) under equilibrium conditions is the focus of many studies but the kinetic controls on Hg-NOM complexation in aquatic systems have often been overlooked. We examined the rates of Hg-NOM complexation both in a contaminated Upper East Fork Poplar Creek (UEFPC) in Oak Ridge, Tennessee, and in controlled laboratory experiments using reducible Hg (Hg(R)) measurements and C(18) solid phase extraction techniques. Of the filterable Hg at the headwaters of UEFPC, >90% was present as Hg(R) and this fraction decreased downstream but remained >29% of the filterable Hg at all sites. The presence of higher Hg(R) concentrations than would be predicted under equilibrium conditions in UEFPC and in experiments with a NOM isolate suggests that kinetic reactions are controlling the complexation between Hg and NOM. The slow formation of Hg-NOM complexes is attributed to competitive ligand exchange among various moieties and functional groups in NOM with a range of binding strengths and configurations. This study demonstrates the need to consider the effects of Hg-NOM complexation kinetics on processes such as Hg methylation and solid phase partitioning.

Coordination chemistry and hydrolysis of Fe(III) in a peat humic acid studied by X-ray absorption spectroscopy

The speciation of iron (Fe) in soils, sediments and surface waters is highly dependent on chemical interactions with natural organic matter (NOM). However, the molecular structure and hydrolysis of the Fe species formed in association with NOM is still poorly described. In this study extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the coordination chemistry and hydrolysis of Fe(III) in solution of a peat humic acid (5010–49,200 μg Fe g −1 dry weight, pH 3.0–7.2). Data were analyzed by both conventional EXAFS data fitting and by wavelet transforms in order to facilitate the identification of the nature of backscattering atoms. Our results show that Fe occurs predominantly in the oxidized form as ferric ions and that the speciation varies with pH and Fe concentration. At low Fe concentrations (5010–9920 μg g −1; pH 3.0–7.2) mononuclear Fe(III)–NOM complexes completely dominates the speciation. The determined bond distances for the Fe(III)–NOM complexes are similar to distances obtained for Fe(III) complexed by desferrioxamine B and oxalate indicating the formation of a five-membered chelate ring structure. At higher Fe concentrations (49,200 μg g −1; pH 4.2–6.9) we detect a mixture of mononuclear Fe(III)–NOM complexes and polymeric Fe(III) (hydr)oxides with an increasing amount of Fe(III) (hydr)oxides at higher pH. However, even at pH 6.9 and a Fe concentration of 49,200 μg g −1 our data indicates that a substantial amount of the total Fe (>50%) is in the form of organic complexes. Thus, in environments with significant amounts of organic matter organic Fe complexes will be of great importance for the geochemistry of Fe. Furthermore, the formation of five-membered chelate ring structures is in line with the strong complexation and limited hydrolytic polymerization of Fe(III) in our samples and also agrees with EXAFS derived structures of Fe(III) in organic soils.

Influence of NOM on the Mobility of Metal(loid)s in Water-Saturated Porous Media

In this work, the interaction of natural organic matter (NOM) with metal(loid)s (Cu, Pb, Zn, Pt, As) and the role of NOM on the metal(loid) transport in a water-saturated quartz sand column were investigated. For detailed information, size exclusion chromatographic (SEC) measurements and “short pulse” laboratory transport experiments with online metal(loid) and NOM detection were used. The SEC measurements showed the formation of metal–NOM complexes. Cu, Pb, Zn and Pt were predominantly bound to the high molecular mass NOM molecules. The binding capacity of the NOM for metals increased with increasing pH value and in the following order: Zn < Pb < Cu < Pt. No evidence for the formation of As–NOM complexes was found. The transport experiments showed no significant influence of NOM on the mobility of Cu, Pb and Zn. The metal–NOM complexes detected in the SEC experiments were obviously sorbed completely onto the grain surfaces in case of the quartz sand system, or they were dissociated partially during passage through the column. No influence of NOM was observed on the transport of As as well. Inorganic Zn and As species were transported through the column with increasing retardation as the pH value increased. Pt showed a high mobility at a pH of 5, and it decreased at a pH of 7 especially in the presence of NOM. The results support the known fact that a decrease in the pH value results in enhanced transport of inorganic metal(loid) species in water-saturated porous media. On the other hand, the presence of NOM can immobilise the metals through metal–NOM complex formation and the deposition of the complexes onto the stationary phase.

Dissociation kinetics of Fe(III)– and Al(III)–natural organic matter complexes at pH 6.0 and 8.0 and 25 °C

The rate at which iron– and aluminium–natural organic matter (NOM) complexes dissociate plays a critical role in the transport of these elements given the readiness with which they hydrolyse and precipitate. Despite this, there have only been a few reliable studies on the dissociation kinetics of these complexes suggesting half-times of some hours for the dissociation of Fe(III) and Al(III) from a strongly binding component of NOM. First-order dissociation rate constants are re-evaluated here at pH 6.0 and 8.0 and 25 °C using both cation exchange resin and competing ligand methods for Fe(III) and a cation exchange resin method only for Al(III) complexes. Both methods provide similar results at a particular pH with a two-ligand model accounting satisfactorily for the dissociation kinetics results obtained. For Fe(III), half-times on the order of 6–7 h were obtained for dissociation of the strong component and 4-5 min for dissociation of the weak component. For aluminium, the half-times were on the order of 1.5 h and 1-2 min for the strong and weak components, respectively. Overall, Fe(III) complexes with NOM are more stable than analogous complexes with Al(III), implying Fe(III) may be transported further from its source upon dilution and dispersion.

Characterization of Iron(III) in Organic Soils Using Extended X-ray Absorption Fine Structure Spectroscopy

The distribution of different iron (Fe) species in soils, sediments, and surface waters has a large influence on the mobility and availability of Fe, other nutrients, and potentially toxic trace elements. However, the knowledge about the specific forms of Fe that occurs in these systems is limited, especially regarding associations of Fe with natural organic matter (NOM). In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize Fe(III) in organic soils (pH 4.6-6.0) with varying natural Fe content. The EXAFS data were subjected to wavelet transform analysis, to facilitate the identification of the nature of backscattering atoms, and to conventional EXAFS data fitting. The collective results showed the existence of two pools of iron: mononuclear Fe(III)-NOM complexes and precipitated Fe(III) (hydr)oxides. In the soil with lowest pH (4.6) and Fe content mononuclear organic complexes were the completely dominating fraction whereas in soils with higher pH and Fe content increasing amounts of Fe (hydr)oxides were detected. These results are of environmental importance, as the different iron pools most likely have markedly different reactivities.

Effects of background cations on the fouling of polyethersulfone membranes by natural organic matter: Experimental and molecular modeling study

Adsorptive fouling of a polyethersulfone (PES) membrane by natural organic matter (NOM) in the presence of common metal cations was investigated with both experimental and computational molecular modeling techniques. NOM, calcium, magnesium and silicon were identified as important foulants through chemical analysis of two Midwestern surface waters. A model feed solution mimicking the lake waters and containing only NOM (Suwannee River NOM) and Ca 2+ resulted in a fouling pattern similar to the surface waters; Mg 2+ and Na + caused much lower fouling at the same ionic strength as the calcium solution. Molecular modeling of the model solution allowed detailed probing of the fouling process. This work suggests that divalent ions (Ca 2+ and Mg 2+) may cause membrane fouling not by forming “ionic bridges” between the negatively charged functional groups on membrane surface and the negatively charged functional groups of NOM, but by promoting the aggregation of NOM molecules in solution. The carboxyl groups of NOM strongly associate with the divalent ions, while the sulfonyl groups in the polyethersulfone do not. Although Ca 2+ and Mg 2+ are both coordinated to the NOM carboxyl groups predominantly by outer-sphere-type complexation, Ca 2+ associates with the carboxyl groups more strongly than Mg 2+ due to the looser second hydration shell structure of Ca 2+. The stronger Ca 2+-NOM complexation is also manifested by the decreased mobility (diffusion coefficients) of the Ca 2+ bound to the NOM.

Development and validation of a chronic copper biotic ligand model for Ceriodaphnia dubia

A biotic ligand model (BLM) to predict chronic Cu toxicity to Ceriodaphnia dubia was developed and tested. The effect of cationic competition, pH and natural organic matter complexation of Cu was examined to develop the model. There was no effect of cationic competition using increasing Ca and Na concentrations in our exposures. However, we did see a significant regression of decreasing toxicity (measured as the IC25; concentration at which there was a 25% inhibition of reproduction) as Mg concentration increased. However, taking into account the actual variability of the IC25 and since the relative increase in IC25 due to additional Mg was small (1.5-fold) Mg competition was not included in the model. Changes in pH had a significant effect on Cu IC25, which is consistent with proton competition as often suggested for acute BLMs. Finally, natural organic matter (NOM) was added to exposures resulting in significant decreases in toxicity. Therefore, our predictive model for chronic Cu toxicity to C. dubia includes the effect of pH and NOM complexation. The model was validated with Cu IC25 data generated in six natural surface waters collected from across Canada. Using WHAM VI, we calculated Cu speciation in each natural water and using our model, we generated “predicted” IC25 data. We successfully predicted all Cu IC25 within a factor of 3 for the six waters used for validation.

沿岸域における有機鉄の解離及び錯形成動力学―有機リガンドの性質と反応速度定数の関係―

The kinetics of the formation and dissociation of the Fe(III)-NOM (natural organic matter) complex in coastal waters was investigated using the competitive ligand method. The rate constants for the formation and dissociation of the Fe(III)-NOM complex were determined by fitting a model to observed data. The iron-binding capacity of NOM, which is required for the determination of the rate constants, was initially measured by the ion exchange method. The rate constants of the formation of the Fe(III)-NOM complex ranged from 2.6×106 to 7.3×106 M-1·s-1. A discrete ligand model for the dissociation kinetics of the Fe(III)-NOM complex yielded rate constants from 6.9×10-4 to 2.0×10-3 s-1 for weak-binding ligands and from 1.6×10-5 to 8.3×10-5 s-1 for strong-binding ligands, respectively. The kinetics of the Fe(III)-NOM complex in coastal water was then predicted using the estimated rate constants. The model prediction showed that the dissociation of the complexes is relatively slow with a half-life longer than 120 hr. The adsorption peak ratios of oxygen-containing functional groups of NOM calculated from FTIR spectra showed good correlation with both the iron-binding capacity and the rate constants, implying that the ratio is a useful factor for assessing the kinetics of organically complexed iron in coastal water.

Significance of ternary bacteria–metal–natural organic matter complexes determined through experimentation and chemical equilibrium modeling

In this study, we conducted experiments that tested for the existence of ternary surface complexes involving bacteria, metal cations and natural organic matter (NOM). We performed a variety of batch complexation experiments in single (NOM only), binary ( Bacillus subtilis–metal, NOM–metal, B. subtilis–NOM), and ternary ( B. subtilis–metal–NOM) systems in order to determine the significance of ternary complexation. We conducted experiments as a function of 1) pH, 2) component concentrations, 3) fraction of NOM (humic, fulvic, or bulk), and 4) metal of interest (Pb, Cu, Cd, and Ni). Our investigative approach was to quantify the differences between the binary and ternary experimental systems under these different conditions. The concentration of NOM bound in ternary form was calculated directly from experimental data by comparing binary and ternary systems. The concentration of metal bound in ternary form was calculated using chemical equilibrium modeling. Experimental results demonstrate that bacteria–metal–NOM complex formation is a rapid, fully-reversible chemical process. The pH of the aqueous system is the biggest factor in the stability, and therefore significance of ternary complexes. As pH decreases, bacteria–metal–NOM complexes become more stable. The fraction of NOM involved and the identity of the aqueous metal cation have a comparatively modest impact on the stability of ternary complexes. All NOM fractions form ternary complexes to similar extents at circumneutral pH, but humic acid becomes the dominant NOM fraction in ternary complexes at low pH. The abundance of humic acid in ternary form is greatest in experimental systems with Ni or Cd and less in systems with Pb and Cu. These observations suggest that bacteria–metal–NOM complexes are abundant under the conditions present in most natural waters. Hence, ternary complexes may impact the mobility of aqueous metal cations in natural systems by changing dissolved NOM–metal complexes to colloidal bacteria–metal–NOM complexes.

Coagulation of humic acid: The performance of preformed and non-preformed Al species

Inorganic polymeric Al coagulants have been being widely used in water treatment due to their advantages of applicability within broad pH and temperature ranges, high capacity of charge neutralization as well as less alkalinity consumption. However, the removal of dissolved organic matter (DOM) by such kind of coagulants has not been well understood because of the complexity of natural organic matter and the diversity of such polymeric Al coagulants. In this paper, coagulants with different Al speciation characteristics were prepared to conduct coagulation of humic acid (HA). The humic acid used was peat origin and was characterized physicochemically. The results showed that coagulants with preformed Al species were less effective than conventional Al salt in removing humic acid with large molecular and hydrophobic properties. The flocs formed by preformed Al species were smaller than those formed by conventional Al salt. Decreasing pH could improve the coagulation performance of all coagulants. Coagulation of humic acid might not follow the same charge neutralization rules associated with coagulation of mineral colloids. In the presence of humic acid, Al 13 could be decomposed during coagulation process.

Lime-Soda Softening Process Modifications for Enhanced NOM Removal

In this research, a number of process modifications to the lime-soda softening process were examined, including utilization of high Mg-content lime, addition of MgCl 2 , and the recycling of softening sludge, in order to improve the removal of natural organic matter (NOM) and reduce the formation of disinfection byproducts (DBPs). Jar test results showed that dissolved organic carbon (DOC) removal increased and trihalomethane (THM) formation was reduced as the magnesium in hydrated lime increased, and was directly correlated with the amount of magnesium removed from the system. However, a dolomitic quick lime hydrated under atmospheric conditions resulted in less effective DOC removal due to a lack of available Mg, and subsequently, less co-precipitation of Mg(OH) 2 -NOM complexes. The addition of MgCl 2 to the raw water also increased DOC removal and reduced THM formation in both the presence and absence of softening sludge, with DOC removal increasing as softening sludge and magnesium dosages increased. As high as 43% removal of DOC was achieved at the stoichoimetric lime-soda ash dose in the presence of 457 mg/L sludge and 7.5 mg/L MgCl 2 , as compared to only 13% removal in the absence of sludge and MgCl 2 . The recycling of softening sludge had little or no effect on the hardness and the level of inorganic elements in treated water. The results presented here provide new approaches for improving DBP precursor removal during lime-soda softening without significantly increasing lime and soda ash dosage or the generation of waste sludge.