Cu(II) complexation of high molecular weight (HMW) fluorescent substances in root exudates from a wetland halophyte (Salicornia europaea L.)

High molecular weight (HMW) fractions are important components in root exudates. However, there is little available information concerning complexation of Cu(II) to the HMW fractions in root exudates. In the present study, complexation of root exudates from Salicornia europaea L. with Cu(II) was investigated using excitation emission matrix (EEM) fluorescence spectroscopy. Two protein-like fluorescence peaks were identified in the EEM spectra of root exudates. Fluorescence of both peaks was clearly quenched by Cu(II). The increase of conditional stability constant with increasing temperature indicates that the fluorescence quenching of the protein-like fluorescence by Cu(II) may be controlled by a dynamic process. The values of conditional stability constants (logK(a)) were in the range of 4.32-4.69, which were close to those of complexation of fulvic acid with Cu(II). This shows that the HMW fluorescent substances in root exudates from S. europaea L. were strong organic ligands for Cu(II). Our study suggests that the HMW fluorescent substances may affect chemical forms, mobility, and thus the fate of copper in wetland.     

Character and chlorine reactivity of dissolved organic matter from a mountain pine beetle impacted watershed

Lodgepole pine needle leachates from trees killed by the mountain pine beetle epidemic in Colorado were evaluated for dissolved organic matter (DOM) character, biodegradation, treatability by coagulation and disinfection byproduct (DBP) formation. An average of 8.0 (±0.62) mg-DOC/g-dry weight of litter was leached from three sets of needle samples representing different levels of forest floor degradation. Fluorescence analysis included collection of excitation and emission matrices, examination of peak intensities and development of a 4-component parallel factor (PARAFAC) analysis model. Peak intensity and PARAFAC analyses provided complementary results showing that fresh leachates were initially dominated by polyphenolic/protein-like components (60-70%) and humic-like fluorescence increased (40-70%) after biodegradation. Humic-like components were removed by coagulation (20-64%), while polyphenolic/protein-like components were not, which may create challenges for utilities required to meet OM removal regulations. DBP formation yields after 24 h chlorination were 20.5-26.4 μg/mg-DOC for trihalomethanes and 9.0-14.5 μg/mg-DOC for haloacetic acids for fresh leachates; increased after biodegradation to 19.2-64.2 and 7.1-30.9 μg/mg-DOC, respectively; and decreased after coagulation (fresh: 11.3-17.7;5.7-7.6 μg/mg-DOC, respectively; biodegraded: 12.0-27.3 and 2.9-7.2 μg/mg-DOC, respectively), reflective of changes in concentration of humic material. Humic-like PARAFAC components and peak intensities were positively correlated (R(2) ≥ 0.45) to DBP concentrations, while polyphenolic/protein-like components were not (R(2) ≤ 0.17).     

Characteristics of dissolved organic matter in Baltic coastal sea ice: allochthonous or autochthonous origins?

The origin of dissolved organic matter (DOM) within sea ice in coastal waters of the Baltic Sea was investigated using parallel factor (PARAFAC) analysis of DOM fluorescence. Sea ice DOM had distinctly different fluorescence characteristics than that of the underlying humic-rich waters and was dominated by protein-like fluorescence signals. PARAFAC analysis identified five fluorescent components, all of which were present in both sea ice and water. Three humic components were negatively correlated to salinity and concluded to be terrestrially derived material. Baltic Sea ice DOM was found to be a mixture of humic material from the underlying water column incorporated during ice formation and autochthonous material produced by organisms within the ice. Dissolved organic carbon (DOC) and nitrogen (DON) concentrations were correlated to the humic fluorescence, indicating that the majority of the organic carbon and nitrogen in Baltic Sea ice is bound in terrestrial humic material trapped within the ice. This has implications for our understanding of sea ice carbon cycling in regions influenced by riverine input (e.g., Baltic and Arctic coastal waters), as the susceptibility of DOM to degradation and remineralization is largely determined by its source.     

Behavior of reoccurring PARAFAC components in fluorescent dissolved organic matter in natural and engineered systems: a critical review

Fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) has been widely used to characterize dissolved organic matter (DOM). Characterization is based on the intensity and location of independent fluorescent components identified in models constructed from excitation-emission matrices (EEMs). Similar fluorescent components have been identified in PARAFAC studies across a wide range of systems; however, there is a lack of discussion regarding the consistency with which these similar components behave. The overall goal of this critical review is to compare results for PARAFAC studies published since the year 2000 which include one or more of three reoccurring humic-like components. Components are compared and characterized based on EEM location, characteristic ecosystems, and behavior in natural and engineered systems. This synthesis allows PARAFAC users to more confidently infer DOM characteristics based on identified components. Additionally, behavioral inconsistencies between similar components help elucidate DOM properties for which fluorescence spectroscopy with PARAFAC may be a weak predictive tool.     

Variations of DOM quality in inflows of a drinking water reservoir: linking of van Krevelen diagrams with EEMF spectra by rank correlation

Elevated concentrations of dissolved organic matter (DOM) such as humic substances in raw water pose significant challenges during the processing of the commercial drinking water supplies. This is a relevant issue in Saxony, Central East Germany, and many other regions worldwide, where drinking water is produced from raw waters with noticeable presence of chromophoric DOM (CDOM), which is assumed to originate from forested watersheds in spring regions of the catchment area. For improved comprehension of DOM molecular composition, the seasonal and spatial variations of humic-like fluorescence and elemental formulas in the catchment area of the Muldenberg reservoir were recorded by excitation emission matrix fluorescence (EEMF) and ultrahigh-resolution mass spectrometry (FT-ICR-MS). The Spearman rank correlation was applied to link the EEMF intensities with exact molecular formulas and their corresponding relative mass peak abundances. Thereby, humic-like fluorescence could be allocated to the pool of oxygen-rich and relatively unsaturated components with stoichiometries similar to those of tannic acids, which are suspected to have a comparatively high disinfection byproduct formation potential associated with the chlorination of raw water. Analogous relationships were established for UV absorption at 254 nm (UV(254)) and dissolved organic carbon (DOC) and compared to the EEMF correlation.     

Parallel factor analysis of excitation-emission matrix fluorescence spectra of water soluble soil organic matter as basis for the determination of conditional metal binding parameters

Organic matter-metal complexes in soil solution and aquatic systems are involved in important environmental and ecological processes such as plant nutrient availability and the solubilization and transport of metals. Our work presented here extends the use of fluorescence spectrometry for determining conditional stability constants for such complexes. We combine the use of excitation-emission matrix (EEM) fluorescence spectrometry and parallel factor analysis (PARAFAC) to determine the stability constants of the chemically meaningful components modeled by PARAFAC. Water-soluble organic matter (WSOM) from O-horizon soils of deciduous and coniferous forest stands were extracted and titrated at pH = 4.7 with iron(lll) (Fe) and aluminum (Al) which are important metals in acid soil systems. The EEM spectra were then recorded and PARAFAC analysis showed that the WSOM contained three humic-substance-like components. Fe titration led to fluorescence quenching of the three components, while Al titration enhanced fluorescence for two components and quenched one of the components. The average Ryan-Weber stability constants at pH 4.7 ranged from log K of 4.28 to 4.91 for Fe and 4.84 to 5.96 for Al. The conditional stability constants were similar for Fe binding for deciduous and coniferous stand-derived WSOM, while they were stronger for Al binding with coniferous stand-derived WSOM. This difference in binding strengths for Al may affect the chemical behavior of Al in soil and aquatic systems. Determining the individual binding parameters of organic matter components with metals represents a significant advance over current approaches that utilize fluorescence quenching at a single excitation-emission wavelength pair to characterize organic matter-metal interactions.     

Fluorescence spectroscopy reveals ubiquitous presence of oxidized and reduced quinones in dissolved organic matter

Excitation-emission matrixes (EEMs) of 379 dissolved organic matter (DOM) samples from diverse aquatic environments were modeled by parallel factor analysis (PARAFAC). Thirteen components likely representing groups of similarly fluorescing moieties were found to explain the variation in this data set. Seven of the thirteen components were identified as quinone-like based on comparison of their excitation and emission spectra to spectra of model quinones. These quinone-like fluorophores were found to vary in redox state and degree of conjugation. Two components were identified as amino acid-like based on comparison to tyrosine and tryptophan fluorescence spectra. The other four components are not yet associated with any class of molecules. The quinone-like fluorophores account for about 50% of the fluorescence for every sample analyzed, showing that quinone-like fluorophores are an important and ubiquitous fluorescing moiety and in natural waters. Further, the distribution of the quinone-like fluorophores was evaluated as a function of environmental and laboratory redox gradients. Under reducing conditions, the contribution of the reduced quinone-like fluorophores increased concurrentwith a decrease in the oxidized quinone-like fluorophores, indicating that DOM fluorescence is a function of redox state of quinone-like moieties. Lastly, a ratio of two quinone-like fluorophores was found to explain the variation in the fluorescence index. These results provide new insight into the redox reactivity of DOM and have implications for the application of fluorescence spectroscopy as a tool to characterize DOM.     

Molecular fractionation of dissolved organic matter with metal salts

Coagulation of dissolved organic matter (DOM) by hydrolyzing metals is an important environmental process with particular relevance, e.g., for the cycling of organic matter in metal-rich aquatic systems or the flocculation of organic matter in wastewater treatment plants. Often, a nonremovable fraction of DOM remains in solution even at low DOM/metal ratios. Because coagulation by metals results from interactions with functional groups, we hypothesize that noncoagulating fractions have a distinct molecular composition. To test the hypothesis, we analyzed peat-derived dissolved organic matter remaining in solution after mixing with salts of Ca, Al, and Fe using 15 T Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FT-ICR-MS). Addition of metals resulted in a net removal of DOM. Also a reduction of molecular diversity was observed, as the number of peaks from the ESI-FT-ICR-MS spectra decreased. At DOM/metal ratios of ～9 Ca did not show any preference for distinct molecular fractions, while Fe and Al removed preferentially the most oxidized compounds (O/C ratio >0.4) of the peat leachate. Lowering DOM/metal ratios to ～1 resulted in further removal of less oxidized as well as more aromatic compounds ("black carbon"). Molecular composition in the residual solution after coagulation was more saturated, less polar, and less oxidized compared to the original peat leachate and exhibited a surprising similarity with DOM of marine origin. By identifying more than 9200 molecular formulas we can show that structural properties (saturation and aromaticity) and oxygen content of individual DOM molecules play an important role in coagulation with metals. We conclude that polyvalent cations not only alter the net mobility but also the very molecular composition of DOM in aquatic environments.     

Nonideal binding between dissolved humic acids and polyaromatic hydrocarbons

Interactions between hydrophobic organic chemicals (HOCs) and dissolved organic matter (DOM) are of environmental significance due to their influence on mobility and bioavailability of HOCs. The linear dissolution concept has been widely used to describe the interactions between HOCs and DOM, but it may not be correct. To date there is no systematic evaluation of nonideal interactions between HOCs and DOM. Therefore, this study employed a dialysis method to investigate sorption, desorption, and competition of two polyaromatic hydrocarbons (PAHs), phenanthrene (PHE) and pyrene (PYR), by two DOMs at pHs of 4, 7, and 11. Nonlinear interactions between PAHs and DOM and desorption hysteresis were consistently observed. The isotherm nonlinearity factor, nvalue, increased significantly after the addition of cosolutes, indicating the occupation of specific binding sites by the cosolute molecules. Significant influence of pH on PAHs-DOM interaction was also observed (higher binding coefficients, stronger desorption hysteresis, and increased nonlinearity at lower pH). This study for the first time systematically showed the nonideal binding behavior of PAHs by DOM. A more complete model rather than linear distribution is required to describe the interactions between HOCs and DOM. Conformation changes of DOM molecules were proposed to explain the interactions between HOCs and DOM.     

Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil

The complexation of heavy metals with dissolved organic matter (DOM) in the environment influences the solubility and mobility of these metals. In this paper, we measured the complexation of Cu, Cd, Zn, Ni, and Pb with DOM in the soil solution at pH 3.7-6.1 using a Donnan membrane technique. The results show that the DOM-complexed species is generally more significant for Cu and Pb than for Cd, Zn, and Ni. The ability of two advanced models for ion binding to humic substances, e.g., model VI and NICA-Donnan, in the simulation of metal binding to natural DOM was assessed by comparing the model predictions with the measurements. Using the default parameters of fulvic and humic acid, the predicted concentrations of free metal ions from the solution speciation calculation using the two models are mostly within 1 order of magnitude difference from the measured concentrations, except for Ni and Pb in a few samples. Furthermore, the solid-solution partitioning of the metals was simulated using a multisurface model, in which metal binding to soil organic matter, dissolved organic matter, clay, and iron hydroxides was accounted for using adsorption and cation exchange models (NICA-Donnan, Donnan, DDL, CD-MUSIC). The model estimation of the dissolved concentration of the metals is mostly within 1 order of magnitude difference from those measured except for Ni in some samples and Pb. The solubility of the metals depends mainly on the metal loading over soil sorbents, pH, and the concentration of inorganic ligands and DOM in the soil solution.     

Interactions of hydrophobic fractions of dissolved organic matter with Fe(3+) - and Cu(2+)-montmorillonite

Interactions of dissolved organic matter (DOM) with clays can significantly affect a variety of soil processes. We studied adsorption and fractionation of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions of DOM on Cu(2+)- and Fe(3+)-montmorillonite. Adsorption of both samples was higher on Fe(3+)-montmorillonite than on Cu(2+)-montmorillonite. A pH increase of about one unit was recorded followed by HoA adsorption by Fe(3+)-montmorillonite. This suggested exchange of negatively charged DOM groups on surface hydroxyl groups of Fe(3+)-montmorillonite surfaces. Adsorption of HoA on Cu(2+)-montmorillonite and HoN on Fe(3+)- and Cu(2+)-montmorillonites was governed mainly by van der Waals interactions. Spectroscopic analyses showed a distinct HoA fractionation by molecular size and aromaticity only by Fe(3+)-montmorillonite. On the basis of the pH measurements (increase in pH following adsorption of acid components) and enhanced DOM fractionation by molecular size and aromaticity we suggest that DOM reacted with Fe(3+)-montmorillonite similar to goethite.     

Fluorescence excitation-emission matrix regional integration to quantify spectra for dissolved organic matter

Excitation-emission matrix (EEM) fluorescence spectroscopy has been widely used to characterize dissolved organic matter (DOM) in water and soil. However, interpreting the > 10,000 wavelength-dependent fluorescence intensity data points represented in EEMs has posed a significant challenge. Fluorescence regional integration, a quantitative technique that integrates the volume beneath an EEM, was developed to analyze EEMs. EEMs were delineated into five excitation-emission regions based on fluorescence of model compounds, DOM fractions, and marine waters or freshwaters. Volumetric integration under the EEM within each region, normalized to the projected excitation-emission area within that region and dissolved organic carbon concentration, resulted in a normalized region-specific EEM volume (phi(i,n)). Solid-state carbon nuclear magnetic resonance (13C NMR), Fourier transform infrared (FTIR) analysis, ultraviolet-visible absorption spectra, and EEMs were obtained for standard Suwannee River fulvic acid and 15 hydrophobic or hydrophilic acid, neutral, and base DOM fractions plus nonfractionated DOM from wastewater effluents and rivers in the southwestern United States. DOM fractions fluoresced in one or more EEM regions. The highest cumulative EEM volume (phi(T,n) = sigma phi(i,n)) was observed for hydrophobic neutral DOM fractions, followed by lower phi(T,n) values for hydrophobic acid, base, and hydrophilic acid DOM fractions, respectively. An extracted wastewater biomass DOM sample contained aromatic protein- and humic-like material and was characteristic of bacterial-soluble microbial products. Aromatic carbon and the presence of specific aromatic compounds (as indicated by solid-state 13C NMR and FTIR data) resulted in EEMs that aided in differentiating wastewater effluent DOM from drinking water DOM.     

Separation and characterization of NOM by high-performance liquid chromatography and on-line three-dimensional excitation emission matrix fluorescence detection

By using high-performance size exclusion chromatography (HPSEC) and reversed-phase high-performance liquid chromatography with on-line three-dimensional excitation emission matrix fluorescence detection, we measured fluorescence properties of natural organic matter (NOM) as a function of molecular size (MS) and polarity. The work was carried out with Suwannee River Fulvic Acid, Aldrich Humic Acid, and a naturally occurring river NOM sample. Significant differences in fluorescence maximum pattern were found as NOM was separated chromatographically based on MS and polarity. There existed a strong relationship between MS, fluorescence pattern, and polarity. Humic-, fulvic-, and protein-like fluorescence fractions had distinct hydrophilic/hydrophobic nature. The results suggest that HPSEC may be better for characterizing major fulvic-like fluorescence and smaller MS fractions but not those having humic- and protein-like fluorescence and larger MS, which may be strongly adsorbed onto the HPSEC column because of their hydrophobic nature. This study has significant implications for further understanding the nature of NOM and its complexation with trace metals.     

Proton and trivalent metal cation binding by dissolved organic matter in the opalinus clay and the callovo-oxfordian formation

We investigated the proton and trivalent metal binding of dissolved organic matter (DOM) in in situ pore water and anoxic rock extracts of two potential host rocks for the disposal of radioactive waste, i.e., the Opalinus Clay (OPA) and the Callovo-Oxfordian formation (COx). The proton, curium, and europium binding properties of the OPA pore water and the extracted DOM of both rocks were studied with acid-base titrations, time-resolved laser fluorescence spectroscopy (TRLFS), and voltammetry, respectively. Protons were mostly buffered by inorganic compounds. DOM contributed to the total proton buffering capacity of the samples only to a small extent. Significant complexation of curium by DOM was observed for OPA pore water by TRFLS in contrast to little complexation by DOM detected in the OPA and COx extracts. The data on europium binding in OPA pore water were described by the presence of 14.3 microM organic ligands exhibiting a conditional affinity constant of log beta = 6.50. Calculations of europium speciation under in situ conditions indicated that carbonates largely controlled the speciation of europium in OPA and COx. In the OPA formation, the presence of DOM may enhance the solubility of europium by 5 x 10(-8) M, representing about one-third of total dissolvable europium.     

Effects of physical-chemical characteristics on the sorption of selected endocrine disruptors by dissolved organic matter surrogates

Sorption coefficients (K(oc) values) of selected endocrine disruptors for a wide variety of dissolved organic matter (DOM) were measured using fluorescence quenching and solubility enhancement. 17beta-Estradiol, estriol, 17alpha-ethynylestradiol, p-nonylphenol, p-tert-octylphenol, and dibutylphthalate were selected as endocrine disruptors. Aldrich humic acid, Suwannee River humic and fulvic acids, Nordic fulvic acid, alginic acid, dextran, and tannic acid were selected as DOM surrogates. The resulting sorption coefficients (log K(oc)) were independent of octanol-water partitioning coefficients (log K(ow)) of the selected endocrine disruptors, indicating the hydrophobic interaction is not the predominant sorption mechanism. Moreover, the K(oc) values for the selected endocrine disruptors, especially the steroid estrogens, correlated much better with UV absorptivity at 272 nm (A272) and phenolic group concentration of the DOM than with either the H/O or the (O+N)/C atomic ratio of the DOM. This suggests that the sorption mechanism is closely related to the interaction between pi-electrons and the hydrogen bonds, i.e., the affinity between phenolic groups of the steroid estrogens and DOM is suggested to provide a relatively large contribution to the overall sorption and yield the K(oc) values of the steroid estrogens as high as those of the alkylphenols and dibutylphthalate, which are suggested to be dominated by nonspecific hydrophobic interaction.     

Mercury(II) sorption to two Florida Everglades peats: evidence for strong and weak binding and competition by dissolved organic matter released from the peat

The binding of mercury(II) to two peats from Florida Everglades sites with different rates of mercury methylation was measured at pH 6.0 and 0.01 M ionic strength. The mercury(II) sorption isotherms, measured over a total mercury(II) range of 10(-7.4) to 10(-3.7) M, showed the competition for mercury(II) between the peat and dissolved organic matter released from the peat and the existence of strong and weak binding sites for mercury(II). Binding was portrayed by a model accounting for strong and weak sites on both the peat and the released DOM. The conditional binding constants (for which the ligand concentration was set as the concentration of reduced sulfur in the organic matter as measured by X-ray absorption near-edge structure spectroscopy) determined for the strong sites on the two peats were similar (Kpeat,s = 10(21.8 +/- 0.1) and 10(22.0 +/- 0.1) M-1), but less than those determined for the DOM strong sites (Kdom,s = 10(22.8 +/- 0.1) and 10(23.2 +/- 0.1) M-1), resulting in mercury(II) binding by the DOM at low mercury(II) concentrations. The magnitude of the strong site binding constant is indicative of mercury(II) interaction with organic thiol functional groups. The conditional binding constants determined for the weak peat sites (Kpeat,w = 10(11.5 +/- 0.1) and 10(11.8 +/- 0.1) M-1) and weak DOM sites (Kdom,w = 10(8.7 +/- 3.0) and 10(7.3 +/- 4.5) M-1) were indicative of mercury(II) interaction with carboxyl and phenol functional groups.     

Tannic acid adsorption and its role for stabilizing carbon nanotube suspensions

Dissolved organic matter (DOM) has been reported to stabilize carbon nanotube (CNT) suspensions, which increases concern over the subsequent transport and behavior of CNTs. However, it is unknown exactly which compounds or functional groups cause the stabilization of CNTs in natural environments. Naturally occurring tannic acid (TA), which has a large number of aromatic functional groups, was used as a surrogate of DOM to investigate its interaction with CNTs. CNT suspendability in TA solution increased with increasing CNT diameter without the aid of sonication. Sorption affinity of CNTs for TA increased with decreasing CNT diameter, positively related to their surface area. A two-stage sorption model was proposed to illustrate the interaction between CNTs and TA. TA molecules may be adsorbed first onto CNTs with aromatic rings binding to the surface carbon rings via pi-pi interactions, until forming a monolayer; the TA monolayer then further sorbed the dissolved TA by hydrogen bonds and other polar interactions. The sorbed TA increased the steric repulsion between individual CNTs, which might disperse the relatively loose CNT aggregates and result in the stabilization of large-diameter CNTs in TA solution. The sorption and suspending processwere also examined bytransmission electron microscopy, providing further evidence for the above proposed CNT-TA interactions. This study implies that widely distributed TA may promote the mobility and transport of CNTs in natural aqueous environments.     

Combining spectroscopic and potentiometric approaches to characterize competitive binding to humic substances

In an area that contains high concentrations of natural organic matter, it is expected that it plays an important role on the behavior of rare earth elements (REE), like europium, and of trivalent actinides. Competitive interactions with H+, inorganic species, major cations, e.g. Ca(II) or Mg(II), could influence these metals transport and bioavailability. Competitive experiments between cations, which can bind differently to humic substances and Eu3+, will bring an improved understanding of the competitive mechanisms. The aim of this study is to acquire data for Eu(III)/Cu(II) and Eu(III)/Ca(II) competitive binding to a sedimentary originated humic acid (Gorleben, Germany). The NICA-Donnan parameters for Ca2+, Cu2+, and Eu3+ obtained from competitive binding experiments using Ca2+ or Cu2+ ion selective electrodes were used to model time-resolved laser fluorescence spectroscopy (TRLFS) measurements. Eu3+ and CU2+ are in direct competition for the same type of sites, whereas Ca2+ has an indirect influence through electrostatic binding.     

Molecular and quantitative analysis of metal ion binding to humic acids from sewage sludge and sludge-amended soils by fluorescence spectroscopy

Humic acids (HAs) isolated from sewage sludge (SS) and control and SS-amended soils were characterized by fluorescence spectroscopy. The main feature of fluorescence spectra was a broad band with the maximum centered at excitation/emission wavelengths that were much shorter for SS-HA (340/438 nm) than for any soil HA (440/510 nm). Titration with Cu(II), Zn(II), Cd(II), and Pb(II) ions decreased fluorescence intensities of HAs. Titration data were fitted to a single-site fluorescence quenching model, and metal ion complexing capacities of each HA sample and stability constants of metal ion-HA complexes were calculated. The binding capacities of HAs and strengths of metal ion-HA complexes followed the order Pb(II) > Cu(II) > Cd(II) > Zn(II). With respect to the control soil HA, SS-HA, which showed the smallest contents of acidic functional groups and the lowest humification degree, was characterized by much smaller binding capacities and stability constants. The binding capacities and affinities of SS-amended soil HA were intermediate between those of native soil HA and SS-HA but closer to the former, thus suggesting a partial incorporation of HA fractions of SS into native soil HAs. These effects are expected to have a great impact on the behavior of metals in SS-amended soils.     

Metal complexation properties of freshwater dissolved organic matter are explained by its aromaticity and by anthropogenic ligands

Dissolved organic matter (DOM) in surface waters affects the fate and environmental effects of trace metals. We measured variability in the Cd, Cu, Ni, and Zn affinity of 23 DOM samples isolated by reverse osmosis from freshwaters in natural, agricultural, and urban areas. Affinities at uniform pH and ionic composition were assayed at low, environmentally relevant free Cd, Cu, Ni, and Zn activities. The C-normalized metal binding of DOM varied 4-fold (Cu) or about 10-fold (Cd, Ni, Zn) among samples. The dissolved organic carbon concentration ranged only 9-fold in the waters, illustrating that DOM quality is an equally important parameter for metal complexation as DOM quantity. The UV-absorbance of DOM explained metal affinity only for waters receiving few urban inputs, indicating that in those waters, aromatic humic substances are the dominant metal chelators. Larger metal affinities were found for DOM from waters with urban inputs. Aminopolycarboxylate ligands (mainly EDTA) were detected at concentrations up to 0.14 μM and partly explained the larger metal affinity. Nickel concentrations in these surface waters are strongly related to EDTA concentrations (R2=0.96) and this is underpinned by speciation calculations. It is concluded that metal complexation in waters with anthropogenic discharges is larger than that estimated with models that only take into account binding on humic substances.