Functional speciation of metal-dissolved organic matter complexes by size exclusion chromatography coupled to inductively coupled plasma mass spectrometry and deconvolution analysis

High performance size exclusion chromatography coupled to inductively coupled plasma mass spectrometry (HP-SEC–ICP-MS), in combination with deconvolution analysis, has been used to obtain multielemental qualitative and quantitative information about the distributions of metal complexes with different forms of natural dissolved organic matter (DOM). High performance size exclusion chromatography coupled to inductively coupled plasma mass spectrometry chromatograms only provide continuous distributions of metals with respect to molecular masses, due to the high heterogeneity of dissolved organic matter, which consists of humic substances as well as biomolecules and other organic compounds. A functional speciation approach, based on the determination of the metals associated to different groups of homologous compounds, has been followed. Dissolved organic matter groups of homologous compounds are isolated from the aqueous samples under study and their high performance size exclusion chromatography coupled to inductively coupled plasma mass spectrometry elution profiles fitted to model Gaussian peaks, characterized by their respective retention times and peak widths. High performance size exclusion chromatography coupled to inductively coupled plasma mass spectrometry chromatograms of the samples are deconvoluted with respect to these model Gaussian peaks. This methodology has been applied to the characterization of metal–dissolved organic matter complexes in compost leachates. The most significant groups of homologous compounds involved in the complexation of metals in the compost leachates studied have been hydrophobic acids (humic and fulvic acids) and low molecular mass hydrophilic compounds. The environmental significance of these compounds is related to the higher biodegradability of the low molecular mass hydrophilic compounds and the lower mobility of humic acids. In general, the hydrophilic compounds accounted for the complexation of around 50% of the leached metals, with variable contributions of humic and fulvic acids, depending on the nature of the samples and the metals.     

Complexation of europium and uranium by humic acids analyzed by capillary electrophoresis-inductively coupled plasma mass spectrometry

Investigations of the mobility of radioactive and nonradioactive substances in the environment are important tasks for the development of a future disposal in deep geological formations. Dissolved organic matter (DOM) can play an important role in the mobilization of metal ions due to complexation. In this study, we investigate the complexation behavior of humic acid (HA) as a model substance for DOM and its influence on the migration of europium as homologue for the actinide americium and uranium as the principal component of nuclear fuel. As speciation technique, capillary electrophoresis (CE) was hyphenated with inductively coupled plasma mass spectrometry (ICP-MS). For the study, 0.5 mg·L?1 of the metals and 25 mg·L?1 of (purified Aldrich) HA and an aqueous solution sodium-perchlorate with an ionic strength of 10 mM at pH 5 were used. CE-ICP-MS clearly shows the different speciation of the triple positively charged europium and the double positively charged uranyl cation with HA.     

Fractionation analysis of trace metals in humic substances of soils irrigated with wastewater in Central Mexico by particle induced X-ray emission

The Mezquital Valley in Central Mexico has received wastewater from Mexico City for nearly 100 years. Wastewater brings in organic matter and nutrients but also trace metals. Humic substances, the main components of organic soil matter, are responsible for retaining and regulating the mobility of trace metals in soils. In this study, humic substances were extracted from the soil and separated into distinct fractions (humic acids, fulvic acids and humins). The particle induced X-ray emission (PIXE) technique was applied to determine the metal content in bulk soil as well as in humic acids and fulvic acids not soluble in H₃PO₄. In order to assess whether the long-term input of organic matter and metals modifies the metal association with these humic substances, parcels irrigated for three time periods (5, 47 and 89 years) were selected for this study. It was observed that metals such as Zn and Cu are mainly associated with the humic acids. Fulvic acids retain mainly Cr while Pb is distributed among humic and fulvic acids. It was also observed that in general, metal retention by humic substances increases with irrigation time. Depth also affects metal association with the humic substances.     

Comparative study for differentiation of aquatic humic-type organic constituents by capillary zone electrophoresis using polyvinyl alcohol-coated capillary

Capillary zone electrophoresis (CZE) with UV detection (254 nm) was applied to characterize aquatic dissolved humic matter (DHM) from different environmental sources (lake, river and sea waters, in all 15 different samples). A series of separation examples of DHMs using a polyvinyl alcohol (PVA)-coated silica open tubular capillary were carried out in a phosphate buffer (40 mM) as a background electrolyte at neutral acidity (pH 6.8). The separative power of electropherograms was reasonable and the reproducibility was above the mark. Each electropherogram was characteristic of the corresponding humic sample. Special functional fulvic and humic acids or their overall mixtures separated with XAD, DAX and DEAE sorbing solids as well as the original dissolved organic matter (DOM) were nicely differentiated according to their environmental sources. The PVA coating of open tubular silica capillaries seems to be very potential in electrophoretic characterization and separation of different humic solutes at neutral acidities with low sample concentrations thus permitting a workable technique, in a growing series of CZE studies, for better compared results from different studies.     

Determination of dissolved organic matter based on UV-light induced reduction of ionic silver to metallic nanoparticles by humic and fulvic acids

We describe a novel solution-based method for the determination of dissolved organic matter (DOM) relying on the formation of silver nanoparticles (AgNPs) via photo-stimulated reduction of silver ions by humic and fulvic acids. The method is based on natural driven formation of nanoscale materials yielding a direct relationship between DOM concentration and AgNPs formation. The aqueous dispersion of the formed AgNPs show strong and uniform UV–Vis absorption bands between 450 and 550 nm irrespectively of DOM nature and properties (i.e. humic or fulvic acids). The ensuing chromatic shift accompanying the appearance of the new absorption bands is easily conceivable by a simple spectrophotometer and the bare eye, holding great promise for the on-site, instrumental-free screening of DOM levels. Under the optimum experimental conditions the determination of DOM was successfully demonstrated to various water samples with high sensitivity (<1.0 mg L⁻¹), satisfactory recoveries (87.5–123.5%) and reproducibility (5.87–6.73%).     

Insights into colloid-mediated trace element release at the soil/water interface

Organic or inorganic colloids play a major role in the mobilization of trace elements in soils and waters. Environmental physicochemical parameters (pH, redox potential, temperature, pressure, ionic strength, etc.) are the controlling factors of the colloidal mobilization. This study was dedicated to follow the colloid-mediated mobilization of trace elements through time at the soil/water interface by means of an experimental approach. Soil column experiments were carried out using percolating synthetic solutions. The percolated solutions were ultrafiltrated with various decreasing cutoff thresholds to separate the different colloidal phases in which the dissolved organic carbon and trace element concentrations were measured. The major results which stem from this study are the following: (i) The data can be divided into different groups of organic compounds (microbial metabolites, fulvic acids, humic acids) with regard to their respective aromaticity and molecular weight. (ii) Three groups of elements can be distinguished based on their relationships with the colloidal phases: the first one corresponds to the so-called "truly" dissolved group (Li, B, K, Na, Rb, Si, Mg, Sr, Ca, Mn, Ba, and V). The second one can be considered as an intermediate group (Cu, Cd, Co, and Ni), while the third group gathers Al, Cr, U, Mo, Pb, Ti, Th, Fe, and rare earth elements (REE) carried by the organic colloidal pool. (iii) The data demonstrate that the fulvic acids seem to be a major organic carrier phase for trace elements such as Cu, Cd, Co, and Ni. By contrast, the trace elements belonging to the so-called colloidal pool were mostly mobilized by humic acids containing iron nanoparticles. Lead, Ti, and U were mobilized by iron nanoparticles bound to these humic acids. Thus, humic substances allowed directly or indirectly a colloidal transport of many insoluble trace elements either by binding trace elements or by stabilizing a ferric carrier phase. (iv) Finally, the results demonstrated also that REE were mostly mobilized by humic substances. The REE normalized patterns showed a middle REE downward concavity. Therefore, as previously shown elsewhere humic substances are a major control of REE speciation and REE fractionation patterns as well since the humic substance/metal ratio was the key parameter controlling the REE pattern shape.     

Ultraviolet absorbance titration for determining stability constants of humic substances with Cu(II) and Hg(II)

We describe an ultraviolet (UV) absorbance titration method that can be used to determine complexing capacities (C_L) and conditional stability constants (log K) of humic substances (HSs) with metal ions such as Cu(II) and Hg(II). Two fulvic acids (FA) and one humic acid (HA) were used for this study. UV absorbance of HSs gradually increased with the addition of Cu(II) or Hg(II) after blank correction, and these increases followed the theoretical 1:1 (ligand:metal ion) binding model. The results from the absorbance titration calculation for HSs with Cu(II) and Hg(II) compared well with those from fluorescence quenching titration. The titration of the model compound l-tyrosine with Cu(II) proved the validity of this method, and the K and C_L were within 2.3% and 7.4% of the fluorescence quenching titration. The results suggest that the UV absorbance titration can be used to study the binding capacities of HSs and/or dissolved organic matter (DOM) with trace metals. The advantages and disadvantages of the absorbance titration method were also discussed.     

Identification of diffuse and point sources of dissolved organic carbon (DOC) in a small stream (Alb, Southwest Germany), using gel filtration chromatography with high-sensitivity DOC-detection

Using gel filtration chromatography and high-sensitivity UV and organic carbon (OC) detection, the quality and quantity of organic matter in a stream (the Alb, Southern Germany) has been studied in the winter season. In the catchment area and upstream of two sewage plant effluents the organic load has been low (below 1 mg/l TOC) and has been dominated by humic substances (more than 80%). Downstream of the sewage plants, TOC increased up to 5 mg/l while humic constituents decreased down to 30%. A detailed analysis showed that in the catchment area the humic fraction has been composed of fulvic acids with relatively high amounts of aliphatic structures. Downstream of the sewage plants, the humic fraction has been composed of fulvic acid precursors with associated non-humic matter. It is concluded that the quality and quantity of organic constituents in the stream has been dominated by the secondary effluents.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Complexation as the most important factor in the fate and transport of heavy metals in the Dnieper water bodies

The results of long-term investigations of the concentrations of dissolved forms of some heavy metals (Mn, Cu, Zn, Pb, Cr, Cd) and their species in the water of the Dnieper reservoirs and the Dnieper-Bug estuary are considered. Chemiluminescent methods, anodic stripping voltammetry, membrane filtration, ion-exchange, and gel-permeation chromatography were used for study of the speciation of the metals. It has been found that binding of heavy metals into complexes with dissolved organic matter (DOM) is the dominant factor of their stabilization in solution. The molecular weight distribution of organic metal complexes and their chemical nature, as well as the potential complexing ability of DOM were investigated. Humic substances, particularly fulvic acids, play a major role in the complexation. These ligands bind from 45 to 80% of metals in the form of organic complexes. Metal complex compounds of relatively low molecular weight (<5 kDa) predominated in the organic complexes.     

Some pitfalls in PAGE-LA-ICP-MS for quantitative elemental speciation of dissolved organic matter and metalomics

An experimental approach to evaluate the capabilities and limitations of polyacrylamide gel electrophoresis (PAGE) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for quantitative elemental speciation is presented. Two metalloproteins (superoxide dismutase, containing Cu and Zn, and thyroglobulin, containing I) with high binding affinity for metals, and metal-dissolved organic matter (DOM) complexes (from a compost leachate sample) which show different types of metal binding are studied. Iodine can be quantitatively detected in thyroglobulin after PAGE-LA-ICP-MS using either sodium dodecyl sulfate (SDS) PAGE or native PAGE. However, detection of Cu and Zn in superoxide dismutase after PAGE-LA-ICP-MS depends on the conditions of the PAGE method because possible metal losses can occur (either with SDS-PAGE or with native PAGE). The use of PAGE-LA-ICP-MS to study the contribution of DOM to the mobilization of metals from environmental samples is possible, but it depends also on the PAGE separation conditions owing to disequilibrium effects of metal-DOM complexes. Presented at the 4th International Conference on Trace Element Speciation in Biomedical, Nutritional and Environmental Sciences, 25–29 May 2008, Munich-Neuherberg, Germany.     

复合功能树脂对水体中天然有机酸的吸附

合成了胺基修饰复合功能树脂WJN-07, 研究了富里酸、单宁酸和没食子酸在树脂上的吸附行为. 与去除DOM的商品树脂XAD-7相比, 树脂WJN-07对3种天然有机酸有较高的吸附容量, 这是由于新型树脂表面具有修饰的化学官能团和丰富的微孔结构所致. 选择单宁酸作为吸附质进行吸附热力学研究, 结果表明, 树脂WJN-07对单宁酸的吸附为吸热过程, 以化学吸附为主导. 溶液中重金属离子Pb²⁺作为影响树脂吸附性能的重要因素在实验中得到证实, 其作用机理也得到合理解释.     

Determination of organically-associated trace metals in estuarine sea-water by solvent extraction and atomic-absorption spectrometry

Chloroform extraction of trace metals (Ni, Cu, Mo, Mn, Cd and Pb) in estuarine sea-water was studied at pH 8 and pH 3, on the basis that the metals would be associated with dissolved organic matter (DOM), which has recently been characterized by reversed-phase liquid chromatography. Ni, Cu, Mo and Mn were extracted more at pH 8 than at pH 3. Cd and Pb were not associated with the DOM at either pH 8 or 3. The percentage of the total dissolved trace metals in sea-water associated with DOM varied from 0 to 14%. The metals extracted into chloroform at pH 8 were assumed to be associated with neutral or weakly basic DOM while at pH 3 they could be associated with either the neutral (or weakly basic) DOM or two types of acidic DOM.     

Assessing the trihalomethane formation potential of aquatic fulvic and humic acids fractionated using thin-layer chromatography

The significance of this research is the application of thin-layer chromatography (TLC) to fractionate well-characterized aquatic humic materials coupled with the novel evaluation of the trihalomethane formation potential (THMFP) of the fractionated materials. Disinfection by-products such as trihalomethanes (THMs) form when natural water is treated by chlorination. Nordic Aquatic and Suwannee River fulvic and humic acids, obtained from the International Humic Substances Society, were prepared at pH 6 and 9 and fractionated on silica gel plates using a mobile phase consisting of methanol and ethyl acetate (2:1, v:v). Based on retention factor (R(f)) values, three common fractions were identified in all substances examined. Additionally, other fractions were noted that were characteristic of specific humic substances. Each of the three primary fractions derived from Nordic Aquatic fulvic acid at pH 6 demonstrated the potential to contribute to formation of THMs. This research provides data to support the hypothesis that differences in the chemical structure and composition of natural organic matter (NOM) significantly affect the potential to react with chlorine to form THMs.     

Influence of humic substances on sorption and methylation processes of inorganic- and organo-tin species

The influence of humic substances on sorption and methylation processes for inorganic- and organotin species is presented. Four sediment samples from different locations of the Rivers Elbe, Mulde and Spittelwasser, Germany, with different organotin and humic contents were selected to extract the humic and fulvic acids. The various fractions—the original sediment, the humic acid, the fulvic acid and the residual sediment—were analysed for their organotin content. The individual buyltin species show quite different distribution patterns. Monobutyltin is found mostly associated with humic acids. Dibutyltin shows a nonunique behaviour. At low total organotin content, dibutyltin is found bonded to humic and fulvic acids, whereas at high organotin content dibutyltin is distributed more with the residual sediment. Most of the tributyltin remains in the sediment unextracted; only small quantities of it are in the fulvic acid fraction. Tetrabutyltin is only in the humic acid fraction when it binds to humic matter; it mostly remains in the sediment. General observations indicate that ionic butyltin species bind to fulvic acids whereas the non-polar tetrabutyltin is not found in the fulvic acid fractions in any of the samples. The appearance of monomethyl- and dimethyl-tin species in the humic and fulvic acid fractions after the alkaline extraction was surprising. There is a correlation between the humic content of the sample and the formation of methyltin species. Evidence is provided by experiments that humic substances act as methylation agents.     

Characterization of groundwater humic and fulvic acids of different origin by GPC with UV/Vis and fluorescence detection

Gel permeation chromatography (GPC) was applied for recognizing the origin of groundwater humic and fulvic acids. GPC was performed with Fraktogel TSK HW-50 in 0.1 M NaCl, pH 8.5 (0.05 M phosphate buffer), 1 mM EDTA, with 10% by volume methanol added. Humic substances from groundwaters and sediments of four different aquifer systems in Germany were isolated, purified and characterized. Both UV/Vis and fluorescence detection were applied. UV/Vis detection was found to be more powerful in identifying differences between the various humic and fulvic acids. The four aquifer systems investigated (“Gorleben”, “Fuhrberg”, “Franconian Albvorland” and “Munich”) differed from one another with respect to hydrological and geochemical conditions. The results showed that the GPC-elution behavior reflects the geochemical environment and origin (source material and generation process) of aquatic humic and fulvic acids.     

Photodegradation study of the antifouling booster biocide dichlofluanid in aqueous media by gas chromatographic techniques

The aquatic photochemical behavior of the biocide dichlofluanid has been studied under natural sunlight conditions as well as under artificial solar irradiation in different types of natural waters (sea, river and lake water) as well as in distilled water. In order to examine the effect of dissolved organic matter (DOM), the photodegradation of the tested biocide was investigated also in the presence of various concentrations of humic and fulvic acids. It was found that the photodegradation proceeds via first-order reaction in all cases and that the presence of various concentrations of DOM inhibits the photolysis reaction. Kinetic experiments are monitored with GC–ECD with half-lives varied between 8 and 83 h. The major photodecomposition products identified by GC–MS were dichlorofluoromethane, aniline, and DMSA. Based on this byproduct identification a possible degradation pathway is proposed for the photolysis of dichlofluanid in aqueous media.     

Influence of Pb(II) on the radical properties of humic substances and model compounds

The influence of Pb(II) ions on the properties of the free radicals formed in humic acids and fulvic acids was investigated by electron paramagnetic resonance spectroscopy. It is shown that, in both humic acid and fulvic acid, Pb(II) ions shift the radical formation equilibrium by increasing the concentration of stable radicals. Moreover, in both humic acid and fulvic acid, Pb(II) ions cause a characteristic lowering of the stable radicals' g-values to g = 2.0010, which is below the free electron g-value. This effect is unique for Pb ions and is not observed with other dications. Gallic acid (3,4,5-trihydroxybenzoic acid) and tannic acid are shown to be appropriate models for the free radical properties, i.e., g-values, Pb effect, pH dependence, of humic and fulvic acid, respectively. On the basis of density functional theory calculations for the model system (gallic acid-Pb), the observed characteristic g-value reduction upon Pb binding is attributed to the delocalization of the unpaired spin density onto the Pb atom. The present data reveal a novel environmental role of Pb(II) ions on the formation and stabilization of free radicals in natural organic matter.     

Flocculation of humic substances with metal ions as followed by capillary zone electrophoresis

Humic and fulvic acids from various sources have been shown to give different electropherograms by capillary zone electrophoresis (CZE), depending on the pH of the electrolyte. This CZE work is extended here through investigations involving the titration of humic and fulvic acids with Fe(III) and Cu(II) cations. As increasing amounts of these cations were added to the humic substances (HUS), flocculation of metal-humic complexes occurred. This is believed to be caused by binding of the metal cations with negative carboxyl and phenolic sites on the HUS, resulting in a decrease of the repulsive forces that keep the HUS in a conformation more suitable for water solubility. The flocculated complexes were separated from the supernatant by centrifugation, and the supernatants were characterized as to total organic carbon (TOC) content, molecular weight (MW) using gel permeation chromatography, and average electrophoretic mobility (AEM) using CZE. The extent of flocculation correlated with both TOC and quantitative CZE measurements. The MW of the HUS remaining in solution actually decreased, presumably because of precipitation of larger molecules as they became insoluble because of reactions with the metals. Humic acids showed total precipitation of TOC with both metals at a concentration equivalent to their measured acidity. CZE demonstrated that certain fulvic acid fractions (low molecular weight phenolic acids) remained in solution even at high metal concentrations. In summary, changes in electrophoretic behavior of the soluble HUS could be related to changes in charge-to-mass ratios (charge densities) of both humic and fulvic acids with increasing metal cation concentration (neutralization). The copper treated HUS showed changes in their electrophoretic behavior even at low metal concentrations before flocculation, whereas the iron treated HUS flocculated uniformally over the range of added iron without significant changes in AEM. Thus these changes in CZE patterns illustrate different specific binding sites of the HUS for each metal.     

Flocculation of humic substances with metal ions as followed by capillary zone electrophoresis

Humic and fulvic acids from various sources have been shown to give different electropherograms by capillary zone electrophoresis (CZE), depending on the pH of the electrolyte. This CZE work is extended here through investigations involving the titration of humic and fulvic acids with Fe(III) and Cu(II) cations. As increasing amounts of these cations were added to the humic substances (HUS), flocculation of metal-humic complexes occurred. This is believed to be caused by binding of the metal cations with negative carboxyl and phenolic sites on the HUS, resulting in a decrease of the repulsive forces that keep the HUS in a conformation more suitable for water solubility. The flocculated complexes were separated from the supernatant by centrifugation, and the supernatants were characterized as to total organic carbon (TOC) content, molecular weight (MW) using gel permeation chromatography, and average electrophoretic mobility (AEM) using CZE. The extent of flocculation correlated with both TOC and quantitative CZE measurements. The MW of the HUS remaining in solution actually decreased, presumably because of precipitation of larger molecules as they became insoluble because of reactions with the metals. Humic acids showed total precipitation of TOC with both metals at a concentration equivalent to their measured acidity. CZE demonstrated that certain fulvic acid fractions (low molecular weight phenolic acids) remained in solution even at high metal concentrations. In summary, changes in electrophoretic behavior of the soluble HUS could be related to changes in charge-to-mass ratios (charge densities) of both humic and fulvic acids with increasing metal cation concentration (neutralization). The copper treated HUS showed changes in their electrophoretic behavior even at low metal concentrations before flocculation, whereas the iron treated HUS flocculated uniformally over the range of added iron without significant changes in AEM. Thus these changes in CZE patterns illustrate different specific binding sites of the HUS for each metal.     

Determinations of humic substances and other dissolved organic matter and their effects on the increase of COD in Lake Biwa.

Humic substances and other dissolved organic matter (DOM) in Lake Biwa and the surrounding rivers were investigated to elucidate their origins and behavior. An annual increase in chemical oxygen demand (COD) has been observed in the northern basin of Lake Biwa since 1985. The concentrations of dissolved organic carbon (DOC) in the northern and southern basins of Lake Biwa were 1.7-2.4 mgC/l and 1.9-2.6 mgC/l, respectively. The DOC concentrations tended to be high in summer and low in winter, and the seasonal changes in the concentrations of humic substances were small. The humic substances content of DOM was considered to be comparatively small because the ratio of the concentration of humic substances to DOC was in the range of 0.14-0.32. From the results of the fractionation of DOM in lake waters, it was estimated that hydrophobic acids, such as humic substances and hydrophilic acids, were about 25% and 45%, respectively. The main origin of hydrophobic acids in Lake Biwa may be humic substances from soils around the rivers that flow into Lake Biwa, while hydrophilic acids may be due to the inner production by phytoplankton. Therefore, the increase of COD in the northern basin of Lake Biwa may be attributed to the contributions of not only humic substances but also hydrophilic acids.