This study uses sorption of specifically interacting compounds from different solvents to address soil organic matter (SOM) structure. Sorption of phenol, pyridine, and atrazine on Pahokee peat was measured as a function of solute activity in water and hydrocarbon solution (Graber and Borisover, 1998). Sorption of phenol and pyridine from water at low solute activities was the same as from n-hexadecane; sorption at high solute activities was much greater from water than from n-hexadecane. Uptake of atrazine at high activities was not significantly influenced by peat hydration. Sorption isotherms measured in non aqueous systems were more non linear than those measured in water. For non polar compounds, hydrated conditions resulted in slightly decreased sorption to peat compared with dehydrated conditions.
The increase in sorption on hydrated peat at high pyridine and phenol activities, and lack of effect for non polar compounds, is attributed to an increase in polar sorption sites resulting from penetration of polar solute molecules between polar peat contacts which are either previously solvated by water or solvated during penetration of the solute. High solute activities are necessary for simultaneous disruption of multiple points of contact in the hydrated SOM macromolecular complex.
To test and elucidate this interpretation of SOM structure, kinetic and sorption experiments for pyridine were carried out from acetone, acetonitrile (MeCN) and methanol (MeOH). Pyridine uptake from MeOH was complete within 1 day, while pyridine sorption from MeCN reached equilibrium only after 40 days. The total sorbed amount was similar from both solvents. At low pyridine activity in the solution phase, sorption uptake from acetone and MeOH is similar to that previously observed from water and n-hexadecane. At low pyridine activity in the solution phase, sorption from MeCN is greater than from the other four solvents. At higher pyridine activity in the solution phase, sorption of pyridine from MeOH and MeCN was higher than uptake from water, while sorption uptake from acetone was the same as from n- hexadecane. High sorption uptake at higher solution activities in polar solvents such as water, MeOH, and MeCN supports the interpretation of disrupted polar contacts inside SOM and demonstrates the importance of polar crosslinks in SOM structure.
Graber ER, Borisover, MD, EST, 32, 258-263, (1998).
The interactions between natural organic matter (NOM) from two differents sources (river and lake) and Al and Fe salts were studied from the atomic to the macroscopic scale. Upon mixing of the NOM with the metal salts aggregation occurred. Analyses of the NOM contained in the flocs revealed that the metal salts have a differential affinity for certain types of biopolymers: Iron (III) binds preferentially polyhydroxy aromatic (PHA) compounds, whereas Al(III) has a high affinity for polysaccharides (PS). The analyses of the clarified water confirmed these findings. At the local scale, the speciation of Fe(III), as determined by EXAFS and SAXS, was found to mainly consist of Fe trimers. No larger Fe polymers were detected. Whatever the pH, each Fe atom is complexed by 2 to 3 organic ligands. The structure of the aggregates is controlled by the NOM as shown by the high fractal dimension (Df > 2.1). The speciation of aluminum in the flocs consists of uncondensed monomers. Modelling of the SAXS curves demonstrated that these monomers are evenly spaced by 8-9 angstrom within the flocs. This suggests that Al binds on a specific and "odered" structure within the NOM. Molecular modeling indicates tentatively the nature of this Al binding sites.
The complexation of Cu and Cd by humic and fulvic acids have been indirectly determined with ligand-exchange and voltammetry at the ambient concentration levels of natural waters (Xue and Sigg 1998, Xue and Sunda 1997). The performance of the used techniques was first evaluated in FA and HA suspensions, and gave comparable results with the literature values for the same materials, according to published models and parameters: 5-site model (Cabaniss and Shuman1988), NICA model (Kinniburgh et al., 1996). Complexation parameters (conditional stability constants and ligand concentrations) of Suwannee River fulvic acids (FA), of purified peat humic acids (HA) and of ligands in lakewater samples are compared, using the same methods and the same titration ranges of Cu, Cd and organic carbon concentrations. The comparison indicates that stronger ligands than FA and HA are present at low concentrations in the lakewaters. Model calculations using the Wham model for FA and HA (Tipping 1994) also confirm these results. Specific strong ligands occur in particular in eutrophic lake waters, whereas in a lake with higher metal concentrations and low biological productivity the ligands more closely match the fulvic or humic acid characteristics. It is likely that FA and HA play an important role for metal complexation in systems with high DOC concentrations and relatively high metal concentrations, especially if DOC mostly originates from soils or wetlands. In systems with high biological productivity and relatively low DOC, such as eutrophic lakes, the more specific ligands may be more relevant.
Cabaniss SE & Shuman MS, Geochim. Cosmochim. Acta, 52, 185-193, (1988).
Kinniburgh DG, Milne CJ, Benedetti MF, Pinheiro JP, Filius J, Koopal LK & Van Riemsdijk WH, Environ. Sci. Technol, 30, 1687-1698, (1996).
Tipping E, Computer Geosci, 20, 973-1023, (1994).
Xue HB & Sigg L, Anal. Chim. Acta, 363, 249-259, (1998).
Xue HB & Sunda WG, Environ. Sci. Technol, 31, 1902-1909, (1997).
Total luminescence spectroscopy (TLS) provides qualitative information on mainly aromatic structures in dissolved and extractable soil organic matter (SOM) and its complexes with aluminum. First, structural assignments of fluorescence signals most characteristic for SOM are discussed based on experiments with model compounds and literature data. In a second part, the potential of the method for the characterization of NaOH extractable SOM is demonstrated using data from a cryptopodzolic soil from southern Switzerland, an andosol from Oregon (U.S.A.) and a vertisol from southeastern Mali. In particular, the possibilities of a combined interpretation of fluorescence and NMR spectra and implications for SOM translocation and stability within a soil profile are discussed. Finally, TLS is shown to be an excellent tool to monitor dissolved organic matter (DOM) in soil solutions and freshwater. The following aspects are discussed: (i) relationship between the litter source and DOM for two acid forest soils, (ii) detection of aluminum organic complexes in the soil solution of the same soils, and (iii) relationship between TLS and residence time of DOM in freshwater systems adjacent to acid forest soils.
Infrared spectroscopy is a traditional tool in investigations of humic acids. In the search for molecular indicators for a differentiation between humic acids of marine and terrestrial origin we have analysed >100 samples of the <63 µm fraction obtained plants, terrestrial soils, peats as well as freshwater and marine sediments using this technique. A noticeable feature was the high ash content of up to 28% in these isolates. Treatment with hydrofluoric acid considerably reduced the proportion of inorganic compounds but also led to a marked change in IR spectra. Thus, the amide band at around 1650 cm-1 and the carbonyl band at around 1700 cm-1 became more pronounced while the C-O band at around 1030cm-1 almost completely disappeared. A group of peaks in the 1200 to 1400 cm-1 region also became more prominent. Furthermore, the methyl and methylene signals in the 2900 cm-1 region became more expressed as the OH trough became less broader. In the fingerprint region, especially for marine samples, a pronounced poeak appeared at aroud 640 cm-1.
As humic acids with low ash content isolated from various peats did not change their IR spectra after HF treatment we conclude that the spectral differences in treated vs. untreated isolates are due to the the presence/absence of clay minerals or other silicates. This implies that some data in the literature may have received incorrect interpretations when these were based on IR spectra alone. This especially holds for the C-O band (1030 cm-1) which has been used as an indication for the presence of carbohydrate moieties. It is therefore advocated that a complete characterisation with the aim of distinguishing marine and terrestrial humic acid sources should encompass as many analytical techniques as possible.
Analysis of the spectral information obtained suggests that for North Sea samples with a geographical coverage from the southern to the northern North Sea the carbonyl/amide peak ratio might be used to classify humic acids as more marine or more terrestrial.
As terrestrial plants and marine primary producers derive the carbon and nitrogen that is incorporated in their biomass from different sources it should be possible to distinguish humic acids or humic acid precursors derived from these sources on the basis of their isotopic signatures. We have therefore obtained humic acids from various plants from both realms and analysed these isolates for their carbon, nitrogen and sulphur stable isotopic composition.
The data show that the isolates reflect the C and N sources in a similar way as whole plants do, i.e. terrestrial plants are isotopically lighter than their marine counterparts. Thus 13C of terrestrial plants varied from -30 to -34 and 15N from 1 to -2. Fucus vesiculosus had values of -20 and 12, respectively. Halimione sp. a salt marsh plant, had a terrestrial carbon signal while the nitrogen signal was with 10 in the marine range.
Surface sediments from the Skagerrak and the South China Sea hads 13C from -22 to -25 and 15C from 6 to 8. The data points clearly fell on a mixing line between marine and terestrial samples indicating that even in the deep South China Sea terrestrial contributions may be found. Applying a linear mixing model to the suite of Skagerrak samples a terrestrial contribution to the humic acid pool of 33% (carbon) and 50% (nitrogen) can be calculated.
Sulphur isotopes show a somewhat different behaviour as isotopically light S is incorporated into humic acids from Skagerrak sediments. This suggests that at least part of the sulphur of sedimentary humic acids is not part of a precursor compound but incorporated during the very first steps of diagenesis. Furthermore, especially for the terrestrial samples isitopic variation of rainwater sulphate has to be taken into account.
Organic macromolecules (fulvics, humics, polysaccharide rich biopolymers) will influence the transport of inorganic colloidal particles by modifying their stability in aquatic systems. The precise role of the organic macromolecules will be determined, in part, by their structure. Unfortunately, only a limited number techniques are available to study the structure of organic macromolecules in the absence of sample perturbation.
Two fairly new techniques have been developed in our laboratory for use in the determination of the size and conformation of natural organic macromolecules, including humic substances. Atomic force microscopy (AFM) is a widely used technique which is to study macromolecules because of its high resolution and its ability to image samples in solution or under non-vacuum conditions. We have employed AFM to determine variations in the heights of adsorbed humic macromolecules across a range of physicochemical conditions. Generally, the adsorbed thickness of a humic macromolecule ranged between 0.5 and 2 nm depending upon the source of the humic substance and the physicochemical conditions.
The diffusion coefficients of the humic macromolecules in solution have been determined in solution by fluorescence correlation spectroscopy (FCS) for concentrations as low as 1 mg L-1. Diffusion coefficients were quite dependent upon the pH and the humic substance studied. For standard fulvic and humic acids from the International Humic Substances Society measured under a variety of solution conditions (pH, ionic strength, [Ca]), diffusion coefficients ranged from 2-3 x 10-10 m2s-1, which for a small, compact sphere represents a hydrodynamic diameter between 1.5 and 2.0 nm.
Both the AFM and FCS gave similar results, i.e. for environmentally relevant concentrations of humic substances (10 mg L-1), the measured dimensions are coherent with single macromolecules undergoing limited aggregation. Nonetheless, the adsorbed thickness of humic macromolecules determined by AFM was systematically smaller than the estimated hydrodynamic diameter of the molecules in solution as determined by FCS. The results are compared and differences discussed in order to explain why the two techniques give complementary but not identical information into the size and conformation of the humic substances.
Natural organic matter (NOM) contributes significantly to the behavior of anthropogenic chemicals in surface water, soils and sediments. Humic substances (HS) can constitute up to 50% of NOM and are known to bind chemicals, governing the transport and bioavailability of xenobiotics and thus participating in the flux of contaminants to water reservoirs. The structural characterization of these ubiquitous organic materials and of the main reactive groups are the first steps in the understanding of these interaction mechanisms. Capillary electrophoresis (CE) has been found to be a useful tool for the analysis of the electrophoretic behavior of anionic polyelectrolytes like humic substances. Capillary zone electrophoresis (CZE), capillary isoelectric focusing (CIEF) and capillary gel electrophoresis (CGE) were used to analyze synthetic/natural humic substances and natural organic matter (NOM) from different sources like lake and marine water, sediments and soils (Schmitt-Kopplin et al. 1999 and 1998a, b, c). The humic substances give an homogeneous signal in CE with a distribution of the detection signals around an average electrophoretic mobility corresponding to the charge density distribution of the humic substances governed by their molecular size and their acidities. The correct mathematical description of this distribution in terms of molecular sizes and charges gives important parameter when studying their reactivity towards organic and metallic species. This presentation is intended to illustrate the present knowledge and limitations of capillary electrophoresis methods in the characterization of humic substances and to give examples of the use of CZE and CGE in the rapid analysis of different synthetic/natural humic mixtures.
Schmitt-Kopplin Ph, Freitag D, Kettrup A, Schoen U & Egeberg P, Environment International, 25(3/4), 259-274, (1999).
Schmitt-Kopplin Ph, Hertkorn N, Garrison AW, Freitag D & Kettrup, A, Anal. Chem, 70(18), 3798-3808, (1998).
Schmitt-Kopplin Ph, Hertkorn N, Schulten HR, Freitag, D & Kettrup, A, Environ. Sci. Technol, 32(17), 2531-2541, (1998).
Schmitt-Kopplin Ph, Garisson AW, Perdue, M, Freitag D & Kettrup A, J. Chromatogr. A, 807(1), 101-109, (1998).
Recent studies have given evidence that sorption is a major process in the preservation of organic matter (OM) in marine sediments. It is also assumed that sorptive preservation may contribute to the accumulation of refractory OM in soil. The aim of our work was to elucidate the potential of sorption of dissolved organic matter (DOM) to soil minerals as a OM conserving process in soil. We tried this by (i) investigation of dissolved organic matter sorption/desorption features, (ii) comparison of the chemical characteristics of DOM and OM associated with clay-sized separates, and (iii) relating the soil content of mineral phase associated OM (OM in soil separate fraction with a density > 1.6) to the soil surface area. The results show that there is a strong retention of DOM in soils containing considerable amounts of clay and metal oxides. In turn, desorption of OM from soils and metal oxides proved to be negligible under conditions similar to those during the sorption. The chemical properties of DOM and OM in the clay-sized soil separates are quite similar. This indicates that sorption of DOM may contribute to the accumulation and preservation of OM in soil. Enhanced microbial decomposition of OM desorbed from mineral soils confirms this assumption. However, the relationship between mineral-associated OM and the soil surface area is not as clear as it was found for marine sediments. Some top soils, especially those from Mollisols, show a close relationship between the two parameters. In contrast, in subsoil horizons from Spodosols, which have received high amounts of DOM from percolation water, mineral-associated OM is much closer related to the content of metal oxides than to the surface area. Possibly reasons for the differences in the relationship of OM vs. surface area between different soils, different soil compartments, and between soils and marine sediments are: (i) the different degree to which the OM sorption complex is saturated, (ii) the different mineralogy, and (iii) different environmental conditions (e.g., oxic vs. anoxic conditions). In summary, we conclude that sorptive preservation of OM may also be important in the soil environment but that it is a more complex process than in marine sediments.
The vital role dissolved organic carbon (DOC) plays in the translocation of contaminants in soils has stimulated research on its sources. Under forests, DOC in soils may derive from precipitation and percolation water of the organic forest floor. In addition, the organic carbon (OC) in the mineral soil may also contribute to DOC due to chemical and microbial solubilization. The portion of soil organic carbon witch is potentially soluble is generally approached by batch extraction with aqueous solutions free of DOC or by calculation from batch sorption data, i. e., the reactive soil pool (RSP) of the initial mass approach (Nodvin et al., 1986). The kinetics of DOC release is estimated by stirring soil samples with DOC-free solutions for different time intervals. The quantity of RSP can be compared to the cumulative DOC output from a soil column percolated with DOC-free solution. Kinetics of DOC desorption can be quantified conducting multiple stop-flow events of increasing duration. The reaction rate of DOC desorption is thereby determined by the analysis of the DOC effluent concentration as a function of time. We compared batch and flow-generated DOC release from subsoils in order to verify results from batch experiments. In a batch reactor, repeated extraction of a soil sample with DOC-free solutions results in a complete exhaustion of soluble OC. In the column experiment, the rinsing with a DOC-free solution results in a rapid decrease of soluble OC concentrations. In contrast to the batch experiments, the DOC-concentration did not approach the zero level. Numerical simulations of the column experiments showed a better accordance with the column data when an additional process resulting in DOC release was included in the model. Besides chemical dissolution and desorption, we assume microbial degradation of soil organic carbon as an additional source for DOC in the column effluent. Thus, batch-generated sorption data may not be suitable to predict the total DOC release from natural soils under flow conditions.
Nodvin SC, Driscoll CT & Likens GE, Soil Science, 142, 27-35, (1986)
Diagenetic processes cause a multiple-step degradation of organic source substances into organic molecules, whereby the original function of the source substances is lost. Such changes in organic phases have been shown for proteins which desintegrate stepwise to form peptides and free amino acids. Furthermore, amino acids continuously undergo defunctionalization processes to form amines, organic acids, and hydrocarbons. Preliminary investigations of modern marine stromatolites from Highborne Cay, Bahamas, indicate that similar defunctionalization processes of organic substances occuring in polysaccharide-biofilms are essential for the formation of micritic layers being the characteristic features of stromatolites. Intracrystalline colloidal organic matter (COM) extracted from micrites that have formed within biofilms were divided into three size fractions (1-3 nm, 3-10 nm, and 10-1000 nm). Subsequently, the effect of COM of all fractions on the rate of precipitation of CaCO3was determined by recording the decrease in pH of a solution containing 30 µ l of 1 M NaHCO3 and 1440 µ l H2O including COM when 30 µ l of 1 M CaCl was added rapidly. Recordings of a series of precipitation experiments showed major differences between the large (> 10 nm) and the smaller fractions (1-3 nm & 3-10 nm) which had similar precipitation patterns. The pH of the large fraction was relatively stable until nucleation occurred, and it declined towards 7.0, comparable to the reference H2O, as visible precipitate formed. In contrast, the pH of the smaller fractions instantly declined until it was adjusted to approximately 7.4. At that point, precipitates were not as obviously visible as in the large fraction. Consequently, it can be interpreted that the Ca-binding properties of large and small COM have distinct characteristics. Apparently, large COM forms complexes with Ca-ions, whereas Ca-bonds of small COM are weak and pH-sensitive. Thus, we propose that the rate of CaCO3 precipitation as it occurs in biofilms of modern marine stromatolites is a function of the defunctionalization of the organic source substance. Large COM complexing Ca-ions, therefore, is negligible concerning carbonate precipitation.
Upon cultivation, the organic stocks of native soils strongly decrease and the stability soil structure is generally observed to decline as well. Organic matter was shown to mediate such changes in soil structure. Soil clay - organic complexes result from the interaction of plant residues and their biodegradation products with the soil mineral matrix. These clay - organic matter complexes are the basic units of soil structure and are throught to have a major role in soil physical properties. In this study, we aimed at analysing the changes in the composition, microstructure and physical properties of clay - organic matter associations with cultivation and relate it to the changes of aggregate stability of the soils with cultivation.
We sampled silty soils from the terraces of the Pyrenean piedmont in South West France that were under native forest vegetation or cultivated for 7 to 100 years. We separated the < 2 µm fraction of the soil after mechanical dispersion and without destruction of the organic matter. We measured the C and N content of the fraction. anfd they were observed by Transmission Electron Microscopy. The wettability of the fraction was assessed by measuring the contact angles of water drops on oriented deposits of fraction < 2 µm. The cohesion of the clay fractions was evaluated by measuring the amount of clay that was dispersed from soil aggregates after 1hour of agitation in water.
The C content of the < 2 µm fraction strongly decreased with cultivation from 112 to 43 mg C/g fraction. However this organic fraction was the least affected in its organic content among other soil particle size fractions. Transmission electron microscopy showed that the clay particles were less aggregated in the cultivated soils as compared to clay particles from the forest soil. As the C content of the fraction decreased, the wettability of the < 2 µm fraction and its mechanical dispersability both increased. These changes in clay minerals cohesion and wettability contribute to the observed changes in aggregate cohesion and rewetting rate in soils from this cultivation sequence.
The modifications of interfacial characteristics of a kaolinite clay induced by adsorption of humic substances in the presence of 10-4 M aluminium ions were investigated. The adsorption isotherms showed the adsorption amount to be strongly increased in the presence of aluminium ions, due to the concomitant interaction of aluminium ions with clay and organic matter. The extent and nature of surface modifications as determined by aluminium ion and humic acid adsorption were determined to be functions of the available surface area, the initial dosage in humic acids and the resulting degrees of clay and humic acid complexation. These characteristics covered up greatly different situations since the mean degree of hydrolysis of aluminium ions was determined to be function of the concentration of free (non adsorbed on kaolinite) aluminium ions, which was varied from zero to values close to 10-4M. Global changes in the net surface charge characteristics of clay/organic matter complexes were evidenced by determining changes in the electrophoretic mobility of the complexes. In the range of very low degree of humic acid complexation, the humic acids behaves like synthetic polyacids (Ringenbach et al., 1993; Elfarissi et al., 1998)), for which attraction between negatively charged acid groups of the organic substances and positively charged sites of the clay prevailed. In the greater complexed form, the adsorbed humic acid behaved as amphoteric macromolecules (Neyret et al., 1995)- bearing concomitantly dissociated and aluminium ion complexed carboxylic acid groups - for which the extent of interfacial charge segregation was limited by the available surface area and surface charge characteristics of the adsorbent. As a result, the surface potential developed by the clay/acid complexes was determined to depend in a complex manner of these different parameters and evolve from negative to positive values with the degree of acid complexation. Negative values were obtained in the presence of an excess of humic acids while positive ones were determined at very low dosages in humic acids. In addition, depending on the available surface area, two typical behaviours below and above a concentration of 0.05 g clay / l were determined for the variation of the zeta potential as a function of the degree of humic acid complexation. This finding showed that the interfacial conformation of the adsorbed humic acids depended on the adsorption area. These pseudo equilibrium situations might result from the slow interfacial reconformation of adsorbed polymers (Ringenbach et al., 1995, (a)) which determined the rate of aggregate fragmentation (Ringenbach et al., 1995,(b)).
Ringenbach E, Chauveteau G & Pefferkorn E, J. Colloid Interface Sci, 161, 223-231, (1993).
Neyret S, Ouali L, Candau F & Pefferkorn E, J. Colloid Interface Sci, 176, 86-94, (1995).
Ringenbach E, Chauveteau G & Pefferkorn E, J. Colloid Interface Sci, 172, 203-207, (1995 a).
Ringenbach E, Chauveteau G & Pefferkorn E, J. Colloid Interface Sci, 172, 208-213, (1995 b).
Elfarissi F, Nabzar L, Ringenbach E & Pefferkorn E, Colloids Surfaces A:, 131, 281-294, (1998).
Although humic acids are important factors in geochemical cycles and play a role in behaviour of many chemical components in the marine environment, the knowledge about theirt content and properties in marine bottom sediments is very limited. Humic acids can form complexes with cations of metals. That is the reason of their influence on the migration and redistribution of metals between water and sediment, but also on the bioavailability and toxicity of trace metals to biota. Additionally, humic acids can bound many organic chemicals for example chlorinated hydrocarbons. Functions of humic substances in sediments depend on their chemical and physical properties, composition and hydrophobicity which are significantly affected during diagenetic processes. The purpose of this work was to evaluate if in the course of early diagenesis humic acids in marine bottom sediments are transformed and at what rate. Humic acids were isolated from 1 cm thick layers of a 30 cm long sediment cores , collected from at the Bornholm Deep and the Gdansk Deep (the Baltic Sea). Then they were analysed for physico-chemical properties, which included elementary composition, UV/VIS, IR and 1H-NMR spectra and (12C/13C) ratio. The age of the layers of sediment was established by using 210Pb method and validated with 137Cs distribution in the profile. The sediment accumulation rate was close to 2 mm/annum in both cores. The age was used for kinetics calculations of the changes of the analysed properties of humic acids in the core. The changes include the increase of aromatization, increased C/H ratio, decreased A4/6 ratio and systematic changes in the content of humic substances in organic matter of investigated samples. The changes were attributed to the mineralization of organic matter accompanied by biochemical transformation of humic substances in the upper portion of marine bottom sediment. Origin of humic substances in the investigated sediments was established by means of stable carbon isotopes analyses (12C/13C).
Organic ligands may have an important influence on the mobility of Cu, Ni and Cd in groundwater whose fate depends largely on speciation and adsorption on solid phases. Of special interest is the formation of ternary complexes, i.e. the adsorption of complexed metals on surfaces, because they immobilise the metals in the soil. Natural organic ligands in natural water are mostly complex humic substances which are represented in our laboratory system by some simple organic molecules such as oxalic or salicylic acid. These molecules were chosen, because they represent structure elements of humic substances. Goethite is used as a representative solid phase. To study the formation of ternary complexes, the adsorption isotherms in the binary systems (metal-surface system or ligand-surface system) are compared to those in the ternary system (metal-ligand-surface system). From the pH-dependence of the respective isotherms suggestions for the existence of ternary complexes can be deduced. Experiments are first carried out in a well defined laboratory batch system. In the oxalate system ternary complexes are formed with Cu in the acidic pH-range up to pH6, the adsorption of Cd and Ni does not show any differences in the binary and the ternary system. Salicylate does not have any effect on the adsorption of these three metals. Both ligands are not strong enough to compete with surface complexation in the pH-range above 7, which would correspond to pH in natural groundwater, and would so in a well equilibrated system not contribute to the mobilisation of metals. In a second stage, the experiments are repeated with freshly infiltrated natural groundwater and the influence of the natural DOC on the adsorption on metals is analysed and compared to the results in the lab system.
Arsenic is a well spread element in natural soils. However human activities as agriculture and mining led to its accumulation in the environment. The arsenic fate is mainly governed by adsorption/desorption processes. Oxi-hydroxides, clay minerals, and organic matter are the main arsenic sorbing phases. In addition organic matter interacts with both mineral surfaces and pollutants. They have thus a key role on the fate of pollutants in the environment.In this study, the role of organic matter on the arsenic adsorption onto clay minerals was evaluated. Arsenic adsorption batch experiments were conducted onto two solid phases : a crude kaolinite and a humic acid coated one. This organic-clay complex is extensively described in Saada et al. (1). The solid was overnight equilibrated within a 0.01 M Ca(NO3)2 electrolyte. Arsenic nitric solution was then added to the batch with an initial arsenic concentration ranged from 50 to 2500 µg/l. pH was adjusted to 7, using NaOH. The batches were conducted during 24, 48, and 72 hours in order to determine the equilibration time. Solid and liquid phases were separated by centrifugation at 10,000 rpm during 10 min. The solution was filtrated at 0.45 µm and residual arsenic concentration was measured by AAS graphite furnace. First results showed that equilibrium is reached after 24 hours in the case of crude kaolinite and after 48 hours for the humic acid coated kaolinite. In both cases, adsorption isotherms fitted the Freundlich equation. Under the studied conditions, no adsorption saturation phenomenon on the solid phases was recorded. Adsorption onto the humic acid coated kaolinite was significantly higher than onto the crude mineral.
Saada A, Gauthier S, Gaboriau H, & Conil P, J. Conf. Abs., 4, (1999).
Polycyclic aromatic hydrocarbons (PAH) are lasting contaminants in the environment. Their fate is often studied due to their genotoxicity. The transport of these pollutants in water depends on their interactions with soil constituents. The aim of this study is to determine the impact of natural organic matter (humic acids) on the PAH retention capacity of soils.In a first step, we examine the fixation mechanisms of humic acid on a well spread clay mineral (kaolinite). Mineral purity and cristallinity are checked by X-Ray diffraction, infrared spectrometry and thermogravimetric measurements. Humic acid adsorption and desorption are studied in batch experiments with initial acid humic concentrations ranging from 0.1 to 1 g/l. Residual dissolved oganic matter concentration is measured by UV-visible spectrophotometry at two wavelengths (400 and 524 nm). The impact of monovalent (Na+) and divalent cations (Ca++) is regarded. The adsorption isotherms fit the Langmuir equation. The desorption isotherms indicate a non reversible fixation of organic matter onto clay surface. The obtained organo-clay complex was studied by infrared spectroscopy. Alteration of surface hydroxyl bands of kaolinite is observed. This confirms the high reactivity of this type of sites (1). The amount of adsorbed humic acids is related to ionic strength and divalent cation concentration (Ca++). Indeed calcium may form 1) intramolecular bridges, inducing humic acid condensation, and 2) intermolecular bridges, between the clay and the humic acid. In a second step we studied alteration of the surface properties of clay due to humic acid coating. The coating increases the specific surface area and the cationic exchange capacity of the solid. As a final step PAH adsorption capacity of the solids are compared. PAH adsorption is studied using 14C radio labelled PAH (phenantrene). For both solids, the resulting isotherms fit a Freunlich equation. The humic acid coated kaolinite has a higher affinity for PAH than the crude kaolinite, due to a higher organic carbon content.
Saada A, Siffert B & Papirer E, J. Colloid Interface Sci., 174, 185-190, (1995).
Multiple correlations were found to exist between organic matter content and aggregate stability, soil sealing and water infiltration rate (Le Bissonnais et al., 1997). On the other hand, aspects of fractal geometry have been used to give quantitative measurements of soil structure (Anderson et al., 1996; Bartoli et al., 1991). Fractal theory and fractal dimension might be used to predict physical characteristics of soil such as porosity, tortuosity etc... In the present study numerical simulation was employed to determine the average internal cohesion of agglomerates of three-dimensional fractal clusters generated by computer simulation. This parameter was determined by the set of the numbers P(v,i,j) of links established between two clusters of masses i and j when all the possible non overlapping configurations of the stuck clusters were taken into account, the parameter v characterising the number of connections concomitantly established. The agglomerate cohesion was found to be independent of the mass (i+j) of the final agglomerate. The cohesion of an agglomerate of a given mass (i+j) being assumed to be maximal for high values of the connectivity index v, the fragmentation probability under a given stress was found to increase exponentially with lowering v. The model was further applied to soil agglomerates for which the adhesion between pre-existing fractal soil aggregates might be mediated by natural organic matter adsorbed on the solid external surface. It was shown that lake of organic binder contributed to induce less adhesive bonds between aggregates so that agglomerates might become brittle. It has been shown that in some cases synthetic polymers behaved like natural organic matter in destabilisation processes of suspended oxides (Elfarissi et al., 1998). In addition, water soluble polyacrylamides have been proposed as soil amendments for various agricultural purposes (Barvenik, 1994; Grula et al., 1994). The major problem which has to be solved to establish the expected soil stability has been the polymer dosage in irrigation water. Since this step is far from being attained, application of this elementary model on more elaborate soil models may provide a comprehensive approach to predict and interpret polymer mediated soil stability.
Le Bissonnais Y & Arrouays D, Eur. J. Soil Sci, 48, 39-48, (1997).
Anderson AN, McBratney AB & Fitzpatrick EA, Soil Sci. Soc. Am. J, 60, 962-969, (1996).
Bartoli F, Philippy R, Doirisse M, Niquet S & Dubuit M, J. Soil Sci, 42, 167-177, (1991).
Elfarissi F, Nabzar L, Ringenbach E & Pefferkorn E, Colloids Surfaces, 131, 281-294, (1998).
Barvenik F, Soil Sci, 158, 235-243, (1994).
Grula MM, Huang M-L & Sewell G, Soil Sci, 158, 291-300, (1994).
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