Formation of bound residues by naphthalene and cis-naphthalene-1,2-dihydrodiol

The formation of bound residues by naphthalene and its metabolite, cis-naphthalene-1,2-dihydrodiol, in a sediment (1% OC), a silty loam soil (2.9% OC) and a peat (26% OC) was examined. The experiments were carried out under both sterile and nonsterile conditions for up to 35 days. The samples containing bound contaminant were hydrolyzed at an alkaline pH and fractionated using 3,000 and 500 Da molecular weight cutoff ultrafiltration membranes in series. The results for all the geosorbents examined showed that bound residue formation is low for naphthalene and between 5 and 20 times higher for the metabolite. The amount of bound residues released by hydrolysis was higher for the metabolite than for the parent compound for all the samples. The molecular weight distribution of bound radioactivity after hydrolysis showed binding to the high molecular weight components of the sediment organic matter and to the low molecular weight components for soil and peat organic matter when incubated with cis-naphthalene-1,2-dihydrodiol. Experiments performed with naphthalene-UL-(14)C showed larger amounts of bound residue found than in experiments with naphthalene-1-(14)C.     

Characterization of extractable and non-extractable polycyclic aromatic hydrocarbons in soils and sediments from the Pearl River Delta, China

Formation of bound residues of pollutants in soils and sediments is an important process to control the fate of pollutants in the environment. The most of bound residue is not solvent extractable. In this paper, we measured both extractable and non-extractable polycyclic aromatic hydrocarbons (PAHs) in different organic matter fractions of samples from the Pearl River Delta, China. Non-extractable PAHs concentration was 234.45-1424.57 μg/kg and accounted for 33.78-57.44% of total PAHs. 2-3 Ring PAHs were the dominant species and differed in concentration substantially between the samples. The atomic ratio of PAHs over organic-C in the fractions ordered as solvent soluble organic matter > humin > humic acids, matching the content of aliphatic moieties in the fractions of organic matter. The ratio of extractable and non-extractable PAHs may relate to the aging process of PAHs in soil and sediment.     

Bound residues: environmental solution or future problem?

The paper examines the issue of bound residues from the viewpoint of the risk assessment procedures employed for environmental protection. It considers, on one hand, the evidence that such residues are so tightly bound to soil organic matter as to be essentially unavailable; and on the other, the perspective that we should not be loading up the environment with compounds whose future behaviour and release we cannot predict. Existing knowledge of the mechanisms by which residues bind to soil organic matter suggests that release will be closely dependent on soil organic matter breakdown. Simple models of organic matter turnover suggest that the release following single applications of individual compounds will be very slow; but the significance of releases following repeated application over many years of a number of compounds needs to be investigated further. Applying environmental risk assessment techniques is complicated by the difficulty in identifying parent molecules and potential metabolites in the bound residue fraction. The paper concludes that for single additions of individual compounds, bound residues probably do represent an environmental solution. But the long-term significance of bound residues formed from multiple additions of a number of compounds is less clear. The paper recommends that future work should emphasise the biological significance of such residues and their release.     

A correlation between the fate and non-extractable residue formation of 14C-metalaxyl and enzymatic activities in soil

Extracellular, oxidative soil enzymes like monophenol oxidases and peroxidases play an important role in transformation of xenobiotics and the formation of organic matter in soil. Additionally, these enzymes may be involved in the formation of non-extractable residues (NERs) of xenobiotics during humification processes. To examine this correlation, the fate of the fungicide ¹⁴C metalaxyl in soil samples from Ultuna (Sweden) was studied. Using different soil sterilization techniques, it was possible to differentiate between free, immobilized, and abiotic (“pseudoenzyme”-like) oxidative activities. A correlation between the formation of metalaxyl NER and soil organic matter content, biotic activities, as well as extracellular phenoloxidase and peroxidase activities in the bulk soil and its particle size fractions was determined. Extracellular soil-bound enzymes were involved in NER formation (up to 8% of applied radioactivity after 92 days) of the fungicide independently from the presence of living microbes and different distributions of the NER in the soil humic subfractions.     

Influence of humic fractions on retention of isoproturon residues in two Moroccan soils

The influence of different fractions of soil organic matter on the retention of the herbicide isoproturon (IPU) has been evaluated. Water and methanol extractable residues of (14)C labeled isoproturon have been determined in two Moroccan soils by beta -counting-liquid chromatography. The quantification of bound residues in soil and in different fractions of soil humic substances has been performed using pyrolysis/scintillation-detected gas-chromatography. Microbial mineralization of the herbicide and soil organic matter has been also monitored. Retention of isoproturon residues after 30-days incubation ranged from 22% to 32% (non-extractable fraction). The radioactivity extracted in an aqueous environment was from 20% to 33% of the amount used for the treatment; meanwhile, methanol was able to extract another 48%. Both soils showed quantities of bound residues into the humin fraction higher than humic and fulvic acids. The total amount of residues retained into the organic matter of the soils was about 65 % of non-extractable fraction, and this percentage did not change with incubation time; on the contrary, the sorption rate of the retention reaction is mostly influenced by the clay fraction and organic content of the soil. Only a little part of the herbicide was mineralized during the experimental time.     

Investigating roles of organic and inorganic soil components in sorption of polar and nonpolar aromatic compounds

The main objective of the present study was to assess the roles of various soil components in sorption of organic compounds differing in polarity. Removal of the whole soil organic matter decreased sorption by approximately 86% for nonpolar 1,3,5-trichlorobenzene (TCB), but only 34-54% for highly polar 1,3,5-trinitrobenzene (TNB); however, removal of the extractable humic/fulvic acids did not much affect sorption of the two sorbates. With normalization of solute hydrophobicity, TNB exhibits several orders of magnitude stronger sorption compared with TCB to maize burn residue (black carbon), extracted humic acid and Na⁺-saturated montmorillonite clay, suggesting specific sorptive interactions for TNB with the individual model soil components. It was proposed that sorption of TCB to the bulk soil was dominated by hydrophobic partition to the condensed, non-extractable fraction of organic matters (humin/kerogen and black carbon), while interactions with soil clay minerals were an important additional factor for sorption of TNB.     

Influence of humic fractions on retention of isoproturon residues in two Moroccan soils

The influence of different fractions of soil organic matter on the retention of the herbicide isoproturon (IPU) has been evaluated. Water and methanol extractable residues of ¹⁴C labeled isoproturon have been determined in two Moroccan soils by β -counting–liquid chromatography. The quantification of bound residues in soil and in different fractions of soil humic substances has been performed using pyrolysis/scintillation-detected gas-chromatography. Microbial mineralization of the herbicide and soil organic matter has been also monitored. Retention of isoproturon residues after 30-days incubation ranged from 22% to 32% (non-extractable fraction). The radioactivity extracted in an aqueous environment was from 20% to 33% of the amount used for the treatment; meanwhile, methanol was able to extract another 48%. Both soils showed quantities of bound residues into the humin fraction higher than humic and fulvic acids. The total amount of residues retained into the organic matter of the soils was about 65 % of non-extractable fraction, and this percentage did not change with incubation time; on the contrary, the sorption rate of the retention reaction is mostly influenced by the clay fraction and organic content of the soil. Only a little part of the herbicide was mineralized during the experimental time.     

Combined application of non-discriminated conventional pyrolysis and tetramethylammonium hydroxide-induced thermochemolysis for the characterization of the molecular structure of humic acid isolated from polluted sediments from the Ravenna Lagoon

Humic acid (HA) isolated from highly polluted sediment from the Ravenna Lagoon (Italy) was subjected to pyrolysis/tetramethylammonium hydroxide (TMAH)-induced thermochemolysis to reveal the impact of industrial activities on humification. Special effort was made to distinguish between analytes originating from the polymeric humic organic matter network along with sequestered compounds (which cannot be released by solvent extraction), and the solvent-extractable lipid fraction sorbed onto the organic matrix. Exhaustive solvent extraction of the isolated HA proved mandatory to avoid biased results when identifying the origin of the pyrolyzates of untreated samples. Conventional pyrolysis at 750 degrees C of the "degreased" HA revealed a characteristic polycyclic aromatic hydrocarbon (PAH) pattern, which was significantly different than patterns obtained from the pyrolysis of natural humic acids. TMAH-induced thermochemolysis at 500 degrees C provided information on carboxylic acids covalently bound inside the HA network via ester bonds. Thermochemolysis of the "degreased" matrix at 750 degrees C, resulting in the cleavage of C-O and C-C bonds, revealed a significant PAH pattern very similar to that obtained by conventional pyrolysis. The uncommon PAH pattern points to the formation of bound residues as the humification/detoxification pathway. Recalcitrant hopane hydrocarbons showed very similar patterns for both the untreated and exhaustively solvent-extracted samples, which points to hopanes being linked inside the humic network via C-C bonds. The technique of non-discriminating flash pyrolysis used in this study, based on extremely rapid capacitive discharge heating in a Silcosteel capillary, proved to be an excellent method to obtain high-boiling pyrolyzates (PAHs beyond 252Da molecular weight, C(29)-C(31)-hopanes).     

Compost effect on soil humic acid: A NMR study

The humic acid (HA) fraction of a food and vegetable residues compost (CM) was taken as indicator to trace the fate of CM organic matter in four years CM amended soil. (1)H and (13)C NMR spectroscopy were used to investigate the nature of the HA isolates from CM, control soil (S(4)) and amended soil. The result indicated a significant structural difference between CM HA and S(4) HA, and supported the presence of both HA fractions in soil at the end of the amendment trials. However, the nature and content of CM HA in soil did not fully explain the increase of soil cation exchange capacity (CEC) after amendment. All CM humic fractions (i.e., fulvic acid, humic acid and humin) were found to contribute to the change of the soil organic matter composition. It is concluded that although CM HA is a suitable indicator of the survival of compost organic matter in soil during amendment, all three humic fractions should be monitored and analyzed to fully understand changes in the composition and properties of amended soil.     

Characterization of non-extractable residues of the fungicide dithianon in soil using 13C/14C-labelling techniques

The fate of the (14)C-labelled fungicide dithianon in soil is characterized by the formation of non-extractable, "bound" residues of approximately 63% of applied amount in 64 d. Humic acids containing these "bound" residues were isolated after conducting degradation studies of the active ingredient in an orthic luvisol under standardized conditions. In the same way, (13)C-labelled dithianon was incubated in an artificial soil which was produced by humification of (13)C-depleted straw in an incinerated soil. The "bound" residues of the (13)C-labelled dithianon in the humic acid fraction of the artificial soil were analyzed using (13)C-NMR techniques. There was no evidence of a covalent bonding of the residues to the humic substances. Results of polarity gradient high performance thin layer chromatography (AMD-HPTLC) of "bound" residues of the (14)C-labelled dithianon in the humic acid fraction indicate a sequestration process of metabolites into the humic substance as a possible binding mechanism.     

Effects of long-term soil amendment with sewage sludges on soil humic acid thermal and molecular properties

Sewage sludges are frequently used as soil amendments due to their high contents of organic matter and nutrients, particularly N and P. However, their effects upon the chemistry of soil humic acids, one of the main components of the soil organic matter, need to be more deeply studied in order to understand the relation between organic matter structure and beneficial soil properties. Two sewage sludges subjected to different types of pre-treatment (composted and thermally dried) with very different chemical compositions were applied for three consecutive years to an agricultural soil under long-term field study. Thermal analysis (TG–DTG–DTA) and solid-state ¹³C NMR spectroscopy were used to compare molecular and structural properties of humic acids isolated from sewage sludges, and to determine changes in amended soils. Thermally dried sewage sludge humic acids showed an important presence of alkyl and O/N-alkyl compounds (70%) while composted sludge humic acids comprised 50% aromatic and carbonyl carbon. In spite of important differences in the initial chemical and thermal properties of the two types of sewage sludges, the chemical and thermal properties of the soil humic acids were quite similar to one another after 3 years of amendment. Long-term application of both sewage sludges resulted in 80–90% enrichment in alkyl carbon and organic nitrogen contents of the soil humic acid fraction.     

Effects of long-term soil amendment with sewage sludges on soil humic acid thermal and molecular properties

Sewage sludges are frequently used as soil amendments due to their high contents of organic matter and nutrients, particularly N and P. However, their effects upon the chemistry of soil humic acids, one of the main components of the soil organic matter, need to be more deeply studied in order to understand the relation between organic matter structure and beneficial soil properties. Two sewage sludges subjected to different types of pre-treatment (composted and thermally dried) with very different chemical compositions were applied for three consecutive years to an agricultural soil under long-term field study. Thermal analysis (TG–DTG–DTA) and solid-state ¹³C NMR spectroscopy were used to compare molecular and structural properties of humic acids isolated from sewage sludges, and to determine changes in amended soils. Thermally dried sewage sludge humic acids showed an important presence of alkyl and O/N-alkyl compounds (70%) while composted sludge humic acids comprised 50% aromatic and carbonyl carbon. In spite of important differences in the initial chemical and thermal properties of the two types of sewage sludges, the chemical and thermal properties of the soil humic acids were quite similar to one another after 3 years of amendment. Long-term application of both sewage sludges resulted in 80–90% enrichment in alkyl carbon and organic nitrogen contents of the soil humic acid fraction.     

Sorption of naphthalene and phenanthrene by soil humic acids

Humic acids are a major fraction of soil organic matter (SOM), and sorption of hydrophobic organic chemicals by humic acids influences their behavior and fate in soil. A clear understanding of the sorption of organic chemicals by humic acids will help to determine their sorptive mechanisms in SOM and soil. In this paper, we determined the sorption of two hydrophobic organic compounds, naphthalene and phenanthrene by six pedogenetically related humic acids. These humic acids were extracted from different depths of a single soil profile and characterized by solid-state CP/MAS 13C nuclear magnetic resonance (NMR). Aromaticity of the humic acids increased with soil depth. Similarly, atomic ratios of C/H and C/O also increased with depth (from organic to mineral horizons). All isotherms were nonlinear. Freundlich exponents (N) ranged from 0.87 to 0.95 for naphthalene and from 0.86 to 0.92 for phenanthrene. The N values of phenanthrene were consistently lower than naphthalene for a given humic acid. For both compounds, N values decreased with increasing aromaticity of the humic acids, such an inverse relationship was never reported before. These results support the dual-mode sorption model where partitioning occurs in both expanded (flexible) and condensed (rigid) domains while nonlinear sorption only in condensed domains of SOM. Sorption in the condensed domains may be a cause for slow desorption, and reduced availability and toxicity with aging.     

Removal of sulfadimethoxine in soil mediated by extracellular oxidoreductases

Sulfadimethoxine (SDM) is an antibiotic commonly used in concentrated animal feeding operations and released into the environment via manure application on agricultural lands. Transformation of antibiotics in soil impacts the likelihood of their entry to water bodies, uptake by plants, and thus their effect on terrestrial and aquatic organisms. We conducted experiments to incubate SDM in a sandy loam soil in the presence of humification enzymes commonly found in natural soil, laccase, horseradish peroxidase, and lignin peroxidase. Incubation with the enzymes led to significant reduction in the fraction of SDM extractable from soil, indicating the formation of bound residues. Such transformation was enhanced when the organic matter content in soil is increased or when certain chemical mediators were used along with laccase. The study provided a basis for understanding the environmental fate of sulfonamides and help with the development of remediation methods to mitigate the release of sulfonamides from soil to water.     

Formation and long-term fate of non-extractable residues in outdoor lysimeter studies

Long-term outdoor lysimeter studies using (14)C-labelled compounds allow the quantification of the 'non-extractable residue fraction'. More than 20 lysimeter studies under realistic environmental conditions showed that more than 80% of residual carbon of the molecule is retained in the topsoil layer even after several years. Generally, 50-90% of this residual radiocarbon is regarded as 'soil bound residue'. Microbial biomass is present in large quantities in topsoil and continuously influences chemical and biochemical alteration of pesticide molecules that may interact directly with the total soil organic matter. Labelling techniques using radioactive isotopes like (14)C have been used to characterize these residues in the humus matrix. Our studies have been directed to the investigation of extractability and/or bioavailability of these residues in long-term investigations.     

Environmental fate of amitrole: influence of dissolved organic matter

In this study the environmental fate of amitrole in terrestrial and aquatic model ecosystems was investigated. Under aerobic conditions mineralization of amitrole is the main degradation pathway. The experiments revealed that the leaching behaviour is low in the presence or the absence of dissolved organic matter (DOM) despite the high water solubility due to a strong binding of amitrole to soil constituents. Under anaerobic conditions the addition of DOM increases the transport of amitrole in soil columns. The tests with water/sediment model ecosystems showed that the mineralization of amitrole is lower in comparison to aerobic soil experiments. Up to 80.6% of the applied 14C-labelled amitrole transfer into the sediment and about 1/3 of this amount formed bound residues, which are not extractable.     

Studies on bound 14C-prometryn residues in soil and plants

A high-temperature distillation technique was developed for determining and chemically identifying the bound (nonextractable) residues of ¹⁴C-prometryn in an organic soil and plants. A considerable portion of the bound ¹⁴C residues in the incubated organic soil was identified as prometryn. These residues were absorbed by plants grown in the soil. Hono-and di-$\text{N}$-dealkylated metabolites of prometryn were present in the plant bound ¹⁴C residues and a major portion of bound residues as associated with lignin. Soil-bound ¹⁴C residues were also released from soil by microbes. The bound ¹⁴C residues in soil were associated with humin, humic acid, and fulvic acid fractions. Thermoanalytical methods were used to obtained information on the nature and location of ¹⁴C bound residues in soil and humic materials.     

Use of 15N-depleted artificial compost in bound residue studies

Association of bound residues to soil humic matter may be accomplished by different binding mechanisms such as sequestration in hydrophobic interiors of the organic material or covalent linkage to the organic matter. The structures and chemical environments of compounds can be observed by NMR spectroscopy. We applied 15N-NMR spectroscopy to study the soil-bound residues of 15N-labeled simazine. As the 15N-isotope has a low sensitivity and natural abundance 15N-NMR experiments require long measurement times and often result in low signal-to-noise (S/N) ratios. Therefore, in addition to the use of 15N-labeled simazine, 15N-depleted compost was used to reduce the amount of background signal and enhance the sensitivity. The compost was produced from maize and wheat plants grown on sand with 15N-depleted NH4NO3 as sole nitrogen source. The plants were freeze-dried, ground and mixed with sand for composting. After a composting period of 224 days analysis of the compost revealed a 15N-content of 267 ppm as opposed to a natural abundance of 3650 ppm. Characterization of this artificial compost produced parameter values similar to those of a natural compost. The 13C-NMR-spectra of the humic and fulvic acids during different stages of maturity showed that there was a shift from single-bond functional groups to more complex double-bond and aromatic structures. Experiments with this compost showed an increased signal intensity. The improved sensitivity made it possible to obtain interpretable NMR signals in contrast to experiments with 15N-simazine on native soil where no signals were detectable. The data indicated that the bound residues of simazine are composed of metabolites resulting from N-dealkylation and triazine ring destruction. Silylation of the bound residues showed a very strong binding of the residues to the matrix as only a small fraction could be solubilized.     

Effects of organic matter on the distribution of uranium in soil and plant matrices

This work studied interactions of uranium with pure organic compounds, such as glutathione, and more complex mixtures, such as humic acid and aqueous plant extracts. High performance liquid chromatography with UV absorption interfaced to inductively coupled plasma mass spectrometry sequential detection was used to detect organouranium complexes in a variety of soils and plant materials, indicating that nearly 100% of the uranium extracted from certain plant tissues was bound to organic ligands. In addition, soil sorption experiments indicated that humic acid generally decreased uranium sorption to soils and promoted subsequent desorption of uranium because of uranium partitioning to the organic phase. These experiments demonstrate that organic compounds influence the mobility and chemistry of uranium in the environment.     

Acetochlor as a soil pollutant

Acetochlor is a widely used herbicide all over the world. Similarly to other organic pollutants, the environmental fate of the acetochlor is strongly related to its adsorption properties. Static adsorption equilibrium measurements were carried out at 25 degrees C on different types of Hungarian soils characterized by varying amounts of organic matter and pH values. Isotherms obtained under different conditions, as well as on various soils, exhibit a similar shape, thus indicating a two-step adsorption process. The plots cannot be interpreted according to the classes of isotherms suggested by Giles. The adsorption coefficients (K) were estimated from the initial slope of the curves. These values were determined not only by the type of the soil, but also by the composition of the aqueous media. The organic matter adsorption coefficients (Kom) were also calculated and they were approximately identical for soils of high organic matter. Due to the low value of the adsorption coefficients, the acetochlor is a rather mobile pollutant of the soil posing a potential danger to the aquatic environment. For soils with high organic content, the Kom values are similar to each other and, due to the significant coverage of the inorganic surfaces, the adsorption is controlled by the solute-organic substance interactions. The adsorption of weakly polar compounds, however, is remarkable even on those soils where the organic content is low. In this case, the binding is governed by the solute-inorganic surface interactions. This conclusion is nicely proved by the adsorption of the acetochlor on quartz. According to our hypothesis, the second part of the adsorption step is controlled by the solute-adsorbed solute interactions. The role of the organic matter in this region of the isotherm is probably negligible. As the organic matter content of the soils plays an important role in the acetochlor adsorption, humic substances must have a strong influence upon the transport of this compound. Experiments to obtain adsorption isotherms of further pesticides and the development of a quantitative model are in progress.