Potential contributions of clay minerals and organic matter to pentachlorophenol retention in soils

Sorption of pentachlorophenol (PCP) by pure minerals and humic acids were measured to obtain additional perspective on the potential contributions of both clay minerals and soil organic matter (SOM) to contaminants retention in soils. Four types of common soil minerals and two kinds of humic acids (HAs) were tested. The sorption affinity for PCP conformed to an order of HAs > K-montmorillonite > Ca-montmorillonite > goethite > kaolinite. Such a difference in sorption capacity could be attributed to the crucial control of HAs. Clay minerals also had their contribution, especially K-montmorillonite, which played an important, if not dominant, role in the controlling process of PCP sorption. By removing 80% (on average) of the organic carbon from the soils with H(2)O(2), the sorption decreased by an average of 50%. The sorption reversibility had been greatly favored as well. Considering the uncharged mineral fractions in soil before and after H(2)O(2)-treated, the main variation in sorption behavior of the soil might thus be related to the removed organic carbon and the reduced pH. This testified rightly the interactive effect of SOM and clay minerals on PCP sorption as a function of pH.     

Sorption of U(VI) onto an artificial humic substance-kaolinite-associate

An artificial humic substance-kaolinite-associate (HSKA) was synthesized as a model substance for natural clays containing organic matter in clay formations, soils, and sediments. The U(VI) sorption onto this model substance was studied in batch experiments as a function of pH and compared to the U(VI) sorption onto kaolinite in absence and presence of separately added humic acid (HA). The HSKA has a TOC content of 4.9 mg g(-1). It was found that the humic matter associated with kaolinite exhibits an immobilizing as well as an mobilizing effect on U(VI). Between pH 3 and 5, humic matter causes an increase of the U(VI) sorption onto kaolinite, whereas at pH above 5 the release of humic matter from the associate into the solution and the formation of dissolved uranyl humate complexes reduces the U(VI) sorption. The U(VI) sorption onto the synthetic HSKA differs from that of U(VI) in the system U(VI)/HA/kaolinite with comparable amounts of separately added HA. Separately added HA causes a stronger mobilizing effect on U(VI) than humic matter present in HSKA. This can be attributed to structural and functional dissimilarities of the humic substances.     

Influences of preparative methods of humic acids on the sorption of 2,4,6-trichlorophenol

Humic acids (HAs) are a major component of soil organic matter which strongly affects the sorption behavior of organic contaminants in soils. To assess the sorption-desorption characteristics of organic compounds on HAs, the organic adsorbent is usually isolated using an acid-base extraction method followed by air-drying or freeze-drying. In this study, a peat soil from the Yangming mountain area of Taiwan was sampled and repeatedly extracted followed by either air-drying or a non-drying treatment (denoted DHAs and NDHAs, respectively). The sorption of 2,4,6-TCP on HAs was evaluated using the batch method. Kinetic sorption results indicated that DHAs exhibited a two-step first-order sorption behavior, involving a rapid sorption followed by a slow sorption. The slow sorption may be attributed to the diffusion of 2,4,6-TCP through the condensed aromatic domains of HAs. On the contrary, the sorption of 2,4,6-TCP on NDHAs was extremely rapid, and the sorption data did not fit existing kinetic models. Each HA sample exhibited a nonlinear sorption isotherm. Sorption nonlinearity (represented by Freundlich N values) and K(oc) had a positive relationship with aliphaticity for DHAs; however, nonlinearity and K(oc) correlated positively with aromaticity when NDHAs adsorbents were used. We conclude that the air-drying technique may artificially create a more condensed area, which strongly affects the sorption characteristics of HAs. Thus, an incorrect evaluation of the sorption capacity and its relationship with the chemical composition of HAs would arise following use of the air-drying method.     

Clear effects of soil organic matter chemistry, as determined by NMR spectroscopy, on the sorption of diuron

Organic matter has long been recognized as the main sorbent phase in soils for hydrophobic organic compounds (HOCs). In recent times, there has been an increasing realization that not only the amount, but also the chemical composition, of organic matter can influence the sorption properties of a soil. Here, we show that the organic carbon-normalized sorption coefficient (K(OC)) for diuron is 27-81% higher in 10 A11 horizons than in 10 matching A12 horizons for soils collected from a small (2ha) field. K(OC) was generally greater for the deeper (B) horizons, although these values may be inflated by sorption of diuron to clays. Organic matter chemistry of the A11 and A12 horizons was determined using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. K(OC) was positively correlated with aryl C (r2=0.59, significance level 0.001) and negatively correlated with O-alkyl C (r2=0.84, significance level <0.001). This is only the second report of correlations between whole soil K(OC) and NMR-derived measures of organic matter chemistry. We suggest that this success may be a consequence of limiting this study to a very small area (a single field). There is growing evidence that interactions between organic matter and clay minerals strongly affect K(OC). However, because the soil mineralogy varies little across the field, the influence of these interactions is greatly diminished, allowing the effect of organic matter chemistry on K(OC) to be seen clearly. This study in some way reconciles studies that show strong correlations between K(OC) and the chemistry of purified organic materials and the general lack of such correlations for whole soils.     

Separating the effects of organic matter-mineral interactions and organic matter chemistry on the sorption of diuron and phenanthrene

Even though it is well established that soil C content is the primary determinant of the sorption affinity of soils for non-ionic compounds, it is also clear that organic carbon-normalized sorption coefficients (K(OC)) vary considerably between soils. Two factors that may contribute to K(OC) variability are variations in organic matter chemistry between soils and interactions between organic matter and soil minerals. Here, we quantify these effects for two non-ionic sorbates-diuron and phenanthrene. The effect of organic matter-mineral interactions were evaluated by comparing K(OC) for demineralized (HF-treated) soils, with K(OC) for the corresponding whole soils. For diuron and phenanthrene, average ratios of K(OC) of the HF-treated soils to K(OC) of the whole soils were 2.5 and 2.3, respectively, indicating a substantial depression of K(OC) due to the presence of minerals in the whole soils. The effect of organic matter chemistry was determined by correlating K(OC) against distributions of C types determined using solid-state (13)C NMR spectroscopy. For diuron, K(OC) was positively correlated with aryl C and negatively correlated with O-alkyl C, for both whole and HF-treated soils, whereas for phenanthrene, these correlations were only present for the HF-treated soils. We suggest that the lack of a clear effect of organic matter chemistry on whole soil K(OC) for phenanthrene is due to an over-riding influence of organic matter-mineral interactions in this case. This hypothesis is supported by a correlation between the increase in K(OC) on HF-treatment and the soil clay content for phenanthrene, but not for diuron.     

Role of cupric ions in the H2O2/UV oxidation of humic acids

The purpose of this study is to reveal the role of cupric ions as a natural water contaminant in the H2O2/UV oxidation of humic acids. Humic acids are naturally occurring organic matter and exhibit a strong tendency of complexation with some transition metal ions. Chlorination of humic acids causes potential health hazards due to formation of trihalomethane (THM). The removal of THM precursors has become an issue of public concern. The H2O2/UV process is capable of mineralizing humic acids due to formation of a strong oxidant, hydroxyl radicals, in reaction solution. Experiments were conducted in a re-circulated photoreactor. Different cupric concentrations (0-3.8 mg/l) and different pH values (4-9) were controlled to determine their effects on the degradation of humic acids, UV light absorbance at 254 nm, and H2O2. The presence of cupric ions inhibits humic mineralization and decreases the rate of destruction of humic acids which absorb UV light at 254 nm. On the other hand, the higher the cupric concentration, the lower the H2O2 decomposition rate. In the studied pH range, the minimum of total organic carbon (TOC) removal occurs at pH = 6 in the presence of 2.6 mg/l of cupric ions; both acidification (pH = 4) and alkaline condition (pH = 9) lead to a better removal of TOC. It is inferred from this study that the cupric-complexed form of humic acids is more refractory than the non-complexed one.     

Sorption of humic substances on aquifer material at artificial recharge of groundwater

Experiments in batch equilibrium system were carried out to evaluate the importance of physical and chemical factors determining the sorption efficiency of humic substances (HS) on aquifer material, which has been used for artificial recharge of groundwater (ARG) in drinking water production. Results showed that an increase of the amount of clay in the aquifer material and a decrease of pH in water increased the sorption efficiency. The sorption of higher molecular weight, more hydrophobic and aromatic HS (Aldrich and forest soil humic acids) were greater than the sorption of acidic HS (river fulvic acids), either on the aquifer material or to its representative sorbing phases, clay and organic matter. The sorption on the aquifer material was largely due to physical sorption (hydrophobic attractions). This study showed the importance of HS composition on their removal during ARG and contributed to an understanding of the HS sorption mechanisms in this process.     

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.     

Sorption and degradation of imidacloprid in soil and water

Imidacloprid, the major component of many widely used insecticide formulations, is highly persistent in soils. In this study, the sorption of imidacloprid by six soils as well as its photodegradation and hydrolysis in water were studied. The soils differed significantly in organic matter content and other physical and chemical properties. Sorption increased with increasing soil organic matter content but was not significantly correlated with other soil properties. Removal of organic matter via H2O2 oxidation decreased the sorption. By normalizing the Freundlich coefficients (Kf) to organic matter contents, the variability in obtained sorption coefficient (Kom) was substantially reduced. These results indicate that soil organic matter was the primary sorptive medium for imidacloprid. The low heat of sorption calculated from Kom suggests that partition into soil organic matter was most likely the mechanism. The photodegradation and hydrolysis of imidacloprid in water followed pseudo-first-order kinetics; however, the latter process needed a six-time-higher activation energy. While both processes produced the same main intermediate, they occurred via different pathways. The hydrolysis of imidacloprid was not catalyzed by the high interlayer pH in the presence of metal-saturated clays, which appeared to result from the lack of the pesticide adsorption in the interlayers of clays.     

Sulfamethoxazole sorption by sediment fractions in comparison to pyrene and bisphenol A

The environmental behavior of antibiotics has attracted great research attention. However, their sorption mechanisms in soils/sediments are still unknown. Comparison of the sorption properties between the widely-studied hydrophobic organic contaminants (HOCs) and antibiotics may provide valuable insight to antibiotic sorption mechanisms. Thus, in this study batch experiments for pyrene (PYR), bisphenol A (BPA), and sulfamethoxazole (SMX) sorption were conducted on a sediment sample and its separated fractions. Our results showed the high sorption of PYR on black carbon and organic matter. Although high sorption of SMX was observed for both separated organic fractions (humic acids) and inorganic mineral particles, the original sediment particles showed relatively low sorption. Competitive sorption between SMX and dissolved humic acid on mineral particles was observed in this study. This competitive interaction is a unique process for antibiotic sorption in soils/sediments compared with apolar HOCs and may be one of the important factors controlling the antibiotic sorption.     

Carbendazim sorption-desorption in Vietnamese soils

Four Vietnamese soils (denoted AG, CT, ST and TG) which differed with respect to pH (pH 2.9-5.4), clay (17-50%) and organic matter (0.3-9.8%) content, were selected for sorption and desorption studies of carbendazim using the batch equilibration technique. Sorption increased with increasing organic carbon (OC) and clay content. Kd values for carbendazim sorption on AG, CT, ST, TG soils at initial concentration of 20 microg/g were 12.5, 127, 8.1 and 9.6 ml/g, respectively. The OC partition coefficients (Koc) for AG, CT, ST and TG were 1140, 1300, 2700 and 960 ml/g, respectively. Carbendazim was strongly sorbed and the binding was less reversible in the acid sulfate soil (CT), than in the other soils. The CT soil had both the highest OC content (9.8%) and the highest clay content (49.8%). The influence of pH on carbendazim sorption was studied in the ST and CT soils. Sorption of carbendazim by the sandy ST soil (OC 0.3%; clay content 26.3%) increased as the pH decreased, while sorption of carbendazim by the CT soil decreased as pH decreased.     

Studies on the sorption behaviors of nitrobenzene on marine sediments

The sorption behaviors of nitrobenzene on marine sediments were systematically investigated in this study. The nitrobenzene sorption on both HCl-treated and untreated sediments accorded well with the linear sorption isotherm. It occurred primarily through partition function of organic carbon of sediments. In comparison, the sorption behavior of nitrobenzene on H2O2-treated sediments was nonlinear and conformed to Langmuir isotherm. Sorption of nitrobenzene on H2O2-treated sediment was mainly through surface function of sediment minerals such as clays. With the increase of ionic strength (salinity), solubility of nitrobenzene in solution would decrease. At the same time, the release of dissolvable part of organic carbon into water solution would also decrease. As a result, partition coefficient and saturate adsorption amount of nitrobenzene on marine sediments increased with increasing salinity of seawater. Contrary to the influence of salinity, partition coefficient and saturate adsorption amount of nitrobenzene decreased with increasing temperature.     

Oxidation of chlorophenols in soil at natural pH by catalyzed hydrogen peroxide: the effect of soil organic matter.

This investigation reports on the effects of soil organic matter (SOM) during the oxidation of chlorophenols with Fe2+-catalyzed H2O2 (Fenton oxidation) system. The soil pH was 7.1 and was not altered. Sorption experiments of soil pre-treated under various oxidation conditions were performed. Concentrations of organic matter in the liquid phase and soil before and after oxidation were analyzed. The results were correlated to the observation in batch Fenton oxidation tests. They showed that the oxidation of chlorophenols at natural soil pH depended on the dose of H2O2 and Fe2+. The soil organic content did not vary significantly after various Fenton treatments, while the sorption of chlorophenols was 10-25% less by the oxidation. The concentration of chlorophenols in the liquid phase exhibited a "decrease and rebound" phenomenon in the batch Fenton oxidation tests. It appeared that the oxidation of SOM resulted in the release of sorbed chlorophenols which were then oxidized by the excess H2O2. An "oxidation-desorption-oxidation" scheme was proposed to describe one of the interaction mechanisms among the oxidant, SOM, and chlorophenols during oxidation.     

Evaluation of impacts of soil fractions on phenanthrene sorption

Phenanthrene sorption to soils and soil fractions was investigated using two contrasting soils with different clay mineral and organic carbon (OC) contents in an attempt to evaluate the contribution of each soil fraction to phenanthrene sorption and the applicability of the carbon-normalized distribution constant (K(OC)) in soils. Sorbents were characterized using surface analysis, solid-state (13)C NMR analysis, and glass transition temperature (T(g)) analysis to gain a insight into the chemical nature of OC in soils. Dissolved organic carbon (DOC) in the soil solution impeded the phenanthrene sorption, while humins accounted for the predominant phenanthrene sorption in soils. The contribution of OC to phenanthrene sorption in soil would be overestimated if only a K(OC)-approach was adopted, since clay minerals could account for much of the sorption, especially when OC was low in soils. Nitrogen gas was shown to be inappropriate for probing non-polar sorption capacity. The results obtained highlight the importance of clay minerals in governing the sorption of phenanthrene in soil, and emphasize the inapplicability of the carbon-normalized distribution coefficient K(OC) in soils.     

Sorption of 3,4-dichloroaniline on four contrasting Greek agricultural soils and the effect of liming

Sorption of 3,4-dichloroaniline (3,4-DCA) on four typical Greek agricultural soils, with distinct texture, organic matter content and cation exchange capacities, was compared by using sorption isotherms and the parameters calculated from the fitted Freundlich equations. The sorption process of 3,4-DCA to the soil was completed within 48-72 h. The 3,4-DCA sorption on all soils was well described by the Freundlich equation and all sorption isotherms were of the L-type. The sandy clay loam soil with the highest organic matter content and a slightly acidic pH was the most sorptive, whereas the two other soil types, a high organic matter and neutral pH clay and a low organic matter and acidic loam, had an intermediate sorption capacity. A typical calcareous soil with low organic matter had the lowest sorption capacity which was only slightly higher than that of river sand. The 3,4-DCA sorption correlated best to soil organic matter content and not to clay content or cation exchange capacity, indicating the primary role of organic matter. The distribution coefficient (K(d)) decreased with increasing initial 3,4-DCA concentration and the reduction was most pronounced with the highly sorptive sandy clay loam soil, suggesting that the available sorption sites of the soils are not unlimited. Liming of the two acidic soils (the sandy clay loam and the loam) raised their pH (from 6.2 and 5.3, respectively) to 7.8 and reduced their sorption capacity by about 50 %, indicating that soil pH may be the second in importance factor (after organic matter) determining 3,4-DCA sorption.     

Sorption of tebuconazole onto selected soil minerals and humic acids

The aim of the present study was to investigate tebuconazole sorption on common soil minerals (birnessite, ferrihydrite, goethite, calcite and illite) and humic acids (representing soil organic matter). Tebuconazole was used (i) in the commercial form Horizon 250 EW and (ii) as an analytical grade pure chemical. In the experiment with the commercially available tebuconazole, a significant pH-dependent sorption onto the oxides was observed (decreasing sorption with increasing pH). The highest sorption was found for ferrihydrite due to its high specific surface area, followed by humic acids, birnessite, goethite and illite. No detectable sorption was found for calcite. The sorption of analytical grade tebuconazole on all selected minerals was significantly lower compared to the commercial product. The sorption was the highest for humic acids, followed by ferrihydrite and illite and almost negligible for goethite and birnessite without any pH dependence. Again, no sorption was observed for calcite. The differences in sorption of the commercially available and analytical grade tebuconazole can be attributed to the additives (e.g., solvents) present in the commercial product. This work proved the importance of soil mineralogy and composition of the commercially available pesticides on the behavior of tebuconazole in soils.     

Effect of clays, metal oxides, and organic matter on rhamnolipid biosurfactant sorption by soil

Rhamnolipids produced by Pseudomonas aeruginosa have been proposed as soil washing agents for enhanced removal of metal and organic contaminants from soil. A potential limitation for the application of rhamnolipids is sorption by soil matrix components. The objective of this study is to empirically determine the contribution of representative soil constituents (clays, metal oxides, and organic matter) to sorption of the rhamnolipid form most efficient at metal complexation (monorhamnolipid). Sorption studies show that monorhamnolipid (R1) sorption is concentration dependent. At low R1 concentrations that are relevant for enhancing organic contaminant biodegradation, R1 sorption followed the order: hematite (Fe(2)O(3))>kaolinite>MnO(2) approximately illite approximately Ca-montmorillonite>gibbsite (Al(OH)(3))>humic acid-coated silica. At high R1 concentrations, relevant for use in complexation/removal of metals or organics, R1 sorption followed the order: illite>humic acid-coated silica>Ca-montmorillonite>hematite>MnO(2)>gibbsite approximately kaolinite. These results allowed prediction of R1 sorption by a series of six soils. Finally, a comparison of R1 and R2 (dirhamnolipid) shows that the R1 form sorbs more strongly alone than when in a mixture of both the R1 and R2 forms. The information presented can be used to estimate, on an individual soil basis, the extent of rhamnolipid sorption. This is important for determining: (1) whether rhamnolipid addition is a feasible remediation option and (2) the amount of rhamnolipid required to efficiently remove the contaminant.     

Sorption of nonpolar neutral organic compounds to humic acid-coated sands: contributions of organic and mineral components

The contributions of organic matter and the mineral surface to the overall sorption of six nonpolar neutral organic compounds (1,2,4-trichlorobenzene, 1,4-dichlorobenzene, chlorobenzene, m-xylene, toluene, benzene) by five humic acid (HA)-coated sands with different fractions of organic carbon (f(oc)) ranging from 0.024% to 0.154% were evaluated on the basis of measured data and four different sorption models. Sorption of all six sorbates to both uncoated and heated sands was nearly linear due to the coverage of hydrophilic mineral surface with the ordered vicinal water region. Sorption of all six sorbates to the HA-coated sands was also essentially linear, and resulted from a combination of sorption to both organic matter and the mineral surface, with the dominance of either contribution depending on the properties of the sorbents (e.g., f(oc)) and the sorbates (e.g., K(ow)). A proposed two-component model for sorption including blocking effect was appropriate for quantifying the contributions of organic matter and the mineral surface to the overall sorption. However, conventional sorption models considering the contributions of both organic matter and the mineral surface provided essentially as good agreement between predicted and measured distribution coefficients as the more complicated, two-component model for sorption that takes into account mineral surface blocking by HA.     

Persistent organic pollutants in soil density fractions: distribution and sorption strength

The sorption strength of persistent organic pollutants in soils may vary among different soil organic matter (SOM) pools. We hypothesized that polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were unevenly distributed and had different soil organic carbon (SOC)-water partition coefficients (K(OC)) among soil density fractions. We determined the concentrations and K(OC) values of 20 PAHs and 12 PCBs in bulk samples and three density fractions (light, <2.0, medium, 2.0-2.4, and heavy, >2.4 g cm(-3)) of 11 urban topsoils (0-5 cm) from Bayreuth, Germany. The K(OC) values were determined using sequential extraction with methanol-water mixtures (35% and 65% methanol) at 60 degrees C. The sum of 20 PAH concentrations in bulk soil ranged 0.4-186 mg kg(-1), and that of 12 PCB concentrations 1.2-158 microg kg(-1). The concentrations of all PAHs and PCBs decreased in the order light>medium>heavy fraction. When normalized to the SOC concentrations, PAH concentrations were significantly higher in the heavy than in the other density fractions. The K(OC) values of the PAHs in density fractions were 3-20 times higher than those of the PCBs with similar octanol-water partition coefficients (K(OW)). The K(OC) values of individual PAHs and PCBs varied up to a factor of 1000 among the studied soils and density fractions. The K(OC) values of 5- and 6-ring PAHs tended to be highest in the heavy fraction, coinciding with their enrichment in this fraction. For the other PAHs and all PCBs, the K(OC) values did not differ among the density fractions. Thus, there is no relationship between sorption strength and distribution among density fractions, indicating that density fractionation is not a suitable tool to distinguish among differently reactive PAH and PCB pools in soils.