Improved retention of imidacloprid (Confidor) in soils by adding vermicompost from spent grape marc

Batch sorption experiments of the insecticide imidacloprid by ten widely different Spanish soils were carried out. The sorption was studied for the active ingredient and its registered formulation Confidor. The temperature effect was studied at 15 degrees C and 25 degrees C. The addition of a vermicompost from spent grape marc (natural and ground), containing 344 g kg(-1) organic carbon, on the sorption of imidacloprid by two selected soils, a sandy loam and a silty clay loam, having organic carbon content of 3.6 g kg(-1) and 9.3 g kg(-1), respectively, was evaluated. Prior to the addition of this vermicompost, desorption isotherms with both selected soils, were also performed. The apparent hysteresis index (AHI) parameter was used to quantify sorption-desorption hysteresis. Sorption coefficients, K(d) and K(f), for the active ingredient and Confidor(R) in the different soils were similar. Sorption decreased with increasing temperature, this fact has special interest in greenhouse systems. A significant correlation (R(2)=0.965; P<0.01) between K(f) values and the organic carbon (OC) content was found, but some soils showed higher sorption coefficients than that expected from their OC values. The normalized sorption coefficients with the soil organic carbon content (K(oc)) were dispersed and low, implying that other characteristics of soils could contribute to the retention capacity as well. The spent grape marc vermicompost was an effective sorbent of this insecticide (K(f)=149). The sorption of imidacloprid increased significantly in soils amended with this vermicompost. The most pronounced effect was found in the sandy loam soil with low OC content, where the addition of 5% and 10% of vermicompost increased K(f) values by 8- and 15-fold, respectively. Soil desorption of imidacloprid was slower for the soil with the higher OC and clay content.     

Development of a kinetic basis for bioavailability of sorbed naphthalene in soil slurries

The degradation of naphthalene in soil-slurry systems was studied using four different organisms and two soils. Organisms with zero-order, first-order, and Michaelis-Menten rates were selected. The soils had substantially different sorption distribution coefficients. Sorption and desorption was evaluated in abiotic soil-slurry systems. The desorption process was described by a model that accounts for equilibrium, rate-limited and non-desorbing sites. Biodegradation parameters were measured in soil-extract solutions. Bioavailability assays, inoculated soil slurries, were conducted and both liquid- and sorbed-phase naphthalene concentrations were measured over time. For the less sorptive soil, the results could be explained by sequential desorption and degradation processes. For the other soil, enhanced degradation was clearly observed for the organisms with first-order and Michaelis-Menten rates. Several explanations are explored for these observations including direct sorbed-phase degradation and the development of elevated substrate concentrations at the organism/sorbent interface. No enhancement was found for the organism with zero-order kinetics.     

Effect of soil composition and dissolved organic matter on pesticide sorption

The effect of the solid and dissolved organic matter fractions, mineral composition and ionic strength of the soil solution on the sorption behaviour of pesticides were studied. A number of soils, chosen so as to have different clay mineral and organic carbon content, were used to study the sorption of the pesticides atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), 2,4-D ((2,4-dichlorophenoxy)acetic acid), isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea) and paraquat (1,1'-dimethyl-4,4'-bipyridinium) in the presence of low and high levels of dissolved organic carbon and different background electrolytes. The sorption behaviour of atrazine, isoproturon and paraquat was dominated by the solid state soil components and the presence of dissolved organic matter had little effect. The sorption of 2,4-D was slightly affected by the soluble organic matter in the soil. However, this effect may be due to competition for adsorption sites between the pesticide and the soluble organic matter rather than due to a positive interaction between the pesticide and the soluble fraction of soil organic matter. It is concluded that the major factor governing the sorption of these pesticides is the solid state organic fraction with the clay mineral content also making a significant contribution. The dissolved organic carbon fraction of the total organic carbon in the soil and the ionic strength of the soil solution appear to have little or no effect on the sorption/transport characteristics of these pesticides over the range of concentrations studied.     

Partitioning of hydrophobic organic compounds within soil-water-surfactant systems

Understanding the partitioning of hydrophobic organic compounds (HOCs) within soil-water-surfactant systems is key to improving the use of surfactants for remediation. The overall objective of this study was to investigate the soil properties that influence the effectiveness of surfactants used to remediate soil contaminated with hydrophobic pesticides, as an example of a more general application for removing strongly sorbing HOCs from contaminated soils via in-situ enhanced sorption, or ex-situ soil washing. In this study, the partitioning of two commonly used pesticides, atrazine and diuron, within soil-water-surfactant systems was investigated. Five natural soils, one nonionic surfactant (Triton-100 (TX)) and one cationic surfactant (benzalkonium chloride (BC)) were used. The results showed that the cation exchange capacity (CEC) is the soil property that controls surfactant sorption onto the soils. Diuron showed much higher solubility enhancement than atrazine with the micelles of either surfactant. Within an ex-situ soil washing system, TX is more effective for soils with lower CEC than those with higher CEC. Within an in-situ enhanced sorption zone, BC works significantly better with more hydrophobic HOCs. The HOC sorption capacity of the sorbed surfactant (K(ss)) was a non-linear function of the amount of surfactant sorbed. For the cationic surfactant (BC), the maximal K(ss) occurred when around 40% of the total CEC sites in the various soils were occupied by sorbed surfactant. Below a sub-saturation sorption range (~20 g/kg), under the same amount of BC sorbed, a soil with lower CEC tends to have higher K(ss) than the one with higher CEC.     

Sorption and desorption of atrazine and diuron onto water dispersible soil primary size fractions

In this study, a low energy separation method was employed to separate water dispersible clay-, silt-, and sand-sized fractions. The batch equilibrium method was used to conduct atrazine and diuron sorption/desorption experiments with the bulk soils and their size fractions separately. A Freundlich sorption model provided the best fit for all sorption and desorption data. A mass balance calculation, taking into account the pesticide concentration differences in the size fraction and bulk soil, showed that pesticide sorption onto the different size fractions reproduces well the total amount of the pesticide sorbed onto the bulk soils. Due to their higher soil organic carbon content, the clay fractions were much more effective sorbents for the pesticides than the bulk soils, silt, and sand fractions. For all soils, the amount of the pesticide sorbed onto the clay fractions was more than 20% of the total amount of the pesticide sorbed by the bulk soils even though the clay fractions in these soils were only 5.3-14.0% (by weight). The clay fractions had the highest desorption hysteresis among all size fractions and the bulk soils, followed by silt fractions, implying the clay fractions had the strongest bound and least desorbable pesticide molecules. Our results suggest that attention should be paid to the pesticide sorbed to the smallest colloids, the water dispersible fraction, which can be potentially mobilized under field conditions, leading to wide spreading of contamination.     

Atrazine sorption on surface soils: time-dependent phase distribution and apparent desorption hysteresis

Non-equilibrium sorption-desorption behavior of atrazine was studied on two surface soils. Impact of sorption contact time was evaluated by interpreting temporal variations in Freundlich sorption isotherm parameters n(t) and K(F)(t) obtained from the phase distribution relationships. The extent of sorption linearity was very similar (n approximately 0.90) for the two soils at all sorption contact times. K(F)(t) increased with contact time and stabilized upon reaching apparent equilibrium. K(F) for woodland soil was significantly higher than that for agricultural soil. The Apparent Hysteresis Index (AHI) parameter was used to quantify sorption-desorption hysteresis arising from non-equilibrium sorption. AHI was a function of the sorption contact time and correlated well with K(F)(t). The woodland soil sorbed more herbicide due to its higher organic matter content. However, a larger fraction of the herbicide sorbed to this soil was released rapidly (within 24 h) following sorptive uptake. The differences in sorption-desorption behavior of atrazine in the two soils appear to be related to variations in the type and location of organic matter in the two soils. The parameters K(F)(t) and AHI(t) consistently demonstrated the effects that arise when batch systems are not brought to equilibrium during sorption studies.     

Sorption and desorption behaviours of 2,4‐D and glyphosate in calcareous soil from Antalya,Turkey

2,4‐Dichlorophenoxyl acetic acid (2,4‐D) and glyphosate are used extensively as a herbicide in vicinity of Antalya, Turkey. Laboratory batch experiments were conducted to investigate the sorption isotherm and sorption‐desorption characteristics of 2,4‐D and glyphosate. Results indicated that degree of sorption of glyphosate was approximately 50 times higher than 2,4‐D (K_d= 34.43 vs. 0.66 L/Kg). The sorption of 2,4‐D and glyphosate was described by linear and rate‐limited processes for soil. Organic carbon content was most likely responsible for sorption behaviour of 2,4‐D and glyphosate. The rapid desorption can be attributed to soft carbon fraction (humic/fluvic acid and lipids) whereas slower desorption can be responsible by hard carbon fraction (black carbon, kerogen) of soils that led to chemically nonideal behaviour (hysteresis). Sorption of 2,4‐D was low due to most likely deactivation of organic carbon surfaces by excess carbonate fraction, whereas strong binding of glyphosate onto organic carbon causing high sorption behaviour.     

Factors affecting sorption of pentachlorophenol to suspended microbial biomass

Sorption and desorption of pentachlorophenol (PCP) to microbial biomass was studied in a series of laboratory experiments. Equilibrium conditions for sorption and desorption were established within 5 min after a step change in concentrations. Linear sorption isotherms were observed at up to 80 μg/l of dissolved PCP at pH 6, and up to 160 μg/l at pH 8. Linear sorption coefficients were primarily influenced by pH, although ionic strength (owing to pH-buffering) and the concentration of dissolved organic matter also had an impact. The influence of these factors was quantified by a simple mathematical model, which included separate sorption coefficients for the neutral and dissociated fraction of PCP.     

Soil particle-size dependent partitioning behavior of pesticides within water-soil-cationic surfactant systems

Cationic surfactants have been proposed for enhanced sorption zones to contain hydrophobic organic compound (HOC) contamination. Benzalkonium chloride (BC), a cationic surfactant, was selected to study the particle-size dependent sorption behavior of the surfactant and its role in the immobilization of two hydrophobic pesticides (atrazine and diuron) within soil-water-surfactant systems for this application. Five different soils were considered in this study. Our results showed significant particle-size dependent behavior for surfactant sorption and pesticide immobilization in the presence of the sorbed cationic surfactant. The cation exchange capacity (CEC) of the bulk soils and their size fractions (clay, silt, and sand fractions) determined BC sorption capacity. In the absence of BC the sand fractions were the least effective sorbent for the pesticides compared with silts and clays. However, at relatively low BC mass sorbed (<10,000mg/kg) to any of the soil fractions, well below sorption saturation, the sand fractions became more effective sorbents for either pesticide than the clay and silt fractions. The pesticide partitioning coefficient onto sorbed BC (K(ss)) was not constant. Particle CEC, availability of CEC sites for sorption of the cationic surfactant, and the amount of the BC sorbed determined the phase of K(ss). The maximum K(ss) occurred before BC saturation sorption capacity was reached and at different % CEC occupancy for the different size fractions. For the clay fractions, the maximum K(ss) occurred at lower % CEC occupancy ( approximately 30-40%) than for the silt and sand fractions. The maximal K(ss) for the sand fractions occurred at the highest % CEC occupancy among all fractions ( approximately 50-60%). These findings suggest that for an in situ surfactant-enhanced sorption zone it may be better to operate well below the saturation sorption of the cationic surfactant. This would enhance sorption of the HOCs onto the immobile fractions (silt and sand fractions) rather than the potentially mobile clay fractions.     

Long-term sewage sludge application and wastewater irrigation on the mineralization and sorption of 17beta-estradiol and testosterone in soils

The disposal of animal manures, wastewater and sewage sludge to agricultural land can lead to the transfer of steroid hormones like 17beta-estradiol and testosterone into soils, surface and groundwaters. The objective of this study was to investigate the effects of different site histories like wastewater irrigation and sewage sludge application on hormone mineralization and sorption in soils. Two agricultural sites with different long-term treatment histories with wastewater and sewage sludge were sampled. The mineralization of (14)C-17beta-estradiol and (14)C-testosterone was studied during incubations at 20 degrees C over three weeks. Despite the structural resemblance of both hormones the mineralization rate of 17beta-estradiol was about an order of magnitude lower than that of testosterone in all four soils, reaching 5-7% vs. 50-59%, respectively. Estradiol mineralization was significantly lower in soils with long-term wastewater irrigation than in the corresponding soil with freshwater irrigation. Pre-incubation of the soils with unlabeled hormones or application of the hormones within a wastewater matrix had only minor effects on their mineralization. The results indicate that estradiol mineralization occurs co-metabolically and is limited by sorption, whereas testosterone appears to be utilized directly by soil microorganisms. Sorption of (14)C-17beta-estradiol and (14)C-testosterone to sterile and unsterile soils was determined in batch experiments with CaCl(2) or wastewater solution with hormone concentrations of 0.13-0.0013 mug mg(-1). FREUNDLICH sorption isotherms and parameters like K(F) and log K(oc) values were used to describe the results. The K(F) values for estradiol sorption were generally about 1.2 to 1.6-fold higher than for testosterone. The SOC-normalized partition coefficients K(oc) also differ accordingly and indicate quite large differences in soil organic matter qualities relating to hormone sorption between the soils and treatments. When the hormones were added to the soil within a wastewater matrix less estradiol was sorbed in the solid phase than in the controls with pure water, thus indicating that wastewater contains soluble sorbents.     

Assessment of olive cake as soil amendment for the controlled release of triazine herbicides

Organic matter-rich agricultural by-products are being produced in huge quantities and can be applied to soil as a disposal strategy. The application of two different rates (2 and 8% w/w) of olive cake to a Mediterranean calcareous soil resulted in an increased sorption of four triazine herbicides, which was higher for the more hydrophobic compounds (terbuthylazine and prometryn) and lower for the more polar ones (simazine and cyanazine). However, when the sorption coefficients were normalised to the total soil organic carbon (K(oc)), the results did not significantly differ between simazine and cyanazine which is an indication that the olive cake did not exert different sorption capacity for both compounds. On the contrary, K(oc) values for terbuthylazine and prometryn increased in the amended soils. Our results from experiments using mixtures of several pesticides suggest that competition for sorption sites resulted in a decrease of herbicide sorption. Desorption was hysteretical both for the amended and unamended soils, but the addition of olive cake at the highest dose diminished desorption of most of the herbicides. In conclusion, the addition of olive cake behaves as a promising method for reducing the risk of groundwater pollution by pesticides.     

Effect of cow slurry amendment on atrazine dissipation and bacterial community structure in an agricultural Andisol

Atrazine is a commonly used herbicide for maize production in Chile, but it has recently been shown to be ineffective in soils that receive applications of cow slurries generated from the dairy industry. This effect may be caused either by the sorption of the pesticide to organic matter or more rapid degradation in slurry-amended soils. The objectives of this study were to evaluate the effects of cow slurry on atrazine dissipation, the formation of atrazine metabolites and the modification of bacterial community in Andisol. The cow slurry was applied at doses of 100,000-300,000 L ha^− 1. After 4 weeks, atrazine was applied to the slurry-amended soils at concentrations of 1-3 mg kg^− 1. The amounts of atrazine and its metabolites were determined by high performance liquid chromatography (HPLC). The soil microbial community was monitored by measurement of CO₂ evolution and changes in bacterial community using PCR-DGGE of 16S rRNA genes. The results show that cow slurry applications had no effect on atrazine dissipation, which had a half-life of 15-19 days. The atrazine metabolites were detected after 20 days and were significantly higher in soils amended with the slurry at both 20 and 40 days after application of the herbicide. Respiration rates were elevated after 10 days in all soils with atrazine addition. Both the atrazine and slurry amendments altered the bacterial community structures, indicated by the appearance of specific bands in the DGGE gels after 10 days. Cloning and sequencing of the 16S rRNA genes from the DGGE gels showed that the bands represented various genera of β-proteobacteria that appeared in response to atrazine. According to our results, further field studies are required to explain the lower effectiveness of atrazine in weed control. These studies may include the effect of dissolved organic carbon on the atrazine mobility.     

Sorption behavior of potential organic wastewater indicators with soils

Soil-aquifer treatment is a wastewater treatment and reclamation option to facilitate beneficial water reuse. The fate of wastewater originated micropollutants in the soil-aquifer system is important to understand. In the study the sorption behavior of potential wastewater indicators such as two antiepileptic drugs (carbamazepine, primidone), one sulfonamide (sulfamethoxazole), and one corrosion inhibitor (benzotriazole) were determined with three natural soils (Lufa 2.2, Euro Soil 5, and Wulpen sand) that differed in pH, organic carbon content and particle size distribution. As aqueous phase a 0.01 M CaCl₂ solution as well as the effluent of a municipal wastewater treatment plant was used. Affinities of all analytes to the soil increased from Wulpen sand, over Lufa 2.2 to Euro Soil 5, indicating that the organic carbon contents might be crucial for sorption. Isotherms were well described by the Freundlich model. Sorption was mainly close to linear (n = 0.93-1.07) for most target compounds and soils. Desorption gave rise to a small hysteresis only for Euro Soil 5 which was likely artificial, due to slow desorption kinetics beyond 24 h used in the experiment. All sorption studies confirmed that Carbamazepine, Benzotriazole and Primidone are appropriate to be used as wastewater indicator substances based on their low sorption affinity to soils, while the suitability of Sulfamethoxazole is limited due to the formation of non-extractable residues, especially at lower pH values.     

Sorption-desorption behavior of triazine and phenylurea herbicides in Kishon river sediments

Sorption and desorption hysteresis of widely applied triazine and phenylurea herbicides were studied for river sediments. Organic carbon normalized sorption coefficient (K_OC) values for all herbicides were significantly higher for the sediment from the downstream region of the river vs. the upstream sediment. On the basis of the measured K_OC values, the triazine herbicides can be arranged in the following order: terbutryn>terbuthylazine>ametryn>atrazine. Among the phenylurea herbicides, chlorotoluron exhibited higher sorption than isoproturon (K_OC values of 137 vs. 60 and 228 vs. 125 L/kg for the upstream and downstream sediments, respectively). Moreover, chlorotoluron exhibited lower desorption potential as compared with isoproturon (apparent hysteresis index values were 0.2–0.3 for chlorotoluron vs. 0.6–0.9 for isoproturon, measured with the upstream sediment). High sorption affinity of chlorotoluron to the sediments is probably due to stronger H-bonding interactions of the herbicide molecules with the sorbents. For both phenylurea herbicides, desorption hysteresis increased with decrease in sorbed amount. This behavior was opposite to the hysteresis trend observed for the triazines. The Cl-triazines (atrazine and terbuthylazine) exhibited higher desorption hysteresis than the S-triazines (ametryn and terbutryn). Therefore, the apparent hysteresis index values calculated for the Cl-triazines were lower than the values of S-triazines (about 0.4 and 0.7, respectively). Based on the relative strength of H-bonding interactions of Cl- and S-triazines with formate anion and on the desorption hysteresis data we suggest a gradient-derived hole-filling sorption mechanism for the triazine herbicides with the river sediments.     

Solubilization and desorption of methyl-parathion from porous media: a comparison of hydroxypropyl-beta-cyclodextrin and two nonionic surfactants

The removal of hydrophobic organic compounds (HOCs) from soils and sediments by water flushing is often constrained by sorption interactions. The development of improved methods for remediation of contaminated soils has emerged as a significant environmental priority. Increasing HOCs desorption and mobility in soil using surfactants is considered to be one of the most suitable on-site techniques for soil remediation. A major concern regarding the use of surfactants for environmental restoration is the potential loss to the environment of large amounts of surfactant through sorption of nonionic types. A study was conducted to investigate whether surfactants and cyclodextrins can be used to enhance the transport of methyl-parathion in a contaminated soil. At aqueous concentrations of surfactants tested, the proportion of each surfactant sorbed to the soil increased with increasing surfactant concentrations. The maximal adsorbed mass is about 5,130 and 14,200 microg/g for Brij 35 and Tween 80, respectively. In the case of nonionic surfactants, sorption attenuates surfactant effectiveness by increasing the organic carbon content of the soil matrix and retarding transport of methyl-parathion through batch and soil column experiments. However, in contrast with the surfactants, hydroxypropyl-beta-cyclodextrin (HPCD) does not interact with the soil tested. The nonreactive nature of cyclodextrins, combined with its large affinity for HOCs suggests that it should have an advantage versus adsorbing surfactants for decreasing HOC distribution coefficients in subsurface systems.     

Aqueous chemistry and interactive effects on non-ionic surfactant and pentachlorophenol sorption to soil

Non-ionic surfactant addition was investigated as a method to remediate pentachlorophenol (PCP) contaminated soil. The goal was to quantify surfactant (Tergitol NP-10 (TNP10)) and PCP sorption to soil and their interactive effects under varying pH, ionic strength, and soil conditions. Up to 16,700 mg/kg of TNP10 partitioned to soil, with increasing sorption far above the critical micelle concentration (CMC) and with greater amounts of PCP present. Approximately 40-45 times more TNP10 and 20-30 times more PCP sorbed to the finer soil with higher organic matter content. Aqueous TNP10 concentrations well above the CMC (>/=5500 mg/L) were required to enhance PCP desorption from the soil. As pH increased by 0.5-0.85 units, TNP10 sorption decreased by 14-25% and PCP sorption as measured by the log of the equilibrium partition coefficient decreased by 1-1.5. A lower ionic strength of 0.03 versus 0.112 M increased PCP desorption from contaminated soil by 5-17% in the presence of TNP10. This work is relevant to designing ex situ soil washing or surfactant-aided PCP remediation.     

Bio-sorption of atrazine in the press-cake from oilseeds

Oilseed press-cake (PC) is proposed as a novel material for the removal of hydrophobic organic pollutants (HOPs) from water. Sorption of the pesticides carbaryl, atrazine and parathion, with log K(ow) being, respectively, 1.59, 2.55 and 3.83, was demonstrated using cold-pressed rapeseed (Brassica napus), moringa (Moringa oleifera) and soybean (Glycine max) PCs. Linear sorption isotherms have been observed. The partition coefficient of carbaryl, atrazine and parathion using rapeseed PC were determined to be 0.028+/-0.003, 0.144+/-0.003 and 2.52+/-0.24 L/g, respectively. Partition studies of atrazine in PC-extracted oil and defatted PC showed that the sorption mechanism is mainly through absorption in the residual oil in the PC, whereas adsorption on the PC matrix is quantitatively much less significant. It was also shown that the oil content of the PC is not the only parameter determining the partitioning of pesticides. Indeed, sorption using ground seeds was very weak, as demonstrated by the low partition and mass transfer coefficients. This may be due to cell structures blocking the pesticide diffusion to the oil-containing structures within the seeds, while for PC oil they are present in the form of small (10 microm) droplets trapped within the hydrophilic PC matrix, thus presenting less resistance for mass transfer.     

Studies of hexavalent chromium attenuation in redox variable soils obtained from a sandy to sub-wetland groundwater environment

Laboratory experiments were conducted to characterize and quantify the capacity and kinetics of the combined effects of natural attenuation processes, such as adsorption, reduction, and precipitation, for hexavalent chromium [Cr(VI)] in a variable geochemical (i.e. fraction of organic carbon [foc], redox) environment of glaciated soils. Equilibrium attenuation terms: linear sorption (K(d)), estimated capacity, and non-linear Langmuir (K(L), Q) sorption parameters; varied over several orders of magnitude. The pseudo-first-order rate of disappearance of Cr(VI) from aqueous:soil slurries ranged from approximately 10(-5) to approximately 10(-1)/min. An operationally defined kinetic attenuation term, attenuation capacity (AC), describing the quantity of Cr(VI) disappearing from the slurries, ranged from 1.1 to approximately 12 microg Cr(VI)/g soil/7 days. The linear K(d)'s and estimated attenuation capacities were indirectly and directly related to increasing soil pH and foc, respectively. The AC values decreased and increased as a function of increasing soil pH and foc, respectively. The parameters determined in this work were used to evaluate the kinetics, capacity, and stability of chromium attenuation in the sub-wetland saturated soils in Hellerich (2004. A field, laboratory, and modeling study of natural attenuation processes affecting the fate and transport of hexavalent chromium in a redox variable groundwater environment. Ph.D. Dissertation, Department of Civil and Environmental Engineering, University of Connecticut-Storrs) using a statistical simulation framework.     

Partitioning of hydrophobic pesticides within a soil-water-anionic surfactant system

Surfactants can be added to pesticide-contaminated soils to enhance the treatment efficiency of soil washing. Our results showed that pesticide (atrazine and diuron) partitioning and desorbability within a soil-water-anionic surfactant system is soil particle-size dependent and is significantly influenced by the presence of anionic surfactant. Anionic surfactant (linear alkylbenzene sulphonate, LAS) sorption was influenced by its complexation with both the soluble and exchangeable divalent cations in soils (e.g. Ca²⁺, Mg²⁺). In this study, we propose a new concept: soil system hardness which defines the total amount of soluble and exchangeable divalent cations associated with a soil. Our results showed that anionic surfactant works better with soils having lower soil system hardness. It was also found that the hydrophobic organic compounds (HOCs) sorbed onto the LAS-divalent cation precipitate, resulting in a significant decrease in the aqueous concentration of HOC. Our results showed that the effect of exchangeable cations and sorption of HOC onto the surfactant precipitates needs to be considered to accurately predict HOC behavior within soil-water-anionic surfactant systems.     

Sorption of organics on clay and synthetic humic-clay complexes simulating aquifer processes

Sorption/partition of several organic solute (contaminants) of a wide range of hydrophobicities was studied on clay and on clay-humic complexes representing aquifer-soil systems. The role of the mineral and of the organic (humic) fractions was elucidated and a model considering both fractions in the sorption process was proposed. The adsorption constants on humic (organic fraction), K_∝, were 8–20 times higher than on “pure” clay, K_m. But with soils with low to medium organic fractions (ƒ_∝ < 0.05) the contribution of the clay mineral to adsorption was quite significant, in spite of the fact that half of the sorption sites on the mineral surface were blocked by the humic. In the range of very low organic content in aquifer soil a non-linear pattern going through a minimum is observed between the overall partition coefficient and soil-organic fraction, transforming to the familiar linear relationship at higher ƒ_∝s. Both the K_∝ and K_m followed the linear-free energy relationship to the octanol-water partition coefficient K_ow.