Distribution behavior of pyrene to adsorbed humic acids on kaolin

The distribution behavior of pyrene on humic acid (HA)-kaolin complexes, prepared by adsorbing HA on kaolin, was investigated by batch experiments. The distribution coefficient (Kd) of pyrene on the HA-kaolin complex increased with the fraction (f(oc)) of organic carbon adsorbed to the surface of the kaolin. This can be attributed to hydrophobic interactions between pyrene and the adsorbed HAs. The effects of adsorbed HAs were quantitatively evaluated by calculating the distribution coefficient (K(oc)) and affinity constant (K(oc)ads) for pyrene to the adsorbed HAs. A fluorescence quenching method was employed to determine the affinity constant (K(oc)aq) of pyrene to HAs dissolved in an aqueous solution. When the K(oc) values were compared with the K(oc)aq values, the K(oc) values were found to be 4 to 11 times larger than the K(oc)aq values. On the other hand, the K(oc)ads values were 4 to 9 times larger than the K(oc)aq values. These indicate that the affinity for pyrene is enhanced by the adsorption of HAs to kaolin. In addition, the K(oc) values increased with increasing average molecular weights of the HAs. These results demonstrate that HAs, when they are adsorbed to clay minerals, play an important role in the deposition of polycyclic aromatic hydrocarbons (PAHs) in a soil environment.     

Effects of mineral surfaces on pyrene partitioning to well-characterized humic substances

Mineral surfaces can alter the ability of humic substances (HS) to bind hydrophobic organic contaminants. In this study, complete adsorption (i.e., to avoid HS adsorptive fractionation effects) of a small subset of well-characterized terrestrial and aquatic HS on kaolinite and hematite significantly changed their subsequent organic carbon-normalized partition coefficients K(ads)(oc) for pyrene relative to their original respective dissolved organic carbon-normalized partition coefficients K(dis)(oc). Parallel experiments with ultrafiltration (UF) fractions obtained from purified Aldrich humic acid (PAHA) (Aldrich Chemical, Milwaukee, WI) gave similar results. The heterogeneity among the PAHA UF fractions was examined via their mineral surface adsorption characteristics and their subsequent ability to bind pyrene. As expected, variations in maximum adsorption densities (q(max)), Langmuir adsorption constants (K(q)), and pyrene K(ads)(oc) values were observed among the PAHA UF fractions. However, general trends of q(max), K(q), and pyrene log K(ads)(oc) values for the PAHA UF fractions versus the logarithm of their weight-average molecular weights (MW(w)) did not typically match the corresponding trends obtained with the four aquatic and terrestrial HS. In general, an ideal mixture competitive adsorption model gave reasonable predictions for PAHA sorption to kaolinite and hematite based on their corresponding UF isotherm parameters. Ideal mixture predictions of pyrene partitioning to adsorbed PAHA from the corresponding UF fraction results were better for kaolinite versus hematite, indicating that the underlying mineral surface can alter the effects of HS heterogeneity on hydrophobic organic contaminant sorption.     

Sorption of sulfonamide pharmaceutical antibiotics on whole soils and particle-size fractions

Residues of pharmaceutical antibiotics are found in the environment, whose fate and effects are governed by sorption. Thus, the extent and mechanisms of the soil sorption of p-aminobenzoic acid and five sulfonamide antibiotics (sulfanilamide, sulfadimidine, sulfadiazine, sulfadimethoxine, and sulfapyridine) were investigated using topsoils of fertilized and unfertilized Chernozem and their organic-mineral particle-size fractions. Freundlich adsorption coefficients (K(f)) ranged from 0.5 to 6.5. Adsorption increased with aromaticity and electronegativity of functional groups attached to the sulfonyl-phenylamine core. Adsorption to soil and particle-size fractions increased in the sequence: coarse silt < whole soil < medium silt < sand < clay < fine silt and was influenced by pH. Sorption nonlinearity (1/n 相似文献   

Cation-pi bonding: a new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces

Recent molecular modeling and spectroscopic studies have suggested that relatively strong interactions can occur between aromatic pi donors and metal cations in aqueous solutions. The objective of this study was to characterize potential cation-pi interactions between pi donors and exchangeable cations accumulated at mineral surfaces via both spectroscopic and batch sorption methods. Quadrupolar splitting in deuterium nuclear magnetic resonance ((2)H NMR) spectroscopy for d(2)-dichloromethane, d(6)-benzene, and d(8)-toluene (C(6)D(5)- moiety) in aqueous suspensions of a Na-saturated reference montmorillonite unambiguously indicated the ordering of solute molecules with respect to the clay surface. The half line broadening (Delta nu(1/2)) of (2)H NMR of d(6)-benzene in montmorillonite suspensions showed that soft exchangeable cations generally resulted in more benzene sorption compared with harder cations (e.g., Ag(+) > Cs(+) > Na(+) > Mg(2+), Ba(2+)). In batch sorption experiments, saturating minerals (e.g., porous silica gels, kaolinite, vermiculite, montmorillonite) with a soft transition metal or softer base cations generally increased the polycyclic aromatic hydrocarbon (PAH) sorption relative to harder cations (e.g., Ag(+) > Cs(+) > K(+) > Na(+); Ba(2+) > Mg(2+)). Sorption of phenanthrene to Ag(+)-saturated montmorillonite was much stronger compared with 1,2,4,5-tetrachlorobenzene, a coplanar non-pi donor having slightly higher hydrophobicity. In addition, a strong positive correlation was found between the cation-dependent sorption and surface charge density of the minerals (e.g., vermiculite, montmorillonite > silica gels, kaolinite). These results, coupled with the observations in (2)H NMR experiments with montmorillonite, strongly suggest that cation-pi bonding forms between PAHs and exchangeable cations at mineral surfaces and affects PAH sorption to hydrated mineral surfaces.     

Can assessing for potential contribution of soil organic and inorganic components for butachlor sorption be improved?

Sorption of butachlor to various types of common soil components was investigated. Six pure minerals (montmorillonite [Mont], kaolinite [Kaol], Ca homoionic montmorillonite [Ca-Mont] and kaolinite [Ca-Kaol], amorphous hydrated Al and Fe oxides [AHOs-Al, AHOs-Fe]), four soil alkali-extractable pure humic acids (HAs), and the four corresponding HAs originated real unmodified and HO-treated soils were selected as the representative sorbents. Results showed that the HAs played a crucial role, and clay minerals (especially Mont) also showed an important effect in butachlor sorption. The AHOs may likely influence only in a mediator way by enhancing the availability of sorption domains of HAs. By removing 78% (on average) of the total organic carbon (TOC) from the soils with HO, the content ratio of clay to TOC (RCO) increased by an average of 367% and became >60. This change simultaneously decreased the sorption capacity of soils (40%, on average). Considering that the surface sorption domain on clay minerals may be highly exposed and more competitive after the partial removal of soil organic matter (SOM), this reaffirmed the potential contribution from clay minerals. It can thus be inferred that in the real soil where SOM and clay minerals are associated, the coating of clay minerals may have weakened the partition function of SOM or blocked some sorption domain within SOM, resulting in a decreased sorption of butachlor. Therefore, clay minerals, especially 2:1 type expanding minerals, may play a dual function vs. SOM content for the sorption of butachlor in soil.     

Adsorption of isoxaflutole degradates to aluminum and iron hydrous oxides

Isoxaflutole is a preemergence herbicide that has been marketed as a substitute for atrazine. It is rapidly transformed to a more stable and soluble diketonitrile degradate (DKN) after field application and can further degrade to a benzoic acid degradate (BA) within soil. However, no previous research has been conducted to investigate DKN and BA sorption to metal oxide minerals. The primary objective of this research was to elucidate the interactions of DKN and BA with synthetic hydrous aluminum and iron oxides (HAO and HFO, respectively) to understand how variably charged minerals may influence adsorption of these compounds in soil. The herbicide degradates did adsorb to HAO and HFO, and the data were well described by the Freundlich model (R2 > 0.91), with Nvalues ranging from 0.89 to 1.2. Adsorption isotherms and Kd values demonstrate that BA is adsorbed to HFO to a greater extent than other degradate-mineral combinations that were studied. The degree of hysteresis between adsorption/desorption isotherms was characterized as slight (hysteresis index values < 1.7), suggesting weak DKN and BA retention to HFO and HAO oxide surfaces. Degradate adsorption was observed to greatly diminish as suspension pH increased. Attenuated total reflectance-Fourier transform infrared spectra show no evidence that DKN or BA adsorb to mineral surfaces as inner-sphere complexes under hydrated conditions. Instead DKN and BA adsorb to positively charged metal oxide surfaces as outer-sphere or diffuse ion swarm complexes via electrostatic attraction. This research indicates that metal oxides may serve as important retardants for DKN and BA migration through acidic soils enriched with aluminum and iron oxides.     

Sorption of aromatic compounds to clay mineral and model humic substance-clay complex: effects of solute structure and exchangeable cation

Clay minerals and humic substance (HS)-clay complexes are widely distributed in soil environments. Improved predictions on the uptake of organic pollutants by soil require a better understanding of fundamental mechanisms that control the relative contribution from organic and inorganic constituents. Five selected aromatic compounds varying in electronic structure, including nonpolar phenanthrene (PHEN), 1,2,4,5-tetrachlorobenzene (TeCB), polar 1,3-dinitrobenzene (DNB), 2,6-dichlorobenzonitrile (dichlobenil [DNL]), and 1-naphthalenyl methylcarbamate (carbaryl [CBL]), were sorbed separately from aqueous solution to Na(+)-, K(+)-, Cs(+)-, and Ca(2+)-saturated montmorillonites with and without the presence of dissolved HS at pH about 6. Upon normalizing for hydrophobic effects by solute aqueous solubility, the overall trend of sorptive affinity to HS-free K(+)-clay is DNB > DNL, CBL > PHEN, TeCB, indicating preferential adsorption of the polar solutes. With the presence of HS, sorption of PHEN, TeCB, and CBL increases by several times compared with the pure clay, attributed to HS-facilitated hydrophobic partition (PHEN and TeCB) or H-bonding (CBL). The enhanced sorption of PHEN by HS is cation dependent, where Cs(+) shows the strongest facilitative effect. Coadsorption of HS does not affect sorption of DNB and DNL to clays except that of DNB to Ca(2+)-clay because cation-dipole interactions between the polar group (NO(2) or CN) of solute and weakly hydrated exchangeable cations dominate the overall sorption.     

Adsorption of pesticides onto quartz, calcite, kaolinite, and alpha-alumina

The fate of pesticides in aquifers is influenced by the small but not insignificant adsorption of pesticides to mineral surfaces. Batch experiments with five pesticides and four minerals were conducted to quantify the contributions to adsorption from different mineral surfaces and compare adsorption characteristics of selected pesticides. Investigated mineral phases included quartz, calcite, kaolinite, and alpha-alumina. Selected pesticides comprised atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea)], mecoprop [(RS)-2-(4-chloro-2-methylphenoxy)propionic acid], 2,4-D (2,4-dichlorophenoxyacetic acid), and bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide]. Specific surface area and mineral surface charge proved to be important for the adsorption of these pesticides. Detectable adsorption of the anionic pesticides (mecoprop, 2,4-D, and bentazone) was only measured when positive sites were present on the mineral surface. However, when CaCl2 was added as an electrolyte, a detectable adsorption of mecoprop and 2,4-D was also measured on kaolinite (which exhibits a negative surface charge), probably due to formation of Ca-pesticide--surface complexes. Adsorption of the uncharged pesticides (atrazine and isoproturon) was detected only on kaolinite. The lack of adsorption on alpha-alumina indicates that the uncharged pesticides have a greater affinity for the silanol surface sites (=SiOH) than for the aluminol surface sites (=AlOH) in kaolinite. No measurable effect of ionic strength was found for the uncharged pesticides. The results indicate that quartz and calcite play a smaller role than clay minerals.     

Interactions of carbamazepine in soil: effects of dissolved organic matter

Pharmaceutical compounds (PCs) and dissolved organic matter (DOM) are co-introduced into soils by irrigation with reclaimed wastewater. We targeted carbamazepine (CBZ) as a model compound to study the tertiary interactions between relatively polar PCs, DOM, and soil. Sorption-desorption behavior of CBZ was studied with bulk clay soil and the corresponding clay size fraction in the following systems: (i) without DOM, (ii) co-introduced with DOM, and (iii) pre-adsorption of DOM before CBZ introduction. Sorption of the DOM to both sorbents was irreversible and exhibited pronounced sorption-desorption hysteresis. Carbamazepine exhibited higher sorption affinity and nonlinearity, and a higher degree of desorption hysteresis with the bulk soil than the corresponding clay size fraction. This was probably due to specific interactions with polar soil organic matter fractions that are more common in the bulk soil. Co-introduction of CBZ and DOM to the soil did not significantly affect the sorption behavior of CBZ; however, following pre-adsorption of DOM by the bulk soil, an increase in sorption affinity and decrease in sorption linearity were observed. In this latter treatment, desorption hysteresis of CBZ was significantly increased for both sorbents. We hypothesize that this was due to either strong chemical interactions of CBZ with the adsorbed DOM or physical encapsulation of CBZ in DOM-clay complexes. Based on this study, we suggest that DOM facilitates stronger interactions of polar PCs with the solid surface. This mechanism can reduce PC desorption ability in soils.     

Factors affecting the ratio of cation exchange capacity to clay content in lignite overburden

Unusually high cation exchange capacity (CEC) values relative to clay content are frequently reported for lignite overburden and minesoils. The CEC to percent clay ratio is commonly greater than one and would require that the average charge of the clay fraction be greater than 100 cmol(c) kg(-1). A comparison of methods for particle-size distribution suggests that the major reason lignite overburden samples have CEC to percent clay ratios greater than one is incomplete dispersion of aggregates of clay minerals or shale fragments. Preliminary investigations revealed the presence of shale fragments, smectite, and partially weathered mica in the silt fraction. Methods commonly used in soil textural analysis underestimated clay content by approximately 24%. The silt fraction may, therefore, provide a "hidden" source of CEC. Another important factor influencing the CEC to percent clay ratio was the presence of organic materials (lignite) in the samples. Lignite may make a significant contribution to CEC in overburden materials. In a study designed to estimate the pH-dependent charge of both the mineral and organic fractions, the CEC of overburden organic constituents was determined to be approximately 158 cmol(c) kg(-1) at pH 8.2. The high CEC to percent clay ratio in lignite overburden and minesoils may be resolved by adjusting methods for clay determination to optimize dispersion and by accounting for CEC due to organic materials. An alternative approach is to use existing methodology and use correction factors to account for incomplete dispersion of clay minerals and the charge contributions of organic materials.     

Sorption and stability of the polycyclic nitramine explosive CL-20 in soil

The polycyclic nitramine CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) is being considered for use as a munition, but its environmental fate and impact are unknown. The present study consisted of two main elements. First, sorption-desorption data were measured with soils and minerals to evaluate the respective contributions of organic matter and minerals to CL-20 immobilization. Second, since CL-20 hydrolyzes at a pH of >7, the effect of sorption on CL-20 degradation was examined in alkaline soils. Sorption-desorption isotherms measured using five slightly acidic soils (5.1 < pH < 6.9) containing various amounts of total organic carbon (TOC) revealed a nonlinear sorption that increased with TOC [K(d) (0.33% TOC) = 2.4 L kg(-1); K(d) (20% TOC) = 311 L kg(-1)]. Sorption to minerals (Fe(2)O(3), silica, kaolinite, montmorillonite, illite) was very low (0 < K(d) < 0.6 L kg(-1)), suggesting that mineral phases do not contribute significantly to CL-20 sorption. Degradation of CL-20 in sterile soils having different pH values increased as follows: sandy agricultural topsoil from Varennes, QC, Canada (VT) (pH = 5.6; K(d) = 15 L kg(-1); 8% loss) < clay soil from St. Sulpice, QC, Canada (CSS) (pH = 8.1; K(d) = 1 L kg(-1); 82% loss) < sandy soil provided by Agriculture Canada (SAC) (pH = 8.1, K(d) = approximately 0 L kg(-1); 100% loss). The faster degradation in SAC soil compared with CSS soil was attributed to the absence of sorption in the former. In summary, CL-20 is highly immobilized by soils rich in organic matter. Although sorption retards abiotic degradation, CL-20 still decomposes in soils where pH is >7.5, suggesting that it will not persist in even slightly alkaline soils.     

Imazaquin adsorbed on pillared clay and crystal violet-montmorillonite complexes for reduced leaching in soil

Ground water pollution due to herbicide leaching has become a serious environmental problem. Imazaquin [2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)quinoline-3-carboxylic acid] is an herbicide used to control broadleaf weeds in legume crops. Imazaquin is negatively charged at the basic pH of calcareous soils and exhibits high leaching potential in soils. Our aim was to design formulation of imazaquin to reduce herbicide leaching. Imazaquin sorption on pillared clay (PC) and crystal violet (CV)-montmorillonite complexes was studied. The CV-montmorillonite complexes become positively charged with adsorption of CV above the cation exchange capacity (CEC) of montmorillonite, and thus can sorb imazaquin. The Langmuir equation provides a good fit to isotherms of imazaquin sorption on PC and CV-montmorillonite complexes, but for charged complexes an equation that combines electrostatics with specific binding was preferred. Maximal imazaquin sorption was 17.3 mmol kg-1 for PC and 22.2 mmol kg-1 for CV-montmorillonite complexes. The extents of imazaquin desorption into water were 21% for PC and 5% for CV-clay complexes. The presence of anions decreased imazaquin sorption on both sorbents in the sequence phosphate > acetate > sulfate. Reduction of imazaquin sorption by the anions and the extent of its desorption in electrolyte solutions were higher for PC than for CV-clay complexes. Leaching of imazaquin from CV-montmorillonite formulations through soil (Rhodoxeralf) columns was two times less than from PC formulations and four times less than that of technical imazaquin. The CV-montmorillonite complexes at a loading above the CEC appear to be suitable for preparation of organo-clay-imazaquin formulations that may reduce herbicide leaching significantly.     

Factors controlling sorption of prosulfuron by variable-charge soils and model sorbents

Prosulfuron [1-(4-methoxy-6-methyltriazin-2-yl)-3-[2-(3,3,3-trifluoropropyl) phenylsulfonyl]-urea), a relatively new sulfonylurea herbicide, is a weak acid (pK(a) 3.76), and therefore, will undergo pH-dependent speciation and sorption. Understanding prosulfuron sorption in soils is important for predicting its environmental fate. Soil and solution factors controlling sorption were investigated by measuring prosulfuron sorption on five model sorbents (amorphous silica, alpha-alumina, CaSWy1 montmorillonite, commercial humic acid, and anion exchange resin) and 10 variable-charge soils from CaCl(2) and Ca(H(2)PO(4))(2) solutions as a function of pH and ionic strength. Anion exchange of prosulfuron accounted for up to 82% of overall sorption in the pH range from 3 to 7. The relative importance of anion exchange to prosulfuron sorption was positively correlated to the ratio of anion and cation exchange capacities. Comparison between organic carbon (OC)-normalized sorption (K(oc)) versus pH for humic acid and variable-charge soils show similar trends with sorption decreasing with increasing pH. However, K(oc) values estimated from variable-charge soils in the lower pH range where anion exchange has the greatest contribution to sorption was almost one log unit greater than that estimated from humic acid clearly exemplifying the impact of anion exchange. Similarity in K(oc)-pH curves for humic acid and variable-charge soils may result from the fact that (i) cation exchange capacity increases with increasing OC content, thus effective anion exchange capacity is reduced; and (ii) the relative contribution of hydrophobic and hydrophilic sorption mechanisms was fairly constant. Given that both hydrophilic and hydrophobic sorption of prosulfuron decrease with increasing pH, addition of fertilizer and lime amendments may enhance the potential for off-site leaching of recently applied acidic pesticides.     

Phenanthrene sorption to structurally modified humic acids

Several studies emphasize the importance of soil organic matter characteristics in hydrophobic contaminant sorption and outline the strong dependence of sorption on organic matter aromaticity. In this study, the role of organic matter aromaticity in phenanthrene sorption was investigated using humic acids (HAs) from compost, peat, and soil that were structurally modified by bleaching, hydrolysis, oximation, and subcritical water extraction. The HAs were characterized with cross polarization magic angle spinning carbon-13 nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy and used in batch equilibrations with phenanthrene. Bleaching substantially reduced the aromaticity of the samples whereas the other treatments increased the relative aromaticity. Phenanthrene sorption increased, even though there was a substantial reduction in sorbent aromaticity with some samples. The HAs that exhibited comparable CPMAS 13C NMR spectra and aromaticity did not behave similarly with respect to phenanthrene sorption. When the sorption data (K(oc) values) were correlated to sample aromaticity, the correlation coefficients (r2) did not exceed 0.39. Comparisons with the atomic H to C ratio provided slightly better r2 values (up to 0.54). This study demonstrates that macroscopic sorbent characteristics could not explain the observed phenanthrene sorption coefficients, aliphatic structural components of HAs can contribute appreciably to phenanthrene sorption, and organic matter physical conformation may regulate access to organic matter structures. Therefore, the use of only macroscopic sorbent properties, such as aromaticity, to predict and rationalize sorption values cannot solely be used to explain the behavior of organic contaminants in soil environments.     

Adsorption and clay-catalyzed degradation of erythromycin A on homoionic clays

Erythromycin has been widely used in food-producing animals and in humans, and is frequently detected as an organic pollutant in U.S. streams. In batch experiments with homoionic clays, the Freundlich isotherms were determined at 10 and 25 degrees C. The adsorption of erythromycin A was strongly influenced by clay type, exchanged cations, the pH of the bulk solutions, and the acidity of clay surfaces. The formation of clay-erythromycin A complexes was thermodynamically favorable except for K+- and Fe3+-exchanged montmorillonites, since the reactions were exothermic (deltaH(o) > 0) and the systems became stable (deltaS(o) > 0). Clays catalyzed the erythromycin A degradation by the hydrolysis of the neutral sugar and the multiple dehydrations. The surface acidity of clay surface enhanced the rate of clay-catalyzed degradation of erythromycin A. In addition, the Fe3+-exchanged clay minerals seemed to have an electrostatic interaction with the erythromycin A molecule, by which the hydrolysis of the neutral sugar was influenced.     

Effect of heat treatment on sorption of polar and nonpolar compounds to montmorillonites and soils

Batch sorption isotherms of 1,3,5-trichlorobenzene, 1,3,5-trinitrobenzene, and tetracycline to organic-free montmorillonites and soils receiving heat treatment (375°C for 24 h) were compared with those to unheated sorbents. Sorption of the nonpolar 1,3,5-trichlorobenzene to soil was lowered after the removal of humus by heating, consistent with the mechanism of hydrophobic partition into organic matter. For 1,3,5-trinitrobenzene, the enhanced sorption to heated soils was attributed to specific interactions with exchangeable cations facilitated by heating-induced irreversible partial dehydration of the clay interlayer. For tetracycline, an additional mechanism for sorption enhancement could be due to increased exposure of strong complexation sites on clay minerals after removal of the humic coating. These hypotheses were supported by the sorption data to heated and unheated Na-, K-, and Cs-saturated montmorillonites. The combustion method is commonly adopted to measure the content of black carbon in soils and sediments. However, findings from the present study indicate that combustion may greatly modify the structural properties of clay minerals, leading to misinterpreted sorption contributions of different soil components to sorption of polar or ionic compounds.     

Contamination time effect on lead and cadmium fractionation in a tropical coastal clay

The capability of a tropical coastal clay to immobilize lead (Pb) and cadmium (Cd) was investigated in laboratory batch sorption tests conducted under acidic, neutral, and slightly alkaline conditions. The contact time was extended to 65 d. The distribution of Pb and Cd among various sorbed phases was examined using a sequential extraction technique. The sorbed phases were fractionated into the exchangeable, carbonate, reducible, organic, and residual fractions. There were only small changes in the total Pb and Cd sorption beyond a 1-d sorption period. The metal fractionation results show that the amount of Pb and Cd in various fractions changed with sorption time, and the changes were pH-dependent. These changes could be attributed to mineral dissolution and transformation or redistribution of the sorbed phases. Transformation of the sorbed phases resulted in increasing Pb and Cd retention in the more persistent fractions with time, at the expense of reductions in the loosely bound fractions. Nevertheless, Pb and Cd fractionation in the solid phase appeared to reach equilibrium within the 65-d sorption period. These Pb and Cd fractionation results reflect the effect of contamination time on the heavy metal lability and bioavailability in the subsurface environment.     

Soil sorption of acidic pesticides: modeling pH effects

A model of acidic pesticide sorption in soils was developed from theoretical modeling and experimental data, which initially considered a combination of a strongly acidic pesticide and a variable-charge soil with high clay content. Contribution of 2,4-D [(2,4-dichlorophenoxy) acetic acid] anionic-form sorption was small when compared with molecular sorption. Dissociation of 2,4-D was not sufficient to explain the variation in Kd as a function of pH. Accessibility of soil organic functional groups able to interact with the pesticide (conformational changes) as a function of organic matter dissociation was proposed to explain the observed differences in sorption. Experimental 2,4-D sorption data and K(oc) values from literature for flumetsulam [N-(2,6-difluorophenyl)-5-methyl [1,2,4] triazolo [1,5-a] pyrimidine-2-sulfonamide] and sulfentrazone [N-[2,4-dichloro-5-[4-(difluromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl] phenyl] methanesulfonamide] in several soils fit the model.     

Sorption kinetics and equilibria of organic pesticides in carbonatic soils from South Florida

A batch reactor was used to determine sorption kinetic parameters (k2, F, and K*) and the equilibrium sorption coefficient (K). The two-site nonequilibrium (TSNE) batch sorption kinetics model was used to calculate the kinetic parameters. Two probe organic pesticides, atrazine [2-chloro-4-ethylamino-6-isopropylamino-s-triazine] and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] were studied using three carbonatic soils from South Florida (Chekika, Perrine, and Krome), one noncarbonatic soil from Iowa (Webster), and one organic soil (Lauderhill) from South Florida. Carbonatic soils contained more than 600 g kg(-1) CaCO3. Sorption is initially very fast up to 3 h and then slowly reaches equilibrium. All soil-chemical combinations reached sorption equilibrium after about 24 h and all sorption isotherms were linear. The sorption kinetics data were well described by the TSNE model for all soil-chemical combinations except for the marl soil data (Perrine-Atrazine), which were better described by the one-site nonequilibrium (OSNE) model. Diuron, with higher K, undergoes slower sorption kinetics than atrazine. The Lauderhill soil containing organic carbon (OC) of 450 g kg(-1) exhibited slowest sorption kinetics for both pesticides. An inverse relationship between k3 and K was observed for atrazine and diuron separately in Chekika, Webster, and Lauderhill soils but not in Perrine and Krome soils. The sorption kinetic parameters were used to distinguish the sorption behavior between atrazine and diuron and to identify differences between soils. Normalizing the sorption coefficient (K) to OC showed that atrazine and diuron had K oc values in carbonatic soils that were a third of reported literature values for noncarbonatic soils. Using existing literature K oc values in solute transport models will most likely underestimate the mobility of atrazine, diuron, and other neutral organic chemicals in carbonatic soils.     

Size distribution of organic matter and associated propiconazole in agricultural runoff material

Sorption and desorption characteristics of propiconazole (1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl]methyl]-1H-1,2,4-triazole) to different particle/aggregate-size fractions of agricultural runoff material were investigated. Emphasis was put on clay and colloidal size fractions to evaluate their role as potential sorbents and carriers for this pesticide. The runoff material was separated into size fractions ranging from 2 mm to ca. 15 nm by wet sieving, sedimentation, centrifugation, and membrane ultrafiltration. Each fraction was characterized by its organic C content and C/N ratio. Distinctive sorption properties of clay-sized particles and colloids were investigated. The obtained size fractions differed significantly in their organic C concentration, C/N ratio, and sorption properties to propiconazole. Organic matter was mainly associated in aggregates >2 microm. Binding of propiconazole to this coarse fraction made up 80% of the sorbed propiconazole. The distribution coefficient between solid and aqueous phases increased with decreasing particle size. The colloidal fraction (<0.16 microm) exhibited the highest sorbtivity, with a distribution coefficient of 113 L kg(-1), which was more than four times higher than that in the bulk sample (27 L kg(-1)). The fraction <2 microm represented 8% of the total sample weight, but contributed to 20% of the sorbed propiconazole. Strong hysteresis was observed for the sorption-desorption of propiconazole on the runoff material. Under dilution very little sorbed propiconazole will be released into the water phase. Due to its high sorbtivity and mobility and the strong sorption-desorption hysteresis, particles in the fraction <2 microm can be important carriers of propiconazole in runoff suspensions with high sediment load.