Potential contributions of smectite clays and organic matter to pesticide retention in soils

Soil organic matter can delay-but not eliminate-leaching of neonicotinoid insecticides

Soil organic matter (SOM) retains and attenuates many contaminants; however, its interactions with neonicotinoid insecticides under field conditions remain poorly understood. The goal of this study was to determine if SOM influences the persistence or leaching of two neonicotinoid insecticides: thiamethoxam (TMX) and its transformation-product clothianidin (CLO). Thiamethoxam-coated soybean [Glycine max (L.) Merr.] was planted into a clay soil containing different soil organic carbon (SOC) concentrations. Leachate and soil samples were collected for 10 wk after planting and were analyzed for insecticide concentrations using liquid chromatography-tandem mass spectrometry. Single and multiple linear regressions were performed between SOC, leaching volumes, and measured insecticide concentrations, focusing on rainfall events near the beginning, middle, and end of the study. Correlations were also tested between SOC and cumulative mass of leached insecticides. Neither SOC nor per-event leachate volumes explained variability in TMX leaching or residual CLO concentrations in soils; however, by the conclusion of the study residual thiamethoxam concentrations in soil were negatively correlated with cumulative volume of leached water. Initially, the concentration and total mass of leached CLO were significantly and negatively correlated with SOC content; however, this effect faded with time. Leachate dynamics also affected CLO transport, with positive correlations between leachate volume and CLO concentration during the latter events. This analysis demonstrates that SOM can reduce peak loading of neonicotinoids but may not alter cumulative leaching over the entire growing season.

Interference between di(2-ethylhexyl) phthalate and heavy metals (Cd and Cu) in a Mollisol during aging and mobilization

Diffuse pollution of the soil by phthalates and heavy metals causes numerous concerns. Their respective fates when coexisting require further investigation. In this study, di(2-ethylhexyl) phthalate (DEHP) and Cd/Cu were used as subjects, focusing on their behavior in Mollisols under combined pollution based on their concentration, fractionation, and leaching. The results indicated that when the two pollutants coexist, the dissipation rate of DEHP in the soil decreased, and its half-life was extended from 30.81 to 40.53 (Cd) and 35.40 d (Cu). DEHP altered the distribution of Cd and Cu in the soil, and this effect persisted after most of the DEHP had degraded. Leaching tests showed that the interaction of DEHP with Cd and Cu hindered leaching during the first rainfall event, but as DEHP degraded and Cd/Cu stabilized, the trapped pollutants were gradually released in subsequent rainfall events. Additionally, to investigate the partitioning of pollutants between soil water and solid surfaces, a diffusion model of DEHP and metal ions on the surface of montmorillonite (high specific surface area adsorbents abundant in soils) was built using molecular dynamics simulations. Simulations revealed their density distribution on the clay surface increased synergistically, whereas their diffusion was antagonistic. This study provides basic data and theoretical support concerning the ecological risk assessment of combined phthalate and heavy metals pollution in soil.

Emerging Contaminant Imidacloprid in Mediterranean Soils: The Risk of Accumulation Is Greater than the Risk of Leaching

Imidacloprid (IMI) is an extensively used neonicotinoid insecticide whose occurrence in the environment is a worldwide problem. Its sorption/transport properties are recognized as one of the key knowledge gaps hindering policymaking regarding its international routine monitoring in soils. Therefore, we studied IMI transport behaviour in Croatian Mediterranean soils using column experiments. Breakthrough curves were analysed using the two-site adsorption model and compared against dimethoate (DIM). Transport parameters were correlated to soil physicochemical properties. The results indicate that IMI shows a high degree of preference for soil organic matter over any other soil constituent. For IMI, the clay did not exhibit any sorption activity, while hematite did act as an active sorbent. Contrarily, hematite increased the leachability of DIM by blocking the active sorption sites on clay platelets. Both hematite and clay sorption acted as type-2 (i.e., rate-limiting) sites. In all soils, IMI exhibited lower short-term leachability than DIM. Combined with a body of data concerning other aspects of IMI environmental behaviour, the results indicate that the risk of accumulation of IMI in the soil is greater than the risk of contamination by leaching. Thus, continuous monitoring of IMI in soils should be incorporated into future soil health protection programs.

The photodegradation processes and mechanisms of polyvinyl chloride and polyethylene terephthalate microplastic in aquatic environments: Important role of clay minerals

It is well known that microplastics (MPs) may experience weathering and aging under ultraviolet light (UV) irradiation, but it remains unclear if these processes are impacted by natural components, such as clay minerals. In this study, we systematically investigated the photodegradation behaviors of polyvinyl chloride (PVC) and poly (ethylene terephthalate) (PET), two utmost used plastics, in the presence of clay minerals (kaolinite and montmorillonite). The results demonstrated that the clay minerals, particularly kaolinite, significantly promoted the MPs photodegradation, and the aging of PET was more prominent. The photodegradation was the most distinct at pH 7.0, regardless of the presence or absence of the clay minerals. The results of electron paramagnetic resonance and inhibition experiments of reactive oxygen species indicated that the minerals, particularly kaolinite, remarkably facilitated production of •OH, which was the key species contributing to the photodegradation of MPs. Specifically, UV irradiation facilitated the photo-ionization of MPs, producing hydrated electrons and MP radical cations (MP⁺). The Lewis base sites prevalent on the clay siloxane surfaces could stabilize the MP radical cations and prevent their recombination with hydrated electrons, which promoted the generation of •OH under aerobic conditions, and facilitated the degradation of MP. Two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (COS) analysis and ultra-high-performance liquid chromatography coupled to a Q Exactive Orbitrap HF mass spectrometer were used to identify the sequential changes of functional groups, and the degradation products of the MPs. This study improves our understanding on the aging of MPs in the complex natural environment.

Effects of Fe(III) and Cu(II) on the sorption of s-triazine herbicides on clay minerals

The effects of Fe(III) and Cu(II) on the sorption of atrazine (AT) and prometryn (PY) on clay minerals were investigated both preloaded and in solution. For smectite, Fe(III) preloading greatly enhanced AT and PY sorption at pH 4.0 and 6.0 but diminished AT sorption at pH 8.0. Cu(II) preloading promoted AT and PY sorption under alkaline conditions but suppressed AT sorption at pH 4.0. The adverse effects were not obvious for PY. While for illite and kaolinite, Fe(III) and Cu(II) had little or promotion effects due to the lower contents of them in these two minerals. In the co-sorption studies, for smectite, AT sorption remained at pH 4.0 and increased at pH 6.0 and 8.0, while PY sorption was inhabited over the pH range of 4.0-8.0 in the presence of Fe(III). AT and PY sorption were not affected by Cu(II) except for PY at pH 8.0, in which case, the sorption was promoted. For illite and kaolinite, Fe(III) and Cu(II) generally enhanced AT and PY sorption.

Sorption and leaching potential of organophosphorus insecticide dimethoate in Croatian agricultural soils

Dimethoate is an organophosphorus insecticide still used in Croatia and worldwide, with polar structure and high water solubility that make it prone to leaching. This study analyzed how physico-chemical properties of soils affected dimethoate sorption and mobility. For that purpose, five soil samples were collected from three Croatian regions (two coastal and one mountain region). Dimethoate sorption process was analyzed using the batch procedure while its mobility and leaching potential was investigated by column experiment. The results showed that dimethoate sorption can be adequately described by Freundlich model. All isotherms were of L-type with varying degrees of non-linearity, indicating different sorption efficiencies and distribution of sorption sites energies among the soils. Energy distribution was broader in soils richer in organic matter (OM). K_F values indicated relatively low sorption efficiency for all soils, with an increase of K_F values proportional to OM content. Mechanisms involved in dimethoate sorption and mobility were analyzed by fitting the breakthrough curves (BTCs) with two mathematical models, namely one-site equilibrium (ELM) and two-site nonequilibrium sorption model (NELM). Correlations were quantified by Kendall-Tau test, which revealed the strongest correlation of K_F value with OM content, cation exchange capacity and the humic acid content, while correlations with pH, clay content and A465nm/A665nm ratio were negative and insignificant. Based on these findings, a model for prediction of leaching potential was formed. A simplified model for dimethoate sorption/transport was proposed.

Effective removal of two fluoroquinolone antibiotics by PEG-4000 stabilized nanoscale zero-valent iron supported onto zeolite (PZ-NZVI)

In this work, nanoscale zero-valent iron (NZVI) was synthesized via liquid phase reduction method with surfactant polyethylene glycol (PEG-4000) modified and supported onto zeolite to prepare PZ-NZVI composite. SEM-EDS, XPS, BET.etc. characterizations indicated that the sphere NZVI particles were loaded on the zeolite successfully and the aggregation was restrained. The adsorption performance of PZ-NZVI for norfloxacin (NOR) or ofloxacin (OFL), two typical fluoroquinolones (FQs), from water was conducted. The equilibrium studies were demonstrated using Langmuir, Freundlich, Temkin and Elovich isotherms and better agreement was attained with the Temkin model. Compared with NZVI and zeolite, PZ-NZVI had higher FQs removal efficiency, and the Langmuir maximum adsorption capacity was 54.67 mg g^-1 (NOR) and 48.88 mg g^-1 (OFL). The kinetic parameters displayed that two FQs adsorption onto PZ-NZVI followed pseudo-second-order kinetic model. The thermodynamic analysis suggested the adsorption process was spontaneous and exothermic. In addition, the adsorption tests were executed at different influence factors and the adsorbent PZ-NZVI was suitable for a wide pH range (4-10) with the FQs (10 mg L^-1) removal efficiency above 90% in 1 h. Furthermore, it was found that PZ-NZVI can be effortlessly separated from mixed solutions using external magnetic field. Finally, the process of FQs adsorbed onto PZ-NZVI was attributed to the surface complexion (forming bidentate complexes), hydrophobic interaction, pore filling and electrostatic interaction.

Reduced nontronite-activated H2O2 for contaminants degradation: The beneficial role of clayed fractions in ISCO treatments

Clayed fractions in aquifers are generally deemed to be detrimental for in situ chemical oxidation (ISCO) treatments due to the difficulty of oxidant injection/transport and the retention/rebound of contaminants. Using a model clay mineral nontronite and a real sediment, here we show that the component of structural Fe(II) in clay minerals is particularly effective in activating hydrogen peroxide (H₂O₂) to hydroxyl radicals (OH) for contaminants degradation under pH-neutral conditions. Using reduced nontronite (Fe(II)/Fe_total : 40 %) as a model Fe(II)-bearing clay mineral, 2 mg/L trichloroethylene (TCE) was degraded by 82.0 % and 95.3 %at 2.5 min and 30 min, respectively, under the condition of 0.6 g/L reduced nontronite, 0.5 mM H₂O₂and pH 7.5. Reactive structural Fe(II) in nontronite was responsible for the initial quick reaction. The degradation was also efficient for phenol, benzoic, toluene and naphthalene, but exhibited higher efficiencies for those with stronger sorption to nontronite. With similar concentrations of H₂O₂ and Fe(II), nontronite-activated H₂O₂ at pH 7.5 led to similar efficiencies of TCE degradation and H₂O₂ utilization to classic homogeneous Fenton at pH 3. A real clayed sediment showed similar performance in activating H₂O₂ for contaminant degradation. Our findings implicate that clayed fractions in aquifers may probably contribute to contaminants degradation in H₂O₂-based ISCO treatments.

Effect of dissolved organic matter on adsorption of sediments to Oxytetracycline: An insight from zeta potential and DLVO theory

To reveal the adsorption mechanism of sediment to antibiotics with the presence of dissolved organic matter (DOM), batch experiments were carried out by oxytetracycline (OTC) on sediments with decayed plants (PDOM) and composted chicken manure (MDOM), and the zeta potential in the system before and after adsorption was measured. Results showed that the PDOM promoted the adsorption process, while the MDOM inhibited the adsorption. Adding PDOM, the change of zeta potential (Δζ) increased by 40.08% for first terrace sediments (FT) and 63.98% for riverbed sediments (RB), respectively; meanwhile, MDOM decreased by 20.04% for FT and 28.39% for RB, respectively. The results of kinetic fitting models of replacing the adsorption amount with Δζ were consistent with the initial. It indicated that there was a positive correlation between the adsorption amount and Δζ, and the zeta potential can be used to quickly judge the degree of adsorption process. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory describes the interactions of sediment particles. In terms of adsorption amount, zeta potential (absolute value) and total interaction energy all followed the order: RB > FT, RB-PDOM > FT-PDOM, and RB-MDOM > FT-MDOM. The more negative the zeta potential is, the better the dispersion of the particles is. Stronger repulsion is more conducive to adsorbing positively charged OTC. The site energy distribution theory further explained that the distribution of adsorption site in the various states of sediments increased while adding the PDOM and decreased while adding the MDOM.

Cation-group interaction as the predominant force for adsorption of substituted dinitrobenzenes by smectite clays

Elucidation of the interaction between NACs and smectites is important to the understanding of the potential for transport of nitroaromatic compounds (NACs) in soils and to implementation of NAC-contaminated soil remediation. The adsorption of dinitrotoluene isomers (DNTs) and substituted dinitrobenzenes (SDNBs) by smectite was determined by batch equilibration and characterized by FTIR and XPS, along with molecular dynamics simulations. The adsorption of DNTs differed substantially among the isomers, attributed to the overall degree of nitro deflection relative to the aromatic ring plane. The substituents in SDNBs strengthened the electrostatic interaction between smectite K⁺ and nitro groups, facilitating SDNB adsorption to smectite. The competition between 2,4-DNT and 1,3-DNB, as well as the inclusion complexation of K⁺ by crown ether 18c6e, both reduced 2,4-DNT adsorption to smectite by weakening the K⁺-nitro interaction. All the results demonstrated that the electrostatic interaction between smectite K⁺ and nitro of NACs was the predominant force in mediating their adsorption. This was supported by FTIR spectra that the N-O bands shifted due to the weakening of N-O bonds and strengthening of C-N bonds via the electron transfer to cations. The XPS of smectite further manifested the cation-nitro interactions that the binding energies of K 2p 1/2, K 2p 3/2, and Si 2p shifted higher with 1,3-DNB adsorbed. Molecular dynamics simulations indicated the aromatic planes of 2,4-DNP and 2,4-DNAs were parallel to the basal plane of smectite and the oxygens of nitro groups in the molecules were directly coordinated with smectite K⁺.

Adsorption and thermodynamic parameters of chlorantraniliprole and dinotefuran on clay loam soil with difference in particle size and pH

The behavior of chlorantraniliprole (CAP) and dinotefuran (DNF) insecticides was investigated in clay loam soil, a common type of the Egyptian soil. Effect of temperature, pH and particle size of the soil on the adsorption process was studied. Adsorption isotherm by bulk soil and its constituents; humic acid (HA), clay, silt and sand fractions was measured using batch equilibration technique. The results showed that the adsorption of the insecticides tested was significantly affected by the temperature and was a spontaneous interfacial process in the soil. Freundlich model accurately predicted the adsorption behavior of both insecticides. The interaction between soil and insecticides was endothermic and the highest adsorption for CAP and DNF was obtained at pH 9. However, the effect of pH on the adsorption of DNF was lower than that of CAP. Sorption of CAP and DNF on HA fraction was significantly greater than on clay fraction and bulk soil. In addition, the adsorption was significantly increased with particle size decrease. It could be inferred that the adsorption of CAP and DNF on clay loam soil was physical in nature and greatly influenced by the soil components, pH and temperature.

Adsorption characteristics of oxytetracycline by different fractions of organic matter in sedimentary soil

Sedimentary soil was selected as the original sample (SOS). The adsorption fractions were obtained by the removal of dissolved organic matter (SRDOM), removal of minerals (SRM), removal of free fat (SRLF), and removal of nonhydrolyzable organic carbon (SNHC) respectively to investigate the adsorption characteristic of oxytetracycline (OTC) by different fractions of organic matter in sedimentary soil. The adsorption mechanism was investigated by elemental analysis, infrared spectra, and UV-visible spectroscopy. The results showed that the DOM in the sedimentary soil inhibited the adsorption of OTC, but the adsorption of different fractions of organic matter was quite different. The sorption kinetics of OTC were fitted to the pseudo-second-order model and the adsorption capacity of each fraction was: SNHC≈SRDOM > SOS > SRLF> SRM. The adsorption processes of OTC by different fractions were spontaneous. Alkaline pH condition had an effect on the adsorption of four fractions except for SNHC, while neutral and acidic pH affects SOS and SRDOM more obviously, the SNHC fraction was almost free from pH varies. Mechanism analysis showed that the main factors determining the adsorption capacity were the aromaticity and polarity of organic matter fractions. For the organic matter-based fractions (SRM, SRLF, and SNHC), the adsorption coefficient was positively correlated with the aromaticity. Furthermore, for SOS and SRDOM based on inorganic minerals, it was not only related to aromaticity, but also the content and composition of inorganic minerals.

Enhanced Phototransformation of Tetracycline at Smectite Clay Surfaces under Simulated Sunlight via a Lewis-Base Catalyzed Alkalization Mechanism

As an important class of soil minerals and a key constituent of colloidal particles in surface aquifers, smectite clays can strongly retain tetracyclines due to their large surface areas and high cation exchange capacities. However, the research on phototransformation of tetracyclines at smectite clay surfaces is rarely studied. Here, the phototransformation kinetics of tetracycline preadsorbed on two model smectite clays (hectorite and montmorillonite) exchanged with Na⁺, K⁺, or Ca²⁺ suspended in aqueous solution under simulated sunlight was compared with that of tetracycline dissolved in water using batch experiments. Adsorption on clays accelerated tetracycline phototransformation (half-lives shortened by 1.1-5.3 times), with the most significant effects observed for Na⁺-exchanged clays. Regardless of the presence or absence of clay, the phototransformation of tetracycline was facilitated by increasing pH from 4 to 7. Inhibition or enhancement of photolysis-induced reactive species combined with their measurement using scavenger/probe chemicals indicate that the facilitated production of self-photosensitized singlet oxygen (¹O₂) was the key factor contributing to the clay-enhanced phototransformation of tetracycline. As evidenced by the red shifts and the increased molar absorptivity in the UV-vis absorption spectra, the complexation of tetracycline with the negatively charged (Lewis base) sites on clay siloxane surfaces led to formation of the alkalized form, which has larger light absorption rate and is more readily to be oxidized compared to tetracycline in aqueous solution at equivalent pH. Our findings indicate a previously unrecognized, important phototransformation mechanism of tetracyclines catalyzed by smectite clays.

Photodegradation of atrazine in the presence of indole-3-acetic acid and natural montmorillonite clay minerals

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) is a commonly used agricultural herbicide that, as a result, is frequently detected in surface and ground water. In this study, we provide evidence of the photo-oxidation of atrazine under environmentally relevant conditions, specifically, in the presence of natural montmorillonite clay and the ubiquitous phytohormone indole-3-acetic acid (IAA). The reaction is initiated by the generation of hydrated electrons from the photo-ionization of IAA. These electrons react with protons and dissolved oxygen to form hydroxyl radicals, which promote the further degradation of atrazine. Montmorillonite strongly enhances the yield of hydrated electrons and prolongs their lifetime, by stabilizing radical cations through electrostatic attraction by the negative charges embedded in the clay. Moreover, by providing a confined space, montmorillonite markedly increases the probability of contact between atrazine and the active radicals. Other factors strongly influencing the degradation process are the solution pH, the type of exchangeable cations present in the clay interlayer, and the hydration status of montmorillonite. Since both IAA and montmorillonite clay are widely distributed in the environment, the proposed reaction is predicted to play an important role in the degradation of atrazine and perhaps other potentially persistent organic contaminants.

Synchrotron infrared microspectroscopy reveals the roles of aliphatic and aromatic moieties in sorption of nitroaromatic compounds to soils

A consensus on the role of organic carbon moieties as the sorptive domains for nonionic organic compounds in soils is lacking due to the extremely complicated compositions of soil matrices. In this study, synchrotron radiation-based infrared microspectroscopy (IMS) was applied to in situ probe the distributions of four nitroaromatic compounds with varying hydrophobicity (namely, 1,3-dinitrobenzene, 1,5-dinitronapthalene, 3-nitrophenanthrene and 6-nitrobenzo[a]pyrene) and their associations with aliphatic and aromatic organic carbon moieties in soils. The technique revealed that both nitro group (NO₂) from the nitroaromatic compounds and organic carbon moieties were unevenly distributed in the soils at the micron scale. The spatial distribution of nitro groups was positively correlated with that of aromatic carbon (C=C) (r>0.804, p<0.01), indicating that the aromatic moieties of soil organic carbon play a key role in sorption of nitroaromatic compounds to soils. Neither nitro groups nor aromatic carbon showed a close relationship with aliphatic carbon (CH) in the spatial distribution in the soils. Meanwhile, the nitro groups from 1,3-dinitrobenzene and 1,5-dinitronapthalene exhibited a significant correlation with clay minerals (OH) in their distributions (r>0.629, p<0.01) in the soils and the correlation became insignificant for the other two compounds with high hydrophobicity. This study for the first time provides micron-scale spectroscopic evidence for the roles of organic carbon moieties in the sorption of nonionic organic compounds to soils.

Potential impact of the herbicide 2,4-dichlorophenoxyacetic acid on human and ecosystems

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is applied directly to aquatic and conventional farming systems to control weeds, and is among the most widely distributed pollutants in the environment. Non-target organisms are exposed to 2,4-D via several ways, which could produce toxic effects depending on the dose, frequency of exposure, and the host factors that influence susceptibility and sensitivity. An increasing number of experimental evidences have shown concerns about its presence/detection in the environment, because several investigations have pointed out its potential lethal effects on non-target organisms. In this review, we critically evaluated the environmental fate and behavior of 2,4-D along with its eco-toxicological effects on aquatic, plants and human life to provide concise assessment in the light of recently published reports. The findings demonstrate that 2,4-D is present in a low concentration in surface water of regions where its usage is high. The highest concentrations of 2,4-D were detected in soil, air and surface water surrounded by crop fields, which suggest that mitigation strategies must be implanted locally to prevent the entry of 2,4-D into the environment. A general public may have frequent exposure to 2,4-D due to its wide applications at home lawns and public parks, etc. Various in vivo and in vitro investigations suggest that several species (or their organs) at different trophic levels are extremely sensitive to the 2,4-D exposure, which may explain variation in outcomes of reported investigations. However, implications for the prenatal exposure to 2,4-D remain unknown because 2,4-D-induced toxicity thresholds in organism have only been derived from juveniles or adults. In near future, introduction of 2,4-D resistant crops will increase its use in agriculture, which may cause relatively high and potentially unsafe residue levels in the environment. The recent findings indicate the urgent need to further explore fate, accumulation and its continuous low level exposure impacts on the environment to generate reliable database which is key in drafting new regulation and policies to protect the population from further exposure.

Emerging organic pollutants in the vadose zone of a soil aquifer treatment system: Pore water extraction using positive displacement

Trace organic compounds in effluents, water streams and aquifers are amply reported. However, the mobile pool of Emerging Organic Contaminants (EOCs) in the deep parts of the vadose zone is hard to estimate, due to difficulties in extraction of sufficient quantity of pore water. Here, we present a new methodology for depth profiling of EOCs in pore water by Positive Displacement Extraction (PDE): Pore water extraction from unsaturated soil samples is carried out by withdrawal of soil cores by direct-push drilling and infiltrating the core by organics free water. We show that EOC concentrations in the water eluted in the plateau region of the inverse breakthrough curve is equal to their pore water concentrations. The method was previously validated for DOC extraction, and here the scope of the methodology is extended to pore water extraction for organic pollutants analysis. Method characteristics and validation were carried out with atrazine, simazine, carbamazepine, venlafaxine, O-desmethylvenlafaxine and caffeine in the concentration range of several ng to several μg/liter. Validation was carried out by laboratory experiments on three different soils (sandy, sandy-clayey and clayey). Field studies in the vadose zone of a SAT system provided 27 m deep EOC profiles with less than 1.5 m spatial resolution. During the percolation treatment, carbamazepine remained persistent, while the other studied EOCs were attenuated to the extent of 50-99%.The highest degradation rate of all studied EOCs was in the aerobic zone. EOC levels based on PDE and extraction by centrifugation were compared, showing a positive bias for centrifugation.

Changes in soil properties, X-ray-mineral diffractions and infrared-functional groups in bulk soil and fractions following afforestation of farmland, Northeast China

Analysis of soil properties, the compositional traits in bulk soil and different fractions and their responses to afforestation practices may possibly facilitate clarification of the mechanisms underlying soil changes. Soil properties, the compositional functional groups and minerals were determined in the bulk soil and fractions from forests and adjacent farmlands. The afforestation of farmland could induce accumulation of soil organic carbon [SOC] (+18%) and nitrogen [N] (+4%) with pH increase (+4%), and declines in electric conductivity (−15%) and bulk density (−3%). Sand and aggregates [SA] and easily oxidized fraction [EO] mainly contributed to the SOC and N accumulation. Moreover, afforestation-induced changes were observed in O-H & N-H stretching (−26%), feldspar (+52%) and huntite crystallinity (−40%). The changes of soil properties were strongly associated with the changes in functional groups, followed by minerals. Of them, asymmetric COO- & C = O stretching & O-H bending, symmetric COO- stretching, huntite and smectite-vermiculite crystallinity were the key factors responsible for the changes of soil properties. Our findings highlight that degraded farmland afforestation could strongly affect soil properties in the bulk soil, and the changes in fractions (mainly SA and EO) as well as their changes in the compositional traits strongly supported these bulk soil changes.

Effects of γ-irradiation of original and organic matter-amended soils on the sorption of triclosan and diuron from aqueous solutions

Soil γ-irradiation is a well-known method of inhibiting microbial activity in studies of the soil sorption of organic compounds. However, few studies have addressed the possible effect of γ-irradiation on the sorptive ability of soils enriched with different types of organic matter (OM). The objective of this study was to probe the effect of soil γ-irradiation on organic compound-soil interactions in two different situations representing adding OM to soils through land disposal of (a) OM-rich sewage sludge-originating biosolids and (b) olive mill wastewater (OMW). Both situations describe frequent environmental and agricultural scenarios. Comparisons of aqueous sorption on cobalt-60 γ-irradiated and non-irradiated soil sorbents were carried out for (a) triclosan (in a series of three soils and their lab-incubated mixtures with three different types of biosolids), and (b) the pesticide diuron (in two different untreated and OMW-affected soils). In each case, sodium azide was used as a biocide. Soil γ-irradiation affected the sorption of organic compounds by a factor generally not exceeding 2-3. Specifically, for triclosan, the sorbed concentration ratio between irradiated and non-irradiated soils when averaged over all the soil samples was 0.94. No significant effects of γ-irradiation on soil organic carbon or total nitrogen contents were observed. The effect of γ-irradiation on a soil sorbent may be less important when a rough estimate of a soil sorption coefficient of an organic compound is needed. However, it may need to be taken into account in mechanistic sorption studies, specifically, when the shape of sorption isotherms is of interest.

Electrokinetic flushing with surrounding electrode arrangements for the remediation of soils that are polluted with 2,4-D: A case study in a pilot plant

This work aimed to evaluate electrokinetic soil flushing (EKSF) technologies for the removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from spiked soils using an electrode configuration consisting of one cathode surrounded by six anodes (1c6a) and one anode surrounded by 6 cathodes (1a6c). Experiments were conducted for over one month in a bench-scale set-up (175 dm(3) of capacity) that was completely automated and operated at a constant electric field (1.0V cm(-1)). The electrical current, temperature, pH, moisture and pollutant concentration in electrolyte wells were monitored daily, and at the end of the experiments, an in-depth sectioned analysis of the complete soil section (post-mortem analysis) was conducted. Despite the geometric similarity, the two strategies led to very different results mainly in terms of water and herbicide mobilization, whereas pH and conductivity do not depend strongly on the electrode configuration. The volume of water extracted from cathodes with 1a6c is seven times higher than that of the 1c6a strategy. Herbicide was transported to the anode wells by electromigration and then dragged toward the cathode wells by electro-osmotic fluxes, with the first process being much more important. The configuration 1c6a was the most efficient and attained a transfer of 70% of the herbicide contained in the soil to flushing water in 35 days. These results outperform those obtained by the configuration 1a6c, for which less than 8% of the herbicide was transferred to flushing fluids in a much longer time (58 days).

Sorption interactions of organic compounds with soils affected by agricultural olive mill wastewater

The organic compound-soil interactions may be strongly influenced by changes in soil organic matter (OM) which affects the environmental fate of multiple organic pollutants. The soil OM changes may be caused by land disposal of various OM-containing wastes. One unique type of OM-rich waste is olive mill-related wastewater (OMW) characterized by high levels of OM, the presence of fatty aliphatics and polyphenolic aromatics. The systematic data on effects of the land-applied OMW on organic compound-soil interactions is lacking. Therefore, aqueous sorption of simazine and diuron, two herbicides, was examined in batch experiments onto three soils, including untreated and OMW-affected samples. Typically, the organic compound-soil interactions increased following the prior land application of OMW. This increase is associated with the changes in sorption mechanisms and cannot be attributed solely to the increase in soil organic carbon content. A novel observation is that the OMW application changes the soil-sorbent matrix in such a way that the solute uptake may become cooperative or the existing ability of a soil sorbent to cooperatively sorb organic molecules from water may become characterized by a larger affinity. The remarkable finding of this study was that in some cases a cooperative uptake of organic molecules by soils makes itself evident in distinct sigmoidal sorption isotherms rarely observed in soil sorption of non-ionized organic compounds; the cooperative herbicide-soil interactions may be characterized by the Hill model coefficients. However, no single trend was found for the effect of applied OMW on the mechanisms of organic compound-soil interactions.

Slow-release formulations of the herbicide MCPA by using clay-protein composites

BACKGROUND

MCPA [(4-chloro-2-methylphenoxy) acetic acid] is a widely used herbicide showing high leaching in the soil. In this study, clay-protein-based formulations of this herbicide were designed to reduce the risk of water pollution resulting from conventional formulations.

RESULTS

Clay-gelatin formulations of MCPA were prepared, and the influence of synthesis parameters such as pH and the presence of a plasticiser (glycerol) on the active substance content and performance of the new formulations was examined. Differential scanning calorimetry measurements provided information on the stability of the gelatin matrix in the gelatin-clay complex. Fourier transform infrared spectroscopy showed that the herbicide was retained by the formation of hydrogen bonds with side amino groups of the protein backbone and polyion complexation. Clay-protein-based formulations prepared at a pH below the isoelectric point value of the protein and in the absence of glycerol provided the slowest release of MCPA in water. Soil column experiments showed a fourfold reduction in leaching and improved bioactivity in the upper soil layer for the new formulation compared with a commercial product used as a control.

CONCLUSIONS

A reduction in the recommended dose of MCPA can be achieved by employing clay-gelatin, which reduces the environmental risk associated with herbicide applications.

Exfoliation and intercalation of montmorillonite by small peptides

Understanding structural changes in clay minerals induced by complexation with organic matter is relevant to soil science and agricultural applications. In this study, the effect of peptide storage in montmorillonite and the thermal stability of peptide-clay complexes was examined through characterization by X-ray diffraction (XRD), electron microscopy, UV absorption, and thermogravimetric analysis (TGA). XRD analysis of small peptide-montmorillonite clay complexes produced profiles consisting of reflections associated with the smectite 001 reflection and related peaks similar to that produced by a mixed layer clay mineral structure. Shifts in higher order diffraction maxima were attributed to disorder caused by the intercalation with the peptides. Increasing peptide concentrations resulted in greater shifts towards smaller 2θ from 6.37° (1.39 nm) to 5.45° (1.62 nm) as the interlayer space expanded. The expansion was accompanied by broadening of the 001 reflection (FWHM increases from 0.51 to 1.22° 2θ). The XRD line broadening was interpreted as caused by poorer crystallinity resulting from intercalation and tactoid exfoliation. SEM images revealed montmorillonite platelets with upwardly rolled edges that tend toward cylindrical structures with the production of tubules. High-resolution TEM images revealed bending of montmorillonite platelets, confirming exfoliation. The distribution of basal spacings in the micrographs was determined from the spatial frequencies obtained by Fourier analysis of density profiles. The distribution indicated the presence of discrete coherent crystallite domains. XRD and TGA results indicated that higher peptide concentrations resulted in a greater fraction of intercalated peptides and that surface adsorption of peptides mediated intercalation. Therefore, higher peptide concentration led to more stable organoclay complexes. However, UV absorption and TGA found that peptide adsorption onto montmorillonite had a finite limit at approximately 16% by weight.

Impact of soil primary size fractions on sorption and desorption of atrazine on organo-mineral fractions

In the current study, a mechanical dispersion method was employed to separate clay (<2 μm), silt (2-20 μm), and sand (20-50 μm) fraction in six bulk soils. Batch equilibrium method was used to conduct atrazine sorption and desorption experiments on soil organo-mineral fractions with bulk soils and their contrasting size fractions separately. The potential contribution of total organic carbon (TOC) for atrazine retention in different fractions was further investigated. It was found that clay fraction had the highest adsorption but the least desorption capacities for atrazine, while sand fraction had the lowest adsorption but the highest desorption capacities for atrazine. The adsorption percentage of atrazine, as compared with adsorption by the corresponding bulk soils, ranged from 53.6 to 80.5%, 35.7 to 56.4%, and 0.2 to 4.5% on the clay, silt, and sand fractions, respectively. TOC was one of the key factors affecting atrazine retention in soils, with the exact contribution dependent on varying degree of coating with mineral component in different soil size fractions. The current study may be useful to predict the bioavailability of atrazine in different soil size fractions.

Sediment-water distribution of contaminants of emerging concern in a mixed use watershed

This study evaluated the occurrence and distribution of 15 contaminants of emerging concern (CECs) in stream water and sediments in the Zumbro River watershed in Minnesota and compared these with sub-watershed land uses. Sixty pairs of sediment and water samples were collected across all seasons from four stream sites for over two years and analyzed for selected personal care products, pesticides, human and veterinary medications, and phytoestrogens. Spatial and temporal analyses indicate that pharmaceuticals and personal care products (urban/residential CECs) are significantly elevated in water and/or sediment at sites with greater population density (>100 people/km(2)) and percentage of developed land use (>8% of subwatershed area) than those with less population density and land area under development. Significant spatial variations of agricultural pesticides in water and sediment were detectable, even though all sites had a high percentage of agricultural land use. Seasonality in CEC concentration was observed in water but not in sediment, although sediment concentrations of three CECs did vary between years. Average measured non-equilibrium distribution coefficients exceeded equilibrium hydrophobic partitioning-based predictions for 5 of the 7 detected CECs by at least an order of magnitude. Agreement of measured and predicted distribution coefficients improved with increasing hydrophobicity and in-stream persistence. The more polar and degradable CECs showed greater variability in measured distributions across different sampling events. Our results confirm that CECs are present in urban and agricultural stream sediments, including those CECs that would typically be thought of as non-sorptive based on their log Kow values. These results and the observed patterns of sediment and water distributions augment existing information to improve prediction of CEC fate and transport, leading to more accurate assessments of exposure and risk to surface water ecosystems.

Sorption of tetracycline to varying-sized montmorillonite fractions

The influence of particle sizes on sorption of tetracycline by clay minerals is poorly understood. In this study, montmorillonite clay fractions with varying particle sizes were prepared by successive centrifugation, and the effects of particle sizes on sorption of tetracycline were evaluated using an equilibrium dialysis method. Sorption isotherms were nearly overlapped for size fractions ranging from 6.38 to 16.00 μm, except for the finest clay fraction (0.41 μm). The relatively low sorption by the fraction with the smallest particles could be attributed to the colloidal nature and high edge-to-surface ratio, which could lead to reduced accessibility of tetracycline to sorption sites (particularly those at the edges). The impact of solution pH and coexisting Na and Ca ions on tetracycline sorption was found to differ between the finest fraction and other clay fractions. The results demonstrated for the first time that clay particle size greatly influenced tetracycline sorption to clay minerals and consequently might affect their transport and bioavailability in the environment.

Mobility and bioavailability reduction of soil TNT via sorption enhancement using monopotassium phosphate

In this study, the effect of monopotassium phosphate (MKP) on the reduction in mobility and bioavailability of 2,4,6-trinitrotoluene (TNT) was tested. In the test soil, collected from an active firing range, of which cation binding sites were mostly exchanged with H(+) or Al(3+), potassium ions in MKP exchanged the existing cations and hence significantly increased TNT sorption. In addition, a competitive sorption experiment with hexafluorobenzene and 2,4-dinitrotoluene suggests that TNT was specifically sorbed through cation-polar interaction in the test soil. The unit-equivalent Freundlich sorption coefficient of TNT in MKP-amended soil (1370.96 mg-TNT/kg-soil) was about 13 times higher than that in untreated soil (106.23 mg-TNT/kg-soil). Finally, modified synthetic precipitation leaching procedure and hydroxypropyl-β-cyclodextrin extraction result revealed that MKP application could reduce both the leachability and bioavailability of soil TNT. The leachable and extractable fraction of TNT in untreated soil were 87.63% and 94.47% of the initial TNT, respectively, whereas these fractions decreased to 49.15% and 54.85% of the initial TNT in the presence of MKP, respectively. MKP application can be a benign technology which can reduce both mobility and bioavailability of TNT in soil.

Biochar-mediated [14C]atrazine mineralization in atrazine-adapted soils from Belgium and Brazil

Biochar addition to soil has been reported to reduce the microbial degradation of pesticides due to sorption of the active compound. This study investigated whether the addition of hardwood biochar alters the mineralization of (14)C-labeled atrazine in two atrazine-adapted soils from Belgium and Brazil at different moisture regimens. Biochar addition resulted in an equally high or even in a significantly higher atrazine mineralization compared to the soils without biochar. Statistical analysis revealed that the extent of atrazine mineralization was more influenced by the specific soil than by the addition of biochar. It was concluded that biochar amendment up to 5% by weight does not negatively affect the mineralization of atrazine by an atrazine-adapted soil microflora.

Study of sorption of two sulfonylurea type of herbicides and their additives on soils and soil components

The sorption of two sulfonylurea type herbicides (chlorsulfuron: (1-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea; tribenuron methyl: (methyl-2-[N-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)-3-(methyl-ureido)-sulfonyl]-benzoate) was studied on sand and chernozem soil adsorbents. Experimental results for solutions prepared from the pure ingredients were compared to those prepared from the appropriate formulated commercial products. At small concentrations, the extent of adsorption of the active ingredient was higher than from the formulation containing solutions. Environmental fate and effects of the forming agents are less investigated because they rarely have concentration limits recommended by authorities. In addition to the adsorption of active ingredients, therefore, the sorption behavior of a widely used additive Supragil WP (sodium diisopropyl naphthalene sulphonate) was also studied. This dispersant is an anionic forming agent applied in a lot of pesticide formulations. Using three different soils (sand, brown forest, chernozem) as adsorbents two-step isotherms were obtained. The role of the soil organic matter (OM) was significant in the adsorption mechanism because the adsorbed amounts of the dispersant correlated with the specific surface area as well as with the total organic carbon (TOC) content of the soils. The sorption behavior indicates the operation of hydrophobic interaction mechanism between the soil OM and the dispersant. These results are supported by our further sorption experiments on clays, too. Zeta potential measurements seem to be promising for the interpretation of multi-step isotherms. The application of this technique proved that higher concentrations of the anionic forming agent assisted the peptization of soil organic matter (SOM) resulting in stable colloidal solution dominated by negative charges. Since the pesticides investigated are also anionic at the studied pH (7 and 8.3) the dissolved organics lead to the enhancement of the mobility of both the sulfonylureas and dispersant.

Predictability by Box-Behnken model for carbaryl adsorption by soils of Indian origin

This study was undertaken to investigate the adsorption capacity of carbaryl on four Indian soils with different physiochemical properties. A batch adsorption study was carried out in order to evaluate the maximum adsorption capacity of carbaryl using a Response Surface Methodology (RSM). The effects of operating parameter such as initial carbaryl concentration (1-20 mgL⁻¹), adsorbent dosage (0.5-6 g) and contact time (10-180 min) were examined. The proposed quadratic model for Box-Behnken design fits very well to the experimental data because it may be used to navigate design space according to ANOVA results. The regression co-efficient (R²) of the models developed and the results of validation experiments conducted at optimal conditions strongly suggests that the predicted values are in good agreement with experimental results. Contour and response surface plots are used to determine the interactions effects of main factors and optimal conditions of the process. The experiment can be utilized as a guideline for better understanding of carbaryl adsorption onto soil under different operating conditions. The results show that the forest soil is most efficient in binding carbaryl (Sevin) than the other types of soil tested.

Adsorption-desorption and leaching of pyraclostrobin in Indian soils

Pyraclostrobin is a new broad-spectrum foliar applied and seed protectant fungicide of the strobilurin group. In this paper, adsorption-desorption of pyraclostrobin has been investigated in three different soils viz. Inceptisol (sandy loam, Delhi), Vertisol (sandy clay, Hyderabad) and Ultisol (sandy clay loam, Thrissur). Effect of organic matter and clay content on sorption was also studied in Inceptisol of Delhi. Leaching potential of pyraclostrobin as influenced by rainfall was studied in intact soil columns to confirm the results of adsorption-desorption studies. The adsorption studies were carried out at initial concentrations of 0.05, 0.1, 0.5, 1 and 1.5 μg mL(-1). The distribution coefficient (Kd) values in three test soils ranged from 4.91 to 18.26 indicating moderate to high adsorption. Among the three test soils, adsorption was the highest in Ultisol (Kd 18.26), followed by Vertisol (Kd 9.87) and Inceptisol (Kd 4.91). KF value was also highest for Ultisol soil (66.21), followed by Vertisol (40.88) and Inceptisol (8.59). S-type adsorption isotherms were observed in all the three test soils. Kd values in organic carbon-removed soil and clay-removed soil were 3.57 and 2.83 respectively, indicating lower adsorption than normal Inceptisol. Desorption studies were carried out at initial concentrations of 0.5, 1 and 1.5 μg mL(-1). Desorption was the greatest in Inceptisol, followed by Vertisol and Ultisol. Amounts of pyraclostrobin desorbed in three desorption cycles for different concentrations were 23.1-25.3%, 9.4-20.7% and 8.1-13.6% in Inceptisol, Vertisol and Ultisol respectively. Desorption was higher in clay fraction-removed and organic carbonremoved soils than normal Inceptisol. Desorption was slower than adsorption in all the test soils, indicating hysteresis effect (with hysteresis coefficient values varying from 0.05 to 0.20). Low values of hysteresis coefficient suggest high hysteresis effect indicating easy and strong adsorption, and slow desorption, of pyraclostrobin in soils. Higher hysteresis coefficient values in organic carbon removed soil (0.25-0.30) and clay fraction removed soil (0.28-0.36) as compared to normal Inceptisol soil suggest relatively weak adsorption and easy desorption of pyraclostrobin. Results of regression analysis suggest that the organic matter and pH of the soil play a major role in adsorption of pyraclostrobin. Leaching studies were carried out in intact soil columns in Inceptisol. The columns were leached with different amounts of water simulating different amounts of rainfall. The results suggest that most of the pyraclostrobin residues will remain present in the top soil layers even under high rainfall conditions and chances of pyraclostrobin moving to lower soil depth are almost negligible.

Intercalation of trichloroethene by sediment-associated clay minerals

The objective of this research was to examine the potential for intercalation of trichloroethene (TCE) by clay minerals associated with aquifer sediments. Sediment samples were collected from a field site in Tucson, AZ. Two widely used Montmorillonite specimen clays were employed as controls. X-ray diffraction, conducted with a controlled-environment chamber, was used to characterize smectite interlayer d-spacing for three treatments (bulk air-dry sample, sample mixed with synthetic groundwater, sample mixed with TCE-saturated synthetic groundwater). The results show that the d-spacing measured for the samples treated with TCE-saturated synthetic groundwater are larger (~26%) than those of the untreated samples for all field samples as well as the specimen clays. These results indicate that TCE was intercalated by the clay minerals, which may have contributed to the extensive elution tailing observed in prior miscible-displacement experiments conducted with this sediment.

Stabilization of enzymatically polymerized phenolic chemicals in a model soil organic matter-free geomaterial

A variety of remediation methods, including contaminant transformation by peroxidase-mediated oxidative polymerization, have been proposed to manage soils and groundwater contaminated with chlorinated phenols. Phenol stabilization has been successfully observed during cross polymerization between phenolic polymers and soil organic matter (SOM) for soils with SOM >3%. This study evaluates peroxidase-mediated transformation and removal of 2,4-dichlorophenol (DCP) from an aqueous phase in contact with a natural geomaterial modified to contain negligible (<0.3%) SOM. The results are compared with those for soils with higher SOM. The SOM-free sorbent was generated by removing SOM using a NaOCl oxidation. When horseradish peroxidase (HRP) was used to induce polymerization of DCP, the soil-water phase distribution relationship (PDR) of DCP polymerization products (DPP) was complete within 1 d and PDRs did not significantly change over the 28 d of study. The conversion of DCP to DPP was close to 95% efficient. Extractable solute consisted entirely of DPP with 5% or less of unreacted DCP. The aqueous extractability of DPP from SOM-free geomaterial decreased at longer contact times and at smaller residual aqueous concentrations of DPP. DCP stabilization appeared to have resulted from a combination of sorption, precipitation, and ligand exchange between oligomeric products and the exposed mineral surfaces. Modification of the mineral surface through coverage with DPP enhanced the time-dependent retention of the oligomers. DPP stabilization in SOM-free geomaterial was comparable with that reported in the literature with soil containing SOM contents >1%. Results from this study suggest that the effectiveness of HRP-mediated stabilization of phenolic compounds not only depends on the cross-coupling with SOM, but also on the modification of the surface of the sorbent that can augment affinity with oligomers and enhance stabilization. Coverage of the mineral surface by phenolic oligomers may be analogous to SOM that can potentially sorb other xenobiotics. HRP- mediated reactions can be used to stabilize DCP associated with low SOM mineral soils or aquifer media, thereby restricting the transport of phenolic contaminants in the soil environment.

Effects of temperature on sorption-desorption processes of imidacloprid in soils of Croatian coastal regions

Sorption-desorption behavior of imidacloprid in six soils collected from five coastal regions in Croatia at 20, 30 and 40°C was investigated using batch equilibrium technique. Isothermal data were applied to Freundlich, Langmuir and Temkin equations, and the thermodynamic parameters ΔH°, ΔG°, ΔS° were calculated. The sorption isotherm curves were non-linear and may be classified as L-type, suggesting a relatively high sorption capacity for imidacloprid. Our results showed that the K( sor ) ( F ) values decreased for all the tested soils as the temperature increased, indicating that the temperature strongly influences the sorption. Values of ΔG° were negative (-4.65 to -2.00 kJ/mol) indicating that at all experimental temperatures the interactions of imidacloprid with soils were spontaneous processes. The negative and small ΔH° values (-19.79 to -8.89 kJ/mol) were in the range of weak forces, such as H-bonds, consistent with interactions and partitioning of the imidacloprid molecules into soil organic matter. The ΔS° values followed the range of -57.12 to -14.51 J/molK, suggesting that imidacloprid molecules lose entropy during transition from the solution phase to soil surface. It was found that imidacloprid desorption from soil was concentration and temperature-dependent, i.e. at lower imidacloprid concentrations and temperature, lower desorption percentage occurred. Desorption studies revealed that hysteretic behavior under different temperature treatments existed, and it was more pronounced at 20°C in the soils with higher OC content. The study results emphasize the importance of thermodynamic parameters in controlling soil pesticide mobility in different geographical locations, seasons and greenhouse conditions.

Differences in sorption behavior of the herbicide 4-chloro-2-methylphenoxyacetic acid on artificial soils as a function of soil pre-aging

PURPOSE

The sorption behavior of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) to three different artificial soil mixtures was investigated. Artificial soils serve as model systems for improving understanding of sorption phenomena.

MATERIALS AND METHODS

The soils consisted of quartz, ferrihydrite, illite, montmorillonite, and charcoal. In a previous study, several selected mixtures had been inoculated with organic matter, and microbial aging (incubation) had been performed for different periods of time (3, 12, and 18 months) before conducting the sorption experiments. The effect of this pre-incubation time on the sorption behavior was determined. Interaction of MCPA with soil surfaces was monitored by aqueous phase sorption experiments, using high-performance liquid chromatography/ultraviolet and in selected cases Fourier-transformed infrared spectroscopy.

RESULTS AND DISCUSSION

The sorption behavior showed large differences between differently aged soils; Freundlich and linear sorption model fits (with sorption constants K_f , 1/n exponents, and K_d values, respectively) were given for pH = 3 and the unbuffered pH of ∼7. The largest extent of sorption from diluted solutions was found on the surfaces with a pre-incubation time of 3 months. Sorption increased at acidic pH values.

CONCLUSIONS

Regarding the influence of aging of artificial soils, the following conclusions were drawn: young artificial soils exhibit stronger sorption at lower concentrations, with a larger K_f value than aged soils. A correlation with organic carbon content was not confirmed. Thus, the sorption characteristics of the soils are more influenced by the aging of the organic carbon than by the organic carbon content itself.

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.

Assessment of spent mushroom substrate as sorbent of fungicides: influence of sorbent and sorbate properties

The capacity of spent mushroom substrate (SMS) as a sorbent of fungicides was evaluated for its possible use in regulating pesticide mobility in the environment. The sorption studies involved four different SMS types in terms of nature and treatment and eight fungicides selected as representative compounds from different chemical groups. Nonlinear sorption isotherms were observed for all SMS-fungicide combinations. The highest sorption was obtained by composted SMS from Agaricus bisporus cultivation. A significant negative and positive correlation was obtained between the K(OC) sorption constants and the polarity index values of sorbents and the K(OW) of fungicides, respectively. The statistic revealed that more than 77% of the variability in the K(OW) could be explained considering these properties jointly. The other properties of both the sorbent (total carbon, dissolved organic carbon, or pH) and the sorbate (water solubility) were nonsignificant. The hysteresis values for cyprodinil (log K(OW)= 4) were for all the sorbents much higher (>3) than for other fungicides. This was consistent with the remaining sorption after desorption considered as an indicator of the sorption efficiency of SMS for fungicides. Changes in the absorption bands of fungicides sorbed by SMS observed by FTIR permitted establishing the interaction mechanism of fungicides with SMS. The findings of this work provide evidence for the potential capacity of SMS as a sorbent of fungicides and the low desorption observed especially for some fungicides, although they suggest that more stabilized or humified organic substrates should be produced to enhance their efficiency in environmental applications.

Nonpoint source pollution, environmental quality, and ecosystem health in China: introduction to the special section

The rapid economic and industrial growth of China, exemplified by a 10-fold increase in its gross domestic product in the past 15 years, has lifted millions of its citizens out of poverty but has simultaneously led to severe environmental problems. The World Health Organization estimates that approximately 2.4 million deaths in China per year could be attributed to degraded environmental quality. Much of China's soil, air, and water are polluted by xenobiotic contaminants, such as heavy metals and organic compounds. In addition, soil quality is degraded by erosion, desertification, and nutrient runoff. Air quality is further compromised by particulates, especially in heavily populated areas. Research shows that 80% of urban rivers in China are significantly polluted, and poor water quality is a key contributor to poverty in rural China. Economic and industrial growth has also greatly expanded the demand for water sources of appropriate quality; however, pollution has markedly diminished usable water resource quantity. Desertification and diminishing water resources threaten future food security. In recent years, China's government has increased efforts to reverse these trends and to improve ecosystem health. The Web of Science database showed that the percentage of articles on China devoting to environmental sciences increased dramatically in recent years. In addition, the top 25 institutes publishing the papers in environmental sciences were all in China. This special issue includes seven articles focusing on nonpoint source pollution, environmental quality, and ecosystem health in China. The major issues, and results of these studies, are discussed in this introduction.

Effect of lipids on sorption/desorption hysteresis in natural organic matter

The chemical composition and physical conformation of natural organic matter (NOM) play a major role in regulating its capacity to retain hydrophobic organic compounds. Naphthalene and phenanthrene were used to study the correlations between sorption/desorption isotherm nonlinearity and compositional data obtained from quantitative (13)C solid-state DPMAS NMR spectroscopy for soil and peat organic matter with or without lipids. Sorption experiments were conducted using a batch equilibration method. Desorption experiments were carried out immediately following the sorption experiments by three successive decant-refill cycles. Hysteresis was observed in all samples. Nonlinear sorption behavior was increased by removal of lipids from the NOM. The hysteresis index, obtained from the ratio of the Freundlich exponents (N values) for the desorption and sorption isotherms, was lower in the lipid-extracted NOM samples than in the same samples without lipid extraction. The relationship between the extent of hysteresis and the characteristics of the (13)C DPMAS NMR spectra indicates that altering NOM composition through lipid extraction not only increased the proportion of aromatic-C content, but also increased sorption/desorption hysteresis. Our data also suggest that the hysteresis index is negatively related to aromaticity.

Analysis and environmental concentrations of the herbicide dichlobenil and its main metabolite 2,6-dichlorobenzamide (BAM): a review

Dichlobenil is an herbicide which has been applied in many countries for weed control in non-agricultural areas such as railroads, car parks and private gardens. In the aquatic environment it has been used for control of floating aquatic weeds. Dichlobenil is relatively persistent in the environment, and primarily bound to solid matrices. Of great concern is its main degradation product 2,6-dichlorobenzamide which is water soluble and therefore transported downward in aquifers, contaminating groundwater resources. It is often found in concentrations exceeding 0.1 μg/L, which is the maximum allowed concentration of pesticides in groundwater set by the European Commission. In many countries, the usage of dichlobenil and the problems associated with groundwater contamination by 2,6-dichlorobenzamide have resulted in intensive research and monitoring of these compounds. This review gives the first overview of analytical strategies available for determining dichlobenil and 2,6-dichlorobenzamide in environmental matrices. It also summarizes studies presenting measured environmental concentrations of dichlobenil and 2,6-dichlorobenzamide identified in the literature during the past two decades. Thereby a preliminary picture of the distribution of dichlobenil and 2,6-dichlorobenzamide in the environment can be outlined for the first time.

Probing the specific sorption sites on montmorillonite using nitroaromatic compounds and hexafluorobenzene

The objective of this work was to test two possible sorption mechanisms of organic chemicals to montmorillonite: n-π electron-donor-acceptor (EDA) interaction with lone electron pairs of siloxane oxygens (n-donors) and complexation with exchangeable cations. Batch sorption experiments were performed for 1,3-dinitrobenzene, 1,4-dinitrobenzene (π-electron acceptors and cation binders), and hexafluorobenzene (π-electron acceptor only) to homoionic montmorillonites in water or hexane. For all three sorbates, the aqueous sorption affinity showed large cation dependency (Cs(+) > K(+) > Na(+)), wherein sorption of hexafluorobenzene to Cs(+)-montmorillonite was the strongest (K(d) in the order of 10(4) L/kg). Change of the solvent media from water to hexane generally favored sorption, indicating suppressive effect by cation hydration. Cosorption of 1,4-dinitrobenzene prominently decreased sorption of 1,3-dinitrobenzene to all cation-exchanged montmorillonites; however, hexafluorobenzene caused strong competition only to Cs(+)-montmorillonite. Furthermore, complexation of exchangeable cations by 18-crown-6 ether dramatically suppressed sorption of 1,3-dinitrobenzene to K(+)-montmorillonite in water and all cation-exchanged montmorillonites in hexane, but not to the rest. The contrast patterns of binary competitive sorption between nitroaromatics and hexafluorobenzene indicated they sorbed to different sites on montmorillonite. It was proposed that sorption of hexafluorobenzene was dominated by n-π EDA interaction, while sorption of nitroaromatics was dominated by cation-polar interaction.

Reduced metribuzin pollution with phosphatidylcholine-clay formulations

BACKGROUND

Metribuzin is a widely used herbicide that has been identified as a groundwater contaminant. In this study, slow-release formulations of metribuzin were designed by encapsulating the active ingredient in phosphatidylcholine (PC) vesicles and adsorbing the vesicles onto montmorillonite.

RESULTS

The maximum active ingredient content in the slow-release formulations was 246 g kg(-1) . Infrared spectroscopy results revealed that the hydrophobic interactions between metribuzin and the alkyl chains on PC were necessary for encapsulation. In addition, water bridges connecting the herbicide and the PC headgroup enhanced the solubility of metribuzin in PC. Adsorption experiments in soils were performed to evaluate the relationship between sorption and leaching. Funnel experiments in a sandy soil revealed that the herbicide was not irreversibly retained in the formulation matrix. In soil column experiments, PC-clay formulations enhanced herbicide accumulation and biological activity in the top soil layer relative to a commercial formulation. PC-clay formulations also reduced the dissipation of metribuzin by a factor of 1.6-2.5.

CONCLUSIONS

A reduction in the recommended dose of metribuzin can be achieved by employing PC-clay formulations, which reduces the environmental risk associated with herbicide applications. Moreover, PC and montmorillonite are non-toxic and do not negatively affect the environment.