Soil-free Systems Research Articles

Soil-pH and cement influence the weathering kinetics of chrysotile asbestos in soils and its hydroxyl radical yield

Chrysotile asbestos is a toxic and carcinogenic mineral that has been used in a variety of industrial and consumer applications. Much of the fiber- and cement-containing asbestos waste has ended up in terrestrial environments. Chrysotile weathering in soils and the potential for natural attenuation have, however, hardly been examined yet. Here we explored how soil properties influence the dissolution rate of chrysotile, the release of the carcinogenic metals chromium and nickel, and the hydroxyl radical (HO•) generation by chrysotile fibers. Chrysotile dissolution rates in soil suspensions decreased with increasing soil-pH and were lower than reported rates in soil-free systems. Dissolved organic carbon did not markedly accelerate dissolution at circumneutral pH, whereas cement mixed with soil inhibited dissolution because of its alkalinity. The HO•-yield of incubated fibers in non-amended soils eventually decreased by 60–75%. The decline was fastest in an acidic podzol soil, yet was followed by a small rebound. Cement amendment induced the largest HO•-yield reduction (∼90%), presumably due to surface coating of the fibers. Overall, this work demonstrates that the potential for natural attenuation of chrysotile asbestos in soils critically depends on soil chemical parameters and the presence of cement in association with the fibers.

Degradation of p-Nitrophenol by thermally activated persulfate in soil system

In this work, acceleration of oxidation of p-Nitrophenol, a known soil pollutant, by thermally activated persulfate was studied for the application of soil remediation. The effect of various operating parameters viz., reaction temperature, initial concentration of persulfate, pH, and basic constituents of soil matrix Cl−, HCO3− and humic acid (HA) was systematically examined. Further, a comparative study was made between the rate of p-Nitrophenol degradation of soil-free and soil-containing systems under similar experimental conditions. The results indicated that oxidation reaction of p-Nitrophenol followed a pseudo-first-order kinetic model, and the rate constant was found to increase with increasing the temperature and the initial concentration of persulfate as well. The rate of consumption of persulfate was enhanced at elevated temperatures, and 60°C was found to be an optimum condition to activate the persulfate. The pH of the system was found to be having no significant effect on the degradation process. The constituents of soil matter presented in the system, such as Cl−, HCO3− and HA, showed negative impact on the progressive degradation of p-Nitrophenol. The presence of soil in the system made mechanical restrains to the diffusion of sulfate radicals, resulting in an inhibitive effect. The observed decline trend of p-Nitrophenol degradation with the system of HA addition was due to a partial consumption and/or deterioration of the oxidant by the dissolved organic matter. In addition, the results derived from LC/MS analysis showed that 2,4-dinitrophenol, 4-hydroxy-2′,3,4′-trinitrobiphenyl, phenol, p-benzoquinone, and maleic acid were the primary intermediates formed during the degradation of p-Nitrophenol. Accordingly, a plausible pathway for the degradation of p-Nitrophenol could be proposed, which was identical in both the soil-containing and soil-free systems.

Degradation kinetics of chlorinated aliphatic hydrocarbons by methane oxidizers naturally-associated with wetland plant roots

Chlorinated aliphatic hydrocarbons (CAHs) are common groundwater contaminants that can be removed from the environment by natural attenuation processes. CAH biodegradation can occur in wetland environments by reductive dechlorination as well as oxidation pathways. In particular, CAH oxidation may occur in vegetated wetlands, by microorganisms that are naturally associated with the roots of wetland plants. The main objective of this study was to evaluate the cometabolic degradation kinetics of the CAHs, cis-1,2-dichloroethene (cisDCE), trichloroethene (TCE), and 1,1,1-trichloroethane (1,1,1TCA), by methane-oxidizing bacteria associated with the roots of a typical wetland plant in soil-free system. Laboratory microcosms with washed live roots investigated aerobic, cometabolic degradation of CAHs by the root-associated methane-oxidizing bacteria at initial aqueous [CH4] ~1.9mgL−1, and initial aqueous [CAH] ~150μgL−1; cisDCE and TCE (in the presence of 1,1,1TCA) degraded significantly, with a removal efficiency of approximately 90% and 46%, respectively. 1,1,1TCA degradation was not observed in the presence of active methane oxidizers. The pseudo first-order degradation rate-constants of TCE and cisDCE were 0.12±0.01 and 0.59±0.07d−1, respectively, which are comparable to published values. However, their biomass-normalized degradation rate constants obtained in this study were significantly smaller than pure-culture studies, yet they were comparable to values reported for biofilm systems. The study suggests that CAH removal in wetland plant roots may be comparable to processes within biofilms. This has led us to speculate that the active biomass may be on the root surface as a biofilm. The cisDCE and TCE mass losses due to methane oxidizers in this study offer insight into the role of shallow, vegetated wetlands as an environmental sink for such xenobiotic compounds.

Accumulation of Pseudomonas-derived 2,4-diacetylphloroglucinol on wheat seedling roots is influenced by host cultivar

Production of antifungal metabolites, including the polyketide 2,4-diacetylphloroglucinol (2,4-DAPG), is one mechanism by which biocontrol strains of Pseudomonas fluorescens suppress soilborne fungal pathogens. P. fluorescens strains vary in ability to produce 2,4-DAPG, but the role of the host in modulating metabolite accumulation in the rhizosphere is not well defined. To examine 2,4-DAPG production and accumulation during early stages of rhizoplane interactions, we compared metabolite production by two P. fluorescens strains in culture and on seedling roots of three Triticum aestivum L. (wheat) cultivars, Buchanan, Finley, and Tara, in a soil-free system. P. fluorescens strain Q8r1-96, an aggressive colonizer of the wheat rhizosphere, produced 1850 μg mL −1 2,4-DAPG after 48 h of growth in King’s Medium B, significantly ( P > 0.05) more than 19.4 μg mL −1 metabolite produced by the moderately aggressive strain Q2-87V1 under the same conditions. Rhizoplane levels of 2,4-DAPG after 4 d of Q8r1-96 colonization were 1946, 1650, and 2767 ng g −1 for Buchanan, Finley, and Tara, respectively. Metabolite levels obtained for Q2-87V1 colonization were 1468, 366, and 80 ng g −1 on the respective cultivars. Strain Q8r1-96 deposited significantly ( P < 0.05) more 2,4-DAPG than Q2-87V1 on Tara and Finley roots, whereas both strains produced similar ( P > 0.05) amounts of the metabolite on Buchanan roots. In greenhouse experiments, take-all damage was reduced only on Tara roots inoculated with Q8r1-96. To our knowledge, this is the first report to compare 2,4-DAPG accumulation in the rhizoplanes of different cultivars, and to demonstrate that rhizoplane 2,4-DAPG accumulation depends on a cultivar–bacterial strain interaction.

Plant secondary metabolites, biphenyl, and hydroxypropyl- β-cyclodextrin effects on aerobic polychlorinated biphenyl removal and microbial community structure in soils

Biphenyl and several plant secondary metabolites (PSM) (carvone, isoprene, limonene, naringin, and coumarin) and surfactant (hydroxypropyl- β-cyclodextrin, (HP- β-CD)) have been shown to improve aerobic polychlorinated biphenyl (PCB) removal by several bacterial species. The objective of this study was to determine whether these treatments also affect PCB removal and microbial community structure in a high organic matter soil (Pahokee soil series with 67% organic matter) and low organic matter soil (Woolper soil series with 6% organic matter), as determined by monitoring changes in PCB levels and phospholipid fatty acids (PLFA) profiles in laboratory microcosms amended with these compounds. Biphenyl enhanced di-chlorinated and tri-chlorinated biphenyl removal in both soils, but PSM did not improve removal of these congeners. On the contrary, HP- β-CD decreased PCB removal when used in combination with biphenyl. Two-way analysis of variance indicated that HP- β-CD significantly increased tetra- and penta-chlorinated biphenyl removal from the high organic matter soil, but not from the low organic matter soil. Principal components analysis of PLFA data indicated that HP- β-CD increased proportions of 18:1 ω7c associated with Gram-negative bacteria, but decreased 10me16 and 10me17 lipid associated with Gram-positive bacteria, while biphenyl and PSMs had no detectable effects on soil microbial communities. PCB removal was not correlated to any PLFA. In conclusion, PSM previously shown to enhance PCB removal in soil-free systems were not effective in two divergent soils evaluated in this study, and HP- β-CB had increase, decrease, or no effect on PCB removal depending on types of PCB congeners, soils, and co-amendments.

Polycyclic aromatic hydrocarbon behavior in bioactive soil slurry reactors amended with a nonionic surfactant

The effects of an ethoxylated sorbitan fatty ester nonionic surfactant (Tween 80) on the bioavailability of polycyclic aromatic hydrocarbons (PAHs) were examined by using soil-free and dense-slurry (67% solids content, by wt) systems containing a creosote-contaminated field soil. The dispersed-micelle-phase PAHs in soil-free systems were not readily bioavailable to the mixed consortium of microbes indigenous to the creosote-contaminated soil. Instead, the microbes partially and preferentially utilized readily available portions of the surfactant as carbon sources (16-18% of the initial surfactant dose). This selective microbial attack resulted in destabilization of dispersed-phase micelles and significant decreases in molar solubilization ratio and micelle-water partition coefficient values. Remarkably high dosages (>20 g/L) of Tween 80 were required to enhance mobilization of the sorbed PAHs via micelle association because of the sorption of Tween 80 to the soil employed. The PAHs released from the destabilized micelles in soil-slurry systems either associated with sorbed-phase surfactants or readsorbed to soil organic matter too rapidly to be biologically accessed, even by the acclimated PAH-degrading microbes present. The work provides important new information and practical insights to surfactant solubilization and mobilization technology applications for the bioremediation of PAH-contaminated soils and sediments.

A novel solubilization of phenanthrene using Winsor I microemulsion-based sodium castor oil sulfate

Problems associated with polycyclic aromatic hydrocarbons (PAHs) contaminated site in environmental media have received increasing attention. Ex situ soil washing is commonly used for treating contaminated soils by separating the most contaminated fraction of the soil for disposal. Surfactant-enhanced soil washing is being considered with increasing frequency to actually achieve soil-contaminant separation. In this research, a novel solubilization of phenanthrene and extraction of phenanthrene from spiked soil by sodium castor oil sulfate (SCOS) microemulsion was presented and compared with the conventional surfactants, Triton X-100 (TX100), Tween 80 (TW80), Brij35, sodium dodecylbenzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS). Unlike conventional surfactants, SCOS forms stable microemulsion in water and thus behaves much like a separate bulk phase in concentrating organic solutes. The extent of solubility enhancement is linearly proportional to the concentration of SCOS microemulsion, in contrast with the effect of a conventional surfactant in which a sharp inflection occurs in the vicinity of the measured critical micelle concentration. SCOS microemulsion exhibits the largest mass solubilization ratio among the selected surface active agents (SAAs) in both soil-free system and soil–water system. The partitioning coefficients of phenanthrene between the emulsified phase and the aqueous phase, K em, is slightly larger than those between the micellar pseudo phase and the aqueous phase, K mc. The extraction experiments demonstrate high and fast desorption of phenanthrene from spiked soil by SCOS microemulsion perhaps due to its high solubilization capacity compared with the conventional surfactant solutions. The results show that SCOS could be an attractive alternative to synthetic surfactants in ex situ washing for PAH-contaminated soils.

Removal of azinphos methyl by alfalfa plants ( Medicago sativa L.) in a soil-free system

The objective of this study was to investigate the removal of azinphos methyl assisted by alfalfa plants, with special emphasis on the effects of this compound on some plant's physiological parameters. Hydroponic cultures of alfalfa ( Medicago sativa L., var Romagnola) were employed as a model system. These cultures were exposed to a nutrient medium containing 10 mg/l of azinphos methyl. A first-order kinetic approach was used to describe the removal of azinphos methyl from the solution. After 20 days of culture, the initial amount of azinphos methyl was reduced to non-detectable levels in the presence of plants. In the absence of plants, 20% of azinphos methyl remained in the solution after 30 days of treatment. The half-life of the pesticide was reduced from 10.8 to 3.4 days in the presence of plants. The growth index of alfalfa plants exposed to azinphos methyl was negatively affected. Chlorophyll contents were reduced after 24 h of treatment and thereafter the levels were comparable to that of control plants. The peroxidase activity of alfalfa roots was not affected by the presence of azinphos methyl. In conclusion, alfalfa plants were able to survive when exposed to an effective concentration of 10 mg/l of azinphos methyl in the root zone, with some alterations on their physiological parameters.

Removal of azinphos methyl by alfalfa plants (Medicago sativa L.) in a soil-free system

Removal of azinphos methyl by alfalfa plants (Medicago sativa L.) in a soil-free system

Hydrolysis and Soil Adsorption of the Labile Herbicide Isoxaflutole

Isoxaflutole (5-cyclopropyl isoxazol-4-yl-2-mesyl-4-trifluoromethylphenyl ketone) is a new herbicide marketed for broadleaf and grass weed control in corn, but little information has been published on the soil behavior and environmental fate of the compound. The herbicide exhibits an unusual behavior in which it is functionally reactivated by rainfall events, providing control of small weeds that have emerged. Isoxaflutole is extremely labile in aqueous solution, thus measuring equilibrium sorption is challenging. A qualitative kinetic evaluation was performed to characterize the sorption of isoxaflutole, during rapid hydrolysis to its bioactive product, a diketonitrile derivative (2-cyclopropyl-3-(2-mesyl-4-trifluoromethylphenyl)-3-oxopropanenitrile). The transformation was measured over time in a herbicide-treated aqueous solution with or without soil. At 25 °C, 83% of the parent compound remained in solution at 24 h in the soil free system, but only 15% remained in the solution in the presence of soil. The sorbed phase consisted mainly of isoxaflutole, although a small percentage of diketonitrile was also detected in increasing concentrations as the study progressed. Hydrolysis prevented the attainment of sorption equilibrium, thus the apparent Kd of isoxaflutole increased over time, while that of diketonitrile remained close to zero at both 5 and 25 °C. Batch sorption isotherms were conducted with both isoxaflutole and diketonitrile using four Illinois soils of the Drummer, Flanagan, Catlin, and Cisne series ranging in organic carbon (OC) from 1.0 to 2.5%. Freundlich Kd values were 6−12-fold greater for isoxaflutole than diketonitrile, with the greatest difference in the lower organic carbon soils. After removing the hydrolysis effect, sorption of the isoxaflutole and diketonitrile was independent of temperature, suggesting that it was an entropy-driven process. Based on soil OC content, Koc values of 134 and 17 mL g-1 were calculated for isoxaflutole and diketonitrile, respectively. Results suggest that desorption coupled to hydrolysis promotes reactivation of the herbicide's function after rainfall and contributes to the efficacy of the compound by resupplying the soil solution with a bioactive product.

Bioavailability of Soil-Sorbed Biphenyl to Bacteria

Limited bioavailability of organic pollutants in soil may be a detriment to the successful application of bioremediation. The availability of soil-sorbed biphenyl to two biphenyl-degrading bacteria, Pseudomonas putida P106 and Rhodococcus erythropolis NY05, was assessed using a kinetic mineralization assay. Biphenyl was aged in four soils of different organic carbon (OC) contents (0.4−7.8%) for up to 274 days. With a biphenyl-soil contact time of 24 h, the initial mineralization rates (IMRs) ranged from 2.6 to 3.5 μg·L-1·min-1 for strain P106 and from 3.8 to 0.92 μg·L-1·min-1 for strain NY05. These IMRs were higher than those of soil-free controls and those predicted by a coupled desorption/biodegradation model, suggesting both strains of bacteria could access soil-sorbed biphenyl. For strain P106, biphenyl mineralization curves in slurries of four different soils were nearly coincident with those in soil-free systems containing the same total mass of biphenyl. This strain appeared to have immediate and c...

Weed control with atrazine and chlorsulfuron is determined by herbicide availability and persistence in soils

Effectiveness and length of weed control with atrazine and chlorsulfuron can be variable in the field. While some of this may be due to climatic variations, differences in soil properties may also be important. We tested this by recording changes in control of mintweed (Salvia reflexa Hornem.) and turnip weed (Rapistrum rugosum L.) with time in different soils, and comparing these results with the measured changes in plant-available herbicide in the soils. Length of weed control with the same herbicide rate varied from 0 to &gt;15 weeks. Mintweed and turnip weed were controlled (85-100%) only when the soils had ¸ 0·1µ#9839;g available atrazine/g and 0·8 ng available chlorsulfuron/g, respectively. This agreed with the sensitivity data for these weeds when grown in a soil-free system. The herbicides were initially more available in grey clays than in black earths, and soil pH accounted for most of the variations in the persistence of the available residues. Thus, the efficacy of these herbicides in different soils could be estimated if the available residues in the root-zone could be predicted and the sensitivity of different weeds was known.

Later events in suppression of Fusarium root rot of red pine seedlings by the ectomycorrhizal fungus Paxillus involutus

Colonization of primary roots of red pine (Pinus resinosa Ait.) seedlings by two isolates of the fungal pathogen Fusarium oxysporum Schlecht. emend Snyd. &amp; Hans. f.sp. pini in a sterile, soil-free system and a nonsterile rooting medium was suppressed for 2 months if the roots were exposed to the ectomycorrhizal fungus Paxillus involutus (Batsch.) Fr. for 1 week before being inoculated with the pathogen. Roots of protected seedlings harboured only a few hyphae of the pathogen. These were either vacuolated or filled with slightly electron-dense substances and did not penetrate host cells. In contrast, roots of seedlings not inoculated by P. involutus were extensively colonized by F. oxysporum, and host tissue vacuolation and disorganization occurred. Papillae, consisting of either small protuberances or elongate projections, were infrequently formed in response to infection. Chlamydospores of F. oxysporum that formed in the sterile system had numerous lipid deposits and amorphous thick walls but few mitochondria and little endoplasmic reticulum. Key words: biological control, ectomycorrhiza, Fusarium, Pinus resinosa, disease suppression.

Asbestos in the natural environment

Chrysotile asbestos is a toxic and carcinogenic mineral that has been used in a variety of industrial and consumer applications. Much of the fiber- and cement-containing asbestos waste has ended up in terrestrial environments. Chrysotile weathering in soils and the potential for natural attenuation have, however, hardly been examined yet. Here we explored how soil properties influence the dissolution rate of chrysotile, the release of the carcinogenic metals chromium and nickel, and the hydroxyl radical (HO•) generation by chrysotile fibers. Chrysotile dissolution rates in soil suspensions decreased with increasing soil-pH and were lower than reported rates in soil-free systems. Dissolved organic carbon did not markedly accelerate dissolution at circumneutral pH, whereas cement mixed with soil inhibited dissolution because of its alkalinity. The HO•-yield of incubated fibers in non-amended soils eventually decreased by 60–75%. The decline was fastest in an acidic podzol soil, yet was followed by a small rebound. Cement amendment induced the largest HO•-yield reduction (∼90%), presumably due to surface coating of the fibers. Overall, this work demonstrates that the potential for natural attenuation of chrysotile asbestos in soils critically depends on soil chemical parameters and the presence of cement in association with the fibers.

The fate of dichlorvos in soil

Experiments were conducted to determine the factors responsible for the loss (adsorption, chemical hydrolysis, microbial degradation, etc.) of dichlorvos (2,2-dichlorovingl 0 , 0 -dimethyl phosphate) in soil perfusion systems of Houston Black clay. The rate of disappearance from the perfusate (hence the rate of dichlorvos degradation in soil) was related directly to the presence of Bacillus cereus in the perfusion system, the pH of the system, and the extent of dichlorvos adsorption. Gas liquid chromatographic analyses of the perfusates showed that dichlorvos disappearance was rapid when B . cereus was added to a previously sterilized soil perfusion system (50% in 3.9 days). Under sterile conditions, 50% of the added dichlorvos was recovered after 10 days. When B . cereus was added to a mineral salts medium containing dichlorvos as sole ccrbon source, 49% of the initial dichlorvos concentration was degraded in 4 days. The organism was not capable of utilizing dichlorvos as a sole phosphorus source. Chemical hydrolysis of dichlorvos in aqueous, buffered, soil-free systems showed that hydrolysis did not occur in very acid systems (< pH 3.3), but increased with increasing pH values (26% in 4 days at pH 6.9), and was rapid at pH 9.3 (> 99% in 2 days). The extent of dichlorvos adsorption was determined by comparing the initial loss of dichlorvos in a sterile, soil-free extract solution with the initial loss in a sterile soil perfusion system. The rapid initial disappearance of dichlorvos in the presence of sterilized soil was attributed to soil adsorption of the pesticide. After 10 days both systems contained equal concentrations (50%) of dichlorvos. Non-biological mechanisms accounted for 70% of the total degradation of dichlorvos, while bacterial degradation accounted for 30% in the soil perfusion systems.

Soil Degradation of Malathion, a Phosphorodithioate Insecticide

AbstractRates of malathion degradation in soils were related directly to extent of malathion adsorption, suggesting that degradation occurred by a chemical mechanism which was catalyzed by adsorption. Malathion degradation was rapid (50 to 90% in 24 hours, depending on the type of soil) in both sterile and nonsterile soil systems, and no lag phase occurred prior to degradation. In aqueous soil‐free systems inoclulated with a soil extract, a lag phase (7 days) occurred, followed by rapid malathion loss, likely due to microbial degradation. Thus in soils, complete chemical degradation of malathion occurred prior to microbial adaptation to malathion. To obtain information on the pathway and products of malathion degradation, chemical hydrolysis in soil‐free systems was investigated using liquid:liquid partition techniques with 14C‐labeled malathion. Hydrolysis did not occur in acid systems (&gt; pH 2), was slow at pH 9 (&lt; 50% in 20 days) and rapid at pH 11 (&gt; 99% in 1 day). At pH 9 the hydrolysis resulted in formation of thiomalic acid and dimethyl thiophosphoric acid as final products with accumulation of diethyl thiomalate as an intermediate due to unequal rates of hydrolysis of the ester linkages. In soils, both ester linkages are hydrolyzed, although not at the same rate, resulting in the accumulation of diethyl thiomalate in some soils.