Effects of sorption on biological degradation rates of (2,4-dichlorophenoxy) acetic acid in soils

Biosorption of dichlorodiphenyltrichloroethane and hexachlorobenzene in groundwater and its implications for facilitated transport

The potential for enhanced mobility of hydrophobic pollutants by cotransport with bacteria in saturated soils was evaluated from measurements of biosorption of 14C-labeled hexachlorobenzene and dichlorodiphenyltrichloroethane (DDT) to five strains of soil and sewage bacteria. The sorption process could be described by a linear partition equation and appeared to be reversible, but desorption kinetics were slow and/or partly irreversible. The DDT partition coefficients varied with equilibration time, possibly reflecting DDT-induced changes in the physiology of the bacteria. The partition coefficients, normalized to the masses of the bacteria, ranged from 250 to 14,000 for live cells, but the largest coefficients were associated with autoclaved cells of a Pseudomonas sp. The sorptive capacity of the bacterial biomass was greater for DDT than for hexachlorobenzene but was not correlated to overall bacterial hydrophobicity, measured by hydrophobic interaction chromatography. In a column study, 1.2 x 10(9) cells of a Bacillus sp. strain per ml enhanced DDT transport about 8-fold, whereas an advective-dispersive-sorptive equilibrium model for two mobile phases, water and free-living bacteria, suggested a 14-fold enhancement, based on the DDT partition coefficient. The disagreement was in part due to a retarded nonequilibrium movement of the bacteria. Model calculations based on literature data covering a wide range of organisms and compounds suggested that 10(6) cells ml-1 would increase the mobility of very hydrophobic compounds (log octanol-water partition coefficient [K(ow) of greater than or equal to 6), whereas higher densities of bacteria (10(8) cells ml-1) would have a significant impact on compounds with a log K(ow) of greater than or equal to 4.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of inoculant strain and organic matter content on kinetics of 2,4-dichlorophenoxyacetic acid degradation in soil

We monitored rates of degradation of soluble and sorbed 2,4-dichlorophenoxyacetic acid (2,4-D) in low-organic-matter soil at field capacity amended with 1, 10, or 100 micrograms of 2,4-D per g of wet soil and inoculated with one of two bacterial strains (MI and 155) with similar maximum growth rates (mu max) but significantly different half-saturation growth constants (Ks). Concentrations of soluble 2,4-D were determined by analyzing samples of pore water pressed from soil, and concentrations of sorbed 2,4-D were determined by solvent extraction. Between 65 and 75% of the total 2,4-D was present in the soluble phase at equilibrium, resulting in soil solution concentrations of ca. 8, 60, and 600 micrograms of 2,4-D per ml, respectively. Soluble 2,4-D was metabolized preferentially; this was followed by degradation of both sorbed (after desorption) and soluble 2,4-D. Rates of degradation were comparable for the two strains at soil concentrations of 10 and 100 micrograms of 2,4-D per g; however, at 1 microgram/g of soil, 2,4-D was metabolized more rapidly by the strain with the lower Ks value (strain MI). We also monitored rates of biodegradation of soluble and sorbed 2,4-D in high-organic-matter soil at field capacity amended with 100 micrograms of 2,4-D per g of wet soil and inoculated with the low-Ks strain (strain MI). Ten percent of total 2,4-D was present in the soluble phase, resulting in a soil solution concentration of ca. 30 micrograms of 2,4-D per ml.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential bioavailability of soil-sorbed naphthalene to two bacterial species

Prediction of the fate of hydrophobic organic contaminants in soils is complicated by the competing processes of sorption and biodegradation. To test the hypothesis that sorbed naphthalene is unavailable to degradative microorganisms, we developed a simple kinetic method to examine the rates and extents of naphthalene degradation in soil-free and soil-containing systems in a comparison of two bacterial species. The method is predicated on the first-order dependence of the initial mineralization rate on the naphthalene concentration when the latter is below the Michaelis-Menten half-saturation constant (Km) for naphthalene for the organism under study. Rates and extents of mineralization were estimated by nonlinear regression analysis of data by using both a simple first-order model and a three-parameter, coupled degradation-desorption model described for the first time here. Bioavailability assays with two bacterial species (Pseudomonas putida ATCC 17484 and a gram-negative soil isolate, designated NP-Alk) gave dramatically different results. For NP-Alk, sorption limited both the rate and extent of naphthalene mineralization, in accordance with values predicted on the basis of the equilibrium aqueous-phase naphthalene concentrations. For strain 17484, both the rates and extents of naphthalene mineralization exceeded the predicted values and resulted in enhanced rates of naphthalene desorption from the soils. We conclude that there are important organism-specific properties which make generalizations regarding the bioavailability of sorbed substrates inappropriate.

Influence of interfaces on microbial activity

Bacterial adhesion in natural and artificial systems has been critically reviewed to investigate the influences exerted by the presence of interfaces. Numerous investigations have demonstrated that, in the presence of a solid phase, the activity of bacterial cultures is changed. Reviewing relevant literature, two problems were encountered. One is of an experimental nature. Due to lack of similarity in experimental conditions, disparate experiments often cannot be compared; their results may even appear conflicting. The other problem is of an interpretational nature: several hypothetical theories exist which try to explain the effect of surfaces on microbial activity. These theories often confuse changes in the medium and limitations in mass transfer which are due to the presence of solid surfaces (indirect influences) with changes in cell properties (direct influences). Whenever a surface is reported to influence the metabolism of bacteria, the action is found almost exclusively to be due to changes in the medium or environment and is therefore indirect. Based on data reported in the literature, and by using thermodynamic and kinetic considerations, it is concluded that so far neither experimental nor theoretical evidence exists for a direct influence of interfaces on microbial activity.

Dissipation of s-triazines and thiocarbamates from soil as related to soil moisture content

Half-lives (t1/2) of two soil incorporated s-triazines (atrazine and prometon) and two thiocarbamate (EPTC and triallate) herbicides were determined in relation to soil moisture content in two California soils. Treated soils were incubated at three moisture levels in aerated glass vials at 25 +/- 1 degree C and were analyzed at 0, 7, 16, 28, 56 and 112 day intervals. Loss of herbicides in all treatments followed first-order kinetics. The t1/2-values of all herbicides decreased with increasing soil moisture and followed an empirical equation, t1/2 = aM(-b) (where t1/2 is half-life; M the moisture content; and a and b are constants). Soil moisture had a greater effect on carbamates than on s-triazines . Prometon exhibited the longest half-life in both soils, whereas EPTC was least persistent in one soil and atrazine in another. The t1/2-values for atrazine, prometon, EPTC, and triallate with medium moisture levels and 10 microg/g concentration were 34.6, 43.2, 25.4 and 38.1 days in sandy loam and 26.5, 44.4, 44.1 and 25.9 days in loamy sand, respectively. Disappearance of 50% of the applied concentrations of most of the herbicide-soil combinations (except EPTC and triallate in one soil) took longer for lower initial concentrations (1 microg/g) than for higher concentrations (10 microg/g).

Laccase-mediated detoxification of phenolic compounds

The ability of a polyphenoloxidase, the laccase of the fungus Rhizoctonia praticola, to detoxify phenolic pollutants was examined. The growth of the fungus could be inhibited by phenolic compounds, and the effective concentration was dependent on the substituents of the phenol. A toxic amount of a phenolic compound was added to a fungal growth medium in the presence or absence of a naturally occurring phenol, and half of the replicates also received laccase. The medium was then inoculated with R. praticola, and the levels of phenols in the medium were monitored by high-performance liquid chromatography analysis. The addition of the laccase reversed the inhibitory effect of 2,6-xylenol, 4-chloro-2-methylphenol, and p-cresol. Other compounds, e.g., o-cresol and 2,4-dichlorophenol, were detoxified only when laccase was used in conjunction with a natural phenol such as syringic acid. The toxicity of p-chlorophenol and 2,4,5-trichlorophenol could not be overcome by any additions. The ability of the laccase to alter the toxicity of the phenols appeared to be related to the capacity of the enzyme to decrease the levels of the parent compound by transformation or cross-coupling with another phenol.

Reductive Dechlorination of Polychlorinated Biphenyls by Anaerobic Microorganisms from Sediments

Microorganisms from Hudson River sediments reductively dechlorinated most polychlorinated biphenyls (PCBs) in Aroclor 1242 under anaerobic conditions, thus demonstrating PCB dechlorination by anaerobic bacteria in the laboratory. The most rapid dechlorination was observed at the highest PCB concentration used; at 700 parts per million Aroclor, 53 percent of the total chlorine was removed in 16 weeks, and the proportion of mono- and dichlorobiphenyls increased from 9 to 88 percent. Dechlorination occurred primarily from the meta and para positions; congeners that were substituted only in the ortho position (or positions) accumulated. These dechlorination products are both less toxic and more readily degraded by aerobic bacteria. These results indicate that reductive dechlorination may be an important environmental fate of PCBs, and suggest that a sequential anaerobic-aerobic biological treatment system for PCBs may be feasible.

Microbial degradation of acenaphthene and naphthalene under denitrification conditions in soil-water systems

This study examined the microbial degradation of acenaphthene and naphthalene under denitrification conditions at soil-to-water ratios of 1:25 and 1:50 with soil containing approximately 10(5) denitrifying organisms per g of soil. Under nitrate-excess conditions, both acenaphthene and naphthalene were degraded from initial aqueous-phase concentrations of about 1 and several mg/liter respectively, to nondetectable levels (less than 0.01 mg/liter) in less than 9 weeks. Acclimation periods of 12 to 36 days were observed prior to the onset of microbial degradation in tests with soil not previously exposed to polycyclic aromatic hydrocarbon (PAH) compounds, whereas acclimation periods were absent in tests with soil reserved from prior PAH degradation tests. It was judged that the apparent acclimation period resulted from the time required for a small population of organisms capable of PAH degradation to attain sufficient densities to exhibit detectable PAH reduction, rather than being a result of enzyme induction, mutation, or use of preferential substrate. About 0.9% of the naturally occurring soil organic carbon could be mineralized under denitrification conditions, and this accounted for the greater proportion of the nitrate depletion. Mineralization of the labile fraction of the soil organic carbon via microbial denitrification occurred without an observed acclimation period and was rapid compared with PAH degradation. Under nitrate-limiting conditions the PAH compounds were stable owing to the depletion of nitrate via the more rapid process of soil organic carbon mineralization. Soil sorption tests showed at the initiation of a test that the total mass of PAH compound was divided in comparable proportions between solute in the aqueous phase and solute sorbed on the solid phase. The microbial degradation of the PAH compound depends on the interrelationships between (i) the desorption kinetics and the reversibility of desorption of sorbed compound from the soil, (ii) the concentration of PAH-degrading microorganisms, and (iii) the competing reaction for nitrate utilization via mineralization of the labile fraction of naturally occurring soil organic carbon.

Physical and biological parameters that determine the fate of p-chlorophenol in laboratory test systems

Shake-flask and microcosm studies were conducted to determine the fate of para-chlorophenol (p-CP) in water and sediment systems and the role of sediment and nonsediment surfaces in the biodegradation process. Biodegradation of p-CP in estuarine water samples in shake flasks was slow over incubation periods of 300 h. The addition of detrital sediment resulted in immediate and rapid degradation evidenced by the production of 14CO2 from [14C]p-CP. The addition of sterile sediment, glass beads, or sand resulted in approximately four to six times more CO2 evolution than observed in the water alone. Densities of p-CP-degrading bacteria associated with the detrital sediment were 100 times greater than those enumerated in water. Bacteria in the water and associated with the sediment after preexposure of both water and sediment of p-CP demonstrated enhanced biodegradation. In some microcosms, p-CP was degraded completely in the top 1.0 cm of intact sediment beds. Sediment reworking activities by benthic invertebrates from one site were sufficient to mix p-CP deep into the sediment bed faster than biodegradation or molecular diffusion. p-CP was persistent at lower depths of the sediment, possibly a result of reduced oxygen conditions preventing aerobic biodegradation.

Biotransformations of Chloroguaiacols, Chlorocatechols, and Chloroveratroles in Sediments

The occurrence of trichloro- and tetrachloroguaiacols, -catechols, and -veratroles and their transformation was studied in freshwater and brackish water sediments putatively exposed to bleachery discharge. The samples contained both chloroguaiacols and chlorocatechols, of which >90% could not be removed by simple extraction. The bound concentrations varied and ranged from 550 μg kg of organic C −1 for 3,4,5-trichloroguaiacol to 8,250 μg kg of organic C −1 for tetrachlorocatechol. Chlorinated substrates added to the aqueous phase were rapidly bound to the sediment with K p values between 1.3 and 2.8 ml kg of organic C −1 for the chloroguaiacols and chloroveratroles and 22 to 36 ml kg of organic C −1 for the chlorocatechols. Sediment samples incubated aerobically brought about O-methylation of 4,5,6-trichloroguaiacol to 3,4,5-trichloroveratrole in a yield of ca. 25%. Under anaerobic conditions, however, de-O-methylation of both the chloroguaiacols and chloroveratroles took place with synthesis of the corresponding chlorocatechols. In separate experiments, the chlorocatechols were not completely stable under anaerobic conditions, but their ultimate fate has not yet been resolved. Sediment which had been autoclaved twice at 121°C for 20 min was unable to bring about any of these transformations; we therefore conclude that they were mediated by biological processes. These results emphasize that, in determining the fate of chloroguaiacols and related compounds discharged into the aquatic environment, the cardinal roles of sorption to the sediment phase and of the oxygen tension must be taken into account. We propose a hypothetical guaiacol cycle to accommodate our observations.

Adsorption mediated decrease in the biodegradation rate of organic compounds

A negative correlation between adsorption of low molecular weight organic acids and sugars onto a hydroxyapatite surface and biodegradation rates of the compounds in the presence of the mineral was observed. Qualitatively, the effect was the same whether the organics were equilibrated with the surface prior to the addition of organisms or the organisms were preattached to the surface. Glucose, acetic acid, succinic acid, glutamic acid, and citric acid showed equilibrium adsorption values ranging from 0-94% from a 2μM solution. Changes in both respiration and assimilation of the substrates in the presence of hydroxyapatite were inversely correlated with adsorption.