Walter J. Farmer

Phenanthrene binding and sorption to dissolved and to mineral-associated humic acid

Abstract Phenanthrene, a three ring aromatic hydrocarbon, was used as a model substrate to distinguish between binding and sorption to dissolved and to mineral-associated humic acid (HA), with an emphasis on its implication to bioavailability. Binding was analyzed by a fluorescence quenching technique using Aldrich HA and HAs derived from compost, loamy and clay loamy soils. Measurements were made both in buffered water and in a mineral medium to be used later in a bioavailability study. Sorption of phenanthrene to mineral-associated HA was analyzed in batch experiments. Mineral–HA complexes were prepared with goethite, hematite, and Ca2+- and Cu2+-montmorillonite. Interaction of HA with clay minerals was greater at low pH and was dependent on the mineral type. Sorption of phenanthrene to these complexes was substantially higher at lower pH and higher when the HA was associated with montmorillonite rather than goethite or hematite. Binding coefficients (Kb(oc)) obtained for dissolved HA were at least an order of magnitude higher than sorption coefficients (Kp(oc)) obtained for mineral-associated HA. Both binding and sorption coefficient values were substantially higher for Aldrich HA. This work emphasizes the need for a detailed sorption study before conducting well controlled bioavailability experiments.

Bioavailability of phenanthrene sorbed to mineral-associated humic acid

Abstract Phenanthrene, a 3-ring aromatic hydrocarbon, was used as a model substrate to analyze the effect of sorption on the bioavailability of polycyclic aromatic hydrocarbons (PAHs). This study focused on sorption to mineral-associated humic acid (HA). Batch sorption analyses of phenanthrene to mineral-HA complexes were performed as integral part of the bioavailability experiments to account for the sorbed fraction under the various experimental conditions. A mixed culture that was enriched from a coal tar contaminated soil dominated by Pseudomonas sp . served as an inoculum. Phenanthrene mineralization was substantially enhanced upon sorption to mineral-HA complexes and the degree of enhancement was positively correlated with the fraction of sorbed phenanthrene. This stimulation is thought to be related to sorption of both the microorganisms and phenanthrene to the colloidal surfaces. This study suggests that when sorbed contaminants are still bioavailable, the presence of surfaces may stimulate mineralization.

Description of Simazine Transport with Rate‐Limited, Two‐Stage, Linear and Nonlinear Sorption

This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one- and two-stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate-limited sorption and transport of the simazine added simultaneously with the Br. The calibrated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single-stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratory-measured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current protocols.

The effect of sorption on phenanthrene bioavailability

Abstract Critical parameters that need to be considered in a bioavailability study are reviewed and applied to a study on the sorption (binding) of phenanthrene to dissolved humic acid (HA). Sorption coefficients values ( K ∞ ) of phenanthrene to HA were measured for HA from several sources. These values were used to calculate the amount of HA that was needed to bind different fractions of solution-phase phenanthrene. Sorption linearity and complete reversibility were assumed. A mixed culture of phenanthrene degraders, dominated by Pseudomonas sp. was sensitive to changes in dissolved phenanthrene concentrations in the range that was used in this study. Therefore, it was expected that if bound phenanthrene was not available, as is commonly stated in the literature, sorption would affect mineralization rate by reducing the concentration of free phenanthrene. However, it was found that mineralization of phenanthrene was not affected by sorption even when 90% of the phenanthrene was in the bound phase. It was concluded that the organisms were able to use the phenanthrene directly from the bound phase and at the same rate as from the free phase.

Spectroscopic evidence for the rate-limited accumulation of a persistent fraction of 1,2-dichloroethane sorbed onto clay minerals

Diffuse reflectance infrared spectra are collected in situ as a function of time during the sorption/desorption of 1,2-dichloroethane on montmorillonite and kaolinite. The spectra indicate that the sorptive becomes associated with the clay minerals in both a liquid and a labile vapor state, confirming previous results obtained with illite as the sorbent. They also provide evidence for an additional sorbed species that accumulates at a rate quite different from those of the other two. Features characteristic of this species persist in the spectra for periods of days after desorption is initiated. The results are interpreted in the context of sorbent porosity and the existence of a network of narrow pores that limits the rates of both sorption and desorption for the persistent species.

N2, CO2, and 1,2-dichloroethane as molecular probes of soil microstructure

Abstract The lysimeters of the San Dimas Experimental Forest were filled with fine sandy loam in 1937 and then planted with vegetation native to California. Extensive investigation of soils collected from the site in the last several years has revealed an array of soil development processes occurring there. The decomposition of plant litter, the flow of carbonaceous substances in the soil profiles, and the activity of earthworms have all contributed to significant differentiation of soils at the site as a function of depth and vegetation type. The impact of these processes on the microstructural character of soil particles is examined here. Soil samples collected from lysimeters planted with oak, pine, and chamise are included in this study. These samples are characterized with regard to their porosity using both N2 and CO2 as probe molecules. Porosity distributions are derived using density functional theory (DFT) that allows a uniform conceptual basis to be applied over the entire range of gaseous porosimetry. A third probe molecule, 1,2-dichloroethane (DCA), is used to determine whether the dynamics of transport for organic chemicals within soil particles can be related to differences in microstructural properties as defined by porosimetry. Time-resolved sorption/desorption processes for DCA on the various soil sorbents are characterized using in situ diffuse reflectance infrared spectroscopy (DRIFT). With all the sorbents investigated, the spectral sequences indicate the presence of three sorbed species: a sorbed liquid, a labile vapor, and a persistent vapor-phase species that increases in concentration even after prolonged desorption. N2 porosity distributions for these samples indicate that the accumulation of organic matter has significantly diminished the mesoporosity of the surface soils relative to the soils at further depths. The greatest reduction was observed for the soil sample with the highest measured organic C. Band areas resulting from curve-fitting the spectral data showed that levels of DCA sorbed as a liquid are similarly reduced. Microporosity as defined by the CO2-derived distributions exhibit the reverse trend with the highest levels for the surface mineral horizons. Differences in microporosity with depth reflect the activity of earthworms. No correlation was found between the CO2-measured microporosity of the soil samples and band areas for the persistently sorbed species. The presence and time-dependent behavior of this band is interpreted in the context of microstructural porosity and capillary flow. For both the sorbed liquid and persistent vapor, the major effect of C accumulation early in the soil development process appears to be the retardation of molecular transport into and out of soil particles.