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This study examined the adsorption of pentachlorophenol (PCP) on geosorbents (original bulk soil and soils treated to significantly remove organic carbon and oxides of Fe and Mn) under oxic and anoxic experimental conditions. pH effects on adsorption capacities were maximal at pH K a of PCP (4.7). Decrease in redox potential and an increase in dissolve organic carbon inhibited PCP sorption. While contribution of Mn oxide to PCP sorption was negative due to decreasing charge interference, the study equally implied negligible role of surface area compared to organic matter contents and oxides of Fe with R 2 ≤ 0.825. The rate of Fe 2+ and Mn 2+ release under anoxic condition reduced interactions of phenolate with iron oxides and consequently, the distribution coefficients coupled with increased dissolve organic carbon. Pseudo second order and Langmuir isotherm provided the best kinetic and isotherm models respectively, although adsorption processes appeared to involve more than one kinetic stage based on intra-particle diffusion model ( R 2 ≤ 0.982). The study suggests that soil treatment to remove Mn oxide may encourage transitory PCP sinks in some reductomorphic environments.

2,4-dichlorophenoxyl acetic acid (2,4-D, pKa = 2.8) is used extensively as a herbicide in agricultural practices. Its sorption behavior on both untreated and soils treated to significantly remove specific components (organic and iron and manganese [Fe-Mn] oxides and hydroxides phases) was investigated under oxic and anoxic conditions. The chemical and structural heterogeneity of the soil components were characterized by elemental analysis and X-ray diffraction (XRD). The coexistence of the various components seems to either mask sorption sites on the untreated soil surfaces or inhibit interlayer diffusion of 2,4-D. All sorption data conform to the Freundlich description and a pseudo-second-order kinetic model. There was a strong positive correlation between sorption capacity K(d), and surface area (r(2) ≤ 0.704), but a negative correlation was uncovered with both pH and organic carbon (r(2) ≤ -0.860). The results indicate that 2,4-D is preferably sorbed under oxic rather than anoxic conditions and it is greater on soils containing a high Fe content. There was incomplete 2,4-D sorption reversibility, with desorption occurring more rapidly under anoxic conditions. The study suggests that stimulation of Fe III reduction could be used for the bioremediation of a 2,4-D-contaminated site.

Predicting the behavior, fate, and transport potential of a herbicide in any soil involves understanding the sorption characteristics. The sorption characteristics of glyphosate (GPS) on soil and their main components were investigated, indicating that the mineral phase is more important than the organic carbon in adsorption of GPS. Sorption isotherms were determined from each component using the batch equilibrium method at various concentrations (5, 10, 15, 20, 25, and 30 mg L−1) and sorption affinity of GPS was approximated by the Freundlich equation. The sorption strength K f [mg kg−1 (L mg−1)−n] across the various components ranged from 2.1–134.9 while the organic carbon-normalized Freundlich sorption capacity values, K foc, ranged from 1.28–3.53 mg kg−1-OC/(mg L−1)n. Infrared Fourier transform spectroscopy (FTIR) of the components showed significant structural differences. The results suggest that the presence of the oxides and hydroxides iron, in particular in soil solutions, enhanced GPS adsorption...

Chlorobenzoic acids represent crucial recalcitrant metabolites in the environment; thus, the influence of soil components on the sorption of 2,4,6-trichlorobenzoic acid (TCB) under oxic and anoxic conditions was studied. The surficial physiognomies of untreated and isolated soil samples were studied using FTIR, XRD, specific surface area, and PZC determination. The roles of redox potential, dissolved organic carbon (DOC), and pH, particularly under anoxic condition, were appraised. Batch equilibrium adsorption studies on soils of variable Fe/Mn oxides and organic carbon showed that adsorption was low across all components (log K oc = 0.82–3.10 Lg−1). The sorption of 2,4,6-TCB was well described by the pseudo second-order kinetic model. The fluctuation of both redox potential and pH during anoxic experiment had a negative impact on the sorption, partitioning, and the oxidation of organic matter. Linear relationships were observed for K d with both soil total organic carbon (TOC) and surface area (SA). The results showed the existence of DOC-mediated sorption of 2,4,6-TCB which seems to be enhanced at lower pH. The reductive dissolution, particularly of iron compounds, possibly impeded sorption of 2,4,6-TCB under anoxic condition. It could be inferred that habitats dominated by fluctuating oxygen concentrations are best suited for the development of environmental conditions capable of mineralizing 2,4,6-TCB and similar xenobiotics.