Complex behavior of petroleum hydrocarbons in vadose zone: A holistic analysis using unsaturated soil columns.

The migration of petroleum hydrocarbons in vadose zone involves complex coupled processes such as downward displacement and natural attenuation. Despite its significance in determining groundwater vulnerability to petroleum contamination and optimizing the remedial strategy, it has not been comprehensively studied in terms of overall processes under field-relevant conditions. In this study, a series of unsaturated soil column experiments were conducted by simulating subsurface diesel contamination within a vadose zone using different soil textures at different soil bulk densities and initial diesel concentrations, while partly exposing them to simulated precipitation. The results showed that the soil column with less fine fraction was favorable for the downward migration of diesel but unfavorable for its natural degradation. However, precipitation complicated the relative conductivities of multiple fluids (water, air, and diesel) through the pore network, therby decreasing diesel migration and degradation. For example, the downward migration of diesel in the SL column decreased by 8.4% under precipitation, while the overall attenuation rate dropped to almost 0.24% of its original state. Lowering bulk density (from 1.5 to 1.23 g/cm3), however, could enhance the attenuation rate presumably due to the secured void space for the incoming fluids. A high initial concentration of diesel (2%; w/w) inhibited its natural attenuation, while its influence on its vertical propagation after the precipitation was not significant. The present findings provide a mechanistic basis for approximating the behavior of petroleum hydrocarbons in a random vadose zone.

Coupled effect of porous network and water content on the natural attenuation of diesel in unsaturated soils.

The natural attenuation potential of a vadose zone against diesel is critical for optimizing remedial actions and determining groundwater vulnerability to contamination. Here, diesel attenuation in unsaturated soils was systematically examined to develop a qualitative relationship between physical soil properties and the natural attenuation capacity of a vadose zone against diesel. The uniformity coefficient (Cu) and water saturation (Sw, %) were considered as the proxies reflecting the degree of effects by porous network and water content in different soils, respectively. These, in turn, are related to the primary diesel attenuation mechanisms of volatilization and biodegradation. The volatilization of diesel was inversely proportional to Cu and Sw, which could be attributed to effective pore channels facilitating gas transport. Conversely, biodegradation was highly proportional to Cu under unsaturated conditions (Sw = 35-71%), owing to nutrients typically associated with fine soil particles. The microbial community in unsaturated soils was affected by Sw rather than Cu. The overall diesel attenuation including volatilization and biodegradation was optimized at Sw = 35% for all tested soils.

What determines the efficacy of landfarming for petroleum-contaminated soils: Significance of contaminant characteristics.

Identifying the cause of inconsistent landfarming efficacy is critical to designing optimal remedial strategies for petroleum-contaminated sites. We assessed contaminated soils collected from two former military bases in South Korea to better understand the role and influence of different factors. Landfarming remediation was simulated in the laboratory by applying comparable practices (such as tillage and bioaugmentation) and the relevant mechanism was examined. We then systematically examined potential factors affecting petroleum-removal efficacy, including the content of fine soil particles, the initial concentration and composition of petroleum contaminants, and the degree of soil-contaminant interaction. The distribution range of total petroleum hydrocarbons (TPHs) and the size of unresolved complex mixture (UCM) found in gas chromatography data showed that petroleum composed of TPHs with lower carbon numbers and having smaller size of UCM could be treated more effectively by landfarming. Incorporating the evaluation of the distribution range and UCM properties of petroleum, rather than simply considering its total concentration, is a more accurate and efficient method for determining the site-specific suitability of landfarming as a remedial option, as well as for assessing the necessity of supplementary processes.

Activation of Hydrogen Peroxide by a Titanium Oxide-Supported Iron Catalyst: Evidence for Surface Fe(IV) and Its Selectivity.

Iron immobilized on supports such as silica, alumina, titanium oxide, and zeolite can activate hydrogen peroxide (H2O2) into strong oxidants. However, the role of the support and the nature of the oxidants produced in this process remain elusive. This study investigated the activation of H2O2 by a TiO2-supported catalyst (FeTi-ox). Characterizing the catalyst surface in situ using X-ray absorption spectroscopy (XAS), together with X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), revealed that the interaction between H2O2 and the TiO2 phase played a key role in the H2O2 activation. This interaction generated a stable peroxo-titania ≡Fe(III)-Ti-OOH complex, which reacted further with H2O to produce a surface oxidant, likely ≡Fe[IV] ═ O2+. The oxidant effectively degraded acetaminophen, even in the presence of chloride, bicarbonate, and organic matter. Unexpectedly, contaminant oxidation continued after the H2O2 in the solution was depleted, owing to the decomposition of ≡Fe(III)-Ti-OOH by water. In addition, the FeTi-ox catalyst effectively degraded acetaminophen over five testing cycles. Overall, new insights gained in this study may provide a basis for designing more effective catalysts for H2O2 activation.

Targeted removal of trichlorophenol in water by oleic acid-coated nanoscale palladium/zero-valent iron alginate beads.

A new material was developed and evaluated for the targeted removal of trichlorophenol (TCP) from among potential interferents which are known to degrade removal activity. To achieve TCP-targeted activity, an alginate bead containing nanoscale palladium/zero-valent iron (Pd/nZVI) was coated with a highly hydrophobic oleic acid layer. The new material (Pd/nZVI-A-O) preferentially sorbed TCP from a mixture of chlorinated phenols into the oleic acid cover layer and subsequently dechlorinated it to phenol. The removal efficacy of TCP by Pd/nZVI-A-O was not affected by co-existing organic substances such as Suwannee River humic acid (SRHA), whereas the material without the oleic acid layer (Pd/nZVI-A) became less effective with increasing SRHA concentration. The inorganic substances nitrate and phosphate significantly reduced the reactivity of Pd/nZVI-A, however, Pd/nZVI-A-O showed similar TCP removal efficacies regardless of the initial inorganic ion concentrations. The influence of bicarbonate on the TCP removal efficacies of both Pd/nZVI-A and Pd/nZVI-A-O was not significant. The findings from this study suggest that Pd/nZVI-A-O, with its targeted, constant reactivity for TCP, would be effective for treating this contaminant in surface water or groundwater containing various competitive substrates.

Effects of washing solution and drying condition on reactivity of nano-scale zero valent irons (nZVIs) synthesized by borohydride reduction.

Washing and drying processes are essential when synthesizing nano-scale zero valent irons (nZVIs) by borohydride reduction of iron salts in aqueous phase. However, effects of these processes on nZVI reactivity have not been investigated in detail, although different washing and drying conditions might alter surface characteristics of nZVIs and thus vary their reactivity towards reducible contaminants. In this study, effects of three washing solutions and drying conditions on the reactivity of nZVIs for nitrate were investigated. Washing with volatile solvents and drying under anaerobic condition decreased thickness of Fe-oxide layer on nZVIs and increased content of Fe(2+)-containing oxides in the layer, which enhanced nZVI reactivity toward nitrate. Volatile solvent washing could minimize the decrease in nZVI reactivity according to changing anaerobic drying condition to aerobic. Findings from this study suggest that application of washing with volatile solvents and drying under aerobic condition should be recommended as effective processes to obtain nZVIs with maximum reactivity at reasonable costs and efforts.