Angela H. Rhodes

Linking desorption kinetics to phenanthrene biodegradation in soil.

The desorption of polycyclic aromatic hydrocarbons (PAHs) often exhibits a biphasic profile similar to that observed for biodegradation whereby an initial rapid phase of degradation or desorption is followed by a phase of much slower transformation or release. Most investigations to-date have utilised a polymeric sorbent, such as Tenax, to characterise desorption, which is methodologically unsuitable for the analysis of soil. In this study, desorption kinetics of (14)C-phenanthrene were measured by consecutive extraction using aqueous solutions of hydroxypropyl-beta-cyclodextrin (HPCD). The data indicate that the fraction extracted after 24 h generally approximated the linearly sorbed, rapidly desorbing fraction (F(rap)), calculated using a three-compartment model. A good linear correlation between phenanthrene mineralised and F(rap) was observed (r(2) = 0.89; gradient = 0.85; intercept = 8.20). Hence HPCD extraction (24 h) and first-order three-compartment modelling appear to provide an operationally straightforward tool for estimating mass-transfer limited biodegradation in soil.

Influence of activated charcoal on desorption kinetics and biodegradation of phenanthrene in soil.

The observed strong sorption of polycyclic aromatic hydrocarbons (PAHs) to black carbon (BC) presents potential implications for PAH bioaccessibility in soils. The effects of BC on the desorption kinetics and mineralization of phenanthrene in four soils was investigated after 1, 25, 50, and 100 d soil-PAH contact time, using sequential hydroxypropyl-β-cyclodextrin (HPCD) extractions in soils amended with 0, 0.1, 1, and 5% (dry wt. soil) activated charcoal (AC, a form of BC). The rapidly (%F(rap)) and slowly (%F(slow)) desorbing phenanthrene fractions and their rate constants were determined using a first-order two-compartment (biphasic) desorption model. A minimum 7.8-fold decrease in %F(rap) occurred when AC was increased from 0 to 5%, with a corresponding increase in %F(slow). Desorption rate constants followed the progression k(rap) (% h(-1)) > k(slow) (% h(-1)) and were in the order of 10(-1) to 10(-2) and 10(-3) to 10(-4), respectively. Linear regressions between %F(rap) and the fractions degraded by a phenanthrene-degrading inoculum (%F(min)) indicated that slopes did not approximate 1 at concentrations greater than 0% AC; %F(min) often exceeded %F(rap), indicating a fraction of sorbed phenanthrene (%F(slow)) remained microbially accessible. Therefore, HPCD-desorption kinetics alone may not be an adequate basis for the prediction of the bioaccessibility of PAHs to microorganisms or bioremediation potential in AC-amended soils.

Impact of activated charcoal on the mineralisation of 14C-phenanthrene in soils.

The development of phenanthrene catabolism in four soils amended with varying concentrations of activated charcoal (AC) (0%, 0.1%, 1% and 5%), a type of black carbon, was investigated. Mineralisation of (14)C-phenanthrene was monitored after 1, 25, 50 and 100 d soil-PAH contact time; lag phases, rates and extents of mineralisation of the (14)C-phenanthrene to (14)CO(2) were determined. At concentrations >0.1% AC rates and extents of mineralisation were reduced by more than 99%. This revealed that the presence of >0.1% AC in soils may substantially diminish the rate at which the catabolic activity of indigenous soil microflora develops in contaminated soil. Soil C, which had the highest organic carbon (OC) content, consistently exhibited the highest extents of degradation. It is suggested that, in accordance with other researchers, OC may have blocked available phenanthrene sorption sites. This enhanced phenanthrene availability ultimately facilitated a greater level of catabolic activity within this soil. Such results reflect the complex nature of interactions between soil, biota and contaminants and their influence on the degradation of contaminants in the environment.

Assessing biodegradation potential of PAHs in complex multi-contaminant matrices

This study sought to extend validation of a cyclodextrin based extraction method for the assessment of PAH-biodegradation potential to complex multi-contaminant matrices. To this end, four reference materials (RMs) were produced by blending, in different proportions, soils impacted with diesel, lubricating oil and spent oxide. These reference materials had modest summation operatorPAH (16 US EPA) concentrations that ranged from 5.6+/-0.5 to 44.4+/-4.5 mg kg(-1). However, extractable petroleum hydrocarbon (EPH) concentrations were comparatively high (up to 2520+/-204 mg kg(-1)). To complement these RMs, two further soils from a municipal gas plant (MGP) with highly elevated concentration of PAHs ranging from 877+/-52 to 2620+/-344 mg kg(-1) were also tested. Results showed, regardless of matrix complexity, that PAH biodegradation within the four RM substrates, and two MGP soils correlated well with biodegradation predicted by hydroxypropyl-beta-cyclodextrin (HPCD) extraction.

Relationship between cyclodextrin extraction and biodegradation of phenanthrene in soil

A number of soil extraction techniques have been proposed to determine the microbial degradability of organic contaminants in soil. Exhaustive methods using organic solvents have little relevance to the concentration of contaminants actually available to microorganisms. The present study investigated the relationship between sequential hydroxypropyl-p-cyclodextrin (HPCD) extractions and microbial mineralization of [C-14] phenanthrene in four soils over time. The desorption of [C-14]phenanthrene was assessed at 24-h intervals over 10 d and compared to cumulative mineralization of the [C-14] phenanthrene using an enriched pseudomonad inoculum. The cumulative total of [C-14] phenanthrene extracted by HPCD exceeded the mineralization asymptote by more than 20%. The overall total extents mineralized after both single and multiple degrader inoculations, however, were statistically similar to that extracted after the first 24 h by HPCD; the ratios of extractable to mineralizable [C-14] phenanthrene consistently approached one. Furthermore, a good linear correlation between mineralized and extracted phenanthrene was observed (single: r(2) = 0.99, gradient = 0.90, intercept = 3.09; multiple: r(2) = 0;95, gradient = 1.01, intercept = -0.48), suggesting that a single HPCD extraction accurately and reproducibly predicts the total fraction of phenanthrene available for microbial mineralization in all soils tested in the present study.}

Using supercritical fluid extraction to measure the desorption and bioaccessibility of phenanthrene in soils

The aim of this paper was to measure the changing desorbable fraction and bioaccessibility of phenanthrene in two different soils with increasing soil-phenanthrene contact time using supercritical fluid extractions (SFE). Both soils were spiked with 100 mg kg(-1) phenanthrene and aged for 28d. Desorption profiles were measured every 7d using selective SFE conditions and the results were compared to 14C-phenanthrene mineralisation assays. Selective SFE showed significant differences in the rates and extents of desorption in the two soils, likely to be due to different organic matter composition. Post-extraction fitting of data yielded consistent SFE extraction times within ageing soils for bioaccessibility prediction.

Biogenic volatile organic compounds as a potential stimulator for organic contaminant degradation by soil microorganisms.

The effects of monoterpenes on the degradation of (14)C-2,4-dichlorophenol (DCP) were investigated in soils collected from areas surrounding monoterpene and non-monoterpene-emitting vegetation. Indigenous microorganisms degraded (14)C-2,4-DCP to (14)CO(2), after 1d contact time. Degradation was enhanced by prior exposure of the soils to 2,4-DCP for 32 d, increasing extents of mineralisation up to 60%. Monoterpene amendments further enhanced 2,4-DCP degradation, but only following pre-exposure to both 2,4-DCP and monoterpene, with total 2,4-DCP mineralisation extents of up to 71%. Degradation was greatest at the higher monoterpene concentrations (> or = 1 microg kg(-1)). Total mineralisation extents were similar between concentrations, but higher than the control and the 0.1 microg kg(-1) amendment, indicating that increases in monoterpene concentration has a diminishing enhancing effect. We suggest that monoterpenes can stimulate the biodegradation of 2,4-DCP by indigenous soil microorganisms and that monoterpene amendment in soils is an effective strategy for removing organic contaminants.