Assessing the phytoavailability of dieldrin residues in charcoal-amended soil using tenax extraction

Lindane and hexachlorobenzene sequestration and detoxification in contaminated sediment amended with carbon-rich sorbents

Sediment represents a sink for toxic and persistent chemicals such as hexachlorobenzene (HCB) and lindane (γ-HCH). This paper investigates the possibility of reducing the risks associated with the presence of these pollutants in sediments by amending the sediment with carbon-rich materials (activated carbon (AC) and humus (HC)) to sequester the contaminants and render them biologically unavailable. The effects of the dose and contact time between the sediment and the carbon-rich amendments on the effectiveness of the detoxification are estimated. Four doses of carbon-rich amendments (0.5-10%) and four equilibration contact times (14-180 days) were investigated. Results have shown that the bioavailable fraction of γ-HCH and HCB decreased significantly in comparison to the unamended sediment. Regarding the AC amendments, almost 100% for both compounds; and for HC amendments around 95% for γ-HCH, and 75% for HCB. Aging caused further reductions in the bioavailable fraction, compared to the untreated sediment. Phytotoxicity tests showed that Zea mays accumulated significantly higher amount of γ-HCH and HCB from unamended sediment, comparing to Cucurbita pepo and Lactuca sativa. Toxicity of HC and AC amended sediment assessed by Vibrio fischeri luminescence inhibition test and by measuring Zea mays germination and biomass yield was significantly reduced in the amended sediment samples. γ-HCH and HCB accumulation in the Zea mays biomass in the unamended sediment were a significantly higher than in the all HC and AC amended sediment. Both sorbents show potential to be used as remediation agents for organically contaminated sediment, but AC exhibited the better performance.

Bioavailability and bioaccessibility of polycyclic aromatic hydrocarbons from (post-pyrolytically treated) biochars

Bioaccessibility data of PAHs from biochar produced under real world conditions is scarce and the influence of feedstock and various post-pyrolysis treatments common in agriculture, such as co-composting or lacto-fermentation to produce silage fodder, on their bioavailability and bioaccessibility has hardly been studied. The total (C_total), and freely dissolved (i.e., bioavailable) concentrations (C_free) of the sum of 16 US EPA PAHs of 43 biochar samples produced and treated in such ways ranged from 0.4 to almost 2000 mg/kg, and from 12 to 81 ng/L, respectively, which resulted in very high biochar-water partition coefficients (4.2 ≤ log K_D ≤ 8.8 L/kg) for individual PAHs. Thirty three samples were incubated in contaminant traps that combined a diffusive carrier and a sorptive sink. Incubations yielded samples only containing desorption-resistant PAHs (C_res). The desorption resistant PAH fraction was dominant, since only eight out of 33 biochar samples showed statistically significant bioaccessible fractions (f_{bioaccessible} = 1 - C_res/C_total). Bioavailability correlated positively with C_total/surface area. Other relationships of bioavailability and -accessibility with the investigated post-pyrolysis processes or elemental composition could not be found. PAH exposure was very limited (low C_free, high C_res) for all samples with low to moderate C_total, whereas higher exposure was determined in some biochars with C_total > 10 mg/kg.

How to Determine the Environmental Exposure of PAHs Originating from Biochar

Biochars are obtained by pyrolyzing biomass materials and are increasingly used within the agricultural sector. Owing to the production process, biochars can contain polycyclic aromatic hydrocarbons (PAHs) in the high mg/kg range, which makes the determination of the environmental exposure of PAHs originating from biochars relevant. However, PAH sorption to biochar is characterized by very high (10(4)-10(6) L/kg) or extreme distribution coefficients (KD) (>10(6) L/kg), which makes the determination of exposure scientifically and technically challenging. Cyclodextrin extractions, sorptive bioaccessibility extractions, Tenax extractions, contaminant traps, and equilibrium sampling were assessed and selected methods used for the determination of bioavailability parameters for PAHs in two model biochars. Results showed that: (1) the KD values of typically 10(6)-10(9) L/kg made the biochars often act as sinks, rather than sources, of PAHs. (2) Equilibrium sampling yielded freely dissolved concentrations (pg-ng/L range) that were below or near environmental background levels. (3) None of the methods were found to be suitable for the direct measurement of the readily desorbing fractions of PAHs (i.e., bioacessibility) in the two biochars. (4) The contaminant-trap method yielded desorption-resistant PAH fractions of typically 90-100%, implying bioaccessibility in the high μg/kg to low mg/kg range.

Assessment of the direct effects of biogenic and petrogenic activated carbon on benthic organisms

Activated carbon (AC) has long been associated with the capacity to effectively remove organic substances from aquatic and sediment matrices; however, its use in remediation purposes has drawn some concern due to possible impacts on benthic communities. Within the inner Oslofjord, the use of AC has been well documented for reducing the risks associated with dioxins or dioxin-like compounds from contaminated areas. However, benthic surveys performed on areas treated with AC have revealed that the abundance of organisms inhabiting these areas can be reduced significantly in the subsequent years following treatment. The reason for the reduction in the benthic communities is currently unknown, and therefore, an integrated approach to assess the effects of 2 different forms of AC (biogenic and petrogenic) on benthic organisms has been performed. A battery of 3 different benthic organisms with different feeding and life-cycle processes has been used encompassing sediment surface feeders, sediment ingestors, and sediment reworkers. Results of the tests indicated that although AC is not acutely toxic at concentrations up to 1000 mg/L, there may be physical effects of the substance on benthic dwelling organisms at environmentally relevant concentrations of AC at remediated sites.

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.

A review of biochars' potential role in the remediation, revegetation and restoration of contaminated soils

Biochars are biological residues combusted under low oxygen conditions, resulting in a porous, low density carbon rich material. Their large surface areas and cation exchange capacities, determined to a large extent by source materials and pyrolysis temperatures, enables enhanced sorption of both organic and inorganic contaminants to their surfaces, reducing pollutant mobility when amending contaminated soils. Liming effects or release of carbon into soil solution may increase arsenic mobility, whilst low capital but enhanced retention of plant nutrients can restrict revegetation on degraded soils amended only with biochars; the combination of composts, manures and other amendments with biochars could be their most effective deployment to soils requiring stabilisation by revegetation. Specific mechanisms of contaminant-biochar retention and release over time and the environmental impact of biochar amendments on soil organisms remain somewhat unclear but must be investigated to ensure that the management of environmental pollution coincides with ecological sustainability.