Degradation of polycyclic aromatic hydrocarbons in a mixed contaminated soil supported by phytostabilisation, organic and inorganic soil additives.

In soil, mixed contamination with potentially toxic trace elements and polycyclic aromatic hydrocarbons (PAHs) may persist for a long time due to strong adsorption to the soil matrix and to its toxicity to microorganism. We conducted an incubation batch experiment to test the effect of soil amendments (biochar, gravel sludge, iron oxides) on the immobilisation of trace elements. To monitor microbial degradation, a 13C-PHE (phenanthrene) label was introduced to soil for 13C-PLFA (phospholipid fatty acid) analysis. Soil amendments increased soil pH, reduced mobility of NH4NO3-extractable trace elements Cd and Zn, and increased mobile Cu. A small consortium of PHE degraders was identified mainly in the microbial groups of gram-negative bacteria and actinomycetes. The degradation process of PHE peaked 9days after incubation start. PAH concentrations remained constant in the soil within the 30-day incubation, except for the easily available 13C-PHE in the amended treatment. In order to test the effect of plants and soil amendments under more realistic conditions, we also conducted an outdoor pot experiment with black locust (Robinia pseudoacacia Nyirsegi). Furthermore, soil amendments increased the mobility of soil Cu and As and decreased the mobility of Cd, Pb and Sb. The uptake of trace elements to leaves was low. Σ 16 U.S. EPA PAHs were significantly reduced only in the combined treatment of black locust and soil amendments after 12months of plant growth. Soil amendment-assisted phytoremediation showed a high efficiency in PAH dissipation and may be a useful remediation technique for mixed contaminated soils.

Investigations of microbial degradation of polycyclic aromatic hydrocarbons based on 13C-labeled phenanthrene in a soil co-contaminated with trace elements using a plant assisted approach.

Co-contaminations of soils with organic and inorganic pollutants are a frequent environmental problem. Due to their toxicity and recalcitrance, the heterogeneous pollutants may persist in soil. The hypothesis of this study was that degradation of polycyclic aromatic hydrocarbons (PAHs) is enhanced if heavy metals in soil are immobilized and their bioavailability reduced. For metal immobilization and enhanced biodegradation, distinct mineral and organic soil amendments (iron oxides, gravel sludge, biochar) were deployed in an incubation batch experiment. The second part of the experiment consisted of a greenhouse pot experiment applying fast-growing and pollution-tolerant woody plants (willow and black locust). Soil amendments initially immobilized NH4NO3-extractable zinc, cadmium, and lead; after 100 days of incubation, soil amendments showed reductions only for cadmium and a tendency to enhance arsenic mobility. In order to monitor the remediation success, a 13C-phenanthrene (PHE) label was applied. 13C-phospholipid fatty acid analysis (13C-PLFA) further enabled the identification of PHE-degrading soil microorganisms. Both experiments exhibited a similar PLFA profile. Gram-negative bacteria (esp. cy17:0, 16:1ω7 + 6, 18:1ω7c) were the most significant microbial group taking up 13C-PHE. Plants effectively increased the label uptake by gram-positive bacteria and increased the biomass of the fungal biomarker, although their contribution to the degradation process was minor. Plants tended to prolong PAH dissipation in soil; at the end of the experiment, however, all treatments showed equally low total PAH concentrations in soil. While black locust plants tended not to take up potentially toxic trace elements, willows accumulated them in their leaves. The results of this study show that the chosen treatments did not enhance the remediation of the experimental soil.