Mixing of an anthracene-contaminated soil: a simple but efficient remediation technique?

Photochemical transformation of anthracene (ANT) in surface soil: Chlorination and hydroxylation

To reveal the fate of anthracene (ANT) in soil, the photodegradation behavior of ANT was systematically studied using SiO₂ to simulate a soil environment. Under xenon lamp irradiation, more than 90% of ANT loaded on SiO₂ could be removed after 240 min. Moreover, the effects of water content, chloride ions (Cl^-) and humic acid (HA) were examined. It was found that the presence of water and HA can significantly inhibit the photolysis of ANT on SiO₂, while the addition of chloride alone has no obvious effect. However, when water is present, the inhibition effect of chloride became more obvious. According to radical quenching experiments and electron paramagnetic resonance (EPR) spectra, hydroxyl radicals (•OH) and chlorine radicals (Cl•) were formed in the system. Possible reaction pathways were speculated based on products identified by mass spectrometry. ANT was attacked by •OH to form hydroxylated products, which can be further hydroxylated and oxidized with the final formation of ring-opening products. ANT directly excited by light may also react with Cl• to produce chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs). Finally, the experimental results were verified on real soil. This study provides important information for understanding the photochemical transformation mechanism of ANT at the soil/air interface.

Changes in the Bacterial Community Structure of Remediated Anthracene-Contaminated Soils

Mixing soil or adding earthworms (Eisenia fetida (Savigny, 1826)) accelerated the removal of anthracene, a polycyclic aromatic hydrocarbon, from a pasture and an arable soil, while a non-ionic surfactant (Surfynol® 485) inhibited the removal of the contaminant compared to the untreated soil. It was unclear if the treatments affected the soil bacterial community and consequently the removal of anthracene. Therefore, the bacterial community structure was monitored by means of 454 pyrosequencing of the 16S rRNA gene in the pasture and arable soil mixed weekly, amended with Surfynol® 485, E. fetida or organic material that served as food for the earthworms for 56 days. In both soils, the removal of anthracene was in the order: mixing soil weekly (100%) > earthworms applied (92%) > organic material applied (77%) > untreated soil (57%) > surfactant applied (34%) after 56 days. There was no clear link between removal of anthracene from soil and changes in the bacterial community structure. On the one hand, application of earthworms removed most of the contaminant from the arable soil and had a strong effect on the bacterial community structure, i.e. a decrease in the relative abundance of the Acidobacteria, Chloroflexi and Gemmatimonadetes, and an increase in that of the Proteobacteria compared to the unamended soil. Mixing the soil weekly removed all anthracene from the arable soil, but had little or no effect on the bacterial community structure. On the other hand, application of the surfactant inhibited the removal of anthracene from the arable soil compared to the untreated soil, but had a strong effect on the bacterial community structure, i.e. a decrease in the relative abundance of Cytophagia (Bacteroidetes), Chloroflexi, Gemmatimonadetes and Planctomycetes and an increase in that of the Flavobacteria (Bacteroidetes) and Proteobacteria. Additionally, the removal of anthracene was similar in the different treatments of both the arable and pasture soil, but the effect of application of carrot residue, earthworms or the surfactant on the bacterial community structure was more accentuated in the arable soil than in the pasture soil. It was found that removal of anthracene was not linked to changes in the bacterial community structure.

Wheat straw biochar amendments on the removal of polycyclic aromatic hydrocarbons (PAHs) in contaminated soil

Soil amendments of wheat straw biochar (BC), lignocellulosic substrate (LS), BC+LS, and BC+LS+BR (surfactant Brij30) were investigated for the first time in order to remedy polycyclic aromatic hydrocarbons (PAHs)-polluted soil using pilot scale microcosm incubation. We hypothesized that the removal of PAHs could be inhibited due to the adsorption and immobilization of biochar and the inhibition depends on the molecular-weight of PAHs. The removal rates of phenanthrene (PHE) and Benzo[a]pyrene (BaP) ranked as C=BC>LS=LS+BC=LS+BC+BR and C=BC=LS+BC+BR>LS=LS+BC. Wheat straw biochar inhibited the removal of PHE and accelerated BaP removal. The activity of Dehydrogenase (DH) was depressed by the addition of the biochar while the activity of polyphenol oxidase (PPO) was stimulated. Lignocellulose and surfactant are favourable to sustain soil microbiological activity and the removal of PAHs although the diversity of bacterial community was not significantly changed. The findings implied that the components of PAHs are necessary to consider when the amendments are implemented by associated biochar in PAH-polluted soil.