Response of an agricultural soil to phenanthrene and pentachlorophenol pollution and to different bioremediation strategies
Scelza, Rosalia (2008) Response of an agricultural soil to phenanthrene and pentachlorophenol pollution and to different bioremediation strategies. [Tesi di dottorato] (Inedito)
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In the last years, a high number of polluting compounds has been released into the environment because of several industrial and/or agricultural activities. In particular, the rapid industrialization of agriculture, expansions in the chemical industry, and the need to generate cheap forms of energy have all resulted in an ever-increasing reliance on anthropogenic organic chemicals and caused the contamination of a significant number of soil environments by xenobiotic compounds with negative, irreversible effects on environmental quality and health. Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated organic compounds (PCB) are surely the most extensively investigated of the chemical pollutants being toxic to many living organisms, including humans, and extensively used for several industrial activities. Because of their low water solubility, stable aromatic ring system, high chlorine content, these compounds have a high persistence in the environment, and may accumulate in food chains and thus have harmful effects on human health. Bioremediation of contaminated sites relies on the metabolic capacities of living organisms to reduce organic pollutants into harmless or, at least, less dangerous compounds. The process can be realized introducing directly into a contaminated system micro-organisms able to consume selectively the target compound (bioaugmentation) or increasing the microbial indigenous population by addition of nutrients in form of organic and/or inorganic fertilizers such as urea, sawdust, compost, manure and biosolids (biostimulation). The evaluation of the effectiveness of a bioremediation process in contaminated soil, however, should not only look at pollutant removal and/or transformation in non toxic end-products but it should also monitor whether and how soil biological functions are affected by and during the process. Studies were performed on an agricultural soil artificially contaminated with phenanthrene (Phe) or pentachlorophenol (PCP), selected as representative of PAHs and PCBs. In a long-term experiment the efficiency of a phenanthrene-degrading microbial culture, the potentiality of compost and dissolved organic matter in soil decontamination, and the dynamics of the main soil biochemical and biological properties were evaluated. The variations of the major physical and chemical properties were also monitored. The efficiency of the different bioremediating approaches was also evaluated against a Phe-aged (2 years) contaminated soil. The obtained results demonstrated that two complex processes occurred simultaneously in the contaminated soil: natural attenuation and ageing. The investigated soil showed an intrinsic capability of degrading Phe and PCP. The addition of a limited dose of compost, as well as the inoculation with a Phe-degrading bacterial culture strongly stimulated and enhanced the attenuation process. Furthermore, several of the soil properties showed differentiated responses to the presence of the Phe, the compost, and/or the exogenous culture. The results obtained with the soil contaminated with phenanthrene and incubated for 2 years supported the occurrence of the ageing process. The impact of PCP on the properties of the soil was the result of opposite effects. The PCP strongly decreased the levels of some biochemical properties that diminished with increasing the incubation time, thus suggesting a depressing effect on the soil micro flora that not recovered from the initial toxic response towards PCP. Conversely, the presence of the contaminant promoted the development of fungal colonies, contributing to its degradation and consequent production of PCP metabolites, considered more toxic than the parent compound. An ageing phenomenon, also favoured by the presence of the dissolved organic matter and leading to the decrease of the extractable PCP, was also hypothesized. The results obtained in this thesis work suggest that soil biological investigations can give information about the intensity and the kind and duration of the effects of pollutants on the metabolic activity of soil. Although the experiments are limited by the laboratory, controlled conditions adopted, they are well suited for measuring the effects of pollution on soil health and to act as a monitoring tool for the decontamination process of a polluted soil. Furthermore, such investigations may be helpful for further studies aimed at validating and extrapolating the data to natural situations.
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