Effect of rhizosphere pH on PAH availability

International audience

English. Soils contaminated by PAHs (Polycyclic Aromatic Hydrocarbons) are of environmental concern because of their ubiquity, persistence and toxicity. Their persistence in soils is related to their low availability due to adsorption processes and sequestration in aggregates. This pollutant state prevents soil microorganisms from PAHs absorption and degradation. Some soil physico-chemical properties modified by plant roots (e.g. organic matter quantity and quality and rhizosphere pH) are known to control PAHs availability. In this work, we studied the influence of the rhizosphere pH on PAHs availability in industrial wastelands soils. This is the first step of the calibrating of our model for PAHs evolution in a rhizosphere soil. We made the hypothesis that PAHs availability may decrease during acidification and increase for a soil alkalinization. Batch experiments (s/l = 1/10) using three contaminated soils sampled on former industrial site were conducted under anaerobic conditions. The pH was modified by mineral acid or base additions. The available fraction of PAH was determined with butanol extraction of the soil. Results showed that a pH decrease had no significant influence on PAHs available fraction. This is in contradiction with results obtained in the literature. The soil suspension alkalinization had different effects in three soils. In two of them, a pH elevation increased the PAHs availability like it was found by many authors. In the third soil, a moderate alkalinization had no effect but a strong one reduced the PAHs availability. It seems that soil cation content may explain these different effects of the pH. The second step of our work will be to elucidate the role of root exudates on PAHs availability. The applied objective of the work is to evaluate if rhizotechnologies may be sustainable solutions for contaminated wastes land management.