Effectiveness of Poplar-Assisted Bioremediation Under Different Concentration of Persistent Organic Pollutants

Plant-assisted bioremediaton (PABR) is an effective green technology for removing contaminants from soils. It relies on the sinergistic interactions between roots and microrganisms of the rhizosphere for removing, transforming and contain toxic compounds. Poplars were proved to be very efficient in promoting degradation of chloride organic pollutants such as lindane and polychlorobyphenils (PCBs) in both laboratory and field scale experiments. Due to their high molecular stability, low solubility in water and high tendency to adsorb on particulate phase, PCBs are extremely hard to remove from soil. In this work, some crucial aspects of the PABR strategy in recovering two historically contaminated areas, located in Southern Italy, characterized by different PCB amounts (medium and high concentrations, respectively), are illustrated.Two poplar treated areas, inside the survey site, were set up at a distance of two years from each other (2013 and 2015, respectively). In the first one, characterized by a medium PCB contamination level (PCB ~250 ng/g) about 600 poplar cuttings were planted; instead, in the second one, affected by high PCB contamination (PCB >2000 ng/g), about 750 poplar cuttings were planted.Chemical investigations of soil properties and pollutants (PCB congeners) were carried out before plantation and at different times after poplar planting, in both the PABR treated areas. Moreover, microbial analyses such as the total microbial abundance (DAPI counts), cell viability (live/dead method), microbial activity (dehydrogenase activity) and the sequencing of 16S DNA (Miseq-Illumina) were performed for evaluating microbial community structure and functioning in the poplar rhizosphere. Moreover, in March 2019, a compost (CMP) or biochar (BC) treatment was also performed on some contaminated plots inside the highest polluted poplar treated area in order to increase soil quality and promote PCB degradation.The results obtained in the first poplar treated area, affected by medium PCB contamination, showed the effectiveness of PABR in restoring this area, promoting active bacteria able to degrade and contain pollutants and increase soil quality. In the second poplar treated area, the high PCB levels and the soil nutrient impoverishment favouring stress conditions for poplars limiting their growth. The addition of the fertilizers (compost or biochar) promoted an increase in soil quality and microbial activity promoting the bioremediation processes.