A NOVEL BIOLEACHING METHOD TO REMOVE POTENTIALLY TOXIC ELEMENTS (PTE) FROM POLLUTED SOILS USING BACTERIA CHEMICALLY IMMOBILIZED IN EXHAUSTED POLYURETHANE FOAM

Soil PTE pollution in former shooting ranges or ammunition factories/deposits is a widespread type of pollution throughout the world. Cleaning up these soils is usually not simple, since most of the remaining PTE are in not bioavailable or leachable forms. In the present study, two soils collected within a former military area of Taranto are under investigation. In these soils, significant amounts of potentially toxic elements (PTE) such as Cu (229 and 140 mg kg-1), Pb (415 and 292 mg kg-1) and Zn (636 and 574 mg kg-1) were found. Clearly, this posed questions about the actual risk linked to such pollutants and about foreseeing possible remediation interventions. The speciation and the potential bioavailability of the PTE in the soils were in-depth examined with a combination of DTPA extractions, sequential extractions, micro X-ray fluorescence, and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX). The DTPAextractable fraction did not exceed the 12% for Cu, 17% for Pb and 5% for Zn, while sequential extractions showed that more than the 92% of PTE is associated to the recalcitrant phases of soil. Furthermore, micro-XRF mapping performed on soil thin-sections, along with SEM-EDX analysis, revealed that Cu, Zn and Pb are distributed inhomogeneously within soil particles and are spatially correlated each other, mostly concentrating in hotspots. This evidence has suggested that the observed PTE may derive mainly from microscopic fragments of bullets dispersed throughout the investigated soils. Because the limited solubility and bioavailability of PTE, the cleaning of the soil through soil washing or bioremediation (e.g., phytoremediation) is not likely to be successful. Therefore, an innovative approach based on ex-situ bioleaching using Pseudomonas fluorescens bacteria was tested. For the purpose, an exhausted polyurethane elastic foam was exploited as a valorised porous waste to chemically immobilize the bioremediating bacteria. The covalent immobilization of the bacteria passed via the in situ green synthesis of a biomimetic polymer based on hydroxyindole and phenylboronic acid. This approach resulted efficient both to i) decrease the delivery of microplastics from polyurethane foam aged structures and ii) let the immobilized bacteria successfully mobilize toxic metals from contaminated soil, envisaging a possible scaled up solution for environmental applications. The first results showed interesting PTE mobilization from polluted soils and the possibility to reutilize in different bioleaching cycles the immobilized bacteria. Agritech National Research Center, European Union Next-Generation EU PNRR – MISSIONE 4 COMPONENTE 2, INVESTIMENTO 1.4 – D.D. 1032 17/06/2022, CN00000022.