Tecniche di biorisanamento per il recupero di falde contaminate da cromo esavalente

ABSTRACT Cr(VI) detected in groundwater, at a concentration above the legal limit, represents a widespread environmental issue. It is strictly related to the use of Cr(VI) in several industrial applications (energy production, manufacturing of metals and chemicals, tanning, etc.), and the corresponding inadequate waste and wastewater management. Cr(VI) remediation is generally based on the reduction to the less toxic and less mobile trivalent state. Cr(VI) can be easily reduced even in natural environments by sulphur compounds, Fe(II), and organic matter, either dissolved or as particulate. On the contrary, the trivalent form of chromium is stable and its oxidation is not favoured, because of the high E° value of the Cr (III)/Cr (VI) redox couple. Among the strategies commonly used for chromium reduction, bioremediation shows some advantages in terms of economic and environmental sustainability, especially in the case of large volumes of water to be treated. A number of bacteria, isolated from contaminated environmental matrices, show metabolic pathways for minimizing toxicity around the cells, including enzymatic and non-enzymatic Cr(VI) reduction reactions. Several studies on Cr(VI) bioremediation with positive outcomes, using a bacterial single strain, have been published in scientific literature. However, in situ treatments cannot rely on single strain inoculation, because no single strain can compete with native microbial communities. Therefore, the latter have to play a key role for successful bioremediation intervention. In order to study and evaluate the capabilities of native microorganisms to remediate groundwater Cr(VI) pollution, Next Generation Sequencing (NGS) technologies are fundamental. Particularly, the 16S rDNA sequencing allow to investigate the bacterial community composition in environmental samples. The most of bacteria capable of Cr(VI) reduction are heterotrophic, and, in oligotrophic environments, such as groundwater, they need additional nutrients from external sources. A key factor for a successful enhanced natural attenuation process is to select the most suitable amendment, which can sustain both bacterial growth and reduction processes. Yeast extract, sodium lactate, polyhydroxybutirrate are some of the organic substances which have been successfully used as electron donors in Cr(VI) bioremediation experiments. This work details the operating procedures and protocols developed during a preliminary experiment of enhanced natural attenuation of Cr(VI) pollution. Microcosms were set up with groundwater and soil in ratio 4:1. The soil was powdered limestone, and groundwater was spiked with 1000 μg L-1 of Cr(VI) as K2Cr2O7. Four different conditions were tested: abiotic, biotic, biotic amended with yeast extract and biotic amended with lactate. The highest rate of Cr(VI) removal was achieved in the yeast extract-amended microcosm set, reaching almost 99% removal after seven days. Work is currently in progress to optimise the experimental setup and protocol, in order to apply the bioremediation procedure to real environmental matrices collected from an industrial site located in the Barletta Municipality, Apulia, Southern Italy. Previous monitoring campaigns, carried out in the study area in 2016 and 2020, evidenced groundwater Cr(VI) concentration of about 100 μg L-1, which exceeded the Italian legal limit of 5 μg L-1. A multidisciplinary approach, including chemical, molecular and spectroscopic techniques, will be used for a comprehensive investigation of the enhanced natural attenuation of Cr(VI) pollution.

La contaminazione delle acque sotterranee da Cr(VI) in concentrazioni sensibilmente superiori ai limiti di legge è una problematica ambientale ampiamente diffusa, connessa alle numerose applicazioni industriali del Cr(VI) e alla non corretta gestione di acque di processo e sottoprodotti. Il risanamento delle acque contaminate può essere convenientemente realizzato mediante la riduzione del Cr(VI) a Cr(III). A tale scopo, un trattamento biologico può risultare più vantaggioso, in termini di sostenibilità economica ed ambientale, rispetto a strategie di intervento di tipo chimico e fisico, soprattutto nel caso di grandi volumi di acqua. Numerosi batteri, isolati da matrici ambientali contaminate, possiedono vie metaboliche per detossificare l’ambiente circostante la cellula, incluse quelle coinvolte nella riduzione biologica del Cr(VI). Tuttavia, si tratta generalmente di microorganismi eterotrofi, che necessitano di una fonte esterna di nutrienti, al fine di sostenerne la crescita e riproduzione in ambienti oligotrofici, quali sono le acque sotterranee. Pertanto, la scelta di un opportuno ammendante, in grado al contempo di sostenere la produzione di biomassa batterica e fornire elettroni per i processi riduttivi, risulta strategica al fine di potenziare i fenomeni di attenuazione naturale della contaminazione. Il presente lavoro descrive in dettaglio lo schema sperimentale, le procedure operative e le risultanze analitiche ottenute a valle di un esperimento preliminare di biorisanamento in scala di microcosmo, condotto utilizzando matrici artificialmente contaminate da Cr(VI). Il test ha mostrato l’efficacia dell’azione dei microrganismi, con un tasso di rimozione del Cr(VI) dipendente dal tipo di ammendante impiegato. L’effettiva capacità della comunità microbica nativa, opportunamente stimolata, di promuovere il biorisanamento è, tuttavia, sito-specifica. Gli studi in corso e futuri, sono, quindi, orientati all’ottimizzazione di quanto sviluppato per l’applicazione ad un caso studio reale, un’area industriale contaminata da Cr(VI) nel territorio della città di Barletta, in Puglia.