Cleanup of Cr(VI)-polluted groundwater using immobilized bacterial consortia via bioreduction mechanisms. (1st August 2023) - Lin, Wei-Han
Abstract: Cr(VI) bioreduction has become a remedial alternative for Cr(VI)-polluted site cleanup. However, lack of appropriate Cr(VI)-bioreducing bacteria limit the field application of the in situ bioremediation process. In this study, two different immobilized Cr(VI)-bioreducing bacterial consortia using novel immobilization agents have been developed for Cr(VI)-polluted groundwater remediation: (1) granular activated carbon (GAC) + silica gel + Cr(VI)-bioreducing bacterial consortia (GSIB), and (2) GAC + sodium alginate (SA) + polyvinyl alcohol (PVA) + Cr(VI)-bioreducing bacterial consortia (GSPB). Moreover, two unique substrates [carbon-based agent (CBA) and emulsified polycolloid substrate (EPS)] were developed and used as the carbon sources for Cr(VI) bioreduction enhancement. The microbial diversity, dominant Cr-bioreducing bacteria, and changes of Cr(VI)-reducing genes ( nsfA, yieF, and chrR ) were analyzed to assess the effectiveness of Cr(VI) bioreduction. Approximately 99% of Cr(VI) could be bioreduced in microcosms with GSIB and CBA addition after 70 days of operation, which caused increased populations of total bacteria, nsfA, yieF, and chrR from 2.9 × 10 8 to 2.1 × 10 12, 4.2 × 10 4 to 6.3 × 10 11, 4.8 × 10 4 to 2 × 10 11, and 6.9 × 10 4 to 3.7 × 10 7 gene copies/L. In microcosms with CBA and suspended bacteria addition (without bacterial immobilization), the Cr(VI) reduction efficiency dropped to 60.3%, indicating that immobilized Cr-bioreducing bacteria supplement could enhance Cr(VI) bioreduction. Supplement of GSPB led to a declined bacterial growth due to the cracking of the materials. The addition of GSIB and CBA could establish a reduced condition, which favored the growth of Cr(VI)-reducing bacteria. The Cr(VI) bioreduction efficiency could be significantly improved through adsorption and bioreduction mechanisms, and production of Cr(OH)3 precipitates confirmed the occurrence of Cr(VI) reduction. The main Cr-bioreducing bacteria included Trichococcus, Escherichia-Shigella, and Lactobacillus . Results suggest that the developed GSIB bioremedial system could be applied to cleanup Cr(VI)-polluted groundwater effectively. Highlights: Immobilized Cr(VI)-bioreducing bacteria can enhance Cr(VI) bioreduction effectively. GSIB addition causes increased Cr(VI)-bioreducing genes and Cr(VI) bioreduction rates. Cr(VI)-reducing bacteria contain Trichococcus, Escherichia-Shigella, and Lactobacillus. Cr(OH)3 is the main form of Cr(III) precipitates via Cr(VI) bioreduction after GSIB addition. GSIB addition causes Cr(VI) removal via bioreduction and adsorption mechanisms.