In-situ remediation of eutrophic Wolong Lake sediments using novel PVA-SA-Biochar and PVA-SA-zeolite embedded immobilized indigenous microorganisms: A pilot study

Sediment restoration has become a key link in river and lake pollution control. This present study investigated the selection of dominant microbial bacteria, the selection and optimization of microbial immobilized carrier materials, and the effect of embedded immobilized microbial in situ remediation of bottom sediments based on the actual restoration pilot project of eutrophic Wolong Lake. The composite of denitrifying and photosynthetic bacteria at a ratio of 1 : 2 showed the best performance with COD, TN, and TP removal efficiencies of 74.86%, 65.2%, and 67.5%, respectively. Denitrifying bacteria to photosynthetic bacteria optimal composite bacterial solutions with polyvinyl alcohol-sodium alginate (PVA-SA), PVA-SA-zeolite and PVA-SA-biochar carriers were selected, and the effects of different carriers were analyzed and compared in terms of multiple characteristics. PVA-SA-biochar carriers showed the best ammonia-nitrogen transfer performance, mass transfer coefficient (0.681 × 10-9 m2 s-1), specific surface area (76.3 m2 MB g-1) and performed best in mechanical strength and chemical stability. The effects of biochar, PVA and SA contents on COD removal (Y) were analyzed using the 3D-response surface methodology. Biodegradation capacity (G-value) increased from 0.68 × 10-3 kg (kg h)-1 at the beginning of the test to 2.32 × 10-3 kg (kg h)-1 after 80 days of the remediation test with a growth rate of 258.82%. The water quality index has significantly improved, indicating a good restoration effect. Alpha diversity analysis showed that the Shannon and Simpson indexes increased and decreased. The relative abundance of Bacteroidota, Proteobacteria, Planctomycetota and Chloroflexi, closely related to the denitrification, decarbonization and phosphorus removal, increased while Chloroflexi decreased compared with before restoration. Embedded immobilized microbial technology significantly enhances the quality of sediment mud and the overlying water. In the long term, this approach does not release toxic substances into water bodies, thus fostering biodiversity and promoting ecological restoration. It represents a novel restoration strategy that contributes positively to environmental sustainability.