Induced metal redistribution and bioavailability enhancement in contaminated river sediment during in situ biogeochemical remediation

In situ sediment remediation using Ca(NO₃)₂ or CaO₂ for odor mitigation and acid volatile sulfide (AVS) and organic pollutant (such as TPH and PAHs) removal was reported in many studies and fieldwork. Yet, the associated effects on metal mobilization and potential distortion in bioavailability were not well documented. In this study, contaminated river sediment was treated by Ca(NO₃)₂ and CaO₂ in bench studies. Through the investigation of AVS removal, organic matter removal, the changes in sediment oxidation-reduction potential (ORP), microbial activity, and other indigenous parameters, the effects on metal bioavailability, bioaccessibility, and fraction redistribution in sediment were evaluated. The major mechanisms for sediment treated by Ca(NO₃)₂ and CaO₂ are biostimulation with indigenous denitrifying bacteria and chemical oxidation, respectively. After applying Ca(NO₃)₂ and CaO₂, the decreases of metal concentrations in the treated sediment were insignificant within a 35-day incubation period. However, the [SEMₜₒₜ-AVS]/f OC increased near to the effective boundary of toxicity (100 μmol g⁻¹ organic carbon (OC)), indicating that both bioavailability and bioaccessibility of metals (Cu, Zn, and Ni) to benthic organisms are enhanced after remediation. Metals were found redistributed from relatively stable fractions (oxidizable and residual fractions) to weakly bound fractions (exchangeable and reducible fractions), and the results are in line with the enhanced metal bioavailability. Compared with Ca(NO₃)₂, CaO₂ led to higher enhancement in metal bioavailability and bioaccessibility, and more significant metal redistribution, probably due to its stronger chemical reactive capacity to AVS and sediment organic matter. The reactions in CaO₂-treated sediment would probably shift from physicochemical to biochemical heterotrophic oxidation for sediment organic matter degradation. Therefore, further investigation on the long-term metal redistribution and associated mobility as well as bioavailability is recommended.