Evaluation of Sorbent Amendments for Mercury Remediation on the Diversity and Viability of Soil Microorganisms in Contaminated Bank Soil

Date of Award

Spring 2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

Boakai K. Robertson

Second Advisor

Daniel A. Abugri

Third Advisor

Emmanuel Tadjuidje

Abstract

The historical use of mercury (Hg) at industrial scales in facilities operated by the U.S. Department of Energy led to elevated Hg concentrations in soils and sediments. One of these facilities is the Y-12 National Security Complex in Oak Ridge Reservation (ORR), Oak Ridge, TN, USA, established as part of the Manhattan Project during World War II (WWII). The Y-12 used a significant amount of Hg for the separation of lithium isotopes to develop nuclear weapons. Mercury used during the operation was released into the pristine environment, especially the East Fork Poplar Creek (EFPC), which originates from the Y-12. Despite the several initiatives implemented over the years to mitigate Hg migration into the EFPC, Hg movement from identified source points into the environment persists. Recently, as part of the Hg cleanup strategy at EFPC, the deployment of sorbents has been considered. Engineered sorbents, Organoclay PM-199 and Organoclay MRM, have been investigated for Hg immobilization. However, the impact of the sorbents on soil health and microbial diversity is poorly understood. In addition, our curiosity was prompted by the gap in knowledge on the toxicity of the sorbents to soil microorganisms. Hence, we proposed conducting a microcosm study to determine the elemental composition of the sorbents and evaluate their impact on soil microbial community and soil parameters. Part of this study assessed the viability of soil microorganisms in the presence of the sorbents. For our microcosm study, we utilized bank soil samples collected from an uncontaminated site (Hinds Creek), a downstream site (SB 14-8), and the Historical Release Deposit site (SB 5-8) during the fall season. The soil from each site was spiked with HgCl2, amended with 5% MRM, and 5% PM-199, and incubated in aerobic conditions for 70 days. Shotgun sequencing was utilized to evaluate the microbial community structure. The microcosm samples were further assessed for integral soil parameters potentially influenced by sorbent amendments. The Hg sorption capability of the sorbents was determined by analyzing the total Hg (THg) in the soil and pore H2O of the microcosm. The elemental contents of the sorbents were characterized using inductively coupled plasma-optical emission spectrometry (ICP-OES). We also investigated the effect of the sorbents on the viability and biofilm formation of soil microorganisms to determine sorbent toxicity before their usage in field applications for Hg remediation. In replicates of four, cultures (25 mL) of S. marcescens or B. thailandensis were amended with 5% (w/v) and 25% (w/v) PM-199 and MRM, respectively, and incubated for 9 days. After the samples were retrieved, a BacLight staining kit was used to assess bacterial cell viability. Metagenomic analysis showed that 5% of sorbent amendments shifted bacterial community structure by increasing bacterial α-diversity in the uncontaminated Hind Creek soil. However, bacterial α-diversity was relatively similar between the unamended and amended SB 14-8 and SB 5-8 soils. Our analysis also showed that the sorbents, especially Organoclay MRM, preferentially favored microbial communities belonging to Hg-resistant groups. Total Hg analysis revealed that the sorbents were able to reduce THg in Hind Creek, SB 14-8, and SB 5-8 soil by an average of 89–90%, 66–72%, and 11–18%, respectively. Although the sorption efficiency of the sorbents was lower in SB 5-8 (the soil with higher Hg concentrations), the total quantity of Hg sorbed off was higher compared to the other soils. Integral soil parameter evaluation showed that the sorbents significantly influenced pH, electrical conductivity, and redox reaction across all soil sites ( p < 0.05) due to the potential release of elements in the sorbents. Our viability results revealed that, in contrast to the unamended control, the growth of S. marcescens amended with 25% sorbents was inhibited. In addition, bacterial biofilm formation and essential biofilm components were examined using biochemical assays. Our results indicated that biofilm formation by sorbent-amended S. marcescens was adversely impacted. On the other hand, B. thailandensis amended with 5% of MRM showed enhanced growth and discernible variations in biofilm morphology. Overall, our findings suggest that the use of Organoclay PM-199 and MRM may support microbial communities with Hg resistance and nitrogen fixation characteristics because the sorbents provide favorable conditions for the microbial groups. In addition, higher concentrations of Organoclay amendments may have a consequential impact on the growth of specific soil microorganisms that cannot tolerate the amendment conditions.

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