Author ORCID Identifier

https://orcid.org/0000-0002-8989-6522

Date Available

4-22-2021

Year of Publication

2021

Degree Name

Master of Science (MS)

Document Type

Master's Thesis

College

Agriculture, Food and Environment

Department/School/Program

Plant and Soil Sciences

First Advisor

Dr. Elisa D'Angelo

Abstract

Worldwide, selenium is a major contaminant of concern for coal and metal ore mining industries. In coal mining regions of the eastern USA, for example, billions of liters of wastewaters with elevated Se oxyanion concentrations (primarily SeO42- and SeO32-) are generated during coal washing which is often stored in > 700 man-made impoundments built into remote mountain valleys. Because impoundments are unlined systems, they tend to leak contents into nearby streams, which has detrimental effects on sensitive biota, particularly fish. The primary goal of this project was to evaluate the capacity of passive anaerobic bioreactors (PAB) to remove Se from coal slurry leachate, while at the same time not contributing other pollutants (e.g. nutrients, trace elements, greenhouse gases) that could have detrimental effects on water or air quality (“pollution swapping”). Generally, PAB are trenches filled with organic substrates (e.g. Wood Chips) which support the growth of microorganisms that can improve water quality as wastes move through the system. Over the last 20 years, PAB systems have been employed to remove nitrate from agricultural drainage by denitrification, but PAB has not been extensively evaluated for SeO42- reduction to insoluble Se(0), and accompanying biogeochemical processes which might contribute unwanted pollutants in PAB effluents. The project's central hypothesis was Se removal rates and efficiency, and pollution swapping will be significantly related to the type and chemical composition of organic substrates and maturity of PAB systems. Selenate removal and pollution swapping were evaluated in PAB filled with biofuel plant substrates (either Hardwood Chips, Switchgrass, Miscanthus, and Timothy Hay) in triplicate laboratory batch reactors fed synthetic coal slurry at 32-d retention times over 7 months. PAB water and headspace gases were sampled with time in four different maturation periods to analyze pH, electrical conductivity, redox potential, Se and trace elements, nutrients (NH4+, (NO3-+NO2-), PO43-, and SO42-), dissolved organic C, and greenhouse gases (CO2, CH4, and N2O). Over different maturation periods, the concentrations of 13 pollutants in PAB effluents were compared with respective concentrations in pristine Appalachian streams to compute the Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) which is a measure of overall effluent quality. This study showed that selenate concentrations were reduced at significantly different rates and efficiencies depending on type of organic substrate and maturation time in PAB. However, Se concentrations did not reach the current EPA target 0.003 mg Se L-1 in any PAB. Results also showed that there were significant differences in several biogeochemical processes (e.g. microbial respiration, denitrification, sulfate reduction), nutrient release (e.g. phosphate, dissolved organic carbon), and greenhouse gas production (e.g. carbon dioxide and nitrous oxide) depending on type of organic substrate and maturation time in PAB systems. Concentrations of Se and several other chemicals in PAB effluents greatly exceeded concentrations in pristine Appalachian streams which resulted in low CCME-WQI scores < 25 which translated to poor water quality. Future research is recommended to determine factors that restrained Se removal and the maturation time required to achieve improved effluent quality in PAB systems.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.073

Funding Information

This research was supported by the Department of Plants and Soil science at University of Kentucky 2018.

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