Fermentation and Degradation Characteristics of Humic Acid-Based Emulsified Oil for Organic and Inorganic Contaminants in Groundwater

The remediation of groundwater contaminants, such as chlorinated aliphatic hydrocarbons (CAHs) and nitrate, often requires the injection of electron donors. Vegetable oil, commonly used as a cost-effective electron donor, is typically emulsified with surfactants before injection. However, some surfactants pose ecological risks to aquatic ecosystems during prolonged exposure. To overcome this challenge, a humic acid-based emulsified oil was developed, leveraging humic acid's non-toxic, naturally occurring, and affordable properties. This study investigates the fermentation process of the emulsified oil by analyzing degradation byproducts and its long-term electron supply potential. It also evaluates its performance in degrading organic and inorganic contaminants through laboratory- and pilot-scale experiments.
Batch reactors were used to conduct lab-scale fermentation test (LFT) and lab-scale degradation test (LDT) for assessing fermentation and contaminant degradation characteristics. The emulsified oil was added to reactors filled with groundwater at concentrations between 0.1% and 1.0% (v/v) during the LFT, and the reactors were monitored for 200 days. Target contaminants—trichloroethylene (TCE) and nitrate—were tested in the LDT using 0.1% and 0.5% (v/v) emulsified oil concentrations, and results were compared with control reactors. In a pilot-scale degradation test (PDT) conducted in an aquifer contaminated with TCE and PCE in Iksan, South Korea, the effectiveness of emulsified oil was further assessed using push-pull and drift tests.
The LFT revealed sustained lipase activity and byproducts, including fatty acids (e.g., stearic acid), organic acids (e.g., propionic acid), carbon dioxide, and methane, indicating that a 0.1% (v/v) oil concentration supported optimal fermentation. For nitrate in the LDT, degradation rates of approximately –30 mg N/L/d were observed across both tested concentrations, whereas TCE exhibited higher degradation rates under 0.1% (v/v) conditions (–0.51 mg/L/d), about twice as effective as 0.5% (v/v). The PDT demonstrated significant CAH degradation, with TCE's first-order degradation rate constant increasing 17-fold and enhanced production of dechlorination byproducts, such as vinyl chloride (VC) and ethene (ETH). These findings highlight the humic acid-based emulsified oil as an effective electron donor for promoting the biological degradation of both organic and inorganic contaminants, offering a promising solution for groundwater remediation.