Sustained Release of Persulfate and Iron(II) Ions from Inert Inorganic Materials for Groundwater Remediation

Contamination of groundwater, initiated from years of human negligence, continues to persist despite major advancements in water remediation technologies. The generated groundwater plumes can continuously replenish contaminants such as dense nonaqueous phase liquids (DNAPLs) through the aid of slow-moving water. Thus, long- term treatment of this flow system has proven to be problematic. A selection of sophisticated sustained diffusion technologies is needed to help prevent further damage to this prominent freshwater source. Here, a novel in-situ chemical oxidation (ISCO) method was designed to incorporate persulfate into silica mediums such as zeolite, diatom, and silica flour for the treatment of groundwater contaminants at polluted sites. By controlling the release of a strong oxidant solely through inorganic materials, the oxidizing potential is preserved in contrast to being released through organic mediums. Several methods of making the pellets as well as different types of pellets with a range of release time for persulfate have been developed, offering a selection of both methods and pellets for the controlled release of persulfate. These methods typically use an organic material such as candle wax or a polymer as the support in the pellets. Since the support materials are organic, they consume the released persulfate. In the current work, inorganic sol-gel, consisting of primarily silicon dioxide, was used as an adhesive for added internal support for the pellets. A chlorinated DNAPL known as trichloroethylene (TCE) was selected for testing this method. The feasibility of controlling the release of ferrous sulfate as an activator for persulfate was also briefly investigated. Batch treatment of 15 mg L⁻¹1 of TCE solutions showed 73-99% degradation and a maximum persulfate release time of over 25 hours from persulfate-containing pellets. Successful treatment of a TCE-spiked Tennessee River water sample by persulfate and Fe²⁺ ions released from their respective pellets was demonstrated. Tests of the treated river water sample showed no detectable TCE by the silver chloride method. The purpose of this work is to show that these safe and inexpensive methods are potential prospects with further optimization and system scale-up for the continuous treatment of the groundwater in the near future.