Kinetics of soil ozonation: an experimental and numerical investigation☆
Introduction
Soil contamination is a challenging problem because of its extent and the difficulty involved in remediation. Several decontamination technologies, including soil vapor extraction (SVE), in situ aeration (air sparging/venting), chemical oxidation, bioremediation, soil heating, and surfactant-aided flushing are currently being used for soil cleanup. Organic pollutants are a major source of soil contamination and commonly occur as a result of disposal of industrial wastes, leaky underground storage tanks, and accidental spills (Hutzler et al., 1991). Organic compounds exist in the subsurface in several forms based on their chemical properties. They may be sorbed onto the soil matrix, be dissolved in groundwater, exist in vapor phase, or exist as a nonaqueous phase liquid (NAPL) (Volkering et al., 1997).
Chemical oxidation is an attractive alternative for remediating organic-contaminated soils because the byproducts are usually harmless and the removal efficiency is usually high if the chemicals can be delivered properly. Potassium permanganate and ozone are two commonly used chemical oxidants. Because it is delivered in the aqueous phase, potassium permanganate is more effective for groundwater cleanup than for soil cleanup. Ozone, which can be delivered as a gas mixture, is effective for both unsaturated and partially or fully saturated soils. The high aqueous solubility of ozone makes it effective in saturated soils as well. The use of gaseous ozone as the oxidizing agent in soil remediation has a relatively short history. Chemical oxidation with ozone-containing gas has been identified as a promising treatment method for soils contaminated with toxic or persistent wastes Siedel et al., 1993, Voigtländer, 1993. Siedel et al. reported that polycyclic aromatic hydrocarbons (PAHs) in soil could be reduced by as much as 99% by ozonation (with an ozone consumption rate of 45 g/kg) in a rotating reactor. Hsu (1995) reported that when ozone was used in a polishing step, the removal efficiency of trichloroethylene (TCE) was significantly improved over that achieved when only air stripping was employed. Cole et al. (1996) demonstrated that removal efficiencies of greater than 90% were achieved for PAHs when ozone was used. More recently, extensive modeling studies supported by column experiments have been conducted for PAH remediation using ozone Hsu and Masten, 2001, Kim and Choi, 2002, Sung and Huang, 2002. Despite the success of ozonation in soil remediation, only a limited amount of studies have been published. The complex and inhomogeneous nature of soil and the relatively short half-life of ozone demand more research efforts (Hsu, 1995).
The present study systematically investigates the behavior of ozone in soil. The objectives are (1) to evaluate the reaction kinetics of ozone and soil, including ozone self-decomposition, catalytic decomposition by soil, and interaction with soil organic matter (SOM) and soil inorganic matter (SIM); (2) to conduct bench-scale soil column experiments to evaluate the behavior of ozone in soil; (3) to numerically simulate ozone breakthrough via soil columns using experimentally obtained rate constants; and (4) to perform three-dimensional numerical simulations to investigate the efficacy of ozonation for field remediation.
Section snippets
Materials and methods
Experiments were conducted using the type of bench-scale soil column reactor shown in Fig. 1. Ozone was produced by a corona discharge ozone generator (Model CD-06, Aqua-Flo). The air-ozone mixture was then passed through a gas-washing bottle to saturate the mixture with moisture. The moisture-saturated air–ozone mixture was intended to mimic both vadose- and saturated-zone ozonation. The flow rate of the system was monitored by a volumetric flowmeter and manually controlled. After passing
Batch experiments
Ozone in soil can be consumed in several ways, including empty-column self-decomposition in the gas phase and at the surfaces of the system; catalytic decomposition through interaction with soil particles; and reactions with moisture, SOM, and SIM. Interactions of ozone with soil are complex and thus difficult to understand completely. However, to a certain extent, it is possible to identify experimentally the effects of soil on ozone consumption. These effects are expressed by the kinetic rate
Conclusions
This study investigated soil–ozone interactions in a sandy soil mixture from the Savannah River Site in South Carolina. The interactions of ozone with soil were expressed by linear and nonlinear reaction kinetics. Cycling batch experiments were conducted to quantify the appropriate rate constants for ozone self-decomposition, catalytic decomposition in the presence of soil, and ozone-SOM and ozone–SIM interactions.
Column experiments were also conducted to measure ozone breakthrough from a soil
Acknowledgements
This research was supported by Lynntech. Partial support was provided by NSF (BES-9702356) and DOE under contract DE-AC05-00OR22725 with UT-Battelle, LLC. The authors are grateful to Dr. David DePaoli for his comments during the course of the work and to Ms. Marsha Savage for editing the manuscript. The authors also appreciate the constructive criticisms provided by Dr. Hoigne and other reviewers in improving this manuscript.
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2020, ChemosphereCitation Excerpt :Prior publications that have focused on O3 injection for contaminant remediation have generally focused on gas phase injection for oxidation of organic contaminants (Masten and Davies, 1997; Hsu and Masten, 2001; Choi et al., 2002). There are a few prior publications focused on gaseous O3 decomposition in batch experiments and experiments that examined the kinetic limitations of gas transport through soil-packed columns (Choi et al., 2002; Kim and Choi, 2002; Pierpoint et al., 2003; Shin et al., 2004). These previous papers that examined gas phase O3 using bulk (multiple process) kinetics were likely influenced by a number of processes including reactivity with the gas and aqueous phases, reactivity with the soil solid phase, water saturation and multiphase permeability and fluid flow, and mass transfer between the gas and aqueous phases.
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