Recalcitrant organic groundwater contaminants, such as 1,4-dioxane (1,4-D), may require strong oxidants for complete mineralization. Ozone is a strong oxidant and capable of mineralizing recalcitrant organic contaminants. However, the efficiency of O3 for in-situ chemical oxidation (ISCO) is limited by oxidant decay, short half-life, and reactivity. Hydroxypropyl-β-cyclodextrin (HPβCD) was examined for its ability to stabilize aqueous phase ozone (O3) and prolong oxidation potential through inclusion complex formation. The research presented here to determine the inclusion complexes of O3 and three common co-contaminants (trichloroethene, 1,1,1-trichloroethane, and 1,4-dioxane) as guest compounds with hydroxypropyl-β-cyclodextrin. Both direct (ultraviolet or UV) and competitive (fluorescence changes with 6-p-toluidine-2-naphthalenesulfonic acid as the probe) methods were used to evaluate the binding constants K (mM-1) of oxidant and contaminants; and their comparability between these two methods. Impacts of changing pH and ionic strength on K (mM-1) for oxidant and contaminants were also assessed because groundwater environment will have variability in pH and ionic strength. Measured binding constants for oxidant (O3) and contaminants (1,4-D, TCE, and TCA) from each of the two different methods (direct UV and competitive fluorescence method) were statistically comparable to each other. Binding constants increased with pH and with ionic strength, which was attributed to variations in guest compound solubility.
The efficiencies of all those complexes towards removal kinetics of 1,4dioxane (1,4-D), trichloroethylene (TCE), and 1,1,1-trichloroethane (TCA) using O 3 in single and multiple contaminant systems (with and without HPβCD) were quantified. It was observed that there was a partial degradation of HPβCD by O3. Although HPβCD was partially transformed by O 3, HPβCD proved sufficiently recalcitrant. Reactivity of O 3 was stabilized by forming HPβCD:O3 clathrate complex, and O3 release from HPβCD cavity including indigo oxidation confirmed that the inclusion complex was reversible. Pseudo first order reaction rate constants k (min-1) for 1,4-D, TCE, and TCA were higher in presence of HPβCD compared to absence of HPβCD. The reason behind this is the possibility of forming ternary complexes of oxidant- HPβCD-contaminant, which likely can increase the reactivity by increasing reactants proximity. Removal rate constants of 1,4-D was higher in presence of chlorinated co-contaminants compared to absence of co-contaminants. Additionally, experiments with increases in salinity (i.e., NaCl or NaHCO3) had increased 1,4-dioxane removal rate constants suggesting potential chloride radical formation, and NaHCO 3 did not act as a radical scavenger for 1,4-dioxane removal in the presence of HPβCD. Except TCE, pH changes (from 4 to 10) did not show any significant impact on degradation rate constants.
Historically, O3 has been injected as gas-bubble in saturated and unsaturated porous media for remediation of contaminants. However, gas-phase O 3 has some limitations such as bubbling effect due to gas injection into water, influence on lateral movement of contaminants due to water table mounding, gas-water capillary pressure, etc. There is research needed to evaluate the feasibility of aqueous-phase O3 injection in saturated porous media for oxidant delivery to transform groundwater contaminants. For evaluating the efficiency of aqueous-phase O3 injection, we studied the transport behavior of aqueous-phase O3 into a saturated porous media. This research evaluated the reaction kinetics of O3 in saturated porous media and compared that with batch kinetics experiments. Transport model CXTFIT 2.1 was used to evaluate the hydrodynamic parameters of non-reactive tracer Pentafluoro Benzoic Acid (PFBA), and to predict the pseudo first order reaction constants of O3. Both column and batch equilibrium experiments showed the pseudo first order reaction kinetics of O3 and reaction rate constants were comparable. Breakthrough curves of O3 from column experiments and reactive transport models for different column lengths and flow rates were well fitted with experiments including transport modeling showing adsorption of O3 under prevailing conditions. Pseudo first order reaction rate constants of O3 (k min-1) were well predicted with the model using the same parameters from moment analysis. Degradation rate constants (k min-1) for aqueous-phase O 3 did not have significant change due to changes in flow rate (range of pore water velocity was 0.814 cm/min to 3.37 cm/min), due to changes in column length (20 cm, 60 cm, and 152 cm). Mass recovery of O3 was proportional in relationship with pore water velocity and inversely proportional to transport distance.
In summary, the results suggest the use of clathrate stabilizers, such as HPβCD, enabled O3 stabilization and enhanced treatment kinetics for both aboveground treatment and subsurface ISCO of groundwater impacted by recalcitrant contaminants. Moreover, injection of aqueous-phase O3 in saturated porous media is an efficient alternative to gas-phase O3 injection and has implications for subsurface ISCO of contaminated groundwater.
|Advisor:||Carroll, Kenneth C.|
|School:||New Mexico State University|
|School Location:||United States -- New Mexico|
|Source:||DAI-B 80/01(E), Dissertation Abstracts International|
|Subjects:||Hydrologic sciences, Natural Resource Management, Geochemistry|
|Keywords:||1.4 Dioxane, Crichloroethene, Cyclodextrin, In-situ Chemical Oxidation, Oxidant Transport, Trichloroethane|
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