Enhanced Biodegradation of Hydrocarbon Contaminants Using Photocatalytic Pretreatment

Photocatalysis was investigated as a pretreatment for hydrocarbon groundwater contamination at an abandoned oil field in Guadalupe, CA. A laboratory photoreactor, with a UV fluorescent lamp submerged in a suspension of TiO2 catalyst particles, was used to determine oxidation rates and the increase in biodegradability of the hydrocarbon mixture. Chemical oxidation demand (COD) and 5-day biological oxygen demand (BOD5) were measured as a function of exposure time using standard methods. During eight-hour photoreactor runs in a nitrogen atmosphere, the COD of diluent solutions in de-ionized water and diluent solutions in simulated groundwater both decreased by approximately 50%, while the BOD5 increased by a factor of 2 to 3. The biodegradability quotient, BOD5/COD, increased from initial values of between 0.03 and 0.06 up to final values of between 0.27 and 0.29 during the eight-hour photoreactor runs. Little or no decrease in the chemical oxygen demand was observed in a control with no UV light and a control with no TiO2, confirming the role of photocatalysis in these experiments. Based on the experimental oxidation and biodegradation kinetics, design parameters for ex-situ and in-situ remediation applications are presented. INTRODUCTION Improper handling and disposal of hazardous wastes has led to widespread groundwater contamination at thousands of sites throughout the world (Ollis et al., 1989). Ideally, groundwater contaminants could be removed via natural attenuation processes mediated by indigenous microorganisms. Unfortunately, some contaminants are not readily biodegradable or the kinetics of biodegradation are so slow that concentration limits designated by federal and local agencies cannot be met in a reasonable time period. Thus there has been a tremendous amount of research conducted to develop technologies that accelerate natural attenuation by enhancing or augmenting biodegradation via the introduction of additional nutrients, electron acceptors, or microorganisms to the subsurface environment. A more novel approach, explored in the present research, is to increase the biodegradability of the contaminants by physical-chemical pretreatment using photocatalysis. This approach could conceivably be applied to both above ground treatment schemes and in-situ remediation. Photocatalysis is a promising advanced oxidation processes (AOP) that has proven useful for the oxidation of many organic hazardous waste compounds, including alkanes, alkenes, haloalkanes, haloalkenes, haloaromatics, aromatics (e.g. phenol, nitrobenzene, nitrotoluenes), surfactants, herbicides, and even some toxic metals (Ollis et al., 1989). Photocatalysis or photocatalytic oxidation (PCO) is a type of heterogeneous catalysis in which a semiconductor material is irradiated with photons of ultraviolet wavelength (λ < 385 nm), creating energetic electrons and holes on the surface of the semiconductor. In an aqueous environment, these charged carriers then react with dissolved oxygen and/or water to produce highly reactive hydroxyl radicals that readily oxidize the contaminants. PCO effectively oxidizes a wide variety of environmental contaminants, yet the most economically efficient means of remediation may be a combination of PCO and natural attenuation. Through oxidation, chemical contaminant structure is changed, often resulting in compounds that are much more biodegradable. In the present study, the feasibility of such a hybrid physical-biological treatment scheme was investigated for remediation of hydrocarbon-contaminated groundwater at a site on the central coast of California. The field site is an abandoned oil field that was contaminated by a mixture of C10 to C30 hydrocarbons (similar to diesel fuel), which was used as a diluent for pumping viscous crude oil. Groundwater contaminated with this diluent was collected and treated with a laboratory photoreactor using suspended TiO2 catalyst. The kinetics of photo-oxidation and biodegradation determined in this study were then used to design potential ex-situ and in-situ hybrid remediation systems. MATERIALS AND METHODS Preparation of Contaminated Water. Samples of hydrocarbon diluent were collected during groundwater sampling from the Guadalupe Dunes Oil Field. A stock solution of contaminant was prepared by continuously mixing 200 mL of raw diluent with 3 L of filtered (5 μm) de-ionized water or simulated groundwater. Simulated groundwater contained 50 mg/L CaCl2 (Fisher Chemical, Fisher Scientific, Fair Lawn, NJ) and 200 mg/L NaHCO3 (Spectrum Chemical Co., Redondo Beach, CA). Contaminated water samples were allowed to come to equilibrium for 5 days before being used in the photoreactor. Photoreactor Apparatus. The photoreactor used for these experiments was an annular re-circulating batch reactor as previously described (Turchi and Ollis, 1989). Briefly, the system is modular in design, with a pump, a slurry vessel and an annular flow tube. The slurry vessel is a 2 L, jacketed, round bottom flask with three threaded sampling ports. The annular reactor column is composed of two 18-inch long concentric Pyrex® tubes 30 mm and 54 mm in diameter. Within the inner concentric tube was an 18-inch, 15-watt fluorescent ultraviolet lamp (λ ≈ 365 nm, Visual Effects, Inc., New York, NY). Slurry flow was maintained at 6.4 L/min using a 2.3 amp sealed bearing centrifugal pump (March Mfg, Inc., Glenview, CA). The temperature of the photoreactor was kept at 25°C by pumping ice water into the jacket of the slurry vessel using a 1-100 rpm adjustable peristaltic pump (Cole Parmer, Chicago, IL). Experiments were conducted with either air or nitrogen sparging (A & R Welding, San Luis Obispo, CA) through a porous stone into the reaction vessel. Photoreactor Operation and Sampling. Prior to the photoreactor runs, titanium dioxide (P25, Degussa Corp.) was added to the contaminated water at a concentration of 1 g/L and manually stirred. Once the TiO2 was thoroughly mixed with the contaminated water, it was poured into the reaction vessel. 40 mL samples were collected at 1 hr intervals and filtered through a 0.45 μm Nylon membrane (Whatman, Maidstone, England, Cat. No. 7404002) into collection vials. Chemical Oxygen Demand (COD) Analysis. COD was analyzed using the Accutest mercury-free Micro-COD System® (Bioscience, Inc., Bethlehem, PA). Potassium biphthalate (KHP) (Spectrum Chemical Co., Redondo Beach, CA), was used as a COD standard. Absorbance of KHP standards of known concentrations were measured and the COD in mg O2/L was calculated using the stoichiometric relation between KHP and oxygen (Eqn. 1). A calibration curve was prepared to determine COD levels in the samples taken from the reactor. 4 K C8H5O4 + 29 O2 AE 32 CO2 + 10 H2O + 4 K (1) Biological Oxygen Demand (BOD) Analysis. The initial microbial seed for BOD analyses was prepared using Polyseed® BOD Seed Inoculum (Interbio, Woodland, TX). An enrichment culture acclimated to growth on diluent was prepared from Polyseed® and used as the seed for all BOD measurements. Each 60 mL BOD bottle (Wheaton, Millville, NJ) contained 10 mL of reactor effluent sample, 2 mL acclimated seed inoculum, 12 μL of concentrated buffered mineral solution (Hach, Loveland, CO) and dilution water. The dissolved oxygen (DO) concentration in each bottle was measured using a portable DO meter with a selfstirring, polarometric DO probe (YSI, Yellow Springs, OH). The BOD bottles were incubated at 20°C for five days. DO measurements were repeated every twenty-four hours for five days, and the BOD5 was calculated from the observed O2 depletion.