Environ. Sci. Technol. 2000, 34, 3018-3022
Sustained Perchlorate Degradation in an Autotrophic, Gas-Phase, Packed-Bed Bioreactor JOEL P. MILLER† AND BRUCE E. LOGAN* Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania16802
An autotrophic packed-bed biofilm reactor was operated in unsaturated-flow mode and continuously fed water containing perchlorate (ClO4-) (as an electron acceptor) and a gas mixture of hydrogen (5%) and carbon dioxide. The reactor was inoculated with a perchlorate-reducing, hydrogen-oxidizing autotrophic bacterial consortium and run for 10 days at a perchlorate feed concentration of 50 mg/L to build up biofilm on the reactor packing. The reactor feed was then switched to a lower influent perchlorate concentration of 740 µg/L. Over a 140-day period at a constant hydraulic loading rate of 0.45 cm/min, 38 ( 9% of the perchlorate was removed in the reactor at detention times of 1.1-1.3 min producing an average removal rate of perchlorate of 230 µg L-1 min-1. To study in more detail perchlorate degradation kinetics, a hydrogen-oxidizing microorganism (Dechlorimonas sp. JM) was isolated from the bacterial consortium. Batch kinetic tests indicated hydrogen and perchlorate half-saturation constants of KH ) 0.036 ( 0.014 mM H2(l) and KP ) 0.15 ( 0.06 mM, respectively. The high perchlorate degradation rates and longterm performance of this system demonstrate the feasibility of this novel unsaturated hydrogen gas-phase fixed-film bioreactor for treatment of perchlorate-contaminated water.
Introduction Perchlorate (ClO4-) contamination of groundwater may affect the drinking water supplies of at least 12 million people in the United States (1, 2). Extensive soil and groundwater contamination at several locations in the United States has been linked to the manufacture and use of perchlorate. Ammonium perchlorate is used as the oxidizer in solid rocket fuel (up to 70 wt %) and in automobile air bag inflation systems (3, 4). Perchlorate also occurs naturally in nitrate deposits in Chile (4-6) and in some fertilizers (4, 7). Perchlorate is a human health concern at high doses due to its ability to interfere with iodide uptake and the ability of the thyroid to regulate hormone production and metabolism. Health risks at lower doses are not well understood, and studies are being conducted by the EPA in order to determine safe levels in drinking water. California was the first state to set a provisional drinking water standard for perchlorate of 18 µg/L (1). Perchlorate has been added to the Federal Contaminant Candidate List (CCL) under the Safe Drinking Water Act (SDWA), and the EPA is considering setting a drinking water standard of 32 µg/L (8). * Corresponding author phone: (814)863-7908; fax: (814)863-7304, e-mail:
[email protected]. † Present address: C.C. Johnson & Malhotra, P.C., 5101 Wisconsin Ave. NW, Suite 210, Washington, DC 20016. 3018
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
As recently as September of 1997, it was concluded by a panel of drinking water experts that there was no proven method to remove perchlorate from drinking water (8). Although highly oxidized, perchlorate is stable and quite soluble in water (>1 kg/L at 25 °C for sodium perchlorate) (1, 5). Perchlorate does not react with zero-valent iron, has long half-lives with other rare earth metals such as Ti3+ (0.83 yr) and V2+ (11.3 yr) (10), adsorbs poorly to activated carbon, and can be difficult to desorb from strong-base anion resins (11). Microbiological reduction of perchlorate by some strains of bacteria however is possible (2, 12). The first engineered systems for biological perchlorate reduction (3, 13, 14) were designed to treat highly contaminated rocket wash wastewater (1000-3000 mg/L) to 40 mg/L) perchlorate concentrations. The system influent was disrupted two times: on day 2, the influent pump failed, and on day 7, the effluent line clogged. using pure nitrogen gas. Sterile controls were prepared in the same manner to quantify hydrogen gas losses. Samples were analyzed for hydrogen gas concentrations every 2 h for 12 h while tubes were shaken horizontally on a shaker table to transfer hydrogen from the headspace into the liquid. Uptake rates were obtained from plots of aqueous hydrogen concentration versus time (corrected for abiotic losses). Overall kinetic parameters assuming Michaelis-Menten kinetics were obtained using a nonlinear fitting routine (Sigma Plot 5.0, Jandel Scientific) for uptake data plotted as a function of initial hydrogen concentrations. For perchlorate uptake experiments, duplicate samples (100 mL in VG-2 medium) were prepared as described above, with an initial absorbance of A600 ) 0.135 (61 µg DW/L) in 700-mL bottles (Wheaton Scientific) fitted with rubber septa screw caps. Bottles were purged with a 10.4% H2/89.6% N2 gas. Hydrogen gas concentrations were measured at the beginning and at the end of the experiment. The headspace volume in these bottles was large enough to ensure that hydrogen concentrations in the gas phase were not appreciably changed ( 0.998. Hydrogen in headspace of samples was measured using 100-µL gas injections (Alltech gas-tight syringe with Mininert adapter) and a gas chromatograph (GC; model 310, SRI Instruments, Torrence, CA) equipped with a thermal conductivity detector and a molecular sieve column (Molesieve 5A; Alltech) with a nitrogen carrier gas. Gas-phase concentrations were converted to liquid concentrations using a Henry’s law constant of 50.4 nM/nM (29). Cell concentrations were converted to dry weight by correlating absorbance (A600) readings to dry weights using preweighed filters (Poretics, 0.2 µm) and samples dried at 103 °C for >8 h. Samples were cooled prior to weighing on a microbalance (Mettler Toledo UMT2) to within (1 µg.
Results Reactor Startup. The gas-phase bioreactor was operated for a 10-day startup period at a high perchlorate concentration (50 mg/L) to build up biofilm in the reactor (Figure 2). During this startup period, perchlorate was reduced in the effluent by an average of 10.1 ( 1.4 mg/L (based on 6 of 9 data). On 3020
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
FIGURE 3. Hydrogen-oxidizing column reactor performance at low influent perchlorate concentrations. (A) Influent and effluent perchlorate concentrations and (B) calculated perchlorate removals (( SD). The arrows indicate column mixing. Measurements for days 0-60 are averaged daily grab samples and for days >60 are daily measurements made in triplicate. the first startup day, the reactor was loaded at a much higher loading rate (2.2 cm/min) than in all other experiments (0.45 cm/min); therefore, this point was excluded from the analysis. In addition, data were not used from day 2 when the influent pump ran dry and on day 7 when effluent tubing from the column clogged. Long-Term Reactor Performance. Perchlorate concentrations in the reactor influent were lowered to
