Vapor-phase biodegradation and natural attenuation of petroleum VOCs in the unsaturated zone: A microcosm study.

Traditional natural attenuation studies focus on aqueous process in the saturated zone while vapor-phase biodegradation and natural attenuation in the unsaturated zone received much less attention. This study used microcosm experiments to explore the vapor-phase biodegradation and natural attenuation of 23 petroleum VOCs in the unsaturated zone including 7 monoaromatic hydrocarbons, 6 n-alkanes, 4 cycloalkanes, 3 alkylcycloalkanes and 3 fuel ethers. We found that monoaromatic hydrocarbon vapors were easily attenuated with significantly high first-order attenuation rates (9.48 d-1-43.20 d-1) in live yellow earth, of which toluene and benzene had the highest rates (43.20 d-1 and 28.32 d-1, respectively). The 13 aliphatic hydrocarbons and 3 fuel ethers all have relatively low attenuation rates (<0.54 d-1) in live soil and negligible biodegradation contribution. We explored the effects of soil types (black soil, yellow earth, lateritic red earth and quartz sand), soil moisture (2, 5, 10, and 17 wt%) contents and temperatures (4, 15, 25, 35 and 45 °C) on the vapor attenuation. Results showed that increasing soil organic matter (SOM) content, silt content, porosity and soil microorganism numbers enhanced contaminant attenuation and remediation efficiency. Increasing moisture content reduced the apparent first-order biodegradation rates of monoaromatic hydrocarbon vapors. The vapor-phase biodegradation had optimal temperature (∼25 °C in yellow earth) and increasing or decreasing temperature slowed down biodegradation rate. Overall, this study enhanced our understanding of vapor-phase biodegradation and natural attenuation of petroleum VOCs in the unsaturated zone, which is critical for the long-term management and remediation of petroleum contaminated site.

Transport and natural attenuation of benzene vapor from a point source in the vadose zone.

The vadose zone is a very dynamic and active environment that directly affects natural attenuation and vapor intrusion of volatile organic compounds (VOCs). Therefore, it is important to understand the fate and transport of VOCs in the vadose zone. A column experiment combined with model study was conducted to investigate the influence of soil type, vadose zone thickness, and soil moisture content on benzene vapor transport and natural attenuation in the vadose zone. Vapor-phase biodegradation and volatilization to atmosphere for benzene are two main natural attenuation mechanism in the vadose zone. Our data showed that biodegradation in black soil is the main natural attenuation mechanism (82.8%) while volatilization is the main natural attenuation mechanism in quartz sand, floodplain soil, lateritic red earth and yellow earth (>71.9%). The R-UNSAT model-predicted soil gas concentration profile and flux were close with four soil column data except for yellow earth. Increasing the vadose zone thickness and soil moisture content significantly reduced the volatilization contribution while increased biodegradation contribution. The volatilization loss decreased from 89.3% to 45.8% when the vadose zone thickness increased from 30 cm to 150 cm. The volatilization loss decreased from 71.9% to 10.1% when the soil moisture content increased from 6.4% to 25.4%. Overall, this study provided valuable insights into clarifying the roles of soil type, moisture, and other environmental conditions in vadose zone natural attenuation mechanism and vapor concentration.

Degradation of 1,2,3-trichloropropane by unactivated persulfate and the implications for groundwater remediation.

Persulfate (PS) is widely used as an in situ chemical oxidation (ISCO) technology for groundwater and soil remediation. While conventional theory generally assumes that PS needs to be "activated" to produce reactive radicals for pollutant degradation, herein, PS without explicit activation system was discovered for the degradation of 1,2,3-TCP with the generation of reactive oxidation species (ROS). Comparison of five common ISCO oxidants (PS, peroxymonosulfate, hydrogen peroxide, potassium permanganate, and sodium percarbonate) indicated that only unactivated PS was able to degrade 1,2,3-TCP in both pure water and 12 natural water samples. 50 µM 1,2,3-TCP degradation can be continued as long as there is enough PS (50 mM). The degradation rate of 1,2,3-TCP increased 450 % when the PS concentration increased from 10 mM to 50 mM and 500 % when the temperature increased from 25 °C to 45 °C. Electron paramagnetic resonance (EPR) analyzes, hydroxyl radicals (·OH) probe reaction and radical quenching experiments confirmed the involvement of both sulfate radicals (SO4·-) and ·OH that were responsible for 1,2,3-TCP degradation and ·OH played a more important role. HCO3-, Cl- and NOM are three groundwater matrix species that are most likely to inhibit PS oxidation of 1,2,3-TCP. Compared to activated PS, unactivated PS is more promising and more practical for groundwater remediation, since it has several advantages: (1) longer lifetime and better long-term availability; (2) ability of enduring contaminant degradation; (3) applicable for low-permeability zones remediation and potential to alleviate contaminant rebound or tailing problems; (4) environmental friendly; and (5) lower cost. Overall, results of this study show that unactivated PS is a promising in situ remediation technology that may be a good candidate for the most challenging low permeable zone remediation.