Petroleum and Soil Pollution

The use of ozone to oxidize contaminants may be advantageous over other oxidants because it can be readily generated on-site using an air source and because of its additional reactivity as it decomposes to secondary oxidants, such as hydroxyl (HO_) and peroxyl (HOO_) radicals. These ozone decomposition products are among the most reactive oxidizing species (Eo(HO_) = 2.8V vs Eo(F) = 3.1V).

In contrast to ozone, these radicals exhibit non-selective reaction chemistry and are produced in chain reactions where HO_itself or HOO_act as initiators. Ideally, ozone could selectively attack unsaturated carbon-carbon linkages as well as decompose to yield reactive radical intermediates that could in-turn degrade saturated hydrocarbons. In addition, application of an oxidizing technology using ozone may preclude the addition of undesirable metals (i.e. Fe in Fenton's reagent, Mn in KMnO4) to the treated source.

Consequently, the development of chemical oxidation processes using ozone to remediate polyaromatic hydrocarbon (PAH) contaminated soil sites have received considerable interest. Ozonation of soil samples containing "spiked" phenanthrene have indicated that 95% reduction of the contaminant can be achieved.

In contrast, degradation of higher molecular weight PAH's namely, chrysene and pyrene, in the same soil system was unsuccessful (ca. : Bioremediation can be a cost-effective method to remediate soils containing crude oil but may not meet regulatory criteria.

Chemical oxidation using ozone, O3, was used as a polishing step to degrade hydrocarbon species that resisted microbial oxidation during bioremediation. The effect of oxidation on moist and dry bioremediated soils containing weathered crude oil (20,600 mg/kg Total Oil and Grease, 40% w/w of total hydrocarbons > 525 o C boiling point) was investigated.

Soil column experiments have shown that moisture content and prior exposure to microbial oxidation control the effectiveness of the degradation process. Moist and dry soil samples were treated with ozone and in both cases significant reductions in extractable hydrocarbons (ca. ~90% w/w) were observed.

In dried soil, however, hydrocarbon contaminant removal was markedly enhanced, a result that is suggested to correlate to the soil temperatures achieved during oxidation. Carbon dioxide trapping from the gas effluent indicated approximately 30% w/w of the hydrocarbons present in the soil had been mineralized.

Bioremediated Soil. The soil used for this study had previously been subjected to three seasons (115 days/season) of ex situ biotreatment using organic and nutrient amendments. It was determined that prior to bioremediation, the crude oil/hydrocarbon content was approximately 40,000 mg/kg defined as total oil and grease (TOG by CH2Cl2 extraction) and 30,000 mg/kg by standard mineral oil and grease (MOG) methods.

Simulated distillation of the extracted hydrocarbons indicated a crude oil composition of broad boiling point range with a significant fraction of the material (ca. 40%) having a boiling point greater than 525 o C. Although bioremediation had resulted in a significant reduction in the extractable hydrocarbons (TOG 20,600 mg/kg, MOG 13,000 mg/kg) within the soil after three seasons of treatment, biodegradation rates had declined and additional biotreatment was not deemed effective.

Moreover, the Canadian Alberta Environment regulatory standards of 1000 mg/kg (MOG) were not achieved (Alberta Env., 1994). The GC results showed that the bioremediation process had a measureable impact on the lower boiling components yet only moderately affected the higher boiling (or molecular weight) species

Carbon Dioxide Wet-Chemistry

Indeed, CO2 formation in the gas effluent was detected. In fact, the analysis of the gas effluent over time indicated that up to 30% of the hydrocarbons had been converted to CO2 (Figure 4). However, only 55% mass balance was achieved, likely because of the formation of highly polar species that would tend to form stronger coordination complexes/interactions with the soil matrix thereby resulting in less recoverable contaminants via extraction.

Soil column experiments indicate that bioremediation followed by ozone oxidation is an effective remediation technology provided key variables are controlled. The oxidation is considerably more effective in dry soil than in moist soil. Analysis of the gas effluent indicated that mineralization of the contaminants occurred.

Moreover, temperature monitoring of the soil during oxidation provided a straightforward:

1. Hydrocarbons + CO2 H2O +(1) O3
2. CO2 + NaOH (aq) NaHCO3 (aq) (2)
3. CO2 H2O ++ NaHCO3 (aq) HCl (3)
4. CO2 + NaOH(s)/SiO2 NaHCO3(s)/SiO2 (4)
5. CO2 Formation (%) versus Oxidation Time.

REMEDIATION OF A SITE IMPACTED WITH CHLORINATED AND PETROLEUM HYDROCARBONS

An integrated, full-scale remedial program was implemented at a former dry cleaning site in Salinas, CA from April to September 2001 that combined the use of multiple technologies to address spatially separated petroleum and chlorinated hydrocarbon impacts in soil and shallow groundwater.

Approximately 2,300 cubic meters (m 3 ) of the petroleum hydrocarbon-impacted soil (exhibiting total petroleum hydrocarbon [TPH] concentrations up to 2,040,000 _ g/kg and total benzene, toluene, ethylbenzene, and xylenes [BTEX] concentrations up to 152,700 _ g/kg) were excavated in April 2001 and treated off-site using low temperature thermal desorption.

The underlying petroleum hydrocarbon-impacted, shallow groundwater (exhibiting TPH concentrations up to 28,300 _ g/L and total BTEX concentrations up to 10,320 _ g/L), is being addressed using an in situ aerobic bioremediation system. The chlorinated hydrocarbon-impacted shallow groundwater (with concentrations of total chlorinated volatile organic compounds [VOCs] up to 5,520 _ g/L) is being addressed using an in situ anaerobic bioremediation approach. To address the approximately 230 m 3 of tetrachloroethene [PCE]-impacted soil (with PCE concentrations up to 4,100,000 _ g/kg), a three-month bench-scale study (involving whey, molasses, and nutrients at several application rates and combinations and using a representative batch of the impacted soil) was conducted.

The study identified the optimum biological treatment regimen that exhibited the highest biodegradation rate of PCE while minimizing daughter product accumulation. The PCE-impacted soil was then excavated, amended with the bench study-identified organic and nutrient additives, placed back into the lined excavation, and sealed. Chlorinated VOC concentrations in ten soil samples collected from the entombed soil after 7 and 8 months of treatment were almost all below the cleanup goals.

Quarterly monitoring for dissolved oxygen (DO) levels, oxidation-reduction potential (ORP), and the concentrations of petroleum and chlorinated hydrocarbon compounds in groundwater has been conducted. The results indicate that the in situ bioremediation systems have generally created aerobic and anaerobic conditions in the appropriate shallow saturated zone areas.

However, DO levels in the petroleum hydrocarbon-impacted groundwater have tended to decline over time and must be increased periodically by deliveries of batches of peroxide-based oxygen sources. Dissolved TPH concentrations in the treated groundwater initially increased from pre-treatment levels and petroleum hydrocarbon compounds that had not been observed previously were detected in groundwater samples from this site area.

Presumably, the excavation of the TPH-impacted soil resulted in releases of previously trapped and “novel” hydrocarbons to the shallow groundwater. Groundwater sampling results in the region impacted by chlorinated hydrocarbons indicate that appreciable biodegradative activity had not begun as of May 2002.