2. LITERATURE REVIEW 13
2.2. Remediation of PAHs Contaminated Environment
2.2.1. Physical methods
Of the various physical methods, incineration, soil vapor extractions (SVE), thermal de-sorption, soil washing/solvent extraction, disposal at hazardous land fills or deep well injection are quite well studied (Lagadec et al., 2000). While each of these techniques has some advantages, there are serious concerns regarding their usage either from the consideration of the environment or economics or both. Incineration is the only destruction technology that completely degrades the toxic residues on soil, but is expensive. Similarly, solvent extraction requires expensive solvent regeneration, and thermal desorption produces air pollutants, which often necessitates secondary treatment of the off gases (Sahle-Demessie and Richardson, 2000). On the other hand, land filling and deep well injection methods are mere transfer of pollutants from one phase to the other and only postpones the PAH pollution problem to the future.
Soil vapor extraction (SVE) is one of the most widely used simple, efficient and cost effective in-situ technology used for removing volatile and semi volatile organic compounds from the contaminated soil, where vacuum is applied to the soil matrix that produces air flow in the soil, and, which due to free volatilization and desorption from the soil transports the contaminants to extraction wells (Figure 2.2). The off gas is usually treated before it reaches the atmosphere. However, this method is applicable for treating LMW PAHs and not ideally suited for HMW ones, mainly because of their less volatile nature. Moreover, important limitation of this remediation technology is the difficulty of
predicting the required time for cleaning up PAH contaminated environment (Albergaria et al., 2006).
Figure 2.2: Schematic of soil vapor extraction system.
* Reproduced from USEPA technology fact sheet EPA 542-F-96-008
Harmon et al. (2001) conducted a feasibility study in support of a soil venting- thermal desorption (SVTD) process, which couples SVE with in situ heating, for remediating lampblack-impacted soil containing 11 PAHs totaling about 4100 mg kg-1 total PAH (TPAH). The results indicated that temperatures above 250°C were sufficient to mobilize most of the PAHs, where TPAH load in the soil was reduced to less than 100 mg kg-1 within 10 d.
Park et al. (2005) conducted laboratory and field pilot studies to evaluate the effectiveness of soil vapor extraction (SVE) system for the removal of semi-volatile organic contaminants (SVOCs) including PAHs from soils. They found that increased rate of air flow results in higher removal of contaminants, which, however, resulted in mass transfer limited volatilization at very high air flow rate. Field pilot study of a hot air
injection for the remediation of diesel-contaminated soil showed dramatic reduction of total TPAH concentrations (> 95%) within 30 d of test operation.
Hot water extraction or pressurized hot water extraction (PHWE) have been used largely in the recent past for extraction of hydrophobic PAHs from contaminated soil and sediments (Andersson et al., 2002; Smith, 2002; Rivas et al., 2008). In PHWE, the temperature of water is kept at 100°C and the critical temperature (Tc) at 374°C.
Furthermore, the solvating properties of water are easily altered through changes in temperature and pressure. For example, at 250 °C, the dielectric constant of water is equal to the dielectric constant of methanol under ambient conditions (Andersson et al., 2002), and, therefore, more polar contaminants can be extracted at lower temperatures;
where as contaminants with low polarity could be extracted at higher temperatures, thus providing possibility of class selective extraction using this technique (Yang et al., 1997).
The major advantage of PHWE is because of the use of water, which is known to be cheaper and far more environmentally compatible than any known solvent. Dadkhah and Akgerman (2002) studied a small-scale batch extraction with/without in situ wet oxidation of PAHs in spiked and aged soils using subcritical water. They observed that removal of phenanthrene, anthracene, chrysene and benzo[a]pyrene from spiked soil in extraction-only experiments was from 79 to 99% depending on their molecular weight, which was however, in the range of 99.1% to 100% for the combined extraction and oxidation. In a more recent study by Dadkhah and Akgerman (2006), semi-continuous experiments with residence times of 1 and 2 h were performed using aged soil at 250°C and hydrogen peroxide as the oxidizing agent. In all combined extraction and oxidation
flow experiments, residual PAHs in the soil were undetectable in the liquid phase after the first 30 min of the experiments.
Supercritical extraction uses supercritical fluids (SCFs) such as carbon dioxide (mixed with co-solvent as methanol) or water for the extraction of volatile and persistent organic pollutants including PAHs from environmental matrices. Figure 2.3 represents a schematic of supercritical fluid extraction system. In addition to having higher affinity for contaminants than the accompanying solid matrix, the chosen SCF should have liquid- like density, low viscosity, high diffusivity and no surface tension for extracting the contaminants at optimum temperature, pressure and flow rate conditions.
Figure 2.3: Schematic of a super critical fluid extraction system.
Anitescu and Tavlarides (2006) recently reviewed the existing literature about SCE methods. It has been a general finding that SCE does not destroy contaminants rather extracted pollutants are highly concentrated that can be subsequently destroyed
more cost effectively. Librando et al. (2004) studied extraction efficiency of 11 PAHs including pyrene from spiked soil using CO2 in the supercritical phase at 50-80°C, at a pressure of 230-600 bar, with three different organic solvents (methanol, n-hexane and toluene) added at 5% v/v. Using methanol as the co-solvent, an increase in the yield of recovered PAHs was observed, but higher temperature than 80°C caused a negative effect. The recovery yield for PAHs from the spiked soil sample was measured and found to be greater than 90%, and, in general, the species with a HMW showed better recovery yields than LMW compounds.