2. LITERATURE REVIEW 13
2.2. Remediation of PAHs Contaminated Environment
2.2.2. Chemical methods
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.
Fenton’s reagent
Hydrogen peroxide (H2O2) is a very strong non-selective liquid oxidizing agents that has widely been used in environmental applications. Though hydroxyl radicals ( ) generated from hydrogen peroxide is capable of reacting with aromatic compounds, the reaction rate is quite slow. Therefore in Fenton's reagent, hydrogen peroxide is dosed together with a solution of a transition metal (mostly iron) for dramatically increasing the peroxide oxidative strength by enhancing the radical formation (Flotron et al., 2005; Watts and Teel, 2005). The solubility of ferric ions generated from ferrous ions by hydroxyl radicals declines at higher pH (>3) due to precipitation of ferric ions as an oxyhydroxide complex (Bohn et al., 1985), which could however be manipulated to occur at near-neutral pH by stabilizing the solubility of ferric ions with chelating agents. In modified Fenton's system, the radical formation is enhanced by the addition of chelating agents and/or by high peroxide concentrations where numerous reacting species in addition to hydroxyl radical are generated, including hydroperoxide radicals, superoxide anions and hydroperoxide anions (Watts and Teel, 2005). The classical Fenton’s free radical mechanism in the absence of the target pollutants mainly involves the following sequence of reactions (Deng and Englehardt, 2006).
•OH
2 3
2 2
Fe ++H O →Fe++•OH OH+ − (2.1)
3 2
2 2 2
Fe ++H O →Fe HO+ •+H+
−
(2.2)
2 2 2 2
OH H O HO H O
• + → • + (2.3)
2 3
OH Fe Fe OH
• + +→ ++ (2.4)
3 2
2 2
Fe++HO• →Fe ++O H+
2 2
(2.5)
2 3
2 2
Fe ++HO•+H+ →Fe ++H O (2.6)
2 2 2
2HO• →H O +O (2.7)
The generated hydroxyl radicals can attack the organic pollutants either by radical addition, hydrogen abstraction, electron transfer, or radical combination, where organic radicals (R˙), formed may rapidly and irreversibly react with O2 generating intermediates, which may further continue to react with hydroxyl radicals and O2 leading to further decomposition and even final mineralization to water and CO2.
Nam et al. (2001) have observed Fenton’s reagent to be very efficient in the destruction of a mixture of naphthalene, fluorene, phenanthrene, anthracene, pyrene, chrysene and benzo[a]pyrene in spiked soil. The degradation was more noticeable for pyrene (84.5%) and benzo[a]pyrene (96.7%). In soil from a contaminated site, the same treatment method destroyed more than 80% of 2- and 3-ring and 20-40% of 4- and 5-ring PAHs. However, use of modified Fenton’s reagent (with catechol or gallic acid as chelator) resulted in a decline in overall performance relative to the unmodified Fenton's reagent.
Lindsey et al. (2003) showed carboxymethyl cyclodextrins (CMCD) as effective agent for degradation of pyrene by Fenton’s degradation in the presence of soil organic matters, humic acid or hydroxyl radical scavenger. However, it was observed by the authors that with increase in the amounts of CMCD, the pyrene degradation fell, though the CMCD protected the hydroxyl radicals from scavenging action of Cl-.
Jonsson et al. (2007) investigated as to how the chemical degradability of PAHs in soil samples is influenced by soil characteristics and by PAH physico-chemical properties in relatively mild, slurry-phase Fenton's reaction conditions. The authors observed LMW PAHs to be degraded to a greater extent (up to 89 and 59% with two and three rings, respectively) than highly hydrophobic HMW variants (0-38%). Anthracene,
benzo[a]pyrene and pyrene were found to be more susceptible to degradation compared to other structurally similar PAHs.
Lundstedt et al. (2006) studied effect of ethanol pre-treatment on efficiencies of Fenton’s oxidation in remediation of a PAH-contaminated soil from a former gasworks site. Although the authors observed facilitated desorption and enhanced depletion of all PAHs in the soil, some PAHs, particularly anthracene, benzo[a]pyrene and perylene were more extensively depleted than others.
Ozonation
Ozone is a highly reactive and powerful oxidizing agent that has traditionally been used in the chemical industry and also in the treatment of drinking water (Rositono et al., 2001). There has been considerable interest in using ozone to remediate contaminated soils which are otherwise non-responsive toward conventional soil venting.
Moreover, ozone can be used in the form of gas or liquid (Choi et al., 2001) which reverts back to atmospheric oxygen after a short period of treatment time leaving no residual contaminants in the soil. Goi and Trapido (2004) have reported the usefulness of ozone for the transformation of PAHs in contaminated soil where the intermediates generated were more soluble in the aqueous phase for easy biodegradation by microbes.
Kornmüller and Wiesmann (2003) studied ozonation kinetics of benzo[e]pyrene in oil/water-emulsions simulating the contaminated sites. Benzo[e]pyrene degradation rate constant for the ozonation process (1.02 min−1) in oil/water-emulsions system was even higher when compared to ozonation process involving dissolved benzo[e]pyrene in
water. The results confirmed the applicability of ozone treatment in real contaminated site containing adsorbed PAHs in an inhomogeneous NAPL water mixture.
Bernal-Martínez et al. (2005) studied combined effect of anaerobic digestion with ozonation in removal of all 13 PAHs (including anthracene, pyrene, benzo[a]anthracene and chrysene) in an urban sludge. The authors observed improved PAH removal rate (61%) due to ozonation of anaerobically digested sludge, compared to untreated anaerobically digested sludge (50%). Moreover, PAH removal rate increased up to 81%
when hydrogen peroxide was added during ozonation. However, higher doses of ozone treatment from the optimal level (1.5 g l-1) did not improve PAH removal noticeably. In another recent study by the same group (Bernal-Martínez et al., 2007), combining ozonation with anaerobic digestion increased biodegradability or bioavailability of each PAH (12 PAHs comprising HMW and LMW), and removals were well correlated to the PAH solubility.
O’Mahony et al. (2006) studied the use of ozone for the removal of phenanthrene from several different soils, both individually and in combination with microbial biodegradation. These authors observed negative impact of water content of the soil in the ozone treatment efficiency, and more than 50% and 85% removal in phenanthrene levels was achieved in air-dried soil and sandy soils, respectively, when treated with ozone for 6 h at 20 ppm. However, pre-ozonation did not enhance (or even some time retarded) subsequent removal of phenanthrene in the soils; the authors attributed this effect due to the possible release of toxic intermediates in this soil during ozonation.