Conversely, subsurface systems are often characterized by a high buffer capacity with pH range usually close to neutral values (e.g., pH 68), making the acidification of the system quite complicated (Petri et al. 2011). To overcome this issue, soluble ligands, such as ethylene diamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), hydroxyethyliminodiacetic acid (HEIDA), oxalic acid and sodium pyrophosphate, were considered for their capacity to keep the iron in solution as an organo-metallic complex at near-neutral pH ranges (Gan et al. 2012; Innocenti et al. 2014; Sun et al. 1992; Schmidt et al. 2011; Watts et al. 2005, 2007; Vicente et al. 2011; Piscitelli et al. 2015). In the last years, humic acids (HA) have also been proposed and tested as amendments for the Fenton process (e.g. Georgi et al. 2007; Lipczynska-Kochany et al. 2008; Santos et al. 2016). Humic acids (HA) together with fulvic acids (FA) are one of the major constituents of the organic matter of soil and sediments and are produced by the chemical and biological degradation of plant and animal residues and by the synthetic activities of microorganisms (Schnitzer and Khan, 1972). Due to the high content of functional groups (carboxylic, phenolic, alcoholic, quinone, amino and amido groups) humic acids are able to perform ionic exchange, complex formation and oxidationreduction processes (e.g. Davies et al. 1997). Potentially important effect of Humic Substances (HS) on the Fenton and Fenton-like processes still remains unclear as their relationship is complicated and the published results are conflicting (Lipczynska-Kochany et al. 2008). Some authors suggested that the presence of HS inhibits (Lindsey and Tarr, 2000a, 2000b; Bogan and Trbovic, 2003) or has no significant effect (Tyre et al. 1991; Li et al. 1998; Bissey et al. 2006) on the Fenton processes, while others (Huling et al. 2001; Yeh et al. 2002; Vione at al., 2004; Georgi et al. 2007; Kochany and Lipczynska-Kochany, 2007) reported that HS enhance the oxidation efficiency in such systems.
Namely, Georgi et al. (2007) observed that at pH 5, 6 and 7 benzene degradation is significantly accelerated by the presence of HA due to the formation of soluble complexes with iron at circumneutral pH, which are able to activate H2O2. According to the authors, the increased benzene degradation rate resulted from the acceleration of Fe3+ reduction, although the generation of nonselective OH? radicals was observed to induce the humic acid oxidation, with a progressive deactivation of the modified Fenton system.
Accordingly, Lipczynska-Kochany et al. (2008) have observed that in the presence of humic substances, the contaminant removal at pH 7.0 is comparable to the one observed operating at pH 3.5 without HS, whereas HS addition at pH 3.5 significantly decreased the removal of all organic pollutants. The authors concluded that in the presence of humic substances the acidification of wastewater may not only be unnecessary but it can even hinder the degradation of organic pollutants.
Also Santos et al. (2016) investigated the effect of a hybrid system that combines Fenton like reagent and HA for removing PFOA, observing a beneficial effect of HA. Only 10% of PFOA removal was achieved after 20 h adopting a classic Fenton treatment whereas the complete PFOA removal observed in the presence of HA was attributed by the authors to the sorption of PFOA on the particles of humic acid precipitated after oxidation.
Most of the studies performed so far investigated the adoption of a combined HA-Fenton system for treating wastewater contaminated by various substances. However, this treatment could be also positively applied for removing contaminants present in groundwater. Furthermore, all the above cited studies adopted humic acids produced at commercial level from lignite or leornardite. However, several types of organic wastes, such as compost, present large amounts of humic substances due to the microbial degradation of organic matter. Hence, in agreement with the Circular economy concept, introduced by the European Council, that encourages to eliminate waste finding added values in products for as long as possible (European Commission, 2014, COM(2014)398) it is conceivable to extract soluble organic substances from bio-wastes and use them in Fenton’s application, instead of commercial humic acids. In this direction, in a previous study we have shown the beneficial effect of humic acids extracted from compost on the Fenton-like treatment of a soil-water system artificially contaminated by 3-chlorophenol, used as model compound (Zingaretti et al. 2018). However, the effectiveness of using the humic acids extracted from compost as amendment in a Fenton-like process adopted for treating an aged contamination, has still to be investigated. The weathering indeed entails a reduction in the availability of the contaminants for the oxidation and hence more difficulties are encountered for reaching a significant removal (McAlexander et al. 2015; Lemaire et al. 2019).
In the present study, we extend the application of HA extracted from compost to the Fenton-like treatment of a soil collected in a former gasoline station characterized by a diesel contamination. Namely, Fenton-like lab-scale tests with different dosages of the extracted humic acids were carried out on soil samples collected from the site, evaluating hydrogen peroxide lifetime and diesel removal. In order to compare the performance of this system, several tests were also carried with the addition of a traditional stabilizing agent (KH2PO4) and, as reference, control tests applying only KH2PO4 were also performed.