environments Article Wet Oxidation of Fine Soil Contaminated with Petroleum Hydrocarbons: A Way towards a Remediation Cycle Maria Cristina Collivignarelli 1 ID , Mentore Vaccari 2 ID , Alessandro Abbà 1,* ID , Matteo Canato 1 and Sabrina Sorlini 2 1 Department of Civil and Architectural Engineering, University of Pavia, via Ferrata 1, 27100 Pavia, Italy; [email protected] (M.C.C.); [email protected] (M.C.) 2 Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, via Branze 43, 25123 Brescia, Italy; [email protected] (M.V.); [email protected] (S.S.) * Correspondence: [email protected]; Tel.: +39-0382-985-314; Fax: +39-0382-985-589 Received: 16 May 2018; Accepted: 6 June 2018; Published: 8 June 2018 Abstract: The aim of this experimental study was to assess the feasibility of using a wet oxidation (WO) process for treating fine soil with a high level of total petroleum hydrocarbons (TPHs). Two samples of soil were spiked with two different contaminants (motor oil, and motor oil + diesel). The samples were subjected to a WO bench plant test, where the effect of the main process parameters (i.e., temperature and reaction time) on the removal of TPHs was investigated. Results show that the WO process is effective for the decontamination of hydrocarbons, and a strong reduction (>85%) can be obtained with the typical working conditions of a full-scale plant (temperature = 250 ◦C, reaction time = 30 min). The solid residue resulting from the WO process was characterized in order to evaluate the recovery options. In terms of chemical characterization, the contents of the pollutants comply with the Italian regulations for commercial and industrial site use. Moreover, the results of the leaching test suggested that these residues could be reused for ceramic and brick manufacturing processes. Keywords: chemical oxidation; contaminated soil remediation; leaching test; recovery options 1. Introduction The rapid and effective removal of total petroleum hydrocarbons (TPHs) from soil can be theoretically obtained through chemical–physical treatments, such as solvent extraction, low temperature thermal desorption, or chemical oxidation. The application of these technologies enables the attainment of high TPHs removal efficiency, even if some drawbacks must be taken into account. For instance, when solvent extraction is applied, the toxicity of the solvent is an important factor, as traces of solvent may remain within the treated soil [1], thereby compromising its quality. On the other hand, for chemical oxidation, reagent costs have to be carefully evaluated during the design phase [2]; for instance, in Fenton’s process, high carbonate and bicarbonate concentrations in the soil (as well as the organic matter) can lead to a high dosage of reagents [3], with respect to the stoichiometric amount. Furthermore, the composition of the soil (in terms of granulometry) can also affect the decontamination process; to give an example, soils characterized by a high content of silt and clay cannot be treated effectively with cheaper biological processes, or by soil vapor extraction, because of their low air permeability. Moreover, due to the tendency of the fine fraction of soil to form aggregates, pre-treatments such as shredding could be necessary prior to the decontamination process, increasing the treatment costs. Environments 2018, 5, 69; doi:10.3390/environments5060069 www.mdpi.com/journal/environments Environments 2018, 5, 69 2 of 13 In addition, it must be taken into account that due to the tendency of contaminants to accumulate in the fine fraction of soil [4], in many cases, that fraction is not treated, but instead directly landfilled, and the remediation cycle is not completed; something that occurs, as reported by Dermont et al. [5], in soil washing treatment. Another aspect to be taken into account is the change in the partition of the contaminant amongst the different fractions of the soils due to the washing process [6]. In recent times, the European Commission has adopted an ambitious Circular Economy Package [7], aiming to stimulate Europe’s transition towards a circular economy. The proposed actions will contribute to “closing the loop” of product life cycles through greater recycling and reuse, and will bring benefits for both the environment and the economy. Therefore, the technicians are motivated to improve or find alternative remediation technologies finalized to close the remediation cycle, and give a new life to waste. A technology that can be useful in reaching the aforementioned targets is the wet oxidation (WO) process, which is reliable and effective in the treatment of a wide spectrum of organic aqueous waste, even toxic waste, produced by various branches of industrial activity [8,9], and sewage sludge [10]. Typical treatment conditions for sludge and hazardous wastes reported in the literature are as follows: 200–325 ◦C for temperature; 5000–17,500 kPa for pressure; and 15–120 min for the reaction time. The preferred COD load ranges from 10–80 kg·m−3. One of the biggest advantages related to the WO process is that decontamination is obtained with low polluting output gases, composed mainly of CO2, water steam, and oxygen, without hazardous by-products and low organic content in the final residue [8,11,12]. In addition, the WO effluent can be conveniently treated in a conventional wastewater treatment plant because of its high biodegradability, and, in some cases, it could also be used as a carbon source for the denitrification process, and as a substrate for the production of biopolymers [12]. Moreover, other researchers have also demonstrated that WO effluent represents an interesting energy vector; in fact, if it is treated by anaerobic digestion, biogas can be produced, and electric and thermal energy could be recovered [13]. Today, about 200 plants are in operation around the world (two of them in Italy), mostly to treat waste streams from petrochemical, chemical, and pharmaceutical industries, as well as residual sludge from urban wastewater treatment [12]. Detailed reviews about the WO process and its application are reported in Debellefontaine and Foussard [8], and Bhargava et al. [14]. According to Italian regulations [15], the soil and stones not containing dangerous substances (classified using the European Waste Code 17 05 04 [16]) can be recovered, in simplified procedure, for the following uses: ceramic and brick manufacturing processes, environmental recovery, road embankments, and foundations. With the exception of the first recovery route, the release of pollutants (by means of a leaching test provided for UNI EN 1247-2 [17]) must be lower than the limit values reported in the regulations. According to Italian regulations, the simplified procedure (where no actual permission is necessary, only the notification of competent local authorities) is provided for a limited class of recovery operations that are completely defined by the law, both in terms of qualitative description, and the threshold quantities to be handled. These residues could also be recovered by an ordinary procedure for further applications. The conventional use of fine particles falls mainly in the civil construction field, employed for the following uses: in the production of bitumen-based binder [18]; as unbound aggregate (usually mixed with lime) in road bases and subbases [19–21]; in the production of cement and concrete [22–25] and bricks [26,27]. These kinds of recovery could be limited due to the uncertainty regarding the physical properties of residues, and the effects on the products obtained. The objective of this work was to carry out a detailed investigation of the WO technology as a treatment option for the decontamination of fine-grained soil, and to demonstrate that the remediation of the soil is achievable. To this end, two samples of fine soil, spiked with contaminants based on petroleum hydrocarbons, were subjected to the WO treatment. The first sample was contaminated with motor oil, while the second one was contaminated with a mix of motor oil and diesel only. Environments 2018, 5, 69 3 of 13 This work is original. In fact, few studies have been reported that deal with the chemical oxidation process for the treatment of wastewater originating from the soil washing process [28–30], or with hot water extraction combined with in situ wet oxidation for the removal of polycyclic aromatic hydrocarbons (PAHs) from soil [31,32]. To our knowledge, no research dealing solely with the direct application of the WO process for the remediation of contaminated soils has been published. 2. Materials and Methods 2.1. Soil Characteristics For the tests, two samples of fine soil (about 90% of soil passes through an ASTM (American Society for Testing and Materials) 200 mesh sieve) were spiked with the following: motor oil (soil #1) and motor oil + diesel (soil #2). The contamination with motor oil was carried out using a heavy-duty synthetic engine oil designed for the lubrication of diesel engines (15W-40 type), while the other contamination was achieved using a commercial diesel fuel. The artificial contamination was carried out by spiking the contaminants on 5 kg of samples. Specifically, the soil samples were put into two different vessels, after which the contaminants were spread on the soils (by means of a spray equipment). The soils were also mixed with trowels. Two days of aging (in a covered basin) were also provided for. The main characteristics of the soil samples (before and after the artificial contamination) are summarized in Table1. The soils, from the granulometric point of view, are similar to the fine fraction of residues derived from mining activities. About 90% of the soil tested is silt and clay. Table 1. Characteristics of the soils treated by wet oxidation. Soil #1 Soil #2 Parameter Measurement Unit As Raw Motor Oil As Raw Motor Oil + Diesel Contaminating medium Total Petroleum mg·kg −1 <20 1455 ± 220 <20 4736 ± 600 Hydrocarbons (TPHs) dw Total Polycyclic Aromatic mg·kg −1 <0.1 0.12 ± 0.10 <0.1 0.20 ± 0.05 Hydrocarbons (PAHs) dw pH - n.a.