1. Introduction
At present, there are more than 20 different types of electric wires available, designed for applications ranging from transmission to heavy industrial use. Electric wires consist of one conductor, which is usually copper or aluminum, covered with insulating material, which can be polyvinyl chloride (PVC), polyethylene (PE), rubber, etc. [
1].
Electric wires are designed with a limited useful life [
2], normally they work for 10–30 years, so they become obsolete after a certain period of time. Consequently, electric wires waste generation has increased noticeably in recent years. Therefore, it is essential to take appropriate measures about the fate of this waste to avoid a negative environmental impact. Prevention of the generation of this waste, reuse and recycling are essential to preserve the environment and public health [
3].
Recycling is crucial, not only to diminish the quantity of waste requiring treatment, but also to recover some of the raw materials, such as plastics and base metals, as well as to eliminate the hazardous compounds (heavy metals such as mercury, nickel, lead, cadmium, etc. and brominated flame retardants) [
4]. The stages of recycling process are: (1) disassembly, (2) separation of metallic and non-metallic fractions, and (3) recycling of both fractions separately. The metallic fraction, which represents 50% of the total weight of electric wires [
5], is considered the most valuable fraction and becomes part of new industrial processes, on the one hand, and, on the other, the non-metallic fraction is used for recycling for material, chemical, or energy recovery [
2].
When plastics of electric wires waste are PVC, thermal treatment is turned into a very problematic way of recovery because important quantities of toxic or harmful compounds, such chlorophenols (ClPhs), chlorobenzenes (ClBzs), and polychlorodibenzo-p-dioxins and furans (PCDD/Fs), are emitted during the thermal decomposition of PVC in an inert and oxidizing atmosphere [
6] and the thermal decomposition of PVC electric wires in air [
5]. One solution is the development of removal techniques of chlorine from PVC electric wires before its thermal treatment.
Catalytic dechlorination and hydrothermal treatment with subcritical or supercritical fluids are methods for removal of chlorine present in PVC wastes. Catalytic dechlorination involves the selective break of one or more C‒Cl bonds, lowering toxicity and giving rise to reusable materials [
7]. Such a phenomenon is due to addition of large volume of catalyst. Catalyst poisoning may be a problem during scale-up of the procedure, as well as the high operation temperature (400–1000 °C), nonuniform heat transfer, low yields obtained and excessive char and gas formation [
8].
Subcritical or supercritical fluids have been successfully applied as a simpler and environmental friendly methods for decomposition of various polymer materials [
8,
9,
10,
11,
12].
Hydrothermal treatment with subcritical or supercritical water have become an effective and promising techniques to remove the chlorine contained in PVC-containing wastes [
13] because these process present simple and fast reaction rates, and homogeneous reactions without limitations of mass transfer occur in these processes. However, subcritical water treatment presents advantages over supercritical water treatment, in addition to requiring lower temperatures and pressures, dechlorination reactions may be enhanced [
14]. Dechlorination of methylene chloride was much faster using subcritical water than when using supercritical water [
15].
Poerschmann et al. [
16] subjected PVC to subcritical water treatment at 180–260 °C and concluded that dechlorination increased with the increase of treatment temperature. Xiu et al. [
17] carried out subcritical water oxidation of waste printed circuit boards and PVC, obtaining a total dechlorination of PVC when the treatment temperature exceeded 250 °C. Kubátová et al. [
14] obtained a solid residue that contained less than 1 wt % of chlorine after subcritical water treatment of PVC at 300 °C. In the same research line, Endo and Emori [
18] examined the hydrothermal degradation of PVC under high pressure and high temperature, concluding that the dechlorination efficiency could reach 100% at 300 °C.
In accordance with the previous studies, the objectives of this research are: (1) to assess the dechlorination efficiency of subcritical water treatment on PVC electric wire waste; (2) to identify the pollutants present in the water after the treatment; (3) to study the thermal degradation of the dechlorinated residues, as well as the decomposition of the original PVC electric wire by thermogravimetry; and (4) to analyze pollutant emissions during the thermal decomposition in inert atmosphere of the dechlorinated residues and the original PVC electric wire, with a particular focus on the chlorinated pollutants.
2. Materials and Methods
The electric wire was supplied by General Cable, Co (Highland Heights, KY, USA). The cover material is PVC, the insulation material is reticulated polyethylene, and the conductor is copper [
19].
As only the non-metallic fraction (cover and insulation material) was used in this study, this fraction was manually separated from the metallic fraction and it was crushed into small particles (<1 mm) using a cutting mill RETSCH SM200 (Haan, Germany).
Figure 1 shows the original electric wire before and after the crushing process.
The sample was characterized. Elemental analysis was performed in elemental microanalyzer Thermo Finnigan Flash 1112 Series (Thermo Fisher Scientific, Waltham, MA, USA). This technique provided total content (wt %) of carbon, hydrogen, nitrogen, and sulfur present in the sample. Oxygen plus ash content was calculated by difference. The ash content was measured following the UNE-EN-14775:2009 [
20] at 550 °C. The analysis of the rest of elements was performed in a sequential X-ray fluorescence spectrometer Philips Magix Pro PW2400 (Malvern Panalytical Ltd., Malvern, UK). The main objective of this technique is the elemental analysis, both qualitative and quantitative, of elements with an atomic number greater than 9.
Table 1 shows the characterization of the cover and the insulation. The concentrations of the PVC wire are calculated as a weighted sum of the cover and insulation.
Dechlorination experiments with subcritical water were performed in a high-pressure batch reactor with stirring (volume of 1 L).
Figure 2 shows the high-pressure reactor used in this study.
The parameters controlling the efficiency of this process are temperature, treatment time and solid/liquid (S/L) ratio [
9]. Dechlorination runs were carried out at 200 °C, 250 °C, and 300 °C during 180 min, with a solid:liquid ratio (S/L) equal to 1:5 g/mL. In this way, treatment time and S/L ratio were not limiting factors in the dechlorination efficiency, based on the previous work done in a similar reactor working on the debromination of printed circuit boards waste [
9].
During the dechlorination experiments, samples from the liquid phase were collected every hour and the remaining liquid from the reaction chamber was also collected after 24 h in order to evaluate the evolution of the dechlorination process with time. These liquid samples were analyzed for chlorine by ion chromatography, and so it was possible to tally the chlorine balance. In addition, these liquid samples were qualitative analyzed for organic compounds by GC-MS (Agilent GC 6890N/Agilent MS 5976N, Agilent Technologies, Santa Clara, CA, USA) following the US EPA method 8270D [
21] as reference. Specific conditions of this analysis have been described in detail in a previous study by Soler et al. [
22].
The solid residues obtained (they will be referred to as R200, R250, and R300, indicating the temperature of the treatment) were dried at 105 °C until they reached a constant weight. Elemental analysis and chlorine content of the dechlorinated residues were also determined.
TGA was performed on a Perkin Elmer (Waltham, MA, USA) thermogravimetric analyzer (model STA6000). PVC electric wires (before and after the dechlorination process) were subjected to three different heating rates (5 °C, 10 °C, and 20 °C/min) from room temperature up to 900 °C in inert and oxidizing atmospheres. The flow rate of carrier gas was set at 100 mL/min. For each run, approximately 8 mg of sample were used.
Prior to each TGA run, a blank run was carried out using the same experimental conditions but with the crucible empty to ensure the system errors were corrected. So, these weight values recorded for each experimental time were subtracted from the values obtained in the TGA runs with sample.
Duplicated runs were done in the thermobalance to prove the outstanding reproducibility of the thermobalance. Really good reproducibility was obtained for the decomposition of these samples.
Pyrolysis of original PVC and residues R200, R250, and R300 were performed in a moving tubular quartz reactor located inside a horizontal laboratory furnace described by Aracil et al. and Moltó et al. [
6,
23].
Once the set-point temperature was reached (850 °C in all runs), the sample was introduced in the horizontal furnace at constant speed (0.5 mm·s−1) using a quartz boat and maintained inside the reactor for 10 min, while the emitted pollutants were sampled for later analysis. A nitrogen flow of 300 mL/min (measured at 1 atm and 20 °C) was introduced in parallel to the sample movement during the experiment. The sample weight was 25 mg approximately in each experiment.
Before the experiments, a control run with no sample was realized under the same conditions (blank). The blank values were subtracted from the values obtained in the pyrolysis runs with sample.
The U.S. EPA method 26 [
24] was applied to analyze the emissions of inorganic chlorine. The evolved gases were passed through two impingers that contained a dilute sulfuric acid solution (0.2 M) and two other impingers that contained a dilute sodium hydroxide solution (0.1 M). The HCl solubilizes in the acidic solution and forms chloride ions, whereas the Cl
2 passes through to the alkaline solution where it is hydrolyzed to form a proton (H
+), the chloride ion (Cl
−), and hypochlorous acid (HClO). To facilitate the formation of a second chloride ion, sodium thiosulfate was added in excess to the alkaline solution. In this way, two chloride ions are formed for each molecule of Cl
2. This experimental setup allows for analyzing HCl and Cl
2 by ion chromatography (IC). Ion chromatograph model 850 Professional IC AnCat (Metrohm, Herisau, Switzerland) with chemical suppression and conductivity detector was employed, using a Metrosep A Supp 7–250 column (250 mm × 4 mm, Metrohm), a 0.8 mL/min flow rate of a 0.0036 M sodium carbonate solution and a 20 mL injection volume. Once that the standard calibration curves for IC analysis of the acid and alkaline solution was realized, a sample of water was injected in the equipment to determine if any Cl
− appeared in the chromatogram. When they were no longer present, the samples were injected in the equipment.
A polyaromatic Amberlite® XAD-2 resin (Supelco, Bellefonte, DE, USA) was used to collect the semivolatile compounds generated during the experiment. This resin was placed at the exit of the furnace. Three internal standards were added to the resin before the extraction to determine the concentration of PAHs, ClBzs and ClPhs. Dr. Ehrenstorfer-Schäfers (Augsburg, Germany) supplied the deuterated standards used for the analysis of the PAHs and Wellington Laboratories (Ontario, Canada) supplied the 13C-labeled standards for ClPhs and ClBzs.
The next step was to extract the semivolatile compounds adsorbed to the resin with a mixture of dichloromethane/acetone (1:1
v/v) using ASE 100
® Accelerated Solvent Extractor (Dionex‒Thermo Fisher Scientific, Waltham, MA, USA) following the U.S. EPA method 3545A [
25]. The resultant extract was concentrated up to 1.5 mL using a rotary evaporator and straightaway with a moderate stream of nitrogen. Lastly, 3 L of 2000 g/mL anthracene-d10 (AccuStandard, New Haven, CT, USA) were added as a recovery standard.
PAHs, ClBzs, and ClPhs were analyzed by GC- MS (Agilent GC 6890N/Agilent MS 5976N, Agilent Technologies, Santa Clara, CA, USA) following the U.S. EPA method 8270D [
21] as reference. Specific conditions of this analysis, the identification and quantification of the compounds have been done in the same way as in the previous work [
22]. A capillary column HP-5 MS (30 m × 0.25 mm i.d.; 0.25 μm film thickness, Agilent, Santa Clara, CA, USA) was used for the analysis of PAHs and ClBzs, whereas a ZB-5MSi (30 m × 0.25 mm i.d.; 0.25 μm film thickness, Phenomenex, Torrance, CA, USA) was employed for the analysis of ClPhs. The injection volume was 1 μL in split 1:25 for the analysis of PAHs and in splitless mode for the analysis of ClBzs and ClPhs. The oven temperature program for the analysis of PAHs, ClBzs and ClPhs consisted of a first isothermal step at 40 °C (5-min hold) then heating up to 290 °C at 12 °C/min (6-min hold) and finally heating up to 320 °C at 20 °C/min (10-min hold). The identification of the 16 priority PAHs [
26] from the analysis in SCAN mode, were performed with a standard of each compound by calibration comparing the spectra and the retention time of the primary ion. The quantification of PAHs was done using the peak area of the corresponding primary ion, according to the U.S. EPA method 8270D [
21]. ClBzs and ClPhs were analyzed in the SIR mode and the identification of each isomer was performed comparing the retention times and the primary/secondary ion area ratio with that obtained in the calibration with the labeled compounds. The quantification of these compounds was based on the sum of areas of the two most abundant ions.
4. Conclusions
Subcritical water treatment for dechlorination of PVC wire was more effective as the treatment temperature increased. Therefore, dechlorination efficiency increased as the process temperature increased, obtaining a maximum efficiency of 95.96 wt % at 300 °C. The concentration of chlorine in solid residue decreased because the chlorine present in PVC wire was nearly completely transferred to the water. In addition, these solid residues obtained high net calorific values, with a maximum value of 37,766 kJ/kg for the solid residue obtained after the treatment at 300 °C, which may consequently be employed as high-quality fuel.
TG curve of the original PVC wire presented three main stages, but as the material was subjected to a more severe dechlorination the TG curve presented fewer stages; the decomposition of R200 was very similar to that of the original wire, while the decomposition of R250 and R300 only presented the first two stages and one stage, respectively. Therefore, dechlorination process produced a more degraded material as the treatment temperature increased.
Regarding pollutant emissions during pyrolysis, emissions of inorganic chlorine significantly decreased when increasing the increase of the dechlorination temperature. R200, R250, and R300 emitted 4.0%, 81.9%, and 92.4% less inorganic chlorine, respectively, than the original PVC wire. PAH formation increased as the temperature of dechlorination process increased. This increase in the emission of PAHs can justify that the formation of chlorine structures was less effective during the pyrolysis. Thus, the emissions of organic chlorinated compounds (ClBzs and ClPhs) decreased with the increase of dechlorination temperature to which the samples had been subjected and total yields originating from the thermal decomposition of dechlorinated residues were lower than the total yield from the original PVC wire. All the materials emitted many more ClBzs than ClPhs. Furthermore, the main chlorinated compounds were monochlorobenzene and monochlorinated phenols (3-+4-).
Therefore, it can be concluded that the subcritical water treatment was an efficient method for the dechlorination of PVC wires and could reduce the formation of chlorinated compounds in subsequent thermal processing of this waste.