DOI: 10.3303/CET2398036 Paper Received: 12 December 2022; Revised: 1 March 2023; Accepted: 8 April 2023 Please cite this article as: Concas A., Delogu F., Lai N., Cao G., 2023, A Preliminary Investigation on the Mechanochemical Degradation of 2,6 Dichlorophenol in Simulated Sandy Soils: Modeling End Experiments, Chemical Engineering Transactions, 98, 213-218 DOI:10.3303/CET2398036 CHEMICAL ENGINEERING TRANSACTIONS VOL. 98, 2023 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Sauro Pierucci, Carlo Pirola Copyright © 2023, AIDIC Servizi S.r.l. ISBN 978-88-95608-97-6; ISSN 2283-9216 A Preliminary Investigation on the Mechanochemical Degradation of 2,6 Dichlorophenol in Simulated Sandy Soils: Modeling End Experiments Alessandro Concasab*,Francesco Delogua, Nicola Laia, Giacomo Cao a,b,c aDepartment of Mechanical, Chemical and Materials Engineering, University of Cagliari, Via Marengo 2, 09123 Cagliari, Italy bInterdepartmental Center of Environmental Science and Engineering (CINSA), University of Cagliari, Via San Giorgio 12, 09124 Cagliari, Italy cCenter for Advanced Studies, Research and Development in Sardinia (CRS4), Loc. Piscina Manna, 09050 Pula (CA), Italy The possibility to degrade 2,6 - Dichlorophenol (2,6 DCP) in matrices simulating sandy soil via ball milling was investigated in this work. The experiments showed that under specific operating conditions a successful degradation of 2,6 DCP could be achieved even when its concentration in the simulated sandy soil was very high. The obtained experimental results were well interpreted by a mathematical model to provide a useful tool to develop a software to design, control and optimize the process at the industrial scale. 1. Introduction Soil contamination is nowadays one of the main concerns to cope with due to the reduced availability of arable lands and the increased demand for food by a growing population (Montinaro et al., 2009). 2,6 - Dichlorophenol (2,6 DCP) is widely employed in the industry to produce pesticides, herbicides, preservatives, antiseptics, disinfectants, and other useful compounds. As a result, it can be frequently found as a dangerous pollutant in the soil (Dai et al., 2021). Relevant features of DCPs are their persistence in the soils and a partial recalcitrance to biological treatments. Therefore, albeit partially biodegradable, their uncontrolled discharge in the environment has resulted in diffuse contamination owing to their notable tendency to accumulate in the environment. Accordingly, various technologies to effectively degrade 2,6 DCP in real environmental matrices are currently the subject of intense research activity. Adsorption, electrocoagulation and chemical coagulation have been demonstrated to suitably reduce 2,6-DCP concentration in liquid matrices (Yadav and Khandegar, 2020). Other technologies capable to degrade this pollutant are photodegradation using TiO2/pillared clays (Kumar et al., 2022) and oxidation through ferrate (VI) CuO (Yan et al., 2021) or activated peroxymonosulfate (Dai et al., 2021). Despite its effectiveness in the treatment of soils contaminated by different pollutants (Concas et al., 2012), mechanochemical methods to degrade 2,6-DCP in soils has not been evaluated so far. This work focuses on the use of mechanochemical degradation of 2,6 DCP using SiO2, that mimics a sandy soil and act as a reducing agent capable to degrade the pollutant. Depending on the specific molar ratios adopted as well as on the operating conditions during milling, the mechanochemical processing induced a gradual degradation of 2,6 DCP. In particular, under specific experimental conditions, removal efficiencies close to 100% could be achieved even when considering high contamination levels. A mathematical model is then proposed and adopted to simulate the experimental data related to the dependence of the degradation efficiency on the energy dose, i.e. the cumulated mechanical energy provided during milling to the unit mixture mass, for the case where gradual degradation took place. The simulation results well capture the experimental data, thus demonstrating the reliability of the proposed model. The latter one can be used for a first stage evaluations of techno-economical feasibility of the proposed technique at the large scale. Finally, a possible proof of concept of a process operating at the industrial scale, involving the use of photovoltaic panels to fuel the mills, is proposed. 213 2. Materials and Methods 2.1 Contaminated sandy soils preparation High purity (99%) SiO2 and 2,6-DCP were purchased from Sigma Aldrich and then mixed in weight ratios equal to 5:1 and 10:1 to simulate sandy soils contaminated by 2.0 × 105 and 1.0 × 105 (ppm) of 2,6 -DCP, respectively. 2.2 Mechanochemical treatment The powder mixtures were inserted, along with a variable number of stainless steel balls of 8 g, 10 mm in diameter, within a stainless steel vial, 606.75 g in weight, having the internal diameter equal to 3.8 cm and the internal height of 5.8 cm. The vial was mounted on a SPEX Mixer/Mill mod. 8000, operating at the standard milling frequency of about 870 rpm. The mechanical treatment was prolonged for times ranging from 6 to 48 hours during which the motion of mill permitted the spheres to hit the soil particles with locally high energies capable to trigger the endothermic reactions leading to 2,6-DCP degradation. At the end of these periods the powder mixture (simulating the contaminated soil) was withdrawn from the vial and then analyzed to determine the degree of degradation achieved because of the treatment. A scheme of the experimental procedure along with a summarizing table of the investigated operating conditions is reported in Figure 1 Figure 1. Scheme of the experimental procedure (a) and summary of investigated experimental conditions (b) At least two replicates of each experimental condition were considered to ensure results reproducibility. 2.3 Treated soil analyses High-Pressure Liquid Chromatography (HPLC) was adopted to evaluate the conversion of 2,6-DCP into the products resulting from the ball milling treatment. An Agilent 1100 Series Chromatograph, equipped with a quaternary pump, a solvent degassing system operating under vacuum conditions, a diode-array detector with a 500-nl flow cell, an auto-sampler and an integrating software for data acquisition was used. A Chromopack Spherisorb ODS-2 column (250 × 4.6 mm; particle size 5 μm) was used. The working temperature of the column was maintained constant at 25 °C through a thermostatic apparatus. A two-component mobile phase (40% methanol, 60% water) was used at a constant flow rate of 1.2 ml min−1. Chromatographic analyses were performed on the liquid solutions obtained by leaching the milled powders with distilled water to transfer the residual 2,6-DCP in the soil towards the liquid phase. The leaching time was such to ensure the complete solubilization of the remaining organic pollutant. Preliminary experiments proved that the solubilization of polar substances was completed in around 1 h. The use of pure water to leach the treated soils might seem arguable since it only allows the dissolution of polar organic species (and in particular of 2,6-DCP) while non-polar species are not dissolved. However, since the present work only aims to verify the possibility of degrading the 2,6-DCP via mechanochemical techniques the use of pure is enough because it can dissolve the residual quantity of 2,6-DCP thus permitting to compare it with the amount of 2,6-DCP in the starting mixture. Ultimately, the 2,6-DCP efficiency could be evaluated. 214 3. Mathematical model The ball milling treatment (BM) can trigger the degradation of 2,6-DCP in the soil into less harmful products according to the schematic reaction: 𝐷𝐶𝑃 𝐵𝑀 → 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠 (1) Let’s now consider to be an effective sphere-soil collision the one capable of triggering the 2,6 DCP degradation. As reported elsewhere, the unreacted fraction 𝜉𝐷𝐶𝑃(𝑚𝑔 𝑘𝑔−1) of 2,6-DCP with respect to the total mass of soil, after a processing time 𝜏 could be evaluated according to an equation of the type (Delogu et al., 2004; Delogu and Takacs, 2018, Carta et al., 2020): 𝜉𝑗 𝐷𝐶𝑃(𝜏) = 𝜉𝐷𝐶𝑃(0) (𝑘 𝜏)𝐽 𝑗! 𝑒− 𝑘 𝜏 (2) where the parameter 𝑘 (ℎ−1) is related to the milling process dynamics via the collision frequency 𝑓(ℎ𝑟−1), the collision energy and the charge ratio 𝐶𝑟 (i.e., the ratio between the mass of sphere and that one of soil). The symbol 𝑗 indicates the number of effective collisions involving the mixture after milling the sample for a time 𝜏. By considering that the mass of residual DCP plus the mass of its degradation products is conserved during the treatment one could calculate the mass fraction of degradation products Pr (𝜓𝑃𝑟) as: 𝜓𝑃𝑟(𝜏) = 𝜉𝐷𝐶𝑃(0) −∑ 𝜉𝑗 𝐷𝐶𝑃(𝜏) 𝐽𝑚𝑖𝑛 𝑗=0 = 𝜉𝐷𝐶𝑃(0) [1 −∑ (𝑘 𝜏)𝑗 𝑗! 𝑒− 𝑘 𝜏 𝐽𝑚𝑖𝑛 𝑗=0 ] (3) In Eq. (3) the symbol 𝐽𝑚𝑖𝑛 represents the minimum number of effective collisions that should be experienced by the soil for the transformation of Eq. (1) to take place. Since the unreacted fraction of 2,6-DCP in the solid phase (𝜉𝐷𝐶𝑃) cannot be directly measured, a strategy similar to the one reported in the literature by Concas et al. (2020) was developed to evaluate 2,6 – DCP degradation degree starting from its concentration in the liquid phase after soil leaching with water. Such a strategy allows a comparison with experimental data by returning the time profile of the leached concentration 𝐶𝑓 𝐷𝐶𝑃(𝜏): 𝐶𝑓 𝐷𝐶𝑃(𝜏) = 𝐶𝑓 𝐷𝐶𝑃(0) [∑ (𝑘 𝜏)𝑗 𝑗! 𝐽𝑚𝑖𝑛 𝑗=0 ] 𝑒− 𝑘 𝜏 (4) This equation quantifies the concentration of the 2,6-DCP transferred from the solid to the liquid phase when the former is first subjected to a mechanical treatment for a time 𝜏 and then undergone to leaching. From this equation one could simply evaluate the time evolution of the degradation degree of 2,6 DCP as 𝜂(𝜏) = 1 − [𝐶𝑓 𝐷𝐶𝑃(𝜏) 𝐶𝑓 𝐷𝐶𝑃(0)⁄ ]. On the other hand, rather than the processing time, the key variable influencing the yield of a ball milling process is the energy supplied to the unit mass of soil. Indeed, degradation reactions can happen depending on the rate at which mechanical energy is provided to the mixture. Therefore, adjustment of the operating parameters affecting milling energy is crucial to achieve high conversions of 2,6 - DCP within a specific operating time 𝜏. To this aim, it is important to express the degradation efficiency as a function of the energy provided to the sample. In this view the so-called milling intensity 𝐼 (𝐽 ℎ𝑟−1) can be introduced: 𝐼 = 1 2 𝑓∑𝑚𝑏,𝑖𝑣𝑏,𝑖 2 𝑛𝑏 𝑖=1 (5) where 𝑛𝑏(/) is the number of milling spheres, 𝑓(ℎ𝑟−1) is their impact frequency, 𝑚𝑏,𝑖(𝑔) their mass and 𝑣𝑏,𝑖(𝑚 𝑠 −1) their relative impact velocity. The specific energy dose 𝐷 (𝐽 𝑚𝑔−1) is the mechanical energy provided to the unit mixture mass at specific time 𝜏 and according to the literature can be computed as: 𝐷 = 𝐼𝜏 𝑚𝑝 = 1 2 𝑓 𝜏 𝑚𝑝 ∑𝑚𝑏,𝑖𝑣𝑏,𝑖 2 𝑛𝑏 𝑖=1 (6) where 𝑚𝑝(𝑔) is the mass of mixture being processed. Since during the the experiments all the spheres had the same mass 𝑚𝑏 and dynamic simulations (Concas et al., 2020) demonstrated that at mill steady-state their impact velocity is constant (𝑣𝑏,𝑖 = 𝑣𝑏), the dose can be evaluated as: 𝐷 = 1 2 𝑛𝑏𝑚𝑏 𝑚𝑝 𝑓𝑣𝑏 2 𝜏 = 1 2 𝐶𝑟𝑓𝑣𝑏 2 𝜏 (7) 215 where the term 𝐶𝑟(/) = 𝑚𝑏/𝑚𝑝 is just the charge ratio, i.e. the ratio between the mass of milling bodies and the mass of soil being processed. Therefore, by dividing k so that the kinetics can be referred to the cumulative energy dose rather than to the process time we get: 𝑘 = 𝑘′ 1 2 𝐶𝑟𝑓𝑣𝑏 2 (8) and substituting in Eq. (4), the following relationship can be obtained: 𝐶𝑓 𝐷𝐶𝑃(𝐷) = 𝐶𝑓 𝐷𝐶𝑃(0) [∑ (𝑘′ 𝐷)𝑗 𝑗! 𝐽𝑚𝑖𝑛 𝑗=0 ] 𝑒− 𝑘 ′ 𝐷 (9) From the latter one the 2,6 - DCP degradation efficiency can be evaluated according to the following Equation. 𝜂(𝐷) = 1 − 𝐶𝑓 𝐷𝐶𝑃(𝐷) 𝐶𝑓 𝐷𝐶𝑃(0) = 1 −∑ (𝑘′ 𝐷)𝑗 𝑗! 𝑒−𝑘 ′ 𝐷 𝐽𝑚𝑖𝑛 𝑗=0 (10) This form was used to fit experimental results by tuning the parameter 𝑘′ and setting 𝐽𝑚𝑖𝑛 = 1. 4. Results and discussions By using the operating conditions specified in figure 1 for each sample ID the experimental results reported in Table 1 were obtained. Table 1. Experimental results obtained with the samples IDs obtained with the operating conditions in Fig. 1. Sample 𝑪𝒇 𝑫𝑪𝑷(𝝉 = 𝟎) (𝒎𝒈 𝑳−𝟏) 𝑪𝒇 𝑫𝑪𝑷(𝝉 = 𝝉𝒇𝒊𝒏𝒂𝒍) (𝒎𝒈 𝑳−𝟏) 𝜼𝒆𝒙𝒑 (/) C5 185.02 126.34 0.32 C1 223.35 146.1 0.35 C4 192.29 79.64 0.59 C2 302.38 173.5 0.43 C3 221.44 28.12 0.87 C6 180.93 34.37 0.81 C9 216.21 9.89 0.95 C8 215.6 0 1.00 C7 217.58 0 1.00 To analyse the evolution of the degradation efficiencies, those obtained with a fixed charge ratio (𝐶𝑟) are reported as function of the milling time in Figure 2a and those, obtained after 24 hr long milling are shown as function of the charge ratio in Figure 2b. 4 8 12 16 20 24 20 40 60 80 100 C5 C4 C3 C6 D e g ra d a ti o n E ff ic ie n c y , h ( % ) Processing time, t(hr) (a) Cr = 4 (g/g) 1 2 3 4 5 6 7 20 40 60 80 100 C1 C2 C3 C8 D e g ra d a ti o n E ff ic ie n c y , h ( % ) Charge Ratio, Cr(g/g) (b) t = 24 (hr) Figure 2. Evolution of degradation efficiency as function of processing time (a) and Charge ratio (b) It is apparent that, at fixed 𝐶𝑟, the prolongation of the milling time leads to an increase in the degradation efficiency of 2,6-DCP. This is because the energy needed to trigger the degradation reactions is accumulated by the solid mixture in the form of defects, dislocations as the mechanical treatment is prolonged (Caschili et al., 2006). While the possible reactions leading to this effect are not well known, based on the current literature a general reacting mechanism can be assumed. Indeed, it is well known that free radicals such as H, 216 OH·, and O· can be produced as a result of high energy milling (Concas et al., 2020). Such free radicals could attack the 2,6-DCP molecule leading to its de-chlorination and possibly the rupture of the aromatic ring producing as a result a harmless, or less dangerous, final compounds (Benitez et al., 2000; Zhu et al., 2022). Although this mechanism should be confirmed by further and more insight experimental activity, a well body of literature corroborate the possibility to perform de-chlorination of organic compounds via ball milling (Cagnetta et al., 2016). An increase of the degradation efficiency can be also observed (Fig. 2b) when the charge ratio is increased by keeping all the remaining operating parameters (including milling time) constant. Indeed, a higher number of spheres typically leads to a higher number of hits per unit time, i.e. a larger collision frequency, that in turn determines to an higher amount of energy transferred to the unit mass of powder per unit time. According to Eq. 7, milling time and charge ratio both contribute to increase the energy dose 𝐷 (𝐽 𝑚𝑔−1) provided to the unit weight of powders that, for the concerned experiments can be calculated as reported in Table 2 by using the values of collision frequency and impact velocity by Concas et al., 2020). Table 2. Energy doses for each experimental condition. Values of 𝑓 and 𝑣𝑖𝑚𝑝 are from Concas et al. (2020). Sample 𝝉 (𝒉𝒓) 𝑪𝒓 (𝒌𝒈 𝒎𝒈−𝟏) 𝒗𝒊𝒎𝒑 (𝒎 𝒔−𝟏) 𝒇 (𝒉𝒓−𝟏) 𝑫 (𝑱 𝒎𝒈−𝟏) 𝜼𝒆𝒙𝒑 (/) C5 6 4.00 × 10-6 4.169 511200 106.62 0.32 C1 24 1.45 × 10-6 4.169 511200 154.60 0.35 C2 24 2.60 × 10-6 4.169 511200 277.21 0.43 C4 12 4.00 × 10-6 4.169 511200 213.24 0.59 C6 18 4.00 × 10-6 4.169 511200 319.86 0.81 C3 24 4.00 × 10-6 4.169 511200 426.48 0.87 C9 15 8.25 × 10-6 4.169 511200 549.76 0.95 C8 24 6.00 × 10-6 4.169 511200 639.72 1.00 C7 48 6.00 × 10-6 4.169 511200 1279.43 1.00 Ultimately, as also reported in Fig. 3a, a degradation efficiencies of 2,6-DCP close to 100% can be achieved (cf. experiments C7 and C8) by suitably increasing milling time and/or the charge ratio, i.e. the energy dose,. It should be stressed here, that such results are obtained by considering high contamination levels of 2,6 -DCP (1 – 2 x 105 ppm) that are higher than the typical one found in real contaminated soils. 0 200 400 600 800 1000 1200 1400 20 40 60 80 100 Experimental Model fitting D e g ra d a ti o n e ff ic ie n c y , h ( % ) Energy Dose, D (J mg-1) Figure 3. Comparison of experimental and model results (a) and conceptual scheme for the sustainable application of the technique at the real scale (b). Therefore, it can be reasonably assumed that, by considering lower concentrations of 2,6-DCP that are typical of contaminated soils, a complete degradation could be obtained by using much lower doses of energy provided to the soil. This would increase the techno-economic feasibility of the proposed technique. Another possible way to increase the feasibility of the process would be for example to use renewable energy from photovoltaic to power the mills. In fact, remediation typically involves large surfaces of soils that can be exploited to gather the energy needed for the milling process according to the scheme reported in Fig. 3b. On the other hand such assumptions should be corroborated by further experiments. Still from Fig. 3a, it can be (a) (b) 217 observed that the experimental results are well fitted by the proposed model when setting the value of Jmin to 1 according to the literature (Concas et al., 2020) and tuning the value of the kinetic constant 𝑘 to 8.4 × 10−3 (𝑚𝑔 𝑔−1). In this way MSE (mean square error) and R2 equal to 9.82 × 10−3 and 0.88, respectively, were obtained. Further experiments should be performed, under different operating conditions, and then simulated by keeping the parameter k fixed to evaluate the predictive capability of the model. Experiments involving larger amounts of soils and larger devices should also be performed to evaluate the scalability of this technology. If such activity were successful, the model would represent a good starting point to develop a software tool to design, optimize and control a plant operating on the real scale for the treatment of soils contaminated by high concentration of 2,6 – DCP. 5. Conclusions High-energy milling has been proved to be a valuable technique to degrade 2,6-DCP in solid matrices simulating highly contaminated sandy soils. In fact, degradation degrees close to 100 % were achieved when using specific energy doses obtained by changing milling time and/or charge ratio. This effect is probably due to the reaction between 2,6 – DCP and the free radicals produced by the mechanical processing of SiO2. However, further research is needed to confirm this mechanism as well as to better identify the 2,6-DCP degradation products and their harmfulness. 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