Sustainable Thermal Regeneration of Spent Activated Carbons by

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Sustainable thermal regeneration of spent activated carbons by solar energy: Application to water treatment Marianne Miguet, Vincent Goetz, Gael Plantard, and Yves Jaeger Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01260 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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Industrial & Engineering Chemistry Research

Sustainable thermal regeneration of spent activated carbons by solar energy: Application to water treatment Marianne Miguet1,2,*, Vincent Goetz1, Gaël Plantard1,3, Yves Jaeger2

1

CNRS Promes, Rambla de la Thermodynamique Tecnosud, 66100 Perpignan, France

2

Veolia eau, 765 rue Henri Becquerel, 34967 Montpellier, France

3

Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 66860 Perpignan, France

* [email protected]

Abstract Our aim was to assess the feasibility of using solar energy for the thermal regeneration of spent activated carbons (ACs). The context was water treatment, for which ACs are commonly used. The target pollutant was perchloroethylene (PCE). Preliminary experiments were performed to select the best-suited adsorbent among five ACs (Aquacarb, surface area BET: 1100 m².g-1 and micropores areas: 870 m².g-1). Water properties influence adsorption capacity; this is lower with natural water (groundwater) than with ultrapure water. The competition between natural organic matter (found in groundwater but not in ultrapure water) and the target pollutant accounts for this difference. Groundwater was used in regeneration tests so as to approximate to field conditions. Operating conditions for regeneration were determined: the working temperature ranged between 130°C and 400°C, and the treatment time was set so as to prevent any kinetic limitation. Four regeneration cycles showed the same decrease in adsorption capacity over the cycles in the working temperature range. The adsorption capacity recovered was about 60% after the fourth cycle. Solar thermal regeneration was then performed. The solar and classical regenerations gave similar results. This first solar thermal regeneration test is promising for its environmental and economical advantages.

Keywords: activated carbon; perchloroethylene; regeneration; sustainability; solar energy

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1. Introduction Groundwater is one of the world's primary freshwater resources.1, 2 Its contamination is a major environmental and health concern. Activated carbons (ACs) are widely used to remove pollutants from water by adsorption processes.3–5 They are efficient, cheap and non-selective. However, they eventually become saturated. Reuse of ACs after thermal regeneration is more and more often chosen as a cost-saving option in environmental protection.6, 7 The energy costs of treating water pollution by separation on ACs arise mainly from the production and regeneration of the adsorbents. For an AC lifetime of 3–5 years, the cost of regeneration amounts to 35–45% of total operating costs in the treatment of drinking water.8 The shorter the lifetime, the higher the percentage: for a lifetime of six months, regeneration represents almost 85% of total costs.9 These costs include transportation, mining, producing the regenerated ACs and heat treatment. The environmental impact of producing and regenerating ACs by thermal treatment has been assessed by life cycle analysis.10, 11 CO2 emissions are 11 kgCO2.kgAC-1 for non-reused adsorbents. Regenerated ACs emit only 1 kgCO2.kgAC-1, a significant decrease of 90% on regeneration. However, regeneration still needs about half of the energy resources used for first producing the adsorbent, owing to the high regeneration temperatures used (700– 1000°C). To reduce energy demand, the regeneration could be performed using renewable energy. This could be solar energy, which would provide sustainable, cost-saving regeneration. Previous studies described solar-powered regeneration using UV radiation applied to ACs doped with a photocatalyst (TiO2).12, 13 They were not addressing thermal regeneration, but rather the possible synergy between adsorption and desorption/degradation of pollutants by photocatalysis.13–16 The principle was a twostep process of adsorption and photocatalysis. Pollutants such as trichloroethene, dichloroethene, toluene and bisphenol-A were tested.12,

15

After the first stage of adsorption, mineralization begins

when the catalyst is photo-excited by UV radiation to degrade pollutants. The regeneration rate thus obtained was acceptable (30–60% after eight cycles13). However, ACs doped with a photocatalyst are not certified for treating water, and they cannot be used in industry. Solar regeneration of certified

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ACs would therefore be of interest because it could be used directly in the field. To our knowledge there is no literature on the thermal regeneration of ACs by solar energy systems. Our study is thus the first one to address this question. The choice of the target pollutant in this study was based on two considerations: (i) high volatility of the pollutant should facilitate desorption and so the regeneration of the adsorbents: a volatile organic compound (VOC) would provide a sufficiently high volatility to work in the most favorable conditions for regeneration, and (ii) our aim was to propose a process as close as possible to what could be implemented in industry. A pollutant commonly found in groundwater was thus the best choice. We therefore chose perchloroethylene (PCE). PCE, a VOC, is toxic and persists in the environment.17–19 It is commonly found in groundwater, especially at existing and former industrial sites and disposal areas. Pollution by PCE is a challenge in many countries, e.g. USA, South Korea, Italy and France.17, 20–22 This work follows on from a previous study that highlighted the efficiency of ACs in a fixed-bed column for removing pollutants such as PCE.23 Here we address the sustainable management of the saturated ACs, specifically their reuse using renewable energy for regeneration. Three aims were set: (i) determine the AC storage adsorption capacity, to assess the efficiency of the adsorbents, (ii) estimate the quality of classical thermal regeneration in terms of regeneration rate, and (iii) determine the feasibility of solar thermal regeneration. Preliminary experiments were carried out to measure isotherms and select the most suitable ACs among a range of adsorbents. Regeneration rate was measured by considering the adsorption isotherms. Operating conditions (working regeneration temperature range and regeneration time) had to be determined. The effect of the number of regeneration cycles and the influence of the regeneration temperature were investigated. The data obtained in the laboratory with classical thermal regeneration were compared with the results of solar regeneration.

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2. Materials 2.1. Perchloroethylene (PCE) Batch experiments were performed using PCE-doped stock solutions made with ultrapure water or groundwater. The groundwater was collected by drilling. This groundwater was already contaminated with PCE. The PCE stock solutions were prepared by adding a set volume (50 µL) of PCE (SigmaAldrich, 99.9%) to a 500 mL headspace-free glass vessel filled with water and sealed with a Teflon cap. PCE concentrations in these ultrapure water or groundwater stock solutions were between 70 and 100 mg.L-1. The concentration of TOC (total organic carbon) in the ultrapure water was less than 0.5 mg.L−1; the TOC concentration in the groundwater used in this study ranged from 1 to 2 mg.L−1. PCE concentrations in the solutions were measured by ultra-high-performance liquid chromatography on a system (UHPLC UltiMate 3000) equipped with a UV-Vis detector and an Accucore C18 column (100 × 2.1 mm, particle size 2.6 µm). Analytes were separated with an acetonitrile:water mixture (72:28, v/v) at a flow rate of 0.4 mL.min-1 in isocratic elution mode with detection at λ = 199 nm. Calibration curves were plotted by repeating the same injection of the standard (purchased from Dr. Ehrenstorfer) three times for each point. Limit of detection was taken as a signal-to-noise ratio of 3. Limit of quantification was 200 µg.L-1.

2.2. Activated carbons (ACs) The ACs selected for the study were Filtrasorb 400, Filtrasorb 200, Aquacarb 207C, AquaSorb 2000 and AquaSorb CX. They are certified for the treatment of drinking water and commonly used in water treatment processes. Their main properties are given in Table 1. The raw material was coconut shell or coal. The porous structures of all ACs were similar. The surface BET of the five ACs were centered around 1000 m².g-1 and the microporous area were centered around 850 m².g-1. ACs are mostly used in the size between 1-2 mm in separation process (fixed bed columns). The use of small particles in this work was chosen because their kinetics is faster. As it was discussed in another work23, the average equilibrium time is 3.7 faster with 112−200 μm particle size than with 1−2 mm particle size. An AC particle range of 1–2 mm was selected by mechanical sieving. These 1–2 mm

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particles were milled and sieved to obtained smaller particle sizes (112–200 µm), which were then degassed by heating to 230°C under low pressure (50 µmHg) using a Micrometrics ASAP 2000 apparatus.

3. Experimental procedures 3.1. Experimental set-up The experiments used two main set-ups: (i) the adsorption capacity (isotherm) was determined with a dedicated test bench and (ii) the regeneration was performed with an electric furnace or a solar concentrator.

Batch adsorption experiments The PCE/Aquacarb isotherms were obtained using a test bench set-up described earlier.23 It comprised a stainless-steel closed circuit containing the AC and allowing the injection of the PCE stock solutions. PCE is particularly difficult to handle owing to its very high volatility, so the test bench was kept airtight while allowing sampling during the experiments. The principle of this experiment was that the PCE adsorption onto the AC occurred stepwise in the test bench, giving successive conditions of equilibrium between the liquid and adsorbed phases. The whole isotherm, from the lowest to the highest concentrations, was constructed. Adsorption capacity q was determined by the following equation: (1) where q is the adsorption capacity (mg.g-1), C0 and Ce are the initial and equilibrium concentrations (mg.L-1) of PCE, respectively, V is the volume of solution (L) and m is the mass of adsorbent (g).

Regeneration experiments Thermogravimetric analysis (TGA) was used in preliminary experiments to determine the working temperature range needed to achieve effective desorption. The thermobalance was a SETSYS

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Evolution 1750 high precision balance (10-4 mg) supplied by SETARAM Instrumentation. About 50 mg of spent AC was heated in a nitrogen stream at a flow rate of 4°C.min -1 up to the working temperature. The regeneration of spent AC was performed with the set-up depicted in Figure 1. The principle was to apply thermal treatment with a heating system (1) to the spent AC (about 0.5 g) in a nitrogen stream. The spent AC (2) was placed in a cell (1 cm3). The nitrogen stream (3) was regulated by a flow meter with a pressure valve (4) at 400 mL.min-1. Thermocouples located at the inlet (T1) and the outlet (T2) of the heating system measured the gas temperature. A thermocouple in direct contact with the AC (T3) indicated the actual temperature of the adsorbents. Another thermocouple placed outside the cell containing the AC (T4) measured the temperature of the set-up. The configuration was the same for the two heating systems (furnace and solar experiment). The heating system in the laboratory was a tubular electric furnace (6 kW Nabertherm R120/1000/12). The ceramic tube containing the sample to be heated was of internal diameter 0.10 m and length 1.4 m. The useful length of tubing in which the temperature was constant was 0.33 m. The AC sample was placed at the center of the tube. The temperature program was a 2 h ramp, followed by a 20 h plateau at the regeneration temperature, after which the heating was stopped. The heating system for the solar experiment was a parabolic trough concentrator (PTC). This set-up is depicted in Figure 2. Like all solar concentration systems, the PTC (1) must be equipped with a monitoring system to follow the apparent path of the sun. In our case, this consisted of a heliostat (2) on which the PTC was fixed. The tests with the PTC took place at the Laboratoire d’Essais Solaires in Perpignan (France). The heliostat was a metal structure that guided the PTC to direct radiation throughout the day following the apparent movement of the sun across the sky. The heliostat was a model SF-70T-PV supplied by Sol-Focus. It was mobile on two axes: azimuth (East-West) and inclination (altitude). Monitoring was automated using the SunDog control unit. This equipment used the calculation of solar ephemeris from an internal clock and geographical installation details. Solar coordinates were then transcribed into angles of rotation to be applied to the axes of the heliostat. The PTC had length 1 m, opening radius 0.5 m and focal distance 0.4 m. The absorber (3) was placed in the focus of the concentrator, a vacuum-jacketed tube of outer diameter 4.8 cm. The AC sample was

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placed in a compartment in the absorber. Measures for direct and global radiation were made respectively by a pyrheliometer (4, Kipp & Zonen CH1) and a pyranometer (4, Kipp & Zonen CMP11). Like in the electric furnace, the nitrogen stream (5) was regulated by a flow meter with a pressure valve set at 400 mL.min-1.

3.2. Method The method consisted in measuring the quality of the separation of PCE by adsorption on the ACs over the regeneration cycle, i.e. the adsorption capacity. The quality of the separation by adsorption was accordingly assessed by the isotherms before and after the thermal treatment. The evolution of the adsorption capacity over the regeneration cycles was then directly evaluated with the isotherms. Equilibrium adsorption was described using the Langmuir isotherm. The Langmuir isotherm assumes that adsorption occurs on a homogeneous surface containing sites with equal energy that are equally available for adsorption.5, 24, 25 Langmuir’s isotherm model is represented by the following equation: (2) where qe is the adsorption capacity at equilibrium (mg.g-1), qm is the maximum monolayer adsorption capacity (mg.g-1), Ce is the equilibrium concentration (mg.L-1) and b is a constant related to the energy of adsorption (L.mg-1). The entire isotherm from the lowest to the highest concentrations has to be measured to determine qm and b. These parameters are identified for each isotherm by minimizing a least-squares criterion that compares the calculated adsorption capacity with the experimental results (Eq. 3). The adsorption in the lowest concentrations (Henry’s zone) is defined by the slope of qmb. The adsorption in the highest concentrations is equal to qm.

(3)

where qexp is the experimental adsorption capacity (mg.g-1), qcal is the calculated adsorption capacity (mg.g-1) and nexp (-) is the number of experimental data in the isotherm. 7



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One regeneration cycle is defined by a first isotherm performed with virgin AC, and a thermal treatment followed by a second isotherm. As many regeneration cycles as desired can be performed, each thermal treatment being preceded and followed by an isotherm. The ACs were contained in a closed cell and were moved from the isotherm bench to the regeneration set-up in this cell with no handling of adsorbents. Thus adsorbents are not lost during the cycles. The parameters studied were regeneration temperature and number of cycles. Operation efficiency was measured with the regeneration rate (Eq. 4) as the comparison between the adsorption capacities of the regenerated AC versus the original AC. (4) where qoriginal is the adsorption capacity of the original AC (mg.g-1) and qn is the adsorption capacity of the AC regenerated n times (mg.g-1).

4. Results and discussion 4.1. Preliminary experiments, adsorption isotherms The five AC isotherms were obtained with ultrapure water in order to determine the best-suited AC. The results are shown in Figure 3 throughout the measurement range up to 30 mg.L−1 and with a close-up of the isotherms at the beginning of the concentrations up to 2 mg.L−1. The close-up is of interest, as most environmental concentrations of PCE-polluted water tend to be lower than a few mg.L−1.20, 26, 27 All the isotherms belong to a IUPAC type-I pattern characterized by a straight line at the low concentrations and a horizontal plateau until saturation. This is typical for a predominantly microporous network and monolayer adsorption. The simulations of each isotherm were performed with the equation of Langmuir. The criterion (Eq. 3) and the parameters are listed in Table 2. At low concentrations, the Langmuir fittings represent the adsorption with a straight line and a slope of bqm. Aquacarb was the AC with the highest slope of

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the straight line at the low concentrations. Our interest was in the low concentrations. Hence Aquacarb proved the most appropriate AC, and so was studied in the following experiments.

The ultimate goal of adsorption using AC was to treat natural water (not ultrapure water). The difference between several isotherms obtained with ultrapure water and groundwater can be seen in Figure 4. The values of the parameters were qm = 518 mg.g−1 and b = 0.798 L.mg−1 for the groundwater and qm = 510 mg.g−1 and b = 1.22 L.mg−1 for the ultrapure water. The uncertainties were calculated based on the uncertainties on the measurements (scale and HPLC) and on the mass of AC (desorbed mass). Level of uncertainty was 2% in concentrations and 12% in adsorption capacities. Groundwater composition was less stable than that of ultrapure water. This fluctuation in groundwater composition was responsible for the variations in the measurements, being the only variable parameter. The adsorption capacities were slightly lower with groundwater than with ultrapure water, especially at the low concentrations. For instance, the adsorption capacity was 280 mg.g−1 with the ultrapure water and 230 m.g−1 with the groundwater for an equilibrium concentration of 1 mg.L−1. This difference is likely due to the presence of natural organic matter (NOM) found in groundwater but not in ultrapure water. The NOM concentration was measured by the TOC concentrations. The difference between the water properties (less than 0.5 mg.L−1 in ultrapure water and from 1 to 2 mg·L−1 in groundwater) was responsible for the distinctive isotherms. Similar results are reported in other studies.28–30 The competition between NOM and the target pollutant accounts for this difference. Carter et al.31 studied the effects of organic matter on the adsorption of trichloroethylene (a substance very similar to PCE) by AC. They concluded that adsorption capacity decreased in the presence of NOM from 20 mg.g−1 (ultrapure water) to 15 mg.g−1 (river water) for an equilibrium concentration of 0.1 mg.L−1. The influence of the water properties (ultrapure/groundwater) was non-negligible. A goal of this study was to approximate to field conditions, and so groundwater was used in all the regeneration experiments described below.

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4.2. Range of the regeneration conditions The first information obtained in the study of the regeneration of ACs saturated with PCE was the regeneration conditions: temperature range and time. The TGA tests were used to measure the mass loss revealing desorption from 50°C to 900°C. The lower temperature limit is the temperature below which the desorption is negligible. The upper temperature limit is the temperature above which the phenomenon of desorption becomes independent of temperature. The first TGA test is plotted in Figure 5. The AC loaded with PCE was heated successively to temperatures of 50°C, 100°C, 200°C, 300°C and 400°C for 8 h each. A final temperature at 900°C was set. The first rapid loss of mass at 50°C is very likely due to liquid evaporation (water). Then, for each temperature between 100°C to 400°C, the mass profile tended to reach a plateau. The complete stabilization of the mass from 400°C indicates that all compounds (water and PCE) are desorbed. The final temperature of 900°C ensures that everything is desorbed. There were no changes in mass between temperature 400°C and 900°C. This indicates that a temperature higher than 400°C does not provide better results (desorbed quantity) than at 400°C. The desorption occurs between 100°C and 400°C and over a period of 20 h. The following experiments consisted in applying a set temperature of 100°C, 200°C, 300°C and 400°C respectively over a period of 40 h. At the end of this first treatment, a second temperature of 1000°C was imposed. This last temperature was to detect any loss that would reveal an incompletely desorbed mass. These tests are shown in Figure 6. All these desorption were obtained from the same batch of AC loaded with PCE. The mass of adsorbed PCE and water is assumed to be the same for all the samples tested (for the entire batch: mass of dried AC=0.47 g, mass of PCE=0.23 g and mass of water=0.20 g). Weight stabilized quickly at temperatures of 200°C, 300°C and 400°C as it can be seen on the close-up on the first 2 h (Figure 6 b). The final temperature of 1000°C confirmed that the AC had been completely desorbed (less than 0.5 mg of mass loss). However, for the treatment at 100°C, the AC mass was not stabilized after 40 h of treatment. A further mass loss was observed during the plateau at 1000°C (more than 4 mg of mass loss), indicating that the desorption was not complete at 100°C.

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The TGA tests showed a minimal desorption temperature between 100°C and 200°C. A temperature higher than 400°C did not lead to a higher mass loss. Most of the mass loss was measured during the first 2 h. No variation of the mass loss was observed after 20 h. These experiments determined the operating conditions of the regeneration: working temperatures ranging from 130°C to 400°C, and 20 h heating time ensuring the maximum amount of PCE is desorbed at the considered temperature. The temperature of 130°C was chosen because it was above the boiling point of the PCE (121°C) while being far enough from the threshold temperature of 100°C.

4.3. Optimization of the regeneration conditions The study of the optimization of the regeneration conditions consisted in determining the role of the temperature and the number of regeneration cycles on the adsorption capacity. The number of regeneration cycles was set at four to obtain a significant change in the adsorption capacity. As stated previously, the groundwater doped with PCE had a composition (NOM) that fluctuated according to the sample (the concentration of TOC in the groundwater used in this study ranged from 1 to 2 mg.L−1). To compare the change in the adsorption capacity of the same AC through cycles, the same groundwater sample was used for all five isotherms from the same regeneration temperature. The regeneration temperatures tested were 130°C, 200°C, 300°C and 400°C over four regeneration cycles. The Langmuir simulation isotherms corresponding to various regeneration temperatures were obtained with the Langmuir model. Langmuir coefficients b and maximum adsorption capacities qm are given in Table 3. The example of the five isotherms from the regeneration temperature of 200°C is shown in Figure 7. For the same AC undergoing several regeneration cycles, the adsorption capacity tended to decrease over the entire isotherms up to 30 mg.L-1. This trend was clearly shown for the regeneration temperatures of 300°C and 400°C with the decreasing values of qm (Table 3). However, it was not obvious for lower temperatures. The adsorbing capacity in the zone of interest (low concentrations) is characterized by the values of the slope bqm (Table 3). These decreased with the increase in the number of regeneration cycles for all

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the regeneration temperatures, meaning a steady decrease of the adsorption capacity. This decline was more or less marked depending on the temperature. For example in Figure 7, adsorption capacity was 215 mg.g-1 for the first isotherm and fell to 104 mg.g-1 for the fifth isotherm from the 200°C regeneration set at an equilibrium concentration of 0.5 mg.L-1.

The change in adsorption capacity was measured with the regeneration rate (Eq. 4) in Figure 8. This is the comparison between the adsorption capacities of the regenerated AC versus the original AC at an equilibrium concentration of 0.5 mg.L-1. The choice of the value of 0.5 mg.L-1 was made because this concentration is in the range of typical PCE-polluted groundwater (lower than a few mg.L-1). Also, the first equilibrium points of the isotherms were below 0.5 mg.L-1. The regeneration rate was then calculated in the range of experimental values and not only by simulation. The regeneration cycle “0” matched the first isotherm, so the regeneration rate was always 100% for the first cycle. The regeneration rate decreased steadily through the regeneration cycles for all the applied temperatures. The average regeneration rate between 130°C and 400°C is also shown in Figure 8. The uncertainties of this average are plotted on the graph. They were calculated based on the uncertainties on adsorption capacities. It is noteworthy that all the data are included in these uncertainties. This shows that the adsorption capacity was restored analogously in the range of working temperatures. It indicates that regardless of the applied temperature, the average regeneration rate was 60% after four cycles. The average of regeneration rate follows a straight line that has a slope of -0.1 (-). If the same change continued after the fourth cycle, it would take about 10 cycles to obtain an adsorption capacity close to zero. The variation in the regeneration rate during adsorption/regeneration cycles is also described in the literature.

For

example,

Moreno-Castilla

et

al.3

performed

up

to

seven

cycles

of

adsorption/regeneration of ACs loaded with different substances (phenol, aminophenol, cresol, nitrophenol) at 700°C under an inert atmosphere (N2). It appears that changes in regeneration rate vary greatly depending on the adsorbed pollutant: a decrease at the beginning of the cycles and then a plateau with cresol (50% at the end of cycles), a steady decline for phenol and aminophenol (80%),

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and a decrease to zero after the sixth cycle for nitrophenol. The influence of the adsorbate/adsorbent is therefore particularly important in the regeneration rate obtained after several cycles.

The temperature defined as the low threshold below which the adsorbent does not desorb was identified at 130°C (identified with the TGA tests). We decided to perform an experiment at 25°C to check the threshold of the working temperatures. The result is shown in Figure 8. The regeneration rate was less than 20% after the first cycle and less than 10% after the second cycle. These very low rates confirm the need to perform the regeneration above 100°C.

4.4. Feasibility of the solar regeneration The study of the feasibility of solar regeneration used a PTC concentrator typically designed to reach temperatures of up to 450°C. This concentrator coupled with a vacuum-jacketed tube (Figure 2) was used because it was perfectly suited to the previously set operating conditions to perform thermal regeneration of the adsorbents. The solar regeneration means several important differences compared to the regeneration performed in the laboratory with an electric oven. Time and adsorbents exposure conditions in the PTC cannot be controlled. The temperature imposed on adsorbents depends on the conditions of solar radiation which is variable. To stay in conditions similar to those in place during regeneration in electric oven, a period close to 20 h at a minimum temperature of 130°C is desired. The device is therefore operated over several days. Indeed, it is not possible to operate more than 8 h per day the PTC with the heliostat. So three days of non-successive operation spread over a week and a half were necessary to work under very different weathers. The solar regeneration took place over three days in March for about 23 h of exposure to obtain a similar duration of regeneration between the solar PTC and the furnace. As shown in Figure 9, the weather fluctuated: the first day with clouds, the second day with strong sunlight and the third day with alternating cloudy and sunny periods. Direct and global radiation and the temperature measured in the compartment containing ACs were recorded (Figure 9). The temperature applied to the ACs was

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directly related to the direct radiation. The average, minimum and maximum temperatures over the three days are listed in Table 4. The temperature was above 168°C during the least favorable day (day 1, clouds). The day with mixed weather (day 3, cloud and sun) showed significant variations in direct radiation and therefore fluctuating temperature. It fell to 101°C, but the average was 310°C with the maximum at 377°C. The most favorable day (day 2, sun) yielded a stable, high temperature of around 390°C. We note that the temperature applied to the adsorbents was between 170°C and 400°C for 23 h during most of the solar regeneration, even on a cloudy day (day 1). These variations were within the range of temperatures used for regeneration under the controlled conditions studied in the laboratory. The solar experiment was tested in the worse conditions to test its reliability (three non-successive days, very different weather for each day, important temperature variation).

The isotherms run before and after the solar regeneration are plotted in Figure 10. We see that the two isotherms are very similar, despite a slight dispersion of the points for high concentrations. In the range of low concentrations, the adsorption capacity is quite repeatable. The regeneration rate at a concentration of 0.5 mg.L-1 was higher than 90%. This result is similar to that obtained after regeneration in the electric furnace in the range of 130–400°C (86%). The principle of solar thermal regeneration feasibility is thus validated.

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5. Conclusion The regeneration of ACs was investigated to assess the possibility of using solar regeneration. The context was water treatment. The target substance was PCE, a common groundwater pollutant and a VOC. Experiments first focused on the adsorption capacity and establishing the isotherms. They enabled us to choose the most appropriate AC among a set of adsorbents. Groundwater was chosen so as to work in conditions approximating field conditions. The regeneration conditions of ACs saturated with PCE were established by preliminary tests. It was found that there was no influence of regeneration temperature so long as this remained in the range of working conditions between 130°C and 400°C. The regeneration rate decreased with increasing number of cycles. After four regenerations and five adsorption isotherms, the adsorption capacity was restored to about 60%. Solar regeneration was tested so as to approximate to the operating conditions in the laboratory. A PTC solar concentrator with a vacuum-jacketed tube as the absorber was used to reach the range of the regeneration temperatures. The solar regeneration gave a regeneration rate similar to that obtained with the classical process. This first promising result opens the way to a new possible use of solar energy. The alternative of solar regeneration would present a significant economic advantage. A further utility is that it limits CO2 emissions and minimizes environmental impact. The choice was made to work only under an inert atmosphere. It would be of interest to use an atmosphere containing oxygen to carry out a moderate oxidation: gasification might restore adsorption capacity more effectively. Moreover, it has to be noticed that the working condition was determined for a VOC that facilitate the desorption. It would be relevant to test the possibility of extending this process on other less volatile contaminants such as pesticides which are also prevalent in groundwater. Finally, extensive studies could be addressed on the solar technology itself. A simpler and non-concentrated system, such as a Compound Parabolic Collector, can reach a temperature above 100°C.

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Aknowledgement This work was supported by the Program “Investissements d’avenir” (Investment for the Future) of the Agence Nationale de la Recherche (National Agency for Research) of the French State under award number ANR-10-LABX-22-01-SOLSTICE.

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List of Figure Captions Figure 1: Test bench set-up for thermal regeneration: heating system (tube furnace (1)), AC contained in a cell (2) with a nitrogen flow (3) regulated with a flow meter with pressure valve (4) and thermocouples (T1, T2, T3 and T4) Figure 2: Test bench set-up for the solar regeneration: PTC (1) fixed on a heliostat (2), absorber (vacuum jacketed tube (3)) containing the AC, measurements of direct and global radiations (4) and nitrogen flow (5) Figure 3: Isotherms ultra pure water/PCE with 5 activated carbons (112-200µm) at 25°C, respectively experimental data and Langmuir fitting: Aquasorb CX (○, ••), Filtrasorb 400 (Δ, ─ ─), Aquacarb (◊, ─), Aquasorb 2000 (*, =) and Filtrasorb 200 (+, ─ •) Figure 4: Six isotherms with groundwater (□) and two isotherms with ultra pure water (◊) at 25°C with Aquacarb (112–200 µm) with Langmuir fitting respectively (‒) and (- -) Figure 5: The evolution of the temperature (─) and the mass of AC (- -) over time Figure 6: The evolution of the mass of AC over time for experiment at 100°C (──), 200°C (─ ─), 300°C (- -) and 400°C (• •) with the second temperature of 1 000°C over the entire test a) and a closeup on the first 2 hours b) Figure 7: Isotherms groundwater/PCE with Aquacarb (112-200µm) at 25°C with the regeneration temperature at 200°C; respectively experimental data and Langmuir fitting: 1 st isotherm (□, ─), 2nd isotherm (○, =), 3rd isotherm (+,••), 4th isotherm (◊,─ ─), 5th isotherm (*,─ •) Figure 8: Regeneration rate at 25°C (+), 130°C (◊), 200°C (□), 300°C(Δ), 400°C (×) and mean value (○) versus regeneration cycle at an equilibrium concentration of 0.5 mg.L-1 Figure 9: Temperature applied to activated carbon (=) and the direct (••) and global (─) radiations measured during the solar regeneration Figure 10: Isotherms groundwater/PCE with Aquacarb (112-200µm) at 25°C with the solar regeneration; respectively experimental data and Langmuir fitting: 1st isotherm (□, ─), 2nd isotherm (○, =)

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Table 1: Main properties of the 5 tested activated carbons Table 2: Parameters of the isotherms with the fittings of Langmuir for the five AC Table 3: Parameters of the isotherms with the fittings of Langmuir for the four regeneration temperatures Table 4: Temperatures measured on three days of the solar regeneration

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Table 1

Filtrasorb 400 Producer

Filtrasorb 200

Aquacarb 207C

AquaSorb 2000

Chemviron Carbon

AquaSorb CX

Pica Jacobi

Raw materials

Coal

Coal

Coconut shell

Coal

Coconut shell

Apparent (kg.m-3)

425

500

450

500

490

Surface area BET (m2.g-1)

1050

850

1100

1000

1150

Micropores (m2.g-1)

811

737

870

797

1036

density

area


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Table 2 Parameters Criterion qm (mg.g-1) b (L.mg-1)

Aquasorb CX 0.018 575 0.567

Aquacarb 207C 0.038 510 1.22

Filtrasorb 400 0.041 488 0.655


Filtrasorb 200 0.063 427 0.824

Aquasorb 2000 0.132 569 0.265


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Table 3

q (mg.g-1)

b (L.mg-1)

1st isotherm 2nd isotherm 3rd isotherm 4th isotherm 5th isotherm 1st isotherm 2nd isotherm 3rd isotherm 4th isotherm 5th isotherm

130°C 517 518 518 523 547 0.683 0.507 0.543 0.545 0.329

Regeneration temperature 200°C 300°C 400°C 532 648 635 576 647 532 572 635 744 533 504 513 598 559 475 1.36 0.885 0.892 1.24 0.847 1.36 0.779 0.516 0.759 0.690 1.07 0.903 0.422 0.736 0.969


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Table 4

Average temperature (°C) Minimum temperature (°C) Maximum temperature (°C)

Day 1, clouds 240 168 311

Day 2, sun 385 300 396


Day 3, clouds and sun 310 101 377


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Figure 1


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Figure 2



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Figure 3


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Figure 5


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Figure 6 a)

Figure 6 b)



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Figure 7


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Figure 9


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Sustainable Thermal Regeneration of Spent Activated Carbons by

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