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Article

Activated Carbons Produced from Hydrothermally Carbonized Prickly Pear Seed Waste

by
Nourhen Hsini
1,
Vahid Saadattalab
2,
Xia Wang
2,
Nawres Gharred
1,
Hatem Dhaouadi
1,
Sonia Dridi-Dhaouadi
1,3 and
Niklas Hedin
2,*
1
Environmental Chemistry and Clean Processes Research Laboratory, Faculty of Sciences of Monastir, University of Monastir, Monastir 5000, Tunisia
2
Arrhenius Laboratory, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
3
Preparatory Institute for Engineering Studies of Monastir, University of Monastir, Monastir 5000, Tunisia
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(21), 14559; https://doi.org/10.3390/su142114559
Submission received: 24 August 2022 / Revised: 2 October 2022 / Accepted: 24 October 2022 / Published: 5 November 2022
(This article belongs to the Section Sustainable Materials)
Browse Figures
Versions Notes

Abstract

:
The agro-sector generates organic waste of various kinds, which potentially could be used to prepare functional materials, lessen environmental problems, and enhance circularity. In this context, the hypothesis that was put forward in this work is that prickly pear seed waste from the Tunisian agro-food industry could be used to prepare activated carbons. The prickly pear seed waste was first hydrothermally carbonized and the resulting hydrochar was activated in CO2 at 800 °C. The yield of the hydrothermal carbonization process is of importance, and it was the highest at intermediate dwell times and temperatures, which was ascribed to the re-precipitation of hydrochar particles on the heat-treated biomass. The hydrochars and activated carbons were characterized with scanning electron microscopy, thermogravimetry, Raman spectroscopy, and N2 and CO2 adsorption/desorption analyses. The activated carbons had micro- (<2 nm) and mesopores (2–50 nm), and specific surface areas and total pore volumes of about 400 m2 −1 and 0.21 cm3 g−1. The study showed that the prickly pear seed waste could be effectively transformed into both hydrochars and activated carbons and that is advisable to optimize the hydrothermal process for the mass yield. A life cycle analysis was performed to assess the environmental impact of the production of typical activated carbons using the approach of this study. Further studies could be focused on enhancing the properties of the activated carbons by further optimization of the activation process.

1. Introduction

The agro-food sector is large and generates solid waste that is often considered harmful to the environment [1]. For example, waste management problems exist in relation certain peels [2], kernels [3], and seeds [4,5]. This type of waste is biodegradable but the rate of the degradation is typically not high enough. The residues tend to accumulate without defined benefit and increase the risks to the surrounding ecosystem. There are many ways to deal with this type of waste. It can be incinerated but also upgraded into functional materials. Among these are activated carbons. Activated carbons have numerous applications that typically make use of the large internal surface areas. They are used, e.g., in processes for gas treatment, gas separation [6], and water treatment [7]. Commercially, activated carbons are typically prepared from coconut shells, petroleum, or coal products [8]. However, research and developments are performed to prepare activated carbons with large specific surface area from various types of agro-food waste such as orange peels [9], apricot seeds, walnut shells [10], and maize straw [11].
Activated carbon can be produced through different processes, but a common feature is that a char is used. It is conventionally prepared by pyrolysis. Slow pyrolysis typically used for the derivation of biochars is characterized by high temperature and a long carbonization. For the activation part, a large amount of toxic gases is also typically produced. On the other hand, hydrothermal carbonization (HTC) of biomass waste is a thermochemical conversion process that allows the conversion of lignocellulosic biomass into refined hydrochar and a liquid phase at low temperature and under pressure. The method is particularly advantageous for wet biomass as it also functions as a dewatering process. Therefore, HTC has better thermodynamics for treating wet biomass than pyrolysis. It probably also has a less negative environmental impact on ecosystems. However, it can be emphasized that the water phase from HTC is often environmentally problematic and needs to be dealt with in a well thought out manner.
Activated carbon can be produced from both types of chars by, e.g., physical activation with CO2 or chemical activation with H3PO4 [12] or KOH [13]. Studies have been performed to activate hydrochars into activated carbons with large specific surface areas [14,15] and high pore volumes [16,17]. CO2 activation has benefits over KOH activation when it comes to cost, and the potential toxicity and other risks of KOH [18]. Activated carbons have been prepared from waste biomass using CO2 activation also in other studies [19,20,21]. In this context, it can be noted that the properties of the hydrochars strongly depend on the water temperature [22] and the dwell time of the HTC reaction [23], which is of importance for the preparation of the activated carbons.
When considering production of activated carbon from agricultural wastes, it is important to note that waste management is linked to sustainability, the environment, and the global energy system [24]. Valorization of biomass plays an important role in the energy transition [25]. In context, it is important to analyze the biomass and its residual products. Take the example of the prickly pear, which is exported fresh (fruit, juice), in dried form (seeds and clacode), and, more recently, in the form of seed oil extract. This oil, rich in sterols [26], fat-soluble vitamins, essential fatty acids, and carotenes [27], is used in the pharmaceutical and cosmetic fields. A large part of the related waste is generated after the extraction of essential oil from these seeds. This waste is rich in lignocellulose [28], which makes it a good precursor for the production of, e.g., activated carbon. In a number of recent studies, valorization of several wild and cultivated plants as well as fruit waste has been reported. These studies include the valorization of Crocus sativus [29], Nigella sativa [30], Azolla pinnata [31], Inula viscosa [32], and melon skin [33]. Hydrochar or biosorbents were developed and studied to be able to remove heavy metals and toxic gases, but also to treat wastewater [34]. To manage the value of resources in waste management, economic, environmental, and social directions should be taken into account. This consideration has circular aspects coming to the energy and material resources. It is within this framework that life cycle analysis (LCA) is commonly used. It is a strategy that offers to assess advantages for the recovery of waste through its reduction, recycling, and reuse. It makes it possible to choose between the different ways of recovering industrial waste in an enlightened manner. Indeed, in relation to LCA it is important to assess the valorization of industrial waste. Here, the derivation of activated carbons might be a means to reduce environmental pollution.
This research is part of the reduction of the environmental impact of waste from the food industry. An activated carbon was produced from a hydrocarbon from prickly pear seed (PPS) waste from the Tunisian food industry. The yield of the hydrothermal process was optimized with respect to time and temperature. The activated carbon was produced by physical activation with CO2 and studied in various ways. A life cycle analysis (LCA) was performed to address the real environmental value of the work.

2. Materials and Methods

2.1. Materials

After extracting oil from the PPS, a solid waste was recovered, powdered, and sieved using stainless steel sieves. Fractions with a diameter of 0.1–1.0 mm were used for the HTC whereas those with a diameter of 0.063–0.100 mm were used for physicochemical characterization.

2.2. Synthesis of Activated Carbon

2.2.1. Hydrothermal Treatment

A set of hydrochars called as HTC-P was prepared from the PPS by using HTC in a set of experiments using the principles of design of experiments. The dwell time and temperature were used as the dependent variables and the effects on the response variable, mass yield, were accessed. The dwell times were 1, 4, 8, 16, or 24 h and the set temperatures were 180, 200, or 220 °C, which corresponded to 15 experiments. For the preparation of each of these hydrochars, 6 g of PPS was mixed with 100 mL of deionized water and stirred for 12 h at ambient temperature. The obtained slurries were placed into Teflon-lined autoclaves (150 mL) and heated in an oven of the brand HERAEUS for a designated time of 1–24 h with an oven heating rate of 6 °C.min−1. The MINITAB software was used to evaluate the design responses with respect to the response variable mass yield.
After the HTC, the resulting hydrochars were filtered from the aqueous phase. The pH of the aqueous phase was measured using a pH paper before and after the HTC. The obtained hydrochars were dried at 100 ± 10 °C in the oven until constant masses were obtained and named as HTC-PXY, where for example HTC-P220-24 was produced at 220 °C for 24 h of HTC. For the mass yield determination, the within-experiment uncertainty was estimated to be in the third digit and related to the mass and volume measurements. The mass yield was calculated using the following Equation (1):
Mass   yield   % =   weight   of   hydrochar   initial   weight   of   PPS × 100

2.2.2. Physical Activation

After HTC, the optimal sample HTC-P200-14 was placed in a tube furnace of the type Carbolite MTF. The heating was first performed in a flow (100 mL.min−1) of N2 at 400 °C, and then in a flow of CO2 (using the same flow rate) at 800 °C. The produced material was denoted as PAC1-800 (activation for 1 h at 800 °C). Note that the optimized conditions for the activation were not the topic of this study.

2.3. Characterization of the Solids

2.3.1. Morphology Analysis

The morphologies of the PPS, the hydrochars, and the activated carbon PAC were studied by using scanning electron microscopy (SEM) with a JEOL JSM-7000F microscope. Small amounts of sample were placed on carbon stubs and overlaid with gold to minimize surface loading. Images were taken at magnifications of ×100 and ×2700 with a working distance of 10 mm and a potential of 15.0 kV. The elemental compositions were determined with SEM energy–dispersive X-ray spectroscopy (EDS). The accuracy of EDS analysis depends on the sample type and element but is about ±5% for rough particles without standards used.

2.3.2. Thermogravimetric Analysis

Thermogravimetric (TG) data were recorded for PPS and HTC-P200-14 with a Discovery TGA7 analyzer (PerkinElmer, Norwalk, CT, USA, and the derivative of the TG traces (DTG) was performed with the associated software. The thermochemical decomposition was studied in a flow of CO2, which was also the gas used for the physical activation of the hydrochars. The hydrochars were placed in a platinum cup and heated from 25 to 800 °C (at a rate of 10 °C min−1).

2.3.3. Raman Analysis

Raman spectra were recorded on a confocal Raman microscope (LabRAM HR Evolution-Horiba). A laser with a wavelength of 532 nm and laser power of 50 mW was used and the resolution was set at 1.7 cm−1 (an objective of ×50 was used). The resulting spectra were recorded in the range of 500–3600 cm−1 with an exposure time of 3s. The spectra were fitted by a second derivative method with adjacent modes.

2.4. Adsorption/Desorption Isotherms

Gas sorption data for the analyses of the surface texture and porosity of the PPS, HTC-P200-14, and PAC800-1 samples were recorded on a Micromeritics ASAP2020 analyzer. Adsorption and desorption isotherms of N2 were recorded at a temperature of −196 °C for all three samples, and CO2 adsorption and desorption isotherms were recorded for the HTC-P200-14 and PAC800-1 at a temperature of 0 °C. All samples were initially degassed at 150 °C using 1440 min dwell time (after an initial heating using a rate of 10 °C.min−1) under conditions of dynamic vacuum. The specific surface areas were calculated within the Brunauer–Emmett–Teller (BET) model from the adsorption isotherms of N2 (SBET(N2)) and CO2 (SBET(CO2.)) by plotting p/(p0−)V in terms of relative pressure (p/p0) (p0 = 26,143 mmHg for CO2). Calculation of SBET(N2) or SBET(CO2) was carried out in the linear range of the adsorption isotherms. For SBET(N2), the p/p0 ranges used for the BET method were selected ensuring a positive intersection with the y-axis and a Rouquerol transformation that increased with the p/p0 [35]. For the BET analysis, the analysis error relates to the mass measurements and the selection of the pressure ranges, and three significant figures were judged to be proper. The SBET(CO2) was determined using Equation (2) as below:
S CO 2 = a co 2 VN A
where a c o 2 is 0.218 nm2, V (mol.g−1) is the molar volume, and NA is the Avogadro number. The total pore volume was determined from the N2 adsorption at a relative pressure of 0.98. The pore size distributions (PSD) obtained from the N2 and CO2 adsorption curves were derived in with a density functional theory (DFT) method assuming a slit-shaped pore geometry for activated carbons. For the DFT analyses, some degree of regularization was used.

2.5. Life Cycle Assessment of Activated Carbon Production

2.5.1. Aim and Scope

The LCA aimed to assess the environmental impacts of the solid waste management. This assessment included processes from the extraction of vegetable oils from the PPS to the production of activated carbons using hydrochars from the HTC of the PPS. The scope was a door-to-door assessment, with an entry of the Tunisian prickly pear fruit valuation industry to the door, which was the exit of the industry. Note that the energy requirements of the main processes of the PPS recovery system were calculated based on the equipment currently used in industry. The system boundary did not consider the transport of the PPS from the production source to the recovery unit. The functional unit was 1 ton of recovered PPS.
In the LCA, the PPS was recovered by production of activated carbon by physical means according to the following main stages: reduction in powder—impregnation—hydrothermal treatment—filtration—activation in CO2 gas—packing in bags. A detailed description of these steps is presented in the supplementary data (Annex S2). An illustration of the upgrading system and its system boundary are given in Figure 1.

2.5.2. Method and Impact Assessment

The LCA analysis was carried out using the Simapro (version 7) software. To convert inputs from the technosphere (electricity, chemistry, etc.) into their equivalent resource consumption/use and into their equivalent atmospheric, water, and solid emissions, the LCA database Ecoinvent was used. The CML 2000 reference method was selected to assess the environmental impacts associated with each recovery system, which enabled them to be compared. Ten impact categories were assessed.

3. Results and Discussion

3.1. Local Maxima in the Mass Yield of the Hydrochars with Respect to Temperature and Time

When turning waste from the agro-sector into functional and sustainable carbon materials, it is important to assure among many things that the yield of the process is high. Here, the yield of the HTC of the PPS waste was studied by varying the dwell time and the temperature. The yield did not monotonically decrease with the dwell time or temperature. Instead, a maximum in the yield with respect to temperature and time was observed as seen in Figure 2a,b, Figure S1, and Table 1. The highest mass yield of 76% was observed under HTC at 200 °C after 14 h. The variations in the mass yield were rationalized by comparing them with literature findings. Above 180 °C, it has been shown that hemicellulose is hydrolyzed and converted to reactive compounds that can be further carbonized during the HTC treatment [36,37]. That such hydrolysis had occurred during HTC of the PPS was consistent with the observed acidity of the aqueous phase after HTC. The pH was reduced from a value of about 7 before the reaction to a value of about 4 after HTC (specific pH values are presented in Table S1 of the Supplementary Materials). Similar observations have been made by, for example, Falco et al. [38] and Fakkaew et al. [39], who explained that the in situ acidity promotes hydrolysis of hemicellulose and isomerization of glucose into fructose, which in turn is dehydrated into HMF (5-hydroxymethylfurfural). The maximum mass yield (see Figure 3) is speculatively related to HMF formed during HTC, which is subsequently sorbed and polymerized on the remaining solid [40,41]. At longer times, the mass yield decreased which may be related to hydrolysis or carbonization of other polysaccharides of the PPS [42], or of other components in the PPS, or further dissolution of soluble components. Note that degradation of cellulose [43,44,45] has been shown not to occur at the HTC temperatures of this study (180–220 °C). The decrease in mass yield observed on increasing the temperature from 200–220 °C was in line with the normally expected dependencies of condensation and dissolution reactions. The analysis of variance (ANOVA) of the yield as a function of the dwell time and temperature is presented in the supplementary information (Annex S1).

3.2. Thermal and Morphological Analyses of the PPS, the Optimal Hydrochar and an Activated Carbon

As expected, the hydrochar had a significantly higher thermal stability compared to the PPS waste. This was deduced from the TG traces in Figure 3. The mass loss at 25–75 °C for HTC-P200-14 was attributed to sorbed water. Hydrochars are comparably hydrophilic [46]. The mass loss of 15% at 350–450 °C was similar to other studies [46] and associated with carbonization [47]. The mass loss at 150–250 °C was related to dehydration reactions [48]. For the PPS waste, a mass loss was also observed at 25–75 °C and attributed to water [49]. It was used to determine the water content of the PPS waste to 5%. The second and third mass losses at 220–320 °C and 420–450 °C corresponded jointly to a weight loss of 95 %. They were tentatively assigned to the degradation of hemicellulose and cellulose [50]. Literature data for the amount of hemicellulose and cellulose in the PPS waste were not found but some information on the cellulosic compounds in raw PPS is available [51]. Note that the thermal analyses were performed in a flow of CO2, instead of in N2 or dry air, which are more commonly applied. This choice of gas was selected as the activation of the HTC-P200-14 was conducted in CO2. CO2 is a weak oxidant.
The morphology of the particles in the PPS waste was different from those in the hydrocarbon and activated carbon, as can be seen in Figure 4. The surfaces of the particles in the PPS waste sample were rough while those in the HTC-P200-14 and PAC800-1 were porous. Spherical particles are observed in the SEM image of HTC-P200-14 in Figure 4b. Such particles have been commonly observed for hydrochars prepared from glucose [52] and from biomass [53]. The spherical particles are commonly related to a phase separation of drops of HMF and a polymerization into hydrochar [54]. In the SEM image of PAC800-1, some spherical particles could still be detected. The general ultrastructures of the PPS waste remained in the hydrochar and the activated carbons. It has been established that the ultrastructures of cellulose-rich waste remain after HTC treatment [55,56] and activation in CO2 [57,58]. The SEM images in Figure 4c indicated that PAC800-1 had higher porosity than HTC-P200-14. The optimization of the conditions for the activation was not the purpose of this study so the activated carbons had somewhat lower surface areas than what would be expected for optimized activation conditions [59,60].

3.3. Atomic Composition and Molecular Features of the Hydrochar and Activated Carbon

Atomic compositions of the PPS waste, hydrochar, and activated carbons were studied by SEM and EDS. The PPS waste contained minor amounts of Ca, Mg, P, and K, which disappeared almost completely for HTC-P200-14 and PAC800-1, see Table 2. The HTC-P200-14 and PAC800-1 were essentially carbon based. EDS spectra are presented in the supplementary information, Figure S2. (The carbon analysis is only qualitative).
Even if the Raman spectra for the HTC-P200-14 and PAC800-1 are seemingly similar as seen from Figure 5, the moieties leading to the bands are different. For HTC-P200-14, the two bands are assigned to the sp2- and sp3-hybridized carbons in the polymeric structures [61]. For PAC800-1, the corresponding bands are assigned to a disordered D band (around 1320 cm−1) and a graphitic G band (around 1580 cm−1) [62,63]. It is established that activated carbon is rich in aromatic structures.

3.4. Adsorption/Desorption of N2 and CO2

Surface area and pore volume were determined by gas adsorption analyses for HTC-P200-14 and PAC800-1. As expected, the SBET(N2) was higher for PAC800-1 than for HTC-P200-14, see Table 3. HTC-P200-14 had a similar value as hydrochars produced from acerola waste [64], corncob and corn straw wastes [65], and bamboo sawdust [56]. The SBET(N2) of the PAC800-1 was lower than in some earlier studies for activated carbons prepared in CO2 [66,67] and relates to the activation not being optimized for achieving the highest possible SBET(N2). The activated carbons prepared from hydrochar from the PPS waste had a relatively low surface area. However, they were still higher than those in some related recent studies, as for example activated carbons from Nauclea diderrichi [68] and date palm seeds [69]. Activated carbons such as those in this study are relevant to many applications [70,71,72]. The SBET(N2) was much higher than the SBET (CO2) for HTC-P200-14, which means that most of the specific surface area corresponded to pores <8 Å [73,74]. In addition, the total pore volume (VT) of PAC800-1 was larger than for HTC-P200-14, which was expected and related to the activation. Furthermore, the VT of the activated PAC800-1 was larger than those observed for activated carbons prepared from barley malt bagasse [75] and hemp hurd [76] when activated in CO2.
The N2 adsorption isotherm (see Figure S3) for HTC-P200-14 was similar to related hydrochars [77], and the N2 adsorption isotherm (in the p/p0 range of 0–0.8) for the PAC800-1 was typical for an activated carbon that contains micropores but mainly mesopores [78]. The CO2 adsorption at 0 °C and at a pressure of 1 bar for the PAC800-1 is shown in Figure S4 and compared with some reported in the literature in Table S3. It should be noted that this study did not optimize the activated carbon with respect to the CO2 uptake. The pore size distributions (PSD) (obtained from N2 adsorption and DFT) for the PAC800-1 showed important fractions of micropores with sizes less than 1.5 nm, as seen in Figure 6a. HTC-P200-14 and PAC800-1 had mesopores with sizes between 18 nm and 44 nm. The PSD in Figure 6b showed that the ultramicropore volume was higher in PAC800-1 than in HTC-P200-14. Speculatively, the chemistry of the hydrochars might lead to a high adsorption of CO2 in the corresponding activated carbon [79]. (For the PPS waste, only the N2 adsorption was measured and it was low).

3.5. Life Cycle Assessment Analysis Results

3.5.1. Data Inventory

The data used for the inputs and outputs of the inventory of the system such as energy use and the data deduced from this study as well as those from the literature are summarized in Table 4 below.

3.5.2. Analysis of the Environmental Impacts

As shown in Figure 7 and Table S4, it is the HTC and the activation processes that have the highest environmental impacts in the production of PAC. Indeed, the carbonization process has a strong impact on global warming (GW) (90%) and photochemical oxidation (PO) (83%) and this result comes from the atmospheric emission of combustion gases from the hydrochar. On the other hand, the impact potentials relating to the ecotoxicity of fresh water (FEW) (60%) and marine (MAE) (60%), to human toxicity (HT) (60%), to the depletion of the ozone layer (DOZ) (58%), terrestrial ecotoxicity (TE) (57%), and abiotic depletion (AD) (55%) are attributed to the HTC process. The main impact contribution is the electricity given the high energy requirements of this process. This contribution leads to the emission of pollutants such as aluminum, its hydroxide, and tri-chloromethane. Here, it can be noted that the energy use can be reduced significantly with thermal management and utilization of the heat generated in the exothermic processes but such enhancements were not included in the LCA. These pollutants have negative effects on HT, FEW, MAE, and on TE (see Annex S3 (7-8-9)). However, to generate electricity, the use of fossil fuels such as coal, crude oil, lignite, and natural gas generates negative impacts on AD, acidification (Ac), and DOZ (see the Annex S3 (1-2-4)). However, the solid-liquid filtration process used to separate the hydrochar from the liquid phase obtained after the HTC treatment strongly contributes to the overall eutrophication impact (Eu) (99.6%), due to the emission in surface waters with loads of organic pollutants (see the Annex S3).

3.6. Discussion of the Practical Implementation of the Method and Limitation of the Study

In this context, it is interesting to also discuss how practically relevant it is with a combined HTC and activation process to be able to produce activated carbon from PPS waste. Here it is important to emphasize that HTC is to some extent commercialized but not on the scale one might have expected from its favorable thermodynamics. The main reason why HTC is not widely used is that the method creates a contaminated water phase that must be managed. This is especially complicated for sludge handling. However, the water phase from HTC-treated PPS waste is judged to be easier to handle because it is not a complex waste. The fact that the method has two steps is also complex industrially. It may therefore be relevant to compare with an alternative process where the PPS waste is first dried and then pyrolyzed and activated into activated carbon. Note in this context that the HTC process has thermodynamic advantages in treating wet biomass as it not only chars but also dewaters the waste.
It should be noted that although we were able to demonstrate that process conditions for the HTC of PPS were important, these studies were performed in a small hydrothermal reactor. For an industrial reactor, the process conditions would need to be re-optimized. However, it is expected that a similar optimum for the yield would be observed for a sufficiently long treatment time at a sufficiently high temperature. Another limitation of this study concerns the process conditions for the synthesis of the activated carbons. They were not optimized. Therefore, optimization would need to if this approach should be applied at scale. Such an optimization would involve variation of the conditions for the activation. The BET surface area or similar could be used as a goal function for using the methods of Design of Experiments.
In summary, it can be said that LCA helps decision-making in different ways. One is to improve sustainability of the processes because the analysis highlights environmental hot spots. However, the limits of LCA should not be overlooked. These include the influence of the functional unit, the system, the inventory database, and the details of the evaluation method. This affects the final result of the environmental impact, which can be both underestimated and overestimated depending on the choices that have been made in the analysis. Future studies should take such limitations into account to achieve robust analyses. Robustness of analysis is important in general and for the hydrochar and activated carbons of this study. For a more robust analysis, Aghbashlo et al. [83] suggests using several computer programs to evaluate the sustainability of a process or a product. Those programs take technical-economic analyses, energy, and exergy into account, complement LCA, and improve on the reliability of the analysis.

4. Conclusions and Prospects

In this study, hydrochar was produced by HTC of PPS waste, and the mass yield was maximal (up to 76%) at intermediate dwell times. As an example of further valorization, activated carbon having both micropores and large mesopores was prepared by CO2 activation of the hydrochar. The LCA analysis showed that the production of the activated carbon was associated with negative impacts such as eutrophication, global warming, and photocatalytic oxidation.
It was shown that the PPS waste could be upgraded to activated carbon via a hydrochar. A new feature in the study was that the maximum yield for the hydrochar was observed at intermediate dwell times. This observation was rationalized with a formation and polymerization of HMF combined with a re-precipitation on the hydrochar derived from the waste biomass. Another new feature was the LCA analysis of both the HTC and CO2 activation. It was concluded that heat integration or at least optimization of the HTC process to limit the specific energy use is advisable.
The PPS waste can effectively be transformed with HTC into hydrochars and then into activated carbons, which could be one approach to use this and similar waste. The specific surface areas of the activated carbon were moderately high and offer room for further optimization. The total pore volume was relatively high.
In addition to the results from the study, it is relevant to further study how to improve the properties of the activated carbon that was produced from hydrochar derived from the PPS biowaste. This could be performed by optimizing the production conditions of the activated carbon. Another perspective could be one to perform detailed environmental and techno-economic evaluations of the combined use of HTC and activation of the biochar for the production of activated carbon. This approach could be compared to a direct pyrolysis and activation of the PPS biowaste. An expanded LCA could be suitable for such a study.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su142114559/s1, Table S1: Measured pH of the aqueous phase before (pH before) and after (pH after) hydrothermal process. Table S2: (a) Estimated regression coefficients for the response, yield%. (b) Analysis of variance for the yield%. Table S3: CO2 adsorption capacities (0 °C, 1 bar) of different activated carbon from HTC precursors cited in the literature. Table S4: Results of the environmental impact assessment for the upgrading system of 1 ton of PPS by activated carbon production by hydrothermal pretreatment. Annex S1: ANOVA statistical analyses. Annex S2: Description of the stages of the system production of activated carbon. Annex S3: Network analysis of the ten environmental impact categories of activated carbon production. Figure S1: Surface plot of yield (%) with respect to time (h) and temperature T (°C) variables. Figure S2: EDS analyses of the precursor PPS, the hydrochar HTC-P200-14, and the activated PAC800-1. Figure S3: N2 adsorption/desorption isotherms at −196 °C for (●) prickly pear seed waste (PPS), (♦) a hydrochar (HTC-P200-14), and (▲) the activated carbon (PAC800-1). Figure S4: CO2 adsorption/desorption isotherms at 0 °C for (♦) the hydrochar (HTC-P200-14) and the corresponding (▲) activated carbon (PAC800-1). Adopted from [83,84,85,86,87,88,89,90,91].

Author Contributions

Conceptualization: N.H. (Nourhen Hsini); Data curation: N.H. (Nourhen Hsini) and N.H. (Niklas Hedin); Formal analysis: N.H. (Nourhen Hsini) and X.W.; Funding acquisition: N.H. (Nourhen Hsini) and N.H. (Niklas Hedin); Investigation: N.H. (Nourhen Hsini), V.S. and X.W.; Methodology: N.H. (Nourhen Hsini), V.S. and N.H. (Niklas Hedin); Project administration: N.H. (Niklas Hedin); Resources: N.H. (Niklas Hedin) and S.D.-D.; Software: X.W. and N.G.; Supervision: S.D.-D. and N.H. (Niklas Hedin); Validation: S.D.-D., N.H. (Niklas Hedin) and H.D.; Visualization: H.D. Writing—original draft: N.H. (Nourhen Hsini) Writing—review and editing: S.D.-D., N.H. (Niklas Hedin) and V.S. All authors have read and agreed to the published version of the manuscript.

Funding

Swedish Foundation for Strategic Environmental Research (Mistra; project Mistra TerraClean, project number 2015/31) and the Green Carbon ITN-project under the Marie Skłodowska-Curie grant agreement No 721991, for financial support. Financial support from Stockholm University and the Ministry of Higher Education and Scientific Research of Tunisia.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This research work financed by Stockholm University and the Ministry of Higher Education and Scientific Research of Tunisia is gratefully acknowledged. NH thanks the Swedish Foundation for Strategic Environmental Research (Mistra; project Mistra TerraClean, project number 2015/31) and the Green Carbon ITN-project under the Marie Skłodowska-Curie grant agreement No 721991, for financial support.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

AbbreviationsNomenclatures
LCALife cycle analysis
AcAcidification
ADAbiotic depletion
BETBrunauer–Emmett–Teller
DFTDensity functional theory
DOZDepletion of the ozone
DTGDerivative of thermogravimetric traces
EDSDispersive X-ray spectroscopy
EuEutrophication
FWEFresh water ecotoxicity
GWGlobal warming
HMF5-hydroxymethyl furfural
HTHuman toxicity
HTCHydrothermal carbonization
PACPhysical activated carbon
MAEMarin aquatic ecotoxicity
POPhotochemical oxidation
PPSPrickly pear seed waste
PSDPore size distributions
SEMScanning electron microscopy
TTemperature
TETerrestrial ecotoxicity
TGThermogravimetric
VT Total pore volume

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Figure 1. Illustration of the system boundary and an upgrading system for the PPS waste by production of activated carbon after hydrothermal pretreatment.
Figure 1. Illustration of the system boundary and an upgrading system for the PPS waste by production of activated carbon after hydrothermal pretreatment.
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Figure 2. (a) The mass% yield of the hydrothermal carbonization of PPS waste. The variations as a function of time and T (°C). (b) A contour plot displaying the variations as a function of time and T (°C).
Figure 2. (a) The mass% yield of the hydrothermal carbonization of PPS waste. The variations as a function of time and T (°C). (b) A contour plot displaying the variations as a function of time and T (°C).
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Figure 3. Thermogravimetric (TG) and differential TG (DTG) traces for (PPS) waste and a corresponding hydrochar prepared by hydrothermal carbonization (HTC) for 14 h at 200 °C (HTC-P200-14).
Figure 3. Thermogravimetric (TG) and differential TG (DTG) traces for (PPS) waste and a corresponding hydrochar prepared by hydrothermal carbonization (HTC) for 14 h at 200 °C (HTC-P200-14).
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Figure 4. SEM images of: (a) the PPS waste; (b) a corresponding hydrochar prepared by hydrothermal carbonization (HTC) for 14 h at 200 °C (HTC-P200-14); and (c) a corresponding activated carbon prepared by activation in CO2 at a temperature of 800 °C (PAC800-1).
Figure 4. SEM images of: (a) the PPS waste; (b) a corresponding hydrochar prepared by hydrothermal carbonization (HTC) for 14 h at 200 °C (HTC-P200-14); and (c) a corresponding activated carbon prepared by activation in CO2 at a temperature of 800 °C (PAC800-1).
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Figure 5. Raman spectra of the hydochar (HTC-P200-14) prepared by hydrothermal carbonization (HTC) for 14 h at 200 °C (green line), and the corresponding activated carbon prepared by activation in CO2 at 800 °C, PAC800-1 (blue line). The D and G bands are indicated in the spectrum of PAC800-1.
Figure 5. Raman spectra of the hydochar (HTC-P200-14) prepared by hydrothermal carbonization (HTC) for 14 h at 200 °C (green line), and the corresponding activated carbon prepared by activation in CO2 at 800 °C, PAC800-1 (blue line). The D and G bands are indicated in the spectrum of PAC800-1.
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Figure 6. Pore size distributions (PSD) obtained from a Density Functional Theory (DFT) of (a) N2 and (b) CO2 adsorption isotherms for (♦) the hydrochar (HTC-P200-14) and (▲) the activated carbon (PAC800-1).
Figure 6. Pore size distributions (PSD) obtained from a Density Functional Theory (DFT) of (a) N2 and (b) CO2 adsorption isotherms for (♦) the hydrochar (HTC-P200-14) and (▲) the activated carbon (PAC800-1).
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Figure 7. Evaluation of the environmental impact of PAC800-1 production. Abbreviations as defined in the text.
Figure 7. Evaluation of the environmental impact of PAC800-1 production. Abbreviations as defined in the text.
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Table
Table 1. Mass yield (%) of hydrochars produced from hydrothermal carbonization (HTC) of the PPS waste.
Table 1. Mass yield (%) of hydrochars produced from hydrothermal carbonization (HTC) of the PPS waste.
HTC Temperature (°C)HTC Time (h)
1481624
18081.960.769.860.161.4
20076.971.265.276.555.9
22073.663.368.549.345.5
Table
Table 2. Elemental compositions of the solids (in weight %), (PPS) waste, and the corresponding hydrochar (HTC-P200-14) and activated carbon (PAC800-1).
Table 2. Elemental compositions of the solids (in weight %), (PPS) waste, and the corresponding hydrochar (HTC-P200-14) and activated carbon (PAC800-1).
SampleC (%)N (%)O (%)Na (%)Mg (%)P (%)K (%)Ca (%)
PPS56.48.5031.10.300.851.130.850.41
HTC-P200-1472.5-26.90.600.460.20--
PAC800-184.1-15.90.280.100.12--
Table
Table 3. Specific surface areas, and total pore volume for the (PPS) waste, a corresponding hydrochar (HTC-P200-14) and activated carbon (PAC800-1).
Table 3. Specific surface areas, and total pore volume for the (PPS) waste, a corresponding hydrochar (HTC-P200-14) and activated carbon (PAC800-1).
MaterialsSBET(N2.) m2 g−1SBET(CO2.) m2 g−1VT (cm3 g−1)
PPS0.30Not measured0.013
HTC-P200-1419.12.820.111
PAC800-138994.50.211
Table
Table 4. Life cycle inventory to produce activated carbon by physical activation.
Table 4. Life cycle inventory to produce activated carbon by physical activation.
Inputs and OutputsUnitQuantity (per Ton of PPS)Reference
Grinding
Inputs
PPSton1This work
Grinding energykWh18.50
Output
PPS powderton1
Impregnation
Inputs
Demineralized waterm3.kg−116.67
Energy usekWh88.80
PPS powderton1
Outputs
PPS impregnated in waterton1
Hydrothermal treatment
Inputs
Hydrothermal carbonization energykWh185
PPS impregnatedton1
Output
Hydrochar (76%)ton0.76
CO2 (2%)kg20[80]
Filtration
Input
Filtration energykWh4
Hydrochar ton0.76
Output
Hydrochar ton0.76
Water discharges
-Hydroxy methyl furfural
-Acetic acid		kg
kg
kg		220
66
22		[81]
Carbonization of hydrochar
Input
Energy use for carbonizationkWh 9
Hydrocharston0.76
CO2 gasmL60
Output
Activated carbon ton0.304
CO2kg336.60
COkg1.59[82]
H2Okg17.03
N2kg18.50
O2kg20.43
H2kg0.70
CH4kg0.15
Storage
Input
Storage energykWh5
Paper bag of 100 kg capacityUnit3
Activated carbonton0.304
Output
Activated carbon in stockton0.304
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Hsini, N.; Saadattalab, V.; Wang, X.; Gharred, N.; Dhaouadi, H.; Dridi-Dhaouadi, S.; Hedin, N. Activated Carbons Produced from Hydrothermally Carbonized Prickly Pear Seed Waste. Sustainability 2022, 14, 14559. https://doi.org/10.3390/su142114559

AMA Style

Hsini N, Saadattalab V, Wang X, Gharred N, Dhaouadi H, Dridi-Dhaouadi S, Hedin N. Activated Carbons Produced from Hydrothermally Carbonized Prickly Pear Seed Waste. Sustainability. 2022; 14(21):14559. https://doi.org/10.3390/su142114559

Chicago/Turabian Style

Hsini, Nourhen, Vahid Saadattalab, Xia Wang, Nawres Gharred, Hatem Dhaouadi, Sonia Dridi-Dhaouadi, and Niklas Hedin. 2022. "Activated Carbons Produced from Hydrothermally Carbonized Prickly Pear Seed Waste" Sustainability 14, no. 21: 14559. https://doi.org/10.3390/su142114559

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