materials Article Physicochemical Characterization of Cherry Pits-Derived Biochar Vladimír Frišták 1, * , Diana Bošanská 1 , Martin Pipíška 1 , Libor Ďuriška 2 , Stephen M. Bell 3 and Gerhard Soja 4,5 1 Department of Chemistry, Trnava University in Trnava, 91843 Trnava, Slovakia; diana.bosanska@tvu.sk (D.B.); martin.pipiska@truni.sk (M.P.) 2 Institute of Materials Science, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, 91724 Trnava, Slovakia; libor.duriska@stuba.sk 3 Institute of Environmental Science and Technology (ICTA-UAB), Universitat Autónoma de Barcelona, 08193 Barcelona, Spain; Stephen.Bell@uab.cat 4 Energy Department, Austrian Institute of Technology GmbH, 3430 Tulln, Austria; Gerhard.Soja@ait.ac.at 5 Institute for Chemical and Energy Engineering, University of Natural Resources and Life Sciences Vienna, 1190 Vienna, Austria * Correspondence: fristak.vladimir.jr@gmail.com; Tel.: +421-33-592-1459 Abstract: Although the suitability of some biochars for contaminants’ sorption separation has been established, not all potential feedstocks have been explored and characterized. Here, we physico- chemically characterized cherry pit biochar (CPB) pyrolyzed from cherry pit biomass (CP) at 500 ◦ C, and we assessed their As and Hg sorption efficiencies in aqueous solutions in comparison to activated carbon (AC). The basic physicochemical and material characterization of the studied adsorbents was carried out using pH, electrical conductivity (EC), cation exchange capacity (CEC), concentration  of surface functional groups (Boehm titration), and surface area (SA) analysis; elemental C, H, N  analysis; and Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy with Citation: Frišták, V.; Bošanská, D.; energy-dispersive X-ray spectroscopy (SEM–EDX). AsO4 3− anions and Hg2+ cations were selected as Pipíška, M.; Ďuriška, L.; Bell, S.M.; model contaminants used to test the sorption properties of the sorption materials. Characterization Soja, G. Physicochemical analyses confirmed a ninefold increase in SA in the case of CPB. The total C concentration increased Characterization of Cherry by 26%, while decreases in the total H and N concentrations were observed. The values of carbonate Pits-Derived Biochar. Materials 2022, and ash contents decreased by about half due to pyrolysis processes. The concentrations of surface 15, 408. https://doi.org/10.3390/ ma15020408 functional groups of the analyzed biochar obtained by Boehm titration confirmed a decrease in carboxyl and lactone groups, while an increase in phenolic functional groups was observed. Changes Academic Editors: Magdalena in the morphology and surface functionality of the pyrolyzed material were confirmed by SEM–EDX Balintova and Adriana Estokova and FTIR analyses. In sorption experiments, we found that the CPB showed better results in the Received: 22 November 2021 sorption separation of Hg2+ than in the sorption separation of AsO4 3− . The sorption efficiency for Accepted: 4 January 2022 the model cation increased in the order CP < CPB < AC and, for the model anion, it increased in the Published: 6 January 2022 order CPB < CP < AC. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: biochar; pyrolysis; sorption separation; Hg; As published maps and institutional affil- iations. 1. Introduction Aqueous organic or inorganic xenobiotics pose several threats to human society, lead- Copyright: © 2022 by the authors. ing to hazardous health issues such as respiratory allergies, skin diseases, gastrointestinal Licensee MDPI, Basel, Switzerland. complications, infertility, and cancer [1]. Organic pollutants can be metabolized, degraded, This article is an open access article or volatilized, but inorganic pollutants such as heavy metals are redispersed in different distributed under the terms and parts of the environment [2]. Although heavy metal ions occur naturally in the environment, conditions of the Creative Commons their concentrations are now often elevated due to the intensification of industrial activities. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Toxic elements enter the soil system through water bodies and subsequently infiltrate the 4.0/). food chain and, ultimately, human bodies [3,4]. As waste and wastewaters are the main Materials 2022, 15, 408. https://doi.org/10.3390/ma15020408 https://www.mdpi.com/journal/materials Materials 2022, 15, 408 2 of 14 source of heavy metals, they need to be treated to immobilize pollutants and/or convert them into less toxic forms. A large part of research is currently devoted to the development of efficient treatment technologies based on physicochemical, electrochemical, or oxidative processes. Physicochemical processes include membrane filtration, chemical precipitation, ion exchange, and adsorption. Commercially available activated carbon as a sorption material has received much of the interest in the field of wastewater treatment and purification due to its valuable characteristics and properties. Thermal stability, homogenous surface morphology, low toxicity hazard, and high porosity and, thus, specific surface area of activated carbon predetermine its various applications in technological processes [5]. However, the cost of activated carbon production is relatively high, mainly due to post-production activa- tion processes by chemical and physical methods [6]. The thermochemical conversion of feedstocks into new and innovative value-added products can be accomplished by several processes such as torrefaction, pyrolysis, hydrothermal carbonization, combustion, and gasification [7]. European Directive 2008/98/EC and Regulation 2019/1009 consider pyrol- ysis and gasification as recovery operations by the re-conversion of organic wastes [8,9]. Biochar, as the main product of thermochemical conversion techniques, is a carbon- and nutrient-rich material that is suitable for agriculture and the environment, and especially safe for humans, animals, and plants. Its increased porosity and surface area, as a result of pyrolysis conversion, allow its use as a soil fertilizer, immobilization agent, and potential new sorption material for the urgent removal of pollutants from contaminated aqueous solutions and liquid wastes [10]. A variety of feedstock materials can be used to produce biochar under strict anoxic conditions at pyrolysis temperatures of 350–700 ◦ C. The wide range of potential feedstocks includes vegetable waste, fibrous vegetable waste from virgin pulp production, processing residues, biowaste, plant residues, and dead organisms or their parts [10–15]. Therefore, in recent decades, there has been a tremendous scientific boom in the field of biochar applications. The main focus of research has been on the optimization of process parameters such as pyrolysis temperature, residence time, and feedstock origin. In general, the transformation of biomass waste into biochar and activated carbon opens up several opportunities: (1) creating a revenue stream from otherwise discarded waste; (2) increasing nutrient use efficiency and soil stability; (3) mitigating drinking water pollu- tion; (4) reducing anthropogenic environmental impacts of industrial food production. In the context of Central Europe, this biomass conversion would use waste from local sources to mediate local problems, thereby reducing the long-distance transport of wastes and byproducts to improve the environment and health of the local population. Total fresh cherry production in recent years in the EU was about 794,000 metric tons/year [16]. The consumption and processing of this fruit by the food industry produces waste in the form of cherry pits that varies in quantity from state to state. For instance, the amount of cherry pits produced in Slovakia based on statistical data was assumed to be 76 tons per year as a result of fruit consumption alone [17]. Additionally, significant quantities of cherry pits also come from other areas of the food and distillery industries. In recent years, there have been several studies on the recovery of sour cherry pits [18–20]. The oils obtained from cherry pits are recommended for use in cosmetics and cooking due to the presence of fatty acids [19]. Studies have demonstrated the potential of pyrolysis of cherry pit waste for recovery as a renewable fuel, and others for co-firing biomass with coal to generate electricity. Other studies have suggested the use of cherry pit bio-waste as catalyst supports, electrode materials, adsorbents of alkaline gases, and as soil amendments for greenhouse crops improvement [20–23]. Kahraman and Pehlivan [24] studied biochar produced from cherry pits (Prunus avium) as a sorption material for Cr6+ ions removal from aqueous solutions. The pyrolysis process of cherry seed in the presence of sodium carbonate has been optimized by Balat [25]. The potential of cherry pits-derived biochar as a tool in nutrient cycles closing was described in the work by Barber et al. [26]. The authors studied the pyrolysis product in filtration experiments with its subsequent application as a soil fertilizer. Diacon et al. [6] showed the potential utilization of chemically activated Materials 2022, 15, 408 3 of 14 cherry pits-derived biochar as carbon-based photocatalysts impregnated with different zinc salt precursors. Unfortunately, despite the potential advantages, there have been limited studies conducted on the recovery of cherry pits for use as activated carbon (specifically, a highly porous material with a wide applicability in gas and liquid purification processes and in catalysis by a two-step physical activation method) [27]. In the whole of Central Europe, while cherry pits are abundantly found as a residual product of industrial cherry production, industry and waste management utilize cherry pits very rarely. In light of this situation and the need for more information to guide future research and industrial practices, this study focused on the preparation and characterization of cherry pits-derived biochar as a starting material for activated carbon through chemical or physical activation. Prepared pyrolysis was tested in a preliminary batch sorption test with As and Hg as model pollutants present in aqueous systems in anionic and cationic forms. Mercury as one of the most hazardous and toxic elements can cause serious damage to animal and human nervous systems. Arsenic has been recognized as a highly poisonous metalloid for flora and fauna and has been classified as carcinogenic for humans. 2. Materials and Methods 2.1. Feedstock Origin and Biochar Pyrolysis Cherry pits (CPs) biomass (Natur-Haus-Kaiser, Neunkirchen, Germany) was washed in deionized water and oven-dried (60 ◦ C). To attain a representative sample, 5 kg of prepared feedstock was homogenized. Noncrushed CPs were pyrolyzed in a continual pyrolysis reactor Pyreka 2.1 (Pyreg, Dörth, Germany) at 500 ◦ C, with 20 min of residence time. To ensure anoxic conditions, N2 was used as a flushing gas (2 L/min). The obtained product, cherry pit biochar (CPB), was homogenized, crushed, and sieved to obtain the fraction of particles between 250 and 1000 µm. For comparison, the nonpyrolyzed biomass of CPs and commercially available activated carbon (AC) were homogenized and sieved to obtain the same particle size. All the obtained materials were stored in polypropylene tubes and used in characterization experiments. 2.2. Physicochemical Characterization pH and EC determination The pH values of CP, CPB, and AC were determined in deionized water (pH 7.10) and 1 mol/L KCl (pH 6.95) (v/v 1/10) by a pH multimeter after 1 h of shaking (150 rpm) and 1 h of stabilization. Electrical conductivity (EC) was determined in biochar suspension with deionized water (v/v 1/7.5) after 24 h of shaking at 45 rpm and 22 ◦ C. CEC determination Cation exchange capacities (CEC) of CP, CPB, and AC were determined with the application of BaCl2 as an extraction agent according to the ISO 11260 method modified by Frišták et al. [2] for biochar samples. Specifically, 6 mL of 0.1 mol/L BaCl2 solution was added to 0.5 g of CP, CPB, and AC each. The samples were stirred for 1 h on a laboratory shaker at 45 rpm and 22 ◦ C. After an hour, the supernatant was separated from the sedimented sample by centrifugation for 5 min at 4000 rpm. This step was repeated twice. In the next step, 3 mL of 0.025 mol/L BaCl2 solution was added to the sample in the tubes and stirred on a laboratory shaker for the next 19 h at 45 rpm and 22 ◦ C. After the desired contact time expired, the supernatant was separated from the sediment sample by centrifugation (5 min, 4000 rpm). Afterward, 3 mL of 0.02 mol/L MgSO4 solution was added to the sample and the suspensions of CP, CPB, and AC were stirred for 19 h at 45 rpm and 22 ◦ C. The supernatant separated by centrifugation (5 min, 4000 rpm) was used in the chelatometric determination of Mg cations using a standard solution of 0.02 mol/L Na2 EDTA. The value of cation exchange capacity was calculated using Equation (1): CEC = (M0 · V0 − M · VV ) · ε · 103 (1) Materials 2022, 15, 408 4 of 14 where M0 represents the molar concentration of Mg added to the sample (mol/L), V0 is the volume of Mg solution added to the sample (L), M represents the concentration of Mg in the titrated sample (mol/L), and VV is the volume of the titrated sample (L). Total carbonates content determination For the determination of carbonates as an inorganic form of carbon in CP, CPB, and AC samples, the volumetric method using a laboratory Janko’s calcimeter was used. Janko’s calcimeter is based on the measurement of the volume of CO2 released during the decom- position of carbonates by HCl according to Equation (2): CaCO3 + MgCO3 + 4 HCl → CaCl2 + MgCl2 + 2 H2 O + 2 CO2 (2) After the calcimeter preparation (filling the tube system with water to zero, closing the three-way valve), 5 g of CP, CPB, or AC sample was added into the decomposition glass flask. The smaller storage flask was filled with HCl (1:1). The reservoir flask with the rubber stopper was inserted into the digestion flask and the valve was turned to such a position that the digestion flask and the calibration tube were connected. By tilting the flask, HCl was poured onto the tested sample. The released CO2 passed through the hose into the first tube, from which it displaced water into the second (open) tube. The method was calibrated by CaCO3 standard application (Sigma Aldrich, Taufkirchen, Germany) Ash content The method according Rehrah et al. [28] to determine the amount of ash in samples of CP, CPB, and AC was used. After weighing 0.25 g of CP, CPB, and AC into cleaned and perfectly dried annealing ceramic crucibles, the samples were put in a laboratory muffle furnace at 700 ◦ C. After 60 min, the samples were transferred into a laboratory desiccator, cooled to laboratory room temperature, and weighed. The ash content was calculated according to Equation (3): ash (%) = (m700 /m0 ) · 100 (3) where m700 is the mass of the sample after combustion at 700 ◦ C in a muffle furnace, and m0 is the original mass of the sample. Elemental and surface area analysis The total C, H, and N contents in CP, CPB, and AC were determined by elemental analysis (CHN-S Elemental Analyzer, EA 1108, Carlo Erba, AIT, Vienna, Austria). Total As and Hg contents were quantified by CV-AFS and ICP-MS (Perkin Elmer, Elan DRCe 9000, AIT, Vienna, Austria) after prior digestion of the samples using the HNO3 /H2 O2 method [29]. Surface area (SA) was analyzed by the gaseous N2 adsorption method (SORPTOMATIC 1990, AIT, Vienna, Austria) using the Brunauer–Emmett–Teller (BET) adsorption isotherm for data evaluation. Surface functional groups (Boehm titration) For the determination of surface functional groups, the method according to Goertzen et al. [30] was used. In plastic tubes (25 mL), 12.5 mL of 0.05 mol/L NaOH, NaHCO3, Na2 CO3 , and HCl was added to 0.25 g of CP, CPB, and AC samples. The tubes were sealed and stirred on a laboratory orbital shaker (Orbital shaker Multi-RS 60, Biosan, Latvia) for 48 h at 45 rpm and 22 ◦ C. At the end of the stirring period, 10 mL from the resulting suspension was removed and transferred to new tubes to which 30, 20, and 30 mL of 0.05 mol/L HCl were added (the addition of the acid matched the above solutions at the beginning of the experiment, with no solution added to the supernatant with HCl). The solutions were stripped of excess CO2 by bubbling with N2 gas for 25 min before titration. After addition of the indicator (methyl red), the sample was titrated with a 0.05 mol/L NaOH solution until the yellow color transition. All measurements were performed in triplicate. The resulting concentrations of carboxyl, lactone, phenolic, and total base surface functional groups were calculated by the following mathematical relationships, following Equations (4)–(6):   V0 Va ·C0 − Vt(NaOH) · Ct(NaOH) n(HCl) = (4) m · Va Materials 2022, 15, 408 5 of 14 h  i V0 (Va ·C0 − VHCl ·CHCl − Vt(NaOH) ·Ct(NaOH) n(NaOH/NaHCO3 ) = (5) m · Va  V ·C −V t(NaOH) ·Ct(NaOH) h i HCl HCl 2V0 (Va ·C0 − 2 n(Na2 CO3 ) = (6) m · Va where n represents the concentration of functional groups on the surface of the CP, CPB, and AC (mmol/g), V0 is the volume of the initial NaOH/NaHCO3 /Na2 CO3 /HCl solution (mL), C0 represents the concentration of the solutions with volume V0 (mol/L), Va is the volume of the solution taken after 48 h of stirring (mL), and VHCl and CHCl are the volume (mL) and the concentration (mol/L) of the titrant (NaOH solution), respectively. The NaOH solution was used to neutralize carboxyl, phenolic, and lactone functional groups. The concentration of carboxyl functional groups was calculated as the difference of n(NaOH) and n(NaHCO3 ), or n(NaOH) and n(Na2 CO3 ), which results in the concentration of carboxyl and lactone functional groups. FTIR analysis Spectral analysis in the infrared region was used to determine the abundance of each functional group on the CP and CPB surface. AC analysis was not feasible, due to the nature of the sample itself. Prior to analysis, representative fractions of CP and CPB samples (0.25–0.1 mm size fraction) were dried at 60 ◦ C for 24 h to remove excess moisture. FTIR analysis was performed using an iS50 FT-IR spectrometer in conjunction with an ATR module, measuring 32 scans, at a resolution of 4 cm−1 in the wavenumber range 4000–450 cm−1 . SEM–EDX analysis The surface morphology of CP, CPB, and AC samples was monitored using scanning electron microscopy (SEM) as well as electron-dispersive X-ray analysis (EDX) to map the abundance of individual elements on the surfaces of the materials. The analysis was carried out using a JEOL JSM 7600F electron microscope (Akishima City, Tokyo, Japan) at 20 kV, a vacuum pressure of 9.0 × 10−4 Pa, and magnifications of 250× and 1500×. 2.3. Batch Sorption Test The sorption potential of the CP, CPB, and AC was verified in model experiments using selected Hg2+ and AsO4 3− sorbates. The sorption experiments were carried out in a batch experimental design according to OECD Methodology No. 106 [31]. In short, 0.25 g of CP, CPB, and AC was added into plastic (for As) and glass (for Hg) tubes, and we pipetted 7.5 mL of AsO4 3− solution with a concentration of 0.2 mmol/L and Hg2+ solution with a concentration of 0.5 mmol/L into these tubes. Stock solutions were prepared from HgCl2 and Na2 HAsO4 . The pH value of both solutions was in the range of 5.5–6.0. The samples were stirred on a laboratory orbital shaker (Orbital shaker Multi-RS 60, Biosan, Latvia) at 45 rpm, for 24 h at 22 ◦ C. After the contact time, the supernatant was separated from the sedimented sample by centrifugation and filtrated through a 0.45 µm nylon filter. The concentration of Hg and As sorbed by CP, CPB, and AC-based adsorbents was determined from the Hg and As present in the supernatant. The quantification of both elements was carried out using an EcaFlow electrochemical analyzer (ISTRAN, Slovakia). All measurements were performed in triplicate. The basic relationship below, Equation (7), was used to calculate the sorption efficiency of Hg and As adsorbents based on CP, CPB, and AC with a fraction size of 0.25–0.1 mm: ( c0 − cm ) η (%) = ·100 (7) c0 where η represents the adsorption efficiency (%), c0 is the concentration of Hg or As before the sorption experiment (at time t0 ) (mg/L), and cm is the concentration of Hg or As after the sorption process (at time t = 24 h) (mg/L). where η represents the adsorption efficiency (%), c0 is the concentration of Hg or As before the sorption experiment (at time t0) (mg/L), and cm is the concentration of Hg or As after Materials 2022, 15, 408 the sorption process (at time t = 24 h) (mg/L). 6 of 14 2.4. Electrochemical analysis of Hg and As The supernatant 2.4. Electrochemical samples Analysis of Hgobtained and As after the sorption experiments were analyzed for freeThe As supernatant and Hg content using an EcaFlow samples obtained after150theelectrochemical analyzerwere sorption experiments withanalyzed an E-T/Au for free As and Hg content using an EcaFlow 150 electrochemical analyzer with For electrode, which operates on the principle of dissolution chronopotentiometry. the an E- actual determination, the application sheets and the manufacturer’s recommended T/Au electrode, which operates on the principle of dissolution chronopotentiometry. For meth- odology the for the determination actual determination, of total As the application in water sheets samples and the (Application manufacturer’s Sheet No. 41, recommended ISTRAN, Slovakia) methodology and low concentrations for the determination of total AsofinHg in water water samples samples (Application (Application SheetSheet No. No. 41, 51, ISTRAN, ISTRAN, Slovakia) Slovakia) wereconcentrations and low applied (Figureof1). Hg in water samples (Application Sheet No. 51, ISTRAN, Slovakia) were applied (Figure 1). Figure Characteristic 1. 1. Figure chronopotentiogram Characteristic chronopotentiogram forfor the determination the determinationofofAsAs(A) and (A) Hg and (B) Hg inin (B) super- super- natant samples after sorption by CP, CPB, and AC-based adsorbents. natant samples after sorption by CP, CPB, and AC-based adsorbents. 3. 3. Results Results and and Discussion Discussion 3.1. Material Characterization 3.1. Material Characterization The resulting pH values of the samples were in the alkaline range for commercial The resulting pH values of the samples were in the alkaline range for commercial activated carbon and biochar prepared from cherry pits (Table 1). Due to the pyrolysis temperaturecarbon activated (500 ◦ C)and biochar used prepared to prepare from cherry the biochar, it was pits (Table expected 1).the that DuepHtovalues the pyrolysis of the temperature (500 °C) used to prepare the biochar, it was expected biochar would be >9. Jindo et al. [32] reported very similar pH values for biochar that the pHsamples values of the biochar prepared fromwould apple be >9. Jindo twigs, et al.oak, rice husk, [32] and reported very similar rice straw, pH pyrolysis and as the values fortemperature biochar sam- ples prepared from apple twigs, rice husk, oak, and rice straw, and increased, the pH values also increased. This phenomenon likely takes place due to the as the pyrolysis tem- perature relative increased, the concentration pH values also of unpyrolyzed increased. inorganic This phenomenon elements present in thelikely takesbiomass. original place due to the relative concentration of unpyrolyzed inorganic elements present The pH value of the cherry pit biomass was in the acidic region, which was also reflected in the original biomass. The pH value of the cherry pit biomass was in the acidic region, when the concentrations of functional groups on the surface of the sorbents were detected which was also reflected when the concentrations of functional groups on the surface by Boehm titration (Table 2). Cherry pit biomass showed higher concentrations of lactone of the sorbents were anddetected carboxyl by functional Boehm titration groups, (Table while2).anCherry pitin increase biomass phenolic showed surfacehigher concentrations functional groups wasof lactone observed andin carboxyl CPB. Thefunctional potential groups, pH values while were anin increase in phenolic the order CP < CPB surface < AC,functional toward thegroups alkalinewas observed in CPB. The potential pH values were in the order CP < CPB < AC, region. toward the alkaline region. Materials 2022, 15, 408 7 of 14 Table 1. Physicochemical properties of CP, CPB, and AC. CP CPB AC pHH2O 6.13 ± 0.25 8.88 ± 0.36 8.54 ± 0.26 pHKCl 5.71 ± 0.32 6.69 ± 0.20 6.74 ± 0.27 EC (µS/cm) 248.55 ± 15.92 19.51 ± 1.40 19.45 ± 1.51 Ash content (%) 2.12 ± 0.11 1.61 ± 0.08 0.95 ± 0.20 CO3 (%) 7.9 ± 0.2 2.4 ± 0.1 ND 2 CEC (meq/100 g) 12.06 ± 0.53 28.21± 0.59 45.87 ± 0.98 SA (m2 /g) 10.31 94.12 852 C (%) 47.74 74.41 92.81 H (%) 9.45 2.14 0.11 N (%) 12.14 4.87 0.88 As (mg/kg)