The Influence of Biowaste Type on the Physicochemical and Sorptive Characteristics of Corresponding Biochar Used as Sustainable Sorbent
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1. Introduction
The use of biomass as a natural resource to produce carbonaceous materials under thermal conditions contributes to long-term environmental protection by purifying aqueous systems and reducing CO2 emissions while favoring sustainability. Biochar is a valuable product obtained from biomass heated under different pyrolytic conditions [1]. Pyrolysis is a thermal process through which organic substances decompose under a limited or very low oxygen atmosphere at a temperature range of 300 to 1300 °C. Biochar can be incorporated into soil as a soil amendment [2], providing organic matter amounts which prevent the physical breakdown of soil [3].
Past studies have shown that biochar obtained at high temperatures (750–900 °C) presents higher surface area and porosity compared to that produced at lower temperatures (300–600 °C) [4]. Another study also demonstrated that biochar yield decreases with increasing pyrolysis temperature [5]. Rasa et al. showed that pyrolysis temperature is a vital parameter affecting produced biochar [6]. Besides the pyrolysis temperature, the biomass type is another factor that determines the production of biochar with desirable properties. The lignin, cellulose, and hemicellulose contents of biomass used for biochar production affect the structure of the final product, whereas inorganic components result in biochar with high ash content and low volatile matter [7]. Both the chemical composition and the physical properties, such as pore size, surface area etc., of biochar depend on the production conditions, such as pyrolysis temperature, atmosphere, and time, along with the feedstock characteristics [8]. Therefore, the use of suitable raw materials and pyrolysis conditions are essential for the production of high performance and low-cost biochar [9,10]. This environmentally friendly material shows great potential as a pollutants (heavy metals, dyes, etc.) sorbent for use in the purification of water. The advantages of biochar for use in place of other materials, such as activated carbon and inorganic sorbents, in the efficient removal of toxic heavy metals from aqueous systems has been extensively reviewed by Shakoor et al. [10].
Coffee is a widespread product and the most famous beverage around the world [11]. In 2021, coffee exports increased by 0.11 million bags compared to 2020 and reached 11.4 million unit [12]. Spent coffee grounds contain several classes of compounds, with polysaccharides comprising 50% of the total mass [13]. Greek coffee is a strongly brewed coffee that is found everywhere in Greece. It is similar to the coffee served in surrounding countries in the Middle East and is an integral part of the country’s culture. Greek coffee is served with grounds in the cup. The grounds are allowed to settle as the coffee is slowly sipped. Therefore, large amounts of Greek coffee sediments end up in sewers without any form of treatment. Greek coffee contains large amounts of organic compounds [14]. Coffee sediments are a widely known pollutant and are hardly treated by conventional municipal wastewater treatment plants due to the fragmentary treatment of caffeine by microorganisms developed in these facilities [15].
Grapes are one of the most popular fruits worldwide. Approximately 75 million tons are produced every year, and 35 million tons are used to produce wine [16,17]. Grape seeds are considered a major winery waste product, comprising approximately 17% of the grape pomace mass [18]. In Greece, in 2021, approximately 695.6 thousand tons of grapes were produced [19], which corresponds to approximately 14 thousand tons of grape seeds in an annual base. These quantities render grape seeds among the popular biowastes found in Greece. Thus, the thermal degradation of grape seeds at elevated temperatures under oxygen-limited conditions seems to be a promising treatment for producing biochar with complex matrices. Grape seeds, after wine production, are also used to produce tsipouro, a Greek national distillate drink. Commercial tsipouro contains alcohol lower than 50% volume after water dilution [20]. Over the last decade, the tsipouro market has increased in size rapidly, producing 18.5 million L in 2012 (IWSR) in Greece. Seeds are the main waste product, among others (e.g., marcs), that should be treated after tsipouro production. However, several studies have shown that tsipouro contains many volatile compounds at significant concentrations [21]; hence, the thermal treatment of seeds remaining after tsipouro production is an issue that could be further explored.
Rice constitutes one of the largest crops in the world, with more than 500 million tons being produced every year. In Greece, the annual national rice harvest is estimated at 0.024% of the global rice production (approximately 120 thousand tons per year). Considering that rice husk represents 20% of the total amount of the grain produced [22], the annual production of rice husk in Greece is estimated approximately at 24 thousand tons per year. Hence, rice husk is an abundant biowaste which can be easily found in the Greek countryside. The high concentration of SiO2 in rice husk compared to other biowastes, with SiO2 comprising 96% of the inorganic compound content in the organic matrix of rice, makes it a favorable feedstock for the production of promising and cost-effective materials such as silica gel [23].
Although the aforementioned biowastes are not toxic, their management is not particularly environmentally friendly. While several extensive articles have reported on the physical and chemical characteristics of biochar obtained from spent coffee grounds [24,25] and rice husk [26], limited literature is available on the physicochemical characteristics of biochar derived from Greek coffee sediment, grape seeds after wine production, and grape seeds after distillation for tsipouro production.
From the above, it is obvious that biochar sorbents produced from biowastes satisfy the principles of circular economy concept, resulting in useful low-cost materials with significant environmental applications. The aim of the current study was to investigate the influence of various types of biowaste on biochar physicochemical and sorptive characteristics in a water purification context. Biochar samples obtained from five biowaste materials, processed via pyrolysis under a limited oxygen atmosphere, were studied. These biowastes, besides their different origin, also underwent different treatments. Rice husk and grape seeds for wine production underwent only mechanical treatment, Greek coffee sediment and grape seeds from tsipouro production underwent strong boiling, and espresso coffee grounds underwent instant contact with hot water. The main objectives of this study are: (a) evaluation of the biochar yield from each raw material, (b) comparison of the physicochemical properties of the different raw materials and biochar samples produced, and (c) investigation of the methylene blue (MB) sorption capability of the studied materials. MB was selected as a probe water pollutant representing cationic dyes found in the effluents of textile industries.
This study aims to increase sustainability and targets different United Nations Sustainability Development Goals (SDGs). It will positively affect SDG 6: Clean Water and Sanitation by improving the index related to anthropogenic wastewater that receives treatment. It also targets SDG 16: Life on Land by sustainably using terrestrial ecosystems, and not for disposing biowaste. SDG13: Climate Action is also targeted, specifically through not allowing the production of CO2 emissions resulting from the degradation of biowaste in disposal sites.
2. Materials and Methods
Material production. Spent coffee grounds (SCG) and sediment from Greek coffee (SGC) were obtained from a coffee shop located on the campus of the University of Patras, Greece. Grape seeds after wine production (GSW) were obtained from Patraiki winery, Patras, Greece. Grape seeds after distillation for tsipouro production (GST) were obtained from a local distillery in the region of Patras, Greece, and rice husk (RH) was sourced from Agrino Company, Agrinio, Greece. The raw materials were oven dried in a Memmet oven at 50 °C for several hours until the moisture was removed and the constant weight of the solid was obtained. The method for biochar production was based on Manariotis et al. [4]. Dried samples were weighed and placed in a custom-made ceramic saggar box, closed with a cap to prevent oxygen from entering the vessel, and pyrolyzed at 850 °C for 1 h in a large electric furnace with a heating range of 30–3000 °C (Type Nabertherm, Controller B 180, Lilienthal, Germany). The pyrolysis temperature of 850 °C for biochar production using several forms of biowaste was chosen according to previous studies [4] to achieve the highest specific surface area (SSA) of biochar. Produced biochar was weighed and yield was calculated according to Equation (1):
Biochar yield (%) = (mass of biochar/mass of dry raw material) × 100%
Ash content. The measurement of ash content was conducted using a standard method based on the findings of Kalaitzidis et al. [27]. For each of the examined samples, 1 g of sample was taken into a pre-calcined crucible for calcination at 750 °C in a furnace for 2 h. The crucible was then cooled to room temperature and reweighed. The ash content was calculated based on the Equation (2):
ash (%) = (mass of ash (g)/dry mass of sample (g)) × 100%
To check the reproducibility of the results, duplicate samples were run.
X-ray diffractions (XRD) analysis. X-ray diffraction examination was performed on the materials studied, and the corresponding patterns were recorded in a 2θ range of 5–70° using a Bucker D8 Advance Diffractometer equipped with a nickel-filtered Cu Kα (1.5418 Å) radiation source. XRD analysis was performed to investigate any possible crystal phase in the materials.
Thermo-gravimetric analysis (TGA). A thermo-gravimetric analyzer was used to reveal the weight loss characteristics of biochar. For each material, 7 mg of powdered sample was heated at a heating rate of 20 °C·min−1 from room temperature to 1000 °C in a Perkin Elmer, Diamond TGA/DTA instrument. To check the reproducibility of the results, duplicate samples were run.
SEM analysis. Morphology visualization, along with the elemental composition of samples, were examined at different magnifications with a JEOL 6300 (SEMJEOL JSM6300, Tokyo, Japan) equipped with an X-ray energy dispersive spectrometer accessory. Photomicrographs were taken in the range of 1.36 to 27.08 K X magnification.
Surface area and porosity. The determination of the specific surface area (SSA), the external surface area, the pore volume, the micropore volume, and the average pore size for each sample was performed using N2 adsorption−desorption isotherms recorded in a Micromeritics TriStar 3000 Analyzer system. Before analysis, the raw materials were degassed at 60 °C under mild nitrogen flow for 2 h, and biochar was degassed at 300 °C under mild nitrogen flow for 1 h.
Suspension pHeq. The suspension pH (pHeq) of raw materials and produced biochar samples was measured according to Manariotis et al. [4]. Approximately 20 mL of an electrolyte solution containing 0.1 M NaNO3 (pH = 7.0) and 0.32 g of each sample were placed in 40 mL glass bottles. The suspension pH of the supernatant was measured after 24 h using a portable multi-parameter pH-meter (Consort C862) equipped with a glass electrode. To calibrate the glass electrode, two different pH buffer solutions, with pH equal to 7.00 ± 0.02 and 4.00 ± 0.02 at 25 °C, were used.
Functional group analysis. The chemical characteristics of the raw materials and produced biochar were identified using a PerkinElmer FTIR spectrometer. Approximately 0.5 mg of dried sample and 50 mg of dried KBr were mixed and turned into a pellet. The wavenumber measurement range was 4000–400 cm−1 and was analyzed by IRSearchMaster 6.0 software.
Sorption tests. Methylene blue (MB) solutions were prepared in synthetic freshwater based on Karapanagioti et al. [28]. Approximately 44 mg·L−1 of CaCl2·2H2O, 14 mg·L−1 of CaSO4, and 17 mg·L−1 of NaHCO3 were added to 4 L of deionized water to adjust the ionic strength and salinity of the solution. MB adsorption experiments were carried out in 40 mL glass vials. A stock solution of 20 mg·L−1 of MB was prepared in synthetic freshwater. About 3 mg of each sorbent (raw materials and biochar samples produced) were placed into the glass vials, and MB stock solution was added. Blanks were also prepared containing only the MB stock solution without sorbent. For all samples, measurements of the aqueous concentration were taken after 24 h and at various time periods up to 137 days for some samples. All samples and blanks were prepared in triplicates.
Aqueous MB concentration was determined by a HACH DR/2400 portable spectrophotometer (Loveland, CO, USA), using a cuvette of 1 cm at 670 nm. The amounts of MB sorption for the tested samples were calculated by Equation (3):
where qt (mg·g−1) is the sorption capacity of the solid at sampling time t, Ct (mg·L−1) is the aqueous concentration of MB at sampling time t, Co (mg·L−1) is the initial aqueous concentration of MB (determined by the blank sample to account for possible method MB losses), also measured at time t, V (mL) is the volume of the MB stock solution used, and m (g) is the sorbent mass added in the vials of 40 mL.
In the current study, the MB adsorption kinetics were well described by a pseudo-first order Equation (4):
where k1 is the constant of the pseudo-first-order rate (min−1), qt is the amount of MB adsorbed per mass of sorbent at time t (mg·g−1), and qe is the amount of MB adsorbed per mass of sorbent at equilibrium (mg·g−1).
4. Conclusions
Biochar samples obtained via pyrolysis of biowaste (spent coffee grains, sediment of Greek coffee, grape seeds remaining after wine production, grape seeds remaining after tsipouro production, and rice husk) exhibit high specific surface areas, low crystallinity, and low population of functional groups. Although these biochar samples are mainly micro-porous materials, they have a significant fraction of pores in the meso-porous range. The specific surface area of the latter pores proved very important for the physical adsorption of methylene blue from aqueous solution. The raw materials with very low SSA had low MB sorption capacity, ranging from 29 to 54 mg·g−1, whereas the biochar samples, with exception of GSW-B (58 mg·g−1), exhibited remarkable MB sorption efficiency, ranging from 99 to 370 mg·g−1. The sorption efficiency trend of the biochar samples is the same as that followed by their SSAExternal values (SGC-B > SCG-B > RH-B > GST-B > GSW-B). This study revealed that different biowastes can be transformed to biochar with interesting sorption properties, satisfying the principles of circular economy. Among the biochar samples studied, those produced from coffee residues proved very promising for MB removal from water solutions.
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