Green synthesis of Quercus coccifera hydrochar in subcritical water medium and evaluation of its adsorption performance for BR18 dye

Water pollution is a global issue (Saleh et al. 2019), and the textile industry is a major source of pollutants as it generates hazardous and colored wastewater that has harmful effects on human life and the environment (Patel & Vashi 2015). Textile wastewater is 3 billion tons over the world (Antczak et al. 2019; Li & Wang 2019). Cationic dyes like Basic Red 18 (BR18) reduce the transparency of water and therefore prevent sunlight from traveling through the water column, which in turn negatively affects the flora and fauna present (Nagpal & Kakkar 2020). Chong et al. (2014) stated that cationic dyes are carcinogenic and do not degrade naturally.

So far, many researchers have tried to treat dyes in wastewater using different methods. For the biological treatment systems, bacteria (Pearce et al. 2006; Lavanya et al. 2014), algae (Elumalai & Saravanan 2016), fungi (Krastanov et al. 2013), enzymes (Darwesh et al. 2019), and yeast (Martorell et al. 2018) have been used in the biodegradation of dyes. Chemical treatment methods have also been used: advanced oxidation processes (AOPs) (Al-Kdasi et al. 2004; Miklos et al. 2018), coagulation-flocculation (Verma et al. 2012; Dotto et al. 2019), electrocoagulation (EC) (Naje et al. 2016), electro-Fenton (EF) (Rosales et al. 2009), and anodic oxidation (AO) (dos Santos et al. 2019). Except for electrochemical methods, chemical treatment is more expensive than biological methods (Crini & Lichtfouse 2019). The mass transfer concept has also been used to remove dyes from wastewater using physical processes. They include the adsorption (Rafatullah et al. 2010), membrane filtration (Jegatheesan et al. 2016), and ion-exchange methods (Bayramoglu et al. 2020). The low cost, simplicity of design, high efficiency, and easy operation make the adsorption process more favorable than others (Asfaram et al. 2017).

The principle of the adsorption process is the attachment of the adsorbate onto the surface of the solid adsorbent. Based on the manner of attachment, the adsorption process can be categorized into physisorption or chemisorption (Gupta & Suhas 2009). Different materials have been used as adsorbents for dye removal: activated carbon (Tan et al. 2008), alumina (Banerjee et al. 2019), zeolite (Brião et al. 2018), bio-adsorbents (Gupta et al. 2014), biochar (Biswas et al. 2020), and subcritical water hydrolysis (Abaide et al. 2019).

Biochar is a stable carbon-rich solid obtained by different methods from different types of biomass. Subcritical water medium (SWM) has been used as an effective medium in synthesis, extraction, oxidation, solubility processes, and many other fields. It also provides a unique medium for converting carbonaceous materials into hydrochar (Nural et al. 2018; Yabalak 2018a; Lee et al. 2019). Subcritical water is defined as water heated in range of 373–647 K and pressurized enough to keep the water liquid at this temperature (Yabalak 2018b). Conversion of biomass to hydrochar in SWM is an advantageous alternative to classical torrefaction, pyrolysis, or gasification because it provides a high conversion yield of biomass to hydrochar and water-soluble components of the biomass that cannot be obtained by other methods (Chuntanapum & Matsumura 2009; Kumar & Gupta 2009). In addition, subcritical water provides a reactive medium based on its molecular properties and SWM is more economical since it does not require a pre-drying step, with its attendant high costs (Chuntanapum & Matsumura 2009). Recently, the production of biochar, especially hydrochar, and their usage in various processes in environmental and agricultural applications have received great attention. Furthermore, it has been reported that biochar is highly effective for removing many organic pollutants from water and soil (Fang et al. 2014). To the best of our knowledge, this is the first attempt to investigate the production of hydrochar from Quercus coccifera L. in unique eco-friendly SWM as well as evaluating its effects as an adsorbent for the removal of dye contaminant in aqueous solution.

Q. coccifera acorns were collected on 21 October 2018 from an open field in Akkent (Yenişehir, Mersin), which is 20–40 m above sea level. The complete fruit (nut and cup) was washed with distilled water to remove the resin and other impurities and kept at room temperature in the dark for 20 days until completely dry.

A home-made stainless-steel reactor, which was described in detail in previous work, was used for the production of hydrochar in the SWM (Yabalak 2018b). The production process is schematized in Figure 1. Briefly, 30 g of dried Q. coccifera acorns were put into the reactor along with 150 mL of pure water. After closing the reactor, the inner pressure was raised to 100 bar. The temperature of the reactor was increased to 513 K and kept constant for 3 hours. At the end of the treatment time, the reactor was depressurized and opened. The obtained hydrochar was filtered through ordinary filter paper and dried in an oven at 373 K. 21.8 g of dry hydrochar was obtained, meaning a 72.8% hydrochar yield from the Q. coccifera. This value is quite high compared to pyrolysis, dry torrefaction, gasification, and even hydrothermal carbonization methods reported in literature (Lee et al. 2019).

In this study, a cationic dye (Basic Red 18; BR18) was examined. The properties of BR18 dye are shown in Table 1. A stock solution (1,000 ppm) was prepared and kept until the end of the experiments. The proposed concentrations were diluted from the stock solution. Sodium hydroxide (NaOH) and sulfuric acid (H₂SO₄) with different molarities (1–5) M were used to adjust the pH.

Brunauer-Emmett-Teller (BET) analysis was employed to determine the surface area, total pore volume, microporous volume, and pore diameter using adsorption and desorption isotherms of N₂ gas (MicroActive for TriStar II Plus 2.00). The adsorbent surface area was determined by plotting the linear form of the BET equation. The total volume was calculated at the relative pressure (p/p₀) 0.99 (Schneider 1995). The microporous volume and the pore diameter were determined by the method of Boer and by the ratio of 4 V_total/BET surface, respectively (Barrett et al. 1951).

The adsorbent surface morphology was observed by scanning electron microscope (SEM; Zeiss, Supra 55).

Fourier transform infrared spectroscopy (FT/IR-6700, Jasco) was utilized to investigate the functional groups present on the surface of the adsorbent. The functional groups were scanned for bands between 450 and 4,000 cm⁻¹.

The surface charge changes of the prepared hydrochar during the pH optimization were measured using zeta potential (Malvern Zeta Sizer Nano ZS).

The effects of pH (2–10), adsorbent size (45–150 μm), adsorbent dose (0.5–1.5 g/L), dye concentration (50–455 mg/L), and contact time (5–120 min) were studied via optimization experiments. The optimum conditions were obtained by studying the factors one by one.

The adsorption isotherm is important for clarifying the interaction between the adsorbent and the adsorbate. Isotherm experiments were carried out by changing the adsorbent dose while the other factors were kept unchanged. The initial concentration, final concentration, the adsorption capacities were plotted using the linear form of the Langmuir (Langmuir 1918), Freundlich (Freundlich 1906), Tempkin (Tempkin & Pyzhev 1940), Dubinin-Radushkevich (D-R) (Dąbrowski 2001), Redlich-Peterson (Redlich & Peterson 1959), and Sips (Blanchard et al. 1984) equations, which are shown in Table 2.

The prepared hydrochar was sieved <45 μm, 45–106 μm, and >106 μm and analysed by BET analysis using the adsorption and desorption of N₂ gas. Three different particle sizes were analysed to determine the physical properties of the adsorbent. BET surfaces areas, the total, micropore and mesopore volumes of the pores, microporous volumes, and the pore diameters were measured; they are shown in Table 3.

As shown in Table 3, the adsorbent with a particle size of less than 45 μm has the maximum surface area, with a value of 25.2718 m²/g. Also, the total pore volume is inversely related to particle size. The maximum pore diameter is 17.88 nm, which is larger than the BR18 molecule. This reflects that the dye can be adsorbed by the pores of the adsorbent.

The hydrochar surface morphology was also identified by SEM analysis before and after adsorption to note the changes, which are shown in Figure 2. As shown in Figures 2(a) and 2(b), the adsorbent has a heterogeneous, rough, and porous surface. The SEM results also confirm the adsorption of BR18 onto the surface of the prepared hydrochar since the pores were filled and no pores were visible after the absorption process (Figures 2(c) and 2(d)).

Energy dispersive X-ray analysis (EDX) was carried out to determine the elements in the adsorbents. A comparison between the raw adsorbent and the adsorbent at the end of the adsorption process is shown in Table 4. Carbon, nitrogen, and oxygen are the dominant elements in the adsorbent. The concentrations of nitrogen and chlorine increased by the end of the adsorption process. This is further proof that the adsorption of BR18 onto the Q. coccifera acorn hydrochar had occurred successfully, as BR18 has nitrogen and chlorine elements in its structure.

FTIR analysis for the adsorbent on its own and with the dye was performed and the results are shown in Figure 3. There are some changes between the FTIR for the adsorbent before and after it. The alkene compound class (functional group C = C bending) observed at peak 790 cm⁻¹ before adsorption disappeared after it. Also, the peak at the band 1,428 cm⁻¹ with functional group O-H bending was not visible at the end of the adsorption process. In contrast, three peaks were recorded for the band from 1,141 to 1,336 cm⁻¹. The peak at the band 1,141.65 cm⁻¹ can be related to an amine compound with a functional group of C-N stretching. The functional group C-O stretching was also visible at the band of 1,213.01 cm⁻¹. The peak 1,336 cm⁻¹ was related to the O-H bending functional group. In addition to these changes, peak shifting was visible at many bands. The FTIR results support the adsorption process.

The concentration of hydrogen ions (pH) is the most important parameter in the adsorption process. It controls the protonation and deprotonation process on the adsorbent surface. To explore the effect of pH on the removal efficiency of the BR18 by adsorption, 1 g/L of the adsorbent was added to 100 mg/L of the dye concentration at constant temperature (25 ± 2 °C) and agitated at a speed of 150 rpm. The effects of pH on the adsorption capacity and removal efficiency are shown in Figure 4(a). The dye uptake and the adsorption capacity increased with increasing pH. The removal efficiency increased from 2% at pH 2 to 72% at pH 10. The capacity increased from 2 mg/g to 80 mg/g.

The zeta potential analysis for the adsorbent and the adsorbate under different pH values was studied. The adsorbent and the adsorbate were dissolved in 25 mL of distilled water and measured using a zeta meter (Malvern Zeta Sizer Nano ZS). The zeta potentials for the adsorbent at all pH values were negative (Figure 4(b)), while for the adsorbate they were positive at all the measured pH values. Because the adsorbent and the adsorbate have different charges, they will be an attractive force between them. The values of the zeta potential for the adsorbent sharply decreased from −5.87 mV at pH 2 to −43.93 mV at pH 10. As the zeta potential for the adsorbent decreased with increasing pH, the attractive force between the hydrochar and BR18 increased, which resulted in the increasing removal efficiency.

The effect of the adsorbent's particle size on the adsorption efficiency was investigated. Three different particle sizes were used in the experiments while the other parameters were kept constant (dye concentration: 110 mg/L, contact time: 60 min, adsorbent dosage: 1 g/L, and volume: 50 mL). The results showed that the maximum removal efficiency was obtained when the particle size was smaller than 45 μm, with an efficiency of 89.1% (Table 5). These results can be explained by the surface area of the adsorbent. As shown in BET results, the particle sizes have an inverse relationship with the surface area.

The effect of the adsorbent amount on the removal effıcency was investigated. Hydrochar was added in different quantities (0.5, 1.0, 1.5 g/L) into solutions with a concentration of 125 mg/L BR18. The results show that the maximum removal efficiency occurred at 1.5 g/L (Table 6).

The effect of contact time (0–120 min) on the uptake of BR18 by Q. coccifera acorn hydrochar at an initial concentration of 280 mg/L was studied and the results are shown in Figure 5(a). The removal efficiency increased sharply for the first 5 min, after which the removal efficiency reached a steady state at 60 min, with a maximum removal of 87.9%. After that, the removal efficiency did not change and hence 60 min was chosen as the optimum contact time. The removal of BR18 from the aqueous solution had occurred in two steps: the rapid step (first 5 min) when the active sites on the adsorbent surface were empty; as the number of active sites decreased (occupied by the dye), the removal speed decreased, which was the second step.

Initial concentration is one of the most important factors affecting the adsorption process. The adsorption capacity for the prepared hydrochar at different initial concentrations is shown in Figure 5(b). The capacity increased sharply with the increase in initial concentration of BR18 up to 280.8 mg/L. Then the graph became flatter. This can be related to the availability of active sites. The capacity increased from 20.9 mg/g at a concentration of 40 mg/L to 194.6 mg/g at a concentration of 454 mg/L.

The obtained data were fitted to Lagergren's pseudo-first-order and pseudo-second-order models to determine the adsorption kinetics. The linear form of Lagergren's pseudo-first-order and pseudo-second-order expressed in Equations (3) and (4) were fitted at different concentrations to obtain the maximum correlation coefficients. The chi-square errors (X²) were tested using the SciDavis program to study the linearization errors. Table 7 shows the kinetics model parameters and also shows the X² results. Because of the high values of the correlation coefficients, the X² errors being very small, and the calculated capacities being very close to the experimental capacities, the adsorption of BR18 onto Q. coccifera acorn hydrochar is fitted to the pseudo-second-order model (Figure 6). According to this model, the rate of the adsorption is proportion to the available sites on the surface of the adsorbent.

According to Guo & Wang (2019), the pseudo-second-order model can occur in three ways: at low concentrations, at the final stage of adsorption, and when the adsorbent has abundant active sites (Qiu & Zheng 2009). The fitting of the current study to the pseudo-second-order model can be due to BR18 attaching to the active sites present on the surface of the prepared adsorbent. Unfortunately, neither Lagergren's pseudo-first-order nor pseudo-second-order models predict solute diffusion into the adsorbent.

The intraparticle diffusion model was used to determine the adsorption mechanism of BR18 onto Q. coccifera acorn hydrochar. The square root of time was plotted against the adsorption capacity, as shown in Figure 7. The intraparticle diffusion model plot shows multiple linear sections, but with none passing through the origin. The multilinearity reflects the effects of different mechanisms. The first mechanism indicates film diffusion, which involves BR18 moving slowly from the boundary layer to the surface of the Q. coccifera acorn hydrochar. The movement from the surface to the adsorbent pores and the attachment to the active pores reflects the second and third mechanisms. As a result of the intraparticle diffusion model graph, the main mechanism of the adsorption of BR18 onto Q. coccifera acorn hydrochar is film diffusion (graph has not passed through the origin), and intraparticle resistance has a role in the adsorption (because of multi-linearity).

The isotherms studies were carried out by exposing different dosages of the adsorbent to an initial BR18 concentration of 125 mg/L. The adsorbent and the adsorbate were agitated at 155 rpm and 25 ± 2 °C. The samples were collected and fitted to the aforementioned isotherms. The isotherm parameters are shown in Table 8. In this study, the experimental data were fitted according to the four-parameter isotherms (Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich (D-R)) and based on the three-parameter isotherms (Redlich-Peterson and Sips). The Langmuir isotherm has a correlation coefficient of 0.8988. The Langmuir assumes the adsorption process will reach the maximum capacity when a saturated monolayer is formed. The Freundlich isotherm has the highest correlation coefficient of 0.9527. This isotherm is an empirical isotherm, which assumes heterogeneous multilayer formation at the end of the adsorption process. The Tempkin isotherm (R² = 0.9192) includes the heat of the adsorption process. It assumes a linear inverse relationship between the heat of the molecule in the same layer and the surface coverage. The Dubinin-Radushkevich has the lowest coefficient, with an R² of 0.6980. The Redlich-Peterson (R² = 0.9483) depends on three parameters to describe the adsorption mechanism. It assumes the mechanism will be a mix of the Langmuir and Freundlich isotherms. According to R-P, no ideal monolayer will be formed. That Sips isotherm also has a relatively high correlation coefficient, with a value of 0.8769. The Sips isotherm can be defined as a combination of the Langmuir and Freundlich isotherms. At high concentrations, the Sips isotherm can be abbreviated to be the Langmuir isotherm. In contrast, it will be reduced to the Freundlich isotherm at low concentrations.

The thermodynamic parameters (Gibbs free energy, enthalpy, and entropy) were determined using Equations (12)–(14). The adsorption experiments were conducted at different temperatures (303, 313, 318, and 323 K). At the end of the adsorption experiments, the relationship shown in Equation (12) was plotted and is shown in Figure 8.

The change in enthalpy was found by multiplying the slope value by the universal gas constant. The multiplication of the universal gas constant with the graph intercept gave the entropy value. The Gibbs free energy for each temperature was determined using Equation (14). Table 9 shows the thermodynamic parameters. Based on the negative value of the Gibbs free energy, the adsorption process of BR18 onto the Q. coccifera acorn hydrochar can be considered to be feasible and spontaneous in nature. The adsorption process can be considered as physisorption since the Gibbs free energy is lower than 20 kJ/mol (Singh & Kaur 2013). The Gibbs free energy at 30 °C was −6.011 kJ/mol, which is higher than the Gibbs free energy for the removal of Methylene Blue onto bagasse hydrochar −1.24 kJ/mol (Biswas et al. 2020). The change in enthalpy was found to be positive, so the adsorption process of BR18 onto Q. coccifera acorn hydrochar can be imagined as being endothermic. The positive entropy value reflects the solid phase randomness and the change in the surface of the adsorbent at the end of the adsorption process. Similar results were obtained by previous work (Afroze et al. 2016; Dawood et al. 2017; Biswas et al. 2020).

Desorption was investigated in the optimum conditions of the adsorption process. Different concentrations of HCl were tested. The desorption percentage increased with the concentration of HCl. The maximum desorption, 80.4%, was obtained at 5 M HCl. The adsorption-desorption process was repeated five times and the removal efficiencies after each cycle are shown in Figure 9. The removal efficiency of the BR18 from the aqueous solution by the prepared hydrochar in the first cycle was 90.4%. The desorption process was carried out several times, and after each cycle, the removal efficiency decreased until it leveled off in the fifth cycle. The removal efficiency in the fifth cycle was 69.2%.

The hydrochar prepared using SWM was used in the adsorption of azo dye. To investigate the feasibility of using this method to produce hydrochar, it was compared with other studies, as shown in Table 10.

In this study, the Q. coccifera acorn hydrochar was prepared successfully in SWM. This is an environmentally friendly method since no chemicals were used in the production of the hydrochar. Also, it can be considered to be an economical method as the preparation temperature did not exceed 240 °C, and the yield reached 72%.

The prepared hydrochar was used as an adsorbent to remove BR18 from aqueous solution. The adsorption process of the BR18 onto the prepared hydrochar can be considered to be fast since a drop in concentration was noticed after 5 min. The adsorption process was significantly affected by pH concentration: the maximum removal efficiency was obtained at pH 10. The adsorption process was fitted to the Freundlich isotherm and pseudo-second-order model. The feasibility of the adsorption process was obtained from the thermodynamic experiments. The adsorption process was found to be spontaneous and endothermic in nature. The adsorbent was used for five cycles, and at the end of the fifth cycle, the efficiency was 70%. According to the these results, the hydrochar prepared by SWM can be used as an effective adsorbent for BR18 dye from and aqueous solution.

The authors have no conflict of interest in relation to this work.

All relevant data are included in the paper or its Supplementary Information.