Effect of temperature and duration of pyrolysis on spent tea leaves biochar: physiochemical properties and Cd(II) adsorption capacity

Heavy metal contamination of surface and underground water is a major environmental concern due to their biotoxicity and low degradability. Cadmium (Cd) is a highly toxic heavy metal pollutant that magnifies rapidly through the food chain, and causes extensive damage to the liver, kidney and bones (Zhou et al. 2018; Gao et al. 2019). The Chinese Ministry of Health (MoH) and the China National Standardization Management Committee have set an acceptable limit of 0.005 mg/L Cd in drinking water (MoH 2006). There have been considerable efforts in recent years to remove Cd(II) from agricultural and industrial effluent (Aghababaei et al. 2017) using chemical precipitation, electrochemistry, ion exchange, coagulation and membrane separation (Tan et al. 2018). However, these methods are costly and ineffective (Roh et al. 2015), which has shifted the attention to biochar-mediated adsorption.

Biochar is a charcoal-like substance produced by pyrolysis or controlled thermal degradation of wood, forest biomass, crop straw, animal manure, sewage sludge and agricultural byproducts at limited oxygen levels and relatively low temperatures (<700 °C) (Park et al. 2015; Aghababaei et al. 2017; Fan et al. 2017; Huang et al. 2018). It has a high adsorption capacity for both organic and inorganic contaminants in aqueous solutions or soil (Liu et al. 2015; Kausar et al. 2016), which in turn depends on various structural and physicochemical factors (Tan et al. 2015; Zhang et al. 2017), such as ash content, pore volume, surface area, surface functional groups etc. (Suliman et al. 2016; Li et al. 2017). Several studies have analyzed the ability of biochar to adsorb Cd(II) from aqueous solutions (Wang et al. 2015a; Cui et al. 2016; Zama et al. 2017). For instance, Cui et al. (2016) found that higher surface area and porosity correlated to increased Cd(II) adsorption capacity, and Zama et al. (2017) showed that the organic functional groups, inorganic minerals and cation content of biochar significantly influenced Cd(II) adsorption. Furthermore, Uchimiya (2014) concluded that the structural properties of biochar depend on the pyrolysis temperature and feedstock.

China is the largest tea producer worldwide in terms of both yield and plantation area. Tea consumption had increased globally on account of its beneficial health effects (Cao et al. 2018), and the large amounts of discarded spent tea leaves (STL) can be used for producing highly adsorbent biochar for water decontamination (Zama et al. 2017). Indeed, STL biochar can successfully remove basic dye (Hameed 2009) and copper (Bajpai & Jain 2010) from aqueous solutions. However, the physiochemical properties of STL biochar that affect its ability to adsorb Cd(II) are still not well understood.

We produced STL biochar using different pyrolysis temperatures and duration, and compared their physicochemical properties and Cd adsorption capacity in order to determine the optimal pyrolysis parameters for generating high adsorbing biochar.

Tea leaves were collected from the Chengdu tea market, washed with distilled water to remove any impurities, and soaked in twice the volume of deionized water for 24 hours. The soaked tea leaves were dried at 55 °C, crushed in a blender and passed through a 0.25 mm sieve. The powdered samples were filled into porcelain crucibles (6 cm in height and 5.5 cm in diameter), closed with a lid and placed in the middle of a muffle furnace (M110 Thermo Scientific, USA). Based on previous findings (Higashikawa et al. 2016; Zhao et al. 2018), the STL samples were pyrolyzed at 300, 400, 500 and 600 °C. The temperature was increased at the rate of 15 °C per minute, and maintained at the requisite point for 1 or 2 h. The resulting biochar was cooled to room temperature for 12 h and weighed to calculate the yield. The ash content was measured by dry combustion at 600 °C for 8 h and calculated (%) as (weight of ash/weight of biochar) × 100 (Wang et al. 2015b). The biochar samples were stored in sealed bags in a dryer.

The pH was determined with a digital pH meter (Orion 2 Star, Thermo scientific, Beverly, MA, USA) at the solid-water ratio 1:2.5. The electrical conductivity (EC) was determined using a conductivity meter, and the cation exchange capacity (CEC) was measured by the ammonium acetate exchange method (Nelissen et al. 2014). The specific surface area (S_BET) and porosity (PV) were determined using a NOVA 1200 surface area pore analyzer (Quantachrome Instruments, Boynton Beach, Florida, USA). Morphological features were analyzed by scanning electron microscopy (SEM, FEI Quanta 650). The functional groups on STL biochar were analyzed by Fourier transform infrared spectroscopy (FTIR) in the wavelength range of 400–4,000 cm⁻¹ with 16 scans at 4 cm⁻¹ resolution (Thermo Fisher Scientific, USA). Elemental composition (C, H, O, N) were measured by CHN Elemental Analyzer (Carlo-Erba NA-1500) operated in CHN mode.

The Cd(II) aqueous solution was prepared by dissolving cadmium chloride (CdCl₂·2.5H₂O) in deionized water. Approximately 0.05 g STL biochar was dispersed in 50 ml Cd(II) aqueous solution (5–200 mg/L) with pH ranging from 2 to 7 (adjusted using 0.01M HCl or NaOH) and incubated at 20–40 °C for 0–48 h with constant shaking at 200 rpm. The suspension was centrifuged and filtered through 0.45 μm pore polypropylene membrane filters. The final concentration of Cd(II) in the supernatant was determined by atomic absorption spectrometry (GTAAS-M6).

The general linear model (GLM) was used to test the influence of pyrolysis temperature and duration on the biochar yield, ash content, pH, EC and CEC, and curve estimation methods were used to determine the correlation between the variables. SPSS Statistics 20.0 (SPSS Inc, Chicago, IL, USA) was used for all statistical analysis.

Several studies have established pyrolysis temperature as a key factor affecting the yield and the physicochemical/structural properties of biochar (Kim et al. 2012; Mimmo et al. 2014; Zhao et al. 2018). Consistent with this, the yield of STL biochar decreased significantly from 52.17% to 26.71% as the temperature was increased from 300 to 600 °C (Table 1; P < 0.01). The yield of STL biochar decreased with increasing temperature because low molecular weight liquid and gas were produced with thermal volatiles’ cracking (Thangalazhy-Gopakumar et al. 2010) or degradation of cellulose and lignin structures (Novak et al. 2009). Interestingly, the most rapid decline occurred between 300 and 400 °C, with a 15.4% loss of the initial feedstock mass (Table 1). This can be explained by the maximum loss in moisture and labile volatile fraction at lower temperatures (Yang et al. 2004; Zhao et al. 2018). The relationship between pyrolysis temperature and yield followed an exponential curve (Figure 1(a); Adj. R² = 0.985). The ash content of STL biochar increased from 5.84 to 17.89% with increasing pyrolysis temperature (Table 1) due to loss of organic matter and accumulation of inorganic compounds, and showed an exponential curve (Figure 1(b); Adj. R² = 0.980). The highest ash content was obtained following pyrolysis at 600 °C for 2 h (17.89%; Table 1), and was higher compared to most wood biochar (e.g. umbrella tree, oak, pine) (Al-Wabel et al. 2013; Lee et al. 2013) but lower than that of straw biochar (Zhang et al. 2015b). The duration of pyrolysis did not significantly affect the biochar ash content (Table 1), which is consistent with the findings of Sun et al. (2017) and Zhao et al. (2018). Furthermore, Zhang et al. (2015b) showed that the ash content of straw biochar increased from 32.33 to 33.57% at 600 °C when the heating time was increased from 1 to 2 h. We observed an increase in only 0.93% at the same parameters.

The average pH of STL biochar increased from 5.75 at 300 °C to 7.2, 7.38 and 7.44 at 400, 500 and 600 °C respectively (Table 1). Zhao et al. (2018) showed that the ash content and acidic functional groups on biochar are the major determinants of the effect of pyrolysis temperature on pH. Likewise, Enders et al. (2012) observed a significant correlation between the ash content and pH of biochar. As shown in Figure 2, the relationship between ash content and pH of STL biochar followed an exponential trend (Adj. R² = 0.936). The STL biochar with pH above 7.3 had more than 10% ash content, while samples with less than 7% ash content were acidic (pH < 7). The temperature-dependent increase in pH can also be attributed to the loss of acidic surface functional groups. Accordingly, the electrical conductivity (EC), which reflects the total amount of dissolved salts in biochar, also increased at higher temperatures (Table 1; P < 0.01) and was maximum in samples pyrolyzed at 600 °C for 2 h. The CEC ranged from 107.97 to 407.79 cmol/kg, and was higher at lower temperatures (300 and 400 °C). This is consistent with the observations on canola and corn biochar (Yuan et al. 2011), and can be partly due to reduced content of functional groups on the biochar surface (Suliman et al. 2016).

The pyrolysis temperature also affected the elemental composition of STL biochar (Table 2). The average C content increased from 64.26 to 81.44%, while that of H, O and N respectively decreased from 3.71 to 1.78%, 23.76 to 14.43% and 2.29 to 0.78% as the pyrolysis temperature was increased from 300 to 600 °C. This indicated an increase in the degree of carbonization at elevated temperatures, and the breakdown of weaker bonds in the biochar matrix (Li et al. 2017). The O/C and H/C atomic ratios are indicative of the degree of aromaticity and polarity of biochar, and ranged from 0.19–0.39 and 0.02–0.06 respectively (Figure 3). In addition, the O/C and H/C ratios were respectively 52.25% and 62.24% lower in the biochar produced at 600 °C compared to that at 300 °C, which can be attributed to a greater degree of dehydration and decarboxylation at high temperatures (Ahmad et al. 2014; Anupam et al. 2016). The C/N atomic ratio, representative of the mobilization and exchange of inorganic N, increased from 28.58 to 106.28 at higher temperatures, indicating removal of labile matter and ignition loss of N during pyrolysis.

High surface area (S_BET) and porosity (PV) are associated with greater biochar adsorption capacity (Luo et al. 2019). The S_BET of STL biochar pyrolyzed at temperatures below 500 °C were extremely low (1.64–8.75 m²/g), and increased to 62.52–67.10 m²/g at 600 °C (Figure 4). This temperature-dependent elevation in S_BET can be attributed to an increase in biochar porosity with heating. The duration of pyrolysis had a negligible effect on S_BET. Likewise, PV value was maximum (0.0301 cm³/g) in the STL biochar produced at 600 °C for 2 h (Figure 4). Consistent with the above results, the SEM micrographs of STL biochar showed a smoother surface and pores blocked with volatile organic compounds at low pyrolysis temperatures (Figure 5(a), 5(b), 5(e), 5(f)). However, pyrolysis at 500 and 600 °C significantly increased the surface cracks and holes, indicating a highly porous structure. In addition, more adherent particles were observed on the surface of STL biochar at a higher temperature (Figure 5(c), 5(d), 5(g), 5(h)), which can be attributed to an increase in crystallization of mineral components. Taken together, pyrolysis at higher temperatures increases the exposed surface area as well as the number of adsorption sites on biochar for heavy metal ions.

The functional groups on the STL biochar were analyzed by FTIR analysis. As shown in Figure 6, the density of the oxygen-containing functional groups, such as hydroxyl (–OH), carboxyl (–COOH) and phenols (C–OH), decreased with increasing temperature. The FTIR bands at 3,400 and 1,600 cm⁻¹ that correspond respectively to the –OH bond and –COOH deviational vibration (Keiluweit et al. 2010; Chen et al. 2015) became weaker at higher temperatures. In addition, the peaks at 1,310 and 780 cm⁻¹ corresponding to C–OH and –COOH deviational vibration (Chen et al. 2015) disappeared completely at 500 and 600 °C. Similar results were observed with the oxygen-containing functional groups on straw biochar (Yuan et al. 2011). The decrease in –COOH, C–OH and –OH peaks is indicative of the cleavage of acidic groups. Since these oxygen-containing functional groups are involved in the immobilization of heavy metal ions (Zhao et al. 2018), our results suggest that they likely do not play any significant role in metal ion sorption by the STL biochar. In contrast, the FTIR peak at 875 cm⁻¹ that is associated with C–H (Li et al. 2017) strengthened at 500 and 600 °C, indicating formation of aromatic structures during pyrolysis (Zhou et al. 2016). Pyrolysis results in a temperature-dependent dehydration of cellulosic and ligneous components, transformation of aromatic C and lignin/cellulose, and progressive condensation of aromatic organic compounds (Keiluweit et al. 2010; Srinivasan & Sarmah 2015). At temperatures below 400 °C, pyrolysis mainly removes the physically adsorbed water and vaporizes aliphatic compounds, resulting in a rapid loss of functional groups (Sun et al. 2017). At higher temperatures, the feedstock decomposes further and the volatiles (e.g. cellulose, hemicellulose and lignin) are completely released (Chandra & Bhattacharya 2019). Taken together, the STL biochar was well carbonized and had sparse oxygen-containing functional groups on the surface.

The adsorption of metal ions on biochar significantly depends on the pH. Cd(II) uptake by the STL biochar varied across the pH range of 2–7 regardless of the pyrolysis temperature (Figure 7(a), 7(b)). Specifically, Cd(II) adsorption increased in acidic solutions of pH 2–5 and decreased as the acidity diminished, with optimal uptake seen at pH 5. This result was unexpected, since the excessive amount of H⁺ on the sorbent surface under acidic conditions competes with Cd(II), and decreases the binding between sorbent and metal cations (Aghababaei et al. 2017). Furthermore, deprotonation at higher pH is known to promote the electrostatic adsorption of Cd(II) on biochar surface (Oh & Seo 2019). Over the studied pH range, STL biochar produced at 600 °C showed the maximum adsorption of Cd(II) and that produced at 300 °C showed the least adsorption capacity. Solution temperature plays an important role in metal ions sorption process. Figure 7(c) and 7(d) show the influence of varying solution temperature from 20 to 40 °C on Cd(II) adsorption. The results indicated that Cd(II) adsorption increased as the temperature increased. The temperature-dependent increase in Cd(II) adsorption points to an endothermic process, since high temperatures would provide sufficient energy for the Cd(II) ions to reach and adsorb onto the biochar (Aksu & Kutsal 1991). Therefore, subsequent studies on adsorption kinetics were performed at 40 °C.

As shown in Figure 8(a) and 8(b), Cd(II) adsorption on the STL biochar was rapid within the first 8 h, likely due to the availability of adsorption sites and stronger attachment to the outer surfaces. The adsorption rates decreased thereafter and approached equilibrium by 48 h. The STL biochar produced at 600 °C for 2 h reached the adsorption equilibrium faster and adsorbed the largest amount of Cd(II) compared to other biochar samples. According to the R² values, the pseudo second-order rather than first-order kinetic model was representative of the Cd(II) sorption kinetics on STL biochars (Table 3). The q_{e, cal} values calculated from the pseudo-second-order kinetic model were closer to the experimental data. These results indicate that Cd(II) adsorption involves valence forces through sharing or exchanging electrons between the functional groups on STL and Cd(II) ions (Zhou et al. 2018).

Adsorption isotherms of Cd(II) were determined by incubating 0.05 g STL biochar with different concentrations of Cd(II) at pH 5 and 40 °C. As shown in Figure 8(c) and 8(d), the adsorption rates increased significantly at Cd(II) concentration below 60 mg/L, and decreased thereafter. In line with the results so far, the STL biochar produced at 600 °C for 2 h showed the highest adsorption capacity. The R² values obtained from the Langmuir isotherm model were higher than that from the Freundlich isotherm model, indicating that the equilibrium kinetics followed the former (Table 4). This strongly indicated that Cd(II) adsorption mainly took place in monolayers, or through a fixed number of homogeneous sites on the surface (Günay et al. 2007). According to the saturation threshold of the best matching model, the STL biochar produced at 600 °C for 2 h showed the maximum Cd(II) adsorption capacity of 97.415 mg/g, which was 1.38–1.63, 1.07–1.26 and 1.01–1.14 times greater than that of biochar samples produced at 300, 400 and 500 °C respectively. This may be attributed to the higher specific surface area and pore volume resulting from pyrolysis at higher temperatures. Interestingly, the adsorption capacity of STL biochar was significantly higher than that reported for Prosopis africana shell biochar (29.9 mg/g) (Elaigwu et al. 2014), rice straw biochar (34.1 mg/g) (Han et al. 2013), Typha angustifolia AC (48.1 mg/g) (Tang et al. 2017) and water hyacinth biochar (70.3 mg/g) (Zhang et al. 2015a).

The pyrolysis temperature was positively correlated to the ash content, pH, EC, specific surface area and pore volume of STL biochar, and negatively to the biochar yield due to loss of oxygen-containing functional groups and greater carbonization at higher temperatures. Biochar produced at 600 °C for 2 hours adsorbed the largest amount of Cd(II), and the sorption kinetics closely followed the pseudo-second-order model. The Langmuir isotherm model fitted well with the experimental data, and the maximum adsorption capacity for Cd(II) was 97.415 mg/g. Thus, STL biochar is a highly suitable low-cost adsorbent for wastewater treatment.

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21507095), the Sichuan Provincial Youth Science and Technology Fund (Grant No. 2017JQ0035), and the Sichuan Province Project Education Fund (Grant No. 16ZA0036).

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