Optimization of the raw materials of biochars for the adsorption of heavy metal ions from aqueous solution

Heavy metals have been employed in many sectors, and the wastewater containing heavy metals has been progressively dumped into the environment (Fu & Wang 2011; Vasudevan & Oturan 2014). Owing to their toxicities and carcinogenicities, heavy metals in wastewater have received a lot of attention (Niu et al. 2021; Zhuang & Zhou 2021). Heavy metals might enter the human body through food chains, resulting in highly hazardous repercussions (Mosbah & Sahmoune 2013). As a result, it is critical to efficiently remove them.

Various technologies, such as coagulation (Yao et al. 2017), chemical precipitation (Chen et al. 2018), adsorption (Nassar 2010), electric treatment (Zhang et al. 2021) and adsorption (Nassar 2010), have been employed, among which, adsorption has been recognized as one of the most promising technologies (Mahdi et al. 2019) because of its evident benefits such as high efficiency, versatility and eco-friendliness (Dai et al. 2019). Prior researches have looked at numerous adsorbents, including silicon dioxide, molecular sieve, activated carbon and biochars (Ahn et al. 2009), which have been used to remove heavy metals.

Biochar is a kind of stable material which is prepared from biomass materials under anoxic and high temperature treatment (Zhang et al. 2012). It has been used in the adsorption of heavy metals because of its porous structure and abundance of functional groups (Qu et al. 2021), and shows good heavy metal adsorption capacity (Darwish et al. 2021). There are many kinds of materials which have been used to make biochars (Ariannezhad et al. 2021). The main raw materials include agricultural wastes, biomass crops, fertilizer, sludge waste, and so on. The char yield, ash content, elements composition and specific surface area are related to the types of the raw materials (Aller 2016), and the removal rates of heavy metals by different biochars vary. It was reported that the rice straw biochar had a substantially greater capability for Pb²⁺ and Zn²⁺ removal than chicken manure and sewage sludge biochars (Zhao et al. 2020). The composite biochar prepared from ferromanganese binary oxide also has a strong adsorption effect on Cu²⁺ and Cd²⁺ in water (Zhou et al. 2018). The adsorption capabilities of animal bone char for Cd²⁺, Cu²⁺ and Zn²⁺ were 53.6, 45.04 and 33.03 mg/g, respectively, and their adsorption data were in line with the film diffusion control model (Choy & McKay 2005). El-Shafey (2010) prepared rice husk biochar at 175–180 °C, and the biochar could strongly adsorb Zn²⁺ and Hg²⁺ through ion exchange. For the same metal, the removal effect varied for different biochars. For example, the adsorption capacity of fish bone char for Pb²⁺ was 1,206.13 mg/g, while the value of rice husk char was only 4.1% of that of fish bone char (Wang et al. 2015; Wang et al. 2017); the adsorption capacities of straw and wood char for Pb²⁺ ranged from 69 to 111 mg/g (Wang et al. 2015; Benguerba et al. 2017).

Pyrolysis is a common carbonization process for preparing biochars. Generally, pyrolysis is carried out under anoxic conditions at 300–900 °C. When the pyrolysis temperature is between 300 and 500 °C, the char yield is 20–35% after pyrolysis for several hours to days, and with the increase of the temperature, the char yield decreases. Pyrolysis under low temperatures and short times can produce biochar products with functional surface groups (Chen et al. 2016). Zhang et al. (2020) prepared biochar from cow dung by pyrolysis, and the char yield increased as the temperature ascended from 300 to 700 °C; when the temperature exceeds 500 °C, 10–15% char yield and higher aromatization degree was obtained. Xue et al. (2019) pyrolyzed sludge by electromagnetic induction to prepare biochar, the yield varied between 89.7% and 51.2% at 300–600 °C, and the adsorption rates of Pb²⁺ and Cd²⁺ for biochar pyrolyzed at 400 °C were higher than other biochar products. Some biochar, such as pine needle (Park et al. 2019) prepared at pyrolysis temperatures of 500–600 °C, had almost no difference in adsorption properties. It is concluded that the increase in pyrolysis temperature increased the C content and decreased the H and O contents, leading to a decrease in the H/C and O/C molar ratios, thus proving the dehydration and deoxygenation of biomass and less oxygen-containing functional groups in biochar (Ahmad et al. 2014; Inyang et al. 2016). The temperature of 350–450 °C was considered to be an ideal pyrolysis temperature range (Kwak et al. 2019).

In this study, the biochar was prepared by pyrolysis method using peanut shell, core stalk and wheat stalk, and the pyrolysis temperatures of 350–550 °C were investigated. The adsorption kinetics of ⁠, Cu²⁺, Pb²⁺, Cd²⁺ and Zn²⁺ by these biochars with different pyrolysis temperatures were investigated, and the biochars suitable for different heavy metals removal were optimized. In addition, the adsorption mechanisms were revealed based on biochar characterization and adsorption kinetics analysis. The research may serve as a sound theoretical and practical foundation for the use of biochars in heavy metal emergency treatment.

The chemical regulators employed in this investigation were all of analytical grade. The regents were utilized without additional purification. NaOH and HNO₃ were obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd (China), while K₂Cr₂O₇, Cd(NO₃)₂·4H₂O, Cu(NO₃)₂·3H₂O, Zn(NO₃)₂·6H₂O and Pb(NO₃)₂ and were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Cr⁶⁺ (in form), Cd²⁺, Cu²⁺, Zn²⁺ and Pb²⁺ stock solutions at 1,000 mg/L were prepared before the experiments. To avoid hydrolysis, the stock and working solutions were held at 4 °C in the presence of HNO₃ (5 wt.%). The deionized water (resistivity >18 MΩ cm) required in the experiments was provided by a Milli-Q system (USA).

Peanut shell, corn stalk and wheat stalk with particle size of 74–149 μm were used as the biomass in this study. The biomass was pyrolyzed in a tubular furnace at 350–550 °C in N₂ atmosphere. The mixture was then washed to neutral with deionized water, and dried at 60 °C to generate biochar.

Scanning electron microscopic (SEM) images were gotten using Zeiss Sigma 300 electron microscope (Germany). The specific surface areas were measured by putting N₂ on a Quadrasorb SI-MP machine (USA).

The adsorption experiment was carried out in a 50 mL tubes with 25 mL heavy metal solution. The biochar of 0.2 g was added into the tube at t₀. The initial concentrations of Cr⁶⁺ (in form), Cd²⁺, Cu²⁺, Zn²⁺ and Pb²⁺ were 10, 2, 10, 40 and 20 mg/L, 20 times of the maximum value of the ‘Integrated Wastewater Discharge Standard’ (GB 8978-1996) in China, respectively. The initial pH value was adjusted at 6.0 ± 0.1 according to the previous studies (Fig. S1). The tubes were then put in a thermostat-controlled shaker, and the adsorption experiment was conducted at 25 °C and 250 rpm stirring. The adsorption capacity (q_t) and the capacity of an adsorbent (q_e) were calculated, and the adsorption kinetics were simulated (Text 1 in the Supplementary Materials).

The samples were filtered through 0.45 μm filters, and acidified with HNO₃ (3 wt.%) before being measured. All materials were analyzed for residual heavy metal contents using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Thermo, 7,000 plus series).

The adsorption kinetics of by several kinds of biochars were fitted using pseudo-first model (Figure 1). As can be observed, adsorption rate of more than 80% was generally achieved in 240 min, corresponding to the fast adsorption stage; after that, the adsorption speed slowed down as residual concentrations and adsorption sites decreased.

Three common kinetic models are employed to correlate the adsorption data (Figs. S2–S4 in the Supplementary Materials), and the linear regression analysis's fitting parameters are presented in Table 1. In terms of parameter fitting (R²), the pseudo-first-order model surpasses both the other two models. It implies that both physisorption and chemisorption process might occur between biochars and heavy metal ions (Maneechakr & Mongkollertlop 2020). Additionally, the values of ‘d’ were not zero, indicating that intraparticle diffusion did not play a significant role in adsorption (Pholosi et al. 2020).

The adsorption kinetics of Pb²⁺, Zn²⁺, Cu²⁺ and Cd²⁺ by biochars, which were fitted using a pseudo-first-order model, were also investigated, and the results are shown in Figures 2–5. The kinetics fitted by other models are illustrated in Figs. S6–S16, and the fitting parameters are listed in Tables S1–S4. As for adsorption, the pseudo-first-order model is the most accurate. Figure 6 illustrates the values of k₁ for the adsorption of heavy metals.

As seen in Figure 6, the largest values of k₁ were in the order of Cr⁶⁺ (⁠⁠, 0.5380 /min) > Pb²⁺ (0.2276 /min) > Cd²⁺ (0.1354 /min) > Cu²⁺ (0.1273 /min) > Zn²⁺ (0.1000 /min). It demonstrates the adsorption variety and preference for various heavy metal contaminants as a result of the enormous disparity in biochar's adsorption ability for various heavy metal ions (Banerjee et al. 2016; Deng et al. 2017). The rate at which heavy metals are absorbed by biochar may be linked to the charging situation and the ion radius of the heavy metals. The is negatively charged and easier to be adsorbed (Zeng et al. 2021). For the divalent heavy metals, the adsorption rate may be positively related to the ion radius which are Pb²⁺ (119 pm) > Cd²⁺ (97 pm) > Zn²⁺ (74 pm) ≈ Cu²⁺ (74 pm).

Figure 6 demonstrates that the adsorption capacities of biochars pyrolyzed at different temperatures for heavy metals are varied. The removal rate of followed the sequence of corn stalk char > peanut shell char > wheat stalk char, whereas that for Cd²⁺ followed wheat stalk char < corn stalk char < peanut shell char. For Cu²⁺ and Zn²⁺, biochars were arranged in the following order: corn stalk char < wheat stalk char < peanut shell char, whereas for Pb²⁺, biochars were arranged in the following order: wheat stalk char > corn stalk char > peanut shell char. Based on their adsorbed capacity, corn stalk, peanut shell, wheat stalk, peanut shell, and peanut shell were shown to be the best materials to make adsorbents for ⁠, Cd²⁺, Cu²⁺, Zn²⁺ and Pb²⁺, respectively. The ratios of cellulose, hemicellulose and lignin in raw materials can affect the elemental composition of biochar, which in turn affects the types and contents of functional groups (Yang et al. 2007). In addition, the pH value and surface charge of biochar prepared from different materials were also significantly different (Nzediegwu et al. 2021). These chemical properties had a significant impact on the adsorption capability of biochars (Tan et al. 2015).

The temperature at which the biomass was pyrolyzed also had an impact on the rate at which heavy metals were absorbed. Within a specific range, the contribution of minerals increased, while the contribution of organic components increased in lockstep with the pyrolysis temperature increased (Wu et al. 2019). The maximum rate of the adsorption of was obtained when the biomass was pyrolyzed at 350 °C. The adsorption rate of Cd²⁺ enhanced as the pyrolysis temperature of the peanut shell rose. The greatest adsorption rates for Cu²⁺ and Zn²⁺ were obtained within the addition of 450 °C pyrolyzed peanut shell char. Previous research has suggested the desorption of Cu and Zn may be due to an increase in mineral composition, polycyclic aromatic hydrocarbons. When the pyrolysis temperatures are elevated, the fixed carbon is translated to be graphite-like and structured carbon, which improves the biochar tenacity and inhibits the release of internally loaded heavy metals (Meng et al. 2018). The maximum Pb²⁺ adsorption rate was observed in the presence of biochar generated from the three biomasses at pyrolysis temperature of 500 °C.

The highest adsorption rate of corn stalk char and peanut shell char for Cd²⁺ was obtained at the pyrolysis temperature of 350 °C. For these three kinds of biochars, the largest removal efficiencies of Cu²⁺ and Zn²⁺ were achieved at the pyrolysis temperature of 350 °C. In addition, except for the corn stalk char, the highest adsorption rate for Zn²⁺ was also gotten at 350 °C. Therefore, in most cases, 350 °C was the optimal pyrolysis temperature.

The char yields of the biomasses were calculated, and the results are shown in Figure 7. The char yields were in the range of 29.0–43.9%. When the pyrolysis temperature was 350 °C, the char yield of wheat stalk, corn stalk and peanut shell were 43.9%, 37.6% and 38.3% respectively. When the pyrolysis temperature was in the range of 350–550 °C, wheat stalk produced more char than other straws, which might be related to the structure of the raw biomass. For example, when the biomass has a high lignin content, the yield of char is high (Demirbas 2004). The char yields of all biomass dropped when the temperature of pyrolysis raised, which was consistent with the study of Zhang et al. (2015). As the pyrolysis temperature increased, more organic matters lost, resulting in the lower char yield (Venkatesh et al. 2022).

Figure 8 shows the SEM micrographs of biochar pyrolyzed at the optimal temperature before and after adsorption. It can be seen that significant pores were present in biochars, and spherical particles were present on the surface of the biochars after adsorption, indicating the adsorption of heavy metals onto the biochar. Among them, the surface of wheat straw was tortuous with honeycomb pores, and the pores were the most obvious. For wheat stalk pyrolyzed at 500 °C, a regular crystal structure was developed on the biochar's surface, which was probably graphite. In general, the graphite-carbon phase and porous structure of biochar could promote organic pollutant adsorption through hydrophobic contact and intra-particle diffusion, but decrease inorganic pollutant adsorption owing to the reduction of oxygen-containing functional groups (Gai et al. 2014).

The isotherms of adsorption and desorption, as well as the distribution of pore sizes of the 450 °C pyrolyzed peanut shell, 350 °C pyrolyzed corn stalk and 500 °C pyrolyzed wheat stalk are shown in Figure 9. The nitrogen adsorption and desorption isotherm of wheat straw biochar had the type IV characteristics, indicating that the material contained mainly mesoporous (2–50 nm), and most of its pore size was less than 20 nm. Under the same P/P⁰ condition, the wheat stalk char had greater adsorption ability than the other two varieties, which was because of its porous structure.

The specific surface areas of the abovementioned biochars were 4.62, 2.68 and 158.98 m²/g, respectively. The wheat stalk char had the highest specific surface area, but lower adsorption capabilities for the removal of ⁠, Cd²⁺, Cu²⁺ and Zn²⁺. It showed that the contribution of specific surface area to the removal of heavy metals was quite limited, and physical adsorption relying on larger specific surface area was not the main adsorption mechanism for heavy metals adsorption. During heavy metal adsorption, the chemical properties of chars are sometimes more important than the surface area (Li et al. 2016). The -OH and -COOH on the char surface can greatly promote the adsorption of heavy metals such as Cd²⁺ and Cu²⁺ (Wang et al. 2015). In this study, the removal of heavy metals might be related to the functional groups of chars. In addition, the point of zero charge (pH_pzc) of the biochars also had a great influence on the adsorption of heavy metals (Li et al. 2017). Tan et al. (Tan et al. 2015) prepared char from waste mangosteen shells to adsorb in water, and found that chemisorption was dominant, which was consistent with the conclusion of this study.

In this study, ⁠, Cd²⁺, Cu²⁺, Zn²⁺ and Pb²⁺ with initial concentrations of 10, 2, 10, 40 and 20 mg/L, was adsorbed with biochars prepared using peanut shell, corn stalk and wheat stalk. The pseudo-first order kinetic was better to fit the adsorption of the abovementioned heavy metals. The adsorption rate was significantly variable in the presence of various biochars, which owed to the varying kinds and amounts of functional groups included in each biochar. The temperature at which the biomass was pyrolyzed also had a significant effect. Based on their adsorbed capacity, 350 °C pyrolyzed corn stalk char, 550 °C pyrolyzed peanut shell char, 450 °C pyrolyzed peanut shell char, 450 °C pyrolyzed peanut shell char and 500 °C pyrolyzed wheat stalk char were shown to be the best adsorbents for ⁠, Cd²⁺, Cu²⁺, Zn²⁺ and Pb²⁺, respectively. The adsorption rates might be linked to the charging situation and the ion radius of the heavy metals. The which is negatively charged is easier to be adsorbed; for the divalent heavy metals, the adsorption rate may be positively related to the ion radius. In most cases, the biochar with higher char yield showed lower adsorption capability. The surface area had little effect on the adsorption rates of ⁠, Cd²⁺, Cu²⁺ and Zn²⁺ at low concentrations, and the removal of these ions might be related to the elements and functional groups of the biochars. For Pb²⁺ which has larger ion radius and higher initial concentration, the surface area and mesoporous structure played an important role.

This work was supported by National Science and Technology Major Project of China (2017ZX07107-005) and the Fundamental Research Funds for the Central Universities of China (FRF-MP-20-33).

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