Comparison of the adsorption capacity and mechanisms of mixed heavy metals in wastewater by sheep manure biochar and Robinia pseudoacacia biochar

The discharge of industrial wastewater has led to a gradual increase in the accumulation of heavy metals in the sediment of surface and groundwater. The issue of heavy metal pollution has gained significant attention as it poses a major environmental threat due to its non-biodegradable nature and tendency to accumulate in living organisms (Zheng et al. 2022). Lead (Pb), copper (Cu), and cadmium (Cd) are the most dangerous heavy metals that can lead to severe health complications, even in small quantities (Ren et al. 2022). Hence, it is crucial to eliminate these metals from wastewater before releasing them into the surroundings to prevent any environmental hazards.

Adsorption technology utilizing biochar as an adsorbent has garnered significant attention in recent times due to its low cost and advantageous physical and chemical properties. Biochar is created through the process of pyrolysis in oxygen-deficient surroundings by utilizing waste biomass. This fascinating compound possesses various physicochemical traits that allow for the effective adsorption and immobilization of heavy metals. The surface of biochar is composed of micro- and nanopores, as well as aromatic structures and surface adsorption sites, all of which contribute to strong adsorption capabilities for heavy metals (Wang et al. 2019).

The physicochemical properties of biochar, including its capacity to adsorb heavy metals, can be influenced by the type of biomass used and the temperature of pyrolysis (He et al. 2018). The properties of livestock manure biochar and plant biochar differ significantly due to variations in the lignin, cellulose, and hemicellulose content in the feedstocks (Rezaee et al. 2021; Xu et al. 2021). The sheep manure (SMB) and Robinia pseudoacacia (RPB), due to their high content of lignocellulose, are not suitable for anaerobic digestion and often are burned directly, causing waste of resources and carbon dioxide emissions. Meanwhile, they were widely distributed in the Mongolian and northeastern regions of China and have received less research attention as biochar raw materials. Thus, in this study, SMB and RPB were chosen as raw materials for biochar production beneficial to waste resource utilization and CO₂ reduction. Furthermore, the pyrolysis temperature significantly impacts the properties of biochar (Zhang et al. 2015). For example, low pyrolysis temperatures can decrease the amounts of H, O, and active surface functional –COOH and −OH groups while increasing the degree of aromatization of biochars, which is unsuitable for the adsorption of heavy metals in wastewater. However, biochar's pore diameter, surface area, and pH can be enhanced through low pyrolysis temperatures, enabling it to efficiently adsorb heavy metals from wastewater. Thus, the adsorption capacity of biochar from different feedstock and from different pyrolysis temperatures should be investigated.

Furthermore, a better understanding of the adsorption mechanisms can lead to improvements in the adsorption performance of biochars via modifications. Despite the widespread investigation of biochar applications in heavy metal adsorption and mechanisms, our literature review reveals that most studies have focused on the qualitative and morphological analysis of before and after adsorption (Fan et al. 2018; Zhang et al. 2018; Wan et al. 2018; Sekulic et al. 2018). Only a few articles in literature focused on the quantitative analysis of adsorption mechanisms. Therefore, it is important to conduct a systematic and quantitative investigation of the adsorption capacities and mechanisms of biochars derived from various feedstocks and pyrolysis temperatures.

The focus of this research was to examine the distinctive physical, chemical, and morphological specifications of biochars produced from livestock waste (specifically SMB) and plant substances (particularly RPB) under various pyrolysis temperatures. Furthermore, the performance of biochar to absorb individual and mixed heavy metals from wastewater was estimated. Specifically, the novelty of this work is combining the analysis of the theoretical models of adsorption and quantitative analysis of the contribution of different adsorption mechanisms. This research conveyed critical insights into the comprehensive recycling utilization of SMB and RPB, thus advancing sustainable development.

SMB and RPB were collected from livestock plants in Inner Mongolia and forestry in Chengde, Hebei, China, respectively. After being air-dried for 7 days, the raw materials were pulverized into particles smaller than 2.0 mm with a grinder, and then dried at 100 °C for 24 h.

Under a N₂ atmosphere, the powdered biomass underwent pyrolysis within a pyrolizer equipped with a stainless-steel tube. The temperature within the internal biomass chamber was gradually elevated at a rate of 10 °C · min⁻¹ until it reached 300, 500, and 800 °C separately, where it was maintained for 90 min. The biochars were referred to as SMB3 and RPB3, SMB5 and RPB5, SMB8 and RPB8 in terms of the pyrolysis temperature. All of the biochar samples were ground to pass through a 0.15 mm sieve prior to use.

The biochar yield was calculated based on the difference in the mass of raw material and the biochar after the completion of the pyrolysis. Elemental analysis was used to determine the C, H, and N contents of SMB and RPB, utilizing the Elemental AnalysisSysteme GmbH in Germany. To measure pH, biochar was combined with deionized water in a ratio of 1:20. O content was calculated based on mass balance: O = 100 − (C + H + N + S + ash). Surface functional groups were determined via Boehm titration (Oickle et al. 2010).

The ash contents of SMB and RPB were determined via the combustion of biochar samples in a Muffle Furnace at 800 °C for 6 h. The suspensions (in triplicate) were prepared by adding 1 g of biochar into 20 mL of deionized (Milli-Q pore) water and shaken for 2 h. The pH of the suspension was measured. The aerogenesis was determined by using GC (6890 Agilent, USA). The metal ions of biochar (K, Ca, Na, and Mg) were measured with an atomic absorption spectrophotometer (900 T, PerkinElmer, USA). The morphological characteristics of biochars were analyzed using a scanning electron microscope (SEM; Merlin, Zeiss, Germany), Fourier transform infrared spectroscopy (FTIR; Nicolet 6700), and a Bruker D8 Advance X-ray diffraction spectrometer. Properties of surface area and porosity were measured by the Brunauer–Emmett–Teller (BET) method using a Quadrasorb Si-MP analyzer, which employed N₂ adsorption isotherms at 77.35 K.

Batch adsorption trials were carried out to determine the heavy metal adsorption capabilities of SMB and RPB. To create the heavy metal blends, Pb²⁺, Cu²⁺, and Cd²⁺ were dissolved in calibrated quantities of deionized (DI) water at a concentration of 100 mg·L⁻¹.

A total of 0.6 g of biochar was introduced to 150 mL of mixture of heavy metal solution in a 250 mL glass container, which was maintained at room temperature. The biochar specimens were positioned in sealed containers, agitated at 150 rpm for 24 h in a mechanical shaker, until equilibrium was attained, and later sieved through a 0.45 μm syringe filter. The residual concentrations of Pb²⁺, Cu²⁺, and Cd²⁺ in the aqueous filtrate were measured using inductively coupled plasma (ICP; PerkinElmer Optical Emission Spectrometer Optima 8300, USA).

In order to investigate the adsorption kinetics, batch adsorption experiments were carried out using SMB or RPB at a fixed initial concentration of 100 mg·L⁻¹ for each metal ion. The experimental procedures and conditions were consistent with those described above. Samples were collected at various time intervals (2, 6, 10, 20, 40, 60, 120, 180, 300, 600, and 1,200 min) to acquire data on the kinetics of adsorption.

The adsorption isotherms for mixed heavy metals were determined by adding 25 mL of a solution containing specific concentrations (20, 40, 60, 80, 100, 120, 150, 200, 300, and 400 mg·L⁻¹) of the metals from a stock solution to a 50 mL Plug Seal Centrifuge Tube containing 0.1 g of biochar by weight.

The quantitative examination of various biochar mechanisms responsible for heavy metal adsorption, such as metal ion exchange (Q_e), mineral precipitation (Q_p), functional group complexation (Q_f), and heavy metal-π coordination (Q_π), was investigated. These mechanisms were ascertained by disregarding other theoretical mechanisms that have minimal contributions (Wang et al. 2019).

• (i)
  The combination of ion exchange (Q_e) and mineral precipitation (Q_p) was identified as the reason behind the interaction between minerals and heavy metals (Q_e+p), as shown in Equation (1):

  

  (1)
• (ii)
  The net quantity of exchanged metal cations (K⁺, Ca²⁺, Na⁺, and Mg²⁺) liberated from the biochar was computed as the ion exchange (Q_e), which is equivalent to the change in cation concentration in the absence or presence of heavy metal, as shown in Equation (2):

  

  (2)

  where Q_K, Q_Ca, Q_Na, and Q_Mg denote the amount of K, released from the biochar into the solution containing heavy metals minus the amount of K, Ca, Na, and Mg released from the biochar to the solution without heavy metals.
• (iii)

  In order to negate the impact of minerals on biochar, SMB3 and RPB8 underwent demineralization using 1 mol·L⁻¹ HCl solution, after which they were denoted as dSMB3 and dRPB8, correspondingly.

The Q_e/Q_t, Q_p/Q_t, Q_f/Q_t, and Q_π/Q_t ratios were calculated to quantitatively evaluate the contribution of different mechanisms to the heavy metal adsorption by SMB3 and RPB8.

The adsorption trials were replicated three times, and the descriptive statistics were utilized to determine the standard deviation. Oringe 8.0 was employed to fit the adsorption kinetics and isotherms, and the R² values were utilized to assess the equations' performance.

As indicated in Table 1, the quantity of biochar obtained is influenced by the type of raw material used and the temperature used for pyrolysis. In particular, when the pyrolysis temperature was increased from 300 to 800 °C, the yields of SMB and RPB decreased from 91.6 to 81.3% and from 64.6 to 33.4%, respectively.

The biochars' biomass primarily comprised of hemicellulose, cellulose, and lignin, undergoing decomposition in the order of hemicellulose (250–350 °C), cellulose (325–400 °C), and lignin (300–550 °C) (Kan et al. 2016). As pyrolysis temperature and biomass degradation increase, the yield decreases accordingly. The yield was higher than that of RPB pyrolyzed at the same temperature. Typically, the lignin content in woody materials constitutes 20–40% of the dry weight (Sharma et al. 2014). Hence, the greater production of biochar from SMB in comparison to RPB can be attributed to the lower amount of lignin present in SMB. This feature suggests that SMB is a more financially feasible option than RPB.

Generally, as the temperature of pyrolysis increases, the pH value of biochar also increases (Wang & Liu 2017). Specifically, the pH value of SMB ranged from 7.79 to 11.72 at pyrolysis temperatures of 300–800 °C, while the pH value of RPB increased gradually from 5.29 to 10.18. This implies that SMB has a stronger alkalinity than RPB, as shown in Table 1. SMB and RPB consist of various minerals, including K, Ca, Na, and Mg. Through pyrolysis, the ash's alkali salts are released due to the organic acid and carbonate decomposition and transformed into hydroxides, such as Ca(OH)₂, Mg(OH)₂, and NaOH (Yuan et al. 2015). Hence, the elevated pH of SMB compared to RPB can be attributed to its abundant mineral composition.

Owing to a certain amount of dry mass loss in the process of pyrolysis, the concentration effect led to a rise in total mineral contents for both SMB and RPB. Specifically, the mineral contents of SMB increased from 43.4 to 71.1 mg·g⁻¹, while RPB's mineral contents increased from 26.8 to 65.7 mg·g⁻¹.

In general, the presence of inorganic elements aids in the elimination of heavy metals by means of ion exchange and precipitation. As a result, the ash content of SMB (ranging from 64.02 to 79.94%) was higher than that of RPB (ranging from 3.3 to 8.1%). This significant ash content in SMB can be explained by the high level of mineral content in the input material.

Table 1 shows the results of the elemental analyses of SMB and RPB. SMB exhibited reduced levels of C (15.9–18.9 wt%), H (1.56–0.24 wt%), and N (1.29–0.54 wt%) in comparison to RPB. However, the O/C ratio groups were higher in SMB as it contains a greater number of O-containing functional groups and has higher polarity.

As the temperature of pyrolysis was raised from 300 to 800 °C, the carbon quantities rose while the nitrogen amounts reduced. The sulfur content of SMB was barely affected by the temperature of pyrolysis. Remarkably, with the increase in temperature of pyrolysis, the oxygen level of SMB fell from 16.96 to 0.08%, while that of RPB decreased from 33.05 to 5.86%.

The primary parameters utilized to determine the aromaticity and polarity of biochars are the molar O/C and H/C ratios (Yuan et al. 2015). As the temperature of pyrolysis increased, the O/C and H/C ratios of RPB and SMB decreased. It can be inferred from the data that SMB8 and RPB8 exhibit greater levels of aromaticity and polarity, while SMB3 and RPB3 possess higher concentrations of oxygen-containing functional groups and exhibit the superior capacity for complexation compared to the other biochars. Table 1 reveals that SMB3 (7.78 mmol·g⁻¹) and RPB3 (3.65 mmol·g⁻¹) have a significantly higher content of oxygen-containing groups on their surfaces compared to SMB8 (0.63 mmol·g⁻¹) and RPB8 (0.49 mmol·g⁻¹). This can be attributed to the fact that, at elevated pyrolysis temperatures, the oxygen-containing groups are transformed into CO and CO₂. The [(O + N)/C] atomic ratios serve as measures of polarity. In this investigation, the overall [(O + N)/C] ratio of SMB surpasses that of RPB. Thus, SMB exhibits greater overall polarity compared to RPB.

As the pyrolysis temperature increased, the surface roughness of SMB and RPB also increased. This trend was consistent with the increase in a specific surface area, as determined by the BET technique. At 800 °C, the highest specific surface areas for SMB and RPB were 22.1 and 43.23 m²·g⁻¹, respectively (Table 1). Furthermore, the larger specific surface areas of RPB than those of SMB could be attributed to the high lignin content of RPB. The process of pyrolysis generates micropores when lignin is transformed into volatilized compounds, thereby contributing to the augmented surface area observed in RPB. Therefore, the specific surface area of RPB increased because of micropore formation, which increased proportionally with pyrolysis temperature.

It has been demonstrated that biochar can remove heavy metals through a variety of mechanisms, including precipitation, cation exchange, electrostatic interaction, physical adsorption as well as metal reduction and complexation (Qambrani et al. 2017; Wang & Liu 2017). As previously noted, for the SMB, as the pyrolysis temperature increased, the adsorption capacity of SMB decreased, which results from the decrease in oxygen-containing functional groups that facilitate the formation of complexes with heavy metals. However, RPB8 exhibited maximum adsorption capacities because RPB8 boasts a high mineral content that fosters cation exchange and precipitation. Compared to RPB, SMB exhibits higher efficiency in eliminating mixed heavy metal solutions that contain Pb²⁺, Cu²⁺, and Cd²⁺, especially SMB3. According to the experimental results, RPB8 shows a relatively ideal adsorption effect on Cu²⁺ compared to RPB3 and RPB5. Therefore, it can be used as a selective adsorbent for single Cu²⁺ in wastewater.

In order to compare the sorption performance of the studied biochars, a review of literature was done about the biochars in other studies used for adsorping heavy metals (Higashikawa et al. 2016; Fan et al. 2018; Gao et al. 2018; Sekulic et al. 2018; Son et al. 2018; Wan et al. 2018; Zhang et al. 2018). In Table 2, it can be seen that SMB3 showed comparably high adsorption capacity for Pb²⁺, Cd²⁺, and Cu²⁺. Therefore, SMB3 is a potential adsorbent for further studies.

Adsorption isotherm models were applied for examining the sorption capacity of RPB and SMB and for exploring the dispersal patterns of heavy metals across solid/liquid interfaces. As shown in Figure 5, both the Langmuir and Freundlich models were employed, and Table 4 provides the parameters for heavy metal sorption by SMB and RPB. It was observed that the Freundlich Langmuir model yielded a superior fit to the Langmuir model, thus suggesting that the adsorption of heavy metals by SMB and RPB occurs via multilayer adsorption. The findings are consistent with the deductions drawn from the kinetics information, indicating that chemical adsorption is the primary mechanism in heavy metal adsorption.

Table 5 presents the quantitative analysis results of the mechanisms of heavy metal adsorption, which include metal ion exchange (Q_e), mineral precipitation (Q_p), functional group complexation (Q_f), and heavy metal-π coordination (Q_π) for SMB3 and RPB8.

With a Q_e/Q_t ratio exceeding 50%, RPB8 demonstrated that the dominant mechanism of heavy metal adsorption was ion exchange due to the high content of metal ions in its composition, as shown in Table 1. RPB8 exhibited the highest Pb Q_e+p/Q_t among all samples (75.1%), implying that minerals, metal ions, and mineral precipitation also contribute to Pb adsorption. On the other hand, SWB3 displayed lower Q_e+p/Q_t (less than 45%) and higher Q_f/Q_t (over 20%) compared to RPB8, which can be attributed to its higher concentration of oxygen-containing functional groups and lower mineral content. RPB8 exhibited the predominance of ion exchange and mineral precipitation mechanisms in terms of adsorbing heavy metals, whereas functional group complexation stood out as the predominant mechanism for SMB3. In the future, the heavy metal adsorption capacity and removal of biomass or biochar from industrial wastewater can be enhanced by incorporating metal ions through impregnation (Sekulic et al. 2018; Jiang et al. 2019).

This investigation examined the physicochemical properties and the heavy metal adsorption capabilities of biochars produced by SMB and RPB at different pyrolysis temperatures. The results showed that the SMB had a higher yield and was more economically viable than RPB. In addition, there are noticeable differences in their physicochemical characteristics among these two kinds of biochar. Therefore, the adsorption capacity of SMB and RPB showed a completely different trend. Results showed that SMB3 is suitable for mixed contaminants, and RPB8 has a selective removal capacity for Cu²⁺. The adsorption kinetics and isotherm model for SMB and RPB indicated that chemical adsorption played a dominant role in heavy metal removal. According to the relative contributions of the various mechanisms involved in the process of heavy metal adsorption, ion exchange and mineral precipitation were the primary mechanisms responsible for heavy metal adsorption by RPB8, while functional group complexation was the dominant mechanism for SMB3.

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

The authors declare there is no conflict.