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Article

The Effects of Three Bean Shell Biochars Under Different Pyrolysis Temperatures on the Adsorption of Cd and Pb in Aqueous Solutions

by
Tao Shen
1,†,
Hongyu Xia
1,†,
Heyi Zhang
1,
Song Guang
1,
Wenwen Hu
1,
Wenrui Zhao
1,
Kuan Zhao
1,*,
Xin Xiao
1,
Shiwen Zhang
2,* and
Aiai Xu
3
1
Key Laboratory of Intelligent Quality Monitoring and Soil Fertility Improvement for Farmland, College of Resources and Environment, Key Laboratory of Aqueous Environment Protection and Pollution Control of Yangtze River in Anhui of Anhui Provincial Education Department, School of Resource and Environment, Anqing Normal University, Anqing 246133, China
2
School of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, China
3
Institute of Resources, Environment and Soil Fertilizer, Fujian Academy of Agricultural Sciences/Fujian Key Laboratory of Plant Nutrition and Fertilizer, Fuzhou 350013, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work and should be considered co-first authors.
Water 2025, 17(7), 918; https://doi.org/10.3390/w17070918
Submission received: 25 February 2025 / Revised: 18 March 2025 / Accepted: 19 March 2025 / Published: 21 March 2025
(This article belongs to the Section Water Quality and Contamination)
Browse Figures
Versions Notes

Abstract

:
Biochar is an eco-friendly material that influences heavy metals adsorption. Three kinds of bean shell biochar, namely, peanut, pea, and soybean biochar, were prepared at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C. The characteristics of these biochars and the physicochemical properties of the biochars were analyzed. The capacities of the different biochars to adsorb Pb and Cd from aqueous solution were determined. With increasing pyrolysis temperature, the relative content of carbon in the biochar increased and that of hydrogen decreased, the porosity decreased, and the specific surface area increased; accordingly, the adsorption capacity of the biochar for Pb and Cd increased. The Pb adsorption capacity of the peanut shell biochar prepared at 400 °C was lower than that of the other shell biochars when the initial Pb concentration was at a low concentration. Three adsorption isotherm models were used to fit the adsorption processes of Pb or Cd on the three different biochars. The Freundlich curves better fit the adsorption capacity of biochar for Cd, and the Freundlich and Langmuir curves better fit the adsorption capacity of biochar for Pb. This work provides a scientific basis for the rational selection of bean shell biochar used in metal-contaminated water.

1. Introduction

Due to industrial processes such as mining, metallurgy, and papermaking, large amounts of heavy metals are released into the environment, causing severe environmental problems [1,2] and exacerbating water scarcity [3]. According to a report of the soil pollution status in China published in 2014, approximately 80% of farmland pollution is caused by irrigation with heavy metal-containing wastewater, which affects 14% of the food produced [4]. The preparation, characterization, and modification of biochar and its application in heavy metal remediation are popular topics in this field. Biochar has a large specific surface area, a well-developed pore structure, and abundant oxygen-containing functional groups, and it is widely used in the removal of heavy metals via adsorption [5]. Studies have shown that biochar can adsorb heavy metals such as lead (Pb) and cadmium (Cd) via mineral precipitation and ion exchange reactions [6,7]. The adsorption capacity of biochar depends on the raw materials and the pyrolysis temperature employed to prepare the materials [8].
Straw is one of the most important sources of biochar materials. Upon pyrolysis at temperatures less than 700 °C in low-oxygen or oxygen-free atmospheres, straw forms a loose, porous, highly aromatic and carbon-rich substance that is rich in nutrients, and this substance is referred to as biochar. Straw biochar mainly contains carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and other elements, and may contain Pb, Cu, and other heavy metal elements. Compared with biochar from other sources, legume-derived biochar materials, such as peanut, pea, and soybean biochar, have better adsorption properties and higher zeta potentials and are widely used in the fields of heavy metal removal and organic pollution treatment [9,10,11]. The preparation of straw biochar is affected by many factors, such as pyrolysis temperature and pyrolysis time [12]. The abundance of various functional groups in biochar gradually decreases with increasing pyrolysis temperature, which results in a decrease in the contents of H, O, and N and an increase in the degree of aromatization, content of fixed carbon, specific surface area, pore volume, and ash content of these materials [13,14,15,16,17,18]. Therefore, the preparation conditions strongly affect the specific surface area and porosity of biochar.
Biochar materials influence the migration and transformation of heavy metals in soils, water, and other environmental media. Functional groups such as carboxyl groups can effectively adsorb heavy metal ions [19]. Yuan et al. [20] reported that the adsorption efficiency of Cu ions in aqueous solution was 96.57% after 2 h of oscillation at 40 °C when 0.6 g of peanut shell biochar material was added to 50 mL solution. Song et al. [21] showed that the Mn adsorption capacity of modified peanut shell biochar was 2.1 times higher than that of unmodified peanut shell biochar for Pb in solution.
Many studies have shown that biochar and modified biochar have strong capacities to adsorb heavy metals, and the related mechanisms are relatively clear. However, systematic and comprehensive comparative studies on the mechanisms by which heavy metals adsorb to biochar prepared from leguminous plants such as soybean, pea, and peanut have not been reported. Soybeans, peas, and peanuts are the most common legumes in China and around the world; soybeans and peanuts are the main sources of edible vegetable oils, and large amounts of waste straw are inevitably generated during legume production and oil refining. However, the shells of many legumes are not fully utilized. Therefore, pea, peanut, and soybean bean shells were taken as the research objects in this study, and the physicochemical and structural characteristics of biochars made from the three bean shells under different pyrolysis temperatures were studied. Pb and Cd adsorption by these biochar materials was investigated, and the mechanism of Pb2+ and Cd2+ adsorption is discussed, providing scientific guidance for the remediation of heavy metal-polluted soil and water environments.

2. Materials and Methods

The methodology of this study involves biochar preparation, biochar characterization, physicochemical properties, adsorption experiments, and adsorption isotherms (the methodological framework is shown in Supplementary Figure S1). The specific experimental methods are shown in Section 2.1, Section 2.2, Section 2.3, Section 2.4 and Section 2.5

2.1. Biochar Preparation

The peanut shells, pea shells, and soybean shells were obtained in the same batch. The peanut shells, pea shells, and soybean shells were cleaned with deionized water and then dried at 100 °C for 12 h in a drying oven until a constant weight was reached. The peanut shell, pea shell, and soybean shell biochar materials were prepared via pyrolysis in a muffle furnace at 400 °C, 500 °C, and 600 °C for 4 h to carry out carbonization under oxygen-limited conditions. These biochar materials were subsequently ground and passed through a 200 mesh nylon sieve. The peanut shell biochar (PB) samples prepared at 400 °C, 500 °C, and 600 °C were named PB400, PB500, and PB600; the pea shell biochar (PW) samples were named PW400, PW500, and PW600; and the soybean shell biochar (PM) samples were named PM400, PM500, and PM600, respectively.

2.2. Characterization and Physicochemical Properties of the Biochar

The micromorphology of the three biochar materials was observed by scanning electron microscopy (SEM, Tescan MIRA LMS, Brno, Czech Republic). The element contents on the surface of these materials were analyzed via X-ray diffraction (XRD, Bruker & Advance, Berlin, Germany), the scanning range was 5–90°, the scanning speed was 2°/min, and the X-ray source was a copper target. The relative contents of C, H, S and N in the materials were determined via an elemental analyzer. The specific surface areas were determined via automatic specific surface and porosity analyses. The pH value of the biochar was determined via a pH electrode at a ratio of biochar to water of 1:20.

2.3. Pb or Cd Adsorption by Biochar

The initial Pb and Cd concentrations were determined to cover both low-dose and high-dose regions. As shown in Table 1, the concentrations of the individual Pb solutions ranged from 0 mg/L to 500 mg/L, the concentrations of the eleven individual Cd solutions ranged from 0 mg/L to 400 mg/L, and the numbers of concentration gradients were twelve and eleven set in Pb solutions and Cd solutions, respectively (Table 1). All solutions were prepared with Pb or Cd NO3− salts.
Biochar samples (15 mg) were weighed and placed in clean 50 mL conical bottles, and 15 mL of the twelve different Pb solutions or the eleven different Cd solutions were added to the conical bottles. According to previous studies [21,22], the biochar pH range the with best adsorption effect on many bivalent metal cations is 4–6. Therefore, we adjusted the pH of the solution to 5.0 ± 0.1 by adding 0.1 mol/L HNO3 or 0.1 mol/L NaOH. Then, the samples were then shaken for 12 h in an orbital shaker and filtered through a 0.22 μm filter membrane, and the concentrations of Cd and Pb in the filtrate were measured via ICP–AES (VISTA-MPX, Varian, Palo Alto, CA, USA).

2.4. Models to Fit the Adsorption Capacity of Biochar for Pb or Cd

On the basis of the adsorption capacity of the biochar and the initial content of Pb or Cd in the solution, adsorption curves were plotted and fitted via three adsorption isotherm models, namely, the Henry, Freundlich, and Langmuir models. The equations for the three isotherms are shown in Table 2.
In Table 2, G represents the adsorption capacity of biochar (mg/g); c represents the initial concentration of heavy metal ions in solution (mg/L); k1 and k2 represent the adsorption constants; and G0 represents the maximum adsorption capacity. n and A are the parameters of the Freundlich and Langmuir equations, respectively, which are related to the adsorption strength and adsorption capacity.

2.5. Statistical Analyses

SPSS 24.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analyses. Differences in the physicochemical properties, such as specific surface area, elemental content, and pH values, of the different bean shell biochars prepared at different pyrolysis temperatures were tested via analysis of variance (ANOVA). The results are presented as arithmetic means ± standard error (SE). Duncan’s multiple range tests at p < 0.05 were subsequently used to compare least significant differences.

3. Results and Discussion

3.1. Physicochemical Properties of the Three Kinds of Biochar

The physicochemical properties of the three biochar materials, including their specific surface areas, elemental contents, and pH values, are displayed in Figure 1A. The three biochar materials were alkaline, which is consistent with the results of Yuan et al. [23]. At the same pyrolysis temperature, the pH value of PB was lower than that of PW and PM. At the pyrolysis temperature of 500 °C, the pH values of PB and PW were higher than those at 400 °C and 600 °C, while the pH values of PM500 and PM600 showed no significant difference. Therefore, the initial pH value may influence the ability to adsorb Cd and Pb.
The specific surface areas of the three types of biochar prepared at different temperatures were different, as shown in Figure 1B. The specific surface areas of PB600 and PM600 were significantly greater than those of PB and PM prepared at pyrolysis temperatures of 400 °C and 500 °C, whereas the specific surface area of PW500 was greater than those of PW400 and PW600. These results indicate that pyrolysis temperature affects the porosity of the three biochar materials, possibly due to changes in the volatile matter or ash content, or the structure of the three types of biochar with increasing pyrolysis temperature [24,25,26,27,28]. Therefore, during biochar formation, the pore structure was destroyed in PW and PM, which decreased the porosity and increased the specific surface area of these materials.
In general, the contents of C, H, N, and S in the three biochar materials significantly differed. With increasing pyrolysis temperature, the content of C increased, and the proportion of C was greater than 60%, whereas the contents of N and H decreased, which indicated that with increasing pyrolysis temperature, the content of volatile substances decreased significantly, and the degree of carbonization increased (Figure 1C–E). At the same pyrolysis temperature, the C content in PB was significantly greater than that in PW and PM, whereas the S content in PW was greater than that in the other two biochar materials (Figure 1C,F).
Pyrolysis temperature affects the adsorption capacity for heavy metals. Generally, biochar prepared at a high pyrolysis temperature has a higher pH, specific surface area, and ash and carbon content, while the content of oxygen-containing functional groups presents an opposite trend [29]. However, the effect of pyrolysis temperature on heavy metal sequestration is not always consistent. The effect of pyrolysis temperature on the bioavailability of heavy metals might be feedstock-dependent [11]. In our experiment, the pH and sulfur content of PW500 were lower than that of PW600. Therefore, the degree of carbonization of PW500 was higher than that of PW600 because the specific surface area of PW500 was higher than that of PW600.

3.2. Structural Characterization of the Three Biochar Materials

3.2.1. SEM Analysis

The surfaces of the three biochar materials were relatively smooth, and all of them were composed of irregular particles and fragments, as shown in Figure 2, which occurred due to the release of volatile substances during the pyrolysis process. At 400 °C and 600 °C, the structure of PB was rougher than that of PW and PM. When the cellulose, hemicellulose, and lignin structures were exposed, fragments appeared on the surface of the biochar material, and most of the particles displayed a certain lamellar stacking structure when the bean shell biochar materials were pyrolyzed. This may have been caused by the destruction of aliphatic hydrocarbon groups and carbonyl groups; the protection of aromatic nuclei during the pyrolysis process; and the formation of lamellar structures characterized by holes, thin lamellae, and layered stacks. However, these structures can provide adsorption sites for heavy metals, which can increase the amount of heavy metals that the materials can adsorb [30,31,32,33].

3.2.2. XRD Analysis

The XRD patterns of the three biochar materials are shown in Figure 3. A large and wide peak appeared at 23° in the three biochar materials prepared at three different pyrolytic temperatures, which indicates that an amorphous carbon structure existed and that the degree of carbonization was high for all these materials. The C contents of PB, PW, and PM increased from 74.55% to 81.84%, from 62.58% to 66.74%, and from 62.75% to 66.78% and (Figure 1), respectively. According to Supplementary Figure S2, the peaks of the three biochar materials became sharper at pyrolysis temperatures of 400 °C and 500 °C, and became relatively flat at a pyrolysis temperature of 600 °C. Accordingly, the adsorption efficiency of the materials increased. PB had the highest and sharpest peak, as shown in Figure 3, which is consistent with the C content described in Figure 1.

3.3. Pb Adsorption by the Three Types of Biochar

The adsorption capacity of the three biochar materials for Pb increased with increasing pyrolysis temperature, as shown in Figure 4. When the initial concentration of Pb in aqueous solution was greater than 200 mg/L, the adsorption capacities of PB600 and PW600 were significantly greater than the capacities of the materials prepared at pyrolysis temperatures of 400 °C and 500 °C. When the initial concentration of Pb in aqueous solution was greater than 100 mg/L, the adsorption capacity of PM600 was significantly greater than that of PM500 and PM400. These results are similar to those of Jia and Jazini et al., who reported similar trends in specific surface area and porosity and noted that biochar adsorption capacity increased with increasing pyrolysis temperature [34,35].
Biochar materials have a certain number of adsorption sites, and these sites may not be fully utilized at a low initial concentration of heavy metals. With increasing initial heavy metal concentration, solute molecules begin to occupy these unused sites, which results in an increase in the adsorption capacity of biochar for heavy metals [36]. Therefore, the adsorption capacity of the three biochar materials increased with increasing initial Pb concentration. The adsorption capacities of the three biochar materials for Pb were similar when the biochar pyrolysis temperature was 600 °C because of the lack of change in ash content. In addition, the increase in Pb adsorption with increasing initial Pb concentration was lo0wer for the three biochar materials prepared at 400 °C and 500 °C than for those prepared at 600 °C, which further indicates that the biochar materials prepared at low pyrolysis temperatures had lower surface areas and fewer adsorption sites.
In general, the Pb adsorption capacity of PB, PM, and PW increased with increasing initial Pb concentration, regardless of the pyrolysis temperature. When the initial concentration of Pb was lower than 100 mg/L, the Pb adsorption capacity of PB400 was lower than that of PW400 and PM400, whereas when the initial concentration of Pb was between 100 mg/L and 500 mg/L, the Pb adsorption capacities of the three biochar materials were similar. There was no significant difference in the Pb adsorption capacity of PB, PM, and PW at the same pyrolysis temperature and the same initial Pb concentration.

3.4. Cd Adsorption by the Three Types of Biochar

The Cd adsorption capacities of the three kinds of biochar materials prepared at different pyrolysis temperatures are shown in Figure 5. The Cd adsorption capacities of PB, PM, and PW increased with increasing initial Cd concentration and were not significantly different with increasing pyrolysis temperature. In general, the Cd adsorption capacities of PB, PM, and PW were the strongest at a pyrolysis temperature of 600 °C, followed by those for PB, PM, and PW prepared at 400 °C and 500 °C. When the initial Cd concentration was lower than 100 mg/L, the adsorption capacities of the three materials did not differ. When the initial Cd concentration was greater than 100 mg/L, the Cd adsorption capacities of PB and PW were greater than that of PM. The results revealed that the Cd adsorption capacity increased with increasing pyrolysis temperature, indicating that there was a relationship between the initial Cd concentration and the adsorption capacity.

3.5. Biochar Adsorption Isotherm Models for Pb and Cd

The previous studies show that the mechanisms by which Pb2+ and Cd2+ are removed from aqueous solution might involve chemical precipitation with carbonate, phosphate, and silicon dioxide in wheat straw and rice straw biochar, which occurs through adsorption on the surface of maize stalk biochar particles that contain large numbers of functional groups; furthermore, the isothermal adsorptions of Pb2+ and Cd2+ fit well with the Langmuir equation [37]. Cao et al. [38] reported that phosphorus (P) in biochar materials precipitates and thereby promotes the adsorption of Pb. Some studies have indicated that the Langmuir model is suitable for single layer adsorption and that the Freundlich model can be used for both single-layer adsorption and multilayer adsorption; however, in most cases, the Freundlich model is suitable for adsorption at medium and high concentrations of adsorbate [39,40]. The adsorption curves, characterized by the initial concentrations of heavy metals and the adsorption capacity of the biochar materials, were fitted with the Henry, Freundlich, and Langmuir adsorption isotherms, and the fitting parameters are displayed in Table 3 and Table 4. The adsorption curves fit well using the three adsorption isotherms (R > 0.9, p < 0.0001). Overall, the Freundlich adsorption isotherms fit the data better for Cd, and the Freundlich and Langmuir adsorption isotherms fit the data better for Pb. The initial concentrations of Pb and Cd were lower than 1000 mg/L; however, the Freundlich model fit the adsorption trends in Pb and Cd because the three biochar materials had a layered and stacked structure. The functional groups on the surface of biochar play a great role in adsorption performance [41]. These functional groups mainly include carboxyl, lactone, phenolic hydroxyl, and carbonyl groups. Heavy metals can interact with these functional groups through metal–ligand complexation. In this study, PB has a higher organic carbon content and more oxygen-containing functional groups among the three types of biochar, and these structures expose more active sites, which are more conducive to chemical adsorption.

4. Conclusions

In this work, PB, PM, and PW were prepared at three pyrolysis temperatures. The pH value, specific surface area, and C content of the three biochar materials increased with increasing pyrolysis temperature. The porosity decreased and the specific surface area increased, as determined by SEM and X-ray diffraction (XRD) analyses, which influenced the adsorption capacity of the biochar materials for Pb and Cd. The amount of Pb and Cd adsorbed to the three materials increased with increasing pyrolysis temperature. PB, PM, and PW pyrolysis at 600 °C resulted in greater amounts of Pb and Cd adsorption than pyrolysis at the other two temperatures. When the initial Pb concentration was low, the amount of Pb adsorbed by PB400 was lower than that adsorbed by PW400 and PM400, but the amount of Cd adsorbed by the three kinds of biochar materials was not significantly different. The isothermal adsorption curves revealed that the Henry-type, Freundlich-type, and Langmuir-type equations fit the Pb and Cd adsorption trends for the three bean shell biochars. The Freundlich equation better fit the adsorption capacity of the three biochar materials for Cd, whereas the Freundlich and Langmuir equations better fit the adsorption trends of the materials for Pb. This study provides more options for the removal of heavy metals in wastewater. In future studies, systematic work is needed to investigate the effect of biochar modification and aging on the heavy metal adsorption capacity, the adsorption mechanism of biochar for heavy metals in industrial wastewater, and the synergistic effects of biochar and microorganisms on combined heavy metal pollution soil.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w17070918/s1, Figure S1: Methodological frame diagram; Figure S2: The partial enlarged drawing of XRD patterns of the three biochars.

Author Contributions

Conceptualization, W.Z. and K.Z.; methodology, T.S., H.Z., S.G. and W.H.; software, T.S. and K.Z.; validation, K.Z., X.X. and W.Z.; formal analysis, T.S.; investigation, T.S., H.Z., S.G. and W.H.; resources, K.Z., W.Z. and A.X.; data curation, T.S., H.Z., S.G. and W.H.; writing—original draft preparation, T.S., H.X. and K.Z.; writing—review and editing, K.Z.; visualization, T.S. and H.Z.; supervision, W.Z. and K.Z.; project administration, H.X. and K.Z.; funding acquisition, W.Z., K.Z., S.Z. and A.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Natural Science Research Projects of Universities in Anhui Province (No. 2023AH050486, No. 2024AH051087), the Opening Foundation of Anhui Province Engineering Research Center of Water and Soil Resources Comprehensive Utilization and Ecological Protection in High Groundwater Mining Area (No. 2023-WSREPMA-04), the Opening Foundation of Fujian Key Laboratory of Plant Nutrition and Fertilizer (No. 2024PNFKL22), and the Open bidding for selecting the best candidates science and technology project of Anqing yangtze delta future industry institute (No. 2023JBGS17, No. 2023JBGS13).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 00918 g001
Figure 1. Physicochemical characteristics of the three biochar materials at different pyrolysis temperatures. Note: (A), pH of the three biochar materials at different pyrolysis temperatures; (B), the specific surface areas of the three biochar materials at different pyrolysis temperatures; (C), the contents of C of the three biochar materials at different pyrolysis temperatures; (D), the contents of H of the three biochar materials at different pyrolysis temperatures; (E), the contents of N of the three biochar materials at different pyrolysis temperatures; (F), the contents of S of the three biochar materials at different pyrolysis temperatures. Values are means (M) ± standard error (SE) (n = 3). Lowercase letters indicate differences between different bean shell biochars at the same pyrolysis temperature (p < 0.05), and uppercase letters indicate differences among the same bean shell biochar prepared at different pyrolysis temperatures (p < 0.05).
Figure 1. Physicochemical characteristics of the three biochar materials at different pyrolysis temperatures. Note: (A), pH of the three biochar materials at different pyrolysis temperatures; (B), the specific surface areas of the three biochar materials at different pyrolysis temperatures; (C), the contents of C of the three biochar materials at different pyrolysis temperatures; (D), the contents of H of the three biochar materials at different pyrolysis temperatures; (E), the contents of N of the three biochar materials at different pyrolysis temperatures; (F), the contents of S of the three biochar materials at different pyrolysis temperatures. Values are means (M) ± standard error (SE) (n = 3). Lowercase letters indicate differences between different bean shell biochars at the same pyrolysis temperature (p < 0.05), and uppercase letters indicate differences among the same bean shell biochar prepared at different pyrolysis temperatures (p < 0.05).
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Figure 2. SEM images of the three types of biochar at different pyrolysis temperatures.
Figure 2. SEM images of the three types of biochar at different pyrolysis temperatures.
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Figure 3. XRD patterns of the three biochars prepared at different pyrolysis temperatures.
Figure 3. XRD patterns of the three biochars prepared at different pyrolysis temperatures.
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Figure 4. Pb adsorption capacity of the three types of biochar prepared at different pyrolysis temperatures. Note: (A), Pb adsorption capacity of PB at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (B), Pb adsorption capacity of PW at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (C), Pb adsorption capacity of PM at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C.
Figure 4. Pb adsorption capacity of the three types of biochar prepared at different pyrolysis temperatures. Note: (A), Pb adsorption capacity of PB at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (B), Pb adsorption capacity of PW at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (C), Pb adsorption capacity of PM at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C.
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Figure 5. Cd adsorption capacity of the three types of biochar prepared at different pyrolysis temperatures. Note: (A), Cd adsorption capacity of PB at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (B), Cd adsorption capacity of PW at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (C), Cd adsorption capacity of PM at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C.
Figure 5. Cd adsorption capacity of the three types of biochar prepared at different pyrolysis temperatures. Note: (A), Cd adsorption capacity of PB at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (B), Cd adsorption capacity of PW at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C; (C), Cd adsorption capacity of PM at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C.
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Table 1. The concentrations of the individual Pb solutions or Cd solution.
Table 1. The concentrations of the individual Pb solutions or Cd solution.
NumberPb Solution Concentrations (mg/L)Cd Solution Concentrations (mg/L)
100
212
354
41010
52020
64040
75050
8100100
9200200
10300300
11400400
12500not set
Table 2. Equations used to analyze the experimental data.
Table 2. Equations used to analyze the experimental data.
EquationAdsorption Isotherm ModelAdsorption Isotherm Formulas
(1)HenryG = k1c
(2)FreundlichG = k2c1/n
(3)LangmuirG = G0c/(A + c)
Table 3. Pb adsorption isotherm models for the three types of biochar prepared at different pyrolysis temperatures.
Table 3. Pb adsorption isotherm models for the three types of biochar prepared at different pyrolysis temperatures.
Biochar MaterialFitting Parameters
Henry TypeFreundlich TypeLangmuir Type
Rp ValueRp ValueRp Value
PB4000.9334<0.00010.9358<0.00010.9364<0.0001
PB5000.9862<0.00010.9949<0.00010.9970<0.0001
PB6000.9997<0.00010.9998<0.00010.9999<0.0001
PW4000.9664<0.00010.9853<0.00010.9898<0.0001
PW5000.9914<0.00010.9972<0.00010.9968<0.0001
PW6000.9999<0.00010.9999<0.00010.9999<0.0001
PM4000.9401<0.00010.9397<0.00010.9299<0.0001
PM5000.9787<0.00010.9877<0.00010.9885<0.0001
PM6000.9999<0.00010.9999<0.00010.9999<0.0001
Note: R is the correlation coefficient of the fitting equation, and p indicates the significance level.
Table 4. Cd adsorption isotherm models for the three types of biochar prepared at different pyrolysis temperatures.
Table 4. Cd adsorption isotherm models for the three types of biochar prepared at different pyrolysis temperatures.
Biochar MaterialFitting Parameters
Henry TypeFreundlich TypeLangmuir Type
Rp ValueRp ValueRp Value
PB4000.9969<0.00010.9983<0.00010.9960<0.0001
PB5000.9572<0.00010.9878<0.00010.9519<0.0001
PB6000.9964<0.00010.9994<0.00010.9937<0.0001
PW4000.9953<0.00010.9952<0.00010.9951<0.0001
PW5000.9869<0.00010.9947<0.00010.9850<0.0001
PW6000.9809<0.00010.9840<0.00010.9798<0.0001
PM4000.9961<0.00010.9980<0.00010.9958<0.0001
PM5000.9622<0.00010.9912<0.00010.9573<0.0001
PM6000.9949<0.00010.9956<0.00010.9940<0.0001
Note: R is the correlation coefficient of the fitting equation, and p indicates the significance level.
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Shen, T.; Xia, H.; Zhang, H.; Guang, S.; Hu, W.; Zhao, W.; Zhao, K.; Xiao, X.; Zhang, S.; Xu, A. The Effects of Three Bean Shell Biochars Under Different Pyrolysis Temperatures on the Adsorption of Cd and Pb in Aqueous Solutions. Water 2025, 17, 918. https://doi.org/10.3390/w17070918

AMA Style

Shen T, Xia H, Zhang H, Guang S, Hu W, Zhao W, Zhao K, Xiao X, Zhang S, Xu A. The Effects of Three Bean Shell Biochars Under Different Pyrolysis Temperatures on the Adsorption of Cd and Pb in Aqueous Solutions. Water. 2025; 17(7):918. https://doi.org/10.3390/w17070918

Chicago/Turabian Style

Shen, Tao, Hongyu Xia, Heyi Zhang, Song Guang, Wenwen Hu, Wenrui Zhao, Kuan Zhao, Xin Xiao, Shiwen Zhang, and Aiai Xu. 2025. "The Effects of Three Bean Shell Biochars Under Different Pyrolysis Temperatures on the Adsorption of Cd and Pb in Aqueous Solutions" Water 17, no. 7: 918. https://doi.org/10.3390/w17070918

APA Style

Shen, T., Xia, H., Zhang, H., Guang, S., Hu, W., Zhao, W., Zhao, K., Xiao, X., Zhang, S., & Xu, A. (2025). The Effects of Three Bean Shell Biochars Under Different Pyrolysis Temperatures on the Adsorption of Cd and Pb in Aqueous Solutions. Water, 17(7), 918. https://doi.org/10.3390/w17070918

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