Optimization of Mixed-Based Biochar Preparation Process and Adsorption Performance of Lead and Cadmium

Abstract

Irreversible pollution by heavy metals such as lead (Pb) and cadmium (Cd) adversely affects the ecological environment and human health. Due to its high adsorption, microporosity, and specific surface area, biochar possesses excellent potential for use in heavy metal pollution remediation. The preparation of mixed-based biochar from sludge and cotton stalk can solve the problems inherent to pure sludge biochar, such as undeveloped pore structure and a small specific surface area, while resourcefully utilizing both waste biomass types. This study investigated the adsorption capacity for Pb²⁺ and Cd²⁺ of mixed-based biochar prepared at different pyrolysis temperatures, different pyrolysis residence times, and different cotton stalks percentages. Response surface experiments revealed the optimum process conditions for preparing mixed-based biochar, which included a pyrolysis temperature of 638 °C, a pyrolysis residence time of 86 min, and an addition ratio of 50% for cotton stalks. The isothermal adsorption experiments revealed that the maximum adsorption capacities of mixed-based biochar for Pb²⁺ and Cd²⁺ were 111.11 and 86.21 mg/g, respectively. Our findings suggest the co-pyrolysis of sludge and cotton stalk as a green and sustainable method for safely disposing of Pb and Cd.

1. Introduction

Heavy metal contamination is characterized by high biological toxicity, a long residence time, and remediation difficulties. This type of contamination can damage ecosystems, restrict biological growth, and cause human health hazards [1,2]. The Agency for Toxic Substances and Disease Registry classifies lead (Pb) and cadmium (Cd) as the second and seventh most hazardous substances, respectively [3,4]. Pb and Cd can inhibit enzyme activity and reduce microbial biomass, ultimately damaging ecological structure and function [5]. They can also affect the absorption of nitrogen, P, and potassium nutrients by crops and inhibit crop growth [6]. Additionally, Pb and Cd can accumulate rapidly in the food chain and can cause severe damage to nerves and kidneys after entering the body [7,8,9].

Due to the accelerated economic development and urbanization in recent years, China has built numerous urban sewage treatment plants that generate large amounts of sludge annually [10,11]. According to the statistical data of China Urban and Rural Construction, the output of sludge in 2019 was approximately 55 million tons, and in 2021, it was approximately 80.6 million tons, and greater than 90 million tons of annual sludge production is expected to be available for biochar production in 2025 [12,13]. The moisture content of sludge is typically 70~80%, and based on this, dehydration treatment is a key issue for rational disposal and resourceful use of sludge [14]. The preparation of biochar via pyrolysis can facilitate harmless sludge reduction, and this results in high economic benefits [15]. Biochar is an eco-friendly adsorbent possessing rich functional groups and a pore structure that can effectively adsorb Cd²⁺ and Pb²⁺ [16,17,18]. Ghorbani et al. discovered that biochar produced from rice stalks could effectively improve the photosynthesis characteristics of plants in acidic soil and mitigate Cd bioaccumulation [17]. Wang et al. demonstrated that biochar derived from sewage sludge/cotton stalks (SCB) was more effective in reducing the mobility and bioavailability of Pb in the soil than biochar derived from sewage sludge alone (SSB) [18].

Xinjiang is one of the three major cotton-producing areas in China where the disposal of cotton stalks is a serious issue [19]. Traditional disposal methods, such as composting, crushing, and incineration, not only cause massive resource waste but also seriously pollute the local atmospheric environment [20]. Pyrolysis is an efficient and eco-friendly method for processing cotton stalks, with the advantages of fast processing speed and high conversion efficiency. Moreover, co-pyrolysis can decrease pyrolysis temperature, shorten pyrolysis residence time and conserve energy consumption [21,22,23]. Pyrolysis temperature, residence time, and biomass are essential factors that affect the adsorption performance of mixed-based biochar [24]. During sludge pyrolysis, adding different biomasses affect the adsorption performance of mixed-based biochar [10,11,25,26]. To date, only a small number of studies have focused on the preparation of mixed-based biochar adsorbent materials via co-pyrolysis of cotton stalks and sludge, and no study has used the response surface analysis method to optimize the process conditions of co-pyrolysis of cotton stalks and sludge to prepare sludge/cotton stalks mixed with biochar for the remediation of lead and cadmium composite pollution.

To prepare a sludge/cotton stalk mixed-based biochar with excellent adsorption performance that meets the demand for the resource utilization of sludge and cotton stalks, we used co-pyrolysis of sludge and cotton stalks to prepare mixed-based biochar in this study. The effects of pyrolysis temperature, pyrolysis residence time, and cotton stalk addition ratio on the iodine adsorption value of the mixed-based biochar were also investigated using single-factor iodine adsorption experiments. A response surface analysis method was used to optimize the co-pyrolysis process conditions, and the adsorption laws and mechanisms of the mixed-based biochar on heavy metal ions (Pb²⁺ and Cd²⁺) were studied in Pb²⁺ and Cd²⁺ adsorption experiments performed in aqueous solution. Our findings provide technical support for further application of the prepared mixed-based biochar for the remediation of Pb and Cd composite heavy metal pollution.

2. Materials and Methods

2.1. Raw Materials

The South Suburban Wastewater Treatment Plant in Karamay City, in the Xinjiang Uygur Autonomous Region of China, provided the sludge source for the experiments. Cotton stalks were purchased from Xinjiang Shenghe Fuchang Stalks Agricultural Technology Co. in Karamay. Previous studies have reported the basic properties of sludge and cotton stalks [27]. Sludge and cotton stalks were air-dried for one week, dried in a laboratory oven at 85 °C for 12 h, and ground through a 0.25-mm sieve.

2.2. Biochar Preparation

Sludge was weighed, and different proportions of cotton sticks were added. The total mass of sludge and cotton sticks in each sample was 50 g. The two substances were thoroughly mixed and placed in quartz tubes. A continuous inflow of N₂ at 0.5 L/min was directed to an electrically heated furnace to maintain an oxygen-free environment before and during pyrolysis [28]. Co-pyrolysis experiments were performed in an electrically heated furnace possessing a heating rate of 20 °C·min⁻¹. Pyrolysis temperatures were set to 350 °C, 450 °C, 550 °C, 650 °C, and 750 °C, respectively. Pyrolysis residence times were set to 30 min, 60 min, 90 min, 120 min, and 150 min, respectively. Additional ratios for cotton stalks were set to 0%, 10%, 30%, 50%, 70%, and 100%, respectively.

2.3. Response Surface Analysis Experiment

According to the results of the single-factor iodine adsorption experiments examining the mixed biochar, the pyrolysis temperature (T), pyrolysis residence time (t), and cotton stalk percentage (C) were selected for the response surface analysis experiments and analyzed using the response surface analysis professional software Design Expert 8.0.5 according to the Box–Behnken central combinatorial experimental design principle with the iodine adsorption value as the response value to obtain the optimal process conditions for the preparation of biochar by co-pyrolysis. The optimum process conditions were determined using Design Expert 8.0.5 (Stat-Ease, Inc., Minneapolis, MN, USA).

2.4. Adsorption Experiments for Pb²⁺ and Cd²⁺ on Mixed-Based Biochar

A total of 100 mg of mixed base biochar was accurately weighed in two 50-mL centrifuge tubes, and 25 mL of heavy metal solution with a Pb²⁺ concentration of 300 mg/L and a Cd²⁺ concentration of 100 mg/L was added. The initial pH of the heavy metal solution was adjusted with a 1% HNO₃ and 1% NaOH solution. The tubes were sealed and centrifuged at 3000 r/min for 5 min in a constant temperature oscillator at 25 °C. The supernatant was collected after filtration, and the concentrations of Pb²⁺ and Cd²⁺ in the solution were determined using inductively coupled plasma mass spectrometry (Optima 5300 dV; Perkin Elmer, Waltham, MA, USA). The amounts of heavy metals (Pb and Cd) adsorbed by the mixed biochar were calculated as follows:

q_t = (C₀ − C_t) V/m,

(1)

where q_t is the amount of heavy metal adsorbed at time t (mg/g), C₀ is the initial concentration of heavy metal (mg/L), C_t is the concentration of heavy metal (mg/L), m is the mass of biochar (g), and V is the volume of the solution (L).

2.5. Statistical Analyses

Statistical analyses were conducted using the SPSS statistical package (version 19.0; IBM Corp., Armonk, NY, USA). The data were subjected to analysis of variance, and the means were separated using the protected least significant difference set at p < 0.05.

3. Results and Discussion

3.1. Single-Factor Iodine Adsorption Experiments

Figure 1a–c present the variation of iodine adsorption values of the mixed-based biochar prepared at different pyrolysis temperatures (350–750 °C), different pyrolysis residence times (30–150 min), and different cotton stalk percentages (0–100%), respectively. The other preparation conditions for the mixed-based biochar included a 2 h pyrolysis residence time with 50% cotton stalks, a pyrolysis temperature of 650 °C with 50% cotton stalks, and a pyrolysis temperature of 650 °C with a 90 min pyrolysis residence time.

Up to a temperature of 650 °C, the iodine adsorption value of the mixed-based biochar increases as the pyrolysis temperature increases. When the pyrolysis temperature exceeds 650 °C, the “ablation” phenomenon will occur, which leads to the interpenetration of several pores within the mixed-based biochar and the loss of specific surface area, ultimately causing the iodine adsorption capacity of the mixed-based biochar to decrease [29]. During the initial pyrolysis stage, the iodine adsorption of the mixed-based biochar increased as the pyrolysis residence time increased. When the pyrolysis residence time exceeded 90 min, the pore “ablation” phenomenon would also occur and would result in poor pore development of the mixed-based biochar, a reduction in the percentage of micropores, and a decline of the specific surface area, ultimately causing the iodine adsorption capacity to decrease. The iodine adsorption capacity of mixed biochar was positively correlated with the amount of cotton stalks that were added. This occurs due to the ability of the addition of cotton stalks to increase the carbon content of the raw material and provide a carbon source for generating more pores. This results in a larger surface area on the mixed-based biochar, increasing the iodine adsorption value.

3.2. Response Surface Analysis Experiments

3.2.1. Secondary Regression Analysis and Optimization of Process Parameters

According to the results of single-factor iodine adsorption experiments examining mixed biochar, the pyrolysis temperatures of response surface analysis experiments were set to 550, 650, and 750 °C. The pyrolysis residence times were 60, 90, and 120 min. The cotton stalk percentages were set to 40, 50, and 60%. The experimental design and results of the response surface analysis are listed in

Table 1. Response surface analysis experimental design and results.

Items	Temperature A/°C	Time B/min	Ratio C/%	Iodine Value Y/(mg·g−1)
1	550	90	40	292.13
2	550	60	50	341.23
3	650	90	50	411.25
4	650	90	50	406.24
5	650	90	50	418.66
6	750	120	50	397.18
7	750	60	50	365.96
8	650	120	60	394.59
9	750	90	40	316.89
10	750	90	60	344.45
11	650	60	40	318.65
12	650	120	40	353.64
13	650	60	60	346.66
14	650	90	50	411.43
15	550	120	50	379.55
16	650	90	50	413.41
17	550	90	60	318.62

.

Multiple regression analysis of the data presented in Table 1 was performed using the response surface analysis professional software design expert 8.0.5, obtaining the regression equation and optimal conditions for the iodine values of the mixed-based biochar (

Table 2. Regression equation and optimal conditions of the biochar.

Regression Equation	Correlation Coefficient R2	SNR	Significant Level	Response Conditions			Iodine Value (mg·g−1)
				Temperature A/°C	Time B/min	Proportion C/%
Y = −2568.90 + 4.75A + 1.04B + 56.29C − 5.92 × 10−4AB + 2.68 × 10−4AC + 0.011BC − 3.83 × 10−3A2 − 3.25 × 10−3B2 − 0.56C2	0.9601	4.45	A, B, C, BC, A2, C2	638.22	86.54	50.23	448.63

). The correlation coefficient (R²) of the model was greater than 0.95, thus indicating that more than 95% of the changes in the response values originated from the selected variables. Therefore, the regression equation can better describe the relationship between the factors and response values that can be used to analyze and predict the optimal conditions for preparing mixed-based biochar. The model parameters exhibiting a probability of p < 0.05 were considered to be significant. A, B, C, BC, A², and C² are significant parameters in this model. The optimal process conditions for preparing mixed-based biochar were obtained by software analysis and included a pyrolysis temperature of 638.22 °C and a pyrolysis residence time of 86.54 min with 50.23% cotton stalks. Under these preparation conditions, the maximum iodine adsorption value of the mixed-based biochar predicted by the model was 448.63 mg·g⁻¹.

The optimized process conditions were used for validation experiments. The actual operation adjusted the above process conditions to a pyrolysis temperature of 638 °C and a pyrolysis residence time of 86 min, with cotton stalks accounting for 50%. The iodine adsorption value of the mixed-based biochar obtained under these conditions was 441.24 mg·g⁻¹. The predicted value is close to the actual measured value, thus indicating that the model is reliable and possesses particular guiding significance for future experiments.

3.2.2. Effects of Interactions among Pyrolysis Factors on the Iodine Adsorption Values of Mixed-Based Biochar

Further response surface analysis was performed to investigate the interactions between two of the three pyrolysis factors and their effects on the iodine adsorption capacity of the mixed biochar. The response surface analysis of these two factors is presented in Figure 2. The results indicated that the interaction between the pyrolysis temperature and pyrolysis residence time was not evident, and there was no interaction between the pyrolysis temperature and the cotton stalk percentage. However, there was an essential interaction between the pyrolysis residence time and the cotton stalk production rate, and this implied that the rate of cotton stalks production could be appropriately reduced when the pyrolysis residence time was increased.

3.3. Adsorption Experiments of Pb²⁺ and Cd²⁺ on Mixed-Based Biochar

3.3.1. Effects of Initial pH of the Solution on the Adsorption Values of Pb²⁺ and Cd²⁺

Under conditions including 25 °C and a mixed-based biochar dosing of 100 mg (100 mesh), the initial concentrations of Pb²⁺ and Cd²⁺ were 300 and 100 mg/L, respectively. The effects of different initial solution pH values on the amounts of Pb²⁺ and Cd²⁺ that were adsorbed are presented in Figure 3.

The initial pH of the solution used in this experiment was between 2.0 and 7.0. As presented in Figure 3a, the adsorption of Pb²⁺ by the mixed-based biochar increased significantly with an increase in the initial pH of the solution, and the trend of increasing the adsorption of Pb²⁺ by the mixed-based biochar was evident when the initial pH of the solution exceeded 5.0. When the solution pH was less than 7.01, Pb²⁺ existed primarily in the ionic state. When the solution pH was greater than 7.01, Pb²⁺ formed a precipitate with -OH groups [30].

The initial pH of the solution in this experiment was set in the range of 2.0–8.0. As presented in Figure 3b, the initial pH of the solution significantly affected the adsorption of Cd²⁺ by the mixed biochar. Within the pH range of 2.0–6.0, the adsorption amount increased rapidly with increasing solution pH, and the increasing trend of Cd²⁺ adsorption by the mixed-based biochar became slower when the initial pH of the solution exceeded 6.0. Cd²⁺ ions exist in a free state and as different hydroxyl-coordinated ions under different pH conditions, such as Cd(OH)⁺, Cd(OH)₂, Cd(OH)₃⁻, and Cd(OH)₄²⁻. When the pH was less than 8.0, Cd²⁺ existed primarily as free Cd²⁺ with a small amount of Cd(OH)⁺. When the pH was greater than 8.0, it existed primarily as anionic complexes, including Cd(OH)₂, Cd(OH)₃⁻, and Cd(OH)₄²⁻.

The pH of the solution exerts a significant influence on the formation of heavy metal ions in the solution and charge distribution on the surface of the adsorbent, thus affecting the adsorption of heavy metal ions in the solution by the adsorbent. When the pH of the solution was low, the surface of the biochar was positively charged, and the metals in the solution predominantly existed in the ionic state. The adsorption of metal ions onto biochar is primarily caused by ion exchange between the metal ions on the surface of the biochar and the adsorbed metals in the solution. Compared to biochar sourced from plant stems, the mixed-based biochar prepared in this study contained a variety of metals such as Ca, Mg, Zn, and Cu that can provide more ion-exchange sites for Pb and Cd ions in solution, thus making adsorption easier. However, the pH of the solution was assumed to be extremely low. In this case, the adsorption of Pb and Cd ions on the surface of the biochar was inhibited due to the competition between H⁺ and Pb and Cd ions for adsorption. When the pH of the solution is high, it causes the biochar surface to be negatively charged, and several Pb and Cd ions in the solution form anionic complexes. The adsorption of Pb and Cd ions by biochar primarily occurred through an electrostatic effect. Compared to the biochar prepared by pure sludge pyrolysis, the mixed-based biochar prepared in this study possessed a larger specific surface area that was beneficial for the adsorption of Pb and Cd when the pH of the solution was high. However, suppose the solution pH is too high. In that case, the formation of Pb and Cd anion complexes reduces the free degree of Pb and Cd ions in the solution, and this is unfavorable for the adsorption of Pb and Cd by hybrid-based biochar. Therefore, either too high or too low pH values are unfavorable for Pb and Cd adsorption by mixed-based biochar. In this study, the optimum pH of the mixed-based biochar for Pb²⁺ adsorption was approximately 5.0, and that for Cd²⁺ adsorption was approximately 6.0.

This study demonstrated a significant difference in the adsorption of Pb and Cd by mixed-based biochar. In the solution, the metal cations all exist as hydrated metal ions. The adsorption of metal ions by an adsorbent is highly dependent on the radii of the hydrated ions in the adsorbed metals. A larger hydration radius of the metal ion results in a stronger hydration capacity and more water films are encapsulated outside the ion. However, external adsorbents do not readily absorb these ions. Studies have demonstrated that the radii of the hydrated Pb and Cd ions are 0.401 and 0.426 nm, respectively [31]. Therefore, Cd is not easily absorbed by the mixed-based biochar.

3.3.2. Adsorption Kinetics Experiments

At temperatures below 25 °C with 100 mg mixed-based biochar (100 mesh), the initial pH was 5.0 with a 300 mg/L initial concentration of Pb²⁺, and the initial pH was 6.0 with a 100 mg/L initial concentration of Cd²⁺. The changes in the adsorption of Pb²⁺ and Cd²⁺ by the mixed-base biochar over time are presented in Figure 4. In the initial stage of adsorption, the adsorption of heavy metal ions by biochar primarily occurs on the biochar surface, where the amount of heavy metal ions adsorbed by the biochar increases rapidly with time; however, this process can be completed quickly. As the reaction time increases, heavy metal ions gradually diffuse from the surface to the interior of the biochar, leading to a decrease in the mass transfer rate and a slower increase in the adsorption amount with time [32,33].

As presented in Figure 4, the amount of Pb²⁺ adsorbed by the mixed-biochar increased rapidly with increasing reaction time during the initial adsorption stage from 0–2 h. The adsorption reached approximately 93.71% of the equilibrium adsorption amount after 2 h. When the reaction time exceeded 2 h, the increasing trend in the adsorption amount tended to level off. When the reaction time exceeded 8 h, the adsorption reached equilibrium. It can be observed that the adsorption amount of Cd²⁺ by mixed-based biochar increased rapidly with the increase in reaction time in the initial stage of adsorption from 0 to 4 h, and the rising trend of adsorption amount with time tended to level off during the period of 4–12 h. When the reaction time exceeded 12 h, the adsorption reached equilibrium.

The adsorption data were fitted using quasi-first-order and quasi-second-order kinetic models. The Pb²⁺ fitting curves are presented in Figure 5a, and the Cd²⁺ fitting curves are provided in Figure 5b.

Adsorption kinetics primarily describes how quickly an adsorbent absorbs a solution [34].

Table 3. The quasi-first-order and quasi-second-order model kinetic fitting parameters of Pb²⁺ and Cd²⁺.

Heavy Metals	Actual Balance Adsorption Amount qe (mg·g−1)	Quasi-First-Order Kinetic Equations			Quasi-Second-Order Kinetic Equations
		qe (mg·g−1)	k1 (/h)	R2	qe (mg·g−1)	k2 (g·mg−1·h−1)	R2
Pb	165.88	158.73	0.04762	0.9176	166.67	0.036	0.9998
Cd	42.00	454.55	11.3636	0.9280	42.37	0.1160	0.9998

displays the results of the kinetic parameter fitting. According to the results of the fitted data, the correlation coefficients (R²) for the quasi-first-order and quasi-second-order adsorption kinetic data exist at statistically significant levels [35]. As presented in Table 3, the correlation coefficients of the quasi-second-order and quasi-first-order kinetic models fitting the mixed-biochar for Pb²⁺ adsorption were both greater than 0.91. The correlation coefficient of the former is significantly higher than that of the latter. The theoretical equilibrium adsorption amount of Pb²⁺ calculated using the quasi-second-order kinetic model was the closest to the actual adsorption amount. The fitting results for Cd²⁺ adsorption by the mixed-based biochar were similar to those for Pb²⁺. Therefore, the quasi-second-order kinetic model can better explain the adsorption mechanism of mixed-based biochar for Pb²⁺ and Cd²⁺ than can the quasi-first-order kinetic model.

The quasi-first-order kinetic model suggests that the internal mass-transfer resistance of the particles limits adsorption. This difference primarily influenced the adsorption behavior regarding the concentration of heavy metal ions on both sides of the interface. The rate of heavy metal occupation of adsorption sites is proportional to the number of unused active sites where the adsorption process is dominated by physical diffusion [36]. In contrast, the quasi-second-order kinetic model suggests that the adsorption process includes both physical diffusion and chemisorption and is primarily dominated by chemisorption. The number of active sites on an adsorbent surface determines the adsorption capacity. The adsorption rate was determined using chemisorption [33,37,38]. Pb²⁺ and Cd²⁺ adsorption processes by the mixed-based biochar were consistent with the quasi-second-order kinetic model, thus indicating that chemisorption and physical adsorption coexist in the adsorption process. However, chemisorption was dominant, and the adsorption rate was primarily controlled by chemisorption.

3.3.3. Isothermal Adsorption Experiments

Adsorption data of mixed-based biochar for Pb²⁺ and Cd²⁺ at room temperature (25 °C) were fitted using the Langmuir and Freundlich models, respectively. The adsorption isotherms are presented in Figure 6.

Langmuir isotherms characterize monolayer adsorption on homogeneous surfaces. The Langmuir adsorption equation assumes that the surface consists of many active adsorption centers and that adsorption occurs only at these active centers. Each active center can adsorb only one molecule [34]. When all active adsorption centers on the surface were occupied, the adsorption capacity reached saturation. The adsorbate is distributed as a single molecular layer on the adsorbent surface. In contrast, the Freundlich isotherm describes multilayer adsorption, and the adsorption capacity continues to increase at high concentrations. Freundlich isotherms are more relevant than Langmuir isotherms regarding nonhomogeneous surfaces and multilayer adsorption [39]. The fitting parameters are listed in

Table 4. Isotherm fitting results of adsorption of Pb²⁺ and Cd²⁺ by the biochar.

Temperature (°C)	Heavy Metals	Langmuir Model			Freundlich Model
		Qm (mg·g−1)	KL (L·mg−1)	R2	Kf (mg·g−1)	1/n (g·L−1)	R2
25 °C	Pb	111.11	0.007679	0.9693	148.4132	3.9667	0.9840
	Cd	86.21	0.091920	0.9577	19.7885	3.5336	0.9796

. As presented in Table 4, by comparing R², the value of R² of the Freundlich isothermal adsorption equation for Pb²⁺ and Cd²⁺ adsorption is greater than that of R² of the Langmuir isothermal adsorption equation, thus indicating that the Freundlich isothermal adsorption equation is more suitable for describing the adsorption isotherms of mixed biochar Pb²⁺ and Cd²⁺. The actual equilibrium adsorption amounts of Pb²⁺ and Cd²⁺ by the mixed-biochar at different initial concentrations were larger than the maximum adsorption amount (Q_m) fitted by the Langmuir adsorption equation, thus indicating that multilayer adsorption occurred on the surface of the mixed-biochar. The 1/n values of mixed-based biochar in the Freundlich model were all within the range of 0.1–0.5 that belonged to preferential adsorption, thus implying that adsorption occurred easily.

In this study, the maximum adsorption capacities of mixed-based biochar for Pb²⁺ and Cd²⁺ were 111.11 and 86.21 mg/g, respectively. Co-pyrolysis of sludge and biomass significantly improved the adsorption capacity of biochar for the heavy metals Pb²⁺ and Cd²⁺, and this may be due to the ability of co-pyrolysis to significantly improve the pore structure and increase the specific surface area of the biochar. The mixed biochar prepared by the co-pyrolysis of cotton stalks and sludge exhibited better performance regarding the adsorption of Pb²⁺ and Cd²⁺.

3.4. Adsorption Mechanism

Unlike the biochar derived from plant stalks, sludge biochar prepared via pyrolysis using sludge as a raw material possessed a high ash content and was a combined carbon-mineral adsorption material due to the many inorganic components present in sludge. Biochar from plant stalks possessed a low ash content, well-developed pores, and a high specific surface area [40]. The adsorption mechanism of heavy metal ions in solution is primarily physical adsorption. In contrast, the adsorption mechanism of heavy metals in solution by biochar prepared from pure sludge was dominated by precipitation, coprecipitation, ion exchange, surface complexation, and physical adsorption. Lu et al. reported that the Ca²⁺ concentration in the solution increases after adding sludge-based biochar to the aqueous solution, thus indicating that Ca²⁺ is released from the sludge-based biochar to facilitate biochar surface ion exchange [41].

The addition of cotton stalks for co-pyrolysis in the sludge pyrolysis process increased the carbon content of the pyrolysis feedstock. The primary components of cotton stalks are cellulose, hemicellulose, and lignin, all of which are promising materials for biochar preparation. Mixed biochar exhibits a larger specific surface area and can provide more adsorption sites than pure sludge biochar. Therefore, the adsorption process of mixed-based biochar for Pb²⁺ and Cd²⁺ in solutions should be performed in combination with physical and chemical adsorption.

4. Conclusions

In this study, we observed that the pyrolysis temperature, residence time, and additional ratio of cotton stalks significantly affected the iodine adsorption capacity of the mixed biochar. Moreover, the initial pH of the solution also exerted a significant influence on the adsorption of Pb²⁺ and Cd²⁺ by mixed-based biochar. The optimum initial pH of the solution for Pb²⁺ adsorption was 5.0, and that for Cd²⁺ was 6.0. The maximum adsorption capacities of mixed-based biochar for Pb²⁺ and Cd²⁺ were 111.11 and 86.21 mg/g, respectively. The experimental results revealed that the kinetic processes of Pb²⁺ and Cd²⁺ adsorption by mixed-based biochar occurred more in accordance with quasi-second-order kinetic equations (R² > 0.99) with adsorption mechanisms, including ion exchange, precipitation, and co-precipitation, complexation, and physical adsorption. The preparation of mixed-based biochar via co-pyrolysis of cotton stalks and sludge can solve the problems of small specific surface area and low adsorption performance observed during the pyrolysis of biochar from sludge alone. However, mixed-based biochar remediation of heavy metal pollution is still in the laboratory research stage. In the future, we must continue to deepen the fundamental theoretical research so that the theory can be applied to practice, and we must produce mixed-based biochar at a low price with high remediation efficiency that is suitable for industrial application.