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Article

Effect of Rape-Straw-Derived Biochar on the Adsorption Properties of Single and Complex Trace Elements

by
Shuai Ma
1,2,
Xinghua Huang
1,
Liangyu Shen
1,
Chen Lv
1,
Weiqin Yin
1,3,
Donghao Liu
4,
Hongjun Wu
4,
Shengsen Wang
1,3,5,
Qiao Xu
1 and
Xiaozhi Wang
1,3,5,*
1
College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127, China
2
College of Hydraulic of Science and Engineering, Yangzhou University, Yangzhou 225127, China
3
Joint International Research Laboratory of Agriculture and Agri-Product Safety of Ministry of Education of China, Yangzhou University, Yangzhou 225127, China
4
Yangzhou Supervision & Inspection Center for Agri-Products, Yangzhou 225009, China
5
Jiangsu Collaborative Innovation Center for Solid Organic Waste Resource Utilization, Nanjing Agricultural University, Nanjing 210095, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(13), 2471; https://doi.org/10.3390/w15132471
Submission received: 23 May 2023 / Revised: 22 June 2023 / Accepted: 27 June 2023 / Published: 5 July 2023
(This article belongs to the Section Wastewater Treatment and Reuse)
Browse Figures
Versions Notes

Abstract

:
Copyrolysis biochar derived from rape straw (RSBC) was prepared through oxygen-limited pyrolysis at 500 °C and utilized to investigate its adsorption capability for single and complex trace elements (Fe2+, Mn2+, Cu2+, and Zn2+) in contaminated solutions. The microstructures, functional groups, and adsorption behaviors of RSBC were determined through scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and batch adsorption experiments, respectively. From these, the single/complex adsorption results showed that the adsorption capacity of RSBC for Fe2+, Mn2+, Cu2+, and Zn2+ was 32.21/23.78, 8.95/3.41, 28.12/7.19, and 13.77/4.92 mg/g, respectively. The Langmuir isotherm model fit better than that of Freundlich in the mixed adsorption system, while the pseudo-second-order kinetic model was the most suitable for single adsorption. Thermodynamic adsorption analysis revealed that the removal rate of the four ions by RSBC was 22.14%, 8.95%, 18.75%, and 13.77%, respectively. Moreover, the adsorption mechanism was primarily chemical adsorption, including ion exchange, precipitation, and complexation, because of the binding effects of aromatic structures and polar groups. Additionally, biochar, with its porous structure and high ash content also provided favorable conditions for adsorption of those ions. Through this simple procedure, this work provides a potential strategy to produce biochar with a high adsorption capacity to remediate trace elements in contaminated solutions.

1. Introduction

With the rapid development of industrialization, trace-element contaminants have become one of the most concerning water pollutants; these have significant harmful effects on human health and food safety due to their nondegradability, bioaccumulation, and toxicity [1,2,3,4]. Iron (Fe) is an essential trace element in the human body and can enter the body through water and food; excessive intake of Fe can have a negative impact on the heart. The adverse effects of manganese (Mn), a necessary trace element responsible for nervous-system disorders, include damage to the digestive system, lungs, and kidneys [5]. Copper (Cu) and zinc (Zn), as other important micronutrients, risk spreading via the food chain and can accumulate in organisms, leading to severe liver injury and anemia after reaching a specific concentration [6]. In addition, the imbalance of trace elements such as Fe, Mn, Cu, and Zn in the solution can easily cause water pollution, so it is necessary to be able to remove trace elements in the solution to ensure human health and environmental safety.
To efficiently remove trace elements from contaminated solutions, various methods, including adsorption, coagulation sedimentation, ion-exchange processes, and membrane systems, alongside other biological methods, have been proposed [7,8,9,10]. Among them, adsorption, a physiochemical method, is a promising technique for the removal of metals because of its high efficiency, ecofriendly status, and low cost [11,12]. Through physiochemical interactions with various amendments (e.g., biochar, chitosan, and sepiolite), metal ions are converted into low soluble and nonbioavailable forms [13,14]. Biochar is a porous carbonaceous solid material obtained under the conditions of oxygen-limited pyrolysis of biomass. Due to the wide availability of its raw materials, biochar can promote the utilization of waste as a resource; so, it is of great interest to both domestic and foreign organizations. With its large specific surface area, porous structures, and rich functional groups, biochar is considered to be a good adsorbent of metal ions in solution [15,16]. Research has shown that the adsorption mechanism is complexation reactions, contributed by the carboxyl and hydroxyl groups of pristine biochar and by the Mn-O and Fe-O groups of all three MnFe2O4 biochars. MnFe2O4 biochar can be reused for three cycles, with the maximum adsorption capacity for Cu (II) of the regenerated biochar declining with the loss of precipitated MnFe2O4 [17]. The main mechanisms of adsorption for Zn (II) were the electrostatic attraction, ion exchange, and functional group involvement [18]. Rape-straw biochar (RSBC) also has a good adsorption effect on methylene blue (MB) in water, dominated by chemical adsorption, specifically involving multiple interaction mechanisms, including electrostatic adherence, hydrogen bond, π-π bond, and ion exchange. NaOH modification obviously improves the structure of RSBC, making biochar have better adsorption and regeneration performance [19].
China has the largest plantation area of rape in the world, and its output accounts for more than 30% of all oil-bearing crops, ranking first among the four major ones (rape, soybean, peanut, and sesame). To realize the efficient utilization of rape straw, we prepared biochar derived from rape straw by oxygen-limited pyrolysis, and its adsorptive behaviors for Fe2+, Mn2+, Cu2+, and Zn2+ contaminated solutions were studied by mixed adsorption and single adsorption at different pH values. Through investigating the adsorption capacity of RSBC for single and complex trace elements in single and mixed adsorption systems, this work provides theoretical support for the remediation of trace elements in contaminated solutions and contributes to ensuring soil health and agricultural product quality.

2. Materials and Methods

2.1. Materials

Rape straw was obtained from Donghai County, Lianyungang City, Jiangsu Province. Ferrous sulfate (FeSO4∙7H2O) (99.0–101.0%), manganese sulfate (MnSO4∙H2O) (≥99.0%), copper sulfate pentahydrate (CuSO4∙5H2O) (≥99.0%), zinc sulfate heptahydrate (ZnSO4∙7H2O) (≥99.5%), sodium hydroxide (NaOH) (≥96.0%), and sulfuric acid (H2SO4) (95.0–98.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All of the reagents were of analytical grade.

2.2. Rape Straw for Biochar Production

The rape straw was crushed to about 0.5 cm. N2 was passed into a tubular furnace (OTF-1500X, China) for 30 min to remove air and then, the temperature was raised to 500 °C at a heating rate of 5 °C/min for 2 h. After naturally cooling to room temperature, the biochar was obtained. Then, the as-obtained samples were weighed to calculate the yield. All experiments were repeated three times, and each index was the mean value of all parameters. The biochar derived from co-pyrolysis was gently crushed and sieved to a small size (<0.5 mm) (Figure 1).

2.3. Characterization

The pH of the biochar was measured using an acidity meter (PHS-3C, Shanghai, Lei Zi) (Carbon: Water = 1:20). The points of zero charge (pHpzc) of the biochar were determined using a nano-size analyzer (Nanolink SZ901, Malvern, Britain). Nitrogen was used as a carrier gas, and the specific surface area of the biochar was measured using a Quadrasorb EVO (Anton Paar, Ashland, Wilmington, DE, USA). A field emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan) was employed to observe the surface structures of the as-obtained biochar. Energy dispersive spectroscopy (EDS) and an elemental analyzer (Vario EL Cube, Elementar, Germany) were used to measure the percentage content of C, H, O, and N elements. A thermogravimetric analyzer (Pyris 1 TGA, PerkinElmer, Waltham, MA, USA) was utilized to analyze the pyrolysis characteristics and stability of the biomass in nitrogen. The functional groups of biochar were determined using a Microinfrared spectrometer (Cary 610/670, Varian, Palo Alto, CA, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi, Waltham, MA, USA) was employed to identify the valence states of the samples. The biochar with the best adsorption capacity in a single adsorption was selected and various characterizations of the biochar after metal adsorption were carried out after drying at 60 °C.

2.4. Batch Sorption Experiment

Monometal, multimetal, and different pH adsorption experiments were conducted to determine the adsorption characteristics of RSBC for trace elements. A total of 0.0200 g RSBC was weighed and poured into a 50 mL centrifuge tube; then, 20 mL single solutions with either Fe2+, Mn2+, Cu2+, or Zn2+ at concentrations of 0, 10, 20, 50, 80, 100, 130, 160, or 200 mg/L, respectively, were added. All solutions were shaken at 200 r/min at 25 °C for 24 h, then filtered with a filter membrane of 0.45 μm × 150 mm size. The concentration of metals in the filtrate was determined by an Inductively Coupled Plasma Emission Spectrometer (ICP-MS) (iCAPQ, Thermo Fisher, Lenexa, KS, USA). The number of metal ions adsorbed by the biochar was calculated according to content changes in the solutions before and after the treatment. Similarly, the mixed solution of Fe2+, Mn2+, Cu2+, and Zn2+ was configured to carry out mixed adsorption of different metals under the same conditions.
In a single adsorption, the adsorption concentration with the best adsorption amount was selected; the concentrations of Fe2+, Mn2+, Cu2+, and Zn2+ ions were 150, 100, 150, and 100 mg/L, respectively. Samples were taken at 30, 60, 120, 240, 360, 600, and 720 min, and shaken at 200 r/min at 25 °C. Kinetic analysis was conducted based on the adsorption capacity of the samples obtained at different times. Thermodynamic analysis was performed at 298, 308, and 318 K, at a speed of 200 r/min for 24 h. The influence of solution pH on metal adsorption by RSBC was investigated under solution pH values ranging from 2 to 6 in a single adsorption system. To avoid the precipitation of the ions, the initial solution pH was adjusted to 2, 3, 4, 5, and 6 with 0.25 M H2SO4. Subsequently, the adsorptive tests under different pH values were carried out in the single adsorption.

3. Results and Discussion

3.1. Characterization of Rape-Straw Biochar

The pH value and ash content of RSBC were 11.83 and 25.61%. Its yield, specific surface area, and total pore volume were 30.84%, 3.5214 m2/g, and 0.006596 cm3/g, respectively. In the process of pyrolysis, some organic acids and carbohydrates might have decomposed into alkali salts which led to the increase in the pH value. Therefore, it is beneficial to carry out the precipitation of metal ions in an alkaline environment.
As demonstrated in Table 1, the elemental analysis provided an explicit contrast between the feedstock and the biochar. Compared with the raw materials, the contents of carbon, nitrogen, and sulfur in the biochar were increased, while the contents of hydrogen and oxygen were decreased. The ratios of H/C, O/C, and (O + N)/C were 0.1558, 0.0391, and 0.1739, indicating that the biochar possessed aromaticity, polarity, and hydrophilicity [20,21].

3.1.1. Surface Morphological Structure

Figure 2 shows the surface structures of RSBC before and after the maximum adsorption of Fe2+, Mn2+, Cu2+, and Zn2+. RSBC is a porous material with a very irregular structure. A porous structure is essential for the remediation of metal ions from their contaminated aqueous solutions [22,23]. Abundant pores caused by the template facilitated the adsorptive process between biochar and metal ions [24]. Before adsorbing the trace elements, its surface was relatively smooth. During the adsorption of trace elements by RSBC, the attached particles significantly increased and the surface of RSBC had become rough, which may have been caused by ion precipitation, indicating that RSBC with a rough and porous structure can provide more potential adsorption sites for metal ions.
A large surface area is considered to be the most essential parameter for better performance of the adsorbent [25]. The N2 adsorption–desorption isotherms of RSBC displayed a Type-I shape, demonstrating that RSBC has a mesoporous structure (Figure 3a). The hysteresis loop of RSBC is not closed due to the content of mesopores. The pore-size distribution curve of the RSBC sample presented a broad range, from 2 nm to above 200 nm, indicating that the RSBC was porous (Figure 3b). This wide pore-size range is sufficient for the adsorption of trace elements.

3.1.2. Thermogravimetric Analysis of Rape Straw

As shown in Figure 4, the weight loss of rape straw showed a wide range of temperatures, and the combustion weight loss of biomass was divided into three stages. When the temperature increased to 135 °C, the first stage was the loss of moisture. The second step related to the pyrolysis of hemicellulose, cellulose, and some lignin; the combustion reaction of volatile components mainly occurred from 135 to 570 °C. The major weight loss occurred at the temperature ranging from 570 to 900 °C, which was caused by the continuous pyrolysis of residual lignin and coke combustion. The pyrolysis process of lignin in biomass is frequently accompanied by weight loss [26]. The spectrum shown in Figure 2 revealed a small peak before the temperature reached 100 °C, which might be due to the evaporation of water physically adsorbed in the rape straw, which only lost 3.43% before reaching 135 °C. With an increase in temperature, the weight loss of the biomass became rapid. Meanwhile, volatile combustion was accompanied by pyrolysis of cellulose, hemicellulose, and lignin. There was an inflection point at 320 °C, and the weight loss reached the maximum, mainly because the volatile combustion was gradually finishing and the fixed carbon in the rape straw had not started to burn. As the temperature continued to rise, the combustion of fixed carbon began, and the weight loss of the sample reached 65.23% at around 570 °C. Subsequently, with the increase in temperature, the weight loss gradually decreased. The combustion process of fixed carbon in the sample basically ended. In the whole process of combustion, with the combustion of physically adsorbed water combined with volatile and fixed carbon, the residue was mainly ash, including sulfur, nonmetallic substances, and metallic substances [27,28]. There was an obvious peak in the DGT curve ranging from 650 to 750 °C, which might have been caused by the second combustion reaction of the coke that had not completely reacted in the previous stage. Consequently, the RSBC was prepared through pyrolysis at the temperature of 500 °C. Due to the high ash content, the diffusion of oxygen onto the surface of the coke was hindered, resulting in the delay of coke combustion. Furthermore, it was possible to observe that only 16.19% of weight loss was obtained up to 800 °C in a nitrogen atmosphere, indicating that the sample exhibited excellent thermal stability. This stability is interesting for the extensive application of the material in processes that require a high temperature.

3.1.3. FTIR Analysis of RSBC before and after Adsorption

FTIR spectra of RSBC before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ are shown in Figure 5. Compared with the spectra of biochar before the adsorption, the wide peak of -OH at 2500–3500 cm−1 decreased or disappeared in the spectrum of biochar after the adsorption, which might be attributed to the interaction between metals and -OH during the adsorption [29]. Peaks located at 1689 and 1580 cm−1 could be ascribed to the vibration of C=O and C=C and the intensity of C=C slightly decreased, which might have been due to oxidation destroying the aromatic ring during the adsorption. Several peaks ranging from 869 to 1114 cm−1 appeared after the adsorption, which might have been associated with the stretching and bending motions of CO32− resulting from the generation of precipitates with metal cations [30]. Additionally, the biochar after the adsorption exhibited a stronger Si-O-Si peak [31], further confirming that the higher SiO2 content was related to the high ash content.

3.1.4. XRD Analysis of RSBC before and after Adsorption

Figure 6 reveals the XRD patterns of the RSBC before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+. The main minerals, including MgCl2, KCl, Ca(PO4)2, CaCO3, and Na2Si4O9, were present in all samples, suggesting the formation of biochar derived from rape straw. Some characteristic peaks were identified after the adsorption of metal ions, demonstrating that the interactions, represented by precipitation and complexation, between these ions and OH− or CO32−, occurred and formed compounds such as CuCO3 and Zn(OH)2.

3.2. Adsorption Experiments

3.2.1. Adsorption Isotherms of Fe2+, Mn2+, Cu2+, and Zn2+ in Single and Mixed Systems

The single adsorption and mixed adsorption properties of trace elements by biochar were fitted by Langmuir and Freundlich isotherms for the analysis of adsorption behaviors.
Langmuir model: qe = KqmCe/(1 + KCe)
Freundlich model: qe = kfCe1/n
Here, qe (mg/g) is the amount of adsorption at equilibrium, Ce (mg/L) is the solution concentration at equilibrium, qm (mg/g) is the maximum adsorption capacity, K (L/mg) is the Langmuir equilibrium parameter, kf (mg/g) is the adsorption capacity, and n is the Freundlich constant, which is a constant for a given system at a certain temperature, related to the physical properties and the temperature of the adsorbent.
The adsorption curves of Fe2+, Mn2+, Cu2+, and Zn2+ in the single and mixed systems fitted by the Langmuir and Freundlich isotherm models are revealed in Figure 7. The adsorption capacity of RSBC for Fe2+, Mn2+, Cu2+, and Zn2+ was different, and the order of adsorption capacity was Fe2+ > Cu2+ > Zn2+ > Mn2+, which might have been due to the increasing hydration radius of Fe2+, Mn2+, Cu2+, and Zn2+ [32]. Whether in the single or mixed adsorption system, the adsorption trend basically showed a two-stage pattern, rising first and then reaching a stable state. In the single adsorption, the maximum adsorption capacity of RSBC for Fe2+, Mn2+, Cu2+, and Zn2+ was 32.21, 8.95, 28.12, and 13.77 mg/g, respectively. In the mixed adsorption at different concentrations, the maximum adsorption capacity of RSBC for Fe2+, Mn2+, Cu2+, and Zn2+ was 23.78, 3.41, 7.19, and 4.92 mg/g, respectively. Under the condition of a low ion concentration, RSBC could provide more adsorption sites. With the increase of the concentration, the adsorption sites on the surface of biochar were limited, with a corresponding decrease in its adsorption ability. Moreover, due to electrostatic repulsion, there were more and more positively charged metal ions on the biochar, contributing to the slow change of adsorption capacity.
The adsorption data obtained from single and mixed adsorptions with different initial concentrations of the trace elements were fitted by Langmuir and Freundlich adsorption isotherm models, as illustrated in Table 2. The parameters that qm obtained in the Langmuir isotherm model were close to the actual adsorption values in both the single and mixed systems and the values of n fitted by the Freundlich equation were all greater than one, indicating that the whole process of adsorption was rapid. In the single adsorption, the Langmuir isotherm model was suitable for the adsorption of Cu2+ and Zn2+, suggesting that the chemisorption of Cu2+ and Zn2+ chiefly occurred on the homogeneous surface of RSBC [33], while the Freundlich isotherm model fitted better for Fe2+ and Mn2+ adsorption. For the mixed system, all adsorption data were fitted by the Langmuir isotherm model, with R2 values ranging from 0.9649 to 0.9848, which might be ascribed to RSBC heterogeneity of chemical performance and physical structure [34]. In terms of the R2 value (Table 2), the Langmuir isotherm model fitted better than the Freundlich isotherm model in the mixed adsorption system, demonstrating that the adsorption process was controlled via the monolayer adsorption mechanism [35]. Additionally, based on K values obtained via the Langmuir isotherm model, RSBC had a stronger binding affinity to Fe2+ in the single adsorption [36].

3.2.2. Adsorption Kinetics

The pseudo-first-order kinetic model, pseudo-second-order kinetic model, Elovich equation, and particle diffusion equation were used to fit the adsorption capacity of RSBC to Fe2+, Mn2+, Cu2+, and Zn2+ under different contact times in order to analyze the kinetic characteristics [37]. The fitting results are shown in Figure 8.
Pseudo-first-order kinetic model: log(qe − qt) = logqe − k1t/2.303
Pseudo-second-order kinetic model: t/qt = 1/(k2qe2) + t/qe
Elovich equation: qt = (lnab + lnt)/b
Particle diffusion equation: qt = kpt0.5 + c
Here, qe (mg/g) and qt (mg/g) refer to the adsorption amount at equilibrium and at time t (min), respectively; k1 (min) is the equilibrium rate constant of pseudo-first-order adsorption; k2 (g·min/mg) is the equilibrium rate constant of pseudo-second-order adsorption; a (mg·min/g) is the initial adsorption rate of the Elovich equation; b (g/mg) is the desorption constant related to the extent of surface coverage and activation energy constant for chemisorptions; c is a constant related to the thickness of the boundary layer; and kp is the diffusion equation.
The fitting parameters for the four adsorption kinetic models discussed previously are shown in Table 3. When the pseudo-second-order kinetic model was used to fit the adsorption experimental data, the determination coefficient (R2) of Fe2+, Mn2+, and Zn2+ were 0.9914, 0.9801, and 0.9498, respectively. When the particle diffusion was used to fit the adsorption experimental data, the determination coefficient (R2) was 0.9969, which was higher than that of the other models. The qe obtained by the pseudo-second-order kinetic adsorption equation was closer to the actual value, indicating that the fitting results using the pseudo-second-order kinetic adsorption equation were better than those of the other three equations. The pseudo-second-order kinetic adsorption equation could better predict the actual adsorption situation. From the microscopic analysis, the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ by RSBC could be divided into two steps. The first was the diffusion of metal ions from the solution towards the surface of RSBC, where they adsorbed onto its surface at a fast adsorption speed. The second was the process of Fe2+, Mn2+, Cu2+, and Zn2+ entering the RSBC mesopores and combining with their internal adsorption sites at a slow adsorption speed. The quasi-second-order kinetic equation further explained that the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ by RSBC is controlled by these two processes.

3.2.3. Adsorption Thermodynamics

For a better understanding of the thermodynamic properties of RSBC adsorption of Fe2+, Mn2+, Cu2+, and Zn2+, we fitted the experimental data of RSBC adsorption reactions obtained for Fe2+, Mn2+, Cu2+, and Zn2+ at reaction temperatures of 298, 308, and 318 K with the following equations. The thermodynamic calculation formula is as follows:
ΔG = −RTlnKL
ΔG = ΔH − TΔS
lnKL = ΔS/R − ΔH/RT
where ΔG (kJ/mol) and ΔH (kJ/mol) represent the Gibbs free-energy change and enthalpy change, ΔS (kJ/(mol·K)) represents entropy change, R (8.314 J/(mol·K)) represents the gas constant, T (K) represents the Kelvin temperature, and KL represents the thermodynamic equilibrium constant. The value of KL is calculated as qe/Ce.
Table 4 displays the spontaneous endothermic nature of RSBC in the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ (ΔH > 0, ΔS > 0, and ΔG < 0). Figure 9a shows that increasing the reaction temperature can facilitate adsorption. The order of the adsorption capacity of RSBC was Fe2+ > Cu2+ > Zn2+ > Mn2+, and as the temperature increased, the trend remained unchanged. It is well known that the adsorption process is primarily physical adsorption when ΔH is less than 20 kJ/mol and it is primarily chemical adsorption when ΔH is larger than 20 kJ/mol. In this study, the ΔH values were less than 20 kJ/mol, so the adsorption processes were dominated by physical adsorption. Additionally, ΔS is greater than zero, indicating that the adsorption process of Fe2+, Mn2+, Cu2+, and Zn2+ by RSBC is primarily driven by entropy and the degree of chaos at the interface between the solid and liquid phases increases during the adsorption process [19].

3.2.4. Influence of pH on Adsorption Characteristics

It can be clearly seen from Figure 10 that the adsorption capacities of Fe2+, Mn2+, Cu2+, and Zn2+ by RSBC showed a gradual increase as pH increased. Fe2+ and Cu2+ showed an obvious change, while Mn2+ and Zn2+ underwent little change under the influence of pH. Since the order of oxidability was Cu2+ > H+ > Fe2+, Cu2+ was more active under acidic conditions, and its adsorption capacity increased at a faster rate than that of Fe2+.
Generally, the deprotonation of a large number of functional groups on RSBC will lead to the formation of a double electron layer structure [38]. Since the isoelectric point of RSBC was 3.1, the pH-dependent adsorption was chiefly ascribed for the changes in zeta potential and the dissociation of acidic functional groups at high pH values, accompanied by changes in the surface properties of RSBC [1]. When the pH value was low, the content of H+ was greater than that of metal ions and competition occurred in the solution. This competition reduced the metal adsorption sites on the RSBC, resulting in a low adsorption capacity. Additionally, in an environment with a low pH, the functional groups on RSBC were positively charged and had a repulsive force with the metal ions in the water, inhibiting the adsorption capacity of the biochar [39,40,41]. With the increase of solution pH, the RSBC gradually developed a negative charge and the content of OH− increased, which combined with H+ on the RSBC. The existence of OH− may maximize ion precipitation in the form of hydroxide, resulting in the decline of H+ competitiveness, so that there were more sites on the RSBC for metal-ion adsorption. Moreover, the negatively charged pH-dependent adsorption sites on the RSBC could also maximize the adsorption capacity of Fe2+ [42].

3.3. Adsorption-Mechanism Analysis of RSBC

The adsorption mechanisms of RSBC for Fe2+, Mn2+, Cu2+, and Zn2+ ions were characterized by XPS. As shown in Figure 11a, the Fe 2p, Mn 2p, Cu 2p, and Zn 2p peaks appeared on the full-range XPS spectra after the adsorption, indicating that the RSBC had an adsorptive ability for Fe2+, Mn2+, Cu2+, and Zn2+. Furthermore, the oxygen-containing functional groups on the surface of the RSBC also played an important role in the adsorption. The C 1s spectra (Figure 11b) were deconvoluted into the four peaks, corresponding to O-C=O, C=O, C-O, and C-C, respectively [43]. The intensity of the O-C=O group decreased after the adsorption of metal ions, indicating the sample could adsorb those ions. The Fe 2p spectrum illustrated in Figure 11c revealed that the peaks occurred at Fe 2p1/2 and Fe 2p3/2, which indicated the coexistence of Fe2+ and Fe3+ in the sample [44]. It was revealed that Fe ions reacted with -OH (Equations (3) and (4)) and the presence of Fe3+ was mainly due to the oxidation of Fe2+. Figure 11d shows that the Mn 2p spectrum consisted of a doublet spin-orbit splitting of Mn 2p1/2 and Mn 2p3/2, respectively. The difference in binding energy between the two positions was around 12 eV, which was ascribed to manganese (Ⅱ) carbonate [45,46]. The peak centered at around 643.97 eV was related to satellite shakeup vibrations of manganese atoms or splitting of Mn-O in the MnO(OH) structure [47]. As shown in Figure 11e, peaks including Cu(OH)2 and copper (Ⅱ) carbonate dihydroxide were identified in the Cu 2p spectrum, indicating that the reaction between Cu2+ and -OH/CO32− ions occurred through precipitation (Equations (6) and (7)). Moreover, the complexation between Cu2+ and RSBC also occurred because the peaks at 933.18 and 936.37 eV suggested the formation of O-Cu and COO-Cu [48,49]. In addition, Figure 11f represents the four peaks that were assigned to Zn 2p1/2 and Zn 2p3/2, with a spin-energy separation of 23 eV and shakeup vibrations of zinc atoms at 1043.22 eV, confirming the existence of Zn4(CO3)(OH)6∙H2O [50,51,52].
One of the main mechanisms for trace element adsorption by RSBC is ion exchange between metal ions and protons on oxygen-containing functional groups, including -OH groups [53]. As demonstrated in the FTIR, the RSBC contained hydroxyl surface functional groups, which was beneficial for the adsorption of Fe2+ and Cu2+. The interaction between Fe2+ and H+ on oxygenated surface functional groups was performed as follows in Equations (3)–(5) [54]. Moreover, the pH value was also a key factor affecting ion exchange. The adsorption behavior was performed under an acidic solution, leading to abundant H+ being close to the saturated metal-adsorption sites. Therefore, the deprotonation of H+ from the surface of RSBC was enhanced, suggesting an increase in the adsorption of Fe2+ on the RSBC.
-OH + Fe2+ → OFe+ + H+
2−OH + Fe2+ → OFeO− + 2H+
Fe2+ + H2O → 2H+ + Fe(OH)2
Cu(OH)2 + 2OH− → [Cu(OH)4]2−
Cu2+ + CO32− + H2O → Cu2(OH)2CO3↓

3.4. Application to Wastewater

When the concentrations of Fe2+, Mn2+, Cu2+, and Zn2+ in the wastewater were 150, 100, 150, and 100 mg/L, the removal rates of these four ions by RSBC were 22.14%, 8.95%, 18.75%, and 13.77%, respectively. Table 5 shows a comparative study of the absorption of Fe2+, Mn2+, Cu2+, and Zn2+ from wastewater by various adsorbent materials. According to the literature review [55,56,57,58,59,60,61,62,63,64,65], biochar derived from various materials has a wide range of Fe2+, Mn2+, Cu2+, and Zn2+ adsorption limits, and RSBC has a certain degree of comparability. In consideration of the industrial application and subsequent soil application, RSBC has an advantage that other adsorbents do not have. As a widely available source of agricultural waste, rape straw can be made into biochar in large quantities to realize the resource utilization of waste, and to reduce environmental pollution.

4. Conclusions

RSBC was utilized for the removal of trace elements, including Fe2+, Mn2+, Cu2+, and Zn2+, from contaminated aqueous solutions. The results of batch adsorption experiments indicated that the RSBC showed an excellent adsorption performance for these metal ions and high selectivity to Fe2+, and revealed that the order of adsorption capacity of RSBC was Fe2+ > Cu2+ > Zn2+ > Mn2+ in both the single and mixed adsorption systems because the ability of trace elements competition for biochar adsorption sites was in the order of Fe2+ > Cu2+ > Zn2+ > Mn2+. In single adsorption, the adsorption capacity of RSBC for Fe2+, Mn2+, Cu2+, and Zn2+ was 32.21, 8.95, 28.12, and 13.77 mg/g, respectively. The Langmuir isotherm model could better describe the adsorption of Cu2+ and Zn2+ by RSBC, while the Freundlich isotherm model was more suitable for the adsorption of Fe2+ and Mn2+. Compared with single adsorption, the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ by RSBC in mixed adsorption decreased by 26.17%, 61.90%, 74.43%, and 64.27%, respectively. The Langmuir isotherm model could better describe the adsorption of RSBC in mixed solutions, indicating that the adsorption of RSBC of mixed metal ions is mainly surface adsorption. The kinetic model indicated that RSBC exhibited rapid surface adsorption of Fe2+, Mn2+, Cu2+, and Zn2+, followed by ions entering the mesopores and combining with internal adsorption sites, resulting in a slower adsorption rate. The fitting effect of the pseudo-second-order adsorption kinetic equation was better than that of the other three models, as it was closer to the actual situation. Thermodynamics indicated that the adsorption of RSBC is mainly physical adsorption, which is a spontaneous endothermic process. The results at different pH values revealed that metal adsorptions onto RSBC were pH-dependent, indicating that the hydroxyl and carboxyl groups in biochar could form complexes with the metal ions. Based on the adsorption-mechanisms investigation, the removal ability of RSBC can be summarized as follows: The porous microstructure and high ash content offered more active sites for adsorption, which were available for ion exchange, and the precipitation mechanism for metal ion removal. In addition, aromatic structures, polar groups, and high pH values could greatly improve the adsorption capability via chemical adsorption. Accordingly, RSBC could be used for specific adsorption in water polluted by iron, although the related adsorptive mechanisms still need to be expanded for the exclusive use of biochar to address pollution by other elements. Due to the high yield and pH of RSBC, it can be applied to acidic soil, which can improve the pH of the soil and fix iron in the soil, improving the yield and quality of crops.

Author Contributions

Conceptualization, S.M., X.H. and L.S.; methodology, S.M. and C.L.; validation, S.M., L.S. and W.Y.; investigation, S.M. and X.H.; resources, S.W. and X.W.; data curation, S.M., D.L., H.W. and Q.X.; writing—original draft preparation, S.M.; writing—review and editing, S.W., Q.X. and X.W.; supervision, X.W.; project administration, X.W.; funding acquisition, S.W. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 41977085), the Qing Lan Project, the 333 Project in Jiangsu Province (No. BRA2020300), the Six Talent Peaks Project in Jiangsu Province (No. 2018-TD-JNHB-012), and the National Key Research and Development Program of China (No. 2021YFD1700800).

Data Availability Statement

No new data were created.

Acknowledgments

Jinyong Xu of Yangzhou University is acknowledged for his help in the preparation of the original draft.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 02471 g001 550
Figure 1. Schematic of the synthesis of rape straw biochar.
Figure 1. Schematic of the synthesis of rape straw biochar.
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Figure 2. SEM images of RSBC before (a) and after (b–e) the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ ions in a single adsorption.
Figure 2. SEM images of RSBC before (a) and after (b–e) the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ ions in a single adsorption.
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Figure 3. (a) N2 adsorption–desorption and (b) pore-size distribution of RSBC.
Figure 3. (a) N2 adsorption–desorption and (b) pore-size distribution of RSBC.
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Figure 4. TG−DTG of rape straw.
Figure 4. TG−DTG of rape straw.
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Figure 5. FTIR spectra of RSBC before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ in a single adsorption.
Figure 5. FTIR spectra of RSBC before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ in a single adsorption.
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Figure 6. XRD patterns of RSBC before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ in a single adsorption system.
Figure 6. XRD patterns of RSBC before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ in a single adsorption system.
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Figure 7. Adsorption isotherm data fitted by the Langmuir and Freundlich models for Fe2+, Mn2+, Cu2+, and Zn2+ adsorption on RSBC in a single (a) and mixed (b) system.
Figure 7. Adsorption isotherm data fitted by the Langmuir and Freundlich models for Fe2+, Mn2+, Cu2+, and Zn2+ adsorption on RSBC in a single (a) and mixed (b) system.
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Figure 8. Effect of contact time on single adsorption: (a) pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model, (c) Elovich equation, (d) particle diffusion equation.
Figure 8. Effect of contact time on single adsorption: (a) pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model, (c) Elovich equation, (d) particle diffusion equation.
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Figure 9. Adsorption capacity (a) and adsorption thermodynamic fitting (b) at different temperatures.
Figure 9. Adsorption capacity (a) and adsorption thermodynamic fitting (b) at different temperatures.
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Figure 10. Influence of solution pH on Fe2+, Mn2+, Cu2+, and Zn2+ adsorption by RSBC.
Figure 10. Influence of solution pH on Fe2+, Mn2+, Cu2+, and Zn2+ adsorption by RSBC.
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Figure 11. XPS analysis of (a) survey spectra and (b) C 1s spectra before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+; (c) Fe 2p spectrum, (d) Mn 2p spectrum, (e) Cu 2p spectrum, and (f) Zn 2p spectrum after adsorption in a single adsorption system.
Figure 11. XPS analysis of (a) survey spectra and (b) C 1s spectra before and after the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+; (c) Fe 2p spectrum, (d) Mn 2p spectrum, (e) Cu 2p spectrum, and (f) Zn 2p spectrum after adsorption in a single adsorption system.
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Table
Table 1. The main elemental compositions of raw materials and biochar.
Table 1. The main elemental compositions of raw materials and biochar.
SampleC/%H/%O/%N/%S/%O/CH/C(O + N)/C
Feedstock40.075.8622.531.150.88---
Biochar66.212.5910.301.211.660.15560.03910.1738
Table
Table 2. Fitting parameters of the Langmuir and Freundlich isotherm model in single and mixed adsorption systems.
Table 2. Fitting parameters of the Langmuir and Freundlich isotherm model in single and mixed adsorption systems.
SingleMixed
LangmuirFreundlichLangmuirFreundlich
Fe2+qm = 30.8241 mg/gkf = 20.0893 mg/gqm = 26.5663 mg/gkf = 8.1321 mg/g
K = 1.1379n = 10.8282K = 0.0641n = 4.4954
R2 = 0.6304R2 = 0.7197R2 = 0.9848R2 = 0.9363
Mn2+qm = 9.2601 mg/gkf = 1.4959 mg/gqm = 5.2078 mg/gkf = 0.2192 mg/g
K = 0.0414n = 2.8853K = 0.0123n = 1.8149
R2 = 0.9644R2 = 0.9867R2 = 0.9649R2 = 0.9524
Cu2+qm = 27.2133 mg/gkf = 13.0384 mg/gqm = 7.1689 mg/gkf = 2.8856 mg/g
K = 0.5837n = 6.3003K = 0.1942n = 5.5015
R2 = 0.9869R2 = 0.8441R2 = 0.9760R2 = 0.8120
Zn2+qm = 12.4077 mg/gkf = 4.1508 mg/gqm = 5.6048 mg/gkf = 0.8298 mg/g
K = 0.1062n = 54.5873K = 0.0329n = 2.8706
R2 = 0.7785R2 = 0.7531R2 = 0.9675R2 = 0.9609
Table
Table 3. Adsorption kinetics fitting parameters of RSBC.
Table 3. Adsorption kinetics fitting parameters of RSBC.
Pseudo-First OrderPseudo-Second OrderElovichParticle Diffusion
qe
(mg/g)		K1
(min)		R2qe
(mg/g)		K2
(g·min/mg)		R2b
(g/mg)		R2Kp
(mg/g·min−0.5)		R2
Fe2+14.93620.00250.896729.77080.000710.99140.25650.96040.56570.9557
Mn2+5.22730.19560.92736.79670.002460.98011.24620.76580.12410.8928
Cu2+17.06550.22310.991921.98770.000710.98770.32200.94850.46090.9969
Zn2+6.34590.00360.634818.22820.000550.94980.31660.89270.46120.9020
Table
Table 4. Effect of temperature on the thermodynamic parameters.
Table 4. Effect of temperature on the thermodynamic parameters.
Temperature (K)ΔH (kJ/mol)ΔS (J/mol)ΔG (kJ/mol)
Fe2+29812.451466.7266−7.4332
308−7.6823
318−7.9321
Mn2+29815.134754.1358−0.9978
308−1.0313
318−1.0648
Cu2+29812.46363.1735−6.3622
308−6.5766
318−6.7891
Zn2+29815.044862.3191−3.5263
308−3.6447
318−3.7630
Table
Table 5. Comparison study of the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ with other adsorbents identified in the literature.
Table 5. Comparison study of the adsorption of Fe2+, Mn2+, Cu2+, and Zn2+ with other adsorbents identified in the literature.
AdsorbentAnalytesAdsorption Capacity (qmax, mg/g)Ref.
Waste orange and lemon activated carbon (WOLAC)Fe2+99.9[55]
Pistachios shellsFe2+89.12[56]
Multiwalled carbon nanotubes (MWCNTs)Mn2+4.8[57]
BC350Mn2+6.63[58]
Fe2O3-HCMn2+2.319[59]
PMSBMn2+, Zn2+29.61[60]
H3PO4 impregnated red-gram biochar–MnO2 nanocompositesCu2+493.34[61]
N-RSBCCu2+40.56[62]
Novel pectin–cellulose–biochar composite with SDSCu2+769.68[63]
calcium alginate–nZVI–biochar composite (CANRC)Zn2+71.77[64]
Hematite-modified biochar (FeB)Zn2+14.09[65]
RSBCFe2+, Mn2+, Cu2+, Zn2+32.21, 8.95, 28.12, 13.77This work
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Ma, S.; Huang, X.; Shen, L.; Lv, C.; Yin, W.; Liu, D.; Wu, H.; Wang, S.; Xu, Q.; Wang, X. Effect of Rape-Straw-Derived Biochar on the Adsorption Properties of Single and Complex Trace Elements. Water 2023, 15, 2471. https://doi.org/10.3390/w15132471

AMA Style

Ma S, Huang X, Shen L, Lv C, Yin W, Liu D, Wu H, Wang S, Xu Q, Wang X. Effect of Rape-Straw-Derived Biochar on the Adsorption Properties of Single and Complex Trace Elements. Water. 2023; 15(13):2471. https://doi.org/10.3390/w15132471

Chicago/Turabian Style

Ma, Shuai, Xinghua Huang, Liangyu Shen, Chen Lv, Weiqin Yin, Donghao Liu, Hongjun Wu, Shengsen Wang, Qiao Xu, and Xiaozhi Wang. 2023. "Effect of Rape-Straw-Derived Biochar on the Adsorption Properties of Single and Complex Trace Elements" Water 15, no. 13: 2471. https://doi.org/10.3390/w15132471

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