Removal of Cadmium from Aqueous Solution by Magnetic Biochar: Adsorption Characteristics and Mechanism

doi:10.21203/rs.3.rs-3317118/v1

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Removal of Cadmium from Aqueous Solution by Magnetic Biochar: Adsorption Characteristics and Mechanism

https://doi.org/10.21203/rs.3.rs-3317118/v1

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published 28 Dec, 2023

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Experiments were conducted to investigate the potential for the efficient resource utilization of waste cow manure and corn straw in an agricultural ecosystem. In this study, magnetic biochar of cow manure and straw was synthesized by co-precipitation method, and cadmium was removed by adsorption in aqueous solution. Several physicochemical characterization techniques were applied, including scanning electron microscopy, Brunauer-Emmett-Teller (BET), Zeta potential analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy. The effects of pH value, magnetic biochar content, kinetics and isotherm on the adsorption of cadmium were investigated. The physicochemical characterizations revealed that the physical and chemical properties of the magnetic biochar were substantially changed compared to the unmodified biochar. The results showed that the surface of biochar became rough, the number of oxygen-containing functional groups increased, and the specific surface area increased. The results of adsorption experiments show that the adsorption capacity is affected by pH, magnetic biochar addition, cadmium concentration and adsorption time. The adsorption kinetics and isothermal adsorption experiments showed that the Cd adsorption processes of the cow manure and corn straw magnetic biochars were consistent with the Freundlich model and quasi-second-order kinetic model. The results of this study also showed that Cd adsorption effect of cow manure magnetic biochar was found to be more effective than that of corn straw magnetic biochar. In conclusion, the magnetic biochar of cow dung is an effective adsorbent for the absorption of cadmium in wastewater.

Adsorption

Cadmium

Heavy metals

Magnetic biochar

Water resources are increasingly affected by a variety of toxins, with particular concerns regarding heavy metal pollution (Han et al.2021; Dickin et al. 2016). Heavy metals are one of the most concerning water pollutants due to their non-degradable nature, which leads to their accumulation in the body, especially in fatty tissues, muscles, and bones (Zhai et al. 2022). The accumulation of heavy metals in the body can cause serious problems, including lack of appetite, nausea, mental and neurological disorders, heart disease, skin cancer, increased risk of lung cancer, damage to the immune system, and DNA damage (Sarker et al.2022; Briffa et al. 2020). The heavy metals typically present in wastewater include mercury, cadmium, lead and chromium. Cd is a non-essential element for biological growth and development, and is considered one of the most toxic and dangerous environmental pollutants (Gaur et al. 2021). Cd in water presents a threat to ecosystems and human health because of its non-degradability and toxicity (Alengebawy et al. 2021).

The currently available methods for treating wastewater containing Cd²⁺ include phytoremediation, biological removal, and physicochemical treatment. Adsorption is a commonly used method that is considered to be one of the most effective physicochemical treatments. It involves the use of porous adsorption materials to adsorb and treat heavy metals in sewage (Gupta et al. 2021). The most widely used adsorbents are diatomite, activated sludge, and activated carbon. In recent years, the potential for biochar to be used as an adsorbent has been widely studied (Yang et al. 2019). Compared with traditional adsorbents, biochar has a lower cost and better adsorption effectiveness (Xiao et al. 2018).

Biochar is a porous, C-rich, and highly aromatic solid material formed by high temperature pyrolysis under oxygenated conditions (Islam et al. 2021). It has a porous structure, large specific surface area, and many oxygen (O)-containing surface functional groups, and it can exist in a stable manner in aquatic environments (Castiglioni et al. 2021; Law et al. 2022; Fang et al. 2022; Lonappan et al. 2018). Biochars derived from agricultural wastes have been shown to be effective at removing heavy metals (e.g., Pb, Cd, and Cu) and organic pollutants from wastewater through adsorption (Hong et al. 2019; Kalinke et al. 2016). Some studies have reported a higher potential for the adsorption of heavy metals from biochars derived from corn stalks and other residual raw materials when applied to sewage. The adsorption capacity of biochar prepared from water hyacinth has been reported to be 46 mg/g.

To enhance the sorptive capacity of biochar, some researchers have used magnetic modification to increase the surface area and pore volume (Rajapaksha et al. 2016; Huang et al. 2020). Feedstock composition, pyrolysis temperatures, and pH strongly affect the physicochemical characteristics of the biochar, including the adsorptive capacity of heavy metals (Xu et al. 2022; Sugawara et al. 2022). It has been found that the maximum Cd removal was achieved at a pyrolytic temperature of 700°C in a nitrogen gas environment (Fu et al. 2021). Biochar has the characteristics of a small particle size and low density, which results in it being difficult to separate from solution. It is therefore necessary to develop a new method to better separate biochar from solution (Ma et al. 2014). It has been speculated that the separation effect of biochar can be promoted by various modifications. In particular, the magnetization of biochar could change the surface and porous structures of biochar, generate additional adsorption sites, and improve the adsorption capacity (Agrafoti et al. 2014; Pan et al. 2014; Wang et al. 2014; Li et al.2020).

To improve the adsorption capacity of Cd, magnetic biochars containing iron oxide were synthesized in this study using cow manure and corn straw as raw materials, respectively. The pristine and magnetic biochar were characterized by pplying scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), Zeta potential analysis (Zeta), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) .The effects of pH value, addition amount, initial concentration and adsorption time on the adsorption process of magnetic biochar were studied. Consecutively the adsorption kinetics and isothermal adsorption were also investigated.

Cow manure and corn straw were used as biomass raw materials to synthesize unmodified and magnetic biochars. Cow manure and corn straw were collected from local farms. The dried cow manure and corn straw were packed into porcelain crucibles for compaction, placed into a vacuum tube furnace, and heated for 3 h at 400, 600, and 800°C in the absence of oxygen. Once the vacuum oven had cooled down to room temperature, the materials were removed, passed through a 100 mesh sieve, and stored in self-sealing bags for later use (Chen et al. 2019).

The magnetic biochar was prepared by completely blending the sifted biochar with FeCl₃, FeCl₂, hydrochloric acid, and ammonia in beakers. During the addition, vigorous stirring was performed for 30 min with a magnetic stirrer, after which the beakers were left to stand for precipitation. The supernatant was discarded, and the precipitate was washed with ultra-pure water three to five times. The precipitate was oven dried for several hours at 80°C to obtain the dried magnetic biochar, which was then placed into self-sealing bags for later use (Zhao et al. 2018; Wulandari et al. 2017; Singh et al. 2018).

Subsequently, the physicochemical characterization of the biochars was achieved by the following methods. The surface morphology and area of unmodified and magnetic biochar was observed by SEM (ZEISS SIGMA HD, DEU) and BET (Micromeritics 3Flex, USA). The Zeta potential of unmodified and magnetic biochar was measured by a Zeta (Zetasizer Nano ZS, UK) potential analyzer. The surface functional groups of unmodified and magnetic biochar were analyzed by FTIR (Thermo Scientific Nicolet iS50, USA). An elemental analysis of unmodified and magnetic biochar was conducted by XPS (Thermo Fisher Scientific K-Alpha, USA). The distribution of iron in magnetic biochar was specifically analyzed by high-resolution Raman spectroscopy (Lei Nishao 2000).

Adsorption of Cd²⁺ by magnetic biochar

To determine the adsorption capacity and mechanism of the two magnetic adsorbents for Cd in water, the cow manure and corn stalk magnetic biochars were tested for Cd adsorption.

A series of 50 mL centrifuge tubes was prepared. NaNO₃ at a volume of 10 mL and a concentration of 0.01 mol/L was placed in each centrifuge tube as an electrolyte solution. Then, 0.05 g of cow manure or corn straw magnetic biochar and 10 mL of a 20 mg/L Cd solution were added. In order to obtain the best adsorption performance, different parameters were used to treat the solution in this study. The parameters were the pH of the solution, the amount of magnetic biochar added, the initial concentration of cadmium and the adsorption time.

The pH of the fixed solution ranged from 5 to 9. The test doses of cow manure magnetic biochar were 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, and 0.16 g, and the test doses of corn straw magnetic biochar were 0.02, 0.04, 0.06, 0.08, 0.1, and 0.12 g. The absorbance of each Cd solution to be tested was measured by a visible light photometer 24 hours later.

The centrifuge tubes used in the experiments to determine the effects of different pH values and different magnetic biochar doses on the Cd adsorption process were allowed to stand for precipitation. The supernatant from each tube was filtered through a 0.45 µm aqueous phase filter membrane, and 1 mL of each filtered solution was transferred to a 50 mL constant volume bottle. Then 3 mL of 1 moL/L H₂SO₄, 4 mL of 20% potassium iodide-ascorbic acid, 2 mL of 10 g/L polyvinyl alcohol, and 1.8 mL of rhodamine B solution were added to each bottle. The mixture was then diluted to scale with distilled water, shaken well, and allowed to stand. The absorbance of the solution in the constant volume bottles was measured by a visible light photometer. The wavelength selected for determination was 600 nm. Before determination, the standard curve was plotted using a Cd standard solution, and then the equilibrium adsorption amount Qe (mg/g) of Cd in the magnetic biochip solution was calculated.

The amount of Cd adsorbed by a unit adsorbent (Qe, mg/g) was calculated according to the following Eq. (1):

$$Qe=（C0-Ce）V/m$$

In the equation, C0 is the initial Cd²⁺ concentration in the solution, mg/L; Ce is the Cd²⁺ concentration in the solution at equilibrium, mg/L; V is the volume of the solution, L; and M is the mass of magnetic biochar added, g.

Isotherm and Kinetics Analysis

For isothermal sorption analysis, different initial concentrations (of 0, 5, 10, 20, 40, 60, 80, or 100 mg/L) were chosen for the Langmuir and Freundlich models. The solution conditions were as follows: 0.14 g of cow manure magnetic biochar with a pH of 5 was added and the tube was heated to 800°C, or 0.12 g of corn straw magnetic biochar with a pH of 8 was added and the tube was heated to 600°C. All centrifuge tubes were oscillated at a constant temperature for 24 h (25°C and 130 r/min), and for each solution there were three parallel groups. The absorbance of each Cd solution to be tested was measured by a visible light photometer 24 hours later.

The study of adsorption isotherms is of great significance for exploring the adsorption system and adsorption mechanism of adsorbents. The relationship between the solute concentration remaining in the solution after adsorption and the solute mass adsorbed by the adsorbent per unit mass is represented by an adsorption isotherm. It is therefore necessary to establish the equilibrium relationship between the solid and liquid phase Cd concentrations under the equilibrium conditions. In this study, the adsorption capacity of Cd by cow manure and corn straw magnetic biochars were investigated with the Langmuir and Freundlich adsorption isotherms (Liu et al. 2023).

The Langmuir isotherm has been widely used for assessing the adsorption of heavy metals, dyes, and organic pollutants. The Langmuir isotherm is a single molecular layer adsorption. In this isotherm all sites have equal adsorbate affinity and the adsorbed molecules do not affect the adsorption of adjacent sites. The maximum adsorption capacity was obtained from the Langmuir isotherm. The isotherm equation is given by Eq. (2):

$$Qe=Qm·Kl·Ce/(1+Kl·Ce)$$

where C0 is the initial concentration of the adsorbate (mg/L); b is the adsorption equilibrium constant that is related to the Henry constant (Kl = $\text{B}\text{Q}\text{m}$); $\text{Q}\text{m}$is is the maximum adsorption capacity of the adsorbent (mg/g); and $\text{Q}\text{e}$ is the amount adsorbed on a unit mass of the adsorbent (mg/g), when the equilibrium concentration is Ce (mg/L). Eq. (2) can be rearranged as Eq. (3):

Ce /$\text{Q}\text{e}$ = 1/$\text{B}\text{Q}\text{m}$+ Ce/ $\text{Q}\text{m}$ (3)

the value of the Langmuir constant can be estimated from a plot of Ce/qe versus Ce.

This Freundlich isotherm model indicates that in different concentrations, the ratio of adsorbate adsorbed onto a given mass of adsorbent to the concentration of adsorbate in the solution is not constant. The isotherm equation is given by Eq. (4):

$$lgQe=lgKf+1/n·lg Ce$$

Equation (4) can be rearranged as Eq. (5):

$$qe = Kf\text{C}\text{e}1∕\text{n}$$

where Kf (mg^1–1/n L^1/n g^− 1) is the adsorption capacity of the adsorbent; and n is the constant depending on the heterogeneity properties of the adsorbent.

2.3.2 Adsorption kinetic experiment

For the adsorption kinetic analysis, different adsorption times (0, 15, 30, and 60 min and 2, 4, 8, 12, 24, and 48 h) were selected for pseudo-first-order and pseudo-second-order kinetic models. Kinetic models were used to describe the adsorption process of Cd and explain the mechanism of the adsorption rate from aqueous solution (Zeng et al. 2022; Chen et al. 2022).

A pseudo-first-order reaction model showed that the adsorbate adsorption rate on the adsorbent is based on the adsorption capacity. The linear form of the pseudo-first-order kinetic equation is given by Eq. (6):

$$Qt/Qe=1-1/（ek1t）$$

where$\text{k}1$(min^− 1) is the adsorption operation rate constant of the pseudo-first-order model; $\text{Q}\text{e}$is the adsorbed amount of adsorbate (mg/g) at equilibrium time; and $\text{Q}\text{t}$ is the adsorbed amount of adsorbate (mg/g) at any time, t.

Only the value of k1 can be obtained from the pseudo-first-order kinetic model, and it cannot obtain the value of $\text{Q}\text{e}$. Therefore, for the estimation of $\text{Q}\text{e}$ pseudo-second-order kinetics have to be tested. The pseudo-second-order kinetic equation is given by Eq. (7):

$Qt/Qe=k2t/(1+ Qek2t)$ Qe (7)

where k2 (gm g^− 1 min^− 1) is the rate constant of the pseudo-second-order kinetic model.

Characterization of Biochar and magnetic biochar

Through the preliminary experiment of cadmium adsorption by magnetic biochar at different temperatures, it was found that the cow manure magnetic biochar at 800°C (C8N1) and corn straw magnetic biochar at 600°C (C6J1) had the highest Cd²⁺ adsorption capacity. Therefore, these magnetic biochars and the corresponding unmodified biochars were selected for an SEM analysis (Fig. 1). The surface morphology of the unmodified and magnetic biochars was substantially different. The SEM results of cow manure biochar, cow manure magnetic biochar, corn straw biochar, and corn straw magnetic biochar at 800°C are shown in Fig. 1. The SEM photographs showed the presence of rough asymmetric holes on C6J1 and C8N1 with few surface pores, while the cow manure biochar at 800°C (C8N) and corn straw biochar at 600°C (C6J) had more pores and smaller pore sizes. In contrast, the morphological analysis (Fig. 1) also revealed that irregular iron oxide particles were dispersed on the surface of the C6J1 and C8N1. Additionally, there were Cd²⁺ polymers on the surface of the magnetic biochars of C6J1 and C8N1 after adsorption that were tightly adsorbed in the pores, forming a pile of straw (Chen et al. 2023).

Table 1 summarizes the changes in physical properties before and after biochar modification. According to the table, it can be found that the specific surface area, average pore size and total pore volume of the modified biochar increased. Among them, the specific surface area of C6J1 is higher than that of C8N1. However, in the adsorption experiment, the adsorption capacity of C8N1 is higher, which may be because the average pore size of C8N1 is more conducive to the diffusion of metal ions into the interior of the magnetic biochar void and adsorption.

Combined with Fig. 1 and Table 1, it can be inferred that straw magnetic biochar has a large specific surface area, which is caused by the porous granular structure of its surface depression. The large pore size of cattle manure magnetic biochar is caused by its regular fibrous void structure.

Table 1

Physical properties of biochar and magnetic biochar.
Sample	Surface Area (m²/g)	Average pore Diamete (nm)	Total pore volume (cm³/g)
C6J	3.879	1.43133	1.388
C6J1	144.838	5.31102	1.923
C8N	3.444	9.11473	7.847
C8N1	47.086	6.751	7.948

Zeta potential can measure the surface charge of adsorbent(Fig. 2). When the number of cations on the adsorbent surface exceeds the number of anions, the surface is positively charged; when the number of anions exceeds the number of cations, the surface is negatively charged. Figure 2 shows that the potential peaks of C6J1 and C8N1 were significantly shifted. Therefore, the negative charge of the cow manure and corn straw biochars after magnetization modification increased, indicating that magnetic biochar was more inclined to agglomerate, i.e., the attraction force exceeds the repulsion force, and the adsorption effect of heavy metal cations is more obvious (Zhu et al. 2018; Wang et al. 2022).

An FTIR analysis was conducted to detect the main components of unmodified and magnetic biochars, and each functional group has its corresponding wave number. Figure 3 shows the FTIR spectra indicating that the adsorption of metals on the surface of biochar was influenced by the pyrolysis temperature and the raw materials used, as also reported in other studies. The main peaks representing the vibration of functional groups in biochar materials before and after modification and the modified biochar after Cd adsorption were located at 3470 cm^− 1 (-OH stretching), 1630 cm^− 1 (C = C stretching), and 1030 cm^− 1 (C-H stretching). A comparison of the spectra of the unmodified and modified biochar materials revealed that the strong absorption peak at 579 cm^− 1 was caused by the Fe-O bond of iron oxide. This indirectly confirmed the presence of Fe particles in the modified biochar materials³⁶. In addition, it can be found from the figure that the Fe-O functional group content of cattle manure magnetic biochar is more than that of straw magnetic biochar. Combined with the adsorption experiment, this may also be the reason for the better cadmium adsorption effect of cattle manure magnetic biochar. The peak at 3470 cm^− 1 in the magnetic biochar significantly weakened after adsorption due to the interaction between Cd(II) and hydroxyl groups that had formed on the magnetic biochar (Xia et al. 2021; Pajda et al. 2022). In addition to that, the peak intensity of the -OH group is weakened after heavy metal adsorption, which may be due to the involvement of the -OH group in surface complexation and ion exchange.

The measured Raman spectra of the C6J, C6J1, C8N, and C8N1 are shown in Fig. 4. The Raman spectrum can be used for characterizing and analyzing the C state of an adsorbent material. In the Raman spectrum, the disordered sp3 hybridized C band at about 1330 cm^− 1 was the d-band peak, and the ordered graphite sp2 hybridized C band at about 1591 cm^− 1 was the g-band peak (Toan et al. 2023). The total peak area of the Raman spectra reflected the degree of graphitization of the nanocomposites. Therefore, the peak area ratio of the D-band to G-band (I_D/I_G) could be used as an indicative value for determining the degree of structural disorder in C materials. The I_D/I_G ratio of C6J1 was 3.29, slightly lower than the values of C6J (I_D/I_G = 3.60) and C8N1 (I_D/I_G = 3.20), indicating that the sp2 hybridized domain increased and was consistent with the SEM characterization in Fig. 1(Yao et al. 2020). Comparing the peak area ratio of the D-band to G-band (I_D/I_G) of biochar and magnetic biochar, it can be found that the I_D/I_G ratio of magnetic biochar decreases, which indicates that magnetic modification will reduce the degree of graphitization of biochar.

X-ray photoelectron spectroscopy was used to identify and analyze the elements on the surface of biochar. By comparing the surface elements of unmodified and magnetic biochar, it could be determined whether the magnetization modification was successful or not. Figures 5(a), (b), (c), and (d) are the full spectra of C8N1, C8N, C6J1, and C6J, respectively. Combined with the earlier figures, they show that the biochars before and after modification contained large amounts of C and O. Figures 5(a) and (c) show the characteristic peaks for Fe2p, indicating that C6J1 and C8N1 were successfully loaded with Fe(Wang et al. 2020). The proportions of the various atoms in the full spectrum were as follows: 90.82% of C and 9.18% of O in C8J; 79.16% of C, 15.77% of O, and 5.07% of Fe in C8J1; 87.73% of C and 12.27% of O in C8N; and 67.56% of C, 24.80% of O and 7.64% of Fe in C8N1. After modification, the C content decreased and the O content increased, at the same time the Fe content increased. The Fe content in C8N1 was higher than that in C6J1. The reduction in the C content also confirmed the conclusion drawn from the Raman spectrum that the defect degree of the modified biochar increased. According to Figs. 6(a), 6(b), 6(c), and 6(d), the C = O bond content decreased after modification, which indicated that the C = O bond participated in several chemical reactions during the modification (Datsyuk et al. 2008). Figure 6(e) and 6(f) show the Fe 2p spectra of C8N1 and C6J1, respectively, both of which contained two main peaks and three satellite peaks. The main peak in the Fe2p chromatogram confirmed the presence of Fe³⁺ and Fe²⁺, suggesting the possible presence of Fe₃O₄ or γ-Fe₃O₃ in C8N1 and C6J1 (Hu et al. 2017).

Adsorption experiments with different influence conditions

The pH is one of the most significant parameters that influences the adsorption capacity. A change in pH will lead to certain changes in the form of heavy metals and the charge density on the surface of biochar. A low pH is not conducive to adsorption. When the pH is high, Cd ion precipitation occurs (Jefferson et al. 2015; Rao et al. 2010). The influence of pH on adsorption capacity was studied over a pH range of 5–9, while the other parameters were kept constant. Figure 7 shows the effect of solution pH on the adsorption of Cd²⁺ by the cow manure and corn straw magnetic biochars at different temperatures.

The amount of Cd adsorbed in the solution by the cow manure magnetic biochar at 400°C and the cow manure magnetic biochar at 600°C increased as the pH of the solution increased. Adsorption on the cow manure magnetic biochar at 800°C first decreased and then slowly increased with pH, but the increase in amplitude was small, which indicated that the adsorption capacity of cow manure magnetic biochar also increased with an increase in pH. The adsorption effect of cow manure magnetic biochar was more significant at 800°C than at the other two temperatures. Its adsorption capacity reached the maximum value of 78.97 mg/g, at a pH of 5.0.

When the pH was 5–6, the adsorption capacity of cow manure magnetic biochar at 400 and 600°C increased rapidly, but the adsorption capacity of cow manure magnetic biochar at 800°C decreased. When the pH value was low, the low adsorption capacity of magnetic biochar from cow manure was likely due to the high H⁺ concentration in the solution. The H⁺ competed with Cd²⁺ for adsorption sites on the surface of magnetic biochar, which was not conducive to Cd²⁺ adsorption by the magnetic biochar. When the pH was 6–9, the adsorption capacity of the magnetic biochar from cow manure at all three temperatures increased with the increase in pH. When the pH increased, the H⁺ concentration in the solution decreased and the competition with Cd²⁺ was weakened, which resulted in Cd²⁺ easily attaching to the binding sites on the surface of the magnetic biochar.

Figure 7 shows that with an increase in pH, the adsorption capacity of corn straw magnetic biochar at 400 and 600°C first decreased and then increased. The adsorption effect of the corn straw magnetic biochar was most significant at 600°C. The adsorption capacity reached the maximum value of 66.07 mg/g at a pH of 8.0. The decrease in the adsorption capacity at pH values ranging from 8.0 to 9.0 might be due to the partial precipitation of Cd²⁺ in the solution caused by the higher pH value, which weakened the adsorption effect of the corn straw magnetic biochar. The reasons for the decrease in the pH of the 400 and 600°C corn straw magnetic biochars over the pH range from 5.0 to 7.0 may be that excessive H⁺ in the solution protonated some of the active groups on the surface of the corn straw magnetic biochar to form positively charged groups, increasing the repulsive force between the surface of the adsorbent and the positively charged metal cation, and further inhibiting the adsorption effect. Hence, 800°C cow manure magnetic biochar at pH 5.0 and 600°C corn straw magnetic biochar at pH 8.0 were selected for this study (Chowdhury et al. 2022).

Figure 8 shows the adsorption of Cd²⁺ for cow manure magnetic biochar at 800°C and pH 5.0 and corn straw magnetic biochar at 600°C and at pH 8.0 in solutions with different amounts of magnetic biochar added. Adsorbent dose is another significant parameter because it controls the adsorbent-adsorbate interaction and balances the system (Qu et al. 2021). The adsorption capacity of Cd²⁺ was studied in response to different doses of magnetic biochar, ranging from 0.02 to 0.2 g.

When the dose was 0.14 g, the adsorption capacity of 800°C cow manure magnetic biochar for Cd²⁺ was 79.40 mg/g and when the dose was 0.12 g, the adsorption capacity of 600°C corn straw magnetic biochar for Cd²⁺ was 67.11 mg/g. Furthermore, the adsorption of Cd²⁺ by cow manure magnetic biochar at 800°C and corn straw magnetic biochar at 600°C first increased and then decreased with the increase in dose. This was probably because for a given Cd²⁺ concentration in the solution, the adsorption sites on the surface of the magnetic biochar correspondingly increased as the magnetic biochar dose increased, and a large number of active sites competed for a limited amount of heavy metal ions, thus reducing the number of heavy metals adsorbed by the magnetic biochar per unit mass. Consequently, cow manure magnetic biochar at 800°C and dose of 0.14 g and corn straw magnetic biochar at 600°C and dose of 0.12 g were selected for this study (Zhang et al. 2021).

Adsorption isothermal

The Freundlich and Langmuir models were applied to describe the adsorption isothermal model for treating solutions containing Cd²⁺. Monolayer and multilayer adsorptions onto different adsorbent surfaces (homo and heterogenous) were analyzed by the Langmuir and Freundlich isotherm models, respectively (Jobby et al. 2018).

The results of the isotherm analysis are shown in Fig. 9 and Table 2. The maximum R² values of the tested isotherms indicated that the model was a good fit for the experimental results. By comparing the correlation coefficients of the two isothermal models it was found that the most appropriate model for the cow manure and corn straw magnetic biochars was the Freundlich model. The results showed that the cow manure magnetic biochar had a higher specific surface area, stronger affinity for Cd²⁺, and more active sites than the corn straw magnetic biochar, and also had a better adsorption capacity. In the Freundlich model, the n and Kf reflected the effect of the heavy metal concentration on the adsorption capacity. When the n value was small, the Kf value was large, which indicated a better adsorption effect. The Kf of the cow manure magnetic biochar was higher than that of the corn straw biochar, which confirmed that the cow manure magnetic biochar had a stronger affinity for Cd²⁺.

The high adsorption capacity of magnetic biochar may be due to the strong affinity of Cd for iron oxide on magnetic biochar, which was also reflected by the FTIR characterization that indicated that the surface functional groups (-COOH, -OH) generated stable internal spheres (Liu et al. 2022).

Table 2

Fitting parameters of the adsorption isotherm model.
Adsorbent	Langmuir			Freundlich
Adsorbent	R²	Qm(mg/g)	K_l(L/g)	R²	n	Kf(mg^{− 2−1/n}·L^1/n)
C8N1	0.9625	58.7984	0.0273	0.9658	1.9300	4.1840
C6J1	0.9300	22.9200	0.0627	0.9961	2.8006	4.0591

Adsorption kinetics

The kinetics study was applied to the adsorption of Cd²⁺ on the cow manure and corn straw magnetic biochars (Trakal et al. 2016; Wang et al. 2021). Figure 10 shows the variation of adsorption over time under the initial Cd²⁺ concentrations. The adsorption of Cd²⁺ on magnetic biochar could be divided into two processes: rapid adsorption and slow adsorption. The adsorption of Cd²⁺ by the two magnetic biochars increased rapidly in the first 2 h, but at different rates. Figure 10 shows that most of the Cd²⁺ was adsorbed within 4 h until the internal active adsorption sites were fully occupied and equilibrium was reached. The adsorption of Cd²⁺ by cow manure magnetic biochar was greater than that achieved by corn straw magnetic biochar, indicating that it had a better adsorption performance than biochar (Choudhary et al. 2020). The fitting parameters of the kinetic model indicated that the pseudo-second-order kinetic model fitted well. This result indicates that physical adsorption may occur in the process of cadmium adsorption by magnetic biochar.

Combining Fig. 10 and Table 3, it can be found that the adsorption of cadmium by the two magnetic biochar biochar can reach the adsorption equilibrium in a relatively short time, which is consistent with the conclusions of most studies. The main reason may be that the negatively charged surface of magnetic biochar allows it to quickly adsorb positively charged metal ions. In addition, due to the large specific surface area of straw magnetic biochar, the adsorption rate of straw magnetic biochar was significantly faster than that of cattle manure magnetic biochar. This feature can also be reflected according to the parameter k in the kinetic equation.

Table 3

Fitting parameters of the adsorption isotherm model
Adsorbent	Pseudo-first-order			Pseudo-second-order
Adsorbent	R²	Qe(mg/g)	k₁(h)	R²	Qe(mg/g)	k₂(g·h^− 1·mg^− 1)
C8N1	0.9464	31.3545	0.7758	0.9868	34.2344	0.0294
C6J1	0.9765	22.6082	0.9613	0.9802	24.3916	0.0531

The optimal conditions of magnetic biochar were obtained by adsorption experiments. Combined with characterization analysis and adsorption experiments, it was found that the adsorption efficiency of cattle manure magnetic biochar was high. Moreover, the surface structure, charge and functional groups of the magnetic modified biochar were changed, which not only proved the successful preparation of magnetic biochar, but also proved that the magnetic modification improved the adsorption ability of biochar. In addition, according to the isothermal adsorption and kinetic fittings of the Cd adsorption process for the cow manure and corn straw magnetic biochars, it was found that both magnetic biochars were most consistent with the Freundlich model and a quasi-second-order kinetic model. These results indicate that physical adsorption and functional groups are involved in the process of cadmium adsorption by magnetic biochar. The adsorption mechanism is mainly related to surface complexation and ion exchange. The magnetic biochar prepared in this study could improve the current situation of heavy metal pollution in water in a short term, and provide a new strategy for the rational use of cattle manure and straw and the treatment of heavy metal pollution in water at a low cost.

Author Contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Zhiwen Li, Ruiyan Niu , Jiaheng Yu, Liyun Yu, Di Cao. The first draft of the manuscript was written by Zhiwen Li and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding: This research was supported by the Heilongjiang Science and Technology Department (LH2019C050, GA21C028), Heilongjiang Science and Education Department (UNPYSCT-2018084) and Heilongjiang Bayi Agricultural University (xyb2013-18).

Data Availability: Data available on request from the authors.

Ethics approval: The authors guarantee that this research’s process, content, and conclusion do not violate the theory and moral principles.

Consent to participate: Not applicable.

Consent for publication: Not applicable.

Competing Interests: The authors have no relevant financial or non-financial interests to disclose.

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Editorial decision: Major Revision
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Removal of Cadmium from Aqueous Solution by Magnetic Biochar: Adsorption Characteristics and Mechanism

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Journal Publication

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Abstract

Figures

Introduction

Materials and Methods

Adsorption of Cd²⁺ by magnetic biochar

Isotherm and Kinetics Analysis

Results and discussion

Characterization of Biochar and magnetic biochar

Adsorption experiments with different influence conditions

Adsorption kinetics

Conclusion

Declarations

References

Status:

Journal Publication

Version 1

Removal of Cadmium from Aqueous Solution by Magnetic Biochar: Adsorption Characteristics and Mechanism

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials and Methods

Adsorption of Cd2+ by magnetic biochar

Isotherm and Kinetics Analysis

Results and discussion

Characterization of Biochar and magnetic biochar

Adsorption experiments with different influence conditions

Adsorption kinetics

Conclusion

Declarations

References

Status:

Journal Publication

Version 1

Adsorption of Cd²⁺ by magnetic biochar