Preparation of composite soybean straw-based materials by LDHs modifying as a solid sorbent for removal of Pb(II) from water samples

In this study, the nanocomposites from biomass (soybean straw) and layered double hydroxides (LDHs), denoted as B/LDHs, were fabricated using the mechanical-hydrothermal method. The obtained B/LDHs nanocomposites were characterized by TEM, SEM, FT-IR, and N2 adsorption–desorption techniques. Adsorption of the heavy-metal ions Pb(II) on the B/LDHs was determined at 25°C and pH 6.0 using a batch technique. The experimental results demonstrated that biomass contributed to the sorption process. The pseudo-second-order, Langmuir, and Freundlich models well fitted the sorption process, indicating chemisorption and monolayer adsorption were the main adsorption mechanisms. Meanwhile, it is found that there is an obvious effect of adsorbent concentration in the studied adsorption system. In comparison with soybean straw and Mg–Al LDHs, the B/LDHs nanocomposites exhibit significantly enhanced sorption capacities. It is evident from this study that the construction of B/LDHs nanocomposites is an effective strategy for improving the sorption capacity of LDHs, and the modified LDH-based adsorbent shows a good potential in the removal of heavy metals fromwater. More importantly, it solves the problem of a large number of agricultural waste disposals. And, it achieved the goal of a win-win situation.


Introduction
Biomass [1][2][3][4][5] refers to all kinds of organisms produced by photosynthesis through the use of atmosphere, water, and land. Its advantages are renewable, low pollution, and wide distribution. Representative biomass includes crops, crop waste (wheat straw, corn cob, soybean stalk, orange peel, coconut shell, rice husk, etc.), wood, wood waste, and animal waste. Straw, as a kind of agricultural waste, is produced in large quantities every year. At present, its use is still at a very inefficient level in China. It is of great strategic significance to pay attention to the development and utilization of renewable biomass resources in the face of rapid resource consumption and environmental degradation.
Layered double hydroxides (LDHs) [6][7][8][9][10] are commonly used adsorbents. And, its chemical composition is as follows: [M II (1−x) M III x (OH) 2 ] x+ [A n− x/n ] x− ·mH 2 O, where M II and M III refer to divalent and trivalent metal cations, respectively; A stands for the interlayer anion; x is for per mole of LDHs M III moles; and m is the number of moles of crystal water in the middle layer per mole LDHs. LDHs have a lamellar crystal structure, and the lamellar plates are positively charged with exchangeable anions. Different types of metal ions (M II and M III ) and interlayer anions (A n− ) lead to different LDHs properties. Therefore, LDHs are a kind of new inorganic lamellar materials. It has broad application prospects in the fields of catalyst, adsorbent, ion exchange agent, combustion promoter, pesticide slow-release preparation, and liquid flowtype regulator, especially in the fields of pharmaceutical transport carrier and sewage treatment, etc.
In recent years, the pollution of global freshwater resources is one of the main environmental problems faced by human beings. In particular, heavy metal ions have aroused widespread concern. With the rapid development of petrochemical, printing, dyeing, textile, plastic, leather, food, and other industries, industrial wastewater containing heavy metal ions are discharged into soil and water in large quantities, which seriously threatens the living space of human beings. To ensure the supply of freshwater resources, reducing water pollution or the treatment and recycling of polluted water resources are paid more and more attention by various countries.
Recently, the synthesis of biomass/LDH materials from LDH and biomass provides a win-win strategy. There are two main reasons for explaining the rationale of adding LDH to straw: first, cellulose contained in soybean straw can provide substrate for LDH growth. Second, lignin contained in soybean straw has abundant hydroxyl, methoxy, and carbonyl groups on its surface, which forms hydrogen bond with hydroxyl groups and water molecules on and between LDH laminates. Therefore, combining biomass with synthetic materials is oftentimes quite advantageous [31]. It not only solves the problem of a large number of agricultural waste treatments but also has a certain breakthrough in the preparation of new adsorbent. Due to its particle stability, pore characteristics, and surface activity, more and more attention has been paid to these kind of materials [32][33][34][35]. For example, Wang et al. [32] found that the maximum As(V) sorption capacity of Ni/Fe-LDHs-biochar composites was 4.38 mg/g, which was approximately three times as large as the mechanical mixture of Ni/Fe-LDHs and biochar. Xue et al. [33] reported that the biochar/ MgFe-LDHs composite has a strong sorption ability to nitrate with the maximum adsorption capacity of 24.8 mg/g. Wan et al. [34] indicated that the biochar/MgAl-LDHs composite containing 40% MgAl-LDHs exhibited the highest phosphate removal rate with >95%. Huang et al. [35] examined that the BC@EDTA-LDHs' nano-adsorbent was used to remove Cr(VI) with the maximum adsorption capacity of 38 mg/g. However, the above biomass materials are calcined or lye washed before being used for adsorption. Calcining brings a series of problems, such as adsorption capacity decreases, greenhouse effect, and air pollution.
More importantly, the pretreatment of biomass will consume more resources. To achieve the full utilization of resources and environmental protection, we used un-calcined or unlye washing biomass to obtain a new adsorbent.
The objectives of this study were as follows: (1) to synthesize the compounds of Biomass (soybean straw)/ MgAl-LDHs (B/LDHs) using the mechanical-hydrothermal method [36]; (2) to analyze the properties of B/LDHs using various characterization methods; and (3) to study the adsorption performance of B/LDHs for heavy metal ions Pb(II).

Materials
Al(OH) 3 , Pb(NO 3 ) 2 , NaNO 3 , and HNO 3 were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Mg (OH) 2 was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. All chemicals were of analytical grade and used as received. Soybean straw was from local agricultural waste. Soybean straw was cleaned and dried. Then, it was crushed to obtain the powder. At last, the powder was passed through a 100-mesh sieve and standby. Water was purified with a Direct-Pure UP water purification system (Rephile, China).

Preparation of B/LHDs nanocomposites
The B/LHDs nanocomposites were prepared by the mechanical-hydrothermal method in our previous studies [36]. Different from previous studies, soybean straw (1.0 g) powder was added in the first step. After a series of reaction treatments, it finally obtained the B/LDHs' nanocomposite sample. This sample was abbreviated as 10% B/LDHs (see supporting information for specific preparation).
Under the same conditions, the 50% B/LDHs and the Mg-Al LDHs without the soybean straw were prepared.

Characterization
Under the conditions of 10 kV acceleration voltage and gold spraying, the morphology of the samples were analyzed using a JSM-6700F scanning electron microscopy (SEM, JEOL, Japan) and a JEM-2100 transmission electron microscopy (TEM, JEOL, Japan). Fourier transform infrared spectra (FTIR) (Nicolet 5700 Spectrometer, USA) of adsorbents were recorded under the conditions that the resolution is better than 0.09 cm −1 , the wavenumber accuracy is 0.01 cm −1 , and the quick scanning speed is 65 times/second. FTIR of adsorbents was recorded in the range of 4,000-400 cm −1 . The samples were pressed with KBr pellets. The N 2 adsorption-desorption isotherms were determined using an Autosorb IQ-MP system (Quantachrome Instruments, USA), and the test samples were degassed at 120°C for 5 h under vacuum before measurement. The specific surface area (A s ) and pore volume (V p ) of the samples were calculated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.

Adsorption experiments
The heavy metal ion Pb(II) was used as the target to test the adsorption capacity of B/LDHs. The sorption experiments were conducted under a batch of equilibration techniques at a temperature of 25.0°C, solubilizing Pb(II) solutions at different concentrations ranging 0-500 mg/L in water containing 0.010 M of NaNO 3 to get Pb(NO 3 ) 2 solutions. NaNO 3 was applied as a maintainer for the constant ionic strength of the solutions. With the use of 0.1 M HNO 3 and NaOH solutions, the pH values of the Pb(II) solutions were adjusted to 6.0. The mass-given adsorbent sample with 25 mL of the pollutant solutions was blended in polyethylene centrifuge tubes. The centrifuge tubes were allowed to swing in a thermostatic water bath vessel (Jiangsu Medical Instrument Factory, China) at a temperature of 25.0 ± 0.2°C for 24 h, and then, the supernatant solution was filtered through a 0.45 µm membrane. The residual pollutant concentrations in the filtrates were measured using the AAS-3600 flame atomic absorption spectrometry (Shanghai Metash Instruments Co., Ltd., China) for Pb(II).
The sorption amount (Γ t ) was determined by the difference between the initial and the residual concentrations of Pb(II): where Γ t (mg/g) is the sorption amount at time t, C 0 (mg/L) and C t (mg/L) are the initial and the remaining concentrations at time t, respectively, and C s (g/L) is the sorbent dosage.
Sorption kinetic tests showed that t = 10 h was required to reach equilibrium. To ensure sorption equilibrium, t = 24 h was selected in the equilibrium sorption tests.
The adsorption capacity and the removal rates (E R ) in this study were calculated as follows: where the C e refers to the equalized one and Γ e indicates the equilibrium adsorption capacity.
The tests shall be conducted three times, and the final values were taken as the average of overall measurements, with a relative error of less than 5%.
Ethical approval: The conducted research is not related to either human or animal use.  (Figure 1a-d). Furthermore, with the decrease of soybean straw contents, a large number of flake structures could be observed growing on the soybean straw (Figure 1c and d). In addition, hexagonal crystals, typical of LDHs, were observed, and the lateral size of the LDH crystals was ∼200 nm, which has been reported in our previous studies [36]. Furthermore, the soybean straw showed irregular block structures. Figure 2 shows the FT-IR spectra of 50% B/LDHs, 10% B/LDHs, Mg-Al LDHs (LDHs), and soybean straw (B) samples. In the spectrum of soybean straw (B), the bands at 3,464 and 1,620 cm −1 were attributed to the hydroxyl group and water deformations, the band at 2,880 cm −1 arose from the characteristic peak of methylene on the molecular chain of total cellulose, and 1,080 cm −1 arose from the stretching vibration peak of C-H on total cellulose. In the spectrum of the LDH sample, the strong broad band centered at approximately 3,700 cm −1 was assigned to the O-H stretching modes of the hydroxyl groups in the LDH layers and the interlayer water, and another band corresponding to water deformation was recorded at approximately 1,620 cm −1 ; the band at 1,362 was attributed to the ν 3 vibrations of  The N 2 adsorption-desorption isotherms of the 50% B/LDHs, 10% B/LDHs, Mg-Al LDHs (LDHs), and soybean straw (B) samples ( Figure 3) showed a type IV adsorption isotherm with a type H3 hysteresis loops when P/P 0 ratio was greater than ∼0.4 [37], which was manifested as fractured pores formed by the accumulation of flap-like particles. The A s , V p , and average pore size (D p ) of the samples are listed in Table 1. With the increase of soybean straw contents, A s and pore data (D p , V p ) of the B/LDHs samples decreased significantly. In addition, the mesoporous structure of D p of ∼3.4-9.3 nm remained in the nanocomposites, which is conducive to its application as an adsorbent.

Removal performance of B/LDHs
composites for Pb(II)

Effects of pH
The pH value of the solution is one of the important parameters affecting the adsorption process. Under the conditions of C 0 = 50 mg/L, C s = 1.0 g/L, C NaNO3 = 0.010 M, and 25°C, the effects of pH on the adsorption of Pb(II) by B, LDHs, and 50% B/LDHs were studied (Figure 4), respectively. As the pH value of 4-10, Γ e and E R for Pb(II) increased, similar results are reported in the literature [27]. In addition, the removal of Pb(II) under high pH (above 8.5) can be attributed to the precipitation of Pb(II) hydroxides [38].

Adsorption kinetics and removal efficiency
First, B/LDHs adsorption kinetics test for Pb(II) (C 0 = 100 mg/L, pH = 6.0) was performed to evaluate the contact time required for adsorption equilibrium, where the sorbent dosage (C s ) was 1.0 g/L, the concentration of NaNO 3 (C NaNO3 ) was 0.010 M, and the temperature was 25°C. The results showed that the adsorption capacity increased significantly in the first 3 h and then increased slowly until reaching equilibrium ( Figure S1(a) in the supporting information). It took about 12 h to reach equilibrium. The contact time of 24 h was to choose the decision Γ e ensures adsorption equilibrium. In addition, it is found that the sorption kinetic processes could be described by the pseudo-second-order kinetic model ( Figure S1(b) in the supporting information).    Under the conditions of C s = 1.0 g/L, C NaNO3 = 0.010 M, and 25°C, the removal rates of Pb(II) (C 0 = 30 mg/L, pH = 6.0) on the B, B/LDHs, and LDHs samples from aqueous solutions were determined, as shown in Figure 5. The E R values of the B and LDHs were 43.7 and 59.8%, respectively. The E R value of the 10% B/LDHs was 47.9%, which was close to the B sample. This may be due to the small specific surface area of the 10% B/LDH composites. However, the E R value of the 50% B/LDHs was 97.3%. Although the specific surface area of 50% B/LDH composites was relatively small, its adsorption capacity is large. This indicates that a large amount of soybean straw powder provides an adequate growth site for LDHs. However, if the soybean straw content exceeds 50%, the adsorption amount decreases again, the reason is not clear at present. The above results indicate that the goal of "treating waste with waste" could be achieved. Under the same conditions, the E R values of the B/LDHs were 98.9% when Pb(II) concentration was very low at C 0 = 0.25 mg/L. The residual Pb(II) after adsorption was ∼2.75 μg/L, which was lower than the World Health Organization standard for drinking water quality (10 μg/L for Pb(II)) [38]. These results indicate the potential of the 50% B/LDHs composites as the sorbents for wastewater and drinking water treatments.
To evaluate the recyclability of 50% B/LDHs composite materials, the recovery tests were carried out under the conditions of C s = 1.0 g/L, C NaNO3 = 0.010 M, and 25°C, in which Pb(II) C 0 = 50 mg/L and pH = 6.0. The desorption efficiency (E D ) of adsorbing Pb(II) from 50% B/LDHs was determined. The results showed that the E D values of Pb(II) were about 95.6% in the hydrochloric acid solution with pH = 4.0. Therefore, the hydrochloric acid solution with pH = 4.0 was used as the eluent for the recovery test. After 4 cycles, the E R values of Pb(II) decreased by only less than 8% ( Figure S2 in the supporting information).
These results indicate that the 50% B/LDH composites have a good recyclability for removing heavy-metal pollutants from wastewater.

Sorption isotherm
To evaluate the adsorption capacity of B, B/LDHs, and LDHs samples to Pb(II), the adsorption isotherm was determined at C s = 1 g/L, C NaNO3 = 0.010 M, pH = 6.0 for Pb(II), and 25°C, as shown in Figure 6.
Adsorption isotherms are usually described by Langmuir or Freundlich isotherms. Langmuir model can be expressed as follows: (4) or in a linear form as The Freundlich model can be expressed in a nonlinear form as or in a linear form as In these equations, the maximum sorption amount is Γ m , the Langmuir equilibrium constant is K L , and the Freundlich constants are K F and n F .
The linear and nonlinear regression Langmuir and Freundlich isotherms were used to fit the adsorption data of different composites. The nonlinear model diagram is consistent with the experimental data ( Figure 6), and the linear model diagram is a straight line ( Figure S3 in the supporting information). The best values of the model parameters, Γ m , K L , K F , n F , and the correlation coefficient (R 2 ), are listed in Table 2 (and Table S1 in the supporting information). The correlation coefficient (R 2 ) of each model fitting curve was high, indicating that both Langmuir and Freundlich models could well describe the adsorption isotherms of different composites. In addition, the optimal fitting values of model parameters using nonlinear regression and linear regression are very similar. Furthermore, it can be seen from Table 2, Γ m values increase with the increase of the content of B. This is due to the availability of more adsorption sites (or functional groups) from straw lignin and LDH nurturing substrate effect of straw cellulose. It is worth noting that the Γ m value of 50% B/LDHs increased by 15% over 10% B/LDHs composites. From the point of view of saving resources, it is necessary to replace hydroxides with a large amount of soybean straw.

Effect of sorbent dosage
The amount of adsorbent used (C s ) in adsorption test is an important factor affecting the adsorption capacity of solid adsorbent in aqueous solutions [39,40]. It is of great theoretical and practical significance to study the effect of C s on adsorption properties. Taking 50% B/LDHs as an example, the adsorption isotherm of Pb(II) at different C s values was measured. Using the 50% B/LDHs as an example, the sorption isotherms for Pb(II) at different C s values were measured under the conditions of C NaNO3 = 0.010 M, pH = 6.0, and 25°C, as shown in Figure 7. The adsorption isotherm decreased significantly with the increase of C s . The C s dependence of adsorption isotherms is a well-known solid (or adsorbent concentration) effect (C s effect) [41,42]. Langmuir and Freundlich isotherms were used for nonlinear and linear regression to fit adsorption data under different C s values. The nonlinear model diagram was consistent with the experimental data (Figure 7), and    Figure S4 in the supporting information). The best values of the model parameters, Γ m , K L , K F , n F , and R 2 , are listed in Table 3 (and Table S2 in the supporting information). The high R 2 values of various model fitting curves indicated that the Langmuir and Freundlich models fully described the adsorption isotherms of any given C s value. In addition, the optimal fitting values of model parameters using nonlinear regression and linear regression are very similar. Furthermore, it can be seen from Table 3 that the Γ m values decreased with increasing C s . In fact, all Langmuir and Freundlich parameters change with the change of C s . Similar results have been reported in the literature. The C s -dependence of the model parameters violates the predictions of the Langmuir and Freundlich models.

Conclusion
The B/LDHs nanocomposites were successfully fabricated using a mechanical-hydrothermal route. The so-obtained nanocomposites were efficient adsorbents to remove heavy metal ions. The experimental results demonstrated that biomass was donated in the sorption process. The pseudo-second-order, Langmuir and Freundlich models well fitted the sorption process, indicating chemisorption and monolayer adsorption were the main adsorption mechanisms. Meanwhile, it is found that there is an obvious effect of adsorbent concentration in the studied adsorption system. Since biomass was from local agricultural waste, the use of biomass as a carrier for LDHs was environmentally friendly. The B/LDHs nanocomposites are potential sorbents for wastewater treatment. Furthermore, the synthesis of biomass/LDHs materials from LDHs and biomass provides a win-win strategy.
Acknowledgments: The authors are grateful to Zheng Niu, Qingqing Guo, and Shaokang Fang for sample preparations for the experiments.

Conflict of interest:
On behalf of all authors, the corresponding author states that there is no conflict of interest.
Data availability statement: All data generated or analyzed during this study are included in this published article.