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Using Al(OH)3 and Mg(OH)2 as raw materials, MgAl layered double hydroxides (LDHs) modified from birch leaves were prepared by the ball-milling–hydrothermal method, denoted as LDHs/BL, and used for the treatment of wastewater containing Cu(II). The morphology, crystal shape, and specific surface area of the samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and BET-specific surface area analyzer. The results showed that the prepared LDHs/BL composites had good crystal shape, large specific surface area, and suitable pore structure. Langmuir and Freundlich adsorption isotherm models were used to analyze the adsorption. The results showed that the adsorption conforms to the Freundlich adsorption isotherm model. Under the same adsorption conditions, the adsorption capacity of the LDHs/BL complex was higher than that of LDH, the removal efficiency of the former was 92%, and that of the latter was 68%. The adsorption mechanism includes metal precipitation and isomorphic replacement. More importantly, this study provides a green method to prepare LDHs. LDHs/BL composites are expected to be used as a new kind of environmental adsorbent for wastewater treatment. This work provides a new idea for the utilization of agricultural and forestry wastes.

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  • Nanocomposites of LDHs/BL were fabricated.

  • The LDHs/BL nanocomposites exhibit enhanced sorption capacities for Cu(II).

  • The nanocomposites are potential sorbents for wastewater treatment.

  • The synthesis of LDHs/BL nanocomposites provides a win–win strategy.

adsorption, birch leaves, heavy metal, layered double hydroxides, mechanical hydrothermal method
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The development of human society is accompanied by the consumption of various resources, which urges people to increasingly turn their attention to renewable resources, among which the development and utilization of biomass resources has been widely researched and valued (Shen et al. 2019; Sun et al. 2019; Shahnaz et al. 2021). As a type of biomass resource, agricultural and forestry waste is a new hot spot in energy research. Different waste resources have different possible utilization rates. At present, the reuse of agricultural and forestry wastes is mainly reflected in three aspects: (1) Gasification of agricultural and forestry waste to generate electricity. At high temperature, the waste is reacted with a gasification agent to produce small molecules of combustible gas to generate electricity. (2) Wood moulding fuel. Heat is obtained by compressing wood waste under high pressure into a hard, rod-like, granular form. (3) Liquefaction to prepare fuel ethanol. Liquefaction is the chemical conversion of waste into liquid products. However, the reuse of these agricultural and forestry wastes has the disadvantages of high temperature, high pressure or using chemical reagents. It is particularly important to carry out efficient and high-value utilization of agricultural and forestry wastes in an environmentally friendly way. As a common agricultural and forestry waste, the utilization of birch leaves (BL) has not been widely studied.

Copper is one of the essential trace elements needed for life. However, it is a common heavy metal ion in electroplating wastewater, which posed a great threat to both human health and the ecological environment (Stern 2010; Yue et al. 2017). Numerous methods (Elmsellem et al. 2014; Li et al. 2016; Vilela et al. 2016; Kobielska et al. 2018; Li et al. 2019; Liu et al. 2019), including chemical precipitation, electro-chemical treatment, ion exchange, membrane filtration, and adsorption technologies, have been used to remove heavy metal ions from wastewater. Among them, adsorption technology appears to be an efficient and economic method for the removal of heavy metals from electroplating wastewater owing to the advantages of low cost, facile operation and regeneration of adsorbents (Sarma et al. 2019; Szewczuk-Karpisz & Wiśniewska 2019).

Layered double hydroxides (LDHs) (Lv 2022; Wu & Wang 2022; Yang et al. 2022) are hydroxides consisting of two or more metallic elements with a layered crystal structure of hydrotalcite. The formula is [M2+(1-x)M3+x(OH)2]q+[An−]q/n·yH2O. The metal ions (M2+ and M3+) in the laminates can be replaced by isocrystalline ions, and the interlayer anions (An−) can be replaced by ions in solution. LDHs is a type of new adsorbent with good application prospects in wastewater treatment (Yuan et al. 2013; Lei et al. 2017; Ahmed et al. 2020; Alagha et al. 2020; Natarajan et al. 2020). Its preparation method is a hotspot of current research, commonly used methods including coprecipitation, hydrothermal methods, urea hydrolysis, mechanical methods and mechanical hydrothermal methods. Among these methods, mechanical method are green methods conforming to atom economy. It mainly uses mechanical energy to induce chemical reactions, or induces the change in material structure, structure and properties to prepare materials or modify materials. There are many reports on the processing and utilization of non-metallic minerals (Lei et al. 2018; Li et al. 2018).

In this study, the BL, Al(OH)3 and Mg(OH)2 were mechanically ground to generate activation sites through friction and collision, and then the MgAl layered double hydroxides modified by the BL were prepared by hydrothermal treatment (denoted LDHs/BL) and used for the treatment of wastewater containing Cu(II). We aimed to understand the removal behavior of Cu(II) by LDHs/BL. Therefore, the main objectives were: (1) comparing the removal efficiency and adsorption capacity between LDHs and LDHs/BL on the adsorption properties of Cu(II); and (2) determining the mechanism for removal of Cu(II) by LDHs/BL.

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Reagents

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Mg(OH)2 was purchased from Aladdin Reagent (Shanghai) Co., Ltd, China. Al(OH)3 and CuSO4·5H2O were purchased from Tianjin Kermel Chemical Reagent Co., Ltd, China. HNO3 was purchased from Sinopharm Chemical Reagent Co., Ltd, China. All chemicals were used without further purification. The BL were collected from the north side of the comprehensive experimental building on the Campus of Heze University. Water was purified using a Hitech-Kflow water purification system (Hitech, China).

Preparation of LDH/BL composites

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The LDHs composites were synthesized via a ball-milling hydrothermal method. First, the collected BL were washed with water to removal dust, and dried in an electric thermostatic air-blowing drying oven. In order to investigate the effects of various components contained in BL on composites, the BL were treated (BL-T) with 1 M NaOH solution to remove the lignin content and with H2O2 dissolving the hemicellulose according to the methods described previously (Shahnaz et al. 2021). The resulting products were denoted BL-T. The BL and BL-T were broken down with a pulverizer, and sifted through a 100-mesh sieve, then set aside.

Next, we took 1.56 g Al(OH)3, 3.48 g Mg(OH)2 and 2.0 g BL (or BL-T), and put them in a 500 mL ball mill pot for milling 1 h under the conditions of the ratio of ball to material of 49: 1 and a rotational speed of 480 r/min. The 0.5 g milling mixture with 50 mL water and added to a 100 mL reaction kettle in an oven at 120 °C for 24 h. The resulting dispersion was centrifuged, and the solid precipitate was washed with water and dried at 60 °C, and labeled as LDHs/BL (or LDHs/BL-T). Next, we took 0.156 g Al(OH)3, 0.348 g Mg(OH)2 and 0.2 g BL, and put them in a 100-mL reaction kettle with 50 mL water to treat at 120 °C for 24 h, the sample obtained was denoted as NMgAl/BL.

For the comparison, MgAl–LDHs were prepared in the same way but with the absence of BL (or BL-T), labeled as LDHs. All samples were sifted through a 100-mesh sieve, then stored for later use.

Characterization

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X-ray diffraction (XRD) patterns of the samples were recorded on a D/max-rA diffractometer (Bruker, Germany) using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA with a scanning rate of 10 °/min. The sample morphology was analyzed using a JSM-6700F scanning electron microscope (SEM, JEOL, Japan) under the conditions of 10 kV acceleration voltage and gold spraying. The samples were dispersed in ethanol solution and characterized using a JEM-2100 transmission electron microscope (TEM, JEOL, Japan). Fourier transform infrared spectra (FT-IR) (Nicolet 5700 Spectrometer, USA) of samples were recorded in the range of 4,000–400 cm−1. The N2 adsorption–desorption isotherms were determined using a 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 (As) and pore volume (Vp) of the samples were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

Sorption experiments

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The heavy metal ion Cu(II) was used as the target to test the adsorption capacity.

The Cu removal test was carried out at room temperature. The initial concentrations of Cu(II) from 50 to 500 mg/L were prepared by dissolving CuSO4·5H2O in deionized water. A specified amount of adsorbent was weighed and put into a polyethylene tube containing a certain volume of the above solution, and oscillated at 150 r/min on a thermostatic oscillator until the specified time. The initial pH values of the solutions were adjusted to 5 with 1 mol/L HCl and 1 mol/L NaOH solution. Then an appropriate amount of the solution was passed through a 0.45-μm filter membrane, and the concentration of copper ions in the filtrate tested by an atomic absorption spectrophotometer (AAS) (AAS-3600, Shanghai Metash Instruments Co., Ltd, China) equipped with an air–acetylene flame.

To ensure sorption equilibrium, t = 24 h was selected in the equilibrium sorption tests.

The equilibrium adsorption amount and the removal efficiency (ER) in this study were calculated as follows:
(1)
(2)
of which, the C0 (mg L−1) is the initial concentrations, and Ce refers to the concentration of solution at adsorption equilibrium. Cs (g L−1) is the sorbent dosage. Γe indicates the equilibrium absorption amount.

The tests were conducted three times and the final values taken as the average of overall measurements, with the relative error less than 4.5%.

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Characterizations

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The LDHs/BL (or LDHs/BL-T) composites were fabricated using a ball-milling hydrothermal route, as shown in Figure 1. The as-obtained LDHs/BL (or LDHs/BL-T) composites were characterized using XRD, SEM, FT-IR, and BET specific surface area analyzers.
Figure 1
Schematic illustration for the preparation process of LDHs/BL (or LDHs/BL-T).
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Schematic illustration for the preparation process of LDHs/BL (or LDHs/BL-T).

Figure 1
Schematic illustration for the preparation process of LDHs/BL (or LDHs/BL-T).
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Schematic illustration for the preparation process of LDHs/BL (or LDHs/BL-T).

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XRD analysis

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Figure 2 shows the XRD patterns of the samples in the range of 10−70°. It can be seen that the diffraction peak positions of samples BL and BL-T are basically consistent, which are the characteristic diffraction peaks of the typical natural cellulose II crystal structure (Gan et al. 2015). Both of them had a bulge in the range of 2θ = 12° and 28°. In particularly, the diffraction peak of 2θ near 22.6° corresponded to the (002) crystal plane of type I cellulose, which is consistent with previous published reports (Hornok et al. 2005; Zhang & Xia 2012). However, the peak strength of BL-T is strong, which was may be because there is more cellulose. The XRD patterns of the LDHs samples displayed obviously the characteristic reflections of the MgAl–OH LDHs phase: (003), (006), (012), (015), (110) and (113) (Zhang et al. 2012). The d003 value of LDHs was 0.775 nm, which is in agreement with the reported value for MgAl–OH LDHs (Tongamp et al. 2007). The characteristic diffraction peaks of LDHs were found in both LDHs/BL and LDHs/BL-T samples, and no change in the layer spacing was found.
Figure 2
XRD patterns of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.
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XRD patterns of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.

Figure 2
XRD patterns of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.
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XRD patterns of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.

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SEM analysis

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Figure 3 presents the SEM and TEM of samples. The BL-T showed ribbons with dimensions of micron grade and micron-sized pores (Figure 3(a)), whereas the MgAl/BL-T sample showed a large number of LDHs flakes, resembling flowers, growing on it (Figure 3(b)). This may be because the main component of BL-T is cellulose. In addition to the ribbon structure, the BL also showed some folds and clumps (Figure 3(c)). The LDHs/BL-T sample showed some LDHs lamellar structures and large areas of folds (Figure 3(d)). In the absence of BL-T and BL, the LDHs exhibit a unique hexagonal morphology (Figure 3(e) and 3(f)) (Zhang et al. 2015a; Zhang & Hou 2018). The reasons for the above results were that, on the one hand, biomass contains cellulose fully as the LDH growth provides the growing point. On the other hand, hydroxyl and other functional groups carried by lignin contained in biomass were fully exposed, and then intercalated with LDHs, and some hydroxyl groups entered the LDHs laminates. The TEM images of (Figure 3(g)) LDHs/BL and (Figure 3(h)) LDHs/BL-T samples showed clearly the same results (LDHs, cellulose and lignin are marked with red arrows and dashed lines respectively). However, it is worth noting that some hydroxyl groups enter between LDH laminates without changing the layer spacing of LDH, which is consistent with the XRD results of the samples (Figure 2).
Figure 3
SEM image of samples: (a) BL-T; (b) LDHs/BL-T; (c) BL; (d) LDHs/BL; (e) LDHs. TEM image of samples: (f) LDHs; (g) LDHs/BL; (h) LDHs/BL-T.
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SEM image of samples: (a) BL-T; (b) LDHs/BL-T; (c) BL; (d) LDHs/BL; (e) LDHs. TEM image of samples: (f) LDHs; (g) LDHs/BL; (h) LDHs/BL-T.

Figure 3
SEM image of samples: (a) BL-T; (b) LDHs/BL-T; (c) BL; (d) LDHs/BL; (e) LDHs. TEM image of samples: (f) LDHs; (g) LDHs/BL; (h) LDHs/BL-T.
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SEM image of samples: (a) BL-T; (b) LDHs/BL-T; (c) BL; (d) LDHs/BL; (e) LDHs. TEM image of samples: (f) LDHs; (g) LDHs/BL; (h) LDHs/BL-T.

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FT-IR analysis

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Figure 4 presents the FT-IR spectra of samples. The strong absorption peak of the BL-T sample at 3,700 cm−1 is due to the abundance of hydroxyl groups on the cellulose surface. Whereas the weak absorption peak of LDHs at 3,700 cm−1 is caused by the vibration absorption of hydroxyl groups contained in Mg(OH)2 and Al(OH)3 in the raw material, which implied that the raw material is almost completely involved in the reaction. The wide absorption peak at 3,450 cm−1 was attributed to the −OH stretching vibrations, which was from the free water and the adsorbed water on the surface of samples. The absorption peaks at 2,880 cm−1 and 2,800 cm−1 in BL and BL-T disappeared in LDHs/BL samples, while the absorption peaks at 1,740 showed a blue shift, which may be due to the hydrogen bonding between the hydroxyl groups of LDH layer plate and the lignin contained in BL and the functional groups carried by hemicellulose (methoxyl and carbonyl group). It is worth noting that the C-O stretching vibration and C-C skeleton vibration existed at 1,022 cm−1 and 1,250 cm−1, and the positions of these peaks changed after the adsorption reaction. The shoulder peak at 1,620 cm−1 indicates the hydrogen bond of interlayer water and metal hydroxide. The adsorption peak at 1,370 cm−1 was the antisymmetric stretching mode of CO32− groups. The low frequency peak at 1,088 cm−1 was the symmetric stretching mode of CO32− groups. The carbonate in the sample came from carbon dioxide in the atmosphere. The low frequency peak in the 400–800 cm−1 may be attributed to the stretching mode of M-O and O-M-O.
Figure 4
FT-IR spectra of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.
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FT-IR spectra of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.

Figure 4
FT-IR spectra of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.
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FT-IR spectra of samples: (a) BL; (b) BL-T; (c) LDHs; (d) LDHs/BL; (e) LDHs/BL-T.

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BET analysis

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Figure 5 shows the N2 adsorption–desorption isotherms of BL, BL-T, NMgAl/BL, LDHs, LDHs/BL and LDHs/BL-T samples. The sample prepared under the same conditions without ball milling is abbreviated as NMgAl/BL. The surface area (As), average pore size (Dp) and pore volume (Vp) of the samples are listed in Table 1. The specific surface areas of LDHs and LDHs/BL are 47.9 and 106 m2 g−1, respectively. After modification by BL, the specific surface areas of LDHs more than doubled, which is advantageous for use as an adsorbent. When the p/p0 ratio is greater than ∼0.4, the LDHs/BL sample shows the type IV adsorption isotherm and the type H3 hysteresis curve, indicating that it is the fissure pore formed by the aggregation of flake particles (Hu et al. 2013). In addition, the average pore size (Dp) and pore volume (Vp) of samples were 8.58 nm and 0.103 cm3 g−1 of LDHs, 6.16 nm and 0.164 cm3 g−1of LDHs/BL, respectively. By comparison, it can be seen that the LDHs/BL sample had a smaller pore size and a larger pore volume, which is also conducive to adsorption. What is more, the NMgAl/BL sample had a smaller specific surface area and pore volume, which were not conducive to adsorption. These above results indicated that the BL and ball-milling operation played a very important role in the preparation of the LDHs/BL sample.
Table 1

Specific surface area, average pore size and pore volume of samples

SamplesAs (m2 g−1)Dp (nm)Vp (cm3 g−1)
NMgAl/BL 4.16 7.23 0.00753 
LDHs 47.9 8.59 0.103 
LDHs/BL 106 6.16 0.164 
BL 6.51 3.42 0.0082 
LDHs/BL-T 63.4 6.56 0.178 
BL-T 4.06 3.78 0.0078 
SamplesAs (m2 g−1)Dp (nm)Vp (cm3 g−1)
NMgAl/BL 4.16 7.23 0.00753 
LDHs 47.9 8.59 0.103 
LDHs/BL 106 6.16 0.164 
BL 6.51 3.42 0.0082 
LDHs/BL-T 63.4 6.56 0.178 
BL-T 4.06 3.78 0.0078 
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Figure 5
N2 adsorption–desorption isotherms of BL, BL-T, NMgAl/BL, LDHs, LDHs/BL and LDHs/BL-T samples.
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N2 adsorption–desorption isotherms of BL, BL-T, NMgAl/BL, LDHs, LDHs/BL and LDHs/BL-T samples.

Figure 5
N2 adsorption–desorption isotherms of BL, BL-T, NMgAl/BL, LDHs, LDHs/BL and LDHs/BL-T samples.
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N2 adsorption–desorption isotherms of BL, BL-T, NMgAl/BL, LDHs, LDHs/BL and LDHs/BL-T samples.

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The results on Cu (ΙΙ) adsorption

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The removal efficiencies (ER) of Cu(ΙΙ) by different adsorbents of BL, LDHs, LDHs/BL and LDHs/BL-T were investigated at C0 = 50 mg/L, Cs = 1.0 g/L, pH = 5.0 and 25 °C (Figure 6). The ER of LDHs/BL was 92%, which was the best among the three sorbents for Cu(II) removal. The ER of BL and LDHs were 32 and 68%, respectively. When the LDHs was modified with BL, the removal efficiency increased by 35.3%. Whereas the ER of LDHs/BL-T was 81%, which increased by 13.6% compared with LDHs. The reason for the former is probably mainly due to the role played by lignin and cellulose contained in BL. The reason for the latter may be that only cellulose plays a role in BL-T.
Figure 6
The ER of Cu(II) by different adsorbents of BL, LDHs, LDHs/BL and LDHs/BL-T. (C0 = 50 mg/L, Cs = 1.0 g/L, pH = 5.0 and 25 °C.).
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The ER of Cu(II) by different adsorbents of BL, LDHs, LDHs/BL and LDHs/BL-T. (C0 = 50 mg/L, Cs = 1.0 g/L, pH = 5.0 and 25 °C.).

Figure 6
The ER of Cu(II) by different adsorbents of BL, LDHs, LDHs/BL and LDHs/BL-T. (C0 = 50 mg/L, Cs = 1.0 g/L, pH = 5.0 and 25 °C.).
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The ER of Cu(II) by different adsorbents of BL, LDHs, LDHs/BL and LDHs/BL-T. (C0 = 50 mg/L, Cs = 1.0 g/L, pH = 5.0 and 25 °C.).

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The amount of adsorbent is an important factor affecting the adsorption effect (O'Connor & Connolly 1980; Voice & Weber 1985). It is of great theoretical and practical significance to study the effect of sorbent dosage on adsorption performance. Taking the LDHs/BL as an example, the sorption isotherms for Cu(II) at different Cs values were measured at pH = 5.0 and 25 °C, as shown in Figure 7. The adsorption results showed that the adsorption amount decreased with the increase in the amount of adsorbent. The Cs-dependence of the sorption isotherms is a well known solid (or sorbent concentration) effect (Cs-effect) (Zhao & Hou 2012; Zhao et al. 2013).
Figure 7
Sorption isotherms of Cu(II) on LDHs/BL at different sorbent dosages. 25 °C, pH 5.0. The dots represent experimental data, the solid lines represent Langmuir model fits, and the dashed lines represent Freundlich model fits.
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Sorption isotherms of Cu(II) on LDHs/BL at different sorbent dosages. 25 °C, pH 5.0. The dots represent experimental data, the solid lines represent Langmuir model fits, and the dashed lines represent Freundlich model fits.

Figure 7
Sorption isotherms of Cu(II) on LDHs/BL at different sorbent dosages. 25 °C, pH 5.0. The dots represent experimental data, the solid lines represent Langmuir model fits, and the dashed lines represent Freundlich model fits.
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Sorption isotherms of Cu(II) on LDHs/BL at different sorbent dosages. 25 °C, pH 5.0. The dots represent experimental data, the solid lines represent Langmuir model fits, and the dashed lines represent Freundlich model fits.

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Langmuir and Freundlich isotherms were used to fit the adsorption data of different Cs values with nonlinear regression. Langmuir and Freundlich models were as follows:
(3)
(4)

All of the nonlinear model plots coincided with the experimental data (Figure 7). Table 2 lists the best-fit values of the model parameters, Γm, KL, KF, nF, and R2. Higher R2 values for the various model-fitting plots indicated that the Freundlich models adequately described the sorption isotherms for any given Cs value better than the Langmuir model. Furthermore, it can be seen from Table 2 that the Γm values decreased with increasing Cs. Similar results were reported in our previous studies (Zhang et al. 2015b, 2019, 2021). Compared with other adsorbents (Table 3) (Yu et al. 2000; Abia et al. 2003; Annadurai et al. 2003; Abdić et al. 2018; Liu et al. 2020a, 2020b; Tomczyk et al. 2020; Zhuang et al. 2020), the adsorption capacity of LDHs/BL prepared in this study was relatively ideal, which is expected to be applied to the treatment of heavy metal wastewater.

Table 2

Nonlinear-fit data of model parameters for Cu(II) sorption on LDHs/BL at different Cs

Cs (g/L)Langmuir isotherm
Freundlich isotherm
Γm
mg/g		KL
L/mg		R2KF
LnF mg1−nF/g		nFR2
1.00 58.9 0.0173 0.953 10.06 0.267 0.979 
2.00 37.8 0.0297 0.938 9.259 0.216 0.998 
6.00 27.6 0.0184 0.722 8.488 0.158 0.959 
Cs (g/L)Langmuir isotherm
Freundlich isotherm
Γm
mg/g		KL
L/mg		R2KF
LnF mg1−nF/g		nFR2
1.00 58.9 0.0173 0.953 10.06 0.267 0.979 
2.00 37.8 0.0297 0.938 9.259 0.216 0.998 
6.00 27.6 0.0184 0.722 8.488 0.158 0.959 
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Table 3

Performance comparison for Cu(II) removal by various adsorbents

Various types of adsorbentspHMonolayer saturation capacity of the adsorbents from Langmuir model Γm (mg/g)References
Modified pineapple bran cellulose by potassium permanganate 7 29.0 Zhuang et al. (2020)  
Mixture of sunflower husks and rapeseed pomace biochar 5 17.1 Tomczyk et al. (2020)  
Sunflower husks biochar 5 13.2 Tomczyk et al. (2020)  
Wood waste biochar 5 4.6 Tomczyk et al. (2020)  
Mn modified biochar 5–6 187.7 Liu et al. (2020a)  
Si modified biochar 5–6 152.6 Liu et al. (2020a)  
Tangerine peel 4 2.4 Abdić et al. (2018)  
Orange peel 5 3.7 Annadurai et al. (2003)  
Banana peel 5 4.8 Annadurai et al. (2003)  
Cassava waste 4–5 56.8 Abia et al. (2003)  
Sawdust 7 1.8 Yu et al. (2000) 
Straw-based adsorbent 3 17.8 Liu et al. (2020b)  
LDHs/BL 5 59.9 This study 
Various types of adsorbentspHMonolayer saturation capacity of the adsorbents from Langmuir model Γm (mg/g)References
Modified pineapple bran cellulose by potassium permanganate 7 29.0 Zhuang et al. (2020)  
Mixture of sunflower husks and rapeseed pomace biochar 5 17.1 Tomczyk et al. (2020)  
Sunflower husks biochar 5 13.2 Tomczyk et al. (2020)  
Wood waste biochar 5 4.6 Tomczyk et al. (2020)  
Mn modified biochar 5–6 187.7 Liu et al. (2020a)  
Si modified biochar 5–6 152.6 Liu et al. (2020a)  
Tangerine peel 4 2.4 Abdić et al. (2018)  
Orange peel 5 3.7 Annadurai et al. (2003)  
Banana peel 5 4.8 Annadurai et al. (2003)  
Cassava waste 4–5 56.8 Abia et al. (2003)  
Sawdust 7 1.8 Yu et al. (2000) 
Straw-based adsorbent 3 17.8 Liu et al. (2020b)  
LDHs/BL 5 59.9 This study 
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Possible mechanism of Cu(ΙΙ) adsorption on LDHs/BL composites

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The solubility product constant of copper hydroxide is 2.2 × 10−20. When copper ion concentration is 1.0 mol/L the concentrated copper hydroxide begins to precipitate. The product of copper ion concentration and hydroxide concentration squared should be greater than 2.2 × 10−20. The minimum pH of copper hydroxide precipitated by copper ions is related to the concentration of copper ions. When the concentrations of copper ions are 50 and 500 mg/L, the pH of the copper hydroxide precipitate is 5.73 and 6.23, respectively. In this study, in order to eliminate copper hydroxide precipitation caused by free OH- in the solution, the pH of the adsorption experiment was controlled as 5. To verify the influence of pH during the adsorption process, the effects of pH on the uptake capabilities of BL, LDHs and LDHs/BL for Cu(II) were investigated at C0 = 50 mg/L, Cs = 1.0 g/L and 25 °C (Figure 8). With increasing pH from 4–9, the ER for Cu(II) was increased. When the pH was 4, the adsorption capacity is lowest, which may be due to the partial dissolution of the adsorbent in the weak acid environment.
Figure 8
Effects of pH on the removal efficiency of BL, LDHs and LDHs/BL for Cu(II). C0 = 50 mg/L, Cs = 1.0 g/L and 25 °C.
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Effects of pH on the removal efficiency of BL, LDHs and LDHs/BL for Cu(II). C0 = 50 mg/L, Cs = 1.0 g/L and 25 °C.

Figure 8
Effects of pH on the removal efficiency of BL, LDHs and LDHs/BL for Cu(II). C0 = 50 mg/L, Cs = 1.0 g/L and 25 °C.
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Effects of pH on the removal efficiency of BL, LDHs and LDHs/BL for Cu(II). C0 = 50 mg/L, Cs = 1.0 g/L and 25 °C.

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Based on the above analyses, a possible mechanism of Cu(II) adsorption (Yu et al. 2000; Abia et al. 2003; Annadurai et al. 2003; Abdić et al. 2018; Liu et al. 2020a, 2020b; Tomczyk et al. 2020; Zhuang et al. 2020) on LDHs/BL composites is proposed, as illustrated in Figure 9. The process may be as follows: as LDHs/BL are added into the solution, the OH− in the interlayer of LDHs slightly and on the BL at a greater amount combined with the surrounding Cu(II) to form a Cu(OH)2 precipitate deposited on the surface of composites, promoting the removal rate of Cu(II). Furthermore, Cu(II) was also partly fixed through isomorphic substitution of Mg(II) in the layers of the LDHs. The whole reactions can be described by Equations (5) and (6):
(5)
(6)
Figure 9
Schematic diagram of possible mechanism of Cu(II) adsorption on LDHs/BL composites.
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Schematic diagram of possible mechanism of Cu(II) adsorption on LDHs/BL composites.

Figure 9
Schematic diagram of possible mechanism of Cu(II) adsorption on LDHs/BL composites.
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Schematic diagram of possible mechanism of Cu(II) adsorption on LDHs/BL composites.

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This study demonstrates for the first time that LDHs/BL composites are an efficient absorbent for the removal of Cu(II) at different concentrations. The new findings include: (1) Under the same adsorption conditions, the removal efficiency of the former was 92%, whereas that of the latter was 68%. (2) There is an obvious effect of adsorbent concentration in the process of adsorption. (3) Possible adsorption mechanisms included metal precipitation and isomorphic replacement. (4) This study provides a new idea for the utilization of agricultural and forestry wastes. In summary, LDHs/BL composites are expected to be used as a new kind of environmental adsorbent for wastewater treatment.

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This work is supported financially by the Science and Technology Program for Colleges and Universities of Shandong Province (No. J18KA104).

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All relevant data are included in the paper or its Supplementary Information.

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The authors declare there is no conflict.

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