Characterization of amphoteric bentonite-loaded magnetic biochar and its adsorption properties for Cu²⁺ and tetracycline

Materials

Dodecyl dimethyl betaine was used as the amphoteric modifier, which abbreviated as BS-12 (AR; produced by Tianjin Xingguang Reagent Factory, Tianjin City, China). The B used was sodium B, which purchased from Henan Xinyang Bentonite Produce Company and purified by washing method before use (Shah et al., 2018). The basic physicochemical properties of the purified B are: cation exchange capacity (CEC) is 1,000.33 mmol/kg, pH is 9.59, and total organic carbon (TOC) is 4.98 g/kg. FeCl₃·6H₂O, FeSO₄·7H₂O and NaOH were all purchased from Chengdu Kelong Chemical Reagent Factory, Chengdu City, Sichuan Province, China. TC was purchased from Sigma (St. Louis, MO, USA) and had a purity of 99.9%. Cu²⁺ solution was used as pollutant, and the solution was prepared by using CuSO₄·5H₂O (analytical reagent) purchased from Chengdu Kelon Chemical Reagent Factory. Figures 1A and 1B show the structural formula of BS-12 and TC, respectively.

BS-B was prepared by wet method (Li et al., 2016b). BS-12 solution was added into 10 g of purified B (the mass ratio between solution and soil was 10:1), then reacted for 6 h at 40 °C; Centrifuged at 4,800 r/min for 10 min, washed 3 times with deionized water (dH₂O), dried, grinded and sieved through 60-mesh nylon sieve to obtain BS-B. The amount use of BS-12 modifier was defined by Eq. (1):

(1) $$W=m\times \mathrm{C}\mathrm{E}\mathrm{C}\times \mathrm{M}\times {10}^{-6}\times R_{\mathrm{B}\mathrm{S}}/C_{\mathrm{B}\mathrm{S}}$$where W_BS stands for the mass (g) of BS-12; m stands for the mass (g) of bentonite; CEC stands for the cation exchange capacity of the bentonite (mmol/kg); M_BS stands for the relative molar mass of BS-12 (g/mol); R_BS stands for the modification ratio (50% or 100%) of BS-12; C_BS stands for the content (mass fraction) of BS-12.

A. philoxeroides was washed with dH₂O, dried to constant weight under 60 °C, grinded and sieved through a 200-mesh nylon sieve, and fired for 8 h under 400 °C by oxygen-limiting high-temperature pyrolysis to obtain BC. Co-precipitation method was used to prepare MBC (He et al., 2019). In this method, 20.00 g BC was dispersed in 2.0 L dH₂O and stirred for 30 min. Under strictly anaerobic conditions, 0.4 M FeCl₃·6H₂O and 0.2 M FeSO₄·7H₂O were successively added to 60 °C water, fully stirred for 2 h, and heated to 75 °C. The pH was adjusted to 10 with 5 mol/L NaOH solution and the MBC was separated by magnets after continuous stirring for 1 h and natural cooling. MBC was obtained after washing several times with dH₂O. MBC was dried at 60 °C and then passed through a 60-mesh sieve. A wet process was used to prepare BS-B/BC and BS-B/MBC. In this process, 100 g BC or MBC was slowly added to 1.0 L dH₂O, and BS-B was added again. After stirring at 40 °C for 3 h, the samples were separated, washed by dH₂O thrice and dried at 60 °C. Then, BS-B/BC and BS-B/MBC were dried at 60 °C for 12 h and passed through a 60-mesh sieve.

Experimental design

BS-B samples with BS-12 proportions of 0%, 50%, and 100% were prepared and named as B (bentonite only), 50BS-B, and 100BS-B, respectively. BC and MBC were used as the control (CK). BC, B/BC, 50BS-B/BC, 100BS-B/BC, MBC, B/MBC, 50BS-B/MBC, and 100BS-B/MBC (8 samples in total) were analyzed for the pH, CEC, and Brunaer–Emmett–Telle specific surface area (S_BET), and characterized by scanning electron microscopy (SEM), thermogravimetry (TG), Fourier transform infrared (FT-IR) spectroscopy, and vibration sample magnetometry (VSM).

The pre-experiment of Cu²⁺ adsorption showed that the adsorption isotherm began to turn at 300–400 mg/L, so the Cu²⁺ concentration in isothermal adsorption experiment was set to 0, 20, 50, 100, 150, 200, 300, 400, and 500 mg/L for a total of nine concentration gradients. The concentration of tetracycline was set to 0, 2, 5, 10, 20, 40, 60, 80, and 100 mg/L nine concentration gradients by their pre-experiment. Three replicates were set for each treatment.

Experimental methods

pH value was determined using a HQ411D table pH meter (Hash Company, Vancouver, WA, USA; refer to the test method of soil sample, solid–liquid ratio was 1:5). CEC was determined by sodium acetate–ammonium acetate method. S_BET was analyzed by by multi-point BET method using a V-Sorb2800P analyzer. SEM was performed using a Japanese Hitachi S-4800 scanning electron microscope. TG was performed using a STA449F3 synchronous thermal analyzer (NETZSCH) under the following conditions: temperature range 25–900 °C; sample quality, 10–15 mg; heating rate, 10 °C/min; N₂ atmosphere. FT-IR analysis was performed on a Nicolet 5DX type Fourier transform infrared spectrometer, and the 2D FTIR spectra were analyzed by 2DShige software. Magnetic curves were determined by Lakeshore 665 VSM method.

Nine samples (0.5000 g) of each composite material were separately packed in 50 mL plastic centrifuge tubes, added with 20 mL of Cu²⁺ (TC) solutions under different concentration gradients, shaken at room temperature for 12 h (200 r/min), and centrifuged at 4,800 r/min for 15 min. The supernatant was collected to determine the Cu²⁺ (TC) concentration, and the actual adsorption amount of the test material was calculated by subtraction (Zhang et al., 2020; Zou et al., 2020). The experiments were carried out at 20 and 40 °C respectively for calculating the thermodynamic parameters of Cu²⁺ and TC adsorptions. The Cu²⁺ content was determined via flame atomic absorption spectrophotometry, and background absorption was corrected through the Zeeman effect. The TC concentration was determined by SP-2100 UV-VIS spectrophotometer at 365 nm. The above measurements were all inserted into standard solutions for analysis quality control.

Data processing

The equilibrium adsorption amount of Cu²⁺ and TC was calculated using Eq. (2):

(2) $$q={\displaystyle \frac{V\times (C_0-C_{\mathrm{e}})}{W_0}}$$where C₀ (mmol/L) and C_e (mmol/L) are the initial and equilibrium concentrations of Cu²⁺ (or TC) in the solution, respectively. V (mL) is the volume of Cu²⁺ (or TC) solution added. W₀ (g) is the weight of the tested material. q (mmol/kg) is the equilibrium adsorption amount of Cu²⁺ (or TC) on the tested material.

The adsorption rate AR (%) of Cu²⁺ and TC was calculated using Eq. (3):

(3) $$AR={\displaystyle \frac{100\times (C_0-C_{\mathrm{e}})}{C_0}}$$

The Langmuir isotherm was selected on the basis of the adsorption isotherm trend and the isothermal equation Eq. (4) is as follows (Zhang et al., 2020):

(4) $$q={\displaystyle \frac{q_{\mathrm{m}}b{C}_{\mathrm{e}}}{1+b{C}_{\mathrm{e}}}}$$where q_m indicates the maximum adsorption amount of Cu²⁺ (or TC) on the different materials, mmol/kg; b represents the apparent equilibrium constant of the Cu²⁺ (or TC) adsorption, which can be used to measure the affinity of adsorption.

Parameter b in the Langmuir model is equivalent to the apparent adsorption constant of equilibrium constant, and the thermodynamic parameter calculated by b = K or K_a is called the apparent thermodynamic parameters; Eqs. (5)–(7) are as follows (Zhao et al., 2021):

(5) $$\mathrm{\Delta }G=-RT\ln K$$

(6) $$\mathrm{\Delta }H=R\left(\frac{T_1\cdot T_2}{T_2-T_1}\right)\cdot \ln \left(\frac{K_a,T_2}{K_a,T_1}\right)$$

(7) $$\mathrm{\Delta }S=\frac{\mathrm{\Delta }H-\mathrm{\Delta }G}{T}$$where ∆G is the standard free energy change (kJ/mol), R is a constant (8.3145 J/mol/K), T is the adsorption temperature (T₁ = 293.16 K, T₂ = 313.6 K), ∆H is the enthalpy of adsorption process (kJ/mol), and ∆S is the entropy change of adsorption process (J/mol/K).

CurveExpert 1.4 fitting software was used in isothermal fitting, and SigmaPlot 10.0 software was adopted to improve data plotting. SPSS 16.0 statistical analysis software was used to process the experimental data for variance and correlation analysis (Li et al., 2020). SigmaPlot 10.0 software was adopted to improve data plotting. The data were expressed as the means with standard deviation, and different letters indicate significant differences among various amendments (Lopez et al., 2012). Analysis of variance was performed to determine the effects of amendments, followed by Tukey’s honestly significant difference test. Differences of p < 0.05 were considered significant (Ana et al., 2013).

Basic physicochemical properties of the tested materials

Figure 2 shows the physicochemical characteristics of each test material. The pH and CEC of BC increased when B was loaded but decreased when BS-B was loaded, and the amplitude decreased with the increase in the BS-12 proportion of BS-B. S_BET decreased when BC was loaded with B and BS-B, and further decreased with the increase in the modification ratio of BS-12 on BS-B. B/BC had a slight increase in pH and a higher increase in CEC, which may be caused by the similar pH and larger CEC of B than those of BC. BS-12 on BS-B can neutralize the alkalinity of B, and the long carbon chains of BS-12 can cover the surface of B and thus reduce the CEC of BS-B. Therefore, when BC was loaded with BS-B, the pH and CEC of BS-B/BC materials decreased with the increase in the modification ratio of BS-12. Moreover, when BC was loaded with BS-B, the interlayer or surface pores of BC were covered by BS-B, which increased the average particle size of BS-B/BC and resulted in the decrease in S_BET. Compared with the unmagnetized ones, the magnetized materials had slightly reduced pH and CEC and remarkably increased S_BET because the increase in Fe₃O₄ particles increased the roughness of a material’s surface and then increased its surface area (He et al., 2019).

SEM images of the test materials

The SEM image of the surface morphology of each test material is shown in Fig. 3. BC had a smooth surface and a regular pore structure. When BC surface was loaded with B, the surface of B/BC became rough, a few number of B particles were attached to the surface of BC, and some pores were also filled with B particles. When BC was loaded with 100BS-B, the surface smoothness of 100BS-B/BC increased, because the hydrophobic long carbon chain of BS-12 could form a layer of organic phase on the surface of 100BS-B, which could decreased the surface roughness (Zhang et al., 2020). When each composite material was loaded with Fe₃O₄, its surface became rough compared with the unmagnetized ones. A large number of Fe₃O₄ particles were attached to the surface of the material, and some pores were also filled with Fe₃O₄ particles. The results show that Fe₃O₄ had a large effect on the surface morphology of the magnetic material, and the iron oxide particles were dispersed on the carbon matrix, which increased the S_BET (Xin et al., 2017).

TG analysis of the tested materials

The TG curves of the different materials are shown in Fig. 4. The test materials had different degrees of weight loss after high-temperature pyrolysis (900 °C), and the weight loss rates of the materials were greater after magnetization. The weight loss of the test materials is in the order: BC > 100BS-B/BC > 50BS-B/BC > B/BC, 100BS-B/MBC > 50BS-B/MBC > B/MBC > MBC. As the modification ratio of BS-12 increased, the weight loss rate of BS-B/BC and BS-B/MBC materials gradually increased, mainly because the surface active agent BS-12 was decomposable organic matter (He et al., 2019). Change in the TG curve showed three stages at the temperatures of 0–250 °C, 250–600 °C, and 600–900 °C, which represent the material water loss, organic matter decomposition, and crystal layer collapse, respectively.

Table 1 shows the weight loss rate and differential thermogravimetric (DTG, mass change rate with time, %/min) peak temperature of each material in the three stages of the TG curve. The DTG peak temperatures of the test materials were kept between 67 °C and 81 °C in the water loss stage (Stage 1). The water loss rates of the test materials are in the order: 100BS-B/BC (8.26%) > B/BC (8.07%) > 50BS-B/BC (7.51%) > BC (6.68%) > 100BS-B/MBC (5.02%) > 50BS-B/MBC (4.74%) > MBC (4.59%) > B/MBC (4.17%). In the organic carbon decomposition stage (Stage 2), the DTG peak temperatures of the test materials was kept between 400 °C and 530 °C. The carbon loss rate of BC was only 11.6%, whereas the carbon loss rates of the other test materials were in the range of 15.1–23.48%. Carbon loss rate increased with the increase in the modification ratio of BS-12. Almost no weight loss was observed in all the tested materials in the crystal layer collapse stage (Stage 3), and the DTG peak temperatures were in the range of 550–900 °C. This part of weight loss change is only related to the composition and structure of the material itself and has nothing to do with the organic modification.

FTIR and magnetic characteristics of the test materials

The 2D infrared spectra of the test materials are shown in Fig. 5A (synchronous correlation diagram) and Fig. 5B (asynchronous correlation diagram), The red and blue marks in the figures indicate positive and negative reactions, respectively. The test materials presented a strong characteristic absorption peak of Fe–O bond near 659–695 cm⁻¹, which indicates that Fe₃O₄ was modified to the surface of BC materials. The peak in the vicinity of 990 cm⁻¹ is the O–H surface modal vibration absorption on the carboxyl group, which is related to the carboxyl group and the amine group in the BS-12 molecule. This result indicates that BS-12 bound to MBC. A characteristic absorption peak containing the C=O functional group appeared at 1,600 cm⁻¹, and the deformation vibration characteristic absorption peak of the C–H bond appeared near 2,920 cm⁻¹. The absorption peak at 3,438 cm⁻¹ was produced by the stretching vibration of the –OH bond. These results confirm that BS-B and Fe₃O₄ were loaded on the BC surface.

The results of the hysteresis curve in Fig. 6 show that MBC and BS-B/MBC have good magnetic separation performance. The saturation magnetization of MBC was 43 emu/g. After BS-B loading, the magnetism of BS-B/MBC decreased slightly with the increase in BS-12 modification on BS-B. BS-B/MBC could still be separated by applying a magnetic field, which was of great importance for the recycling of BS-B/MBC materials.

Isothermal adsorption and thermodynamic parameters of Cu²⁺

The Cu²⁺ adsorption isotherms and adsorption rates of the test materials are shown in Fig. 7. The adsorption capacity of Cu²⁺ increased with the increase in equilibrium concentration, and the adsorption isotherm of the test materials to Cu²⁺ all conformed to the Langmuir model. The Cu²⁺ adsorption capacity of BC increased obviously after B loading. The Cu²⁺ adsorption capacity of BS-B/MBC increased with the increase in the modification ratio of BS-12 on BS-B. Compared with the unmagnetized ones, the magnetized materials had increased Cu²⁺ adsorption capacity.

Table 2 shows the fitting parameters and thermodynamic parameters of Cu²⁺ adsorption by Langmuir model. The correlation coefficient reached an extremely significant level (p < 0.01). The maximum adsorption capacity (q_m) of different materials changed from 135.07 mmol/kg to 322.00 mmol/kg. Compared with unmagnetized ones, the magnetized materials had higher q_m for Cu²⁺. 100BS-B/MBC had the best adsorption effect on Cu²⁺; its q_m reached 322.00 mmol/kg, and its average adsorption rate (AR_e) was 91.92%. The values of the adsorption constant (b) of magnetized materials for Cu²⁺ were smaller than those of unmagnetized ones, which indicates that magnetization reduced the adsorption affinity of BC materials to Cu²⁺. This outcome might be due to the fact that Fe₃O₄ particles block the pores of BC materials, which results in the reduction of exchangeable cation sites on the surface. The b values of BS-B/BC and BS-B/MBC were larger than those of BC and MBC, which indicates that the adsorption affinity to Cu²⁺ became greater after BS-B loading. B had a larger CEC and could have an ion exchange reaction with more Cu²⁺, and B loading could promote Cu²⁺ adsorption. The value of b was the largest when BC was loaded with B but decreased with the increasing in BS-12 modification rate on BS-B when BC was loaded with BS-B. This result indicates that BS-12 formed an organic coating on the surface of BC and reduced its adsorption affinity for Cu²⁺.

At 20 and 40 °C, the Gibbs free energy (ΔG) values for the Cu²⁺ adsorption of each test material were less than 0, which indicates that the adsorption was a spontaneous reaction. The adsorption enthalpy change (ΔH) of each tested material to Cu²⁺ was greater than 0, which indicates that the adsorption was an endothermic reaction, and increasing temperature was conducive to the occurrence of adsorption. The entropy change (∆S) of each test material was greater than 0, which indicates that the disorder of the system in the Cu²⁺ adsorption process was increased by the tested materials.

Isothermal adsorption and thermodynamic parameters of TC

Figure 8 shows the adsorption isotherm and adsorption rate of TC on the tested materials. The TC adsorption capacities of the materials increased with the increase in equilibrium concentration and were in accordance with the Langmuir model. Table 3 shows the Langmuir model fitting parameters for TC adsorption. The fitting of the TC adsorption isotherms of the test materials reached a very significant correlation level (p < 0.01). The q_m for TC adsorption changed from 118.60 to 602.83 mmol/kg, B/MBC had the best q_m for TC adsorption, and 50BS-B/MBC had the largest AR_e of 97.76%. Compared with the unmagnetized materials, the magnetized materials could reach the TC adsorption equilibrium quickly and had higher q_m values for TC adsorption. After B and BS-B loading, the b values of the materials for TC adsorption were larger than those of BC and MBC. At 20 and 40 °C, all test materials for TC adsorption had ΔG < 0, which indicates that the adsorption was a spontaneous reaction; ΔH > 0, which indicates that the adsorption was an endothermic reaction; and ∆S > 0, which indicates that the disorder of the system increased in the TC adsorption process.

Correlation between adsorption and physicochemical properties

The q_m and AR_e values of each test material for Cu²⁺ and TC were linearly fitted to the physicochemical properties of the material, and the fitting results are shown in Table 4. CEC had positive correlations with the q_m and AR_e for Cu²⁺ and TC adsorptions; pH and S_BET had positive correlations with the q_m and AR_e for TC adsorption but had negative correlations with the q_m and AR_e for Cu²⁺ adsorption; pH had a negative correlation with the AR_e for TC adsorption. Moderate correlations between q_m and pH, and between AR_e and S_BET were observed in Cu²⁺ adsorption (r > 0.5); and the other indexes maintained a low degree of correlation. q_m and S_BET in TC adsorption were moderately correlated, and the other indexes maintained a low degree of correlation. The results indicate that the pH and S_BET of the material have a greater influence on Cu²⁺ and TC adsorptions respectively, than CEC.

Adsorption difference of Cu²⁺ and TC on BS-B/MBC

Figure 9 shows the adsorption difference of Cu²⁺ and TC on BS-B/MBC. The S_BET, pore structure, and functional groups of BC determined its adsorption capacity for Cu²⁺ and TC, but the adsorption capacity of original BC was limited. Solid–liquid separation could be realized under the action of external magnetic field by magnetizing the material with the magnetic medium. Fe₃O₄ loading increased the S_BET and effective adsorption sites on the surface of BC. MBC and BS-B/MBC had good magnetic separation performances and could be separated by applying a magnetic field.

B had strong adsorption capacity and ion exchange capacity, and amphoteric modification could remarkably improve its adsorption capacity for heavy metals and organic pollutants (Li et al., 2016c). Therefore, BS-B had high adsorption capacity for Cu²⁺ and TC. When MBC was loaded with BS-B, BS-B/MBC had a strong affinity for heavy metal ions and could effectively fix Cu²⁺, and which increases the adsorption capacity for Cu²⁺. The adsorption mechanism of Cu²⁺ comes from ion exchange, complexation, and electrostatic attraction by BC and from complexation and electrostatic attraction by BS-B (Zhang et al., 2020). TC has multiple ionizable functional groups, contains two groups of positive and negative charges, and has hydrophilicity (Khanday & Hameed, 2018). When the pH of the solution changed, the adsorption behavior of BS-B/MBC became more complicated. The relatively high surface area and large pore size of BS-B/MBC provided more active sites for TC molecules (Jang et al., 2018). The rapid adsorption stage dominated the adsorption process of TC. The adsorption mechanism of TC comes from ion exchange and hydrophobic combination by BC and from complexation and hydrophobic combination by BS-B (Zou et al., 2020). Moreover, the FTIR spectra of 100BS-B/MBC before and after Cu²⁺ and TC adsorptions were compared (Fig. 10). The results showed that the C=O and C–H bonds on the surface of the material shifted after Cu²⁺ adsorption, which indicates that C=O and C–H on the surface of 100BS-B/MBC were involved in the Cu²⁺ and TC adsorption processes. The appearance of O–H bonds on 100BS-B/MBC after TC adsorption showed that hydroxyl group participates in the TC adsorption process. Additionally, the movement of the peak to a higher wave number means that the energy required for vibration is lower, which indicates that the group is more stable.