Novel clivia biochar adsorbing daptomycin (DAP) was prepared by microwave digestion–anaerobic carbonization in this work. Fe/Ag submicron particles were introduced to the biochar surface based on the reducibility of biochar to enhance its adsorption capacity. Characterization confirmed that modified biochar (AF-biochar) had a higher particle size (126 μm), larger specific surface area (521.692 m2 g−1), richer pore structure, and higher thermal stability. The effects of the main variables (e.g., the solution pH, contact time, initial DAP concentration, and temperature) were investigated during adsorption. The results showed that AF-biochar could reach the adsorption equilibrium at pH 4.8 for 85 min. Besides, the adsorption capacity was 48.25 mg g−1, and the adsorption efficiency was 96.50% when the concentration of DAP was 25 mg L−1. The pseudo-second-order kinetics (R 2 = 0.9997), Langmuir equation (R 2 = 0.9999), and thermodynamics (R 2 = 0.9631) of AF-biochar fit well, indicating that the main adsorption process of AF-biochar was spontaneous, exothermic, and monolayer. Their adsorption was analyzed by physical and chemical adsorption. The main adsorption mechanisms included the electron donor–acceptor interaction, electrostatic force interaction, Lewis acid–base interaction, and H-bond interaction.
Over the past decade, pharmaceutical compounds have received increasing attention for their potential environmental effects . Antibiotics are widely used in preventing and treating bacterial infections of humans, animals, and plants . However, some antibiotics cannot be completely metabolized and excreted out of the body, thus becoming one of the sources of antibiotics in the aquatic environment . With the widespread use of antibiotics, one can detect antibiotics in surface water, groundwater, hospital wastewater, and even in drinking water . Therefore, antibiotics are considered a new worldwide water-borne pollutant , and developing an effective strategy for removing antibiotics in water is essential .
Daptomycin (DAP) is a new cyclic lipopeptide antibiotic, which is acidic, highly water-soluble, and active against Gram-positive organisms [7,8,9]. It is extracted from the fermentation of streptomyces used for treating complex skin and skin structure infections as well as right-heart infective endocarditis caused by Staphylococcus aureus . Although the concentration of DAP in aquatic environments is low, its continued use poses a potential threat to aquatic and terrestrial organisms .
Recently, various techniques and methods for removing antibiotics based on physical, chemical, and biological principles have been reported , including adsorption, nanofiltration membrane, biodegradation, reverse osmosis, and catalytic degradation [13,14,15]. However, compared to the existing treatment technologies, using biochar adsorption is more practical and environmentally friendly . Biochar is a carbonaceous biomass-derived material, namely negative carbon material [17,18]. It can be used to adsorb some hydrophilic organic compounds and inorganic pollutants because of its aromatics and hydrophobicity . Biochar can be produced by the thermal degradation of wood wastes, crop residues, or animal wastes, such as pine sawdust, corn stalks, and poultry wastes . The use of biochar as a low-cost adsorbent is well documented in ref. [11,16,17,24]
Anshan clivia is a specialty of Anshan, Liaoning Province, China. In 2016, more than 50,000 people planted Anshan clivia, and an industrial base of clivia covering an area of more than 120 acres has been applied. Anshan clivia is an excellent new variety of clivia, successfully bred by crossing Japan clivia as the female parent and clivia “round head short leaf monk” in Anshan as the male parent. It has an elegant and solemn plant type, with wide and thick leaves and beautiful flowers. There are 40–50 μm onion-shaped stomas on the far-axis leaves surface of Anshan clivia, and the developed internal structure and excellent specific surface area have become one of the important reasons as experimental objects. Clivia leaf is a cheap and easy-to-obtain biological residue, but there are few studies on the biochar of clivia leaves, so it is of great significance to generate low-cost biochar from the biological residue by thermal degradation.
The adsorption capacity of most carbon-based materials mainly depends on the specific surface area, and the materials have high affinity and high selectivity to pollutants [4,12]. Iron-based magnetic nanocomposites are widely used in environments, energy, industry, and medicine. Fatimah et al. reviewed various methods for synthesizing iron oxide nanocomposites. The magnetic nanocomposites synthesized by microwave digestion have good electrochemical properties and cyclic stability . Moreover, in their subsequent research on magnetic nanocomposites, magnetic silicon nanocomposites have been well applied in catalysts, photocatalyst drug delivery, and sensors . It is also believed that magnetic nanomaterials have the prospect of sustainable development in easy separation, recyclability, chemical stability combined with adsorption, catalysts, and photocatalysts . An appropriate amount of metals (such as Au, Ag, Pb, Cu, and Al) on the catalyst surface can improve their catalytic activities . The Ag/ZnO/C photocatalyst was synthesized by Xue et al. using calcination and photodeposition. Ag has the visible light response ability of Ag–ZnO composites, which can separate and transfer photoelectrons and increase the reactive sites. The photocatalyst has an excellent photocatalytic activity for degrading tetracycline hydrochloride .
Previously, our research group used two magnetic ultra-fine wood-based biochars for the adsorption and magnetic separation of DAP . On this basis, AgNO3 and Fe(NO3)3 were loaded onto clivia leaves by microwave digestions to obtain magnetically separable biochar. Then unmodified biochar (Biochar) was used as blank control to analyze the adsorption mechanism of modified biochar (AF-biochar), with element composition, structure, surface morphology, functional groups, and magnetic properties of characterized biochar. The pH, contact time, initial DAP concentrations, and temperatures of solutions were experimentally studied. Moreover, the adsorption mechanism was described by the adsorption isotherm, kinetic model, and thermodynamics. The retrievability of biochar was studied by five repeated experiments.
2 Materials and methods
Anshan clivia leaves were purchased from the flower market (Anshan, China). They were repeatedly rinsed with distilled water to remove impurities and then dried with blotting papers. The blades were cut into pieces with scissors and then placed on the filter paper. They were then placed in a dryer at 70°C for 240 min. Samples were ground using a lapping machine mill for 3 min. Finally, the obtained particles were sieved.
DAP (purity > 98%) was purchased from Guangzhou Kafen Biotech Co., Ltd (Guangzhou, China). Table S1 (Supplementary Information) lists the chemical structure and basic physicochemical properties of DAP. All the other reagents were above the analytical grade.
2.2 Synthesis of raw materials
Magnetic clivia biochar (AF-biochar) was prepared by the microwave digestion–anaerobic carbonization method. Then 0.5 g clivia leaf powders, 2 mL of 0.5 M Fe(NO3)3 solution, 2 mL of 0.5 M AgNO3 solution, and 6 mL of deionized water were added into the digestion tube. The pH was adjusted between 10 and 11 with NaOH solution. They were digested at 150, 180, and 210°C for 10, 10, and 15 min in a microwave digestion instrument, respectively, and then carbonized at 850°C for 240 min in the carbonization furnace in the N2 environment. Under the same conditions, clivia biochar was prepared by anaerobic carbonization with Anshan clivia as a blank control group.
2.3 Characterization of the synthesized adsorbents
Main substances and functional groups on the biochar surface were detected by X-ray diffraction (XRD) (Bruker Discover D8, Germany) and Fourier transform infrared (FTIR) spectroscopy (FT/IR-410, JASCO, Japan), respectively. A laser particle size analyzer (BT-9300S, Battersize, China) was used to measure the particle-size distribution of the microparticles. Measurement was repeated to determine the mean value. The adsorption–desorption of nitrogen on the biochar surface was measured using a surface area and porosity analyzer (Micromeritics Tristar II 3020, USA) at 77 K. Also, the specific surface area and pore-size distribution of sediments were estimated by Brunner-Emmet-Teller (BET) and Barret-Joyner-Halenda. Elements C, H, S, and N in biochar were determined using an elemental analyzer (Elemental model Vario MICRO Cube, Germany). The room-temperature Raman spectra were obtained using a confocal Raman microscope (Renishaw RM2000, UK) with a laser wavelength of 785 nm.
The scanning electron microscope (SEM) (SU8020, Japan) was used for the morphological survey of biochar and energy dispersive X-ray (EDX) (Zeiss model Sigma 500, Germany). The contact angle of biochar was measured by a contact angle meter (Dataphysics OCA50, Germany) to test the hydrophilicity of biochar before and after modification. Ash contents were measured by heating the biochar samples at 800°C for 240 min in the muffle furnace. Thermal gravimetric analysis (TG-DTA) of samples was carried out using a thermogravimetric analyzer (Perkin Elmer model Diamond 6300, USA) at a heating rate of 10°C min−1 from the room temperature to 850°C in the N2 atmosphere. Inductively coupled plasma optical emission spectrometry (Jena Model PQ9000, Germany) was used to qualitatively and quantitatively analyze Fe and Ag in the samples. The point of zero charge (PZC) was measured with potentiometric mass titration using an automatic titrator (Rex Electric Chemical model PHS-3C, China). The vibrating sample magnetometer (VSM) (model WSM-01; Changchun Great Wall Teaching Instrument Co., Ltd, China) was used to measure the magnetic properties of the samples. DAP concentration was measured using an ultraviolet spectrophotometer (model UV-1700SPC; Shanghai Meishan Instrument Co., Ltd, China). All drawings were completed using Origin2019 software.
2.4 Analysis of DAP
Prepare a 25 mg L−1 solution of DAP in a brown conical flask with a stopper. Add 25 mg adsorbent to the aqueous solution, and then put it into the thermostatic water-bath oscillator at 25°C in a dark place for 240 min. Filter with a 0.45 μm membrane before measurement. The concentration of DAP in the filtrate was analyzed by an ultraviolet spectrophotometer. The maximum wavelength used in the detection was 221 nm. Based on these results, the adsorption capacity of the adsorbent to DAP was calculated.
where q e (mg g−1) is the adsorption amount of DAP adsorbed at equilibrium; q t (mg g−1) the adsorption amount of DAP adsorbed at time t; C 0 (mg L−1) and C 1 (mg L−1) are the initial and equilibrium concentrations of DAP; C t (mg L−1) the concentration of DAP at time t; V (L) the volume of the solution; and M (g) is the mass of the adsorbent.
2.5 Recyclability tests of adsorbent
Prepare 14 groups of 50 mL DAP solutions with 25 mg L−1 concentration. Then 25 mg AF-biochar was added to seven groups, and the remaining seven groups were added with 25 mg Biochar. Adsorption experiments were carried out under the same conditions. When the concentration of the solution reached equilibrium, a small amount of the solution was taken to measure its concentration. The modified biochar was sucked out of the remaining solution by magnets, and unmodified biochar was filtered out by the vacuum pump. Then rinsed and filtered four times, and dried at 50°C for 300 min. After weighing the adsorbents, 25 mg was added into the conical flask, to keep the solid–liquid ratio constant. Finally, repeat this process five times.
2.6 Data analysis
The adsorption of DAP was fitted on biochar by the pseudo-first-order (3), pseudo-second-order (4) and particle internal diffusion models (5). The theory behind each model is given in Text S1 (Supplementary Information).
Langmuir (6), Freundlich (7), Tempkin (8), and D–R (9) models were used to analyze the adsorption isotherm model. The detailed procedure for calculating adsorption is explained in Text S2.
The thermodynamic parameters of the standard Gibbs free energy (ΔG°), the enthalpy change (ΔH°), and the entropy change (ΔS°) (10–13) were calculated to evaluate the thermodynamic behaviors for adsorbing DAP (Text S3 for detailed calculation).
3 Results and discussion
3.1 Characterization of biochar adsorbents
Figure 1 and Table 1 show the preparation of sample particles. The mean particle sizes of AF-biochar and Biochar were observed to be 126.0 and 55.1 μm, 37.6 and 10.6 μm for D10 (particle diameter corresponding to 10%-cumulation [from 0% to 100%] undersize-particle-size distribution), and 279 and 226 μm for D90 (particle diameters corresponding to 90%-cumulation (from 0 to 100%), undersize-particle-size distribution), respectively. The average particle size of modified biochar decreases because the introduced AF material has a strong squeezing effect on biochar.
|Elemental composition (%, mass based)||C||17.490||55.100|
|BET surface area (m2 g−1)||521.692||232.196|
|Average pore diameter (nm)||2.488||2.834|
|Total pore volume (cm3 g−1)||0.325||0.165|
|The largest pore (nm)||210.4||185.9|
Table 1 shows the main element composition and content of AF-biochar and Biochar. After introducing Ag and Fe, C, H, N, and S in biochar decrease significantly, and the ash content increases significantly after combustion at 800°C. The ratios of H/C and O/C can reflect the aromaticity and polarity of biochar . After the contact-angle test (Figure S1), the water contact angle before and after modification was 153.6° and 132.7°, respectively, indicating that the biochar before and after modification was hydrophobic. However, the water contact angle of modified biochar is significantly reduced, indicating that the addition of elemental Fe and Ag promotes biochar toward hydrophilicity, and the increased O/C value can also demonstrate this view. The H/C value of AF-biochar was 0.0102 lower than that of unmodified biochar (0.0315), indicating that modified biochar contains fewer organic plant residues than modified biochar.
The magnetic field-related behavior is measured using VSM to determine the magnetism of AF-biochar. The hysteresis loop in Figure 2 shows typical ferromagnetic characteristics. Magnetization increases sharply with the increased external field, and then gradually approaches saturation. The low magnetization of biochar is because submicron Ag particles reduce the coercivity of AF-biochar , so the saturation magnetization of AF-biochar is relatively weak (9.6 Am2 kg−1). Besides, solid–liquid separation can be completed under the action of magnets.
TG-derivative thermogravimetry (TG-DTG) was used to study the thermal stability of biochar. TG image in Figure S2(b) shows that the weight losses of biochar before and after modification are 75.5 and 10.2%, respectively. AF materials can greatly improve the thermal stability of carbon materials. DTG image in Figure S2(a) shows that the weight losses mainly have four stages. The decomposition peaks at 40 and 58°C are likely attributed to the free water loss, and the weight losses at 467 and 500°C are due to the pyrolyzed carboxyl and carbonyl functional groups of hemicellulose, cellulose, and lignin. The decomposition peaks at 699 and 698°C are because part of the carbon skeleton is pyrolyzed, which causes weight loss. Weight loss at 886°C may be due to the pyrolysis of a more heat-resistant structure in biochar.
The specific surface area and pore volume of AF-biochar increased significantly through detecting the biochar structure (Table 1) compared with those before modification. It indicates that some Ag and Fe particles were pressed into biochar, resulting in more pores. However, in contrast, the average pore size of modified biochar decreased slightly due to the introduced submicron Ag and Fe particles blocking the pores of some biochars and preventing the entry of N2. N2 adsorption isotherm in Figure S3(a) conforms to the type-I adsorption isotherm, belonging to monolayer adsorption . Under relatively low pressure, the rapid adsorption and high adsorption of N2 may be caused by the adsorbing micropores. Under the relative medium pressure, N2 adsorption is relatively stable and forms hysteresis loops under medium and high pressures, indicating macropores in biochar . When P/P 0 approaches 1.0, the adsorption capacity of macropores increases rapidly.
XRD analysis of AF-biochar was carried out to study the crystallization behaviors of biochar. In Figure 3(a), there are mainly eight group peaks, and the peaks at 2θ = 38.11°, 44.30°, 64.43°, 77.39°, and 81.52° are considered for submicron element Ag (JCPDS No. 01-087-0717). Besides, the peaks at 2θ = 44.67°, 65.03°, and 82.34° are consistent with element Fe (JCPDS No. 00-006-0696). Therefore, AF-biochar materials are considered to be mainly loaded with Fe and Ag. Raman spectroscopy (Figure 3(b)) was performed to further study the structure of AF-biochar materials. In Raman spectroscopy, the D peak represents the defects of the carbon atomic lattice, and the G peak represents the in-plane stretching vibration of sp2 hybrid carbon atoms . The I D/I G (the ratios of integral regions of D and G bands) value of 2.86 indicates that the number of crystal defects in AF-biochar structure is large. The main reason may be that Ag and Fe nanoparticles are uniformly dispersed in the crystal structure of biochar, which indicates that AF materials are successfully embedded in biochar .
SEM was used to compare the microcosms before and after biochar modification (Figure 4). The pore distribution of biochar before modification (Figure 4(a) and (b)) is extremely uneven with porous layers. Irregular holes of different sizes and shapes can be observed, indicating that the unmodified biochar mechanism is in heterogeneous structures. After modification (Figure 4(c) and (d)), different spherical particles and pores are on the biochar surface. Compared with the unmodified biochar, the AF-biochar surface becomes rougher with the increased pores due to the successful grafting of biochar with small particles formed after adding AF materials.
EDX spectrometry was performed for AF-biochar (Figure 5). Particles of different sizes can be found, with diameters distributed between submicrons. The proportions of Ag, Fe, and O are 76.34, 20.80, and 2.86%, respectively. Therefore, these particles may be submicron Ag and Fe particles, and solids and liquids are separated under the action of magnets due to submicron Fe. In addition, C was not detected in the measured area, indicating that Ag and Fe were thickly distributed and covered the biochar surface.
3.2 Effect of solution pH on DAP adsorption
The pH values of the key factors affect the adsorption capacity. In this experiment, the pH value of the solution is mainly adjusted by different concentrations of HCl and NaOH solutions (all drops were less than three drops). Figure S4 shows that the general trend of AF-biochar is similar to that of Biochar, but AF-biochar has better adsorption. The adsorption capacity increases with the increased initial solution at pH 2–4.8; when the pH further increases, the adsorption capacity decreases sharply and tends to be stable at pH 10–12. For further analysis, see Section 3.7.
3.3 Adsorption kinetics
The relationship between adsorption capacity and time is measured at pH 4.8 and 25°C (Figure 6(a)). The results show that AF-biochar and Biochar reach the adsorption equilibrium in 85 min. The adsorption rate of AF-biochar is faster than that of most adsorbents in Table S3, compared with the equilibrium time reached by other adsorbents in references. The initial adsorption capacity of AF-biochar changes more obviously than that of Biochar with time. This excellent performance can be ascribed to the abundant availability of active sites and the high driving force in mass transfer. Pseudo-first-order (Figure 6(b)), pseudo-second-order (Figure 6(c)), and intramolecular diffusion (Figure 6(d)) models are used to fit experimental data (Table 2) to study the adsorption kinetics. The pseudo-second-order model (for AF-biochar, R 2 = 0.9997; for Biochar, R 2 = 0.9993) well fits the adsorption of DAP on two adsorbents. The maximum adsorption values are 47.96 (AF-biochar) and 40.03 mg g−1 (Biochar) estimated by pseudo-second-order kinetics, close to the experimental results of 48.25 mg g−1 (AF-biochar) and 40.35 mg g−1 (Biochar). It indicates that chemisorption dominates the experiment and controls the adsorption process. It can be seen from Figure 6(b) that the pseudo-first-order kinetic models of AF-biochar and Biochar have poor overall fitting effect and are not suitable for discussing their adsorption kinetics, so this study is no longer discussed. However, it can be seen from Figure 6(a) that the adsorption rate of the adsorbent was the highest in the first 15 min, indicating that AF-biochar had good hydrophobicity in the early stage of adsorption .
|Kinetic model||Parameters||Value of the parameters|
|Pseudo-first-order||k f (min−1)||8.5680||8.0720|
|Pseudo-second-order||(g mg−1 min−1)||0.0199||0.0235|
|Intraparticle diffusion||(mg g−1 min−1/2)||14.6709||9.6670|
|(mg g−1 min−1/2)||5.8840||4.0057|
|(mg g−1 min−1/2)||0.6210||0.6974|
The model of intraparticle diffusion can be fitted at three stages (Table 2). In the first stage, the straight line is relatively steep due to the large specific surface area and large pore distribution on the surface, which increases the absorption rate. The gradual flattening in the second and third stages may be because the pores in the macropores are occupied and reduced, and DAP is mainly adsorbed by the macropores, leading to the decreased absorption rate.
3.4 Adsorption isotherm
The relationship between concentration and adsorption capacity was determined at pH 4.8 and 25°C to study the adsorption mechanism of biochar (Figure S5). The lower the concentration of DAP, the better the adsorption capacity of biochar. The adsorption capacity gradually declines with the increased concentration when the adsorption of biochar reaches saturation.
When the system reaches equilibrium, the adsorption isotherm is generally used to describe the distribution of adsorbed molecules between liquids and solids. In this experiment, Langmuir (Figure S6(a)), Freundlich (Figure S6(b)), Tempkin (Figure S6(c)), and D–R (Figure S6(d)) were used for analysis, and Table 3 shows the main parameters. In terms of the fitting effect of all models, the fitting degree of the Langmuir model (both R 2 = 0.9999) is in complete agreement with experimental data and an optimal fitting effect can be obtained, indicating that the adsorption process was homogeneous and monolayer within the experimental concentration range .
|Isotherm model||Parameters||Value of the parameters|
|K L (L mg−1)||88.976||227.960|
In Table 3, the maximum adsorption capacity of AF-biochar is 81.53 mg g−1, and that of Biochar is 44.81 mg g−1. The former is 1.82 times better than that of the latter. The adsorption effect of modified biochar is far better than that of unmodified biochar because submicron Ag and Fe on the surface of AF-biochar can be complexed with DAP. Besides, submicron Ag can promote the degradation of DAP into small molecular organics, thus accelerating DAP into a smaller hole. Therefore, the adsorption capacity of DAP is accelerated. This is also one of the reasons for introducing submicron Fe and Ag particles in this experiment. Table S3 compares the maximum adsorption capacity or removal efficiency of macrolide antibiotics with other adsorbents in references. The maximum adsorption capacity of AF-biochar is not particularly prominent, but it can be considered an efficient adsorbent for removing macrolide antibiotics.
In the Freundlich model, the values of 1/n are 0.760 (AF-biochar) and 0.455 (Biochar), indicating that the physical adsorption effect is favorable . The K T of the Tempkin model is related to the thermodynamic properties of the adsorption process. The K T of two adsorbents are 29.728 (AF-biochar) and 11.807 (Biochar), respectively, which are greater than 1. It indicates that the adsorption of the two adsorbents to DAP is exothermic within the range of concentration studied .
E (average adsorption energy) in the D–R model can be used to distinguish whether the adsorption process is physical or chemical adsorption . The E values of two adsorbents in this experiment were 0.169 kJ mol−1 (AF-biochar) and 0.220 kJ mol−1(Biochar), both less than 8 kJ mol−1. It indicates that the adsorption process may be physical adsorption , consistent with Freundlich’s judgment.
3.5 Adsorption thermodynamics
Thermodynamic-energy adsorption reflected the internal changes in the adsorption process. Adsorption experiments were carried out between 285 and 320 K at a certain temperature. Figure S7 shows that the adsorption capacity varies with the increased temperature, indicating that the adsorption capacity depends on the temperature .
According to the reports, the free energy of chemical adsorption is at −400 to 80 kJ mol−1, and the free energy of physical adsorption is at −80 to 0 kJ mol−1. Table S2 shows the main thermodynamic parameters. The adsorption process of this experiment is mainly physical adsorption, indicating that adsorption occurs spontaneously. ΔH° > 0 of Biochar indicates that the adsorption of unmodified biochar has endothermic properties. ΔH° < 0 of AF-biochar indicates that the adsorption of modified biochar has exothermic properties because the introduced AF materials change the absorption and exothermic properties in the adsorption process. ΔS° of the adsorption process of two adsorbents is greater than 0, indicating that the adsorption process is entropy-increasing and irreversible. It is conducive to stable adsorption.
3.6 Regeneration and reusability
Recyclability of adsorbents is essential in reducing process costs and in environmental protection. Five cycles of recycling experiments were carried out under the same conditions (Figure S8 for the results). The adsorption capacity of AF-biochar was stronger than that of Biochar in the first three cycle experiments. However, the adsorbability of two adsorbents gradually decreased with more experiments. In the third-cycle experiment, the adsorption capacity of AF-biochar was 46.35% of the first adsorption, while that of unmodified biochar was 51.85%. In the fifth-cycle experiment, the adsorption capacity of AF-biochar was only 14.01% of the first adsorption, and that of biochar was 32.31%. The recoveries of AF-biochar and biochar were more than 94 and 70%, respectively. It is because parts of Ag and Fe on the biochar surface were gradually precipitated out, and parts of the micropores were occupied by the previously adsorbed DAP with the increased experiments. Therefore, it is easy to be separated from the solution by magnets, but with weak reusability.
3.7 Mechanism of adsorption
The morphology and properties of some functional groups of adsorbents may be significantly affected by variable pH. In general, the ionization, solubility, and hydrophilicity of organic chemicals increase with increased pH, leading to the reduced adsorption capacity of adsorbents . However, when the pH increases, the interaction between ionized amino groups and electron donor–acceptor (EDA) is strengthened, and the adsorption capacity increases .
The acidity coefficient (pK a) of DAP is 4.1 . When pH < 4.1, the solution is dominated by cations; when pH > 4.1, the solution is dominated by anions. That is, anions in the solution increase with increased pH, so they are dominant.
In Table 1, the points of zero charge (pHpzc) of biochar and AF-biochar are around 4.8. When pH < 4.8 for AF-biochar, the biochar surface has mainly positive charge; when pH > 4.8, the surface of biochar has mainly negative charge.
Figure 7 shows the strong electrostatic repulsion between DAP cations and the positive charges on the AF-biochar surface seriously reduce adsorption when pH < 4.1. The strong interaction between them gradually weakens with the increased pH, so the adsorption capacity gradually increases. DAP mainly exists in the form of DAP−, and a strong electrostatic attraction exists between DAP and biochar when 4.1 < pH < 4.8. Also, DAP contains carboxyl and amino groups. The interaction between the Lewis acid and adsorbability is enhanced when pK a < pH .
When pH is about 4.8, the adsorption capacity reaches the maximum of 47.71 mg g−1, consistent with the experimental results. Its hydrophilicity also increases with increasing pH, which is offset by the electrostatic force; therefore, the adsorption capacity gradually decreases.
When pH > 4.8, the biochar surface has negative charges, and there is electrostatic repulsion between biochar and DAP. Electrostatic repulsion and hydrophilicity increase with the increased pH, so the adsorption capacity gradually decreases.
The interaction between the π hydrogen bond and hydrogen bond affects non-ionization adsorption . Hydrogen bond receptors on biochar at a lower pH are more likely to bind to H in an aqueous solution, thus reducing the hydrogen bond between DAP and biochar. At a higher pH, the hydrogen bond donor on the biochar surface is ionized and interacts with H in an aqueous solution. Besides, the hydrogen bond donor on the DAP surface interacts with the hydrogen bond acceptor and π donor on the biochar surface, which weakens the influence of hydrophilicity and electrostatic repulsion on adsorption. Therefore, the adsorption capacity gradually flattens at a higher pH, consistent with the experimental results.
FTIR spectra before and after modification and adsorption are made to study the adsorption mechanism of biochar (Figure 8). Compared with the biochar before modification, two groups of new characteristic absorption peaks appear at 1,698 and 467 cm−1 after modification, caused by the C═O bond stretching of the aldehyde group and metal–oxygen stretching, respectively. Besides, the obvious displacement of some peaks may be caused by electronic attractions and transfers between metal ions and functional groups, indicating that submicron Fe and Ag are grafted with biochar.
Submicron Ag can be complexed with biochar to decompose DAP into small molecular organic matters. Then DAP easily enters the pores and can be easily absorbed, which accelerates the adsorption of biochar. Moreover, submicron Fe loaded on biochar can magnetize biochar, which separates solids and liquids and recycles biochar. This is the main reason for introducing Fe and Ag in this experiment.
The work studied FTIR spectra before and after adsorbing DAP by AF-biochar. Peaks at 3,479–3,316 cm−1 were due to the –NH stretching of amino groups or the O–H stretching of alcohol and carboxyl groups . Double peaks near 1,646 cm−1 were caused by the C═C double-bond stretching of olefin, and the absorption peak at 1,462 cm−1 was attributed to the skeleton vibration in the benzene ring (υC═C) . The peak at 1,398 cm−1 was mainly caused by the O–H inward bending of a primary alcohol, while that at 1,158 cm−1 was probably caused by the C–N bond stretching in the amino group.
The modified biochar contained the amino group, carbon–carbon double bond, benzene ring, hydroxyl group, aldehyde group, and other functional groups. However, the absorption band appeared near 3,181 cm−1 after adsorbing DAP, caused by the H-bond extension in the carboxyl group. The oxygen-containing functional group in AF-biochar formed charge with DAP−, confirming the connection between the carboxylic acid group in DAP and the oxygen-containing group in biochar.
The peak at 3,479 cm−1 disappeared and decreased from 3,406 to 3,316 cm−1, indicating that the N–H bond on biochar was grafted to the oxygen-containing group on DAP. The increased double-peak value at 1,646 cm−1 and the obvious shift were caused by the electron-induced effect, indicating that the π–π interaction adsorption mechanism existed between C═C and DAP . The disappearance of the C═O bond stretching at 1,698 cm−1 was due to the interaction with the Lewis acid on DAP.
AF-biochar was prepared by microwave digestion–anaerobic carbonization in a furnace. According to the pseudo-second-order kinetics and adsorption thermodynamics, both chemical and physical adsorption played an important role in adsorbing DAP. The average pore size of modified biochar was 126.0 μm, with a specific surface area of 521.692 m2 g−1 and a total void volume of 0.325 cm3 g−1.
The Langmuir function fits the adsorption isotherm data, showing that the adsorption process was homogeneous and monolayer with more adsorption sites. The analysis of the FTIR spectrum, pH value, and pHpzc showed that EDA interaction, electrostatic force interaction, Lewis acid–base interaction, H-bond interaction, and submicron silver could be complexed with biochar and decompose DAP into small molecular organics in the adsorption process. Thus, chemical adsorption had a good advantage in the adsorption process. In conclusion, AF-biochar has a more efficient adsorption capacity than unmodified biochar in the adsorption process of DAP and can be separated by the magnetic field.
We acknowledge Dr Pengcheng Li and Professor Zhao Li of the School of Mining Engineering, University of Science and Technology Liaoning for the support and data analysis of XRD, SEM, EDX, and FTIR instruments in this study.
Funding information: This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51874168, 51574146, and 52174254), the Innovation and Entrepreneurship Program of University of Science and Technology Liaoning (Grant No. X202010146234), and Xingliao Yingcai Science and Technology Innovation Leading Talent Project (Grant No. XLYC2002028).
Author contributions: Conceptualization: T.A., S.D.; data curation: L.Z., T.A.; formal analysis: T.A., L.Z.; funding acquisition: T.A.; investigation: L.Z., C.X., Y.W., Z.Y.; project administration: T.A., X.T.; resources: T.A., S.D.; software: L.Z.; validation: L.Z., C.X., Y.W., Z.Y.; writing – original draft: L.Z.; writing – review and editing: T.A., S.D., L.Z.; methodology: T.A., S.D. All authors have read and agreed to the published version of the manuscript.
Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.
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