Simultaneous Removal of Cu2+, Cd2+ and Pb2+ by Modified Wheat Straw Biochar from Aqueous Solution: Preparation, Characterization and Adsorption Mechanism

As an eco-friendly and efficient adsorbent for removal of potential toxic metals from aqueous solution, biochar has received widespread attention. In the present study, wheat straw biochar (BC) and corresponding modified biochar (HNC) were used to remove Cu2+, Cd2+ and Pb2+ from an aqueous solution. The influence of the environment factors on metals adsorption and adsorption mechanism were discussed in detail. The results showed that the HNC had porous structures and owned ample functional groups (−OH, −COOH and C−N groups) compared with the BC. In the single system, the adsorption capacities of HNC for Cu2+, Cd2+ and Pb2+ at a pH of 5.5 were 18.36, 22.83 and 49.38 mg/g, which were 76.89%, 164.36% and 22.75% higher than that of the BC, respectively. In addition, the adsorption process of Cu2+ and Cd2+ on BC and HNC fitted to the Langmuir isotherm model and pseudo-second-order kinetics, but the adsorption of Pb2+ on BC and HNC fitted to the Langmuir isotherm model and pseudo-first-order kinetics. Adsorption isotherms indicated that the adsorption of Cu2+, Cd2+ and Pb2+ by BC and HNC was a spontaneous endothermic process. The competitive adsorption of mixed metal ions (Cu2+, Cd2+ and Pb2+) revealed that HNC was more preferential to adsorb Cu2+ compared with Cd2+ and Pb2+. Furthermore, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses revealed that the main adsorption mechanisms were surface complexation and precipitation, and the adsorbed Cu2+, Cd2+ and Pb2+ on HNC mainly exist as CuO, Cd(OH)2, Pb3O4 and Pb(OH)2.


Introduction
The rapid development of industrial, mining and metallurgical industry has resulted in large amount of metals-containing wastewater being discharged into the natural aquatic environment, especially for Cu 2+ , Cd 2+ and Pb 2+ , which are common pollutants in the environment [1,2]. These metals are persistent, toxic and biologically accumulating [3][4][5]. When plants absorb nutrients from the polluted environment, these metals can also be accumulated in the organism through the food chain, then causing a long-term threat to the health of humans and ecosystems [6]. Therefore, it is imperative to reduce the discharge of metals into the aquatic environment. NaOH, HNO 3 , KOH and NH 3 (aq). Ultrapure water with a resistivity of 18.25 MΩ cm was adopted in experiments.

Synthesis of BC and HNC
The wheat straw was cleaned successively with tap water and deionized water, then cut into small sections of about 2 cm, dried and pyrolyzed in a tube furnace under limited oxygen (under the protection of N 2 ) at 350 • C for 2 h. After naturally cooling to room temperature, the pristine biochar was obtained, grounded, sieved through 100 mesh sizes (0.150 mm) and named as BC. Based on the results of previous studies [29,30], BC was modified successively with KOH (2 mol/L), HNO 3 (1 mol/L) and ammonia solution (5%, w/w), then washed repeatedly with deionized water until neutral. After drying, the aminomodified biochar was obtained and named as HNC. The detailed modification process is provided in the Supplementary Materials.

Characterization of BC and HNC
The point of zero charge (pH pzc ) of BC and HNC was measured according to the method described by Ren et al. [31]. The content of oxygen-containing functional groups on the surface of BC and HNC was determined by Boehem titration [32]. The element (C, H, N, S and O) content of BC and HNC was determined by an elemental analyzer (FLASH 2000HT, Thermo Fisher Scientific, America). The morphology and structure of BC and HNC were observed by scanning electron microscopy (SEM, JSM-7001F, JEOL, Japan). The surface functional groups were obtained via Fourier transform infrared spectroscopy (FT-IR, Vertex 70, Bruker, Chicago, IL, USA) within 4000-400 cm −1 . The composition and valence states of major elements were acquired via X-ray photoelectron spectroscopy (XPS, AXIS ULTRA, Kratos, Manchester, UK). The surface area and porosity of BC and HNC were measured with a high precision gas/vapor adsorption measurement instrument (BELSORPmax II, Microtrac BEL, Japan). HNC before and after the adsorption of Cd 2+ , Pb 2+ and Cu 2+ were analyzed by FT-IR and XPS to investigate the potential adsorption mechanism.
The adsorption capacity (qe, mg/g) as well as the removal percentage (R, %) were calculated by the following formula (Equations (1) and (2)): where C 0 and C e are the initial and equilibrium concentrations of Cu 2+ , Cd 2+ and Pb 2+ (mg/L), respectively; q e is the adsorption amount at equilibrium; V is the volume of the solution (L) and M is the mass of the adsorbent (g).
To better understand the adsorption mechanism, a Pseudo-first-order (Equation (3)), and Pseudo-second-order models [Equation (4)] were used to fit the adsorption process [33].
ln q e = ln K F + 1 n ln C e (6) where C e refers to the concentration of Cu 2+ , Cd 2+ and Pb 2+ at equilibrium (mg/L); qmax is the theoretical maximum adsorption amount (mg/g); q e is the adsorption amount of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC at equilibrium (mg/g); K L is the Langmuir constant (L/mg); K F [mg/g (L/mg)1/n] and 1/n are Freundlich constants. R is the universal gas constant (8.314 J/mol K); T is the temperature (K) and A(L/g) and B are the Temkin constants.

Adsorption Mechanism
To further explore the metals adsorption mechanism of HNC, the HNC before and after the adsorption of Cu 2+ , Cd 2+ and Pb 2+ (pH 5.5, dosage of 2 g/L for Cu 2+ and Cd 2+ and 1 g/L for Pb 2+ , time of 240 min, initial concentration of Cu 2+ , Cd 2+ and Pb 2+ of 50 mg/L and temperature of 298.15 K) were analyzed by FT-IR and XPS.

Characterization of Biochar
The morphological structures of BC and HNC are shown in Figure 1. The structure of BC is tube-shaped, sieve-pored, layered and skeleton-like (Figure 1a-d), which is relatively complete. Conversely, the layered structure was broken (Figure 1e), and the tube-shaped, sieve-pored and skeleton-like structure was collapsed after the modification (Figure 1f-h). In addition, the pore size of HNC was larger than that of the BC, and the average pore diameter increased from 10.20 to 13.46 nm. Compared with BC, the specific surface area, pore volume and micropore volume of NHC all decreased obviously ( Table 1). The element analysis indicated that the C content of BC decreased dramatically from 76.29% to 57.55% after the modification. Correspondingly, the content of H, O and N increased from 3.88%, 18.38% and 0.51% to 6.26%, 27.52% and 4.44%, respectively. The atomic ratios of H/C, O/C and (O+N)/C were 0.05, 0.24 and 0.25 for BC, which were increased to 0.11, 0.48 and 0.56, respectively, of that of the HNC. These results indicate that HNC had stronger hydrophilicity and greater polarity, and the aromaticity was weakened [34,35], which may be more conducive for the adsorption of metals [36].  The pH pzc of BC and HNC were 4.77 and 2.69, respectively ( Table 1). The decrease of HNC pH pzc (compared with BC) may be favorable for metal cations adsorption due to the enhanced electrostatic attraction [37]. In addition, the modification increased the carboxyl and lactonic groups from 1.251 and 1.206 to 2.320 and 1.336 mmol/g. However, the hydroxyl group decreased correspondingly from 1.975 to 1.670 mmol/g, respectively ( Table 1). The FT-IR spectrum of BC and HNC are shown in Figure S1. The spectrum of BC contains peaks at 3417 cm −1 (−OH stretching) [38,39], 2921 cm −1 (−CH 2 stretching) [40], 2378 cm −1 (C≡N stretching), 1627 cm −1 (C=O or −COOH stretching) [41] and 1402 cm −1 (C−O or C−N stretching) [42]. The FT-IR spectrum of HNC is similar to that of BC, with some differences in peak intensity and position. The most important changes are the enhanced intensity at 1627 cm −1 and 1402 cm −1 , indicating that the number of −COOH and C−N on the surface of HNC increased. In addition, the decrease of −OH was attributed to the increase of −COOH or C−N. The increase of oxygen-and nitrogen-containing functional groups may enhance the metals removal capacity of HNC through complexing [43].
As shown in Figure 2a, the peaks that appeared at binding energies of 284.75, 532.5 and 400.60 eV can be assigned to C1s, O1s and N1s, respectively. The C1s (Figure 2b [46]. The atomic% of N1s increased from 0.68% to 5.78% after the modification, and the new peaks also appeared in the N1s spectrum at about 405.77 eV, which confirmed that aminos were successfully grafted on HNC.

Effect of Initial pH
The effects of initial pH (1.5-6.5) on the removal of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC are shown in Figure 3. The removal percentage of Cu 2+ , Cd 2+ and Pb 2+ was the lowest at the pH of 1.5, which can be attributed to the fact that the initial pH is lower than the pH pzc of BC (pH pzc = 4.77) and HNC (pH pzc = 2.69) ( Table 1). With increasing of the initial pH from 1.5 to 4.5, the removal percentage of metals by BC and HNC increased from 1.5% and 6.6% to 39.4% and 70.6% for Cu 2+ , 0.4% and 2.4% to 33.0% and 90.4% for Cd 2+ and 0% and 2.7% to 80.6% and 98.6% for Pb 2+ , respectively. The change of these removal percentages was nonsignificant when the initial pH increased from 4.5 to 6.5, except for Pb at pH 6.5 by BC. These results demonstrate that the initial pH had a significant impact on metals adsorption by BC and HNC. In fact, the solution pH can influence the surface charge of the adsorbent and the speciation of metal ions in solution [47]. A lower pH can result in protonation and more positive surface charges of the adsorbent, which can compete (even through electrostatic repulsion) with metal ions and result in a low removal percentage of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC [48]. However, with the initial pH increased from 1.5 to 4.5, the surface charge changed from positive to negative for HNC, and the surface positive charge decreased for BC, both of which increased the removal percentage of these metals by BC and HNC [16]. In general, the initial pH of 5.5 is higher than the pH pzc of BC and HNC, and a higher initial pH could not further enhance the removal percentage of Cu 2+ , Cd 2+ and Pb 2+ in solution. Therefore, a pH value of 5.5 was the optimum condition for the removal of Cu 2+ , Cd 2+ and Pb 2+ in the aqueous solution.

Effect of Adsorbent Dosage
The adsorption of Cu 2+ , Cd 2+ and Pb 2+ by different dosages of BC and HNC are shown in Figure 4. When their dosage increased from 0.4 g/L to 8.0 g/L, the removal percentage of these metals by BC and HNC increased from 8.56% and 20.12% to 92.75% and 99.59% for Cu 2+ , 6.83% and 24.18% to 89.12% and 99.87% for Cd 2+ and 31.53% and 68.40% to 99.15% and 99.89% for Pb 2+ (Figure 4a-c). In general, the removal percentage of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC increased gradually with the increase of their dosage. In contrast, the adsorption capacity decreased from 9.63 mg/g and 24.34 mg/g to 5.63 mg/g and 6.14 mg/g for Cu 2+ , 7.56 mg/g and 25.48 mg/g to 5.03 mg/g and 5.31 mg/g for Cd 2+ and 30.48 mg/g and 64.84 mg/g to 4.84 mg/g and 4.87 for Pb 2+ (Figure 4d-f).
Previous studies indicated that the number of the adsorption sites and surface area are positively correlated with the dosage [49], but excessive dosage will result in the agglomeration of the adsorbent, overlapping of the adsorption sites and blocked diffusion of metals, which may be the main reason for the decrease of the adsorption capacity in the present study [50]. In summary, the dosage of 2 g/L for the adsorption of Cu 2+ and Cd 2+ , and 1 g/L for adsorption of Pb 2+ was selected for the subsequent adsorption experiments.

Adsorption Kinetics
The adsorption kinetics fitted by the pseudo-first-order and pseudo-second-order models are shown in Figure S3. Figure S3 shows that the adsorption rate at the initial stage was faster, and about 80-90% of these metals were adsorbed within about 1 min, and the adsorption equilibrium was reached at about 20 min. Previous reports indicate that a perfect adsorbent should have a fast adsorption rate and short equilibrium time [51], indicating that HNC may have a good prospect for metals removal in aqueous solutions. In addition, the rapid adsorption at the first stage was mainly controlled by physical diffusion; Cu 2+ , Cd 2+ and Pb 2+ occupied the available adsorption sites quickly on the. BC and HNC surface, while chemical adsorption controlled the slow adsorption process until the adsorption equilibrium was reached [52].
The R 2 of the pseudo-second-order model for Cu 2+ and Cd 2+ adsorption by HNC was 0.9880 and 0.9900, respectively, which was higher than that of the pseudo-first-order model ( Table 2), indicating that the adsorption of Cu 2+ and Cd 2+ by BC and HNC was controlled by chemisorption, such as surface complexation and mineral co-precipitation [53]. In contrast, Pb 2+ adsorption by HNC fitted better (R 2 = 0.9992) to the pseudo-first-order kinetics, indicating that the adsorption was controlled by both physical adsorption and chemisorption, especially for the physical adsorption [54]. In addition, the adsorption capacity for these metals by HNC is much higher than that of BC. These results further indicated that the modification significantly increased the metal adsorption capacity of BC.

Adsorption Isotherms
The fitting parameters of Langmuir, Freundlich and Temkin isotherms are shown in Figure 5 and Table S1. The adsorption of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC fitted the Langmuir models better than the Freundlich and Temkin models (Table S1), indicating that the adsorption of Cu 2+ , Cd 2+ and Pb 2+ by both biochars was monolayer chemisorption that occurred on the homogeneous surface [55]. The maximum adsorption capacity of HNC at 288. 15 2+ . The adsorption capacity of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC increased gradually with the increasing temperature. This result indicates that the adsorption of these metals by BC and HNC was an endothermal reaction, which is similar to the previous reports [56,57].

Competitive Adsorption
Langmuir, Freundlich and Temkin isotherms were used to describe the Cu 2+ , Cd 2+ and Pb 2+ adsorption process by BC and HNC in a ternary system ( Figure 6 and Table 3). As shown in Table 3, the adsorption of Cu 2+ , Cd 2+ and Pb 2+ in the ternary system by BC and HNC is similar with that of the monometal system, which fitted better with the Langmuir models than the Freundlich and Temkin. The maximum adsorption capacities (mg/g) for these metals under a ternary system followed the following order: Pb 2+ (17.95) > Cu 2+ (8.86) > Cd 2+ (4.74) by BC, and Pb 2+ (46.84) > Cu 2+ (19.79) > Cd 2+ (10.37) by HNC. The maximum adsorption capacities of these metals by BC and HNC under a ternary system were much lower than those under a monometal system, which might be attributed to the limited adsorption site on both biochars [2]. In addition, compared with the monometal system, the decrease of the maximum adsorption capacity of Cd 2+ was the most, followed by Pb 2+ and Cu 2+ . This result may mean that Cd 2+ and Pb 2+ could be exchanged and substituted by other metals easily compared with Cu 2+ . Furthermore, HNC showed a higher affinity toward metals compared with BC, which further verified that HNC has a stronger adsorption capacity for metals.

Potential Adsorption Mechanisms
Fourier transform infrared spectroscopy was used to characterize the adsorption mechanisms of Cu 2+ , Cd 2+ and Pb 2+ on HNC (Figure 7a). After the adsorption of Cu 2+ , Cd 2+ and Pb 2+ , the peaks at 3419 cm −1 (−OH group) and 1402 cm −1 (C−O or C−N group) shifted slightly. Meanwhile, the area of these peaks decreased obviously, demonstrating that these groups participate in the metal adsorption [38,39,42]. At the same time, the peak at 2304 cm −1 disappeared, which confirmed that the O=C=O group also participates in the adsorption of metals [58]. Furthermore, the peak at 1622 cm −1 (C=O or −COOH) was shifted and sharpened after the adsorption of Cu 2+ and Cd 2+ but weakened after the adsorption of Pb 2+ , which indicates that aromatic C=O and −COOH also participate in the metals adsorption [33]. The above results verified that multiple functional groups (−OH, C−O or C−N, C=O or −COOH and O=C=O) participated in the adsorption of metals through electrostatic interaction and surface complexation. The X-ray photoelectron spectroscopy analysis indicated that new peaks (Cu 2p, Cd 3d and Pb 4f) appeared after the adsorption of Cu 2+ , Cd 2+ and Pb 2+ (Figure 7b). The binding energy of C1s (Figure 7c) at 284.77 and 286.74 eV can be assigned to C−C/C=C and C=O, respectively. After the adsorption of Cu 2+ , Cd 2+ and Pb 2+ , the new peaks representing C−O−C/C−OH appeared in the C 1s spectrum at about 285.91, 285.87 and 285.65 eV, respectively [59]. Meanwhile, the peak area of C−C/C=C decreased from 72.37% to 57.84%, 60.10% and 51.34% after the adsorption of Cu 2+ , Cd 2+ and Pb 2+ , respectively, indicating that C−C/C=C played an important role in the adsorption of metals. The N1s (Figure 7d)) spectra of HNC before the metal adsorption can be divided into 400.07 (−NH−) and 505.77 eV (-NH 2 /-NO 2 ), while it just shifted slightly after the adsorption of Cu 2+ , Cd 2+ and Pb 2+ , indicating that −NH− and -NH 2 /-NO 2 played a small role in the metal adsorption process. After the adsorption of Cu 2+ , Cd 2+ and Pb 2+ , the proportion of C−O (Figure 7e) significantly increased from 56.70% to 85.90%, 80.20% and 78.94%, respectively. At the same time, the proportion of O−H decreased from 43.3% to 14.1%, 19.8% and 21.06%, respectively. These results imply that the adsorption of metals is accompanied by the breaking of hydrogen bonds [59].

Conclusions
In conclusion, the modification dramatically changed the physical and chemical characteristics of BC. HNC had a higher adsorption capacity for Cu 2+ , Cd 2+ and Pb 2+ compared with BC. The chemisorption controlled the overall adsorption for Cu 2+ and Cd 2+ by BC and HNC, and both physical adsorption and chemisorption controlled for Pb 2+ adsorption by BC and HNC. The Langmuir model could precisely describe the adsorption isotherm. In addition, HNC was more preferential to adsorb Cu 2+ in the ternary system compared with Pb 2+ and Cd 2+ . The predominant metals sorption mechanism by HNC was the surface complexation and precipitation. These results indicate that HNC is a potential adsorbent for the remediation of metals contamination in aqueous environments.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/toxics10060316/s1. Figure S1: FT-IR spectra of BC and HNC; Figure S2: Pseudo-first-order kinetic and pseudo-second-order kinetic fitting of Cu 2+ (a), Cd 2+ (b) and Pb 2+ (c) adsorption by BC and HNC; dosage of 2 g/L for Cu 2+ and Cd 2+ and 1 g/L for Pb 2+ ; pH of 5.5; initial concentration of Cu 2+ , Cd 2+ and Pb 2+ of 50 mg/L and temperature of 298.15 K; Table S1: Isotherm parameters for the adsorption of Cu 2+ , Cd 2+ and Pb 2+ by BC and HNC.
Author Contributions: Conceptualization and writing-original draft preparation, Y.W. and X.R.; methodology, Z.J.; investigation, K.Z. and S.G.; data curation, S.N. and S.F.; writing-review and editing, Y.W.; supervision, W.Z. All authors have read and agreed to the published version of the manuscript.