Abstract
The impact of Ba-modified peanut shell biochar (Ba-PSB) on Pb(II) removal was studied and BaCl2 was used as a modifier. It was shown that the PSB obtained at 750 °C had the best adsorption effect, and the Ba-PSB had a larger specific surface area and a good adsorption effect on Pb(II). At pH = 5, concentration was 400 mg/L, time was 14 h, and temperature was 55 °C, the loading amount of black peanut shell biochar (BPSB), red peanut shell biochar (RPSB), Ba-BPSB, and Ba-RPSB reached 128.050, 98.217, 379.330, and 364.910 mg/g, respectively. In addition, based on the non-linear fitting, it was found that the quasi-second-order kinetic model, and isothermal model could be applied to describe Pb(II) adsorption on PSB and Ba-PSB. The adsorption behavior of PSB unmodified and modified was a spontaneous process. Moreover, chemical modification of BPSB, RPSB, Ba-BPSB, and Ba-RPSB for hindering of –COOH and –OH groups revealed 81.81, 77.08, 86.90, and 83.65% removal of Pb(II), respectively, which was due to the participation of –COOH, while 17.61, 21.70, 12.77, and 15.06% was from –OH group, respectively. The increase of cation strength (Na+, K+, Ca2+, and Mg2+) will reduce the adsorption capacity of PSB for Pb(II).
HIGHLIGHTS
Ba-modified PSB was prepared and its adsorption property was examined.
The modification of PSB could greatly improve its loading amount for Pb(II).
The effect of –COOH and –OH groups from PSB on Pb(II) sorption was scrutinized.
Ba-modified PSB was an excellent green adsorbent for Pb(II) removal from water.
INTRODUCTION
Water contamination caused by heavy metal ions (HMIs) is a highly concerning issue that has captured significant attention. This is because HMIs in water cannot be naturally degraded and finally accumulate in human bodies through food chain, which seriously threatens people's health (Lu et al. 2022; Shakya et al. 2022). Consequently, efficient techniques are crucial for addressing HMI pollution in water. Governments and regulatory bodies have made significant efforts to minimize the presence of HMIs in wastewater by imposing stringent limits on their allowable levels. For example, the maximum allowable discharge concentration of Pb2+, Hg2+, Cd2+, Cr3+, Cr6+, Ni2+, and Be2+ in the industrial discharge sewage in the domestic set by governing authorities were 1.0, 0.05, 0.1, 1.5, 0.5, 1.0, and 0.005 mg/L, respectively (He et al. 2013). Among these toxic metal ions, Pb is particularly worrying. This is because Pb is a very toxic metal ion that can affect the central nervous system of humans and brings about many serious diseases and poses a risk to the environment (Zhang et al. 2019). The removal of toxic Pb ions from water is a matter of concern (Islam & Patel 2009). At present, many innovative methods are proposed to remove Pb(II) and other HMIs from water. These methods primarily involve precipitation, electrodialysis, ion exchange, and biochemical approaches (Shan et al. 2020; Bashir et al. 2022). When the concentration of HMIs in wastewater becomes dilute, these methods will show a number of limitations, such as high cost, ineffective, high requirements for reagents, and easy to produce toxic sludge (Popuri et al. 2009; Futalan et al. 2011), thus their applications in industrial processes will be limited (Rhazi et al. 2002). Differencing from these methods, the removal of HMIs using adsorption had many merits, such as low cost, simple process, and high adsorption efficiency (Liu et al. 2013a). Biomass carbon (or biochar) adsorbents are particularly advantageous due to their low-cost, reusability, and are environmentally friendly, so it is widely used as an adsorbent to remove HMIs from wastewater (Islam et al. 2018; An et al. 2019; Bai et al. 2020; Deng et al. 2020; Sasidharan et al. 2022; Moussout et al. 2023). Therefore, biochar adsorbents (BCAs) attract much interest.
As a typical biomass source, waste peanut shells (WPSs) have also captured much attention. Especially, China has a large amount of WPSs, but only a tiny part of WPSs was reasonably utilized. Most of the rest was discarded. As a result, environmental pollution and a significant waste of resources were initiated. With the implementation of ‘double carbon’ goal, the utilization of agricultural wastes captures much attention. This is because the raw materials of WPSs were easy to obtain and low-cost. Rich porous structures, large specific surface area (SSA), high porosity, and abundant functional groups were possessed by most biomass carbons (Yang et al. 2019; Zhao et al. 2022). The structural performances and different functional groups of peanut shell biochar (PSB), prepared under diverse pyrolysis conditions, can influence the adsorption performance of HMIs (Jafri et al. 2018; Shi et al. 2019; Xiao et al. 2020). For example, Liu et al. (2013a) explored the impact of WPSs prepared at 350, 450, and 550 °C on the removal of HMIs, and confirmed that the removal of HMIs might be related to the elements and functional groups of biochar (BC). In addition, Cobbina et al. (2018) reported that PSB prepared at 350 and 700 °C was used to remove Cd(II), Hg(II), and Pb(II) in water. All (100%) of the binary mixed solution of Pb(II) and Cd(II) could be removed by groundnut shell biochar 700, while 100% of the binary ionic solution of Pb(II) and Hg(II) could only be removed by groundnut shell biochar 350. Clearly, PSB has excellent adsorption performances for Pb(II).
As one of the peanuts, black peanuts (BPs) also have special characteristics. Compared with ordinary red peanuts (RPs), BPs were rich in arginine, and their amounts were much higher than the common ones in terms of protein, potassium, and calcium (Wang et al. 2014; Zhao et al. 2015). In addition, BPs are also rich in amino acids and have high nutritional value. Hence, they are beneficial to human health. But, they produce a large amount of WPSs. Therefore, it is urgent to apply such types of WPSs as bioadsorbents. However, few studies were performed to do such a job. Hence, the present study is insufficient and further exploration needs to be conducted. Interestingly, as a biomaterial, black WPSs fit the current research direction of ‘green chemistry’ (Moussout et al. 2023). For such a purpose, some applications of black WPSs are proposed. For example, black WPSs can be used as carbon precursors in the preparation of adsorbent materials and in the removal of the adsorption of Pb(II) (Franco et al. 2023). To raise the loading amount of black WPSs (which was obtained from a special product of Sixian county of Anhui province, China) for Pb(II) from water, herein, the black WPSs pyrolyzed at 750 °C was modified with BaCl2, which was used as a modified biochar (labeled as Ba-PSB, the original black WPSs pyrolyzed at 750 °C was labeled as BPSB). As a model HMI, Pb(II) was used to inspect the modification effect. For comparison, ordinary red WPSs pyrolyzed at 750 °C were also modified by BaCl2 (labeled as Ba-RPSB, the original red WPSs pyrolyzed at 750 °C was labeled as RPSB) and its adsorption for Pb(II) was also examined. The innovation of this study is that (1) black WPSs modified by BaCl2 were used as bioadsorbent for Pb(II) removal; (2) the effect of –COOH and –OH groups from PSB on Pb(II) adsorption was scrutinized; and (3) adsorption mechanism was explored by traditional theoretical equations. It is hoped that such research is favorable to the potential application of WPSs in the treatment of metal-containing wastewater and the improvement of poverty alleviation goals.
MATERIAL AND METHODS
Material and chemicals
Black peanut shells (BPSs) (a special product of Sixian, Anhui, China), and red peanut shells (RPSs) were purchased from the market. BaCl2·2H2O (AR-grade) was purchased from the Xilong chemical plant (Guangdong, China).
Preparation of PSB
First, the WPSs were rinsed repeatedly with deionized water and subsequently desiccated at 125 °C. Then the desiccated WPSs were crushed with a crusher. The WPS powder was soaked for 24 h and dried after removing the suspended matter. The obtained PS powder was put into a sealed bag for the next step. Second, 5 g of pretreated PS powder was added into 0.12 mol/L BaCl2 solution. The mixed solution was placed in the sonicator for 2 h. After sonication, the mixed solution was filtered and desiccated at 105 °C, and the modified PS powder was thus obtained. Third, a certain amount of PSs and Ba-modified PSs were put into a crucible, covered, and placed in a muffle furnace for calcination at 450, 550, 650, and 750 °C for 2 h, respectively. The pyrolyzed BC was washed with deionized water and dried at 105 °C to a constant weight. Thus, the PSBs and Ba-PSBs samples were acquired.
Characterizations
The physicochemical properties of biochar including yield, pH, and industrial analysis (moisture, ash, volatile fractions, and fixed carbon) were evaluated. Where BC yield was determined by the fraction of the mass of the PSs raw material. The pH measurement involved mixing BC with water at a ratio of 1:20 (w/v). The moisture content of PSB was obtained by measuring the mass loss at 150 °C. The ash and volatile organic fractions were obtained through remaining weight measurements after calcination at 400 ± 20 °C for 3 h and at 500 ± 20 °C for 30 min, respectively. The fixed carbon content was calculated by subtracting moisture, ash, and volatile fraction from 100% of the dry basis (Liu et al. 2017). The elemental analysis (C, H, O, N) was shown by using a vario EL cube (Elementar Analysensysteme GmbH, Elementar, Germany). The micromorphology, textural properties, elemental composition, crystal structure, molecular structure, chemical composition, surface area, and pore volume of PSs, PSB, and Ba-PSB were determined using a cold field emission scanning electron microscope (SEM, SU8010, Hitachi, Japan), X-ray photoelectron spectroscopy (XPS, ES-CALAB 250Xi, Thermo, UK), energy dispersive spectrometer (EDS, SU8010, Hitachi, Japan), X-ray diffractometer (XRD, TD-3500, Dandong Tongda), Fourier transform infrared spectrometer (FTIR, Thermo Fisher company, USA), thermogravimetric analysis (TGA, Q-500, T-a company, USA), and Brunauer–Emmett–Teller (BET) surface area analysis (Autosorb-iQ, Quantachrome company, USA).
The determination of –COOH and –OH effect
According to the method mentioned in this article (Iqbal et al. 2009), the impact of –COOH from PSB in this study was determined as follows: 0.6 g of PSB was added into a solution containing 42 mL of methanol and 0.36 mL of concentrated hydrochloric acid. The resulting mixture was continuously stirred at 200 rpm for 5 h using a magnetic stirrer. The solution was filtered by extraction to remove excess methanol and concentrated hydrochloric acid. Finally, the product was dried for adsorption experiments.
To determine the effect of –OH from PSB, a mixture of 5 g of PSB and 100 mL of formaldehyde solution was prepared, and the subsequent steps were similar to that of –COOH from PSB.
Adsorption experiment
The adsorption of Pb(II) by PSB after chemically blocking the hydroxyl carboxyl groups was measured as follows: 0.08 g of PSB with and without chemical blocking of hydroxyl and carboxyl groups were separately suspended in 80 mL of Pb(II) solution at a concentration of 140 mg/L. The adsorption capacity was analyzed using an atomic absorption spectrophotometer after 24 h of static adsorption.
The adsorption of PSB for Pb(II) at different ionic strengths (0.01 and 0.05 mol/L of Na+, K+, Ca2+, and Mg2+) was evaluated. The starting concentration was set as 140 mg/L. Then 0.08 g of PSB was taken and mixed with 140 mg/L of Pb(II) and statically adsorbed for 24 h until adsorption equilibrium was reached. Pb concentration was measured after sample collection. All adsorption experiments were repeated thrice.
Adsorption kinetics and thermodynamics experiments were conducted by adding 0.08 g PSB into an 80 mL conical flask. When exploring the sorption kinetics, the initial ion concentration was 400 mg/L, and the pH = 5, the conical flask was shaken using a shaker at 120 rpm, and the shaker temperature was set as 45 °C. All experiments were performed in triplicates. The times were set as 0.5, 1, 1.5, 2, 4, 6, 8, 10, 12, and 14 h. When exploring the adsorption thermodynamics, the experimental temperatures were adjusted to 25, 35, 45, and 55 °C. In order to explore sorption isotherms, the concentrations of Pb(II) were set as 200, 250, 300, 350, and 400 mg/L. Other schedules were the same as the sorption kinetics experiments.
RESULTS AND DISCUSSION
The determination of calcination temperature
Qe of PSB pyrolyzed at different temperatures for Pb(II), in which BPSB represented the black PSB, RPSB was the red PSB, Ba-BPSB was Ba-modified black PSB, and Ba-RPSB was Ba-modified red PSB.
Qe of PSB pyrolyzed at different temperatures for Pb(II), in which BPSB represented the black PSB, RPSB was the red PSB, Ba-BPSB was Ba-modified black PSB, and Ba-RPSB was Ba-modified red PSB.
Clearly, the Qe of Ba-BPSB for Pb(II) was greater than that of Ba-RPSB at any pyrolysis temperature, and the Qe of the modified BC was higher than those of the unmodified BC. The reason can be assigned to the difference of functional groups on its surface (see Table 2) and larger SSA (as presented in Table 3). This finding demonstrated that the modification of WPSs using BaCl2 as a modifier could effectively increase the Qe of PSB. Moreover, the increase in calcination temperature was helpful in elevating the adsorption of BPSB, RPSB, Ba-BPSB, and Ba-RPSB for Pb(II). The theoretical explanation to such phenomena can be seen as follows. Generally, the elevation of calcination temperature can increase the pore size and surface area. Further, an increase in BC will activate more functional groups on the surface of BC. Accordingly, it will result in the improvement of the loading amount. Based on such a result, the pyrolysis temperature was set as 750 °C for further study.
Physical and chemical properties
Table 1 presents the physical and chemical properties of yield, moisture, ash, volatile organic matter (VOM), fixed carbon, and pH of BPSB, RPSB, Ba-BPSB, and Ba-RPSB at 750 °C.
Physical and chemical properties of BPSB, RPSB, Ba-BPSB, and Ba-RPSB prepared at 750 °C
Materials . | Yield (wt.%) . | pH . | Moisture (%) . | Ash (%) . | VOM (%) . | Fixed C (%) . | Elemental content (wt. %) . | Atomic ratio (dB) . | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
C . | H . | O . | N . | H/C . | O/C . | (O + N)/C . | |||||||
BPSB | 8.14 | 8.98 | 10.89 | 6.54 | 26.33 | 56.23 | 85.14 | 1.049 | 12.921 | 0.89 | 0.012 | 0.152 | 0.162 |
RPSB | 8.33 | 8.74 | 10.64 | 5.95 | 28.35 | 55.06 | 84.56 | 1.250 | 12.950 | 1.24 | 0.015 | 0.153 | 0.168 |
Ba-BPSB | 9.22 | 8.53 | 7.88 | 21.25 | 27.11 | 43.77 | 68.94 | 0.897 | 29.133 | 1.03 | 0.013 | 0.423 | 0.438 |
Ba-RPSB | 10.16 | 8.29 | 7.10 | 20.83 | 33.32 | 38.76 | 69.19 | 0.948 | 28.512 | 1.35 | 0.014 | 0.412 | 0.432 |
Materials . | Yield (wt.%) . | pH . | Moisture (%) . | Ash (%) . | VOM (%) . | Fixed C (%) . | Elemental content (wt. %) . | Atomic ratio (dB) . | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
C . | H . | O . | N . | H/C . | O/C . | (O + N)/C . | |||||||
BPSB | 8.14 | 8.98 | 10.89 | 6.54 | 26.33 | 56.23 | 85.14 | 1.049 | 12.921 | 0.89 | 0.012 | 0.152 | 0.162 |
RPSB | 8.33 | 8.74 | 10.64 | 5.95 | 28.35 | 55.06 | 84.56 | 1.250 | 12.950 | 1.24 | 0.015 | 0.153 | 0.168 |
Ba-BPSB | 9.22 | 8.53 | 7.88 | 21.25 | 27.11 | 43.77 | 68.94 | 0.897 | 29.133 | 1.03 | 0.013 | 0.423 | 0.438 |
Ba-RPSB | 10.16 | 8.29 | 7.10 | 20.83 | 33.32 | 38.76 | 69.19 | 0.948 | 28.512 | 1.35 | 0.014 | 0.412 | 0.432 |
From Table 1, the yields of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were 8.14, 8.33, 9.22, and 10.16%, respectively. Clearly, the yield of BPSs was lower than that of RPSs both modified and unmodified products. By comparing the yields of the two modified PSBs, it can be seen that the yields of the modified PSB were significantly higher than those of the unmodified PSB. In addition, the moisture, ash, VOM, and fixed carbon contents of BPSB were 10.89, 6.54, 26.33, and 56.23%, respectively; the corresponding values for RPSB were 10.64, 6.54, 28.35, and 55.56%, respectively. As for the modified Ba-BPSB, the values were 7.88, 21.25, 27.11, and 43.77%, respectively. With regard to the modified Ba-RPSB, the values were 7.10, 20.83, 33.32, and 38.76%, respectively. The moisture and fixed carbon contents of the modified PSB were lower than those of the unmodified PSB, whereas the ash and VOM of the modified PSB were higher than those of the unmodified PSB.
The theoretical explanation for these trends was that the loaded Ba was not easily removed during the pyrolysis process and largely remained in ash in the modified biochar. Additionally, the presence of Ba might promote the generation of volatile products during the pyrolysis process (Guo et al. 2018) and the reduction of fixed carbon in the modified PSB was also mainly caused by the increase in ash content.
Taking the modified and unmodified PSBs into account, the contents of moisture, ash, and fixed carbon of the black PSB were higher than those of the common PSB. Conversely, the contents of VOM from the black PSB were lower than those from the common PSB. The differences in PSB between the black PSB and the common PSB might be attributed to the differences in the composition of raw materials; this is because the black PSB contain higher arginine, protein, and amino acids than the common peanuts (Deng et al. 2018).
Moreover, the pH values of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were 8.98, 8.74, 8.53, and 8.29, respectively. Notably, the pH values of the black PSB were higher than those of the regular PSB both modified and unmodified. Further, the ash content of the black PSB was found to be higher than that of the common PSB, suggesting that the ash content was correlated with the pH of biochar (Li et al. 2017). Interestingly, the modification of Ba(II) resulted in a decrease in the pH of both types of PSBs.
The elemental analysis (C, H, O, and N) revealed that PSB was mainly composed of carbon, and the C content of BPSB, RPSB, Ba-BPSB, and Ba-RPSB was 85.14, 84.56, 68.94, and 69.19%, respectively. The aromaticity and polarity of the charcoal are commonly assessed using elemental atomic ratios (H/C, O/C, and (O + N)/C), which are due to the removal of polar functional groups and the formation of aromatic structures. As a result, the carbonization of organic materials is easily performed during pyrolysis.
From Table 1, the H/C of the black PSB was lower than that of the normal PSB both modified and unmodified. Specifically, H/C values were 0.012 and 0.013 for BPSB and Ba-BPSB, and 0.015 and 0.014 for RPSB and Ba-RPSB, which further indicated the higher aromaticity of common PSs. Compared to the unmodified PSB, the O/C value of Ba-BPSB and Ba-RPSB (0.423 and 0.412, respectively) were significantly higher than those of BPSB and RPSB (0.152 and 0.153, respectively), indicating an increase in surface polarity and hydrophilicity (Chen et al. 2019). The atomic ratios of (O + N)/C for Ba-BPSB and Ba-RPSB (0.438 and 0.432, respectively) were higher than those of BPSB and RPSB (0.162 and 0.168, respectively), further suggesting that the modification of Ba(II) increased the polar functional groups of PSB.
Effect of –COOH and –OH groups from PSB on Pb(II) adsorption
To examine the effect of –COOH and –OH groups from PSB on Pb(II) adsorption, the binding ability of PSB with and without chemically blocked –COOH and –OH for Pb(II) is shown in Table 2.
Sorption of Pb(II) by PSB with and without chemically blocked –COOH and –OH groups
Materials . | Unblocked (mg/g) . | –COOH blocked (mg/g) . | Reduction rate (%) . | –OH blocked (mg/g) . | Reduction rate (%) . | Total reduction rate (%) . |
---|---|---|---|---|---|---|
BPSB | 81.04 | 14.74 | 81.81 | 66.77 | 17.61 | 99.42 |
RPSB | 67.22 | 15.40 | 77.08 | 52.64 | 21.70 | 98.79 |
Ba-BPSB | 138.78 | 18.18 | 86.90 | 121.06 | 12.77 | 99.67 |
Ba-RPSB | 138.65 | 22.68 | 83.65 | 117.76 | 15.06 | 98.71 |
Materials . | Unblocked (mg/g) . | –COOH blocked (mg/g) . | Reduction rate (%) . | –OH blocked (mg/g) . | Reduction rate (%) . | Total reduction rate (%) . |
---|---|---|---|---|---|---|
BPSB | 81.04 | 14.74 | 81.81 | 66.77 | 17.61 | 99.42 |
RPSB | 67.22 | 15.40 | 77.08 | 52.64 | 21.70 | 98.79 |
Ba-BPSB | 138.78 | 18.18 | 86.90 | 121.06 | 12.77 | 99.67 |
Ba-RPSB | 138.65 | 22.68 | 83.65 | 117.76 | 15.06 | 98.71 |
When the effect of –COOH from PSB was blocked, the adsorption of Pb(II) was severely inhibited. After blocking the –COOH from PSB, the loading amount of BPSB, RPSB, Ba-BPSB, and Ba-RPSB on Pb(II) decreased from 81.04, 67.22, 138.78, and 138.65 mg/g to 14.74, 15.40, 18.18, and 22.68 mg/g, resulting in an 81.81, 77.08, 86.90, and 83.65% down in loading amount, respectively. These results revealed the crucial role of –COOH from PSB in Pb(II) adsorption. It was reported (Gardea-Torresdey et al. 1990) that methanol-modified algal biomass showed decreases in metal ions binding. It was likewise demonstrated that the –COOH in biomass had an important role in the binding of metal ions.
Similarly, after the blocking of –OH from PSB, the adsorption of Pb(II) on PSB decreased to 66.77, 52.64, 121.06, and 117.76 mg/g, with corresponding reduction in loading amount of 17.61, 21.70, 12.77, and 15.06%, respectively. It was confirmed that –OH groups also produce a significant role in the adsorption of Pb(II). Such a trend was similar to the results reported in an article (Iqbal et al. 2009). It was reported that the formaldehyde modified natural mango peel biomass was found to have a reduction in metal ion adsorption capacity. The role of hydroxyl groups in the adsorption of HMIs was confirmed.
Obviously, the adsorption of Pb(II) by PSB after blocking the –COOH groups was lower than that by PSB after blocking the –OH groups, indicating that the –COOH groups played a major role in the adsorption of Pb(II) on BPSB and RPSB. Note that the loading amount of the black PSB after blocking the –COOH was lower than that of the red PSB, while the loading amount of the black PSB after blocking the –OH groups was higher than that of the red PSB. It can be presumed that the –COOH groups of the black PSB is richer than that of the red PSB.
SEM analysis
The SEM images of WPSs (a, b) and PSB (c–j). (a) BPS; (b) RPS, (c) BPSB, (d) RPSB, (e) Ba-BPSB, (f) Ba-RPSB, (g) BPSB after adsorbing Pb(II), (h) RPSB after adsorbing Pb(II), (i) Ba-BPSB after adsorbing Pb(II), and (j) Ba-RPSB after adsorbing Pb(II).
The SEM images of WPSs (a, b) and PSB (c–j). (a) BPS; (b) RPS, (c) BPSB, (d) RPSB, (e) Ba-BPSB, (f) Ba-RPSB, (g) BPSB after adsorbing Pb(II), (h) RPSB after adsorbing Pb(II), (i) Ba-BPSB after adsorbing Pb(II), and (j) Ba-RPSB after adsorbing Pb(II).
In Figure 3(a) and 3(b), the surface of uncalcined PSs was smoother, and a dense lamellar structure was found. The reason could possibly be ascribed to the existence of a large amount of lignocellulose contained in PSs. In Figure 3(c) and 3(d), the character of the PSs was changed after high-temperature pyrolysis. Compared with the character of the PSs, the character of the PSB became rougher, and a large number of irregular particles with uneven particle size were distributed on its surface. Due to high-temperature pyrolysis, lignocellulose and lignin in PSs were degraded. As a result, a fractured lamellar stacking structure appeared in most of the particles. In Figure 3(e), many mesoporous structures were generated on the surface of Ba-BPSB, the surface particles were agglomerated and the surface roughness was increased. In Figure 3(f), more particles were produced in Ba-RPSB, which increased the surface roughness of biochar to a certain extent. In Figure 3(e) and 3(f), Ba-modified PSB indicated the excellent loading amount. In Figure 3(g) and 3(h), the surface morphology of the BC adsorbed Pb(II) did not change significantly. However, as seen in Figure 3(g), a relatively obvious pore structure was produced in BPSB after adsorbing Pb(II). Similarly, the pore structure of Ba-BPSB became much more evident after adsorbing Pb(II), and the pore size was increased. Based on these changes in the pore structure of PSB for Pb(II), it can be reasoned that the loading amount of PSB for Pb(II) could be improved by increasing pore size to a certain extent.
XPS analysis
XPS spectra of BPSB, RPSB, Ba-BPSB, and Ba-RPSB. (a, b) the wide scan spectra before and after adsorbing Pb(II), respectively; (c) Pb4f-spectrum after adsorbing; (d, e) Ba3d-spectrum before and after adsorbing Pb(II), respectively; (f, g) C1s spectrum before and after adsorbing Pb(II); and (h, i) O1s spectrum before and after adsorbing Pb(II), respectively.
XPS spectra of BPSB, RPSB, Ba-BPSB, and Ba-RPSB. (a, b) the wide scan spectra before and after adsorbing Pb(II), respectively; (c) Pb4f-spectrum after adsorbing; (d, e) Ba3d-spectrum before and after adsorbing Pb(II), respectively; (f, g) C1s spectrum before and after adsorbing Pb(II); and (h, i) O1s spectrum before and after adsorbing Pb(II), respectively.
As shown in Figure 4, the predominant chemical elements of PSB were C and O, and chemical elements such as C, O, and Ba mainly existed in the PSB after modification by Ba(II). In Figure 4(a), the peaks at 284.82 and 532.97 eV were C1s and O1s, respectively, and 781.12 and 797.54 eV were Ba3d, which further indicated that Ba ions were successfully loaded on the surface of PSB. Besides, the peaks at 137.69 and 145.21 eV were Pb4f, 286.66 and 532.19 eV were C1s and O1s, respectively, and 782.23 and 797.27 eV were Ba3d, which indicated that Pb ions were successfully adsorbed on the surface of PSB (Figure 4(b)). In Figure 4(c), the peaks of Pb4f binding energy, and the peaks at binding energies of 143.89 and 139.03 eV in the unmodified PSB after Pb ion adsorption, which are corresponded to Pb4f5/2 and Pb4f7/2, respectively. The peaks at a binding energy of 143.99 and 139.12 eV in Ba(II)-modified black PSB and the peaks at a binding energy of 144.08 and 139.23 eV in Ba(II)-modified safflower shell biochar also corresponded to Pb4f5/2 and Pb4f7/2, respectively. The strength ratio of Pb4f5/2 and Pb4f7/2 was 3:5, and Pb ions existed as the +2 valence form.
The peak of Ba3d binding energy of 795.48 and 780.18 eV in Ba ion-modified PSB corresponded to Ba3d3/2 and Ba3d5/2, respectively (Figure 4(d)). After the Pb(II) adsorption, the peak at binding energy of 796.49 and 781.29 eV in Ba-modified BPSB, and the binding energy of 796.22 and 780.84 eV in Ba-modified RPSB also corresponded to Ba3d3/2 and Ba3d5/2, respectively (Figure 4(e)). The intensity ratio was 2:3, proving that Ba ion mainly existed as the +2 valence form in PSB.
The C1s spectrum had three different peaks corresponding to various carbon functional groups, namely O–C=O, C–O, and C–C/C=C. These peaks were observed at binding energy values of 289.70, 286.08, and 284.8 eV for BPSB (Khater et al. 2022), 289.95, 285.99, and 284.8 eV for RPSB, 289.20, 285.99, and 284.8 eV for Ba-BPSB, and 289.93, 286.10, and 284.8 eV for Ba-RPSB, respectively (Figure 4(f)). After adsorption, the carbon functional groups of C–C/C = C, C–O, and O–C = O corresponding to binding energy values of 289.22, 286.02, and 284.8 eV for BPSB, 289.72, 285.98, and 284.8 eV for RPSB, 290.02, 286.19, and 284.8 eV for Ba-BPSB, and 289.85, 285.96, and 284.8 eV for Ba-RPSB, respectively (Figure 4(g)). After adsorption of Pb ions, the binding energy of C1s peak changed and the peak width became narrower, indicating that the chemical environment might have been changed when Pb(II) was adsorbed by PSB.
The surface of the PSB adsorbent was rich in O functional groups (Sattar et al. 2019). The O1s spectrum also had three different peaks, which represented different functional groups, such as C–OH, C–O–H, and C = O (Murali et al. 2019), corresponding to binding energy values of 533.57, 532.41, and 531.38 eV for BPSB, individually and 533.79, 532.77, and 531.68 eV for RPSB, respectively. Compared with BPSB and RPSB, the O1s peak width of Ba-BPSB and Ba-RPSB was narrower. Each of the three function groups represented the binding energy of 533.71, 532.42, and 531.42 eV for Ba-BPSB and 534.41, 533.42, and 531.64 eV for Ba-RPSB (Figure 4(h)). After adsorption, the function groups of C–H, C–O–H, and C = O corresponded to binding energy values of 533.92, 533.08, and 531.60 eV for BPSB, respectively; 535.44, 533.45, and 531.63 eV for RPSB, respectively; 533.79, 531.88, and 531.81 eV for Ba-BPSB; and 533.57, 532.37, and 531.44 eV for Ba-RPSB (Figure 4(i)). This indicated that the O element mainly existed in the form of −2 valence before and after the Pb(II) sorption.
EDS analysis
EDS chart of (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB, respectively; (e–h) were the adsorbed BPSB, RPSB, Ba-BPSB, and Ba-RPSB by Pb(II), respectively.
EDS chart of (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB, respectively; (e–h) were the adsorbed BPSB, RPSB, Ba-BPSB, and Ba-RPSB by Pb(II), respectively.
Figure 5(c) and 5(d) shows that Ba(II) was successfully loaded on the surface of PSB, and the atomic proportion of Ba element was 4.81 and 0.88%, respectively. Figure 5(e)–5(h) shows that Pb(II) was loaded on the surface of PSB, and the atomic proportions of Pb element are 0.39, 0.35, 1.53, and 2.18%, respectively. In Figure 5(g) and 5(h), the peak of Ba(II) was missing–this is because Ba(II) peak was overlapped by Pb(II) peak due to its relatively low energy. In addition, the EDS map of PSB in Figure 5 showed that the BC contains Si element. This is because Si tablets were used as the substrate in the testing process. Moreover, Si was clearly observed on the surface of BC in Figure 5 because Si sheets were used as the substrate in the testing process.
XRD analysis
XRD patterns of (a) BPSs RPSs, (b) two PSBs before and after adsorbing Pb(II), and (c) Ba-modified two PSBs before and after adsorbing Pb(II).
XRD patterns of (a) BPSs RPSs, (b) two PSBs before and after adsorbing Pb(II), and (c) Ba-modified two PSBs before and after adsorbing Pb(II).
In Figure 6(a), the XRD patterns between BPSs and RPSs were similar. The pronounced dispersion peaks at 2θ = 16.06° and 2θ = 24.1° would be probably due to the existence of fatty chains, which were composed of amorphous hexagonal carbon. Moreover, the PSs had a narrow diffraction peak at 2θ = 34.77°. In Figure 6(b), the pronounced dispersion peaks appeared at 2θ = 25.5°, 29.58°, and 43.19°; while the dispersion peaks occurred at 2θ = 25.5° and 43.19°, indicating that PSB was amorphous. The dispersion peak of PSB disappeared after Pb(II) adsorption. The prominent diffraction peaks were generated at 2θ = 24.61°, 27.07°, and 33.97°, and the diffraction angle gradually shifted to a lower grade, indicating that Pb ions adsorbed on PSB could increase the crystal plane spacing and reduce the diffraction angle.
As seen in Figure 6(c), the Ba-modified PSB had pronounced diffraction peaks at 2θ = 24.22°, 29.39°, 34.37°, 42.53°, 44.73°, and 46.9°. The diffraction peaks of Ba-modified RPSs were similar to those of it. Further, the diffraction peaks of PSB modified by Ba ions increased significantly, which would be possible because as the Ba(II) was added, the pore size and pore structure of PSB became more complicated. The disappearance of the dispersion peak indicated that the doping of Ba ions resulted in the crystallization of PSB and the formation of microcrystalline structure under high-temperature pyrolysis. While the diffraction peak intensity of the BPSB modified by Ba(II) was higher than that of the RPSB, which was different from the observations in Figure 6(a) and 6(b). More diffraction peaks with significant diffraction intensity were produced onto the modified PSB after Pb(II) adsorption. Further, the diffraction angle shifted to a lower grade, because Pb ions and Ba ions exchanged with cations or were precipitated on the surface of PSB.
FTIR analysis
FTIR profile of PSB: (a) two kinds of WPSs, (b) BPSB and RPSB, (c) Ba-BPSB and Ba-RPSB; and (d) BPSB, RPSB, Ba-BPSB, and Ba-RPSB after adsorbing Pb(II), respectively.
FTIR profile of PSB: (a) two kinds of WPSs, (b) BPSB and RPSB, (c) Ba-BPSB and Ba-RPSB; and (d) BPSB, RPSB, Ba-BPSB, and Ba-RPSB after adsorbing Pb(II), respectively.
In Figure 7(a), for BPSs and RPSs, their curves had similar change trends. In contrast, BPSB, RPSB, Ba-BPSB and Ba-RPSB also had such phenomena as illustrated in Figure 7(b) and 7(c). The peaks observed at 3,445 and 3,193 cm−1 corresponded to the vibration of O–H stretching (Li et al. 2018), which was the vibration of free and associated –OH groups. Additionally, the prominent peaks at ∼1,625, ∼1,400, and ∼1,045 cm−1 were the vibration of –C=O stretching, the vibration of C–H bending, and the stretching vibration coupled with C–O, respectively. In addition, the absorption peaks located at 2,853 cm−1 from BPSs and at 2,925 cm−1 from RPSs were the stretching vibration of C–H. In Figure 7(b), the peaks sited at ∼3,537 and ∼3,197 cm−1 from the BPSB, at ∼3,451 and ∼3,154 cm−1 from the RPSB were the vibration of O–H stretching. Moreover, the adsorption peaks at 1,630 and 1,396 cm−1 were the stretching vibration of –C = O and the stretching vibration from C–H (Lee et al. 2021), respectively. In Figure 7(c), the peaks at 3,423 and 3,135 cm−1 were the stretching vibration of –O–H, indicating the existence of phenolic and –O–H groups. The absorption peaks sited at ∼1,635, ∼1,488, and ∼1,396 cm−1 could be interpreted as the vibration of C=C from benzene ring skeleton and the vibration of C–H bending, respectively.
After Pb(II) adsorption, changes in the absorption peaks were observed, indicating that chemical changes occurred during the Pb(II) adsorption process. The peaks at 3,437 and 3,126 cm−1 were assigned to the stretching vibration of O–H. Further, peaks near ∼1,633 and 1,399 cm−1 were the stretching vibration of –C = O and the stretching vibration of C–H. Peaks ∼ 834 and 671 cm−1 were associated with the bending vibration of σC–H (Figure 7(d)).
TGA analysis
In Figure 8, for both BPS and RPS (unmodified and modified), the change in weight loss percentage indicated the same changing trends. The weight loss percentage of PSBs could be pated into three stages. For both BPS and RPS, the weight loss decreased by 7.13% during the first stage. In the second stage, the PSs exhibited a weight loss percentage of 57.41%. This significant decrease in weight indicated that the sample underwent pyrolysis. Subsequently, in the third stage, the weight loss percentage curve gradually reached a plateau, indicating that the thermal decomposition of the PSs was basically completed and the weight loss gradually tended to be stable (the trends for Ba-PSs were similar). For Ba-PSs, due to the reduction of adsorbed water and the pyrolysis of the samples, the weight loss percentage of Ba-PSs was reduced by 8.15% in the first stage and 48.59% in the second stage, respectively.
Based on these results, it is clear that the pyrolysis temperature should exceed 400 °C (Figure 8(b)). Therefore, the starting temperature of pyrolysis was selected as 450 °C.
Determination of BET
Aperture distribution
Aperture distribution diagram of (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB.
Aperture distribution diagram of (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB.
Specific surface characteristic parameters of unmodified and modified PSB
PSB . | SSA (m2g−1) . | Pore volume (cm3 g−1) . | Pore diameter (nm) . |
---|---|---|---|
BPSB | 0.000 | 0.27350 | 3.817 |
Ba-BPSB | 789.129 | 0.28910 | 3.878 |
RPSB | 550.699 | 0.30970 | 19.118 |
Ba-RPSB | 392.847 | 0.29300 | 19.124 |
PSB . | SSA (m2g−1) . | Pore volume (cm3 g−1) . | Pore diameter (nm) . |
---|---|---|---|
BPSB | 0.000 | 0.27350 | 3.817 |
Ba-BPSB | 789.129 | 0.28910 | 3.878 |
RPSB | 550.699 | 0.30970 | 19.118 |
Ba-RPSB | 392.847 | 0.29300 | 19.124 |
As shown in Figure 9, the PS structure unmodified and modified was mesoporous, and the pore size was mainly distributed in 3.817, 19.118, 3.878, and 19.124 nm, respectively. Whether before or after modification, the pore size distribution of the BPSB was much smaller than the pore size distribution of the RPSB, so it can be speculated that the adsorption effect of BPSB will be better.
As shown in Table 3, the RPSB had larger SSA than that of the BPSB, but the BPSB exhibited a smaller pore volume and narrower pore size distribution. Consequently, the overall adsorption performance of BPSB was found to be superior to that of RPSB. Additionally, the pore volume and pore diameter of Ba-BPSB were slightly larger than those of BPSB. However, the SSA of Ba-BPSB (789.129 m2/g) was significantly greater than that of BPSB (0.000 m2/g). This suggests that the modification of Ba ions can effectively boost the SSA of the BPSB. On the contrary, the modification of Ba ions will decrease the SSA of RPSB, i.e., from 550.699 of RPSB to 392.847 m2/g of Ba-RPSB. Furthermore, comparing Ba-BPSB with Ba-RPSB, it was found that Ba-BPSB had larger SSA and smaller pore diameter, although their pore volumes were close to each other. Obviously, the surface performances of Ba-BPSB and Ba-RPSB were significant, which will produce a larger difference in Pb(II) adsorption as discussed later.
N2 adsorption–desorption isotherm
N2 isotherm curves of adsorption–desorption for (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB.
N2 isotherm curves of adsorption–desorption for (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB.
In Figure 10(a) and 10(c), type I isotherms were conformed to the adsorption–desorption curves of BPSB and Ba-BPSB, and the equilibrium was reached at the saturation pressure point (P/P0 = 1), which is consistent with the Langmuir isotherm. This finding directed that the sorption of the BPSB belonged to the monolayer process, and adsorption could be quickly saturated.
In Figure 10(b) and 10(d), the adsorption–desorption curves of RPSB and Ba-RPSB were consistent with type II isotherm, indicating that RPSB was a single multilayer reversible process. The equilibrium was reached by the loading amount of the monolayer at the inflection point of P/P0. However, with an increase in relative pressure, the second layer was gradually formed. At the saturation pressure point (P/P0 = 1), the number of adsorbed layers was infinite.
Moreover, mesoporous hysteresis rings were found in the curves of RPSB, Ba-BPSB, and Ba-RPSB. Especially, no prominent saturation adsorption platform was found in the hysteresis isotherm of RPSB and Ba-RPSB, representing that their pore structures were irregular.
Sorption of Pb(II)
Influence of initial pH
Effect of contact time
Impact of contact time on the adsorption performance of PSB for Pb(II).
As shown in Figure 12, the changing trends of the Ba-modified and unmodified PSB are similar. Meanwhile, the Qe data of the Ba-modified PSB for Pb(II) are all larger than those of the unmodified BPSB and RPSB, demonstrating that BaCl2 is an excellent BC modifier. However, the adsorption rate was different. For example, with the elapsed contact time, the loading amount of Ba-BPSB rapidly increased within 0–6 h, then, there was a slight increase in the loading amount of Pb(II), reaching its maximum value after 12 h. Similarly, the upward trend of Ba-RPSB was consistent with that of Ba-BPSB. Besides, the sorption ability of Pb(II) tended to equilibrium after 8 h, and the Qe reached the maximum value at 8 h (156.143 mg/g) and 6 h (109.051 mg/g) of BPSB and RPSB, respectively. Based on these data, the adsorption contact time was set as 14 h.
It should be emphasized that for Pb adsorption on Ba-BPSB and Ba-RPSB, there is a dramatic increase between 0–2 h and 4–6 h, and then a slight increase within 2–4 h and 6–14 h. These phenomena cannot meet the theoretical expectation. Generally, physical adsorption in biochar and GAC usually follows the fast-to-slow trend. Unfortunately, such phenomena cannot be explained in our limited knowledge. Two factors might be responsible for such trends. One is the testing errors. The other possibly is related to the molecular structure of the Ba-BPSB and BA-RPSB after modification by the modifier, resulting in the changes in adsorption behaviors of Ba-BPSB and BA-RPSB.
Influence of starting concentration
Impact of initial solution concentrations on the adsorption performance of PSB for Pb(II).
Impact of initial solution concentrations on the adsorption performance of PSB for Pb(II).
Clearly, the adsorption of Pb(II) on BPSB and RPSB was affected by the starting concentration of solution, and showed trends such as first increasing and then decreasing loading amount. Comparing the Qe of BPSB with that of RPSB, the Qe of BPSB was higher than that of RPSB at different starting concentrations. The reason can be ascribed to the difference in SSA. Similarly, starting concentration also had larger impact on the adsorption of Pb(II) on Ba-BPSB and Ba-RPSB, and the increase of loading amount was promoted with an increase in starting concentration. Further, the Qe of BPSB and Ba-BPSB was greater than that of RPSB and Ba-RPSB, respectively. These results proved that BPSB and Ba-BPSB had better effects on Pb(II) adsorption. In addition, the Qe of the Ba-modified PSB was better than that of the original BC at different starting concentrations, indicating that the absorption efficiency was significantly improved by the modification of Ba ions, i.e., the modification of BC can effectively increase its adsorption performance. As shown in Figure 13, as the starting concentration of Pb ion was equal to 400 mg/L, the loading amount of Ba-BPSB and Ba-RPSB reached the maximum values, which were 369.977 and 329.050 mg/g, respectively.
Effect of solution temperature
Effect of temperatures on the adsorption performance of PSB for Pb(II).
As expected, with the elevated solution temperature, the Qe data of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were also increased and reached the maximum value when the temperature was 55 °C, i.e., the Qe values of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were 128.050, 98.217, 379.330, and 364.910 mg/g, respectively. These results imply that the sorption of PSB for Pb(II) is an endothermic process. Moreover, the Qe data of BPSB were higher than that of RPSB, evidencing that the BPSB had an advantageous adsorption property than the RPSB. Further, the Qe of the Ba-modified PSB was significantly higher than that of the original PSB, which further proved that BaCl2 was an excellent modifier for the modification of BC. Based on these findings, the optimal adsorption temperature was selected as 55 °C for this study.
Notice that solution temperature showed a more significant impact on the Ba-BPSB and Ba-RPSB compared to BPSB and RPSB in Pb removal. Two dominating factors might be responsible for such trends. One can be ascribed to the amount of functional groups on the surface (as presented in Table 2). As the temperature is elevated, the adsorption sites will be activated and easily combined with Pb(II) ions. The other will be correlated to the surface properties (as given in Table 3). After the modification of PSB, SSA will play a key role in the adsorption of Pb(II) on these samples, resulting in an increase in the adsorption capacity.
Effect of ion strength
The influence of cations on Pb(II) loading (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB.
The influence of cations on Pb(II) loading (a) BPSB, (b) RPSB, (c) Ba-BPSB, and (d) Ba-RPSB.
In Figure 15, when 0.01 mol/L of Na+, K+, Ca2+, and Mg2+ was introduced into the mixed system, it was observed that the amount of adsorbed BPSB, RPSB, Ba-BPSB, and Ba-RPSB was highest in the presence of K+, and lowest in the presence of Ca2+. This result was also true as the ionic strength was set as 0.05 mol/L. Such phenomena demonstrated that the ion-exchange process is the main adsorption type. Typically, when the ion concentration was 0.01 and 0.05 mol/L, the impacts of cations were ranked as Ca2+ > Mg2+ > Na+ > K+, i.e., the higher the valence of cations, the larger the effect of cations on Pb(II) adsorption.
Two causes might be responsible for such trends. One cause is related to the ionic radius, which will produce a smaller steric hindrance effect. For example, the ionic radius of K+ (0.133 nm) is similar to that of Pb(II) (0.132 nm), and PSB could adsorb Pb(II) more easily (He et al. 2009). In contrast, among the divalent cations, the ionic radius of Mg2+ is smaller than that of Ca2+, so it was easier to exchange with Pb(II). The other cause can be ascribed to the ionic charge effect. Increasing the valence state of cations will result in a higher charge content per unit concentration, thereby it will increase the competitive ability of cations for negatively charged adsorption sites on the PSB surface. The sorption capacity of Pb(II) decreased as the cation (Na+, K+, Ca2+, and Mg2+) concentration was increased. Such a decreasing trend can be credited to the competitive binding between these cations and Pb(II) for available sorption sites (Jiang et al. 2021).
Adsorption mechanism
Adsorption isotherms
Currently, adsorption isotherms are primarily referred to as Langmuir and Freundlich isotherm equations, which are mainly gained from the dependence on the loading amount of an adsorbent versus equilibrium concentration. Based on these isotherm equations, the adsorption process will be easily predicted.
In which, Qm (mg/g) is the maximum loading amount calculated from isotherm equations, KL is Langmuir model constant.
The parameter comparison of non-linear fitting of adsorption isotherm models for Pb(II) adsorption on PSB
Model . | Parameter . | PSB . | |||
---|---|---|---|---|---|
BPSB . | RPSB . | Ba-BPSB . | Ba-RPSB . | ||
Langmuir | Qm | 123.178 | 95.516 | 281.892 | 269.316 |
KL | −7.47 × 1044 | −1.64 × 1044 | −8.26 × 1045 | −4.02 × 1038 | |
R2 | 0.997 | 0.998 | 0.748 | 0.805 | |
Freundlich | KF | 145.982 | 113.124 | 182.814 | 188.269 |
n | −0.033 | −0.032 | 0.187 | 0.133 | |
R2 | 0.998 | 0.999 | 0.978 | 0.991 |
Model . | Parameter . | PSB . | |||
---|---|---|---|---|---|
BPSB . | RPSB . | Ba-BPSB . | Ba-RPSB . | ||
Langmuir | Qm | 123.178 | 95.516 | 281.892 | 269.316 |
KL | −7.47 × 1044 | −1.64 × 1044 | −8.26 × 1045 | −4.02 × 1038 | |
R2 | 0.997 | 0.998 | 0.748 | 0.805 | |
Freundlich | KF | 145.982 | 113.124 | 182.814 | 188.269 |
n | −0.033 | −0.032 | 0.187 | 0.133 | |
R2 | 0.998 | 0.999 | 0.978 | 0.991 |
Non-linear fitting of isotherm model for Pb(II) adsorption on PSB (a) Langmuir and (b) Freundlich.
Non-linear fitting of isotherm model for Pb(II) adsorption on PSB (a) Langmuir and (b) Freundlich.
When a non-linear fitting was made, the correlation coefficient (R2) of BPSB, RPSB, Ba-BPSB, and Ba-RPSB fitted by the Freundlich model was larger than those fitted by the Langmuir model, and the R2 values were more remarkable than 0.978. Hence, it could be concluded that Pb(II) adsorption on BPSB, RPSB, Ba-BPSB, and Ba-RPSB followed the Freundlich model, and such an adsorption was a multimolecular layers process rather than monolayer adsorption.
Adsorption kinetics
Adsorption kinetics studies the adsorption rate of metal ions on an adsorbent, which can be obtained based on the relationship of the loading amount of metal ions against contact time. At present, kinetic equations chiefly include quasi-first and quasi-secondary kinetic models (labeled as Q-F and Q-S, respectively), intraparticle diffusion (I-P-D), and Elovich model (E-M).
Quasi-first and quasi-secondary kinetic models
Q-F, Q-S, I-P-D, and E-M parameters of PSB for Pb(II) adsorption
Model . | Parameter . | PSB . | |||
---|---|---|---|---|---|
BPSB . | RPSB . | Ba-BPSB . | Ba-RPSB . | ||
Q-F | k1 | 1.190 | 1.773 | 0.545 | 0.651 |
qcal (mg/g) | 148.800 | 104.168 | 329.177 | 266.799 | |
R2 | 0.986 | 0.969 | 0.875 | 0.925 | |
Q-S | k2 | 0.011 | 0.031 | 0.002 | 0.003 |
qcal (mg/g) | 159.815 | 108.801 | 374.477 | 301.062 | |
qexp (mg/g) | 156.143 | 109.051 | 355.438 | 267.635 | |
R2 | 0.995 | 0.984 | 0.917 | 0.952 | |
I-P-D | kp | 33.716 | 20.352 | 87.974 | 70.150 |
xi | 49.487 | 45.319 | 56.605 | 52.266 | |
R2 | 0.759 | 0.609 | 0.908 | 0.879 | |
E-M | α | 751.245 | 11460.821 | 292.883 | 261.278 |
β | 0.045 | 0.097 | 0.014 | 0.018 | |
R2 | 0.978 | 0.970 | 0.940 | 0.957 |
Model . | Parameter . | PSB . | |||
---|---|---|---|---|---|
BPSB . | RPSB . | Ba-BPSB . | Ba-RPSB . | ||
Q-F | k1 | 1.190 | 1.773 | 0.545 | 0.651 |
qcal (mg/g) | 148.800 | 104.168 | 329.177 | 266.799 | |
R2 | 0.986 | 0.969 | 0.875 | 0.925 | |
Q-S | k2 | 0.011 | 0.031 | 0.002 | 0.003 |
qcal (mg/g) | 159.815 | 108.801 | 374.477 | 301.062 | |
qexp (mg/g) | 156.143 | 109.051 | 355.438 | 267.635 | |
R2 | 0.995 | 0.984 | 0.917 | 0.952 | |
I-P-D | kp | 33.716 | 20.352 | 87.974 | 70.150 |
xi | 49.487 | 45.319 | 56.605 | 52.266 | |
R2 | 0.759 | 0.609 | 0.908 | 0.879 | |
E-M | α | 751.245 | 11460.821 | 292.883 | 261.278 |
β | 0.045 | 0.097 | 0.014 | 0.018 | |
R2 | 0.978 | 0.970 | 0.940 | 0.957 |
Non-linear fitting of quasi-kinetic models for Pb(II) adsorption (a) Q-F and (b) Q-S.
Non-linear fitting of quasi-kinetic models for Pb(II) adsorption (a) Q-F and (b) Q-S.
Noticeably, the R2 of Q-S parameters were more significant than those of Q-F, while the qcal data obtained from Q-S all approached the qexp values. Consequently, it can be reasoned that Pb(II) sorption on PSB abided by the Q-S, and such an adsorption was the chemical process.
Intraparticle diffusion
In which, kp was the internal diffusion rate constant, and xi was the constant associated with the thickness of the boundary layer.
As listed in Table 5, the xi value of these PSB was highly larger than 0, signifying that intraparticle diffusion was not the sole determining factor and other adsorption mechanisms exist in the adsorption of BPSB, RPSB, Ba-BPSB, and Ba-RPSB for Pb(II).
Elovich model
Obviously, the R2 of BPSB, RPSB, Ba-BPSB, and Ba-RPSB for Pb(II) was more than 0.940, indicating that they had an excellent fitting degree. These results revealed that Pb(II) adsorption on BPSB, RPSB, Ba-BPSB, and Ba-RPSB followed the E-M and was governed by heterogeneous adsorption processes.
Thermodynamic parameters
Adsorption thermodynamic model parameters of Pb(II) adsorption on PSB
PSB . | T (°C) . | ΔG (kJ/mol) . | ΔS (J mol−1K−1) . | ΔH (kJ mol−1 K−1) . | R2 . |
---|---|---|---|---|---|
BPSB | 25.0 | −14.450 | 76.495 | 8.298 | 0.946 |
35.0 | −15.392 | ||||
45.0 | −15.994 | ||||
55.0 | −16.794 | ||||
RPSB | 25.0 | −13.952 | 61.432 | 4.403 | 0.946 |
35.0 | −14.472 | ||||
45.0 | −15.132 | ||||
55.0 | −15.786 | ||||
Ba-BPSB | 25.0 | −20.180 | 217.725 | 44.397 | 0.957 |
35.0 | −23.210 | ||||
45.0 | −24.937 | ||||
55.0 | −26.818 | ||||
Ba-RPSB | 25.0 | −19.076 | 195.49 | 38.336 | 0.898 |
35.0 | −21.281 | ||||
45.0 | −22.117 | ||||
55.0 | −25.254 |
PSB . | T (°C) . | ΔG (kJ/mol) . | ΔS (J mol−1K−1) . | ΔH (kJ mol−1 K−1) . | R2 . |
---|---|---|---|---|---|
BPSB | 25.0 | −14.450 | 76.495 | 8.298 | 0.946 |
35.0 | −15.392 | ||||
45.0 | −15.994 | ||||
55.0 | −16.794 | ||||
RPSB | 25.0 | −13.952 | 61.432 | 4.403 | 0.946 |
35.0 | −14.472 | ||||
45.0 | −15.132 | ||||
55.0 | −15.786 | ||||
Ba-BPSB | 25.0 | −20.180 | 217.725 | 44.397 | 0.957 |
35.0 | −23.210 | ||||
45.0 | −24.937 | ||||
55.0 | −26.818 | ||||
Ba-RPSB | 25.0 | −19.076 | 195.49 | 38.336 | 0.898 |
35.0 | −21.281 | ||||
45.0 | −22.117 | ||||
55.0 | −25.254 |
In Table 6, the ΔG data of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were all negative, suggesting that the Pb(II) adsorption on BPSB, RPSB, Ba-BPSB, and Ba-RPSB was a spontaneous process. Besides, the ΔS values of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were all positive, signifying that the Pb(II) adsorption on BPSB, RPSB, Ba-BPSB, and Ba-RPSB was an entropy-increasing process, and the adsorption reaction was irreversible. Moreover, the ΔH values of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were all positive, proving that the Pb(II) adsorption on the unmodified and modified PSBs was an endothermic process. Hence, an increase in solution temperature was helpful for the Pb(II) adsorption on BPSB, RPSB, Ba-BPSB, and Ba-RPSB. This discovery is consistent with the finding obtained from the relationship of Qe against solution temperature (as illustrated in Figure 14).
A comparison of loading amount with those reported in references
A comparison of Qe of various BC and modified BC for Pb(II) in references is shown in Table 7. Compared with the results reported in the references, the adsorption of Ba-BPSB for Pb(II) was higher than those of biosorbent, demonstrating that the Ba-modification of the BPSB used as a biochar adsorbent had excellent effectiveness in Pb(II) removal from water.
A comparison of loading amount of various BCs and modified BC for Pb(II)
Biosorbent . | Pyrolysis temperature (°C) . | Original BC . | Modifier . | Qe (mg/g) . | Sorption temperature (°C) . | Reference . |
---|---|---|---|---|---|---|
HP-BC | 350 | Watermelon seeds biochar | H2O2 | 60.87 | 25 | Ahmed et al. (2021) |
MWSC | 700 | Walnut shell biochar | KMnO4 | 70.37 | 25 | Chen et al. (2022) |
HNC | 300 | Wheat straw biochar | KOH, HNO3, and ammonia solution | 161.29 | 35 | Wang et al. (2022) |
HMBC | 400 | PSB | MnCl2⋅4H2O and H2O2–NaOH | 164.59 | 25 | Liu et al. (2022) |
Ba-BPSB | 750 | BPSB | BaCl2 | 379.33 | 55 | This job |
Biosorbent . | Pyrolysis temperature (°C) . | Original BC . | Modifier . | Qe (mg/g) . | Sorption temperature (°C) . | Reference . |
---|---|---|---|---|---|---|
HP-BC | 350 | Watermelon seeds biochar | H2O2 | 60.87 | 25 | Ahmed et al. (2021) |
MWSC | 700 | Walnut shell biochar | KMnO4 | 70.37 | 25 | Chen et al. (2022) |
HNC | 300 | Wheat straw biochar | KOH, HNO3, and ammonia solution | 161.29 | 35 | Wang et al. (2022) |
HMBC | 400 | PSB | MnCl2⋅4H2O and H2O2–NaOH | 164.59 | 25 | Liu et al. (2022) |
Ba-BPSB | 750 | BPSB | BaCl2 | 379.33 | 55 | This job |
CONCLUSION
A series of PSB, RPSB, Ba-BPSB, and Ba-RPSB were prepared from BPSs and discarded common RPSs, and their adsorptions for Pb(II) in an aqueous solution were surveyed. The following results were achieved.
- (1)
The optimal sorption condition for Pb(II) on BPSB, RPSB, Ba-BPSB, and Ba-RPSB was as follows: starting concentration was 400 mg/L, time was around 14 h, and temperature was 55 °C. Based on such conditions, the Qe values of BPSB, RPSB, Ba-BPSB, and Ba-RPSB were 128.050, 98.217, 379.330, and 364.910 mg/g, respectively. These findings suggested that the modified PSB was a favorable bioadsorbent for the elimination of Pb(II) from water.
- (2)
The influence of –COOH and –OH groups from PSB on Pb(II) adsorption disclosed that chemical blocking of –COOH and –OH groups from BPSB, RPSB, Ba-BPSB, and Ba-RPSB could reduce the sorption of Pb(II) by 81.81, 77.08, 86.90, and 83.65%, as well as 17.61, 21.70, 12.77, and 15.06%, respectively. These data approved the involvement of –COOH and –OH groups in Pb(II) adsorption.
- (3)
The rise in ionic strength (Na+, K+, Ca2+, and Mg2+) led to a decline in the adsorption of Pb(II) on PSB, further evidencing that the adsorption of Pb(II) on PSB is an ion-exchange process.
- (4)
Adsorption models revealed that the adsorption of Pb(II) onto BPSB, RPSB, Ba-BPSB, and Ba-RPSB followed Freundlich and quasi-second-order kinetic models.
- (5)
Thermodynamic calculation demonstrated that the adsorption of Pb(II) onto BPSB, RPSB, Ba-BPSB, and Ba-RPSB was an exothermic and spontaneous process.
- (6)
Note that only Pb(II) adsorption on PSB, RPSB, Ba-BPSB, and Ba-RPSB was investigated in this study. Whereas in industrial processes, mixed multiple ions systems may be more real. In this case, competition among metal ions will appear and impact their adsorption performances. Therefore, the adsorption of metal ions on PSB, RPSB, Ba-BPSB, and Ba-RPSB in mixed metal ions solution will be the next job.
ACKNOWLEDGEMENTS
We thank Si'xian Shengli Grain and Oil Planting Family Farm (Anhui, China) for providing black peanuts used as raw biochar materials in the experiment.
FUNDING
The authors did not receive support from any organization for the submitted work.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.