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Optimized Ca-Al-La modified biochar with rapid and efficient phosphate removal performance and excellent pH stability
⁎Corresponding author at: Chongqing Three Gorges University, China. chengfl731566250@163.com (Fulong Cheng)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Biochar-based adsorbents for phosphate removal from wastewater has gained increasing attention and achieved significant progress. However, there is still much room for the enhancement on adsorption performance in terms of removal rate, adsorption capacity and pH application range. Herein, novel Ca-Al-La modified biochar adsorbents in this study were synthesized for enhancing phosphate adsorption in wastewater. The synthesis parameters of adsorbents were optimized by using the response surface methodology (RSM). The Ca-Al-La modified biochar adsorbent synthesized under the optimal conditions (CAL-MBC) exhibited superior adsorption capability towards phosphate. Specifically, CAL-MBC achieved a rapid removal rate to reach the adsorption equilibrium within 1 h, a maximum phosphate adsorption capacity of 152.9 mg/g, and excellent pH stability within a broad pH range of 3.0–11.0. The adsorption mechanisms involving electrostatic interaction, precipitation and inner-sphere complexation via ligand exchange were responsible for the excellent adsorption performance of phosphate on CAL-MBC. This study verifies CAL-MBC as a promising metal modified biochar adsorbent to enhance phosphate removal from wastewater.
Keywords
Ca-Al-La
Biochar
RSM
Phosphate
Adsorption
1 Introduction
Phosphorus (P), an indispensable nutrient in the growth of aquatic organisms, is normally present in the aquatic environment as dissolved phosphate form. Phosphate in effluents is discharged from various sources such as domestic, industrial, and agricultural activities (Liu et al., 2021). However, the inappropriate discharge of phosphate-containing wastewater into freshwater resources will cause eutrophication, which in turn induce seriously environmental problems (Asaoka et al., 2021). Thus, effective control of phosphate concentration in wastewater prior to discharge is essential to suppress eutrophication. Various technologies, such as biological processes, chemical precipitation, ion exchange, electrodialysis, and adsorption, have been used to capture phosphate in wastewater (Palansooriya et al., 2021). Adsorption has been regarded as one of the most desirable technologies for the removal of phosphate from wastewater because of its ease of operation, recyclability, high adsorption capacity and cost-effectiveness (Hao et al., 2019). The development of a cost-effective adsorbent material with outstanding adsorption capability is the key to the application of this technology (Du et al., 2022).
Biochar is a kind of carbon material obtained by pyrolysis of biomass under anoxic or anaerobic conditions. Among many recently investigated adsorbents, such as activated carbon (Nazarian et al., 2021), bentonite (Olu-Owolabi and Unuabonah, 2011), kaolin (Deng and Shi, 2015), zeolite (Yang et al., 2021b), calcite (Lei et al., 2021), etc., biochar has attracted much attention over the past few years due to its porous carbon structure, high specific surface area, and abundant surface functional groups. To date, biochar has been widely used as an excellent adsorbent for the removal of heavy metals and organic pollutants from wastewater (Wang and Wang, 2019). Recently, the utilize of biochar to remove phosphate from wastewater has been proved to be a promising method (Zhang et al., 2020). However, the surface of biochar was enriched with functional groups containing oxygen, nitrogen and sulfur, resulting in a negatively charged surface. Consequently, the binding affinity of biochar to phosphate was greatly limited due to the presence of electrostatic repulsion, which lead to the undesirable adsorption capacity of phosphate (Peng et al., 2019). In recent years, biochar modification, especially metal modification, has been widely employed to enhance the phosphate adsorption capability of biochar (Shyam et al., 2022).
Among the available metal elements, calcium (Ca) is an effective metal for biochar modification due to its wide availability and non-toxicity (Yang et al., 2021a). For example, Ca modified biochar adsorbents presented high phosphate adsorption capacities in the range of 109.7–136.8 mg/g (Cao et al., 2020), (Wang et al., 2021b). However, the phosphate uptake of Ca modified biochar was greatly affected by solution pH, which tended to show high adsorption capacity in the alkaline environment while low adsorption capacity in the acidic environment. Lanthanum (La) is an environmentally friendly rare earth element with stable chemical properties and strong affinity for phosphate (Zhi et al., 2020). Nowadays, the utilize of La modified biochar for phosphate removal from wastewater has been widely reported, such as La modified biochar (Li et al., 2020), La(OH)3 modified biochar (Li et al., 2022), lanthanum carbonate modified biochar (Huang et al., 2022), lanthanum carbonate hydroxide modified biochar (Lan et al., 2022). Although La modified biochar showed excellent adsorption capacity for phosphate, its adsorption ability was significantly inhibited in alkaline environment. Recently, Ca-La bimetal modified biochar (BC-La4) was reported to show stable adsorption performance in a wide pH range (3.0–6.0 and 8.0–11.0) (Feng et al., 2021), but BC-La4 suffered from a slow removal rate with the adsorption equilibrium time over 12 h. Moreover, the use of excess La to modify biochar was not cost-effective. Aluminum (Al), the most abundant metallic element in the crust of earth, is cost-effective and environmentally friendly. (Wang et al., 2021a). Al modified biochar has also been extensively used to eliminate phosphate from water bodies (Zheng et al., 2020). The adsorption of phosphate on the Al modified biochar could reach equilibrium within 6 h (Zhang and Gao, 2013). However, Al modified biochar exhibited a relatively low maximum phosphate adsorption capacity of 57.49 mg/g. In addition, the phosphate adsorption capacity of biochar was significantly inhibited in the alkaline environment (Yin et al., 2018). Based on the above analysis, the introduction of La could compensate for the low adsorption ability of Ca-modified biochar under acidic conditions, while the partial substitution of La by Al was aimed at improving the phosphate adsorption rate and minimizing the cost. Thus, it is speculated that Ca-Al-La modified biochar may be a promising phosphate adsorbent with high adsorption capacity, rapid adsorption rate, strong pH adaptability, and low cost. However, few studies have been performed to optimize the synthesis process of Ca-Al-La modified biochar and its phosphate adsorption characteristics and mechanism.
In this work, Ca-Al-La modified biochar adsorbents were synthesized for phosphate removal from wastewater. Firstly, response surface methodology (RSM) were employed to optimize the synthesis parameters of Ca-Al-La modified biochar, and the obtained Ca-Al-La modified biochar under the optimum synthesis conditions (CAL-MBC) was selected for further adsorption experiments and characterization tests. Subsequently, the effects of various factors such as reaction time, initial phosphate concentration, pH and coexisting anions on the phosphate adsorption capacity of CAL-MBC were analyzed. Furthermore, the actual wastewater adsorption test and the cyclic adsorption–desorption experiment were performed to evaluate the removal efficiency in actual wastewater and the recyclability of CAL-MBC. Lastly, the adsorption mechanism of CAL-MBC was analyzed in detail based on the results of adsorption experiments and various characterization analysis.
2 Experimental section
2.1 Materials
Sawdust feedstock, locally sourced in Chongqing, China, was used for biochar production. Analytical grade calcium chloride (CaCl2), aluminum chloride hexahydrate (AlCl3⋅6H2O), lanthanum chloride heptahydrate (LaCl3⋅7H2O), potassium phosphate monobasic (KH2PO4), sodium hydroxide (NaOH), and sodium carbonate (Na2CO3) were purchased from Aladdin reagent company. The conductivity of the deionized water used for experiments was below 14 MΩ/cm.
2.2 Preparation of Ca-Al-La modified biochar
Firstly, biochar was prepared by the pyrolysis method (Pongkua et al., 2018). Briefly, the sawdust was washed by deionized water, dried at 110 ℃, and ground and passed through a nylon sieve (<0.45 mm). The obtain fine sawdust powder was transferred into a sealed crucible, and then placed in a muffle furnace at 500 ℃ (heating rate of 10 ℃/min) for pyrolysis of 3 h. The resulting black solid was subsequently ground and passed through a nylon sieve (<0.25 mm). The obtained biochar was labeled as MBC.
Secondly, Ca-Al-La modified biochar was prepared via a simple precipitation method. Before starting experiments, the effect of Ca2+/M3+ (where M represents Al and La, and the Al/La molar ratio is fixed to 1) molar ratios (2/1, 3/1, and 4/1) on the phosphate adsorption capacity was examined. As shown in Fig. S1, the optimal phosphate adsorption performance was obtained at the Ca2+/M3+ molar ratio of 3/1. Thus, the Ca2+/M3+ molar ratio of 3/1 was selected for the next experiment. In addition, the metal nanoparticles loaded on the surface of the biochar appeared agglomerated, and some of the metal components were not loaded onto the biochar surface when the amount of biochar was 0.75 g, as shown in Fig. S2. Thus, we chose to investigate the effect of biochar amount on phosphate adsorption capacity in the range of 1 g to 4 g. In brief, a certain amount of CaCl2, AlCl3·6H2O and LaCl3·7H2O were dissolved in 100 mL of deionized water to obtain solution A with a concentration of 0.08 M of total metal ions. The Ca2+ concentration in solution A was fixed at 0.06 mol/L, while the concentrations of Al3+ and La3+ were designed to be variable. In addition, 100 mL of alkaline solution B containing NaOH (1 M) and Na2CO3 (1 M) was prepared. Then, the designed amount of MBC was added to solution A and dispersed well under magnetic stirring at 150 r/min. Subsequently, the alkaline solution B was added dropwise to the solution A until the designed pH was reached. The resulting suspension was continued to be stirred for 12 h. After filtration, washing, and drying at 60 ℃ overnight, the final Ca-Al-La modified biochar was obtained.
2.3 Optimization of synthesis parameters
The response surface methodology (RSM) was utilized for the optimization the synthesis process of the Ca-Al-La modified biochar with three variates, including the molar ratio of La to Al, mass of biochar, and precipitation pH. According to the Box-Behnken design, seventeen Ca-Al-La modified biochar adsorbents were synthesized for the adsorption of 100 mg/L phosphate solution at a dose of 1 g/L. Then, the above data were fitted using the second-order model equation (Eq. S1). This mathematical model could obtain the optimum conditions, and the adsorbent prepared with optimum parameters labeled as CAL-MBC was used for subsequent adsorption experiments and characterization tests.
2.4 Characterization
The crystallographic information of as-prepared materials was detected by a X-ray diffractometer (XRD, Rigaku Ultima IV, Japan). The surface morphologies of as-prepared materials were performed by the scanning electron microscope (SEM, Thermo Scientific Apreo 2C, USA) coupled with an energy dispersive spectroscopy (EDS, Oxford Ultim Max 65, UK) detector. The surface functional groups on CAL-MBC before and after phosphate adsorption were examined by the Fourier transform infrared spectrophotometer (FTIR, Thermo Fisher Nicolet Is10, USA). The surface chemistry of CAL-MBC before and after phosphate adsorption were examined by the X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab Xi+, USA) with Al Kα radiation. The values of pH at the isoelectric point (pHPZC) of CAL-MBC before and after phosphate adsorption were measured by a pH shift method reported in the previous literature (Pashai Gatabi et al., 2016).
2.5 Adsorption experiments
All batch adsorption tests were stirred on a thermostatic shaker (SHA-C, China) at 25 ℃ with a rate of 150 rpm. The adsorption experiments were carried out at pH of 7.0 except for the pH effect experiments. Batch adsorption tests were performed with 50 mL polypropylene tubes. All the adsorption experiments were conducted in triplicates and the average value was taken. The phosphate concentration was determined by the ammonium molybdate spectrophotometric method via a spectrophotometer (UV-2550, Japan). The adsorption capacity (qe) and the removal efficiency at equilibrium (R) were calculated by the Eq. S2 and Eq. S3, respectively.
2.5.1 Adsorption kinetics
Adsorption kinetics experiments were performed at three phosphate concentrations (20, 50 and 100 mg/L). 0.5 g of adsorbent was added into the phosphate solution (500 mL). Subsequently, 2 mL of the solution was taken at the set time for phosphate concentration measurement. The obtained experimental data were then fitted by three kinetic adsorption model equations, including pseudo-first-order equation (Eq. S4), pseudo-second-order equation (Eq. S5), and intra-particular diffusion equation (Eq. S6) (Liu et al., 2022).
2.5.2 Adsorption isotherm
The phosphate adsorption isotherm was performed at a dose of 1 g/L in the phosphate concentration range of 20–300 mg/L. Adsorption temperature and adsorption time were set to 25 °C and 24 h, respectively. The obtained adsorption data were fitted by four isothermal adsorption equations, including Langmuir equation (Eq. S7), Freundlich equation (Eq. S8), Temkin equation (Eq. S9), and Koble-Corrigan equation (Eq. S10) (Yang et al., 2021b).
2.5.3 Effect of pH and coexisting ions
The effect of pH on phosphate adsorption capacity was studied by adding 25 mg adsorbent into 25 mL of phosphate solution (20 mg/L) at pH 3.0–11.0. The pH of solution was adjusted by 0.1 M NaOH solution and 0.1 M HCl solution solution. 50 mg/L, 100 mg/L and 200 mg/L of Cl-, NO3–, HCO3– and SO42- were added into phosphate solution with the concentration of 50 mg/L to examine the influence of coexisting ions.
2.5.4 Actual wastewater adsorption test
The actual wastewater was collected from a municipal drainage ditch in Chongqing, China. CAL-MBC with different doses (0.5–1.5 g/L) were added into 25 mL of actual sewage for phosphate adsorption at 25 ℃ for 2 h. Prior to being treated, raw sewage was filtered through 0.45 μm membrane to eliminate the influence caused by particulate matter.
2.5.5 Desorption and reusability test
After adsorption of 25 mg CAL-MBC added into 25 mL phosphate solution for 24 h, the spent CAL-MBC absorbent was immersed in the 25 mL of mixture of NaOH (1 M) and Na2CO3 (1 M). The effect of desorption time on the phosphate desorption efficiency (calculated by Eq. S11) of CAL-MBC was tested to obtain the optimal desorption time. The regeneration of the spent adsorbent was performed by washing with deionized water and then dried at 60 ℃ for 24 h. In total, five adsorption–desorption cyclic experiments were performed.
3 Results and discussions
3.1 Optimization of synthesis process
RSM was employed to optimize the synthesis parameters of the Ca-Al-La modified biochar with three variates including molar ratio of La to Al, mass of biochar, and precipitation pH. The fitted quadratic model equation was shown in the following equation.
(a) Plots of predicted values versus actual values for phosphate uptake; (b-d) the three-dimensional response plot.
The analysis of variance (ANOVA) for the quadratic model was displayed in Table 1. The p-values of A (<0.0001) and B (<0.0001) were<0.05, which indicated that the molar ratio of La to Al and the mass of biochar had a significant effect on phosphate adsorption. However, the p-value of C (0.8650) was more than 0.05, thereby indicating that the precipitation pH was lack of significance. The coefficient A in Eq. (1) was above zero, indicating that the molar ratio of La to Al had a positive effect on phosphate adsorption. In contrast, the coefficient B in Eq. (1) was less than zero, suggesting that the mass of biochar had a negative influence on phosphate adsorption. (Kang et al., 2021). As shown in 1(b-d), the optimal molar ratio of La to Al was close to 0.60, while the optimal mass of biochar was close to the minimum mass of biochar (1 g). According to this quadratic model, the maximum predicted phosphate uptake of 80.9 mg/g was observed for the following optimal synthesis conditions: molar ratio of La to Al of 0.62, mass of biochar of 1 g, and precipitation pH of 11.0. To verify the accuracy of the model, the actual phosphate uptake of the adsorbent prepared under the above optimized conditions was 80.5 mg/g, which was very close to the value of 80.9 mg/g. Therefore, the Ca-Al-La modified biochar adsorbent synthesized under the optimal conditions (CAL-MBC) was selected for the following adsorption experiments and characterization tests.
Source
Sum of Squares
df
Mean Square
F-value
p-value
Model
4562.93
9
506.99
104.29
< 0.0001
A
935.28
1
935.28
192.39
< 0.0001
B
1740.50
1
1740.50
358.03
< 0.0001
C
0.15
1
0.15
0.03
0.8650
AB
4.62
1
4.62
0.95
0.3620
AC
0.81
1
0.81
0.17
0.6953
BC
0.30
1
0.30
0.06
0.8102
A2
1753.11
1
1753.11
360.62
< 0.0001
B2
14.88
1
14.88
3.06
0.1237
C2
48.82
1
48.82
10.04
0.0157
Residual
34.03
7
4.86
Lack of fit
18.94
3
6.31
1.67
0.3087
Pure error
15.09
4
3.77
Cor. Total
4596.96
16
3.2 Characterization
The XRD patterns of MBC and CAL-MBC were shown in Fig. 2. MBC exhibited a broad diffraction peak in the 2θ range of 15-35°, indicating an amorphous structure of the biochar (Missau et al., 2021). In addition, characteristic diffraction peaks of SiO2 and CaCO3 were observed on MBC, which was similar to the phenomena observed in other literature (Nzediegwu et al., 2021). The peak shape of CAL-MBC was significantly changed compared to MBC. The peaks at 2θ of 23.02°, 23.39°, 35.94°, 39.37°, 43.10°, 47.57° and 48.46° belonged to CaCO3, while the peaks at 2θ of 18.46° and 20.46° corresponded to Al(OH)3. However, La-containing phases were not observed on CAL-MBC, suggesting that the La species might be in the amorphous form, which favored its proximity to phosphate (Li et al., 2017). The SEM images of MBC and CAL-MBC were shown in Fig. 3. MBC presented irregular lamellar structures with rough and uneven surface (Fig. 3(a)), which contributed to the surface loading of metal species (Li et al., 2021). The surface of CAL-MBC consisted of a large number of dispersed nanoparticles (Fig. 3(b-d)), indicating the reduction of metal nanoparticle agglomeration by biochar as a carrier. Furthermore, the EDS results demonstrated that Ca, Al and La were well dispersed on the biochar surface, as shown in Fig. 3(e-g). Therefore, it could be speculated that a large number of metal nanoparticles uniformly loaded on biochar might provide abundant active sites to facilitate phosphate adsorption.
XRD patterns of MB, CAL-MBC and P-loaded CAL-MBC.
SEM images: (a) MBC; (b-d) CAL-MBC, and (e-f) EDS elemental distribution maps of CAL-MBC.
3.3 Adsorption performance
3.3.1 Adsorption kinetics
The experimental data of phosphate adsorption kinetics of CAL-MBC were shown in Fig. 4(a). CAL-MBC achieved 80% of ultimate adsorption capacity within 30 min at initial phosphate concentrations of 20, 50 and 100 mg/L. This rapid adsorption rate was likely due to the uniform dispersion of metal nanoparticles on the surface of biochar, which offered abundant accessible active adsorption sites for phosphate. Notably, the actual adsorption equilibrium time of CAL-MBC was approximately 1 h, which exhibited superior or comparable adsorption rates compared to most recently reported metal-modified biochar adsorbents under similar experimental conditions, as listed in Table 2. Thus, CAL-MBC with rapid adsorption ability had the potential to perform well for phosphate removal in practical wastewater treatment.
The adsorption kinetics curves and intra-particle diffusion model fitting curves of CAL-MBC for phosphate.
Adsorbents
Temperature
(℃)Dose
(g/L)phosphate concentration (mg/L)
Equilibrium time
(h)Ref.
PB-800
25
1
20
12
(Yu et al., 2022)
CBC-La
25
1.7
20
8
(Liu et al., 2022)
BC@Ca + Zn
25
1
50
12
(Chen et al., 2022)
LN-WB
25
1
10, 20, 50
24–48
(Luo et al., 2021b)
RB/MgAl2O4@CL+
25
1
25, 50
10
(Shan et al., 2022)
La/Fe-NBC
25
0.5
50
5
(Lan et al., 2022)
MgO/KBC
20
1
50
12
(Luo et al., 2021a)
BC-Ca5/BC-La4
25
1
50
4
(Feng et al., 2021)
La-TC
25
0.4
10, 20, 30
0.5–3
(Jia et al., 2020)
Mar-BC800
25
0.3
100
3
(Deng et al., 2021)
CS-E0.25-Ca
25
0.3
75
3
(Wang et al., 2021b)
Ce-MSB
25
2
300
1.5
(Feng et al., 2017)
LHB800
25
0.4
50
1
(Huang et al., 2022)
Ce@NLC
25
2
200
0.5
(Jiao et al., 2021b)
CAL-MBC
25
1
20, 50, 100
1
This study
The pseudo-first-order model and pseudo-second-order model were used to fit the kinetic experimental data, as shown in Fig. 4(a). Table 3 summarized the parameters fitted by these two kinetic models. The correlation coefficients of the pseudo-second-order model (R2 = 0.961–0.986) were higher than those of the pseudo-first-order model (R2 = 0.858–0.885) at phosphate concentrations of 20, 50 and 100 mg/L. Furthermore, the adsorption capacities obtained from the pseudo-second-order model were closer to the experimental values than those from the pseudo-first-order model. The above results showed that the pseudo-second-order model can fit the kinetic data well, indicating that chemisorption dominated the phosphate adsorption process of CAL-MBC (Cheng et al., 2022).
Concentrations
(mg/L)Pseudo-first-order model
Pseudo-second-order model
qe
k1
R2
qe
k2
R2
20
19.6
0.782
0.885
19.9
0.056
0.986
50
45.9
0.751
0.858
48.0
0.011
0.946
100
89.1
0.255
0.873
92.6
0.003
0.961
The controlling steps in the adsorption process were further investigated by using the intra-particle diffusion model. As shown in Fig. 4(b), there were three separated lines for each data, representing three adsorption stages (Wu et al., 2009). The first stage was related to the adsorption on the external surface of adsorbent. The second stage was attributed to the internal adsorption of phosphate by diffusion into the pores of the adsorbent. The third stage was the process of gradually approaching adsorption equilibrium. As displayed in Table S3, the values of kid gradually decreased from 2.886, 5.249, and 12.075 to 0.001, 0.041, and 0.081 for 20, 50, and 100 mg/L, respectively, as the adsorption process proceeded. The larger values of kid1 at all thee concentrations indicated that the intra-particle diffusion was the dominant process during the first adsorption stage of CAL-MBC (Qiu et al., 2017). However, the values of a for the three concentrations were far away from zero, indicating that the intra-particle diffusion was not the only control step (Koh et al., 2020a),(Yang et al., 2020).
3.3.2 Adsorption isotherms
Phosphate adsorption isotherm curves of CAL-MBC were shown in Fig. 5(a). It was observed that the adsorption capacity increased with the increase of phosphate concentration. This was attributed to the stronger mass transfer propulsion at higher concentrations. Four adsorption isotherm models including Langmuir, Freundlich, Temkin, and Koble-Corrigan models were applied to fit the experimental data. According to the fitting correlation coefficient (R2) in Table 4, Langmuir, Freundlich, and Koble-Corrigan models (R2 greater than 0.946) better fitted the adsorption process than Temkin model (R2 = 0.902). The maximum phosphate adsorption capacity fitted by the Langmuir model was calculated to 152.9 mg/g, which was higher than that of most reported metal modified biochar adsorbents as shown in Fig. 5(b). Freundlich model with high value of R2 (0.988) indicated that the Freundlich model could well describe the kinetic phosphate adsorption process of CAL-MBC. This result implied that the adsorption process of CAL-MBC for phosphate might involve heterogeneous surface adsorption (Nuryadin et al., 2021). Furthermore, the parameter of 1/n (0.313) in Freundlich model located in the range of 0.1–0.5, which indicated that CAL-MBC had strong affinity for phosphate and the adsorption process was easy to occur (Yang et al., 2021b). In addition, the value of m in Koble-Corrigan model was 0.403, indicating that the adsorption process had characteristics of both Langmuir and Freundlich models (Han et al., 2018). Based on the above analysis, phosphate adsorption on CAL-MBC tended to be monolayer adsorption on heterogeneous surface, which was consistent with previous studies (Jiao et al., 2021a).
(a) Phosphate adsorption isotherm curves of CAL-MBC; (b) Comparison between the maximum adsorption capacity of CAL-MBC towards phosphate with those recently reported metal modified biochar adsorbents.
Langmuir model
Freundlich model
qmax
kl
R2
kf
1/n
R2
152.9
0.078
0.946
32.273
0.313
0.988
Temkin model
Koble-Corrigan model
B
kt
R2
A
D
m
R2
30.797
15.603
0.902
32.618
−0.083
0.403
0.989
3.3.3 Effect of pH
The variation of phosphate removal efficiency with solution pH was shown in Fig. 6(a). The removal efficiency of CAL-MBC for phosphate exceeded 90.8% in the pH range of 3.0–10.0. Even further increasing the pH to 11.0, the removal efficiency of phosphate on CAL-MBC still reached 86.4%. The isoelectric point (pHPZC) of CAL-MBC was determined as 8.1 (Fig. 6(b)). This meant that the positive surface charge of CAL-MBC would be electrostatically attracted to phosphate when the pH was below 8.1, while the negative surface charge would facilitate the interaction between adsorbate and phosphate through electrostatic repulsion when the pH was above 8.1. However, CAL-MBC still maintained stable adsorption performance in the pH range of 8.1–11.0, demonstrating that electrostatic attraction was not the main phosphate removal mechanism in the alkaline environment (pH greater than 8.1). As shown in Fig. 6(a), it was observed that the value of solution pH after phosphate adsorption was lower than that of initial solution pH in the pH range of 8.1–11.0. The decreased concentration of hydroxide ions in solution might be ascribed to the formation of Ca5(PO4)3(OH) precipitation, which was responsible for the stable adsorption performance in the pH range of 8.1–11.0 (Liu et al., 2019; Feng et al., 2021). Actually, the pH stability of CAL-MBC as a phosphate adsorbent was better than most of the reported adsorbents. For example, the La/Al engineered bentonite could maintain stable phosphate adsorption performance only in the pH range of 3.0–6.0, while the loss of adsorption capacity reached 83% at pH of 11.0 (Wang et al., 2022). Similarly, La-doped Mn-Al bimetallic oxides could only show stable phosphate adsorption capacity in the pH range of 3.0–7.0 (Liu et al., 2022). Another example was that the La-Zr-Zn ternary metal oxide exhibited stable phosphate adsorption ability in the pH range of 5.0–9.0 (Wei et al., 2020). The above results evidenced that CAL-MBC had strong pH tolerance, which was promising for its practical application in a wide pH range.
(a) Effect of solution pH on phosphate removal efficiency of CAL-MBC and the variation of solution pH before and after adsorption; (b) the isoelectric point of CAL-MBC before and after adsorption of phosphate.
3.3.4 Effect of coexisting anions
The effect of coexisting anions with different concentrations on the phosphate adsorption capacity of CAL-MBC was investigated. As shown in Fig. 7, there was only 1.1–3.7% loss of adsorption capacity by CAL-MBC even if the concentration of coexisting anions (Cl-, NO3–, and SO42-) was increased from 50 mg/L to 200 mg/L. Compared with the above three anions, coexisting HCO3– anions had a stronger interference effect. When the concentration of HCO3– increased from 50 mg/L to 200 mg/L, the phosphate adsorption capacity of CAL-MBC decreased from 48.2 mg/g to 43.4 mg/g. However, CAL-MBC still maintained more than 90% of the maximum adsorption value. The above results evidenced that CAL-MBC had great selectivity towards phosphate, thus indicating its potential for phosphate removal in actual wastewater.
Effect of coexisting anions on adsorption capacity of phosphate on CAL-MBC.
3.4 Phosphate removal from actual wastewater
The adsorption efficiency of CAL-MBC was investigated in wastewater collected from a municipal drainage ditch. The measured COD, pH, conductivity and total phosphorus concentration in the wastewater were 603 mg/L, 7.3, 876 μs/cm, and 3.820 mg/L, respectively. The influence of CAL-MBC dosage on the phosphate removal efficiency was shown in Fig. 8. At 0.50 g/L dose of CAL-MBC, the phosphate removal efficiency reached 93.7%, and the effluent concentration was 0.242 mg/L that was below the limit value of 0.5 mg/L set by the Chinese government (Zhang et al., 2022). Further increasing the dosage to 1.0 and 1.5 g/L, the residual phosphate concentrations were as low as 0.046 mg/L and 0.027 mg/L, respectively. The above results proved that CAL-MBC had great potential in treating actual wastewater with low concentration of phosphate.
Treatment performance of CAL-MBC at different doses in actual wastewater.
3.5 Cyclic adsorption–desorption performance test
Fig. 9(a) showed the phosphate desorption efficiency in the mixed solution of NaOH and Na2CO3 at different desorption time. The desorption efficiency firstly increased and then remained almost constant with the increase of desorption time. The optimum desorption efficiency of 92.9% was achieved at desorption time of 60 min. Thus, the desorption time of 60 min was selected to further perform the consecutive adsorption–desorption experiment of CAL-MBC. After the first cycle, CAL-MBC maintained 92.9% of phosphate uptake of the virgin adsorbent. The phosphate uptake of CAL-MBC gradually reduced with the increase of cycle times. After five regeneration cycles, the adsorption capacity decreased to 24.7 mg/g, which was only about 51.1% of that of the fresh CAL-MBC.
(a) Phosphate desorption efficiency at different time, and (b) phosphate uptake of CAL-MBC at different cycle times.
3.6 Adsorption mechanism
As shown in Fig. 2, for P-loaded CAL-MBC, characteristic peaks attributed to LaPO4 were observed at 2θ = 14.25°, 30.92° and 41.92°, indicating the formation of LaPO4 precipitation by the amorphous La species with phosphate anions during the adsorption process. Furthermore, new diffraction peaks appeared at 2θ = 18.63°, 25.09°, and 32.35° ascribed to Ca5(PO4)3(OH), confirming the formation of Ca5(PO4)3(OH) precipitation in the adsorption process. Moreover, the intensities of peaks corresponding to CaCO3 were significantly decreased after phosphate adsorption, implying that CaCO3 was responsible for the phosphate adsorption. The above results proved the precipitation mechanism (formation of LaPO4 and Ca5(PO4)3(OH)) was one of the main mechanisms of phosphate adsorption by CAL-MBC.
The surface functional group properties of CAL-MBC and P-loaded CAL-MBC were investigated, as shown in Fig. 10. For P-loaded CAL-MBC, new obvious peaks corresponding to phosphate (O-P-O bending vibration and P-O stretching vibration) were observed at 542 cm−1, 616 cm−1, and 1055 cm−1 (Li et al., 2020), confirming the successful adsorption of phosphate on CAL-MBC. In addition, the intensity of the stretching vibration peaks attributed to CO32– in the range of 1404–1483 cm−1 was significantly decreased, suggesting the formation of inner-sphere complexation through the ligand exchange between CO32– and phosphate anions (Kong et al., 2019).
FTIR spectra of CAL-MBC and P-loaded CAL-MBC.
XPS analysis was used to further clarify the phosphate adsorption mechanism of CAL-MBC. As shown in Fig. 11(a), a new P 2p peak appeared at 134.71 eV after adsorption, proving that phosphate was successfully adsorbed onto CAL-MBC. In Fig. 11(b), the relative percentage of the CO32– peak at 291.21 eV significantly decreased from 41.43% to 19.96% after adsorption, verifying most carbonates ions in CAL-MBC were replaced by phosphate anions (Koh et al., 2020b). In Fig. 11(c), the relative percentage of the –OH peak at 533.60 eV decreased from 44.11% to 35.87% after adsorption, illustrating the ligand exchange between the hydroxyl groups and the phosphate anions (Fang et al., 2018). This was also evidenced by the higher final pH than initial pH in the pH range of 3.0–8.0 (Fig. 6(a)). Furthermore, an obvious shift of 0.36 eV towards lower binding energy was observed in the O 1 s spectrum (Fig. 11(c)) after adsorption, while slight shifts toward higher binding energy was observed in the Ca 2p, La 3d and Al 2p spectra (Fig. 11(d-f)). These changes were attributed to the electron transfer between the phosphate anions and M (Ca, La, Al) atoms, suggesting the formation of inner-sphere complexation via ligand exchange between phosphate anions and carbonates and hydroxyl groups (Wang et al., 2022). This was also supported by the decline of the pHZPC value of CAL-MBC from 8.1 to 6.2 after phosphate adsorption (Fig. 6(b)) (Wu et al., 2017).
XPS full spectra (a) and high-resolution spectra of various elements (b-f).
In summary, the phosphate adsorption mechanism of CAL-MBC was analyzed as follows. Firstly, the surface of CAL-MBC was positively charged at pH below pHpzc (8.1), and phosphate anions were attracted to the surface of CAL-MBC by electrostatic attraction. Subsequently, phosphate anions were exchanged with ligands (carbonates and hydroxyl groups) to form inner-sphere complexes. Meanwhile, Ca5(PO4)3(OH) and LaPO4 precipitations were formed during this process. Furthermore, the Ca5(PO4)3(OH) precipitation mechanism dominated at the pH above 8.1, ensuring that CAL-MBC remained stable phosphate adsorption performance within the pH range of 8.1–11.0. Thus, the adsorption mechanisms involving electrostatic interaction, precipitation and inner-sphere complexation via ligand exchange were responsible for the excellent adsorption performance of phosphate on CAL-MBC. Schematic diagram of mechanisms of phosphate adsorption on CAL-MBC was shown in Fig. 12.
Schematic diagram of mechanisms of phosphate adsorption on CAL-MBC.
4 Conclusion
In this work, a novel CAL-MBC adsorbent was prepared for the enhancement of phosphate removal from wastewater. RSM was used to determine the optimum synthesis conditions. CAL-MBC achieved adsorption equilibrium within 1 h. The maximum adsorption capacity of CAL-MBC reached 152.9 mg/g. CAL-MBC also exhibited stable phosphate adsorption ability in the pH range of 3.0–11.0. The rapid removal rate, high adsorption capacity, and pH stability of CAL-MBC were outperformed than those of most reported metal modified biochar adsorbents. Moreover, CAL-MBC also showed high selectivity toward phosphate and excellent treatment capacity in actual wastewater. Finally, characterization results and adsorption experimental results demonstrated that phosphate adsorption on CAL-MBC included electrostatic attraction, precipitation (Ca5(PO4)3(OH) and LaPO4), and inner-sphere complexation via ligand exchange between carbonates and hydroxyl groups and phosphate anions. Overall, CAL-MBC reported here could be applied as a promising adsorbent for dephosphorization of wastewater.
Data Availability
Data will be made available on request.
Acknowledgements
This work was supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (No. KJQN202201206), the Open Fund Project of Chongqing Key Laboratory of Water Environment Evolution and Pollution Control in Three Gorges Reservior (No. WEPKL2019YB-03), the College Students' Innovation and Entrepreneurship Project of Chongqing of China (No. S202210643019).
Declaration of Competing 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 paper.
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Appendix A
Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2023.104880.
Appendix A
Supplementary data
The following are the Supplementary data to this article:Supplementary data 1
Supplementary data 1