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Article

Efficient Phosphate Removal from Wastewater by Ca-Laden Biochar Composites Prepared from Eggshell and Peanut Shells: A Comparison of Methods

by
Xiaoqi Liu
and
Jialong Lv
*
College of Natural Resources and Environment, Northwest A & F University, Yangling, Xianyang 712100, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(3), 1778; https://doi.org/10.3390/su15031778
Submission received: 17 November 2022 / Revised: 6 January 2023 / Accepted: 9 January 2023 / Published: 17 January 2023
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Abstract

:
Biochar is currently widely used as the adsorbent for phosphorus (P) removal from wastewater. Cheap and green modified materials and efficient preparation methods are the key to obtain efficient and economical engineering biochar. Conventional salt solution and chemical impregnation are common methods for preparing engineered biochar. However, this preparation method is not environmentally friendly or cheap due to the price of salt solutions and the solvent treatment process for chemical impregnation. In this article, Ca-laden biochar was prepared using peanut shells as carbon base materials and discarded eggshells as calcium source. Two methods (ball milling and chemical impregnation) of building the Ca-laden biochar were compared from the perspective of the characterization of biochar, the adsorption performance and the economic cost. The composition and structure of biochar were analyzed by the element content, functional group, X-ray diffraction, energy spectrum and electron microscope scanning etc. The adsorption behavior of biochar was tested in different environments (pH and temperature). The results revealed that the capacity of P adsorption by the Ca-modified biochar was higher than the adsorption by raw biochar, and that the prepared Ca-laden biochar has a wide working environment. Moreover, the Ca-laden biochar prepared by ball milling has a higher specific surface area and more porosity. The Ca-modified biochar through ball milling has a higher amount of adsorbed P than that of through chemical impregnation. This work not only creates a novel method for making excellent P adsorbents, but also offers an environmentally friendly use for agricultural eggshells and peanut shells.

Graphical Abstract

1. Introduction

Phosphorus (P), as an essential nutrient for all life, plays an important role in modern agricultural and industrial production [1,2]. P fertilizers have a positive effect on agricultural production, and they are important components to ensure national food security [3]. However, P fertilizers are mainly derived from phosphate rock (PR), which is a non-renewable and limited reserved resource [4]. In addition, PR is only distributed in specific areas, such as Morocco, Western Sahara, and China, etc. on the globe [5]. More PR needs to be mined, due to the increase in the global population [6]. According to statistics, the PR reserves would only last until the year 2170 if the population grows as forecasted by the UN under the “high” population growth estimation [7]. The sustainable development of P fertilizers is a fundamental prerequisite for ending hunger worldwide by 2030 and achieving global food security, as intended by the United Nations Sustainable Development Goals (SDGs) [8].
Recovering P from wastewater, which contains large quantities of P, can not only purify sewage but also be used as phosphorus fertilizer in agricultural production [9,10]. In fact, the reports of P recovery from wastewater are increasing year by year in recent years [11,12,13]. Many methods have been developed with the advancement of research on the recycling of phosphorus from wastewater, including precipitation processes, electrochemical processes, and biological processes etc. [14,15,16]. Most of these methods are difficult to implement on a large scale, due to complex operation procedures and cost issues [17,18]. Therefore, these methods are yet to be competitive in the market compared to mining [19].
There are more and more reports about P recovery from wastewater using biochar through adsorption methods, due to the advantages of low costs, no pollution, and easy operation [20,21]. Biochar has shown outstanding performance in many fields, such as radionuclides adsorption, different gas/vapor adsorption for cooling–heating applications, CO2 capture and storage applications, and so on [22,23]. Unfortunately, the raw biochar did not exhibit excellent capacity of P removal from wastewater due to the negative charge on the surface of raw biochar [24,25]. Raw biochar is usually modified to enhance its adsorption capacity before it is used to remove P from wastewater. [26]. The charge performance of biochar can be changed by loading metal ions (such as Mg2+, Ca2+, and Fe3+), thus improving the P adsorption capacity of biochar [27,28,29]. Waste eggshells, which contain large amounts of CaCO3, can be used as a calcium source to prepare the Ca-laden biochar instead of conventional salt solutions [30].
However, the impregnation method was often used for the above modifications, which required a large amount of solvent treatment and had the potential to harm the environment [31]. A solvent-free ball milling process may be an efficient, low-energy consumption, and environmental protection method. It is necessary to further study different preparation methods due to the significant differences between the different preparation methods.
Peanut shells and eggshells are common household garbage, which is harmful to the environment. Preparation of the Ca-laden biochar using peanut shells as carbon-based material and eggshells as calcium sources is a green and cost-effective manufacturing technique. The objectives of this study are as follows: (i) Preparing the Ca-laden biochar using waste peanut shells and eggshells; (ii) Exploring the differences of the preparation of the Ca-laden biochar by the impregnation method and ball milling method; and (iii) Evaluating the P adsorption capacity performance of the prepared different biochar from wastewater.

2. Materials and Methods

2.1. Materials

Peanut shells were provided by Shijiazhuang Dasong Agricultural Planting Co., Ltd. (Hebei, Shijiazhuang, China). Waste eggshells were provided by the student restaurant of Northwest A&F University. All chemicals used in the present study were of analytical reagent grade (>99.0% purity).

2.2. Biochar Production

The preparation of biochar was referred to Liu et al. [32]. The detailed preparation process was as follows: Clean and dry peanut shells were crushed to about 0.15 mm (i.e., through a 100-mesh sieve). The 500 g of peanut shell powder was dried at 60 °C for 24 h before being put in a muffle oven. The pyrolysis procedure was as follows: (i) Sample was heated up to 550 °C with heating rate of 15 °C/min; (ii) Sample was pyrolyzed at 550 °C for 2 h under N2 atmosphere. The prepared samples of biochar made from peanut shells were designated as PSB. The elemental content of C, N, and O was displayed in Table 1. The preparation process is shown in Figure 1.
In order to prepare the Ca-laden biochar, eggshells were pre-prepared into hydroxyl-eggshells (ES-OH). The detailed preparation process was as follows: Clean and dry eggshells were crushed to about 0.2 mm. A total of eggshells powder was slowly added in the 500 mL of hydrochloric acid solution (1 mol/L) under constant stirring. When the system reached equilibrium (i.e., no formation of bubbles was observed in the system), 500 mL of sodium hydroxide solution (1 mol/L) was added to the beaker, which instantly converts calcium chloride (CaCl2) into calcium hydroxide [Ca(OH)2]. After the reaction lasting for about 2 h, the solid was separated through filtration. Then, the solids were washed three times with deionized water. Finally, the hydroxyl-eggshells were obtained after being oven-dried at 95 °C for 24 h. Subsequently, hydroxyl-eggshells were crushed with a pestle and homogenized using a 0.2 mm sieve. Then, the PSB sample (25 g) was added to 500 mL of hydroxyl-eggshells solution (the content of hydroxyl-eggshells was 10 g). After a 24-hour incubation at 25 °C with constant stirring, the mixture was filtered. The modified biochar (named Ca-PSB1) was then obtained after the solid was dried at 105 °C for 24 h. Then the mixture (PSB and ES-OH) of the same ratio was milled in a ball mill machine 60 °C for 40 min. Finally, eggshell modified biochar (named Ca-PSB2) was obtained. The preparation process is shown in Figure 1.

2.3. P Adsorption

Herein, monopotassium phosphate solutions (KH2PO4) were used as phosphoric source to study the P adsorption performance of biochar. The P adsorption procedures were as follow: (i) 0.05 g of biochar sample was carefully added to 50 mL P solution (given concentration was 0, 10, 20, 50, 100, and 200 mg P L−1, respectively); (ii) The containers were shaken continuously in oscillator for 24 h with the rate of 180 rpm at 25 °C; (iii) Filtering the mixtures with a 0.22 μm membrane and determining the concentration of P in the filtrate by ammonium molybdate and ascorbic acid method.
The kinetic process of P adsorption by biochar was studied. The specific process was as follows: (i) 0.05 g of adsorbent was added into 50 mL of P solution (200 mg P L−1); (ii) Shaking the mixtures continuously in an oscillator for a given time (0.5, 1, 3, 5, 7, 12, 24 h) with the rate of 180 rpm at 25 °C; (iii) The mixtures were then filtered to determine the concentration of P in the filtrate. The ability of biochar to adsorb P could be characterized by the difference in the concentration of P in the solution before and after the adsorption experiment.
The P adsorption capacity of the adsorbent was calculated according to the following formula:
Q e = C 0 − C e V m
where Qe was the amount of adsorption (mg P g−1), C0 and Ce were the P concentrations (mg P L−1) in the initial and equilibrium solution, respectively. V and m were the volume of solution (L) and the weight of the adsorbent (g), respectively.
Isothermal adsorption and kinetics characteristics were studied by Langmuir equations (Equation (2)) and second-order mathematical models (Equation (3)), respectively.
q e = KQ C e 1 + K   C e
t q t = 1 k q e 2 + t q e
Q was the Langmuir maximum capacity (mg P g−1), Ce was the P concentration in the equilibrium solution (mg P L−1), K and k represent the constants. qt and qe were the adsorption capacity (mg P g−1) at time t and at equilibrium, respectively..
To investigate the P adsorption performance of the prepared biochar at different ambient temperatures, a series of adsorption experiments were examined with different ambient temperatures (15 °C, 25°C, and 40 °C), and the adsorption experiment was performed by mixing 0.05 g of the adsorbent and 100 mL of P solution (200 mg P L−1).
For determining P adsorption performance of the prepared biochar at different pH values, a series of adsorption experiments were conducted in different initial pH (2, 4, 6, 8, 10, and 12). The solution pH was amended by HCl (0.1 mol/L) or NaOH (0.1 mol/L) solution, and the adsorption experiment was conducted by blending 50 mg of biochar and 100 mL P solution (200 mg P L−1).

2.4. Adsorbents’ Regeneration

The recyclability of the adsorbents was measured through five cycles of adsorption–desorption experiment in this work. The specific process was as follows: (i) P adsorption experiment was conducted by blending 0.1 g of biochar and 200 mL P solution (200 mg P L−1); (ii) P desorption experiment was carried out as follows: the P-laden biochar samples after adsorption of P were then added into 100 mL of NaOH solution (3 mol/L) and shaken for 12 h; (iii) The regenerated adsorbents was obtained after washing by deionized water and drying at 80 °C for 12 h. In the subsequent adsorption experiment, the samples were directly reused. The adsorption–regeneration experiments were then carried out for a total of five cycles using the same procedure.

2.5. Characterization

Elemental content (C, H, N and S) of peanut shells and PSB was determined by Elemental Analyzer. The microstructure of PSB and Ca-PSBs were investigated by scanning electron microscopy (SEM). Functional groups of the samples were analyzed by Fourier transform infrared spectrometer (FTIR) (Thermo Fisher Scientific, Massachusetts, USA). BET surface area, total pore volume, and pore diameter of the adsorbents were measured by N2 absorption-adsorption spectrometer (MicrotracBEL, Osaka, Japan). The elemental of the samples was obtained by an energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) analysis of the adsorbents before and after adsorbing P was measured (BRUKER, Karlsruhe, Germany).

3. Results and Discussion

3.1. Characterization of Ca-Laden Biochar

The physicochemical properties of the different biochar samples are presented in Table 2. The pore characteristics of the Ca-laden biochar prepared under the different loading methods were different (Table 2). The BET surface area of the Ca-PSB2 was much higher than that of the PSB, while the value of the Ca-PSB1 was in between the PSB and Ca-PSB2. There were no significant changes in the average pore volume of the PSB and Ca-PSB1. However, the average pore volume of the Ca-PSB2 was 66.7% higher than that of the PSB, maybe because the high energy generated by the ball milling changed its apparent structure, such as opening the microchannels through generating crashes [33]. The average pore diameter of the PSB, Ca-PSB1, and Ca-PSB2 was14.81, 16.34, and 16.32 nm, respectively.
FTIR spectra (Figure 2) were used to examine the changes in the functional groups of the various samples in order to gain a deeper comprehension of the chemical properties of the raw and Ca-modified biochar. The chemical structure of the Ca-PSB1 and Ca-PSB2 was similar, the Ca-PSB2 was used to represent the Ca-laden biochar to characterize its structure in this paper. The -OH, -CH2, -CHO, and C=C groups were assigned to the peaks at 3410, 2924, 1742, and 1618 cm−1, respectively. The disappearance of the peak at 1000–1500 cm−1 after peanut shell powder was converted into biochar (Figure 2 Left-A and B), indicating the rupture of the ether bond during pyrolysis. In addition, the peak at about 581 cm−1 was maybe attributed to the Ca-O composite (Figure 2 Left-C). This may be due to the combination of Ca ions and some other oxygen-containing groups on biochar. This was consistent with the results of the XRD spectra (Figure 3 Right). The peak of the Ca-O band could be observed on the Ca-PSB2 (Figure 3 Right). According to PDF 48-1043, the peaks at 36.342° (2 Theta) were related to calcium formate. This was comparable to the findings of the Wang et al. [34]. The XPS spectra of the PSB and Ca-PSB2 also confirmed that Ca was successfully laden onto PSB (Figure 2 Right). As shown in Figure 2 Right-A, O 1s (531.54 eV), N 1s (399.91 eV), and C 1s (284.40 eV) on the XPS spectrum of the PSB were observed. Compared with the XPS spectrum of the PSB, the Ca 2P (348.23 eV) was observed in the Ca-PSB2 (Figure 2 Right-B), which represent the existence of Ca. The Raman spectrum of the PSB and Ca-PSB (Figure 3 Left) showed two fundamental vibrations at 1350 and 1580 cm−1, which can be assigned to the D band and G band, respectively. The higher ID/IG intensity suggested that the prepared PSB and Ca-PSB2 had high activity [35].
SEM was used to examine the morphology in order to further observe the surfaces characteristic of different samples (Figure 4). After the peanut shell powders were pyrolyzed, there were many micropore on the PSB (Figure 4A,B). After being modified by the eggshell based-Ca(OH)2, there were many small particles attached on the biochar surface (Figure 4C), which probably helped improve the P adsorption performance of Ca-PSB. [36]. The micromorphology of the Ca-laden biochar prepared by the different methods was different (Figure S1). The Ca-PSB2 prepared through ball milling presented more small holes (Figure S1). This may be more beneficial for the attachment site of Ca and subsequently the adsorption of P. The corresponding EDX spectra shown that the surface of the Ca-PSB mainly contains C, O and Ca (Figure 4C1). The elemental compositions and distributions were also investigated using the EDX mapping shown in Figure 4D,E.

3.2. Adsorption of P onto PSB and Ca-PSB Samples

The various P adsorption performance of adsorbents varied significantly (Figure 5). The shape of isothermal P adsorption curve was similar: all first steep and then gentle. However, the maximum P adsorption capacity of Ca-PSB was greatly improved. The maximum amount of P adsorbed by the PSB, Ca-PSB1, and Ca-PSB2 was 72.23, 105.12, and 130.57 mg P g−1, respectively, according to the Langmuir fittings (Table 3). The P adsorption capacity of the Ca-laden biochar prepared by the ball mill method was higher than that of the solution impregnation method. The P adsorption capacity of the Ca-PSB2 was higher than that of the adsorbent reported by Lin et al. [37].
The adsorption kinetic process was determined to gain a deeper comprehension of the P adsorption processes of various adsorbents. As shown in Figure 5B, P was quickly absorbed by the absorbents. (<24 h). The P adsorptions data were accurately described by the second-order kinetic models (R2 > 0.9) (Figure 5B and Table 4). There were two stages to the P adsorption process: first quickly and then slowly. Before saturation adsorption was reached, the P adsorption by the Ca-PSB1 and Ca-PSB2 was faster than the P adsorption by the PSB. The rate coefficients were 3.14 × 10−3, 1.36 × 10−2, and 1.69 × 10−2 g/(mg·min), respectively. This may be due to the presence of Ca, which allowed phosphate to precipitate directly with Ca and created a stronger and faster adsorption [38]. Additionally, the Ca-laden biochar had a higher rate of P adsorption than the raw biochar. This was consistent with the findings of Xu et al. [39]. The presence of Ca had greatly improved the P adsorption performance of biochar, but the P adsorption properties of biochar prepared under different methods were also different. The Ca-PSB2 prepared through ball milling showed better adsorption properties than the Ca-PSB1. This may be because the Ca-PSB2 has excellent pores and more active functional groups than the Ca-PSB1.

3.3. The Effects of Ambient Temperatures and Initial pH on P Adsorption

The prepared biochar had essentially the same pH response. The P adsorption capacity of adsorbent varied slightly with pH between 2 and 8 in the system (Figure 6). When the pH was higher than 10, the P adsorption capacity of the adsorbent clearly decreased (Figure 6), which was similar to that of Jiao et al. [40]. The zero potentials of the PSB, Ca-PSB1, and Ca-PSB2 were 9.30, 9.52, and 9.56, respectively (Figure S3). When pH > 10, the surface of the adsorbents can generate electrostatic repulsion between the adsorbents and phosphate. Moreover, excessive OH- may preferentially react with Ca2+, and the precipitant occupies the adsorption sites and pores. In brief, the prepared PSB and Ca-PSB worked well in a wide range of pH, but showed weak adsorption properties at pH greater than 10.
P adsorption was examined in relation to ambient temperature. The P adsorption capacity increases slightly with the increasing ambient temperatures from 15 °C to 40 °C, but no obvious difference was observed (Figure 6). These results indicated that these adsorbents worked well at normal temperature.

3.4. Recovery of Phosphate

With the number of adsorption-desorption recycling increasing, the P adsorption capacity of biochar showed a downward trend (Figure 7). However, the ability to adsorb P of the different adsorbents showed different downward trends. (Figure 7). For the PSB, there was no significant difference in the P desorption rate, which varies between 93.3% and 96.3%. For the Ca-PSB1 and Ca-PSB2, the P desorption rate was 85.4% and 78.4%, respectively. This result can be attributed to the formation of precipitate between Ca ion and some phosphate. However, the P desorption rate of the Ca-PSB1 was 93.4%, 94.6%, 94.5%, and 94.5% for 2nd, 3rd, 4th, and 5th cycle, and the desorption rate for the Ca-PSB2 was 94.6%, 95.9%, 96.6%, and 97.2%. The adsorption efficiency for the Ca-PSB1 and Ca-PSB2 was reduced by 37.7% and 32.6% after five cycles, this may be attributed to the accumulative loss of Ca ion in the adsorption and desorption process. However, the adsorption capacity of the Ca-PSB2 was still higher than that of the PSB and Ca-PSB1 after five cycles, due to the abundant pore structure.

3.5. Mechanism of the P Adsorption

To confirm the adsorption, the SEM images, EDX spectra, FTIR spectra, XPS 265 spectra of biochar before and after P adsorption were measured. The surface microstructure of the adsorbent before and after P adsorbed is shown in Figure 8. After P adsorption, the pimple-like structures appeared on the exterior-faces of the PSB and Ca-PSB2 (Figure 8B,D), which may be calcium phosphate substance. The flower-shaped crystal substances predominant composition of O, Ca, and P elements was confirmed by EDX spectra and mapping. The surface of the Ca-PSB presented a looser morphological structure, namely more adsorption sites (Figure 8A–D).
The peak (3411 cm−1) attributed to the −OH group was weakened significantly after P adsorption (Figure 9 Left), which was inferred that there may exist a ligand exchange between the PO43− and the −OH [41]. The emerging peaks at about 1050 and 621 cm−1 were attributed to the PO43− and ν (O-P) after P adsorption, respectively, suggesting that phosphate was successfully adsorbed or precipitated by raw and modified biochar [42]. The peak around 508 cm−1 was attributed to the Ca-PO4. The XRD spectra (Figure 9 Right) also confirm that Ca was reacted with phosphate. According to PDF 19-0272, the peaks at 32.172° (2 Theta) were related to the Ca-PO4-CO3 complexes (Figure 9 Right).
The XPS spectra (Figure 10) were measured to further investigate the adsorption mechanism by detecting the elemental compositions and corresponding elemental valence states of various absorbents before and after P adsorption. O 1s (531.21 eV), N 1s (399.97 eV), and C 1s (284.44 eV) on the XPS spectrum of the PSB before P adsorption were observed (Figure 10 Left-a). After being absorbed P by PSB, the P 2p (134.9 eV) was observed (Figure 10 Left-b). Furthermore, the signal of Ca 2p was shifted from 348.17 to 348.82 eV after P adsorption, which may be because when Ca and PO43− were combined, the electron density on Ca was reduced due to the electronegativity of oxygen being large and the binding energy being improved. The peak of Ca 2p was split into two overlapped peaks, corresponding to the Ca-O compound and the Ca-PO4 compound, respectively (Figure S1). The results indicated the Ca3(PO4)2 had been formed during P adsorption.

3.6. Cost Analysis

The synthesis of the Ca-PSBs mainly requires peanut shells, waste eggshells, sodium hydroxide, and hydrochloric acid (37%). Here, the price of discarded eggshells is not considered due to its low cost. The current cost of peanut shells, hydrochloric acid, and sodium hydroxide was about $107.31, $42.36, and $652.66 per ton, respectively. The production of 1 kg of the Ca-PSB1 requires about 1.70 kg peanut shells, 84.5 g sodium hydroxide, and 208.15 g hydrochloric acid (37%). The production of 1 kg of the Ca-PSB2 requires about 1.58 kg peanut shells, 78.5 g sodium hydroxide, and 185.22 g hydrochloric acid. With the consideration of labor and energy consumption, the coating cost for coating one ton of the Ca-PSB1 was about $203.12, while the preparation coat was $194.58 for the Ca-PSB2. Meanwhile, commercially engineered biochar was between $304.16 and $342.18. Therefore, this preparation process can effectively reduce the cost of production of biochar.

4. Conclusions

In this paper, biochar was prepared from discarded peanut shell. Subsequently, the prepared biochar was modified with the waste eggshells as the calcium source, and the Ca-laden biochar was successfully prepared. In addition, we explored the effects of two different preparation methods on the physicochemical properties and adsorption properties of the Ca-laden biochar. After the eggshell based-Ca-modified biochar by ball mill method, many bumps appeared on the surface of the biochar, which further increased the adsorption site of the adsorbent P. Additionally, there were more active functional groups in the Ca-PSB2. Loading of Ca changed the adsorption mechanism of biochar for P. The Ca-laden biochar prepared by the ball mill method was a more efficient green, environmentally friendly, and low-energy method.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15031778/s1, Figure S1. SEM spectra of Ca-PSB1 (A1 and B1) and Ca-PSB2 (A2 and B2). Figure S2. XPS spectra of Ca 2p of Ca-PSB after P adsorption. Figure S3. Zeta potentials of PSB, Ca-PSB2, and Ca-PSB2.

Author Contributions

Investigation, data curation, formal analysis, and writing—original draft preparation: X.L.; investigation, data curation, software, writing—review and editing: J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Special Projects of the Ministry of Science and Technology for the 13th Five Year Plan of China (Project No. 2017YFD0200205); and The Project of Shaanxi Agricultural Science and Technology Innovation Drive (NYKJ-2020-YL-21).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author ljlll@nwafu.edu.cn.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The preparation process diagram of PSB and Ca-PSB.
Figure 1. The preparation process diagram of PSB and Ca-PSB.
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Figure 2. FTIR spectra (Left) of peanut shells (A), PSB (B), and Ca-PSB2 (C); XPS spectra (Right) of PSB (A) Ca-PSB2 (B).
Figure 2. FTIR spectra (Left) of peanut shells (A), PSB (B), and Ca-PSB2 (C); XPS spectra (Right) of PSB (A) Ca-PSB2 (B).
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Figure 3. Raman spectra (Left) of PSB (A) and Ca-PSB2 (B); XRD spectra (Right) of peanut shells (A), PSB (B), and Ca-PSB2 (C).
Figure 3. Raman spectra (Left) of PSB (A) and Ca-PSB2 (B); XRD spectra (Right) of peanut shells (A), PSB (B), and Ca-PSB2 (C).
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Figure 4. SEM images of peanut shells (A), PSB (B), and Ca-PSB2 (C); EDX spectra of peanut shells (A1), PSB (B1), and Ca-PSB (C1); and EDX mapping analysis of PSB (D), and Ca-PSB2 (E).
Figure 4. SEM images of peanut shells (A), PSB (B), and Ca-PSB2 (C); EDX spectra of peanut shells (A1), PSB (B1), and Ca-PSB (C1); and EDX mapping analysis of PSB (D), and Ca-PSB2 (E).
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Figure 5. P absorption isotherms (A) and kinetics (B) by PSB, Ca-PSB1, and Ca-PSB2.
Figure 5. P absorption isotherms (A) and kinetics (B) by PSB, Ca-PSB1, and Ca-PSB2.
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Figure 6. The effects of different initial pH (A) and ambient temperatures (B) on the P adsorption.
Figure 6. The effects of different initial pH (A) and ambient temperatures (B) on the P adsorption.
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Figure 7. P adsorption (A) and desorption (B) cycle of peanut shell-derived biochar.
Figure 7. P adsorption (A) and desorption (B) cycle of peanut shell-derived biochar.
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Figure 8. SEM images (left row) and EDX spectra (middle row) of PSB before P adsorption (A,A1), PSB after P adsorption (B,B1), Ca-PSB2 before P adsorption (C,C1), and Ca-PSB2 after P adsorption (D,D1); EDX mapping analysis (right row) of PSB (B2), and Ca-PSB2 (D2) after P adsorption.
Figure 8. SEM images (left row) and EDX spectra (middle row) of PSB before P adsorption (A,A1), PSB after P adsorption (B,B1), Ca-PSB2 before P adsorption (C,C1), and Ca-PSB2 after P adsorption (D,D1); EDX mapping analysis (right row) of PSB (B2), and Ca-PSB2 (D2) after P adsorption.
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Figure 9. FTIR spectra (Left) and XRD spectra (Right) of (a) PSB before P adsorption, (b) PSB after P adsorption, (c) Ca-PSB2 before P adsorption, and (d) Ca-PSB2 after P adsorption.
Figure 9. FTIR spectra (Left) and XRD spectra (Right) of (a) PSB before P adsorption, (b) PSB after P adsorption, (c) Ca-PSB2 before P adsorption, and (d) Ca-PSB2 after P adsorption.
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Figure 10. XPS spectra of PSB (Left) and Ca-PSB2 (Right): (a) before P adsorption and (b) after P adsorption.
Figure 10. XPS spectra of PSB (Left) and Ca-PSB2 (Right): (a) before P adsorption and (b) after P adsorption.
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Table
Table 1. The C, N, and O element content of peanut shells and PSB.
Table 1. The C, N, and O element content of peanut shells and PSB.
SampleC Element (%)N Element (%)H Element (%)S Element (%)
Peanut shells47.810.665.540.05
PSB77.591.492.2930.27
Table
Table 2. The specific surface area and porosity of different adsorbents.
Table 2. The specific surface area and porosity of different adsorbents.
SampleBET Surface Area (m2 g−1)Total Pore Volume
(cm3 g−1)		Average Pore Diameter (nm)
PSB1830.2414.81
Ca-PSB11470.2216.34
Ca-PSB22610.4016.32
Table
Table 3. Adsorption isotherm parameters of different adsorbents under the Langmuir fittings.
Table 3. Adsorption isotherm parameters of different adsorbents under the Langmuir fittings.
TreatmentQ (mg P g−1)KR2(MBC)
PSB72.236.016 × 10−20.98754.345
Ca-PSB1105.120.25720.978927.036
Ca-PSB2130.570.66120.967286.33
Table
Table 4. Adsorption kinetic parameters of different adsorbents under the pseudo-second-order fittings.
Table 4. Adsorption kinetic parameters of different adsorbents under the pseudo-second-order fittings.
TreatmentR2K (g/(mg·min))Qe (mg P g−1)
PSB0.97653.14 × 10−373.32
Ca-PSB10.99841.36 × 10−2118.58
Ca-PSB20.98531.69 × 10−2138.56
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Liu, X.; Lv, J. Efficient Phosphate Removal from Wastewater by Ca-Laden Biochar Composites Prepared from Eggshell and Peanut Shells: A Comparison of Methods. Sustainability 2023, 15, 1778. https://doi.org/10.3390/su15031778

AMA Style

Liu X, Lv J. Efficient Phosphate Removal from Wastewater by Ca-Laden Biochar Composites Prepared from Eggshell and Peanut Shells: A Comparison of Methods. Sustainability. 2023; 15(3):1778. https://doi.org/10.3390/su15031778

Chicago/Turabian Style

Liu, Xiaoqi, and Jialong Lv. 2023. "Efficient Phosphate Removal from Wastewater by Ca-Laden Biochar Composites Prepared from Eggshell and Peanut Shells: A Comparison of Methods" Sustainability 15, no. 3: 1778. https://doi.org/10.3390/su15031778

APA Style

Liu, X., & Lv, J. (2023). Efficient Phosphate Removal from Wastewater by Ca-Laden Biochar Composites Prepared from Eggshell and Peanut Shells: A Comparison of Methods. Sustainability, 15(3), 1778. https://doi.org/10.3390/su15031778

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