# Biomimetic Synthesis of Hydroxyapatite in Presence of Imidazole-4,5-dicarboxylic Acid Grafted Chitosan for Removing Chromium(VI)

Authors: Zhangxu Chen; Baorong Wu; Xiaodan Huang; Xianxue Li; Yonglong Lin
Journal: Journal of Nanotechnology
 (2018)

---

## Abstract

In order to biomimetic synthesize hydroxyapatite similar to natural bone. Hydroxyapatite (HAP) is biomimetic synthesized in simulated body fluid (SBF) by addition of imidazole-4,5-dicarboxylic acid grafted chitosan (IDACS). The effect of molar ratio of chitosan (CS) to imidazole-4,5-dicarboxylic acid (IDA) on preparation of HAP was investigated. The structure, size, and crystal phase of the obtained hydroxyapatite were observed by Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. The results show that the molar ratio of CS to IDA is 1 : 3, the temperature is 37.0°C, the aging time is 48 h, the synthesized nanorod-like hydroxyapatite with diameter 20–30 nm, and length ranging from 75 to 120 nm presents excellent phase, which disperses well and is similar to the natural bone of HAP. The obtained HAP can be used to remove chromium(VI) by the orthogonal experiments, and the results indicated that the removal rate can reach 95.66% under the optimum conditions. These results suggest that the morphology of the obtained HAP is more affected by the material ratio of chitosan to imidazole-4,5-dicarboxylic acid than its structure, and the obtained HAP can effectively remove Cr(VI), which provides a novel method for biomimetic synthesis of other biomaterials and application in the water purification.


---

## Body

## 1. Introduction

Biomimetic synthesis of biological minerals has received considerable attentions in recent years. Calcium carbonate [1–3] and hydroxyapatite [4–6] are the most abundant biological minerals, which are especially focused all over the world. Up to date, synthesis of calcium carbonate and hydroxyapatite with various strategies has been studied. Among these, ethylene glycol [7] and chitosan [8] were used as organic matrix to biomimetic synthesize calcium carbonate and hydroxyapatite. Moreover, an impersonating physiological condition, simulated body liquid (SBF), has been generally utilized as a splashing medium since it was initially arranged by Kokubo et al. [9], which has specific preferences, such as, small particle sizes and narrow size dispersion. In recent years, biomimetic synthesis of hydroxyapatite was carried out in microwave oven using 1.5 × SBF and 10 × SBF by Bindal et al. [10] and Tolga Demirtaş et al. [11]. It is worthwhile to mention that organic matrix is used as controlled template for biomimetic synthesis of calcium carbonate and hydroxyapatite in SBF solution.Hydroxyapatite (Ca10(PO4)6(OH)2; HAP) is the one of the main biological materials, which can existed in human skeleton, animal bones, and teeth. Herein, the hydroxyapatite was formed through biomineralization and related biorelated processes, which are multifactorial, fantastic, and complex processes to commonly become a nice guide for fabrication of functional materials and keys for understanding of biophenomena [12–14].HAP is used as a good biological material because it has particular structure and properties, such as compatibility, bioactivity, osteoconductivity, nontoxicity, and noninflammation. Therefore, it can be applied in the medical field and potentially water purification, such as an ideal implant material, pesticides, plastic surgery, protein adsorption, and removing heavy metal. As we known, lots of researchers have devoted themselves to the medical field. Unfortunately, it still represents great challenge to apply in water purification [15]. It is well known that heavy metal pollution in groundwater or wastewater is an important environment problem due to its toxic effects and accumulation throughout the food chain and hence in the human body [16].Removing Cr(VI) is the most important and urgent problem to be solved as human body can absorb Cr(VI) easily through the digestive, respiratory, and mucous membrane [17]. During the last decade, numbers of technologies have been adopted to remove Cr(VI) from groundwater or wastewater. However, the available publications on removing Cr(VI) by hydroxyapatite and related composites are still very limited. HAP, HAP/chitin composite, and HAP/chitosan composite were synthesized to remove Cr(VI) by Kousalya et al., and the sorption capacities of them were found to be 2720, 2845, and 3450 mg·kg−1, respectively [18]. Nanohydroxyapatite embedded gelatin composite was prepared by Gopalakannan, which was used as a biocompatible sorbent to the removal of hexavalent chromium from aqueous solution in batch mode. The adsorption capacity can reach 170 mg·g−1, and the sorption of chromium(VI) onto Nanohydroxyapatite embedded gelatin biocomposite follows Langmuir isotherm [19].In our previous research, a series of biological materials, such as calcium carbonate and hydroxyapatite have also been successfully fabricated by using the method of biomimetic synthesis [8, 20, 21]. Encouraged by this, herein, a rational design to biomimetic synthesize hydroxyapatite for removing Cr(VI) was firstly proposed. The obtained results might significantly contribute to the practical application of biological materials as promising composites in the treatment of contaminated water.


## 2. Experimental Section

### 2.1. Materials

Imidazole-4,5-dicarboxylic acid grafted chitosan (IDACS) is synthesized according to our previous reports [20–22]. Disodium hydrogen phosphate and (Na2HPO4), absolute ethanol (C2H5OH), calcium chloride (CaCl2), hydrochloric acid (HCl), and ammonia (NH3·H2O) are all of analytical reagent and are all purchased from Aladdin Reagent Co., Ltd. (China), which are used as starting materials without further purification.


### 2.2. Biomimetic Synthesis of Hydroxyapatite in Presence of IDACS

In this study, simulated body fluid (SBF) solution was used as solvent to replace water [23]. The hydroxyapatite crystals were obtained in pure water system [23]. To investigate the effect of IDACS on the crystallization of hydroxyapatite, different samples (S1–S5) were prepared by using IDACS as a matrix with the different molar ratio of chitosan to imidazole-4,5-dicarboxylic acid (3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3), respectively. In a typical synthesis, 0.050 g of different types of IDACS was added in 25.00 mL of Na2HPO4 (0.06 mol·L−1), which was adjusted to pH 10.0∼10.5 using ammonia or hydrochloric acid with constant stirring (Solution A). CaCl2 (0.10 mol·L−1) was adjusted to pH 10.0∼10.5 using ammonia or hydrochloric acid with constant stirring (Solution B). Then, Solution B was dropwise added into the above Solution A kept at a temperature of 37°C and stirred for 30 min and aged for 48 h. Finally, the obtained white precipitates were separated by centrifugation, rinsed several times with double distilled water and absolute ethanol, respectively, and then were dried in oven at 80°C until a constant weight was achieved. The as-synthesized product was denoted sample 1–5 (S1–5).


### 2.3. Removing Cr(VI) by Obtained Nanorod-Like Hydroxyapa-tite

In order to verify HAP be responsible for environmental remediation, its removing activities for chromium(VI) were investigated. The chromium sorption was studied as follows: 25 mL of chromium(VI) solution with various initial chromium(VI) concentrations were taken in a flask, and then the pH of the above solution can be adjusted by using 0.1 M HCl/NaOH solution. After adjustment, different dosage of HAP5 was added into the above solution. To explore the optimum operating condition, the L16 (45) orthogonal array was used in Table 1, herein, based on parallel single-factor experiment, the effect of pH value, initial concentrations of Cr(VI), dosage of HAP, and adsorption time on the removal of Cr(VI) were examined by our previous report [24].Table 1
Levels and factors of orthogonal experiment design.
Level
pH
Initial concentration of Cr(VI) (mg·L−1)
Dosage of HAP (mg·L−1)
Adsorption time (min)
1
3.00
25
200
90
2
5.00
50
400
120
3
7.00
100
800
150
4
9.00
200
1000
180


### 2.4. Characterization

The sizes and morphologies of hydroxyapatite crystals were investigated by FESEM (Hitachi Model S-4800). Calibrated pellets of calcium carbonate (in proportion of 1 wt.% in KBr powder) were performed and recorded with a Fourier transform infrared spectrometer (BRUKER TENSON 27, Germany) between 4000 and 400 cm−1 with a resolution of 4 cm−1. The X-ray diffraction (XRD) patterns were obtained on SmartLab 3 kW, X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation at a scan rate of 4°C·min−1 was used to determine the identity of crystalline phase. The accelerating voltage and applied current were 30 kV and 40 mA, respectively.The thermal behavior (TG) analysis of hydroxyapatite crystals were performed by using a Netzsch TG 209F3 in the temperature range of 40–1000°C with a heating rate of 10°C·min−1 in a dynamic atmosphere of nitrogen (60 mL·min−1). The surface area and the mean pore size were determined based on the nitrogen adsorption-desorption isotherms at liquid N2 temperature on a Micrometrics (ASAP 2020M + C) instrument. The specific surface area was determined by using the Brunauer–Emmett–Teller (BET) method. The mean pore size was estimated from the desorption branch of the nitrogen adsorption-desorption isotherms using the Barrett–Joyner–Halenda (BJH) method.


## 2.1. Materials

Imidazole-4,5-dicarboxylic acid grafted chitosan (IDACS) is synthesized according to our previous reports [20–22]. Disodium hydrogen phosphate and (Na2HPO4), absolute ethanol (C2H5OH), calcium chloride (CaCl2), hydrochloric acid (HCl), and ammonia (NH3·H2O) are all of analytical reagent and are all purchased from Aladdin Reagent Co., Ltd. (China), which are used as starting materials without further purification.


## 2.2. Biomimetic Synthesis of Hydroxyapatite in Presence of IDACS

In this study, simulated body fluid (SBF) solution was used as solvent to replace water [23]. The hydroxyapatite crystals were obtained in pure water system [23]. To investigate the effect of IDACS on the crystallization of hydroxyapatite, different samples (S1–S5) were prepared by using IDACS as a matrix with the different molar ratio of chitosan to imidazole-4,5-dicarboxylic acid (3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3), respectively. In a typical synthesis, 0.050 g of different types of IDACS was added in 25.00 mL of Na2HPO4 (0.06 mol·L−1), which was adjusted to pH 10.0∼10.5 using ammonia or hydrochloric acid with constant stirring (Solution A). CaCl2 (0.10 mol·L−1) was adjusted to pH 10.0∼10.5 using ammonia or hydrochloric acid with constant stirring (Solution B). Then, Solution B was dropwise added into the above Solution A kept at a temperature of 37°C and stirred for 30 min and aged for 48 h. Finally, the obtained white precipitates were separated by centrifugation, rinsed several times with double distilled water and absolute ethanol, respectively, and then were dried in oven at 80°C until a constant weight was achieved. The as-synthesized product was denoted sample 1–5 (S1–5).


## 2.3. Removing Cr(VI) by Obtained Nanorod-Like Hydroxyapa-tite

In order to verify HAP be responsible for environmental remediation, its removing activities for chromium(VI) were investigated. The chromium sorption was studied as follows: 25 mL of chromium(VI) solution with various initial chromium(VI) concentrations were taken in a flask, and then the pH of the above solution can be adjusted by using 0.1 M HCl/NaOH solution. After adjustment, different dosage of HAP5 was added into the above solution. To explore the optimum operating condition, the L16 (45) orthogonal array was used in Table 1, herein, based on parallel single-factor experiment, the effect of pH value, initial concentrations of Cr(VI), dosage of HAP, and adsorption time on the removal of Cr(VI) were examined by our previous report [24].Table 1
Levels and factors of orthogonal experiment design.
Level
pH
Initial concentration of Cr(VI) (mg·L−1)
Dosage of HAP (mg·L−1)
Adsorption time (min)
1
3.00
25
200
90
2
5.00
50
400
120
3
7.00
100
800
150
4
9.00
200
1000
180


## 2.4. Characterization

The sizes and morphologies of hydroxyapatite crystals were investigated by FESEM (Hitachi Model S-4800). Calibrated pellets of calcium carbonate (in proportion of 1 wt.% in KBr powder) were performed and recorded with a Fourier transform infrared spectrometer (BRUKER TENSON 27, Germany) between 4000 and 400 cm−1 with a resolution of 4 cm−1. The X-ray diffraction (XRD) patterns were obtained on SmartLab 3 kW, X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation at a scan rate of 4°C·min−1 was used to determine the identity of crystalline phase. The accelerating voltage and applied current were 30 kV and 40 mA, respectively.The thermal behavior (TG) analysis of hydroxyapatite crystals were performed by using a Netzsch TG 209F3 in the temperature range of 40–1000°C with a heating rate of 10°C·min−1 in a dynamic atmosphere of nitrogen (60 mL·min−1). The surface area and the mean pore size were determined based on the nitrogen adsorption-desorption isotherms at liquid N2 temperature on a Micrometrics (ASAP 2020M + C) instrument. The specific surface area was determined by using the Brunauer–Emmett–Teller (BET) method. The mean pore size was estimated from the desorption branch of the nitrogen adsorption-desorption isotherms using the Barrett–Joyner–Halenda (BJH) method.


## 3. Results and Discussion

### 3.1. Influence of Molar Ratio of Chitosan/Imidazole-4,5-dicarboxylic Acid on Hydroxyapatite

Figures1–3 present the infrared spectra, XRD patterns, and FESEM images of the HAP crystals (S1–S5) produced with the different molar ratio of chitosan to imidazole-4,5-dicarboxylic acid (3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3) at 37.0°C for aging 24 h, respectively.Figure 1
FT-IR spectra of HAP particles obtained by using IDACS as an organic matrix with different molar ratios of chitosan to imidazole-4,5-dicarboxylic acid. a: 3 : 1; b: 2 : 1; c: 1 : 1; d: 1 : 2; e: 1 : 3.Figure 2
XRD patterns of HAP particles obtained by using IDACS as an organic matrix with different molar ratios of chitosan to imidazole-4,5-dicarboxylic acid. a: 3 : 1; b: 2 : 1; c: 1 : 1; d: 1 : 2; e: 1 : 3.Figure 3
FESEM images of HAP particles obtained by using IDACS as an organic matrix with different molar ratios of chitosan to imidazole-4,5-dicarboxylic acid. (a) 3 : 1; (b) 2 : 1; (c) 1 : 1; (d) 1 : 2; (e) 1 : 3.
(a)
(b)
(c)
(d)
(e)As it is well known, band assignment for FT-IR of HAP was well established [25]. These absorption bands corresponding to the infrared spectra of the synthesized powders are presented in Figure 1. The presence of two bands around 565 and 604 cm−1 correspond to characteristic ν4 (OPO) bending mode, and the 962 cm−1 band in the spectra was assigned to ν1 (PO) symmetric stretching. 1097 and 1038 cm−1 were assigned to ν3 (PO) antisymmetric stretching mode. The bands at around 1631 cm−1 can be indicated to the bending mode of OH− groups. It is worth noting that the weak peaks of 1462, 1423, and 872 cm−1 were observed in the five spectra, 1458 and 1419 cm−1 are corresponding to ν3 (CO32−) antisymmetric stretching mode, and the band at ∼872 cm−1 are indicated to ν2 (CO32−) bending mode, which is indicated that a small amount of carbonate hydroxyapatite was doped in HAP and they are similar to the main constituent of human bone [26], and it is different from HAP obtained in absence of imidazole-4,5-dicarboxylic acid grafted chitosan [23].The same observation can also be found by XRD analysis (S1–S5 in Figures2(a)–2(e)). The diffraction peaks occurred at 2θ = 25.8°, 32.1°, 33.9°, 39.5°, 46.8°, 49.7°, and 53.5°, corresponding to HAP crystal face (002), (211), (202), (310), (222), (213), and (004), respectively (JCPDS number 03-0747) [19, 22]. It indicated that HAP was all formed with different molar ratio of chitosan to imidazole-4,5-dicarboxylic acid (3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3). Moreover, the intensity of X-ray diffraction patterns increased with the increasing the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid, especially in the face of crystal face (211).In Figure3, FESEM images of the HAP particles obtained at various molar ratio of chitosan to imidazole-4,5-dicarboxylic acid show the morphology of HAP. When the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid are 3 : 1, 2 : 1, 1 : 1, and 1 : 2, the obtained HAP presented aggregated spherical structure nanoparticles with mean diameters ∼30 nm (shown in Figures 3(a)–3(d)). As the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid decreases to 1 : 3, nanorod-like HAP is with the well-distributed diameters of 20–30 nm and length ranging from 75 to 120 nm. The above HAP with small particle sizes and narrow size dispersion can be biomimetic synthesized by using SBF solution. Based on the infrared spectra, XRD patterns, and FESEM images, it can be seen that the morphology of the obtained HAP is more affected by the material ratio of chitosan to imidazole-4,5-dicarboxylic acid than its structure.The following reasons may explain these results, as the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid decreases to 1 : 3, that is to say, using more imidazole-4,5-dicarboxylic acid to modify chitosan, the synthesized imidazole-4,5-dicarboxylic acid grafted chitosan (IDACS), the positive charges on its surface are surplus because of the protonation of imidazole and carboxyl groups. Because of molecular recognition and electrostatic interactions, lots of Ca2+ ions will aggregate on the surface of IDACS and increase surface supersaturation of Ca2+ ions. Meanwhile, the bonding site is offered, which would attract OH− and CO32− ions in the chain of IDACS and easily form the stable HAP nucleus and grow in the special crystal phase in special limited area.


### 3.2. Adsorption Studies

The value of the range shown in the orthogonal array indicated the influence of factors on the test index. Based on the intuitive analysis shown in Table2, the range of pH value, initial concentration of Cr(VI), dosage of HAP, and adsorption time are 9.67, 4.76, 13.81, and 2.15, respectively. It can be shown that the pH value and dosage of HAP are the most important key factors, while the others have insignificant effects. As seen on Table 2, the results revealed that the optimum degradation conditions are as follows: the range of pH value is 3.00, initial concentration of Cr(VI) is 50 mg·L−1, dosage of HAP is 1000 mg·L−1, and adsorption time is 180 min. Herein, the optimum degradation condition is different from the highest removal rate (number 4) in Table 2. Therefore, the removal rate need further research under the optimum degradation condition. Verifying test results show that the obtained HAP is feasible for removal of Cr(VI) under the optimum conditions, and the degradation rate can reach 95.66%.Table 2
Orthogonal experimental design and results.
Number
pH
Initial concentration of Cr(VI) (mg·L−1)
Dosage of HAP (mg·L−1)
Adsorption time (min)
Removal rateη (%)
1
3
25
200
90
80.85
2
3
50
400
120
86.32
3
3
100
800
150
91.73
4
3
200
1000
180
94.45
5
5
25
400
150
82.05
6
5
50
200
180
85.71
7
5
100
1000
90
89.83
8
5
200
800
120
88.36
9
7
25
800
180
88.67
10
7
50
1000
150
93.26
11
7
100
200
120
73.95
12
7
200
400
90
80.28
13
9
25
1000
120
87.45
14
9
50
800
90
83.62
15
9
100
400
180
74.37
16
9
200
200
150
69.25
Verifying test
3
50
1000
180
95.66
ADR1∗
88.34
84.76
77.44
83.65
ADR2∗
86.49
87.23
80.76
84.02
ADR3∗
84.04
82.47
88.10
84.08
ADR4∗
78.67
83.09
91.25
85.80
Range
9.67
4.76
13.81
2.15
IF∗
Secondary
Tertiary
Primary
GeneralBased on the analysis of variance shown in Table3, the dosage of HAP and pH value were significant influencing factors. The other factor is not significant factor. These results are agreement with the intuitive analysis.Table 3
Analysis of variance.
Source of variation
SS
df
MS
F
Significance
pH value
211.18
3
70.39
10.20
∗
Initial concentration of Cr(VI)
54.30
3
18.10
2.62
Dosage of HAP
489.07
3
163.02
23.63
∗
Adsorption time
11.12
3
3.71
0.54
Error
20.70
3
6.90
—
Total
786.37
15
—
—
F0.05 (3, 3) = 9.28; F0.01 (3, 3) = 29.46;∗a significant influencing factors.


### 3.3. Adsorption Mechanism

To understand the reason of removing chromium(VI) by HAP, the TG analysis, surface area, and porosity of the HAP materials are presented in Figures4 and 5, respectively. The results obtained from the TG analysis are shown in Figure 4. As can be seen, the HAP5 obtained in SBF is stable, and the mass loss is about 2.4% of water (the weight loss below 100°C), due to the removal of adsorbed water [27], and the weight loss in the temperature range (40–900°C) investigated is ∼12.1%. Furthermore, the isotherm adsorption curves for HAP5 are shown in Figure 5. In all cases, the isotherm curves have identical behaviors and can be classified as Type IV with loop H3, which is the typical characterization of mesoporous materials, suggesting the possibility of formation of slit-shape pores. Its surface area is 125.28 m2·g−1, the larger surface area of HAP is, the better the adsorption is, which makes HAP5 possess good adsorption capacities for chromium(VI). The pore size distribution is centered in the range of 10–20 nm, as shown in Figure 6. The mean pore size during adsorption process was determined by the BET method. This value for HAP5 is 11.59 nm, which is in agreement with the FESEM image of HAP5 in Figure 3(e).Figure 4
Thermal decomposition of HAP5.Figure 5
Pore diameter distributions of HAP5.Figure 6
N2 adsorption-desorption isotherms of HAP5.


## 3.1. Influence of Molar Ratio of Chitosan/Imidazole-4,5-dicarboxylic Acid on Hydroxyapatite

Figures1–3 present the infrared spectra, XRD patterns, and FESEM images of the HAP crystals (S1–S5) produced with the different molar ratio of chitosan to imidazole-4,5-dicarboxylic acid (3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3) at 37.0°C for aging 24 h, respectively.Figure 1
FT-IR spectra of HAP particles obtained by using IDACS as an organic matrix with different molar ratios of chitosan to imidazole-4,5-dicarboxylic acid. a: 3 : 1; b: 2 : 1; c: 1 : 1; d: 1 : 2; e: 1 : 3.Figure 2
XRD patterns of HAP particles obtained by using IDACS as an organic matrix with different molar ratios of chitosan to imidazole-4,5-dicarboxylic acid. a: 3 : 1; b: 2 : 1; c: 1 : 1; d: 1 : 2; e: 1 : 3.Figure 3
FESEM images of HAP particles obtained by using IDACS as an organic matrix with different molar ratios of chitosan to imidazole-4,5-dicarboxylic acid. (a) 3 : 1; (b) 2 : 1; (c) 1 : 1; (d) 1 : 2; (e) 1 : 3.
(a)
(b)
(c)
(d)
(e)As it is well known, band assignment for FT-IR of HAP was well established [25]. These absorption bands corresponding to the infrared spectra of the synthesized powders are presented in Figure 1. The presence of two bands around 565 and 604 cm−1 correspond to characteristic ν4 (OPO) bending mode, and the 962 cm−1 band in the spectra was assigned to ν1 (PO) symmetric stretching. 1097 and 1038 cm−1 were assigned to ν3 (PO) antisymmetric stretching mode. The bands at around 1631 cm−1 can be indicated to the bending mode of OH− groups. It is worth noting that the weak peaks of 1462, 1423, and 872 cm−1 were observed in the five spectra, 1458 and 1419 cm−1 are corresponding to ν3 (CO32−) antisymmetric stretching mode, and the band at ∼872 cm−1 are indicated to ν2 (CO32−) bending mode, which is indicated that a small amount of carbonate hydroxyapatite was doped in HAP and they are similar to the main constituent of human bone [26], and it is different from HAP obtained in absence of imidazole-4,5-dicarboxylic acid grafted chitosan [23].The same observation can also be found by XRD analysis (S1–S5 in Figures2(a)–2(e)). The diffraction peaks occurred at 2θ = 25.8°, 32.1°, 33.9°, 39.5°, 46.8°, 49.7°, and 53.5°, corresponding to HAP crystal face (002), (211), (202), (310), (222), (213), and (004), respectively (JCPDS number 03-0747) [19, 22]. It indicated that HAP was all formed with different molar ratio of chitosan to imidazole-4,5-dicarboxylic acid (3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3). Moreover, the intensity of X-ray diffraction patterns increased with the increasing the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid, especially in the face of crystal face (211).In Figure3, FESEM images of the HAP particles obtained at various molar ratio of chitosan to imidazole-4,5-dicarboxylic acid show the morphology of HAP. When the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid are 3 : 1, 2 : 1, 1 : 1, and 1 : 2, the obtained HAP presented aggregated spherical structure nanoparticles with mean diameters ∼30 nm (shown in Figures 3(a)–3(d)). As the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid decreases to 1 : 3, nanorod-like HAP is with the well-distributed diameters of 20–30 nm and length ranging from 75 to 120 nm. The above HAP with small particle sizes and narrow size dispersion can be biomimetic synthesized by using SBF solution. Based on the infrared spectra, XRD patterns, and FESEM images, it can be seen that the morphology of the obtained HAP is more affected by the material ratio of chitosan to imidazole-4,5-dicarboxylic acid than its structure.The following reasons may explain these results, as the molar ratio of chitosan to imidazole-4,5-dicarboxylic acid decreases to 1 : 3, that is to say, using more imidazole-4,5-dicarboxylic acid to modify chitosan, the synthesized imidazole-4,5-dicarboxylic acid grafted chitosan (IDACS), the positive charges on its surface are surplus because of the protonation of imidazole and carboxyl groups. Because of molecular recognition and electrostatic interactions, lots of Ca2+ ions will aggregate on the surface of IDACS and increase surface supersaturation of Ca2+ ions. Meanwhile, the bonding site is offered, which would attract OH− and CO32− ions in the chain of IDACS and easily form the stable HAP nucleus and grow in the special crystal phase in special limited area.


## 3.2. Adsorption Studies

The value of the range shown in the orthogonal array indicated the influence of factors on the test index. Based on the intuitive analysis shown in Table2, the range of pH value, initial concentration of Cr(VI), dosage of HAP, and adsorption time are 9.67, 4.76, 13.81, and 2.15, respectively. It can be shown that the pH value and dosage of HAP are the most important key factors, while the others have insignificant effects. As seen on Table 2, the results revealed that the optimum degradation conditions are as follows: the range of pH value is 3.00, initial concentration of Cr(VI) is 50 mg·L−1, dosage of HAP is 1000 mg·L−1, and adsorption time is 180 min. Herein, the optimum degradation condition is different from the highest removal rate (number 4) in Table 2. Therefore, the removal rate need further research under the optimum degradation condition. Verifying test results show that the obtained HAP is feasible for removal of Cr(VI) under the optimum conditions, and the degradation rate can reach 95.66%.Table 2
Orthogonal experimental design and results.
Number
pH
Initial concentration of Cr(VI) (mg·L−1)
Dosage of HAP (mg·L−1)
Adsorption time (min)
Removal rateη (%)
1
3
25
200
90
80.85
2
3
50
400
120
86.32
3
3
100
800
150
91.73
4
3
200
1000
180
94.45
5
5
25
400
150
82.05
6
5
50
200
180
85.71
7
5
100
1000
90
89.83
8
5
200
800
120
88.36
9
7
25
800
180
88.67
10
7
50
1000
150
93.26
11
7
100
200
120
73.95
12
7
200
400
90
80.28
13
9
25
1000
120
87.45
14
9
50
800
90
83.62
15
9
100
400
180
74.37
16
9
200
200
150
69.25
Verifying test
3
50
1000
180
95.66
ADR1∗
88.34
84.76
77.44
83.65
ADR2∗
86.49
87.23
80.76
84.02
ADR3∗
84.04
82.47
88.10
84.08
ADR4∗
78.67
83.09
91.25
85.80
Range
9.67
4.76
13.81
2.15
IF∗
Secondary
Tertiary
Primary
GeneralBased on the analysis of variance shown in Table3, the dosage of HAP and pH value were significant influencing factors. The other factor is not significant factor. These results are agreement with the intuitive analysis.Table 3
Analysis of variance.
Source of variation
SS
df
MS
F
Significance
pH value
211.18
3
70.39
10.20
∗
Initial concentration of Cr(VI)
54.30
3
18.10
2.62
Dosage of HAP
489.07
3
163.02
23.63
∗
Adsorption time
11.12
3
3.71
0.54
Error
20.70
3
6.90
—
Total
786.37
15
—
—
F0.05 (3, 3) = 9.28; F0.01 (3, 3) = 29.46;∗a significant influencing factors.


## 3.3. Adsorption Mechanism

To understand the reason of removing chromium(VI) by HAP, the TG analysis, surface area, and porosity of the HAP materials are presented in Figures4 and 5, respectively. The results obtained from the TG analysis are shown in Figure 4. As can be seen, the HAP5 obtained in SBF is stable, and the mass loss is about 2.4% of water (the weight loss below 100°C), due to the removal of adsorbed water [27], and the weight loss in the temperature range (40–900°C) investigated is ∼12.1%. Furthermore, the isotherm adsorption curves for HAP5 are shown in Figure 5. In all cases, the isotherm curves have identical behaviors and can be classified as Type IV with loop H3, which is the typical characterization of mesoporous materials, suggesting the possibility of formation of slit-shape pores. Its surface area is 125.28 m2·g−1, the larger surface area of HAP is, the better the adsorption is, which makes HAP5 possess good adsorption capacities for chromium(VI). The pore size distribution is centered in the range of 10–20 nm, as shown in Figure 6. The mean pore size during adsorption process was determined by the BET method. This value for HAP5 is 11.59 nm, which is in agreement with the FESEM image of HAP5 in Figure 3(e).Figure 4
Thermal decomposition of HAP5.Figure 5
Pore diameter distributions of HAP5.Figure 6
N2 adsorption-desorption isotherms of HAP5.


## 4. Conclusion

Hydroxyapatite can be biomimetic synthesized successfully and fully characterized. The morphology of the obtained HAP is more affected by the material ratio of chitosan to imidazole-4,5-dicarboxylic acid than its structure. The formed HAP can remove chromium(VI) effectively. It can be expected that the obtained biological materials might be used for removing heavy metal in the environmental remediation or in wastewater.



---

*Source: 5431290-2018-06-03.xml*