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. 2024 Jan 2;9(2):2730–2739. doi: 10.1021/acsomega.3c07725

Facet-Controlled Growth of Hydroxyapatite for Effectively Removing Pb from Aqueous Solutions

Hee Jung Kim †, Jin Hyuk Choi †, SangMyeong Lee †, Gill Sang Han ‡,*, Hyun Suk Jung †,§,*
PMCID: PMC10795148  PMID: 38250348

Abstract

graphic file with name ao3c07725_0006.jpg

To address the growing concerns regarding severe water pollution, effective and environmentally friendly adsorbents must be identified. In this study, we prepared hydroxyapatite (HAp, Ca10(PO4)6(OH)2) as an eco-friendly absorbent via simple precipitation and obtained rod- (r-HAp) and plate-shaped HAp (p-HAp). The approach to obtaining p-HAp involved a low pH titration rate, promoting growth along the c-axis due to the adsorption of OH– on the (110) facet. Conversely, r-HAp was obtained by maintaining a high concentration of OH– during the initial stage through rapid pH titration, leading to a stronger restrictive effect on the growth of positively charged a(b)-planes. p-HAp demonstrated superior adsorption capacity, removing Pb through dissolution and recrystallization, achieving an impressive 625 mg/g within a 60 min reaction time compared to r-HAp. Our findings afford insights into the Pb removal mechanisms of HAp with different morphologies and can aid in the development of water purification strategies against heavy metal contamination.

1. Introduction

Water pollution is a severe environmental concern that can accelerate the water shortage and threaten the environmental ecosystems involving water. Heavy metals are notable water pollutants as they are nondegradable and can bioaccumulate. Lead, listed by the World Health Organization as one of the ten chemicals of public health concern, is a toxic heavy metal that can directly affect the brain, kidneys, nerves, and bones.1−5 Several approaches for removing pollutants have been developed, for example, adsorption,6,7 chemical precipitation,8,9 reverse osmosis,10,11 ion exchange,12−14 and membrane-based purification.15−18 Among these methods, adsorption is a promising strategy for water purification owing to its low cost, simplicity, universality, recycling, and reusability. Many absorbents, such as metal oxides,7,19−21 biochar,22−24 carbons,25,26 zeolite,27,28 and calcium phosphate,29−31 have been developed to remediate heavy-metal-contaminated water.

Hydroxyapatite (HAp, Ca10(PO4)6(OH)2), an eco-friendly biomineral, is one of the most promising absorbents, as it has the highest absorption capacity for heavy metals. The adsorption performance of HAp is strongly influenced by structural properties such as crystallinity, defects, surface texture, specific surface area, and morphology. The lower crystallinity observed leads to increased active surface area32 and weaker chemical bonds,33 contributing to enhanced adsorption properties. Additionally, the low crystallinity introduces a large number of defects, making the material highly degradable. Consequently, Ca ions are easily released from HAp, facilitating ion exchange with other metal ions.34 Xu et al.35 employed manganese-doped HAp (MnHAp) to enhance heavy metal adsorption performance. The low-concentration doping of Mn caused lattice distortion and a substantial number of defects, as smaller Mn ions substituted for Ca ions, resulting in an enhanced adsorption capacity. The heavy metal removal mechanism and performance strongly depend on the morphology of HAp,36−39 as different morphologies are associated with different specific surface areas, mechanical properties, and surface characteristics. Many researchers have attempted to develop HAp with different morphologies, such as rod, plate, dandelion, and spherical particles, by controlling the specific facet growth using anionic or cationic surfactants. Hao et al.40 studied the Pb removal performance of rod- (r-HAp), flake-flower-, and dandelion-shaped HAp. The use of ammonium tartrate and trisodium citrate surfactants resulted in the formation of flake-flower and dandelion morphologies, respectively. In contrast, r-HAp was obtained when no surfactants were used. The dandelion-shaped HAp exhibited the highest maximum adsorption capacity of 819.7 mg/g, owing to the presence of multiple adsorption sites. However, the surfactants used in synthesizing HAp affect its physical and chemical properties such as the surface area, acid sites, and Ca/P ratio,41 which considerably influence the Pb removal ability. Additionally, the high sorption capacity of mesoporous nano-HAp for Pb (1438.85 mg/g) was reported.42 Removal and reuse of Pb were realized using Fe-decorated HAp high sorption capacity with fast kinetics.30 Recently, whitlockite (WH, Ca18Mg2(HPO4)2(PO4)12), with a similar phase as that of HAp, was introduced as an adsorbent for removing Pb.29 WH exhibits a higher Pb adsorption capacity (2339 mg/g) than HAp despite a smaller Brunauer–Emmett–Teller (BET) surface area, potentially because the dissolution and recrystallization mechanisms are in play. Nonetheless, despite the numerous studies on controlling the shape of HAp, there has been limited research conducted to understand the influence of HAp morphology on the Pb adsorption performance.

In this study, we aimed to realize the facet-controlled growth of HAp nanoparticles (NPs) using a simple precipitation method without any surfactants. The HAp morphology was noted to depend on the initial pH conditions. At high pH values, the initial phase transformed from amorphous nuclei to r-HAp, whereas under low initial pH conditions, plate-shaped HAp (p-HAp) was obtained, forming hexagonal apatite. The Pb adsorption performance associated with the different morphologies was investigated in terms of their kinetic models and Pb removal mechanisms. The results indicated that p-HAp removes Pb with dissolution and recrystallization as the main mechanisms. In contrast, the Pb removal mechanism of r-HAp changes from initial complexation with Pb to dissolution and recrystallization over time. Consequently, p-HAp exhibits an adsorption capacity of 625 mg/g, which is two times that of r-HAp (294.99 mg/g), which demonstrates the higher effectiveness of p-HAp in Pb removal.

2. Experimental Section

2.1. Materials

Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, 99%), sodium dihydrogen phosphate dihydrate (NaH2PO4·2 H2O, 99%), gelatin (from bovine skin, Type B), sodium hydroxide (NaOH, 98%, pellets), and lead nitrate (Pb(NO3)2, 99%) were purchased from Sigma-Aldrich and used without purification.

2.2. HAp Synthesis

p-HAp and r-HAp were synthesized via precipitation methods. For p-HAp, NaH2PO4·2 H2O (0.15 mol/m3) and gelatin (0.5 g) were dissolved in 250 mL of deionized water heated to 40 °C and mixed with a solution of Ca(NO3)2·4H2O (0.25 mol/m3) dissolved in 250 mL of deionized water. Subsequently, 20 cm3 of a sodium hydroxide (NaOH) solution (1 mol/m3) was added in a dropwise manner using a syringe pump, and the solution was maintained at 40 °C for 24 h. To obtain r-HAp, NaOH (1 mol/m3) was poured into a solution prepared in the same manner as aforementioned until the pH became 7.5. The solution was heated to 90 °C and maintained at this temperature for 24 h.

2.3. Characterization

The morphologies and elemental distribution of samples were investigated by field emission scanning emission microscopy (FE-SEM) and energy dispersive spectroscopy (EDS)-based elemental mapping (JSM-7600F, JEOL Ltd.). The crystal phases of all the samples were analyzed by X-ray diffraction (XRD) (Rigaku, SmartLab SE) patterns obtained at a scanning rate of 4°/min and step of 0.02°, and the crystallite size was calculated using the Scherrer equation.43,44

2.3.

where L is the crystallite size, K is a constant related to crystallite shape (normally taken as 0.9), λ is the wavelength of X-ray, β is the full-width half-maximum (FWHM), and θ is the diffraction angle.

The morphology and crystal structure of the synthesized samples were determined by transmission electron microscopy (TEM) and selected area electron diffraction (SAED), and the d-spacing was calculated through high-resolution transmission electron microscopy (HR-TEM) (JEM-ARM200F, JEOL Ltd.). The zeta potential in a neutral aqueous solution was determined using an electrophoresis analyzer (ELSZ2000-ZS, Otsuka) by the Doppler method. The Hückel equation was used45 and each measurement was conducted three times for accuracy.

2.4. Pb Removal Experiments

Pb solution was prepared by dissolving Pb(NO3)2 in deionized water at concentrations of 400 mg/m3 and 2100 mg/m3. The synthesized p-HAp and r-HAp were added to Pb solutions with concentrations of 2100 mg/L in dosages of 1 g/m3. The samples were dispersed by sonication for 2 min. Each sample was allowed to react with Pb ions for durations ranging from 10 min to 24 h. After the reaction, the solutions were filtered, and the concentrations of Pb and Ca were measured through inductively coupled plasma-atomic emission spectrometry (OPTIMA 8300, PerkinElmer).

For the analysis of adsorption kinetics, we employed the pseudo-first-order (PFO) and pseudo-second-order (PSO) models.46−48 The adsorption capacities were calculated using the following equation

2.4.

where C0 is the initial concentration, Ct is the remaining concentration, V is the volume of solution, and m is the adsorbent mass.

The PFO kinetics are described by the following equation

2.4.

where qe is the equilibrium absorption capacity, qt is the absorption capacity at each contact time t, and k1 is the PFO constant. This equation can also be defined as

2.4.

The PSO kinetics are given by the following equation

2.4.

where qe is the equilibrium absorption capacity, qt is the absorption capacity at each contact time t, and k2 is the PSO constant. This equation can also be defined as

2.4.

2.5. Simulations

Simulations were conducted to examine the crystal structures of HAp. The optimized data of all crystal structures were derived from the Materials Project [hydroxyapatite (mp-41472)].49 Avogadro 2 was used to obtain the simulation data for molecular electrostatic potential mapping.50 Other simulation data were obtained by Quantum Espresso, an open-source density functional theory calculation software package in Materials Square. The molecular electrostatic potential mapping was simulated using a 5 × 5 × 1 superlattice of the (001) and (110) planes. The periodic potential conditions were determined using conventional solid-state pseudopotentials (PSLibrary_PAW_PBE). The cohesive energy was simulated using a 10 × 10 × 10 Γ-centered k-mesh in a conventional hydroxyapatite crystal structure. Additionally, surface energies were simulated using a 5 × 5 × 1 Γ-centered k-mesh in a five-layer stacked (001) surface with 20 of vacuum and a 4 × 4 × 1 Γ-centered k-mesh in a three-layer stacked (110) surface with 20 of vacuum [(001) plane: 240 atoms, (110) plane: 298 atoms]. Moreover, crystal structure images were obtained using VESTA3 software.51

3. Results and Discussion

HAp NPs were synthesized via a simple precipitation method by adjusting the titration rate of NaOH, as illustrated in Figure 1a. The morphology of HAp was easily controlled by adjusting the pH conditions during the synthesis process, which typically affects the electrostatic potential and surface energies of the facets in the HAp crystal. The initial pH of the mixed solution, including calcium and phosphorus, was low (4.6). The pH gradually changed from 4.6 to 9.5 when NaOH was added in a dropwise manner for 24 h. In contrast, in the pouring process, the pH immediately increased to 7.5 and was maintained until the end of the reaction, as shown in Figure S1. These different synthetic pH environments induced different crystal growth in the p-HAP and r-HAp NPs, as shown in Figure 1b,c. The initial phase of r-HAp NPs involved tiny and spherical amorphous calcium phosphate (ACP) particles with sizes less than 50 nm, and this phase then transformed into the hexagonal apatite phase. (Figure 1d, Figures S2 and S3) In general, metastable ACP can easily transform to HAp through dissolution and recrystallization at high pH.52 In contrast, the p-HAp NPs directly formed a hexagonal apatite phase owing to the introduction of appropriate pH conditions in which apatite is energetically stable,53 followed by crystal growth with the initial plate morphology. To clarify the growth mechanism for p-HAp and r-HAp, the (001) and (110) facet surface energies of HAp and its a(b)-plane and c-plane molecular electrostatic potential were calculated using the Quantum Espresso module. The results are presented in Figure 1e and Table 1. Based on the HAp crystal structure, the c-planes, perpendicular to the c-axis in the hexagonal channels, were negatively charged because the structure was rich in phosphate (PO43–) and hydroxide ions (OH–). In contrast, the a(b)-planes were abundant in positive calcium ions (Ca2+), which affected the nucleation and growth of HAp through the electrostatic interaction with negatively charged OH– ions. In the initial stage of dropwise addition, heterogeneous nucleation and growth were dominant as the OH– ions restricted the growth of the (110) plane at low pH. The OH– ions first adsorbed onto the (110) facet of HAp owing to its negative charge and higher surface energy compared with those of other facets of HAp (Table 1), thereby limiting the Ca2+ and PO43– interactions for growth in that facet.54,55 Thus, the HAp nuclei preferred to grow parallel to the (110) direction, resulting in a plate morphology of the initial crystallite along the c-axis, as illustrated in Figure 1a. Subsequently, highly crystalline p-HAp was obtained after the reaction proceeded for 24 h. In contrast, the pouring of NaOH resulted in a high concentration of OH– ions in the initial step, thereby influencing the homogeneous nucleation and growth of the primary particles. The abundance of OH– ions led to growth along the c-axis owing to its stronger restrictive effect on the growth of positively charged facets (a(b)-planes), as depicted in Figure 1a, resulting in the formation of rod-shaped NPs during the phase transformation from ACP to HAp.54−57 The crystallite size of both types of HAp was determined through XRD analysis, as shown in Figure S4. Typically, the (002) peak is employed for calculating the crystallite size of HAp due to its distinctive and sharp peak shape.58−60 The p-HAp exhibited a smaller crystallite size of 26.8 nm, indicating lower crystallinity compared to that of the r-HAp (crystallite size: 38.7 nm). In addition, the crystallite sizes of both HAp types, calculated from (300) peaks, were smaller than those derived from the (002) peak, indicating growth along the c-axis.58,59

Figure 1.

Figure 1

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(a) Schematic of HAp synthesis by adjustment of the pH. FE-SEM images of synthesized (b) p-HAp and (c) r-HAp. (d) XRD patterns of as-synthesized samples. (e) Crystal structures and molecular electrostatic potential surface mappings corresponding to the a(b)-plane and c-plane of HAp.

Table 1. Comparison of Surface Energies of Different Facets of HAp.

material surface surface energy (kg/s2)
HAp (001) 1.8950
  (110) 4.4864
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The structural characteristics of p-HAp and r-HAp were analyzed by HR-TEM. The p-HAP NPs exhibited a plate-type morphology with diameters ranging from 50 to 500 nm and thicknesses lower than 10 nm. In the SAED patterns (inset of Figure 2a), the bright pattern associated with the (002), (211), (112), and (300) planes indicated the presence of highly crystalline HAp. The d-spacings of p-HAp NPs were 0.344 and 0.278 nm with an angle of 36.2°, which were associated with the (002) and (112) crystal planes, respectively (Figure 2b), and indicated the a(b)-plane growth of HAp. Figure 2c shows the fast Fourier transform (FFT) patterns, which indicate that the (004), (002), (112), and (110) planes along the [1–10] zone axis demonstrate growth parallel to the c-axis. Figure 2d shows the rod-shaped particles with a d-spacing of 0.344 nm, corresponding to the (002) plane of HAp. (Figure 2e) The SAED pattern and FFT image can be indexed to the (211), (210), and (002) planes of the crystalline hexagonal HAp structure, suggesting the occurrence of preferentially oriented growth along the [001] direction, as shown in Figure 2d–f. These results are consistent with the growth mechanism illustrated in Figure 1a.

Figure 2.

Figure 2

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HR-TEM and FFT images of synthesized (a,c) p-HAp and (d–f) r-HAp. The insets of (a,d) show the corresponding SAED patterns.

The Pb removal capacities of both synthesized HAp NPs were evaluated at pH 5 in an aqueous solution over a period of 1 h. The sorption kinetics of p-HAp and r-HAp were investigated by fitting PFO and PSO models, as shown in Figure 3a–c. Both p-HAp and r-HAp were well fitted to the PSO kinetic model with higher linear regression coefficient (R2) values than those of the PFO model (Figure 3a,b and Table S1), which indicated that the Pb removal process was dominated by chemisorption.61 p-HAp exhibited slower adsorption kinetics than r-HAp, as evidenced by its lower second-order adsorption rate constant (k2) of 8.28 × 10–4 g/mg min. However, p-HAp had a significantly higher expected maximum adsorption capacity (qe,cal) of 625 mg/g compared to r-HAp, which was well-validated by the experimental data. This adsorption capacity of p-HAp demonstrates a relatively high level, even in comparison with previously reported HAp (Table S3). The lower crystallinity observed in p-HAp (Figure S4) can contribute to an enhanced dissolution behavior, consequently resulting in a higher adsorption capacity.32,33,62 This lower crystallinity also includes a large number of defects, facilitating ion exchange reactions between cations in HAp and the solution.34 In addition, p-HAp had a slightly higher BET surface area of 111.88 ± 0.58 m2/g compared with that of r-HAp (101.01 ± 0.43 m2/g), along with a higher surface energy (Table 1). The surface potential of the a(b)-plane exhibited a charge that was more negative than that of the c-plane of HAp. In other words, the enhanced electrostatic interaction with positively charged Pb ions under these pH conditions resulted in the fast kinetics and large adsorption capacity of p-HAp (Table S2). To clarify the Pb removal reaction mechanism, we analyzed the characteristics of the Pb and Ca ions dissolved in the aqueous solution during the Pb removal process. Figure 3d shows the three mechanisms of Pb removal by HAp, i.e., cation exchange, complexation, and dissolution/recrystallization, expressed as in eqs 1–3.

3. 1
3. 2
3. 3

Figure 3.

Figure 3

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(a) Pb sorption kinetics of p-HAp and r-HAp fitted with PFO and PSO models, and linear fitting plots of the (b) PFO and (c) PSO models [Pb concentration: 2100 mg/m3, HAp amounts: 1 mg/cm3, reaction time: 60 min]. (d) Schematic of three mechanisms for Pb removal by HAp. (e) Ratio of the removed Pb to dissolved Ca during the removal process. A ratio larger than one indicates excess Pb removal compared to Ca dissolution, indicating the occurrence of surface complexation.

Typically, cation exchange and dissolution/recrystallization occur when Pb and Ca are exchanged in a ratio of 1/1, whereas surface complexation removes Pb without any loss of cations in the HAp structure. Thus, a Pb/Ca ratio approaching 1 indicates that cation exchange and dissolution/recrystallization are the dominant mechanisms for Pb removal, whereas a value higher than 1 suggests that surface complexation contributes to Pb removal.63−65 As shown in Figure 3e, the Pb/Ca ratios were ∼1.5 and 1.22 for r-HAP and p-HAp, respectively. In other words, the dominant removal mechanism for r-HAp was surface complexation and that for p-HAp was ion exchange and dissolution/recrystallization. Thus, the Pb removal capacities of r-HAp and p-HAp were expected to be different.

Figure 4a shows the XRD patterns of p-HAp and r-HAp NPs during the Pb removal process. For p-HAp, new peaks were observed at 21.56°, 26.04°, 27.48°, and 30.04°, which corresponded to the (111), (102), (210), and (112) planes of lead hydroxypyromorphite (LHy). The intensities of the (002) and (211) planes of HAp significantly decreased even in the reaction that lasted for only 10 min. The p-HAp NPs immediately transformed to rod-shaped LHy with lengths of a few hundred nanometers after Pb removal, as shown in Figure 4c–e. This dramatic morphology change was attributable to the occurrence of dissolution/recrystallization rather than complexation during the Pb removal process.29,65 In comparison, the r-HAp NPs underwent a slow reaction with Pb ions. Thus, the HAp phase and morphology did not change considerably even after the 60 min reaction, as depicted in Figure 4b,f–h). This phenomenon can be attributed to pseudomorphism: The direct replacement of Ca2+ with Pb2+ by ion exchange helped preserve the initial shape and size of r-HAp.66,67

Figure 4.

Figure 4

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XRD patterns of (a) p-HAP and (b) r-HAp with different sorption time values. FE-SEM images of (c–e) p-HAP and (f–h) r-HAP after 10, 30, and 60 min of removal. (a,c–e) The shape and crystal structure of p-HAp change dramatically from HAp to LHy, whereas (b,f–h) those of r-HAp do not change significantly.

Over the reaction period, p-HAp transformed to a highly crystalline LHy phase, which was rod shaped with a length of several micrometers (Figures 5a and S5(a–c)). The EDS mapping and relative composition under different reaction periods were evaluated (Figure 5c,d, Figure S6, and Table S4). In the EDS mapping results of p-HAp, no Ca was detected after a reaction time of 72 h, suggesting that the p-HAp was completely transformed to LHy. In addition, as shown in Figure 5e, the Pb removal capacity of p-HAp reached equilibrium at the reaction time of 24 h, which indicated that p-HAp completely removed Pb by dissolution/recrystallization within 24 h. In contrast, r-HAp maintained its small rod shape with the coexistence of the low-crystalline HAp and LHy phases at 24 h, but a large rod shape with the highly crystalline LHy phase was obtained after 72 h (Figures 5b and S5(d–f)). The Pb removal capacity of r-HAP gradually increased after 120 h. In the kinetic curves of r-HAp, an inflection point was observed at approximately 24 h (Figure 5e), suggesting a change in the Pb removal mechanism. Figure 5f shows the Pb/Ca ratio during the long-term Pb removal process. The high Pb/Ca ratio of r-HAp gradually decreased over time and approached that of p-HAp (approximately 1.1). This observation indicated that the dominant Pb removal mechanism of r-HAp changed from ion exchange in the initial stage to dissolution/recrystallization in the final stage. However, r-HAp exhibited the presence of Ca even after 120 h (Figure 5d), indicating an incomplete phase change to LHy. Consequently, p-HAp could remove Pb faster and better than r-HAp. Moreover, the final product after the removal reaction was pure LHy without any residual cations, which can promote Pb reuse or recycling.29

Figure 5.

Figure 5

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XRD patterns of (a) p-HAp and (b) r-HAp with different sorption time values. FE-SEM images and corresponding EDS mapping of (c) p-HAp and (d) r-HAp after sorption for 120 h. (e) Pb sorption kinetics of p-HAp and r-HAp [Pb concentration: 2100 mg/m3, HAp amount: 1 mg/cm3, reaction time: 120 h] and (f) ratio of removed Pb to dissolved Ca during the sorption process.

4. Conclusions

HAp with different morphologies (plate- and rod-shaped) was obtained, and the XRD pattern and TEM analysis results were consistent with the HAp crystal structure. We investigated the three removal mechanisms of the as-synthesized HAp: cation exchange, complexation, dissolution, and recrystallization. The Pb removal mechanisms of p-HAp and r-HAp were considerably different. For p-HAp, dissolution and recrystallization were predominantly observed from 10 min. In contrast, the dominant Pb removal mechanism of r-HAp changed from ion exchange to dissolution and recrystallization over time. Overall, the final dominant mechanism for both p-HAp and r-HAp was dissolution and recrystallization. Clarification of the Pb2+ removal mechanism is expected to facilitate the development of effective soil, air, and water purification strategies.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF, 2022M3J1A1064315) funded by the Ministry of Science and ICT (MSIT), and the Korea Evaluation Institute of Industrial Technology (KEIT, 20016588) funded by the Ministry of Trade, Industry and Energy (MOTIE). This Research has been performed as a project NO. KS2322-20 (A Study on the Convergence Materials for Off-Grid Energy Conversion/Storage Integrated Devices) and supported by the Korea Research Institute of Chemical Technology (KRICT). This research was supported by the SungKyunKwan University and the BK21 FOUR (Graduate School Innovation) funded by the Ministry of Education (MOE, Korea) and the National Research Foundation of Korea (NRF).

Glossary

Abbreviations

ACP

amorphous calcium phosphate

EDS

energy dispersive spectroscopy

FFT

fast Fourier transform

PFO

pseudo-first-order

PSO

pseudo-second-order

SAED

selected area electron diffraction

TEM

transmission electron microscopy

XRD

X-ray diffraction

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c07725.

  • Additional pH analysis, FE-SEM images, XRD, EDS mapping, adsorption kinetics analysis, BET, zeta potential, and table for adsorbent comparison (PDF)

Author Contributions

∥ H.J.K. and J.H.C. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao3c07725_si_001.pdf (760.7KB, pdf)

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