# Copper Adsorption Using Hydroxyapatite Derived from Bovine Bone

Authors: Lingchang Kong; Xin Liu; Guocheng Lv; Tianming Liu; Peijun Zhang; Yuxin Li; Bin Chen; Libing Liao
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

Mining and smelting effluent have resulted in heavy metal-contaminated groundwater. Copper-polluted groundwater poses a severe threat to human health and the ecological environment. Permeable reactive barrier (PRB) has been rapidly developed as the in situ remediation technology to control toxic copper migration. Low cost, seepage stability, and great longevity are considered within PRB reactive media. In this paper, hydroxyapatite derived from bovine bone was proven to be a suitable adsorbent owing to cost-effectiveness, great adsorption capacity, and longevity. Batch experiments were carried out to determine the copper adsorption behavior as a function of copper concentration and contact time. Adsorption isotherm was represented by the Langmuir isotherm model, and the adsorption capacity of 25.7 mg/g was superior to most of the adsorbents. A kinetic study was accurately fitted by the pseudo-second-order kinetic model interpreted as a chemical reaction. In addition, the column study confirmed hydroxyapatite has excellent hydraulic performance with no clogging phenomenon happened. At C/C0 = 0.5, the number of pore volume (PV) reached 450. The batch and column experiments also revealed that the overall adsorption process followed up the monolayer chemisorption. Furthermore, systematic analyses demonstrated that surface adsorption was responsible for the copper removal by hydroxyapatite based on experimental analysis and density functional theory (DFT) calculations. This work provides an alternative strategy as filling material for in situ remediation of copper-contaminated groundwater and enriches relevant theoretical references.


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## Body

## 1. Introduction

Groundwater is essential for human life and industrial production as a vital water resource [1]. Nowadays, mining and smelting effluent have resulted in more and more heavy metal-contaminated groundwater [2, 3]. Among the toxic heavy metals, copper has posed a severe threat to human health and the natural ecosystem because of accumulation and degradation resistance [4]. The copper concentration is in the range of 1.2 mg/L–200 mg/L under acid mine drainage and polluted groundwater [5, 6]. The copper concentration even reached 1000 mg/L in industrial wastewater [7]. However, the maximum copper concentration of direct discharge is 0.1–0.5 mg/L in different wastewater discharge standards [8, 9]. Hence, efficient remediation of heavy metal polluted groundwater has attracted extensive attention worldwide [10]. Various technologies aim to address this problem, including ex situ remediation using a pump-and-treat (P&T) system or in situ remediation through the permeable reactive barrier (PRB) [11, 12]. P&T technology has been widely used for controlling contaminant migration, but the complicated management, lack of long-term operation, and secondary pollution have remained unsolved [13]. In contrast, PRB technology has been proven to be a promising strategy due to its cost-effectiveness and sustainability [14]. Thus, PRB was rapidly developed as the green technology for remediating polluted groundwater. The contaminant was immobilized by the reactive media in the filling well through precipitation, adsorption, or reduction into a low-toxicity pollutant [15]. Therefore, the research on filling material for PRB is crucial for heavy metal polluted groundwater [16].High porosity, effect-cost, seepage stability, and longevity of filling media have been increasingly considered for practical application at pilot or field-scale [17]. Zero-valent iron material is widely applied to PRB due to its outstanding reductivity and adsorption performance [18], but easy oxidation and blocking inhibit its application. Nowadays, studies on natural waste resources as PRB reactive material have developed into hot points. Compared with the expensive synthetic methods, hydroxyapatite derived from animal bone, such as fish bone, chicken bone, and bovine bone, has been developed as an adsorbent for heavy metal polluted water or soil because of its porous structure and excellent adsorption capacity [19–29]. The previous finding suggested that hydroxyapatite could serve for heavy metal-contaminated groundwater, such as uranium, lead, manganese, and copper [30–33]. However, study has not involved in the laboratory column experiment, especially copper flow and transport on hydroxyapatite. Laboratory column experiment could simulate the groundwater condition to explore the flow and pollutant transport model in porous media [34].Crystallinity plays an important role in the adsorption performance of heavy metals by hydroxyapatite [35]. The previous finding revealed that crystallinity or morphology can be affected by sintering temperature, especially for animal bone [36]. Under low temperatures (<∼ 600°C), because organic components in the animal bone cannot be completely thermal decomposed, hydroxyapatite derived from the animal bone has low crystallinity with poor mechanical strength; however, it has higher adsorption capacity (e.g., 99.98 mg/g at 400°C) [37, 38]. Under high temperatures (>∼ 600°C), hydroxyapatite has higher crystallinity with excellent mechanical strength; however, it has lower adsorption capacity [39]. Hydroxyapatite would transform into oxy-hydroxyapatite through dehydroxylation and even decomposed into calcium phosphate with higher sintering temperatures (>1000°C) [40, 41]. The systematic effect of mechanical strength, pore structure, and adsorption capacity should be comprehensively taken into account in determining the suitable temperature. Therefore, it is essential to investigate the effect of interaction between crystallinity and calcination temperature on copper removal by hydroxyapatite [42].It was believed that hydroxyapatite has a significant effect on remediation of heavy metal polluted water or soil, and the removal mechanism has been extensively studied [43]. The previous studies demonstrated that surface adsorption was mainly attributable to favorable interaction with the functional group on the surface of hydroxyapatite, such as hydroxyl and phosphate [44, 45]. However, the comprehensive explanation of adsorption sites not be fully revealed completely through traditional analyses [46–48]. Hence, the detailed model with a specific binding site is the key to interpret adsorption process. Simulation calculation was extensively used to study the adsorption behavior on the surface of hydroxyapatite as supplementary approach [49–51].In this paper, batch and column experiments were designed to investigate the adsorption performance of copper by hydroxyapatite derived from the bovine bone. The adsorption mechanism was studied in-depth through the experimental analysis methods, such as X-ray diffraction (XRD) combined with Rietveld refinement [52, 53], X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscope (HRTEM), and DFT calculations. This work provides the primary reference for evaluating the copper removal mechanism on hydroxyapatite as reactive media within PRB for groundwater remediation.


## 2. Materials and Methods

### 2.1. Reagents

Copper nitrate (Cu(NO3)2.2H2O) used in these experiments was supplied by West Long Chemical Co., Ltd. Copper standard solution was purchased from Guobiao (Beijing) Testing & Certification Co., Ltd. The bones of bovine were bought from Weihai, China.


### 2.2. Batch Experiments

The adsorption performance was studied by batch equilibrium experiments. The initial concentration range of copper was 0–500 mg/L. 0.2 g hydroxyapatite was added into 30 mL copper solution for 72 h. The equilibrated suspensions were centrifuged for 5 min at 8000 rpm. Langmuir adsorption isotherm was utilized to simulate the adsorption behavior [54]. For adsorption kinetic, 0.2 g hydroxyapatite was added into a 30 mL copper solution for different reaction times. The pseudo-second-order kinetic model was used to fit the copper uptake kinetics on hydroxyapatite [55]. The above experiments for each condition were conducted in triplicates.


### 2.3. Column Experiment

The experiment used a glass column (length: 15.5 cm; inner diameter: 3.6 cm) equipped with sampling ports at the top. The column was packed with 20 g hydroxyapatite in the middle of the column together with quartz at the top and base of the column. In the column experiment, the 20 mg/L copper solution was pumped upward through the column at a constant flow rate of 0.612 mL/min to simulate a flow rate of 1 m/d for the groundwater environment. Detailed information regarding the column is presented in TableSI.1. The groundwater model software (FEFLOW 6.0) was carried out to simulate the results from the column experiment. The mathematical model was described as the one-dimensional of copper solution undergoing equilibrium sorption in porous hydroxyapatite column based on the adsorption mechanism.


### 2.4. Characterization of Hydroxyapatite before/after Adsorbing Copper

The thermogravimetric behavior of bovine bone was investigated through thermogravimetric-differential scanning calorimetry (Diamond TG-DSC, PE) at a heating rate of 10°C/min under an air atmosphere. The element content of hydroxyapatite was measured by X-ray Fluorescence Spectrometer (XRF, ZSX PrimusII). Nitrogen isotherms were investigated using ratio surface and porosity analyzer (BET, Autosorb-iQ). Surface areas were recorded through Brunnauer–Emmett–Teller (BET) equation fitting. Phase analysis was analyzed by powder X-ray diffractometer (XRD, SmartLab) from 5° to 120° with a step of 0.02, speed of 2°/min with Cu Kα radiation at 40 kV and 200 mA. Rietveld refinement of solids was conducted using FullProf software. Inductively coupled plasma atomic emission spectrometry (ICP-OES, ICAP 7600) was used for the determination of copper concentration. Surface chemical composition was measured through XPS (Thermo Scientific Escalab 250Xi+) equipped with an Al Kα X-ray as the excitation source, and the spectra binding energy was calibrated with the C 1s peak at 284.8 eV. Morphology and element distribution were speculated by a high-resolution transmission electron microscope (HRTEM, TalosF200X). The adsorption energy was calculated through the Materials Studio software package with the CASTEP module and ultrasoft pseudopotentials, where the energy cut-off was 520 eV, and the energy change was less than 1 × 10−6 eV. The optimized structure was adopted to make a (001) surface adsorption model with 14 Å thick bulk hydroxyapatite and a 24 Å thick vacuum layer. The vacuum distance (24 Å) is enough to avoid interaction between periodic configurations. DFT calculations were conducted under the Monkhorst–Pack k-mesh of 3 × 3 × 2 in the Brillouin zone. The adsorption energy is as follows:(1)Eads=ETotal−EHAP−ECu,where E(Total) is the total energy of the hydroxyapatite adsorbing copper, E(HAP) is the energy of bulk hydroxyapatite, and E(Cu) is the energy of the isolated copper atom.


## 2.1. Reagents

Copper nitrate (Cu(NO3)2.2H2O) used in these experiments was supplied by West Long Chemical Co., Ltd. Copper standard solution was purchased from Guobiao (Beijing) Testing & Certification Co., Ltd. The bones of bovine were bought from Weihai, China.


## 2.2. Batch Experiments

The adsorption performance was studied by batch equilibrium experiments. The initial concentration range of copper was 0–500 mg/L. 0.2 g hydroxyapatite was added into 30 mL copper solution for 72 h. The equilibrated suspensions were centrifuged for 5 min at 8000 rpm. Langmuir adsorption isotherm was utilized to simulate the adsorption behavior [54]. For adsorption kinetic, 0.2 g hydroxyapatite was added into a 30 mL copper solution for different reaction times. The pseudo-second-order kinetic model was used to fit the copper uptake kinetics on hydroxyapatite [55]. The above experiments for each condition were conducted in triplicates.


## 2.3. Column Experiment

The experiment used a glass column (length: 15.5 cm; inner diameter: 3.6 cm) equipped with sampling ports at the top. The column was packed with 20 g hydroxyapatite in the middle of the column together with quartz at the top and base of the column. In the column experiment, the 20 mg/L copper solution was pumped upward through the column at a constant flow rate of 0.612 mL/min to simulate a flow rate of 1 m/d for the groundwater environment. Detailed information regarding the column is presented in TableSI.1. The groundwater model software (FEFLOW 6.0) was carried out to simulate the results from the column experiment. The mathematical model was described as the one-dimensional of copper solution undergoing equilibrium sorption in porous hydroxyapatite column based on the adsorption mechanism.


## 2.4. Characterization of Hydroxyapatite before/after Adsorbing Copper

The thermogravimetric behavior of bovine bone was investigated through thermogravimetric-differential scanning calorimetry (Diamond TG-DSC, PE) at a heating rate of 10°C/min under an air atmosphere. The element content of hydroxyapatite was measured by X-ray Fluorescence Spectrometer (XRF, ZSX PrimusII). Nitrogen isotherms were investigated using ratio surface and porosity analyzer (BET, Autosorb-iQ). Surface areas were recorded through Brunnauer–Emmett–Teller (BET) equation fitting. Phase analysis was analyzed by powder X-ray diffractometer (XRD, SmartLab) from 5° to 120° with a step of 0.02, speed of 2°/min with Cu Kα radiation at 40 kV and 200 mA. Rietveld refinement of solids was conducted using FullProf software. Inductively coupled plasma atomic emission spectrometry (ICP-OES, ICAP 7600) was used for the determination of copper concentration. Surface chemical composition was measured through XPS (Thermo Scientific Escalab 250Xi+) equipped with an Al Kα X-ray as the excitation source, and the spectra binding energy was calibrated with the C 1s peak at 284.8 eV. Morphology and element distribution were speculated by a high-resolution transmission electron microscope (HRTEM, TalosF200X). The adsorption energy was calculated through the Materials Studio software package with the CASTEP module and ultrasoft pseudopotentials, where the energy cut-off was 520 eV, and the energy change was less than 1 × 10−6 eV. The optimized structure was adopted to make a (001) surface adsorption model with 14 Å thick bulk hydroxyapatite and a 24 Å thick vacuum layer. The vacuum distance (24 Å) is enough to avoid interaction between periodic configurations. DFT calculations were conducted under the Monkhorst–Pack k-mesh of 3 × 3 × 2 in the Brillouin zone. The adsorption energy is as follows:(1)Eads=ETotal−EHAP−ECu,where E(Total) is the total energy of the hydroxyapatite adsorbing copper, E(HAP) is the energy of bulk hydroxyapatite, and E(Cu) is the energy of the isolated copper atom.


## 3. Results and Discussion

### 3.1. Effect of Calcination on the Hydroxyapatite

Calcination temperature is a critical factor in determining the crystallinity, grain size, and pore structure of hydroxyapatite. To investigate the effect of calcination temperature on the bovine bone, comprehensive analyses are employed to discuss the hydroxyapatite formation from bovine bone. The thermogravimetric analysis is essential for investigating the hydroxyapatite extraction process from bovine bone (Figure1(a)). A slight weight loss of 8% is attributed to the evaporation of adsorbed water at low temperatures (<120°C). The DSC curve shows a low endothermic peak correspondingly. The decomposition of organic matter causes a weight loss of 36.7% at 120–540°C. This reaction is followed by broad endothermic peaks correspondingly. A slight weight loss of 2.4% at 540–900°C can occur due to the dehydroxylation of hydroxyapatite promoted by heat.Figure 1
(a) TG-DSC curve of bovine bone. (b) XRD patterns of hydroxyapatite under different calcination temperatures. (c) N2 adsorption isotherms of hydroxyapatite under different calcination temperatures. (d) Surface parameters of hydroxyapatite under different calcination temperatures.
(a)(b)(c)(d)To control the crystallinity and porosity, a series of characterizations systematically depict the crystallization process of the hydroxyapatite phase during the heat sintering. XRD analysis confirms that all powders under different sintering temperatures are hydroxyapatite (Ca10(PO4)6(OH)2, JCPDS 84-1998). The crystallinity increases with increasing temperature due to the broad diffraction peaks transform into sharp diffraction peaks (Figure 1(b)). These results agree with previous reports [39, 41, 56]. In the process of heat treatment, the crystal phase of hydroxyapatite gradually emerges as the calcination temperature increases between 500°C and 600°C. The organic matter is decomposed into carbon dioxide through carbonation and decarboxylation. The nitrogen adsorption isotherms of hydroxyapatite under different temperatures belong to V-type adsorption isotherms, indicating that hydroxyapatite has a pore structure with the coexistence of micropores and mesopores (Figure 1(c)). The specific surface area, specific volume, and pore diameter also decrease with the increase in calcination temperature (Figure 1(d)). Specific surface area is 11.43 m2/g under 600°C. XRF analysis confirms that hydroxyapatite under 600°C mainly consists of three components, containing calcium, oxygen, phosphorus (Table SI.2). Hence, 600°C is the suitable sintering temperature for hydroxyapatite derived from bovine bone.


### 3.2. Batch Experiment

The adsorption amount as the function of copper concentration is shown in Figure2(a). The adsorption capacity of 25.7 mg/g is superior to most of the adsorbents (Table 1) [53–64]. The copper adsorption isotherm agrees well with the Langmuir model interpreting as homogeneous monolayer adsorption. The detailed parameters are shown in Table SI.3. The R2 value of the copper fitting model reaches 0.9993 (Figure 2(b)). The effect of contact time on the copper uptake is investigated to explore the reaction kinetic, which is shown in Figure 2(c). The adsorption process of copper by hydroxyapatite is fitting well with the pseudo-second-order kinetic model indicating the chemisorption. It can be found that the adsorption capacity of copper increases rapidly in the first 10 h. However, only a slight increase in adsorption capacity is observed after 10 h. It declares that its adsorption process consists of two parts: adsorption by available surface and further transformation from the solid-liquid interface to the pore of hydroxyapatite. The parameter values obtained through the pseudo-second-order kinetics model fitting are shown in Table SI.3. The R2 value of copper fitting models is 0.9868 correspondingly (Figure 2(d)). It can be concluded that copper removal by hydroxyapatite is mainly rate-limited chemisorption.Figure 2
(a) Adsorption equilibrium isotherm of copper on hydroxyapatite. Langmuir model fitting. (b) Linear plot of Langmuir model for adsorption isotherm. (c) Adsorption kinetic of copper on hydroxyapatite. Pseudo-second-order kinetic model fitting (adsorbent dosage: 0.2 g; copper concentration: 500 mg/L; solid/solution ratio: 6.7 g/L at 25 ± 2°C). (d) Linear plot of pseudo-second-order kinetic model for adsorption kinetics.
(a)(b)(c)(d)Table 1
Adsorption capacity of copper by various absorbent.
AdsorbentQm(mg/g)ReferencesRaw pomegranate peel30.12Ben-Ali et al. 2017 [57]Pine fruit14.1Najim et al. 2009 [58]Fe5C2@SiO237.73Ahmadpoor et al. 2019 [59]Bioball5.60Çelebi 2021[60]Chicken feather7.84Solis-Moreno et al. 2021 [61]Chicken bone charcoal15.057Niu et al. 2021 [62]Activated carbon13.05Imamoglu and Tekir 2008 [63]Rice shell2.954Aydin et al. 2008 [64]Hydroxyapatite25.7Present study


### 3.3. Column Experiment

The removal behavior of copper by hydroxyapatite is further evaluated by performing a column study. The breakthrough curve of copper for a column packed with hydroxyapatite is shown in Figure3(a). At C/C0 = 0.5, the number of pore volume (PV) is 450, suggesting excellent column durability [65]. Hydroxyapatite has a stable hydraulic performance to copper because no clogging phenomenon happened in the column. Based on the simulation model, the breakthrough curve of different copper concentrations has been further predicted through FEFLOW software (Figure 3(b)). The breakthrough time decreases with the increase in input copper concentration. The half-life of the reaction reaches 43 d when the input concentration of copper is 15 mg/L. The above results confirm that hydroxyapatite derived from bovine bone is highly effective in in situ remediation of copper-contaminated groundwater as a suitable filling material within PRB. This dynamic column study provides a good prospect for removing copper in field application.Figure 3
(a) Experimental and fitting breakthrough curve of copper for a column packed with hydroxyapatite. The input concentration of copper is 20 mg/L. Adsorbent mass is 20 g. (b) Predicting breakthrough curve of copper for a column packed with hydroxyapatite for different copper concentrations.
(a)(b)


### 3.4. Adsorption Mechanism

XRD combined with Rietveld refinement is used to analyze the product after adsorption shown in Figure4(a). The result represents that the product after adsorbing copper belongs to hydroxyapatite phase (Ca10(PO4)6(OH)2) according to JCPDS 84-1998. Rietveld refinement defines that the calcium site and phosphate site were fully occupied. The fitting coefficients are Rp of 9.61% and Rwp of 13.5%. The functional groups of hydroxyapatites before/after adsorbing copper are identified through FTIR analysis, and corresponding results are presented in Figure 4(b). It can be seen that characteristic peak of hydroxyapatite before/after adsorbing copper is approximately identical. The hydroxyl stretching vibration band at 3560 cm−1 is presented. The characteristic peaks at 1030 cm−1, 609 cm−1, and 545 cm−1 are attributed to asymmetric stretching of PO43−. Besides, the carbonate group is observed at 1407 cm−1, which indicates slight substitution of carbonate group into phosphate group in the hydroxyapatite lattice.Figure 4
(a) X-ray diffraction patterns with Rietveld refinement of hydroxyapatite adsorbing copper. (b) FTIR spectra before/after adsorbing copper by hydroxyapatite.
(a)(b)XPS analysis (Figure5) is consistent with the XRD data. The fitted Cu 2p spectra (Figure 5(a)) show two peaks of Cu 2p1/2 and Cu 2p3/2 at the binding energy of 953.25 eV and 933.5 eV, respectively [66]. The spin-orbit splitting E(Cu 2p1/2) − E(Cu 2p3/2) of 19.73 eV is associated with the Cu2+. The XPS characterization is corresponding to the presence of copper ions adsorbed on the surface of hydroxyapatite. The XPS spectra of Ca 2p, P 2p, and O 1s are characteristic of phosphorus, calcium, and oxygen ions in hydroxyapatite before/after adsorbing copper (Figures 5(c) and 5(d)). The P 2p peaks at 133.8 eV and 132.9 eV are assigned to P of hydroxyapatite (Ca10(PO4)6(OH)2). The Ca 2p spectra are composed by two peaks Ca 2p1/2 and Ca 2p3/2, in which the energy separation value of E(Ca 2p1/2) − E(Ca 2p3/2) is 3.57 eV on representing of Ca2+ ions. The binding energy of O 1s consists of two peaks at 531.58 eV and 530.85 eV associated with the presence of O-P and O-H groups in hydroxyapatite, respectively. The XPS results also show the similar surface chemical status of hydroxyapatite before/after adsorbing copper.Figure 5
XPS spectra before/after adsorbing copper by hydroxyapatite. (a) Cu 2p XPS spectra. (b) P 2p XPS spectra. (c) Ca 2p XPS spectra. (d) O 1s XPS spectra.
(a)(b)(c)(d)Morphology characterization further confirms that copper is adsorbed on the surface of nanorod hydroxyapatite in Figure6. HRTEM images with annular dark field (HADDF-HRTEM) of hydroxyapatite adsorbing copper show complete nanorods. The corresponding EDX elemental mappings also show the core/shell structure formation. Copper is mostly located in the outside area of the nanorod structure. Furthermore, calcium, oxygen, and phosphorus are uniformly distributed on the internal area of the nanostructure. The above results confirm that surface chemisorption is the copper uptake mechanism on hydroxyapatite.Figure 6
HADDF-STEM image and EDX elemental mappings of hydroxyapatite after adsorbing copper.DFT calculation is conducted to further study the adsorption site in Figure7. (001) and (100) are typical planes in hydroxyapatite, in which hydroxyapatite has electronegativity in (001) and electropositivity in (100). Bivalent cation is easier to be adsorbed on hydroxyapatite (001) surface through electrostatic attraction. Hence, (001) plane is considered as a dominant plane. Hydroxyapatite has acidic-basic sites, where acidic groups (≡POH, ≡PO−, and ≡CaOH) could complex with heavy metal on the surface. The calculation result shows that copper prefers to be adsorbed to the Ca(I) site through bonding to three oxygens of phosphate group stably in the hydroxyapatite (001). The calculation adsorption energy before/after adsorbing copper is −1.15 eV, indicating strong copper adsorption on the surface. Hence, a surface chemisorption model has proposed that copper bounds to the oxygen of phosphate through complexation with a Cu-O distance of ∼2.15 Å. The DFT calculations provide new insights into the copper adsorption mechanism on hydroxyapatite as adsorbent.Figure 7
DFT calculation of copper adsorption energy on hydroxyapatite. Copper adsorption on the pristine hydroxyapatite (001). Oxygen, calcium, phosphorus, hydrogen, and copper are in red, blue, pink, yellow, and green, respectively.


## 3.1. Effect of Calcination on the Hydroxyapatite

Calcination temperature is a critical factor in determining the crystallinity, grain size, and pore structure of hydroxyapatite. To investigate the effect of calcination temperature on the bovine bone, comprehensive analyses are employed to discuss the hydroxyapatite formation from bovine bone. The thermogravimetric analysis is essential for investigating the hydroxyapatite extraction process from bovine bone (Figure1(a)). A slight weight loss of 8% is attributed to the evaporation of adsorbed water at low temperatures (<120°C). The DSC curve shows a low endothermic peak correspondingly. The decomposition of organic matter causes a weight loss of 36.7% at 120–540°C. This reaction is followed by broad endothermic peaks correspondingly. A slight weight loss of 2.4% at 540–900°C can occur due to the dehydroxylation of hydroxyapatite promoted by heat.Figure 1
(a) TG-DSC curve of bovine bone. (b) XRD patterns of hydroxyapatite under different calcination temperatures. (c) N2 adsorption isotherms of hydroxyapatite under different calcination temperatures. (d) Surface parameters of hydroxyapatite under different calcination temperatures.
(a)(b)(c)(d)To control the crystallinity and porosity, a series of characterizations systematically depict the crystallization process of the hydroxyapatite phase during the heat sintering. XRD analysis confirms that all powders under different sintering temperatures are hydroxyapatite (Ca10(PO4)6(OH)2, JCPDS 84-1998). The crystallinity increases with increasing temperature due to the broad diffraction peaks transform into sharp diffraction peaks (Figure 1(b)). These results agree with previous reports [39, 41, 56]. In the process of heat treatment, the crystal phase of hydroxyapatite gradually emerges as the calcination temperature increases between 500°C and 600°C. The organic matter is decomposed into carbon dioxide through carbonation and decarboxylation. The nitrogen adsorption isotherms of hydroxyapatite under different temperatures belong to V-type adsorption isotherms, indicating that hydroxyapatite has a pore structure with the coexistence of micropores and mesopores (Figure 1(c)). The specific surface area, specific volume, and pore diameter also decrease with the increase in calcination temperature (Figure 1(d)). Specific surface area is 11.43 m2/g under 600°C. XRF analysis confirms that hydroxyapatite under 600°C mainly consists of three components, containing calcium, oxygen, phosphorus (Table SI.2). Hence, 600°C is the suitable sintering temperature for hydroxyapatite derived from bovine bone.


## 3.2. Batch Experiment

The adsorption amount as the function of copper concentration is shown in Figure2(a). The adsorption capacity of 25.7 mg/g is superior to most of the adsorbents (Table 1) [53–64]. The copper adsorption isotherm agrees well with the Langmuir model interpreting as homogeneous monolayer adsorption. The detailed parameters are shown in Table SI.3. The R2 value of the copper fitting model reaches 0.9993 (Figure 2(b)). The effect of contact time on the copper uptake is investigated to explore the reaction kinetic, which is shown in Figure 2(c). The adsorption process of copper by hydroxyapatite is fitting well with the pseudo-second-order kinetic model indicating the chemisorption. It can be found that the adsorption capacity of copper increases rapidly in the first 10 h. However, only a slight increase in adsorption capacity is observed after 10 h. It declares that its adsorption process consists of two parts: adsorption by available surface and further transformation from the solid-liquid interface to the pore of hydroxyapatite. The parameter values obtained through the pseudo-second-order kinetics model fitting are shown in Table SI.3. The R2 value of copper fitting models is 0.9868 correspondingly (Figure 2(d)). It can be concluded that copper removal by hydroxyapatite is mainly rate-limited chemisorption.Figure 2
(a) Adsorption equilibrium isotherm of copper on hydroxyapatite. Langmuir model fitting. (b) Linear plot of Langmuir model for adsorption isotherm. (c) Adsorption kinetic of copper on hydroxyapatite. Pseudo-second-order kinetic model fitting (adsorbent dosage: 0.2 g; copper concentration: 500 mg/L; solid/solution ratio: 6.7 g/L at 25 ± 2°C). (d) Linear plot of pseudo-second-order kinetic model for adsorption kinetics.
(a)(b)(c)(d)Table 1
Adsorption capacity of copper by various absorbent.
AdsorbentQm(mg/g)ReferencesRaw pomegranate peel30.12Ben-Ali et al. 2017 [57]Pine fruit14.1Najim et al. 2009 [58]Fe5C2@SiO237.73Ahmadpoor et al. 2019 [59]Bioball5.60Çelebi 2021[60]Chicken feather7.84Solis-Moreno et al. 2021 [61]Chicken bone charcoal15.057Niu et al. 2021 [62]Activated carbon13.05Imamoglu and Tekir 2008 [63]Rice shell2.954Aydin et al. 2008 [64]Hydroxyapatite25.7Present study


## 3.3. Column Experiment

The removal behavior of copper by hydroxyapatite is further evaluated by performing a column study. The breakthrough curve of copper for a column packed with hydroxyapatite is shown in Figure3(a). At C/C0 = 0.5, the number of pore volume (PV) is 450, suggesting excellent column durability [65]. Hydroxyapatite has a stable hydraulic performance to copper because no clogging phenomenon happened in the column. Based on the simulation model, the breakthrough curve of different copper concentrations has been further predicted through FEFLOW software (Figure 3(b)). The breakthrough time decreases with the increase in input copper concentration. The half-life of the reaction reaches 43 d when the input concentration of copper is 15 mg/L. The above results confirm that hydroxyapatite derived from bovine bone is highly effective in in situ remediation of copper-contaminated groundwater as a suitable filling material within PRB. This dynamic column study provides a good prospect for removing copper in field application.Figure 3
(a) Experimental and fitting breakthrough curve of copper for a column packed with hydroxyapatite. The input concentration of copper is 20 mg/L. Adsorbent mass is 20 g. (b) Predicting breakthrough curve of copper for a column packed with hydroxyapatite for different copper concentrations.
(a)(b)


## 3.4. Adsorption Mechanism

XRD combined with Rietveld refinement is used to analyze the product after adsorption shown in Figure4(a). The result represents that the product after adsorbing copper belongs to hydroxyapatite phase (Ca10(PO4)6(OH)2) according to JCPDS 84-1998. Rietveld refinement defines that the calcium site and phosphate site were fully occupied. The fitting coefficients are Rp of 9.61% and Rwp of 13.5%. The functional groups of hydroxyapatites before/after adsorbing copper are identified through FTIR analysis, and corresponding results are presented in Figure 4(b). It can be seen that characteristic peak of hydroxyapatite before/after adsorbing copper is approximately identical. The hydroxyl stretching vibration band at 3560 cm−1 is presented. The characteristic peaks at 1030 cm−1, 609 cm−1, and 545 cm−1 are attributed to asymmetric stretching of PO43−. Besides, the carbonate group is observed at 1407 cm−1, which indicates slight substitution of carbonate group into phosphate group in the hydroxyapatite lattice.Figure 4
(a) X-ray diffraction patterns with Rietveld refinement of hydroxyapatite adsorbing copper. (b) FTIR spectra before/after adsorbing copper by hydroxyapatite.
(a)(b)XPS analysis (Figure5) is consistent with the XRD data. The fitted Cu 2p spectra (Figure 5(a)) show two peaks of Cu 2p1/2 and Cu 2p3/2 at the binding energy of 953.25 eV and 933.5 eV, respectively [66]. The spin-orbit splitting E(Cu 2p1/2) − E(Cu 2p3/2) of 19.73 eV is associated with the Cu2+. The XPS characterization is corresponding to the presence of copper ions adsorbed on the surface of hydroxyapatite. The XPS spectra of Ca 2p, P 2p, and O 1s are characteristic of phosphorus, calcium, and oxygen ions in hydroxyapatite before/after adsorbing copper (Figures 5(c) and 5(d)). The P 2p peaks at 133.8 eV and 132.9 eV are assigned to P of hydroxyapatite (Ca10(PO4)6(OH)2). The Ca 2p spectra are composed by two peaks Ca 2p1/2 and Ca 2p3/2, in which the energy separation value of E(Ca 2p1/2) − E(Ca 2p3/2) is 3.57 eV on representing of Ca2+ ions. The binding energy of O 1s consists of two peaks at 531.58 eV and 530.85 eV associated with the presence of O-P and O-H groups in hydroxyapatite, respectively. The XPS results also show the similar surface chemical status of hydroxyapatite before/after adsorbing copper.Figure 5
XPS spectra before/after adsorbing copper by hydroxyapatite. (a) Cu 2p XPS spectra. (b) P 2p XPS spectra. (c) Ca 2p XPS spectra. (d) O 1s XPS spectra.
(a)(b)(c)(d)Morphology characterization further confirms that copper is adsorbed on the surface of nanorod hydroxyapatite in Figure6. HRTEM images with annular dark field (HADDF-HRTEM) of hydroxyapatite adsorbing copper show complete nanorods. The corresponding EDX elemental mappings also show the core/shell structure formation. Copper is mostly located in the outside area of the nanorod structure. Furthermore, calcium, oxygen, and phosphorus are uniformly distributed on the internal area of the nanostructure. The above results confirm that surface chemisorption is the copper uptake mechanism on hydroxyapatite.Figure 6
HADDF-STEM image and EDX elemental mappings of hydroxyapatite after adsorbing copper.DFT calculation is conducted to further study the adsorption site in Figure7. (001) and (100) are typical planes in hydroxyapatite, in which hydroxyapatite has electronegativity in (001) and electropositivity in (100). Bivalent cation is easier to be adsorbed on hydroxyapatite (001) surface through electrostatic attraction. Hence, (001) plane is considered as a dominant plane. Hydroxyapatite has acidic-basic sites, where acidic groups (≡POH, ≡PO−, and ≡CaOH) could complex with heavy metal on the surface. The calculation result shows that copper prefers to be adsorbed to the Ca(I) site through bonding to three oxygens of phosphate group stably in the hydroxyapatite (001). The calculation adsorption energy before/after adsorbing copper is −1.15 eV, indicating strong copper adsorption on the surface. Hence, a surface chemisorption model has proposed that copper bounds to the oxygen of phosphate through complexation with a Cu-O distance of ∼2.15 Å. The DFT calculations provide new insights into the copper adsorption mechanism on hydroxyapatite as adsorbent.Figure 7
DFT calculation of copper adsorption energy on hydroxyapatite. Copper adsorption on the pristine hydroxyapatite (001). Oxygen, calcium, phosphorus, hydrogen, and copper are in red, blue, pink, yellow, and green, respectively.


## 4. Conclusion

Copper-contaminated groundwater is directly associated with human health and the ecological environment. In this paper, cost-effective hydroxyapatite derived from bovine bone was proven as a suitable adsorbent for in situ remediation of copper-contaminated groundwater within PRB. Calcination temperature has a significant effect on the crystallinity and porosity, and 600°C is confirmed as suitable temperature for hydroxyapatite derived from the bovine bone. Hydroxyapatite has a porous structure, and its specific surface area is 11.43 m2/g. Hydroxyapatite is proven as an excellent adsorbent owing to cost-effectiveness, great adsorption capacity, and longevity. Adsorption isotherm is represented by the Langmuir isotherm model, and adsorption capacity of 25.7 mg/g is superior to most of the adsorbents. The kinetic study is accurately fitted by the pseudo-second-order kinetic model interpreted with chemical reaction. In addition, column study confirms that hydroxyapatite has excellent hydraulic performance with no clogging phenomenon happened. At C/C0 = 0.5, the number of pore volume (PV) reaches 450, suggesting stable durability. The model predicates that breakthrough time decreases with the increase in input copper concentration through FEFLOW software. The batch and column experiments also reveal that the overall adsorption process follows up the monolayer chemisorption. XRD combined with Rietveld refinement demonstrated that copper has not incorporated into the calcium sites in hydroxyapatite. Other characterizations such as XPS analysis, EDX mapping images, and FTIR further confirm that copper is adsorbed on the surface of hydroxyapatite. DFT calculation shows that copper was attracted to oxygen atoms to form complexes based on the phosphate group in the (001) plane. The adsorption energy is −1.15 eV with a Cu-O distance of ∼2.15 Å. This work provides an alternative strategy as filling material for in situ remediation of copper-contaminated groundwater and enriches relevant theoretical references.



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*Source: 1026129-2022-05-06.xml*