# Treatment of Cd2+ and Cu2+ Ions Using Modified Apatite Ore

Authors: Phuong Thu Nguyen; Xuyen Thi Nguyen; Trang Van Nguyen; Thom Thi Nguyen; Thai Quoc Vu; Huy Trung Nguyen; Nam Thi Pham; Thanh Mai Thi Dinh
Journal: Journal of Chemistry
 (2020)

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## Abstract

Apatite ore from Lao Cai (Vietnam) has large reserves and low prices. Its main component is fluorapatite. The purification and modification of apatite ore can produce a material that can be used as an absorbent for heavy metals with high efficiency. The molecular structure, phase component, specific surface area, element component, and morphology of modified apatite ore from Lao Cai province, Vietnam, were characterized by IR, XRD, BET, EDX, and SEM methods. The IR and XRD results show that the modified process transformed apatite ore from fluorapatite to nanohydroxyapatite. The specific surface area of modified apatite ore (100.79 m2/g) is much higher than the original ore (3.97 m2/g). The modified apatite ore was used to adsorb Cd2+ and Cu2+ ions in water. The effect of adsorbent mass, pH, contact time, and initial concentration of Cd2+ and Cu2+ on the adsorption efficiency and capacity was investigated. Besides, the isotherm adsorption model was determined using Freundlich and Langmuir theories.


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## Body

## 1. Introduction

Nowadays, water pollution caused by heavy metal is a serious problem for human health and the environment. Heavy metal poisoning will lead to many fatal diseases such as pulmonary edema, kidney failure, cancer. Especially, a high concentration of cadmium (Cd) and copper (Cu) can affect the nervous and skeletal system [1–3]. The content of heavy metal ions in drinking water is very low, maximum content of Pb: 0.05 mg/L; Hg: 0.006 mg/L; As: 0.01 mg/L; Cd: 0.003 mg/L; Cr: 0.05 mg/L; Mn: 0.1 mg/L; Cu: 2 mg/L following WHO standard [4].Heavy metal ions can be removed by many methods using different materials such as activated carbon, zeolite, clay, silica, polymer, and apatite [3, 5–7]. Apatite includes hydroxyapatite (Ca5(PO4)3(OH), fluorapatite Ca5(PO4)3F, and chlorapatite Ca5(PO4)3Cl. Hydroxyapatite powder can remove some pollution ions and substances in water such as Cu2+, Pb2+, Zn2+, Cd2+, Co2+, Cr6+, La3+, Eu3+, phenol, nitrobenzene, NO3−, F− [6–17], and Congo Red [18] with high adsorption ability. Hydroxyapatite can be synthesized by many methods from different calcium and phosphorus sources.One of the research directions of environmental technology is finding the natural original materials with huge reserves, low price, and good treatment ability [19–21]. Using apatite ore for treating heavy metals in soil is applied in many countries in the world, such as in situ remediation techniques and phosphate-induced metal stabilization. Apatite can treat almost heavy metals and radioactive substances such as Cu, As, Zn, Th, Ac, U, Pu, Sr, Cd, Se, Cs, Tc, and Pb to prevent the pollution to the water [22–28]. PO43− groups in apatite can react with heavy metals to form precipitation and Ca2+ ions in the structure of apatite can be replaced by heavy metals ions [29]. Moroccan phosphate rock which has component is carbonate-fluorapatite was used as the calcium and phosphorus source to synthesize nanohydroxyapatite. This modified material removed heavy metal ions such as Pb2+, Cu2+, and Zn2 with a higher adsorption capacity than natural rock [30, 31]. Spongy Ni/Fe carbonate-fluorapatite was synthesized from natural phosphorite (Egypt) that was studied for decontamination of Zn2+, Co2+, and Cu2+ with Qmax values of 149.25 mg/g, 106.4 mg/g and 147.5 mg/g, respectively [3].In Vietnam, apatite from Lao Cai has a huge reserve of about 2550 million tons. The main component of the material is fluorapatite which is used to procedure fertilizer for agriculture. Using this material to adsorb heavy metal ions such as Cd2+, Cu2+ is still limited.In this study, apatite ore was modified by a chemical process and its characterization was analyzed using IR, XRD, EDX, SEM, and BET methods. After that, the material was used to treat Cd2+ and Cu2+ ions in water and investigated the effect of contact time, initial Cd2+ and Cu2+ concentration, pH solution, and adsorbent mass on adsorption capacity and efficiency. From the experiment data, the adsorption isotherms were also determined.


## 2. Materials and Methods

### 2.1. Materials

Apatite ore is a product from Lao Cai province, Vietnam. Apatite ore was milled and dried at 100oC in the oven. And then, it was modified by the procedure as follows: apatite ore was put in the glass reactor containing 1M HNO3 solution and was stirred at a rate of 400 rpm at room temperature. After that, the insoluble matter was separated by filtration. The remaining solution containing Ca2+ and PO43− is neutralized by a concentrated NH4OH solution (25%) to form precipitation of hydroxyapatite. The pH value was maintained at pH > 10 to avoid the formation of other calcium phosphate compounds. The powder of hydroxyapatite was obtained by filtration and washed by distilled water to pH value of 7 to remove the unreacted calcium, phosphate, or ammonia ions, and then dried at 100°C.NaOH 96%, KCl 99.5%, HCl 36%, NH3 25%, HNO3 65%, CuSO4·5H2O 99% are pure chemicals from China; Cd(NO3)2·4H2O 99.5% is pure chemical of Merck.Apatite and modified apatite ore characterized the molecular structure, phase component, morphology, specific surface area, and element component by Infrared (IR, Nicolet iS10, Thermo Scientific), Scanning Electron Microscope (SEM-EDX, SM-6510LV, Jeol-Japan, X-Act, Oxford Instrument-England and Hitachi S-4800-Japan), High-resolution Transmission Electron Microscope (HR-TEM, JEM2100, Jeol-Japan), X-ray Diffraction (XRD, D8 ADVANCE-Bruker, CuKα radiation (λ = 1.54056 Å) with a step angle of 0.030o, the scanning rate of 0.04285 s−1, and 2θ degree in the range of 10–70°) and Brunauer-Emmett-Teller methods (BET, TriStar II, Micromerit, 77 K, N2).


### 2.2. Adsorption Experiments

The adsorption experiments of Cd2+ and Cu2+ ions were conducted with 50 mL of Cd(NO3)2 or CuSO4 solution at different concentrations from 10 mg/L to 1000 mg/L. An amount of modified apatite ore from 0.01 g to 0.1 g was dispersed into the above solution. Initial pH values of solution were adjusted from 2.5 to 8 using solutions of 0.01 M KOH and 0.01 M HCl and controlled by pH meter (1100H VWR pHenomenal, Germany). The mixtures were stirred at a rate of 400 rpm by a magnetic stirrer (Velp Scientifica) at different times (from 5 to 90 minutes). The concentration of Cd2+ and Cu2+ ions in the solutions after treatment was determined by Atomic Absorption Spectrophotometer (AAS) on iCE 3500 Thermo Scientific device (Germany).The adsorption efficiency (H (%)) and capacity (Q (mg/g)) were calculated according to the following equation:(1)H=Co−Ce⋅100Co,Q=Co−CeVm,where Co (mg/L) is the initial concentration of Cd2+ and Cu2+ ions in the solution, Ce (mg/L) is the equilibrium concentration of Cd2+ and Cu2+ ions in the solution after treatment, V (L) is the solution volume (V = 50 mL), and m (g) is the mass of modified apatite ore.


## 2.1. Materials

Apatite ore is a product from Lao Cai province, Vietnam. Apatite ore was milled and dried at 100oC in the oven. And then, it was modified by the procedure as follows: apatite ore was put in the glass reactor containing 1M HNO3 solution and was stirred at a rate of 400 rpm at room temperature. After that, the insoluble matter was separated by filtration. The remaining solution containing Ca2+ and PO43− is neutralized by a concentrated NH4OH solution (25%) to form precipitation of hydroxyapatite. The pH value was maintained at pH > 10 to avoid the formation of other calcium phosphate compounds. The powder of hydroxyapatite was obtained by filtration and washed by distilled water to pH value of 7 to remove the unreacted calcium, phosphate, or ammonia ions, and then dried at 100°C.NaOH 96%, KCl 99.5%, HCl 36%, NH3 25%, HNO3 65%, CuSO4·5H2O 99% are pure chemicals from China; Cd(NO3)2·4H2O 99.5% is pure chemical of Merck.Apatite and modified apatite ore characterized the molecular structure, phase component, morphology, specific surface area, and element component by Infrared (IR, Nicolet iS10, Thermo Scientific), Scanning Electron Microscope (SEM-EDX, SM-6510LV, Jeol-Japan, X-Act, Oxford Instrument-England and Hitachi S-4800-Japan), High-resolution Transmission Electron Microscope (HR-TEM, JEM2100, Jeol-Japan), X-ray Diffraction (XRD, D8 ADVANCE-Bruker, CuKα radiation (λ = 1.54056 Å) with a step angle of 0.030o, the scanning rate of 0.04285 s−1, and 2θ degree in the range of 10–70°) and Brunauer-Emmett-Teller methods (BET, TriStar II, Micromerit, 77 K, N2).


## 2.2. Adsorption Experiments

The adsorption experiments of Cd2+ and Cu2+ ions were conducted with 50 mL of Cd(NO3)2 or CuSO4 solution at different concentrations from 10 mg/L to 1000 mg/L. An amount of modified apatite ore from 0.01 g to 0.1 g was dispersed into the above solution. Initial pH values of solution were adjusted from 2.5 to 8 using solutions of 0.01 M KOH and 0.01 M HCl and controlled by pH meter (1100H VWR pHenomenal, Germany). The mixtures were stirred at a rate of 400 rpm by a magnetic stirrer (Velp Scientifica) at different times (from 5 to 90 minutes). The concentration of Cd2+ and Cu2+ ions in the solutions after treatment was determined by Atomic Absorption Spectrophotometer (AAS) on iCE 3500 Thermo Scientific device (Germany).The adsorption efficiency (H (%)) and capacity (Q (mg/g)) were calculated according to the following equation:(1)H=Co−Ce⋅100Co,Q=Co−CeVm,where Co (mg/L) is the initial concentration of Cd2+ and Cu2+ ions in the solution, Ce (mg/L) is the equilibrium concentration of Cd2+ and Cu2+ ions in the solution after treatment, V (L) is the solution volume (V = 50 mL), and m (g) is the mass of modified apatite ore.


## 3. Results and Discussion

### 3.1. Characterization of Original and Modified Apatite Ore

The infrared spectra and the SEM and TEM images of apatite ore and modified apatite ore are presented in Figures1 and 2, respectively. The spectra show that characteristic bands of PO43− at 1097, 1046, 600, and 564 cm−1 correspond to the asymmetric stretching vibration of the P-O bond and the asymmetric bending vibration of O-P-O, respectively [32]. The peak with strong intensity at 3417 cm−1 is characteristic for the vibration of the OH− group or the absorbed water. In addition, the bending vibration of the OH− group is also characterized by a peak at wave number of 1617 cm−1. The peak at 696 cm−1 which is between 674 and 720 cm−1, is typical for F− ion [33]. Besides, there are peaks at 1458 and 1426 cm−1 that correspond to the vibration of CO32− group [32]. The peak at 1384 cm−1 in modified apatite is attributed to NO3− group [34] in the modification process. It is important to note that there were no visible characteristic peaks for F− and CO32− after modifying, indicating that the modified process has transformed fluorapatite in the apatite ore to hydroxyapatite. This will be confirmed by the XRD result below.Figure 1
FT-IR spectra of apatite ore before (a) and after (b) modifying.Figure 2
SEM images of apatite ore before (a) and after (b) modifying and TEM images of modified apatite ore (c, d).
(a)
(b)
(c)
(d)From SEM images, it can be seen that the particles of apatite ore do not have a uniform particle size. The size of large particles can be up to 15–20μm. The smallest particles are about 200 nm. After modifying, the particles are more uniform in size and significantly smaller than those before modifying with nanoscale. TEM images of modified apatite ore reflected concordance with the results obtained from SEM images. From TEM images, we can see that the cylinder particles are cluster and have a size of about 20–30 nm.To determine the crystallinity and phase composition, the samples of apatite ore before and after modifying are characterized by X-ray diffraction (Figure3). On the X-ray diffraction pattern of apatite ore, there are typical peaks for the phase of fluorapatite and some other peaks specific to SiO2 and Ca2(Al2O7)2. The diffraction angle (2θ) of 32°; 33°; 34° specific to the phase of Ca5(PO4)3F. There are also some other peaks corresponding to the phase of Ca5(PO4)3F with smaller intensity at 2θ of 11°; 26°; 36°; 40°; 47°; 50° …[PDF 00-015-0876, 32]. The phase of SiO2 is characterized by the peaks at 21.0° and 26.7° [PDF 00-046-1045] and Ca2(Al2O7)2 crystal is attributed at 2θ of 31° [PDF 01-070-0801]. The X-ray diffraction pattern shows that the phase composition of the ore sample is mainly fluorapatite and also contains SiO2 and a small content of Ca2(Al2O7)2. For modified apatite ore samples, the characteristic peaks for hydroxyapatite were observed at 2θ of 26.5 and 32.4° corresponding to the (002) and (211) crystal planes [16]. This proves again that the modified process has transformed fluorapatite in the ore to hydroxyapatite.Figure 3
X-ray diffraction patterns of apatite ore before and after modifying.The specific surface area of apatite and modified apatite ore is determined by the BET method, using N2 adsorption at 77 K (Figure 4). The N2 adsorption/desorption isotherm and the pore size distributions were plotted in Figures 4(a) and 4(b). According to the IUPAC classification of pores materials, both apatite and modified apatite exhibit type IV isotherm curve with a hysteresis loop, which indicates the presence of mesopores [19]. The average pore size of apatite ore and modified apatite is 15.88 nm and 24.10 nm, respectively. The results show that the apatite ore sample has a lower significant adsorption capacity of N2 than the modified sample. The specific surface area for apatite ore before and after modifying is 3.97 and 100.79 m2/g, respectively. This result can predict that the modified apatite ore may have better adsorption capacity than the original apatite.Figure 4
The N2 adsorption-desorption isotherm and the pore size distributions of apatite ore (a), modified apatite (b), and BET plot (c) of apatite ore before and after modifying.
(a)
(b)
(c)The composition and percentage of elements in apatite ore are in accordance with the published company production criteria and the XRD results. The EDX results are presented in Figure5 and Table 1. EDX spectra show the characteristic peaks for elements of Ca, P, O, F in apatite ore. Besides, there are several metal elements with small components such as Al, K, Fe, Mn. For the modified apatite sample, the peaks of Ca, P, and O were observed, along with other elements with small components. The ratios of Ca/P for apatite before and after modifying are 1.53 and 1.60, respectively, similar to that in fluorapatite Ca5(PO4)3F and hydroxyapatite Ca5(PO4)6(OH)2 (1.67). For solids after filtering, while the content of Ca, P is very small (about 0.44 and 0.92% by weight), the data for Si is significantly large (33.65% by weight), indicating that the solids contain mainly SiO2.Figure 5
EDX spectra of apatite ore before and after modifying and the solid after filtering. (a) Apatite ore. (b) Modified apatite. (c) Solid after filtering.
(a)
(b)
(c)Table 1
Composition of elements in apatite ore before and after modifying and the solid after filtering.
Element
Apatite ore
Modified apatite
Solid after filtering
%m
%a
%m
%a
%m
%a
O
50.19
68.21
37.67
54.59
47.74
57.49
Ca
21.40
11.61
36.73
21.25
0.44
0.21
P
10.79
7.57
17.75
13.29
0.92
0.57
F
2.76
3.16
2.58
3.15
1.07
1.08
Si
5.35
4.14
0.42
0.35
33.65
23.09
Al
3.06
2.47
0.46
0.40
1.69
1.21
Mn
1.19
0.47
0.80
0.28
Mg
0.36
0.32
0.17
0.16
0.11
0.09
K
0.82
0.46
0.37
0.18
Fe
4.08
1.59
0.66
0.27
2.93
1.01
N
6.91
9.50
C
3.34
6.44
3.28
5.26
Cu
0.16
0.06
0.11
0.03
Cl
0.07
0.04


### 3.2. Adsorption Experiments Results

#### 3.2.1. Calibration Curve

Several solutions with an initial Cd2+ concentration of 0.24–2 mg/L and Cu2+ from 0.12–8 mg/L prepared from a 1000 mg/L standard solution were used to build the calibration curve of Cd2+ and Cu2+ by AAS method (Figure 6). The calibration curve equation y = 0.21208x with correlation coefficient R2 = 0.9993 and y = 0.0693x with R2 = 0.9993 used to calculate the concentrations of Cd2+ and Cu2+, respectively, in the next experiments.Figure 6
The calibration curves of Cd2+ and Cu2+.
(a)
(b)


#### 3.2.2. Effect of the Modified Apatite Mass

The influence of modified apatite ore mass on the adsorption capacity and efficiency of Cd2+ and Cu2+ was shown in Figure 7. The results show that when the mass of modified apatite increases, the adsorption efficiency increases and the adsorption capacity decreases. For Cd2+ (Figure 7(a)), treatment efficiency increased rapidly from 39.64% to 82.08% when the modified apatite mass increased from 0.01 g to 0.05 g. After that, the adsorption efficiency increases slowly in the range of adsorbent mass from 0.05 g to 0.1 g. Therefore, to achieve not only a relatively high treatment efficiency but also high adsorption capacity, 0.05 g modified apatite is selected for further experiments. Similarly, for the treatment of Cu2+ ion, 0.03 g modified apatite is appropriate.Figure 7
Variation of the Cd2+/Cu2+ adsorption capacity and efficiency according to the modified apatite ore mass at pH0, Co = 30 mg/L, t = 60 mins. (a) Cd2+, (b) Cu2+.
(a)
(b)


#### 3.2.3. Effect of Solution pH

To determine pHpzc, 50 mL solution 0.01 M KCl was adjusted pH in the broad pH range from 5 to 9 (pHbefore) and in the small pH range from 7 to 8 by 0.01 M KOH and 0.01 M HCl solution. 0.03 g modified apatite was put in the solution, stir 1 hour with rate 400 rpm at room temperature. Filter and measure the pH value of the obtained solution (pHafter). The curve of ∆pH (= pHafter – pHbefore) follows pHbefore and horizontal axis meet at pHpzc. The result of determining the pH value at the point zero-charge of the modified apatite ore is shown in Figure 8. The result shows that the adsorbent has point zero-charge at pHpzc = 7.49.Figure 8
Graph for determining pHpzc of modified apatite in broad pH range (a) and in small pH range (b).
(a)
(b)The effect of pH on the adsorption efficiency and capacity of modified apatite ore is shown in Figure9. The results show that when the pH of the solution increased from 2.6 to 7.9, the adsorption efficiency and capacity increased. At low pH values, the surface of the material has a positive charge; therefore the interactive force is also electrostatic force. Besides, if the concentration of H+ ions is high, the competition with metal cations in the adsorption process leads to a decrease in efficiency. When pH is higher than pHpzc (7.49), the adsorption capacity and efficiency rise. It can be explained as follows: at high pH value, the surface of adsorbent has a negative charge, but if pH is too high, the precipitation of Cd(OH)2 and Cu(OH)2 can happen [2, 3, 15]. To facilitate the practical application, we choose pH = pHo = 5.5 (for Cd2+) and 5.6 (for Cu2+) to investigate the next experiments.Figure 9
Effect of pH solution on the adsorption capacity and efficiency of Cd2+ ((a) 30 mins, 0.05 g modified apatite) and Cu2+ ((b) 45 mins, 0.03 g modified apatite) (Co = 30 mg/L).
(a)
(b)


#### 3.2.4. Effect of Contact Time

The variation of the adsorption capacity and efficiency according to the contact time was investigated with the conditions as follows: the adsorbent mass is 0.05 g modified apatite ore for Cd2+ adsorption and 0.03 g for Cu2+ adsorption, in 50 mL of the Cd2+ or Cu2+ solution (30 mg/L), pHo, at 25°C with the different contact time changed from 5 to 90 minutes (Figure 10).Figure 10
Variation of Cd2+ (a), Cu2+ (b) adsorption capacity, and efficiency of modified apatite ore according to the contact time.
(a)
(b)When increasing the adsorption time from 5 to 30 minutes, the Cd2+ adsorption efficiency increased significantly from 50% to 84%, corresponding to the increase of adsorption capacity from 14.15 mg/g to 24.24 mg/g. From 30 minutes to 60 minutes, the adsorption efficiency and capacity increase gradually. It can be explained that when the adsorption time increases, the ions fill the pores of the absorbent more, thus the adsorption efficiency and capacity increase. However, after a certain time, the adsorption equilibrium is reached, the adsorption capacity remains stable. Therefore 30 minutes is the optimal Cd2+ adsorption time of the modified apatite ore. Similarly, for the adsorption of Cu2+, 45 minutes was chosen for the removal process by the modified apatite ore. The adsorption efficiency and capacity of the Cu2+ treatment archives 85% and 41.34 mg/g, respectively.


#### 3.2.5. Effect of Initial Concentration of Cd2+ and Cu2+

Figure11 shows the variation of adsorption efficiency and capacity according to the initial concentration of Cd2+ and Cu2+. It can be seen that the adsorption capacity increases, and the adsorption efficiency decreases with the increase of initial concentration. However, when the concentration increases to a certain value, the adsorption capacity is almost stable which corresponds to the adsorption process reaches the maximum state. It can be explained by the fact that an amount of modified apatite ore has a certain number of adsorption centers. When all adsorption centers are occupied by Cd2+ and Cu2+ ions, the ion concentration continues to increase, and the adsorption capacity is almost constant.Figure 11
Variation of Cd2+ and Cu2+ adsorption capacity and efficiency of modified apatite ore according to the initial concentration (pHo, t = 60 mins). (a) C Cd2+ (mg/L), (b) C Cu2+ (mg/L).
(a)
(b)


#### 3.2.6. Adsorption Isotherm

In order to determine the maximum adsorption capacity of modified apatite ore toward Cd2+ and Cu2+ ions, adsorption studies were carried out with conditions as follows: 0.05 g adsorbent with treatment Cd2+ and 0.03 g with Cu2+ was dispersed into 50 mL solution with different initial concentrations from 8.44 to 138.63 mg/L with Cd2+, 17.68 to 947.88 mg/L with Cu2+, at 25°C, pHo; the mixtures were stirred at 400 rpm for 60 mins. Figures 12 and 13 show the linear fits of the experimental data of the Cd2+ or Cu2+ treatment process using Freundlich and Langmuir adsorption isotherm models.Figure 12
Adsorption isotherm curves of Cd2+ follow the Langmuir (a) and Freundlich (b).
(a)
(b)Figure 13
Adsorption isotherm curves of Cu2+ follow the Langmuir (a) and Freundlich (b).
(a)
(b)From the obtained results, it is evident that Langmuir adsorption isotherm model can be used to describe experimental data of adsorption of Cd2+ and Cu2+. From the linear equation, the maximum adsorption capacity value (Qmax) of Cd2+ and Cu2+ is calculated at about 43.94 and 69.59 mg/g, respectively. Compare with other adsorbents, with Cd2+, HAp/CS, 81.1 mg/g [1], HAp, 21 m2/g, 16.7 mg/g [8], HAp, 94.9 m2/g, 142.9 mg/g [9], AlHAp, 205 m2/g, 103 mg/g [15], activated carbon, 178.5 mg/g [5], thiourea-modified Sorghum bicolor Agrowaste, 5.13 m2/g, 17.2 mg/g [35], with Cu2+, HAp, 50 m2/g, 96.6 mg/g [13], HAp, 125 mg/g [36], SDS modified laterite, 185 mg/g [2], Ni/Fe carbonate-fluorapatite, 74.5 m2/g, 147.5 mg/g [3], modified natural phosphate, 150 m2/g, 166 mg/g, Bengurir rock-Morocco, 21 m2/g, 57 mg/g [30], thiourea-modified Sorghum bicolor Agrowaste, 7.98 m2/g, 15.2 mg/g [35], we can conclude that modified apatite ore is a good adsorbent for Cd2+ and Cu2+.


#### 3.2.7. The Phase Component and Molecular Structure of the Absorbent after Absorption

The X-ray diffraction patterns of modified apatite ore before and after Cd2+ adsorption (at pH0, m = 0.05 g, 30 minutes, Cd2+ concentration of 30 mg/L) and Cu2+ adsorption (pH0, m = 0.03 g, 45 minutes, Cu2+ concentration of 30 ppm) are shown in Figure 14(a). It can be seen that the material after adsorption still has the phase structure of hydroxyapatite, indicating that the phase structure of the material does not change.Figure 14
X-ray diffraction patterns (a) and IR spectra (b) of modified apatite ore before and after Cd2+ and Cu2+ adsorption.
(a)
(b)The FTIR spectra of modified apatite ore before and after Cd2+ and Cu2+ adsorption are shown in Figure 14(b). FTIR spectra of the solid residues after adsorption are comparable to one of the initial modified apatite ore, suggesting no other functional groups formed after the heavy metal sorption. Compared with the FTIR spectra before and after adsorption of Cd2+ and Cu2+, there are band shifts, which occur in the groups of apatite that had bound metals. Sorption of Cd2+ and Cu2+ undergoes two-step processes. Rapid complexation of heavy metals on specific sites of the modified apatite ore surface is the first step. A metal diffusion into the modified apatite ore structure or to a heavy metal-containing modified apatite ore formation is the second step in the sorption process [36].


## 3.1. Characterization of Original and Modified Apatite Ore

The infrared spectra and the SEM and TEM images of apatite ore and modified apatite ore are presented in Figures1 and 2, respectively. The spectra show that characteristic bands of PO43− at 1097, 1046, 600, and 564 cm−1 correspond to the asymmetric stretching vibration of the P-O bond and the asymmetric bending vibration of O-P-O, respectively [32]. The peak with strong intensity at 3417 cm−1 is characteristic for the vibration of the OH− group or the absorbed water. In addition, the bending vibration of the OH− group is also characterized by a peak at wave number of 1617 cm−1. The peak at 696 cm−1 which is between 674 and 720 cm−1, is typical for F− ion [33]. Besides, there are peaks at 1458 and 1426 cm−1 that correspond to the vibration of CO32− group [32]. The peak at 1384 cm−1 in modified apatite is attributed to NO3− group [34] in the modification process. It is important to note that there were no visible characteristic peaks for F− and CO32− after modifying, indicating that the modified process has transformed fluorapatite in the apatite ore to hydroxyapatite. This will be confirmed by the XRD result below.Figure 1
FT-IR spectra of apatite ore before (a) and after (b) modifying.Figure 2
SEM images of apatite ore before (a) and after (b) modifying and TEM images of modified apatite ore (c, d).
(a)
(b)
(c)
(d)From SEM images, it can be seen that the particles of apatite ore do not have a uniform particle size. The size of large particles can be up to 15–20μm. The smallest particles are about 200 nm. After modifying, the particles are more uniform in size and significantly smaller than those before modifying with nanoscale. TEM images of modified apatite ore reflected concordance with the results obtained from SEM images. From TEM images, we can see that the cylinder particles are cluster and have a size of about 20–30 nm.To determine the crystallinity and phase composition, the samples of apatite ore before and after modifying are characterized by X-ray diffraction (Figure3). On the X-ray diffraction pattern of apatite ore, there are typical peaks for the phase of fluorapatite and some other peaks specific to SiO2 and Ca2(Al2O7)2. The diffraction angle (2θ) of 32°; 33°; 34° specific to the phase of Ca5(PO4)3F. There are also some other peaks corresponding to the phase of Ca5(PO4)3F with smaller intensity at 2θ of 11°; 26°; 36°; 40°; 47°; 50° …[PDF 00-015-0876, 32]. The phase of SiO2 is characterized by the peaks at 21.0° and 26.7° [PDF 00-046-1045] and Ca2(Al2O7)2 crystal is attributed at 2θ of 31° [PDF 01-070-0801]. The X-ray diffraction pattern shows that the phase composition of the ore sample is mainly fluorapatite and also contains SiO2 and a small content of Ca2(Al2O7)2. For modified apatite ore samples, the characteristic peaks for hydroxyapatite were observed at 2θ of 26.5 and 32.4° corresponding to the (002) and (211) crystal planes [16]. This proves again that the modified process has transformed fluorapatite in the ore to hydroxyapatite.Figure 3
X-ray diffraction patterns of apatite ore before and after modifying.The specific surface area of apatite and modified apatite ore is determined by the BET method, using N2 adsorption at 77 K (Figure 4). The N2 adsorption/desorption isotherm and the pore size distributions were plotted in Figures 4(a) and 4(b). According to the IUPAC classification of pores materials, both apatite and modified apatite exhibit type IV isotherm curve with a hysteresis loop, which indicates the presence of mesopores [19]. The average pore size of apatite ore and modified apatite is 15.88 nm and 24.10 nm, respectively. The results show that the apatite ore sample has a lower significant adsorption capacity of N2 than the modified sample. The specific surface area for apatite ore before and after modifying is 3.97 and 100.79 m2/g, respectively. This result can predict that the modified apatite ore may have better adsorption capacity than the original apatite.Figure 4
The N2 adsorption-desorption isotherm and the pore size distributions of apatite ore (a), modified apatite (b), and BET plot (c) of apatite ore before and after modifying.
(a)
(b)
(c)The composition and percentage of elements in apatite ore are in accordance with the published company production criteria and the XRD results. The EDX results are presented in Figure5 and Table 1. EDX spectra show the characteristic peaks for elements of Ca, P, O, F in apatite ore. Besides, there are several metal elements with small components such as Al, K, Fe, Mn. For the modified apatite sample, the peaks of Ca, P, and O were observed, along with other elements with small components. The ratios of Ca/P for apatite before and after modifying are 1.53 and 1.60, respectively, similar to that in fluorapatite Ca5(PO4)3F and hydroxyapatite Ca5(PO4)6(OH)2 (1.67). For solids after filtering, while the content of Ca, P is very small (about 0.44 and 0.92% by weight), the data for Si is significantly large (33.65% by weight), indicating that the solids contain mainly SiO2.Figure 5
EDX spectra of apatite ore before and after modifying and the solid after filtering. (a) Apatite ore. (b) Modified apatite. (c) Solid after filtering.
(a)
(b)
(c)Table 1
Composition of elements in apatite ore before and after modifying and the solid after filtering.
Element
Apatite ore
Modified apatite
Solid after filtering
%m
%a
%m
%a
%m
%a
O
50.19
68.21
37.67
54.59
47.74
57.49
Ca
21.40
11.61
36.73
21.25
0.44
0.21
P
10.79
7.57
17.75
13.29
0.92
0.57
F
2.76
3.16
2.58
3.15
1.07
1.08
Si
5.35
4.14
0.42
0.35
33.65
23.09
Al
3.06
2.47
0.46
0.40
1.69
1.21
Mn
1.19
0.47
0.80
0.28
Mg
0.36
0.32
0.17
0.16
0.11
0.09
K
0.82
0.46
0.37
0.18
Fe
4.08
1.59
0.66
0.27
2.93
1.01
N
6.91
9.50
C
3.34
6.44
3.28
5.26
Cu
0.16
0.06
0.11
0.03
Cl
0.07
0.04


## 3.2. Adsorption Experiments Results

### 3.2.1. Calibration Curve

Several solutions with an initial Cd2+ concentration of 0.24–2 mg/L and Cu2+ from 0.12–8 mg/L prepared from a 1000 mg/L standard solution were used to build the calibration curve of Cd2+ and Cu2+ by AAS method (Figure 6). The calibration curve equation y = 0.21208x with correlation coefficient R2 = 0.9993 and y = 0.0693x with R2 = 0.9993 used to calculate the concentrations of Cd2+ and Cu2+, respectively, in the next experiments.Figure 6
The calibration curves of Cd2+ and Cu2+.
(a)
(b)


### 3.2.2. Effect of the Modified Apatite Mass

The influence of modified apatite ore mass on the adsorption capacity and efficiency of Cd2+ and Cu2+ was shown in Figure 7. The results show that when the mass of modified apatite increases, the adsorption efficiency increases and the adsorption capacity decreases. For Cd2+ (Figure 7(a)), treatment efficiency increased rapidly from 39.64% to 82.08% when the modified apatite mass increased from 0.01 g to 0.05 g. After that, the adsorption efficiency increases slowly in the range of adsorbent mass from 0.05 g to 0.1 g. Therefore, to achieve not only a relatively high treatment efficiency but also high adsorption capacity, 0.05 g modified apatite is selected for further experiments. Similarly, for the treatment of Cu2+ ion, 0.03 g modified apatite is appropriate.Figure 7
Variation of the Cd2+/Cu2+ adsorption capacity and efficiency according to the modified apatite ore mass at pH0, Co = 30 mg/L, t = 60 mins. (a) Cd2+, (b) Cu2+.
(a)
(b)


### 3.2.3. Effect of Solution pH

To determine pHpzc, 50 mL solution 0.01 M KCl was adjusted pH in the broad pH range from 5 to 9 (pHbefore) and in the small pH range from 7 to 8 by 0.01 M KOH and 0.01 M HCl solution. 0.03 g modified apatite was put in the solution, stir 1 hour with rate 400 rpm at room temperature. Filter and measure the pH value of the obtained solution (pHafter). The curve of ∆pH (= pHafter – pHbefore) follows pHbefore and horizontal axis meet at pHpzc. The result of determining the pH value at the point zero-charge of the modified apatite ore is shown in Figure 8. The result shows that the adsorbent has point zero-charge at pHpzc = 7.49.Figure 8
Graph for determining pHpzc of modified apatite in broad pH range (a) and in small pH range (b).
(a)
(b)The effect of pH on the adsorption efficiency and capacity of modified apatite ore is shown in Figure9. The results show that when the pH of the solution increased from 2.6 to 7.9, the adsorption efficiency and capacity increased. At low pH values, the surface of the material has a positive charge; therefore the interactive force is also electrostatic force. Besides, if the concentration of H+ ions is high, the competition with metal cations in the adsorption process leads to a decrease in efficiency. When pH is higher than pHpzc (7.49), the adsorption capacity and efficiency rise. It can be explained as follows: at high pH value, the surface of adsorbent has a negative charge, but if pH is too high, the precipitation of Cd(OH)2 and Cu(OH)2 can happen [2, 3, 15]. To facilitate the practical application, we choose pH = pHo = 5.5 (for Cd2+) and 5.6 (for Cu2+) to investigate the next experiments.Figure 9
Effect of pH solution on the adsorption capacity and efficiency of Cd2+ ((a) 30 mins, 0.05 g modified apatite) and Cu2+ ((b) 45 mins, 0.03 g modified apatite) (Co = 30 mg/L).
(a)
(b)


### 3.2.4. Effect of Contact Time

The variation of the adsorption capacity and efficiency according to the contact time was investigated with the conditions as follows: the adsorbent mass is 0.05 g modified apatite ore for Cd2+ adsorption and 0.03 g for Cu2+ adsorption, in 50 mL of the Cd2+ or Cu2+ solution (30 mg/L), pHo, at 25°C with the different contact time changed from 5 to 90 minutes (Figure 10).Figure 10
Variation of Cd2+ (a), Cu2+ (b) adsorption capacity, and efficiency of modified apatite ore according to the contact time.
(a)
(b)When increasing the adsorption time from 5 to 30 minutes, the Cd2+ adsorption efficiency increased significantly from 50% to 84%, corresponding to the increase of adsorption capacity from 14.15 mg/g to 24.24 mg/g. From 30 minutes to 60 minutes, the adsorption efficiency and capacity increase gradually. It can be explained that when the adsorption time increases, the ions fill the pores of the absorbent more, thus the adsorption efficiency and capacity increase. However, after a certain time, the adsorption equilibrium is reached, the adsorption capacity remains stable. Therefore 30 minutes is the optimal Cd2+ adsorption time of the modified apatite ore. Similarly, for the adsorption of Cu2+, 45 minutes was chosen for the removal process by the modified apatite ore. The adsorption efficiency and capacity of the Cu2+ treatment archives 85% and 41.34 mg/g, respectively.


### 3.2.5. Effect of Initial Concentration of Cd2+ and Cu2+

Figure11 shows the variation of adsorption efficiency and capacity according to the initial concentration of Cd2+ and Cu2+. It can be seen that the adsorption capacity increases, and the adsorption efficiency decreases with the increase of initial concentration. However, when the concentration increases to a certain value, the adsorption capacity is almost stable which corresponds to the adsorption process reaches the maximum state. It can be explained by the fact that an amount of modified apatite ore has a certain number of adsorption centers. When all adsorption centers are occupied by Cd2+ and Cu2+ ions, the ion concentration continues to increase, and the adsorption capacity is almost constant.Figure 11
Variation of Cd2+ and Cu2+ adsorption capacity and efficiency of modified apatite ore according to the initial concentration (pHo, t = 60 mins). (a) C Cd2+ (mg/L), (b) C Cu2+ (mg/L).
(a)
(b)


### 3.2.6. Adsorption Isotherm

In order to determine the maximum adsorption capacity of modified apatite ore toward Cd2+ and Cu2+ ions, adsorption studies were carried out with conditions as follows: 0.05 g adsorbent with treatment Cd2+ and 0.03 g with Cu2+ was dispersed into 50 mL solution with different initial concentrations from 8.44 to 138.63 mg/L with Cd2+, 17.68 to 947.88 mg/L with Cu2+, at 25°C, pHo; the mixtures were stirred at 400 rpm for 60 mins. Figures 12 and 13 show the linear fits of the experimental data of the Cd2+ or Cu2+ treatment process using Freundlich and Langmuir adsorption isotherm models.Figure 12
Adsorption isotherm curves of Cd2+ follow the Langmuir (a) and Freundlich (b).
(a)
(b)Figure 13
Adsorption isotherm curves of Cu2+ follow the Langmuir (a) and Freundlich (b).
(a)
(b)From the obtained results, it is evident that Langmuir adsorption isotherm model can be used to describe experimental data of adsorption of Cd2+ and Cu2+. From the linear equation, the maximum adsorption capacity value (Qmax) of Cd2+ and Cu2+ is calculated at about 43.94 and 69.59 mg/g, respectively. Compare with other adsorbents, with Cd2+, HAp/CS, 81.1 mg/g [1], HAp, 21 m2/g, 16.7 mg/g [8], HAp, 94.9 m2/g, 142.9 mg/g [9], AlHAp, 205 m2/g, 103 mg/g [15], activated carbon, 178.5 mg/g [5], thiourea-modified Sorghum bicolor Agrowaste, 5.13 m2/g, 17.2 mg/g [35], with Cu2+, HAp, 50 m2/g, 96.6 mg/g [13], HAp, 125 mg/g [36], SDS modified laterite, 185 mg/g [2], Ni/Fe carbonate-fluorapatite, 74.5 m2/g, 147.5 mg/g [3], modified natural phosphate, 150 m2/g, 166 mg/g, Bengurir rock-Morocco, 21 m2/g, 57 mg/g [30], thiourea-modified Sorghum bicolor Agrowaste, 7.98 m2/g, 15.2 mg/g [35], we can conclude that modified apatite ore is a good adsorbent for Cd2+ and Cu2+.


### 3.2.7. The Phase Component and Molecular Structure of the Absorbent after Absorption

The X-ray diffraction patterns of modified apatite ore before and after Cd2+ adsorption (at pH0, m = 0.05 g, 30 minutes, Cd2+ concentration of 30 mg/L) and Cu2+ adsorption (pH0, m = 0.03 g, 45 minutes, Cu2+ concentration of 30 ppm) are shown in Figure 14(a). It can be seen that the material after adsorption still has the phase structure of hydroxyapatite, indicating that the phase structure of the material does not change.Figure 14
X-ray diffraction patterns (a) and IR spectra (b) of modified apatite ore before and after Cd2+ and Cu2+ adsorption.
(a)
(b)The FTIR spectra of modified apatite ore before and after Cd2+ and Cu2+ adsorption are shown in Figure 14(b). FTIR spectra of the solid residues after adsorption are comparable to one of the initial modified apatite ore, suggesting no other functional groups formed after the heavy metal sorption. Compared with the FTIR spectra before and after adsorption of Cd2+ and Cu2+, there are band shifts, which occur in the groups of apatite that had bound metals. Sorption of Cd2+ and Cu2+ undergoes two-step processes. Rapid complexation of heavy metals on specific sites of the modified apatite ore surface is the first step. A metal diffusion into the modified apatite ore structure or to a heavy metal-containing modified apatite ore formation is the second step in the sorption process [36].


## 3.2.1. Calibration Curve

Several solutions with an initial Cd2+ concentration of 0.24–2 mg/L and Cu2+ from 0.12–8 mg/L prepared from a 1000 mg/L standard solution were used to build the calibration curve of Cd2+ and Cu2+ by AAS method (Figure 6). The calibration curve equation y = 0.21208x with correlation coefficient R2 = 0.9993 and y = 0.0693x with R2 = 0.9993 used to calculate the concentrations of Cd2+ and Cu2+, respectively, in the next experiments.Figure 6
The calibration curves of Cd2+ and Cu2+.
(a)
(b)


## 3.2.2. Effect of the Modified Apatite Mass

The influence of modified apatite ore mass on the adsorption capacity and efficiency of Cd2+ and Cu2+ was shown in Figure 7. The results show that when the mass of modified apatite increases, the adsorption efficiency increases and the adsorption capacity decreases. For Cd2+ (Figure 7(a)), treatment efficiency increased rapidly from 39.64% to 82.08% when the modified apatite mass increased from 0.01 g to 0.05 g. After that, the adsorption efficiency increases slowly in the range of adsorbent mass from 0.05 g to 0.1 g. Therefore, to achieve not only a relatively high treatment efficiency but also high adsorption capacity, 0.05 g modified apatite is selected for further experiments. Similarly, for the treatment of Cu2+ ion, 0.03 g modified apatite is appropriate.Figure 7
Variation of the Cd2+/Cu2+ adsorption capacity and efficiency according to the modified apatite ore mass at pH0, Co = 30 mg/L, t = 60 mins. (a) Cd2+, (b) Cu2+.
(a)
(b)


## 3.2.3. Effect of Solution pH

To determine pHpzc, 50 mL solution 0.01 M KCl was adjusted pH in the broad pH range from 5 to 9 (pHbefore) and in the small pH range from 7 to 8 by 0.01 M KOH and 0.01 M HCl solution. 0.03 g modified apatite was put in the solution, stir 1 hour with rate 400 rpm at room temperature. Filter and measure the pH value of the obtained solution (pHafter). The curve of ∆pH (= pHafter – pHbefore) follows pHbefore and horizontal axis meet at pHpzc. The result of determining the pH value at the point zero-charge of the modified apatite ore is shown in Figure 8. The result shows that the adsorbent has point zero-charge at pHpzc = 7.49.Figure 8
Graph for determining pHpzc of modified apatite in broad pH range (a) and in small pH range (b).
(a)
(b)The effect of pH on the adsorption efficiency and capacity of modified apatite ore is shown in Figure9. The results show that when the pH of the solution increased from 2.6 to 7.9, the adsorption efficiency and capacity increased. At low pH values, the surface of the material has a positive charge; therefore the interactive force is also electrostatic force. Besides, if the concentration of H+ ions is high, the competition with metal cations in the adsorption process leads to a decrease in efficiency. When pH is higher than pHpzc (7.49), the adsorption capacity and efficiency rise. It can be explained as follows: at high pH value, the surface of adsorbent has a negative charge, but if pH is too high, the precipitation of Cd(OH)2 and Cu(OH)2 can happen [2, 3, 15]. To facilitate the practical application, we choose pH = pHo = 5.5 (for Cd2+) and 5.6 (for Cu2+) to investigate the next experiments.Figure 9
Effect of pH solution on the adsorption capacity and efficiency of Cd2+ ((a) 30 mins, 0.05 g modified apatite) and Cu2+ ((b) 45 mins, 0.03 g modified apatite) (Co = 30 mg/L).
(a)
(b)


## 3.2.4. Effect of Contact Time

The variation of the adsorption capacity and efficiency according to the contact time was investigated with the conditions as follows: the adsorbent mass is 0.05 g modified apatite ore for Cd2+ adsorption and 0.03 g for Cu2+ adsorption, in 50 mL of the Cd2+ or Cu2+ solution (30 mg/L), pHo, at 25°C with the different contact time changed from 5 to 90 minutes (Figure 10).Figure 10
Variation of Cd2+ (a), Cu2+ (b) adsorption capacity, and efficiency of modified apatite ore according to the contact time.
(a)
(b)When increasing the adsorption time from 5 to 30 minutes, the Cd2+ adsorption efficiency increased significantly from 50% to 84%, corresponding to the increase of adsorption capacity from 14.15 mg/g to 24.24 mg/g. From 30 minutes to 60 minutes, the adsorption efficiency and capacity increase gradually. It can be explained that when the adsorption time increases, the ions fill the pores of the absorbent more, thus the adsorption efficiency and capacity increase. However, after a certain time, the adsorption equilibrium is reached, the adsorption capacity remains stable. Therefore 30 minutes is the optimal Cd2+ adsorption time of the modified apatite ore. Similarly, for the adsorption of Cu2+, 45 minutes was chosen for the removal process by the modified apatite ore. The adsorption efficiency and capacity of the Cu2+ treatment archives 85% and 41.34 mg/g, respectively.


## 3.2.5. Effect of Initial Concentration of Cd2+ and Cu2+

Figure11 shows the variation of adsorption efficiency and capacity according to the initial concentration of Cd2+ and Cu2+. It can be seen that the adsorption capacity increases, and the adsorption efficiency decreases with the increase of initial concentration. However, when the concentration increases to a certain value, the adsorption capacity is almost stable which corresponds to the adsorption process reaches the maximum state. It can be explained by the fact that an amount of modified apatite ore has a certain number of adsorption centers. When all adsorption centers are occupied by Cd2+ and Cu2+ ions, the ion concentration continues to increase, and the adsorption capacity is almost constant.Figure 11
Variation of Cd2+ and Cu2+ adsorption capacity and efficiency of modified apatite ore according to the initial concentration (pHo, t = 60 mins). (a) C Cd2+ (mg/L), (b) C Cu2+ (mg/L).
(a)
(b)


## 3.2.6. Adsorption Isotherm

In order to determine the maximum adsorption capacity of modified apatite ore toward Cd2+ and Cu2+ ions, adsorption studies were carried out with conditions as follows: 0.05 g adsorbent with treatment Cd2+ and 0.03 g with Cu2+ was dispersed into 50 mL solution with different initial concentrations from 8.44 to 138.63 mg/L with Cd2+, 17.68 to 947.88 mg/L with Cu2+, at 25°C, pHo; the mixtures were stirred at 400 rpm for 60 mins. Figures 12 and 13 show the linear fits of the experimental data of the Cd2+ or Cu2+ treatment process using Freundlich and Langmuir adsorption isotherm models.Figure 12
Adsorption isotherm curves of Cd2+ follow the Langmuir (a) and Freundlich (b).
(a)
(b)Figure 13
Adsorption isotherm curves of Cu2+ follow the Langmuir (a) and Freundlich (b).
(a)
(b)From the obtained results, it is evident that Langmuir adsorption isotherm model can be used to describe experimental data of adsorption of Cd2+ and Cu2+. From the linear equation, the maximum adsorption capacity value (Qmax) of Cd2+ and Cu2+ is calculated at about 43.94 and 69.59 mg/g, respectively. Compare with other adsorbents, with Cd2+, HAp/CS, 81.1 mg/g [1], HAp, 21 m2/g, 16.7 mg/g [8], HAp, 94.9 m2/g, 142.9 mg/g [9], AlHAp, 205 m2/g, 103 mg/g [15], activated carbon, 178.5 mg/g [5], thiourea-modified Sorghum bicolor Agrowaste, 5.13 m2/g, 17.2 mg/g [35], with Cu2+, HAp, 50 m2/g, 96.6 mg/g [13], HAp, 125 mg/g [36], SDS modified laterite, 185 mg/g [2], Ni/Fe carbonate-fluorapatite, 74.5 m2/g, 147.5 mg/g [3], modified natural phosphate, 150 m2/g, 166 mg/g, Bengurir rock-Morocco, 21 m2/g, 57 mg/g [30], thiourea-modified Sorghum bicolor Agrowaste, 7.98 m2/g, 15.2 mg/g [35], we can conclude that modified apatite ore is a good adsorbent for Cd2+ and Cu2+.


## 3.2.7. The Phase Component and Molecular Structure of the Absorbent after Absorption

The X-ray diffraction patterns of modified apatite ore before and after Cd2+ adsorption (at pH0, m = 0.05 g, 30 minutes, Cd2+ concentration of 30 mg/L) and Cu2+ adsorption (pH0, m = 0.03 g, 45 minutes, Cu2+ concentration of 30 ppm) are shown in Figure 14(a). It can be seen that the material after adsorption still has the phase structure of hydroxyapatite, indicating that the phase structure of the material does not change.Figure 14
X-ray diffraction patterns (a) and IR spectra (b) of modified apatite ore before and after Cd2+ and Cu2+ adsorption.
(a)
(b)The FTIR spectra of modified apatite ore before and after Cd2+ and Cu2+ adsorption are shown in Figure 14(b). FTIR spectra of the solid residues after adsorption are comparable to one of the initial modified apatite ore, suggesting no other functional groups formed after the heavy metal sorption. Compared with the FTIR spectra before and after adsorption of Cd2+ and Cu2+, there are band shifts, which occur in the groups of apatite that had bound metals. Sorption of Cd2+ and Cu2+ undergoes two-step processes. Rapid complexation of heavy metals on specific sites of the modified apatite ore surface is the first step. A metal diffusion into the modified apatite ore structure or to a heavy metal-containing modified apatite ore formation is the second step in the sorption process [36].


## 4. Conclusions

Apatite ore is modified by a chemical process using HNO3 and NH3. The modified apatite ore has the phase of nanohydroxyapatite with the specific surface area is 100.79 m2/g which increases about 25 times compared with apatite ore. 0.05 g modified apatite powder can remove about 84% Cd2+ in 50 mL Cd(NO3)2 solution of 30 mg/l with an adsorption capacity of 24.24 mg/g. For treatment Cu2+ ions, 0.03 g modified apatite powder can remove about 85% Cu2+ in 50 mL Cu(NO3)2 solution of 30 mg/L with an adsorption capacity of 41.58 mg/g. Both Cd2+ and Cu2+ removal process is best described by the Langmuir adsorption isotherm model with the R2 near 1 (R2 = 0.99997 and 0.99888, respectively). The maximum monolayer adsorption capacity is calculated from the Langmuir adsorption isotherm about 43.94 and 69.59 mg/g for Cd2+ and Cu2+, respectively.



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*Source: 6527197-2020-03-26.xml*