Treatment of Cd and Cu Ions Using Modified Apatite Ore

Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Hanoi 10000, Vietnam Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Hanoi 10000, Vietnam University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Hanoi 10000, Vietnam


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][2][3]. e 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].
One of the research directions of environmental technology is finding the natural original materials with huge reserves, low price, and good treatment ability [19][20][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, , Ac, U, Pu, Sr, Cd, Se, Cs, Tc, and Pb to prevent the pollution to the water [22][23][24][25][26][27][28]. PO 4 3− groups in apatite can react with heavy metals to form precipitation and Ca 2+ 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. is modified material removed heavy metal ions such as Pb 2+ , Cu 2+ , and Zn 2 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 Zn 2+ , Co 2+ , and Cu 2+ with Q max 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. e 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 Cd 2+ , Cu 2+ 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 Cd 2+ and Cu 2+ ions in water and investigated the effect of contact time, initial Cd 2+ and Cu 2+ concentration, pH solution, and adsorbent mass on adsorption capacity and efficiency. From the experiment data, the adsorption isotherms were also determined.

Materials.
Apatite ore is a product from Lao Cai province, Vietnam. Apatite ore was milled and dried at 100 o C in the oven. And then, it was modified by the procedure as follows: apatite ore was put in the glass reactor containing 1M HNO 3 solution and was stirred at a rate of 400 rpm at room temperature. After that, the insoluble matter was separated by filtration. e remaining solution containing Ca 2+ and PO 4 3− is neutralized by a concentrated NH 4 OH solution (25%) to form precipitation of hydroxyapatite. e pH value was maintained at pH > 10 to avoid the formation of other calcium phosphate compounds. e 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%, NH 3 25%, HNO 3 65%, CuSO 4 ·5H 2 O 99% are pure chemicals from China; Cd(NO 3 ) 2 ·4H 2 O 99.5% is pure chemical of Merck.

Adsorption Experiments.
e adsorption experiments of Cd 2+ and Cu 2+ ions were conducted with 50 mL of Cd(NO 3 ) 2 or CuSO 4 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). e mixtures were stirred at a rate of 400 rpm by a magnetic stirrer (Velp Scientifica) at different times (from 5 to 90 minutes). e concentration of Cd 2+ and Cu 2+ ions in the solutions after treatment was determined by Atomic Absorption Spectrophotometer (AAS) on iCE 3500 ermo Scientific device (Germany). e adsorption efficiency (H (%)) and capacity (Q (mg/g)) were calculated according to the following equation: where C o (mg/L) is the initial concentration of Cd 2+ and Cu 2+ ions in the solution, C e (mg/L) is the equilibrium concentration of Cd 2+ and Cu 2+ 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.

Characterization of Original and Modified Apatite
Ore. e infrared spectra and the SEM and TEM images of apatite ore and modified apatite ore are presented in Figures 1 and 2, respectively. e spectra show that characteristic bands of PO 4 3− 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]. e 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 . e 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 CO 3 2− group [32]. e peak at 1384 cm −1 in modified apatite is attributed to NO 3 − group [34] in the modification process. It is important to note that there were no visible characteristic peaks for F − and CO 3 2− after modifying, indicating that the modified process has transformed fluorapatite in the apatite ore to hydroxyapatite. is will be confirmed by the XRD result below.
From SEM images, it can be seen that the particles of apatite ore do not have a uniform particle size. e size of large particles can be up to 15-20 μm. e 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 ( Figure 3). On the X-ray diffraction pattern of apatite ore, there are typical peaks for the phase of fluorapatite and some other peaks specific to SiO 2 and Ca 2 (Al 2 O 7 ) 2 . e diffraction angle (2θ) of 32°; 33°; 34°specific to the phase of Ca 5 (PO 4 ) 3 F. ere are also some other peaks corresponding to the phase of Ca 5 (PO 4 ) 3  e X-ray diffraction pattern shows that the phase composition of the ore sample is mainly fluorapatite and also contains SiO 2 and a small content of Ca 2 (Al 2 O 7 ) 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]. is proves again that the modified process has transformed fluorapatite in the ore to hydroxyapatite. e specific surface area of apatite and modified apatite ore is determined by the BETmethod, using N 2 adsorption at 77 K ( Figure 4). e N 2 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]. e average pore size of apatite ore and modified apatite is 15.88 nm and 24.10 nm, respectively. e results show that the apatite ore sample has a lower significant adsorption capacity of N 2 than the modified sample. e specific surface area for apatite ore before and after modifying is 3.97 and 100.79 m 2 /g, respectively.
is result can predict that the modified apatite ore may have better adsorption capacity than the original apatite. e composition and percentage of elements in apatite ore are in accordance with the published company production criteria and the XRD results. e EDX results are presented in Figure 5 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. e ratios of Ca/P for apatite before and after modifying are 1.53 and 1.60, respectively, similar to that in fluorapatite Ca 5 (PO 4 ) 3 F and hydroxyapatite Ca 5 (PO 4 ) 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 SiO 2 .

Calibration Curve.
Several solutions with an initial Cd 2+ concentration of 0.24-2 mg/L and Cu 2+ from 0.12-8 mg/L prepared from a 1000 mg/L standard solution were used to build the calibration curve of Cd 2+ and Cu 2+ by AAS method (Figure 6). e calibration curve equation y � 0.21208x with correlation coefficient R 2 � 0.9993 and y � 0.0693x with R 2 � 0.9993 used to calculate the concentrations of Cd 2+ and Cu 2+ , respectively, in the next experiments.

Effect of the Modified Apatite Mass.
e influence of modified apatite ore mass on the adsorption capacity and efficiency of Cd 2+ and Cu 2+ was shown in Figure 7. e results show that when the mass of modified apatite increases, the adsorption efficiency increases and the adsorption capacity decreases. For Cd 2+ (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. erefore, 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 Cu 2+ ion, 0.03 g modified apatite is appropriate.

Effect of Solution pH.
To determine pH pzc , 50 mL solution 0.01 M KCl was adjusted pH in the broad pH range from 5 to 9 (pH before ) 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 (pH after ). e curve of ∆pH (� pH after -pH before ) follows pH before and horizontal axis meet at pH pzc . e result of determining the pH value at the point zerocharge of the modified apatite ore is shown in Figure 8. e result shows that the adsorbent has point zero-charge at pH pzc � 7.49. e effect of pH on the adsorption efficiency and capacity of modified apatite ore is shown in Figure 9. e 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 pH pzc (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    Journal of Chemistry pH � pHo � 5.5 (for Cd 2+ ) and 5.6 (for Cu 2+ ) to investigate the next experiments.

Effect of Contact Time.
e 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 Cd 2+ adsorption and 0.03 g for Cu 2+ adsorption, in 50 mL of the Cd 2+ or Cu 2+ solution (30 mg/L), pH o , at 25°C with the different contact time changed from 5 to 90 minutes ( Figure 10).
When increasing the adsorption time from 5 to 30 minutes, the Cd 2+ 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. erefore 30 minutes is the optimal Cd 2+ adsorption time of the modified apatite ore. Similarly, for the adsorption Journal of Chemistry 5 of Cu 2+ , 45 minutes was chosen for the removal process by the modified apatite ore. e adsorption efficiency and capacity of the Cu 2+ treatment archives 85% and 41.34 mg/g, respectively. 2+ and Cu 2+ . Figure 11 shows the variation of adsorption efficiency and capacity according to the initial concentration of Cd 2+ and Cu 2+ . 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 Cd 2+ and Cu 2+ ions, the ion concentration continues to increase, and the adsorption capacity is almost constant.   Figures 12 and 13 show the linear fits of the experimental data of the Cd 2+ or Cu 2+ treatment process using Freundlich and Langmuir adsorption isotherm models. From the obtained results, it is evident that Langmuir adsorption isotherm model can be used to describe experimental data of adsorption of Cd 2+ and Cu 2+ . From the linear equation, the maximum adsorption capacity value (Q max ) of Cd 2+ and Cu 2+ is calculated at about 43.94 and 69.59 mg/g, respectively. Compare with other adsorbents, with Cd 2+ , HAp/CS, 81.1 mg/g [1], HAp, 21 m 2 /g, 16.7 mg/g [8], HAp, 94.9 m 2 /g, 142.9 mg/g [9], AlHAp, 205 m 2 /g, 103 mg/g [15], activated carbon, 178.5 mg/g [5], thiourea-modified Sorghum bicolor Agrowaste, 5.13 m 2 /g, 17.2 mg/g [35], with Cu 2+ , HAp, 50 m 2 /g, 96.6 mg/g [13], HAp, 125 mg/g [36], SDS modified laterite, 185 mg/g [2], Ni/Fe carbonate-fluorapatite, 74.5 m 2 /g, 147.5 mg/g [3], modified natural phosphate, 150 m 2 /g, 166 mg/g, Bengurir rock-Morocco, 21 m 2 /g, 57 mg/g [30], thiourea-modified Sorghum bicolor Agrowaste, 7.98 m 2 /g, 15.2 mg/g [35], we can conclude that modified apatite ore is a good adsorbent for Cd 2+ and Cu 2+ .

e Phase Component and Molecular Structure of the Absorbent after Absorption.
e X-ray diffraction patterns of modified apatite ore before and after Cd 2+ adsorption (at pH 0 , m � 0.05 g, 30 minutes, Cd 2+ concentration of 30 mg/L)

Journal of Chemistry 9
and Cu 2+ adsorption (pH 0 , m � 0.03 g, 45 minutes, Cu 2+ 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. e FTIR spectra of modified apatite ore before and after Cd 2+ and Cu 2+ 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 Cd 2+ and Cu 2+ , there are band shifts, which occur in the groups of apatite that had bound metals. Sorption of Cd 2+ and Cu 2+ 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].

Conclusions
Apatite ore is modified by a chemical process using HNO 3 and NH 3 . e modified apatite ore has the phase of nanohydroxyapatite with the specific surface area is 100.79 m 2 /g which increases about 25 times compared with apatite ore. 0.05 g modified apatite powder can remove about 84% Cd 2+ in 50 mL Cd(NO 3 ) 2 solution of 30 mg/l with an adsorption capacity of 24.24 mg/g. For treatment Cu 2+ ions, 0.03 g modified apatite powder can remove about 85% Cu 2+ in 50 mL Cu(NO 3 ) 2 solution of 30 mg/L with an adsorption capacity of 41.58 mg/g. Both Cd 2+ and Cu 2+ removal process is best described by the Langmuir adsorption isotherm model with the R 2 near 1 (R 2 = 0.99997 and 0.99888, respectively). e maximum monolayer adsorption capacity is calculated from the Langmuir adsorption isotherm about 43.94 and 69.59 mg/g for Cd 2+ and Cu 2+ , respectively.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.  Figure 14: X-ray diffraction patterns (a) and IR spectra (b) of modified apatite ore before and after Cd 2+ and Cu 2+ adsorption.