Abstract

This study is the first report of its type demonstrating the synthesis of mHAP on the basis of magnetic functionalization with nHAP, which were synthesized using Rutilus frisii kutum fish scale as a benign fishery waste by-product. The mHAP was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray diffraction (EDX), and Fourier transform infrared (FT-IR) spectroscopic techniques. The XRD pattern confirmed the formation of a single-phase nHAP without any extra steady phases. It was also found that the pseudo-second-order kinetic model gave a satisfactory fit to the experimental data (R2 = 0.99). The maximum removal percentages of Cu and Zn ions in optimal conditions (adsorbent dosage at 0.1 g, 30 min contact time at 25 ± 1 °C and pH = 5 ± 0.1) by mHAP were 97.1% and 93.8%, respectively. Results also demonstrated that mHAP could be recycled for up to five cycles in the case of copper and zinc. The Langmuir isotherm was proved to have a better correlation compared with that of the Freundlich isotherm. The thermodynamic parameters indicated that it was a spontaneously endothermic reaction. In conclusion, mHAP could be regarded as a powerful candidate for efficient biosorbent, capable of adsorbing heavy metals from aqueous solutions.

INTRODUCTION

As surface water supplies are diminishing, sustaining groundwater resources has become particularly important. Inefficient methods for the management of heavy metals in urban run-off may cause direct or indirect long-term risks for the ecosystem as well as human beings. Considering heavy metals' toxicity, non-biocompatibility, and non-biodegradability, it is essential to remove them effectively from wastewater through suitable remedies. Conventional methods of removing heavy metals from wastewater such as chemical precipitation (Pang et al. 2009), reverse osmosis (Bakalar et al. 2009), absorption (Hegazi 2013), and activated sludge (Banadda et al. 2011) are expensive (Popuri et al. 2009). Adsorption on solid layer biomaterials has been identified as one of the most appropriate methods for removing heavy metals from solution (Feng et al. 2010). However, each method has been limited by cost, complexity, and secondary waste. The application of such biosorbents as seafood processing wastes is one of the emerging adsorption technologies used for the removal of heavy metals. This could be attributed to the fact that this technology is inexpensive, and favourable, used for the removal of heavy metals even at low concentrations (Turhanen et al. 2015).

Annually, about 18–30 million tons of fish waste is discarded all over the world. By weight, about 50% of the whole waste produced by the fisheries is waste, and about 4% of the waste is fish scale. However, this contains valuable organic and inorganic compounds such as hydroxyapatite nanoparticles (nHAP) and collagen (Elvevoll & James 2001). It is worth mentioning that low expense, high sorption capacity, and accessibility are among the noticeable benefits of nHAP.

Among the various methods used for synthesizing nHAP, only a few methods are suitable for the preparation of nHAP. This could be due to their diverse precursors needed in the synthesis, severe aggregation, expensive equipment, and phase impurities during the preparation of nHAP (Sadat-Shojai et al. 2013). An alternative is the synthesis of nHAP from such natural resources as bone, fish scales, and fish bones (Kongsri et al. 2013; Venkatesan et al. 2015). Also, magnetic HAP (mHAP) nanocomposites have recently drawn considerable attention due to their easy separability from the aqueous environment as well as their effective recyclable performance. Few research studies, however, have been conducted so far on the functionalization of magnetic nanoparticles with nHAP (Guo et al. 2011; Bharath & Ponpandian 2015).

Due to the fact that magnetic nanoparticles of Fe3O4 are highly abundant and have a specific surface area, they can play an important role, as environmentally friendly materials, in removing heavy metals. However, magnetic nanoparticles tend to be accumulated. This can lead Fe3O4 to lose both their unique properties and their dispersion due to the anisotropic dipole interactions (Sahika & Ismail 2003). One way to prevent their accumulation is to spread them on a specific area of supports such as hydroxyapatite. In spite of the advantages of HAP, all the adsorbents based on HAP had the common drawback of being inconvenient to separate. The great advantage of magnetic HAP is their easy separation from the aqueous environment as well as their effective recyclable performance.

Many methods for the preparation of mHAP nanocomposite are not desirable as they necessitate stabilizers, the aggregation of products, and much time. Nevertheless, the process we successfully developed in this study provides a synthesized mHAP through the simple process of dispersing nHAP in the pale yellow solution of Fe(III)/Fe(II) with molar ratio 2:1, which was accomplished under magnetic stirring and a nitrogen atmosphere. The nHAP was extracted from fish scales through the alkaline heat treatment method (Kongsri et al. 2013). This bionanocomposite has been applied as a fast recyclable tool for the removal of Cu2+ and Zn2+ from groundwater. Batch adsorption experiments were performed to investigate the effects of various parameters including the pH, the contact time, and the initial zinc and copper concentrations on the adsorption efficiency of mHAP. It is worth noting that mHAP has a high removal efficiency of zinc and copper ions not only in the first but also in the succeeding cycles of the experiments.

EXPERIMENTAL SECTION

Materials

Fish scales (Rutilus frisii kutum from the Caspian Sea) were collected from fish markets. Fish scales were rinsed thoroughly in distilled water for the removal of dirty matter, and then air dried. The chemicals used in this work were purchased at the highest possible grade from Merck Company. The deionized water used in the experiments was Milli-Q gradient (Millipore).

The synthesis of magnetite (Fe3O4) and magnetic hydroxyapatite (mHAP) from fish scales

Magnetite nanoparticles were prepared through the co-precipitation technique, which is the most suitable method (Ahn et al. 2012). Typically, the FeCl3.6H2O and FeCl2.4H2O were dissolved in 3 mL of deionized water under vigorous magnetic stirring for 5 min with degassing of the prepared solution by nitrogen gas (molar ratio of Fe3+/Fe2+ = 2). Then, 10 mL NaOH (1 M) was added to the solution so that it could gradually reach the pH of 8, and which was then stirred for 15 min. Nitrogen gas was continuously passed through the solution during the reaction. The black product was collected with a permanent magnet, washed several times with double distilled water, and then dried under vacuum at 80 °C.

For synthesizing mHAP, the FeCl3.6H2O and FeCl2.4H2O were dissolved in deionized water under vigorous magnetic stirring and a nitrogen atmosphere (molar ratio of Fe3+/Fe2+ = 2). This pale yellow colour solution was stirred for 5 min at 80 °C. Then, 3.52 g (7 mmol) of nHAP was gradually dispersed in the above-mentioned solution. The nHAP was synthesized via the alkaline heat treatment method reported by Kongsri et al. (2013) with a very slight modification. A pale yellow gel was produced, which was diluted by deionized water, and 10 mL of 1 M NaOH was added drop-wise to increase the pH level to 8–10. The initial shift of colour from pale yellow to dark brown was indicative of the formation of mHAP with a molar ratio of nHAP/Fe3O4 = 7/1. Subsequent stirring was continued for 30 min in order to form homogenized mHAP. Finally, the produced mHAP was separated by a permanent magnet, washed several times and dried in oven vacuum at 50 °C overnight.

Characterization

The structure of the synthesized nHAP, magnetite and mHAP was characterized by X-ray diffraction (XRD) through a XEPRT-PRO diffractometer with Cu Kα radiation (λ = 0.154 nm). Scans were performed over 2θ from 20° to 80°. The size and the morphology of the synthesized nHAP, Fe3O4 and mHAP were characterized by scanning electron microscopy (SEM) analysis using a VEGA//TESCAN KYKY-EM3200 microscope, which was coupled with energy dispersive X-ray diffraction (EDX) for elemental analysis. A Fourier transform infrared spectrometer (FT-IR, Bruker Tensor 27) was applied to characterize nanoparticles in the frequency range of 400–4,000 cm−1 and at the resolution of 4 cm−1. The sample was prepared in a KBr pellet. The magnetic properties of mHAP and Fe3O4 were recorded by a vibrating sample magnetometer (VSM) 7400 model with the applied magnetic field sweeping from −10 to +10 kOe at room temperature.

Determination of pHZPC

The zero point charge pH (pHZPC) for the mHAP was measured by the batch equilibration technique. The initial pH values of 0.1 M NaCl (as an inert electrolyte) were set from 2 to 12 by adding 0.1 M HCl or NaOH solution. An amount of 0.1 g of mHAP was added to 25 mL of 0.1 M NaCl solution. The suspensions were permitted to equilibrate for 24 h in a shaker at 30 °C. Then, the mHAP suspensions were separated using a permanent magnet, and the final pH values were measured again.

The adsorption experiments

The adsorption experiments for Cu and Zn ions were performed on simulated groundwater (SGW) according to the batch method. The chemical composition of the SGW was as follows: K2SO4 13 mg/L, Na2SO4 1,284 mg/L, NaHCO3 370 mg/L, CaCl2.2H2O 233 mg/L, and MgCl2.6H2O 136 mg/L. An amount of 0.1 g of mHAP was added to 20 mL of 40 mg/L Cu2+ or Zn2+ solution (SGW) with the initial pH value of 5.0 ± 0.1, and stirred for 30 min at room temperature. After a predetermined time interval, the mHAP was separated using a permanent magnet from the SGW. Then, the residual concentration was determined using novAA 4000P AAS. In order to obtain the optimum conditions, various experimental parameters including the pH of solution, the contact time, and the adsorbent dosage on adsorption efficiency of mHAP were investigated. The equilibrium adsorption capacity (mg/g) of metal ions was obtained using the following equation:  
formula
(1)
where C0 is the initial concentration (mg/L), Ce is the final concentration (mg/L) of each metal ion, V is the volume of the solution (mL), and m is the mass of the biosorbent (g).

The desorption experiments

An amount of 20 mL of 40 mg/L Cu2+ or Zn2+ solution was added to 0.1 g of mHAP and stirred for 30 min at room temperature . Then, the mHAP was separated from the solution with the help of an external magnet and the metal concentrations were determined by a novAA 4000P AAS. Next, the biosorbent was thoroughly washed several times with distilled water and dried at 80 °C. Then, the dried mHAP was dispersed into 20 mL of 0.01 M HCl,0.01 M NaOH, 0.01 M Ca(NO3)2, and 0.003 M EDTA at 25 ± 1 °C under vibration for 30 min. Finally, the metal concentrations were determined in the same way. The above procedure was repeated for up to five cycles.

RESULTS AND DISCUSSION

The properties of the synthesized mHAP bionanocomposite

Figure 1(a) shows the characteristic peaks corresponding to the diffraction pattern of nHAP from fish scale. The main diffraction peaks appeared at 25.9°, 31.8°, 32.2°, 32.9°, 34.1°, 39.8°, 46.7°, 49.5°, 50.5°, and 53.2°, which corresponded to the hexagonal structure of HAP (ICSD-082289). The XRD pattern of the Fe3O4 nanoparticle is shown in Figure 1(c). The XRD pattern indicates that the magnetite nanoparticles were well crystallized (JCPDS card: No. 89–0688). The major diffraction peaks appeared at 30.3°, 35.7°, 43.3°, 53.7°, 57.3°, and 62.9°, which correspond to the (220), (311), (400), (331), (422), and (511) crystal planes of Fe3O4, respectively. The XRD patterns for the mHAP indicates the dual phases of pure nHAp from fish scale and Fe3O4, as appear in Figure 1(b). The intensities and the peak positions of the diffraction peaks of mHAP and nHAP are correspondent. This indicates that the nHAP structure was preserved even after being functionalized with magnetite.

Figure 1

XRD patterns for (a) nHAP, (b) mHAP, (c) magnetite nanoparticles.

Figure 1

XRD patterns for (a) nHAP, (b) mHAP, (c) magnetite nanoparticles.

The FT-IR spectra of the prepared nHAP, mHAP and Fe3O4 in the range of 400–4,000 cm−1 are depicted in Figure 2. The FT-IR spectrum shows all the characteristic absorption peaks of nHAP and Fe3O4. The presence of peaks at 569, 604 and 1,043 cm−1 are due to the bending and asymmetric stretching modes of the group. The band at 1,420 cm−1 indicates a carbonate group. A major peak of the Fe–O bond is located at about 622 cm−1 (Figure 2(b)). The nanopowder produced two characteristic stretching modes of O–H bands at about 3,300–3,500 and 1,690 cm−1, which were reported in all FT-IR spectra of nHAP. Figure 2(a) represents the spectrum for the mHAP showing a similar peak to nHAP at 1,043 cm−1 which indicates that the phosphate lattice is not affected by Fe3O4 functionalization. However, the intensity of the peak has been minimized. The peak of mHAP at 620 cm−1 is similar to that of the magnetite nanoparticles.

Figure 2

FT-IR spectra of (a) nHAP and mHAP, (b) Fe3O4 nanoparticles.

Figure 2

FT-IR spectra of (a) nHAP and mHAP, (b) Fe3O4 nanoparticles.

The hysteresis loops of the magnetite nanoparticles and the mHAP are shown in Figure 3. The saturation magnetization value was 46.52 emu/g compared with the bulk magnetite. The saturation magnetization value of mHAP was 11.47 emu/g, which was less than that of the magnetite nanoparticles. The decrease in magnetic saturation may be attributed to the covered mass of nHAP on the surface of the Fe3O4 nanoparticles. Such an excellent magnetic property means that the nanocomposite can be separated easily from the solution with the help of a permanent magnet.

Figure 3

Magnetic hysteresis loops for (a) Fe3O4 nanoparticles, (b) mHAP.

Figure 3

Magnetic hysteresis loops for (a) Fe3O4 nanoparticles, (b) mHAP.

Figure 4 demonstrates the plot of pHinitial–pHfinal values against the initial values of pHinitial to obtain the pHZPC of mHAP. The sample had an amphoteric character and behaved as a buffer in aqueous systems. The pHZPC values of the mHAP bionanocomposite indicate that its surface was expected to be prevailingly positive at pH values lower than pHZPC = 6.1 and negative at pH values higher than pHZPC = 6.1.

Figure 4

The pHZPC determination for mHAP.

Figure 4

The pHZPC determination for mHAP.

The SEM micrographs of the prepared samples are shown in Figure 5. The results showed that the nHAP, Fe3O4 nanoparticles, and mHAP were spherical shapes with diameters of about 11.7, 18.9, and 23.4 nm, respectively.

Figure 5

SEM-EDX images of (a) nHAP, (b) Fe3O4 nanoparticles, (c) mHAP.

Figure 5

SEM-EDX images of (a) nHAP, (b) Fe3O4 nanoparticles, (c) mHAP.

Figures 5(a) and 5(c) demonstrate the EDX spectra of as-synthesized nHAP and mHAP, respectively. The elemental analysis confirmed that HAP nanopowder was composed of calcium (Ca), phosphorus (P), oxygen (O), and also the iron (Fe) for mHAP biosorbent. The Ca/P molar ratios of nHAP and mHAP were 1.51 and 1.41, respectively. It is noticeable that nHAP from fish scale has a Ca/P molar ratio which is similar to tricalcium phosphates (TCP), Ca3(PO4)2 (Ca/P = 1.5), but it is structurally and chemically similar to HAP (Ca/P = 1.67) (Vallet-Regí & González-Calbet 2004; Märten et al. 2010).

SORPTION STUDY

The effect of pH

The concentration of H+ ions in aqueous media could have an impact on the functional groups of mHAP. The relationship between the initial pH and the adsorption capacity of heavy metals adsorbed on mHAP is shown in Figure 6.

Figure 6

The effect of the pH on (a) Cu2+ and (b) Zn2+ adsorption. Adsorption conditions: 40 mg/L initial copper and zinc ion concentration, adsorbent dosage at 0.1 g and 30 min contact time at 25 ± 1 °C.

Figure 6

The effect of the pH on (a) Cu2+ and (b) Zn2+ adsorption. Adsorption conditions: 40 mg/L initial copper and zinc ion concentration, adsorbent dosage at 0.1 g and 30 min contact time at 25 ± 1 °C.

As can be seen, the qe (mg/g) increased as pH increased from 2 to 5. If pH increased further, then the qe would greatly be diminished. It was also noticed that at optimum pH value (pH = 5), the adsorption uptakes of Zn2+ and Cu2+ were 95.8% and 98.4%, respectively. The surface charge of the adsorbent is positive when solution pH is below pHZPC = 6.1. However, the results showed that the efficiency of copper and zinc ions onto mHAP decreased considerably from 98.4% (95.8%) to 52% (45.6%) when pH increased from 5 to 7. At pH lower than pHZPC = 6.1, the surface charge of mHAP is positive. Therefore, the amounts of the adsorbed copper and zinc ions are not limited by the repulsion of the electrostatic forces between the positively charged surface of the mHAP and Zn2+ and Cu2+ ions.

By enhancing the pH values, the hydroxyl ion concentration can increase. The hydrolysis of heavy metal ions may occur so that various species can be formed in the solution. There may exist Zn(OH)2 and Cu(OH)2 complexes at pH> pHZPC. However, at high concentrations, and complexes may also be produced.

The overall mechanisms, responsible for the Cu2+ and Zn2+ sorptions of mHAP, can include two procedures. At first, the surface of the nanocomposite is dissolved, which can release, , , and functional groups in aqueous solutions. Equation (2) demonstrates the calcium ion exchange with copper or zinc ions (Ma et al. 1993; Lower et al. 1998):  
formula
(2)
And, on the basis of the adsorption mechanism, we can get Equation (3):  
formula
(3)

The effect of mHAP dosage

The effect of the amount of mHAP on heavy metal adsorption is shown in Figure 7. It is evident that the quantity of the heavy metals adsorbed can increase with the dosage increase of mHAP. With the increase in the amounts of mHAP from 0.05 to 0.5 g, the removal percentage increased rapidly, and displayed a plateau for amounts higher than 0.1 g. Any increase in the amount of biosorbent may cause an increase in the number of accessible active sites in the adsorbent. Having used the adsorbed amount of 0.1 g, we found that the removal percentages of mHAP for Cu2+ and Zn2+ were 98.4% and 91.8%, respectively. According to the results, mHAP was an appropriate adsorbent for adsorbing copper and zinc ions. The optimum dose was 0.1 g, which was applied by further studies. On the other hand, the plot of adsorption capacity revealed that the qe was high at low dosages and reduced at high dosages. The decrease in qe with the increase in the adsorbent dosage is mainly assigned to the unsaturation of the adsorption sites through the adsorption process (Han et al. 2006).

Figure 7

The effect of the dosage of mHAP nanocomposite on (a) Cu2+ and (b) Zn2+ adsorption. Adsorption conditions: solution pH = 5 ± 0.1, 40 mg/L initial copper and zinc ion concentration, and 30 min contact time at 25 ± 1 °C.

Figure 7

The effect of the dosage of mHAP nanocomposite on (a) Cu2+ and (b) Zn2+ adsorption. Adsorption conditions: solution pH = 5 ± 0.1, 40 mg/L initial copper and zinc ion concentration, and 30 min contact time at 25 ± 1 °C.

The effect of contact time and the initial concentration of heavy metals

Figure 8 shows the effect of contact time on the adsorption of Cu2+ and Zn2+ on mHAP. The adsorption rate of Cu2+ and Zn2+ by the mHAP is relatively rapid and adsorption equilibrium can be reached in 30 min, and no remarkable changes were observed for longer contact time. It also indicated that the adsorption rate of Zn2+ and Cu2+ on the mHAP bionanocomposite was much faster than the rates of other adsorbents for copper and zinc ions (Liu et al. 2001; Corami et al. 2007). The rapid adsorption of Cu2+ and Zn2+ on mHAP assures that sufficient time is accessible for adsorption equilibration to be acquired under the usual operating conditions of the adsorption experiment. The equilibrium time of different initial Cu2+ and Zn2+ concentrations was also conducted and the results showed that the initial copper and zinc concentrations had little effect on the adsorption equilibrium time.

Figure 8

The effect of the initial concentration of heavy metals and contact time of mHAP nanocomposite on (a) Cu2+ and (b) Zn2+ adsorption. Adsorption conditions: adsorbent dosage at 0.1 g, and pH = 5 ± 0.1 solution at 25 ± 1 °C.

Figure 8

The effect of the initial concentration of heavy metals and contact time of mHAP nanocomposite on (a) Cu2+ and (b) Zn2+ adsorption. Adsorption conditions: adsorbent dosage at 0.1 g, and pH = 5 ± 0.1 solution at 25 ± 1 °C.

The effect of the initial concentration of Cu2+ and Zn2+ ions was found to be crucial because it had overwhelmed the mass transfer constraints of Cu2+ and Zn2+ between the solution and the solid phases. Therefore, higher initial copper and zinc concentrations will increase the adsorption process. Table 1 shows the effect of initial Cu2+ and Zn2+ concentrations on the qe at various contact times. It was clear to see that the qe values increase with the increase in the initial copper and zinc concentrations and contact times. The maximum equilibrium qe values were determined as 4.32 (4.04), 5.64 (5.44), 7.69 (7.47), 11.23 (11.15) and 11.23 (11.15) mg/g for 70 mg/L initial Cu2+ (Zn2+) concentrations at 2, 5, 15, 30 and 40 min, respectively.

Table 1

The equilibrium qe obtained at different initial Cu2+ (Zn2+) concentrations and contact time

Time (min)20 (mg/L)40 (mg/L)70 (mg/L)
qe (mg/g)qe (mg/g)qe (mg/g)
1.53 (1.34) 3.86 (3.74) 4.32 (4.04) 
2.06 (1.86) 5.12 (4.84) 5.64 (5.44) 
15 2.58 (2.46) 6.27 (6.02) 7.69 (7.47) 
30 3.69 (3.62) 7.66 (7.50) 11.23 (11.15) 
40 3.69 (3.62) 7.66 (7.50) 11.23 (11.15) 
Time (min)20 (mg/L)40 (mg/L)70 (mg/L)
qe (mg/g)qe (mg/g)qe (mg/g)
1.53 (1.34) 3.86 (3.74) 4.32 (4.04) 
2.06 (1.86) 5.12 (4.84) 5.64 (5.44) 
15 2.58 (2.46) 6.27 (6.02) 7.69 (7.47) 
30 3.69 (3.62) 7.66 (7.50) 11.23 (11.15) 
40 3.69 (3.62) 7.66 (7.50) 11.23 (11.15) 

Adsorption conditions: adsorbent dosage at 0.1 g, and pH = 5 ± 0.1 solution at 25 ± 1 °C

The desorption experiment

It was indicated that the desorption percentage was maximum in NaOH solution. The amounts of metal ions desorbed by this solution were 84.38% and 84.33% for Cu2+ and Zn2+, respectively. This phenomenon may be due to the reversible competition between the mHAP bionanocomposite surface and the basic solution for metal ions. At high concentrations, and complexes may also be formed. At this stage, the metal ions were removed from the bionanocomposite surface. Figure 9 shows the removal efficiency in each cycle. Each cycle was repeated three times. The removal efficiency was gradually reduced in the later cycles. However, it was still above 80% in the last cycle. As Table 2 shows, the desorption from the EDTA solution with a lower desorption affinity was assigned to the complex formation between the metal ions and EDTA. In addition, the amounts of metal ions desorbed from the acidic solution were 64.3% and 62.4% for Cu2+ and Zn2+, respectively. Under high acidic conditions, was preferred on the surface of the biosorbent, which led to the desorption of heavy metal ions. In Ca(NO3)2 solution, Ca2+ was preferred to be complex with phosphate on the bionanocomposite surface, which resulted in the desorption of metal ions.

Table 2

Desorption of Cu2+ and Zn2+ from loaded mHAP

EluantsDesorbed (%)
Cu2+Zn2+
EDTA (0.003 mol/L) 53.14 52.73 
HCl (0.01 mol/L) 64.30 62.40 
NaOH (0.01 mol/L) 84.38 84.33 
Ca(NO3)2 (0.01 mol/L) 59.31 59.32 
EluantsDesorbed (%)
Cu2+Zn2+
EDTA (0.003 mol/L) 53.14 52.73 
HCl (0.01 mol/L) 64.30 62.40 
NaOH (0.01 mol/L) 84.38 84.33 
Ca(NO3)2 (0.01 mol/L) 59.31 59.32 
Figure 9

The removal efficiency of Cu2+ and Zn2+ in different cycles by mHAP nanocomposites.

Figure 9

The removal efficiency of Cu2+ and Zn2+ in different cycles by mHAP nanocomposites.

The thermodynamic studies

The effect of temperature was further supported by calculating thermodynamic studies. Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) for the sorption of Cu2+ and Zn2+ on mHAP nanocomposite were all determined using the following equations:  
formula
(4)
 
formula
(5)
 
formula
(6)
where CAe is the concentration of copper and zinc ions on mHAP surfaces at equilibrium (mg/g), Ce is the equilibrium concentration of Cu2+ and Zn2+ in solution (mg/L) and Kc is the equilibrium constant of sorption. ΔH° and ΔS° values for copper and zinc sorption can be evaluated from the slope and the intercept of the linear plot of ln Kc vs 1/T (Figure 10 and Table 3). The negative values of the Gibbs energy (ΔG°) indicated the capability and the spontaneous nature of the copper and zinc sorption process, while the positive value of the enthalpy (ΔH°) suggested the endothermic nature of the sorption of Cu2+ and Zn2+ on mHAP. The affinity of the adsorbent for copper and zinc ions is reflected by the positive values of ΔS°. The obtained thermodynamic data can provide invaluable information for the design of improved adsorption schemes, which can help us treat copper and zinc ions from water and wastewater resources through synthetic nanocomposites.
Table 3

Thermodynamic parameters for sorption of Cu2+ and Zn2+ (data in parentheses) onto mHAP nanpcomposite

T (K)Kc (L/g)ΔG° (kJ/mol)ΔS° (kJ/mol.K)ΔH° (kJ/mol)
283 2.31 (2.05) −1.98 (− 1.69) 0.508 (0.287) 143.17 (79.48) 
293 17.87 (11.66) −7.02 (− 5.99) 
303 54.55 (24.64) −10.05 (− 8.06) 
313 499 (39) −16.16 (− 9.52) 
T (K)Kc (L/g)ΔG° (kJ/mol)ΔS° (kJ/mol.K)ΔH° (kJ/mol)
283 2.31 (2.05) −1.98 (− 1.69) 0.508 (0.287) 143.17 (79.48) 
293 17.87 (11.66) −7.02 (− 5.99) 
303 54.55 (24.64) −10.05 (− 8.06) 
313 499 (39) −16.16 (− 9.52) 
Figure 10

Enthalpy determination curves for the sorption of (a) Cu2+ and (b) Zn2+ onto mHAP. Adsorption conditions: Cu2+ and Zn2+ initial concentrations at 40 mg/L , adsorbent dosage at 0.1 g, 30 min contact time and pH = 5 ± 0.1 solution.

Figure 10

Enthalpy determination curves for the sorption of (a) Cu2+ and (b) Zn2+ onto mHAP. Adsorption conditions: Cu2+ and Zn2+ initial concentrations at 40 mg/L , adsorbent dosage at 0.1 g, 30 min contact time and pH = 5 ± 0.1 solution.

The adsorption kinetics

In order to examine the nature of the adsorption mechanism involved during the adsorption process, the pseudo-first-order kinetic and pseudo-second-order kinetic models were examined, which are determined as follows (Meski et al. 2010):  
formula
(7)
 
formula
(8)
where qe (mg/g) and qt (mg/g) are the amounts of copper and zinc adsorbed on the mHAP bionanocomposite when the adsorption reaches equilibrium and at various times t (min), respectively. K1 and K2 are the rate constants of the pseudo-first-order (min−1) and the pseudo-second-order (g/mg. min) models.

Linear plots of log(qeqt) vs t and t/qt vs t are depicted in Figures 11 and 12, respectively; k1, k2 and qe are calculated from the slopes and intercepts of the lines obtained by plotting log (qeqt) against t and t/qt against t are listed in Table 4. According to R2 from Table 4, it can be found that the pseudo-second-order kinetic model gives a satisfactory fit to the experimental data. This model is based on the presumption that the rate-limiting step may be chemisorption, which involves the valence forces through sharing or exchanging electrons between the adsorbent and the adsorbate. Due to this chemical interaction, mHAP in combination with copper and zinc ions is a stationary chemical with physical properties, which will not make secondary pollution in the environment.

Table 4

The pseudo-first-order and pseudo-second-order models for mHAP nanocomposites

 Kinetic modelR2qe,cal.qe,exp.K
Cu2+ Pseudo-first-order 0.96 4.05 7.66 0.073 
Pseudo-second-order 0.99 8.26 7.66 0.039 
Zn2+ Pseudo-first-order 0.94 4.44 7.50 0.075 
Pseudo-second-order 0.99 8.12 7.50 0.037 
 Kinetic modelR2qe,cal.qe,exp.K
Cu2+ Pseudo-first-order 0.96 4.05 7.66 0.073 
Pseudo-second-order 0.99 8.26 7.66 0.039 
Zn2+ Pseudo-first-order 0.94 4.44 7.50 0.075 
Pseudo-second-order 0.99 8.12 7.50 0.037 
Figure 11

The first-order kinetic model of (a) Cu2+ and (b) Zn2+ adsorption onto mHAP at a temperature of 25 °C. Adsorption conditions: adsorbent dosage at 0.1g, 30min contact time and pH = 5 ± 0.1 solution.

Figure 11

The first-order kinetic model of (a) Cu2+ and (b) Zn2+ adsorption onto mHAP at a temperature of 25 °C. Adsorption conditions: adsorbent dosage at 0.1g, 30min contact time and pH = 5 ± 0.1 solution.

Figure 12

The second-order kinetic model of (a) Cu2+ and (b) Zn2+ adsorption onto mHAP at a temperature of 25 °C. Adsorption conditions: adsorbent dosage at 0.1g, 30min contact time and pH = 5 ± 0.1 solution.

Figure 12

The second-order kinetic model of (a) Cu2+ and (b) Zn2+ adsorption onto mHAP at a temperature of 25 °C. Adsorption conditions: adsorbent dosage at 0.1g, 30min contact time and pH = 5 ± 0.1 solution.

The adsorption isotherms

The Langmuir model supposes that adsorption occurs on a monolayer surface at the specific sites of the adsorbent, which is given by (Sundaram et al. 2008):  
formula
(9)
where qmax (mg/g) is the maximum amount of the copper or zinc adsorbed to form a complete monolayer covering the surface at equilibrium copper or zinc concentration Ce (mg/L), qe is the amount of Cu2+ or Zn2+ adsorbed per unit weight of adsorbent at equilibrium, and KL is the Langmuir constant (L/mg), which is related to the surface affinity for copper or zinc. The crucial characteristics of the Langmuir isotherm may be expressed in terms of the equilibrium parameter RL, which is a dimensionless constant, known as the separation factor or equilibrium parameter:  
formula
(10)
The Freundlich adsorption isotherm is the equation to describe the heterogeneous surface, and is represented by the following equation:  
formula
(11)
where KF and nF are the Freundlich constants and the intensity of adsorption, respectively. While the constant KF approximately reflects the adsorption capacity, 1/nF is a function of the adsorption strength in the adsorption process. The fitting plots of Langmuir adsorption and the Freundlich adsorption of copper and zinc onto mHAP bionanocomposites are shown in Figures 13 and 14, respectively.
Figure 13

The Langmuir adsorption plots for (a) copper and (b) zinc adsorption onto mHAP from fish scale at a temperature of 25 ± 1 °C. Adsorption conditions: Cu2+ and Zn2+ initial concentrations at 40 mg/L , adsorbent dosage at 0.1 g, 30 min contact time and pH = 5 ± 0.1 solution.

Figure 13

The Langmuir adsorption plots for (a) copper and (b) zinc adsorption onto mHAP from fish scale at a temperature of 25 ± 1 °C. Adsorption conditions: Cu2+ and Zn2+ initial concentrations at 40 mg/L , adsorbent dosage at 0.1 g, 30 min contact time and pH = 5 ± 0.1 solution.

Figure 14

The Freundlich adsorption plots for (a) copper and (b) zinc adsorption onto mHAP from fish scale at a temperature of 25 ± 1 °C. Adsorption conditions: Cu2+ and Zn2+ initial concentrations at 40 mg/L , adsorbent dosage at 0.1 g, 30 min contact time and pH = 5 ± 0.1 solution.

Figure 14

The Freundlich adsorption plots for (a) copper and (b) zinc adsorption onto mHAP from fish scale at a temperature of 25 ± 1 °C. Adsorption conditions: Cu2+ and Zn2+ initial concentrations at 40 mg/L , adsorbent dosage at 0.1 g, 30 min contact time and pH = 5 ± 0.1 solution.

It can be concluded that the Langmuir isotherm (R2>0.91) fitted the experimental results better than the Freundlich isotherm (R2>0.66) (Table 5). These facts indicate that Cu2+and Zn2+ were evenly adsorbed in the form of monolayer coverage at distributed active sites on the surface of the biosorbent. The RL values indicated that the Langmuir isotherm was favourable (0 < RL < 1). The smaller 1/nF means a greater expected heterogeneity. From the data in Table 5, it can be shown from the values of 1/nF that the sorption of Cu2+ and Zn2+ ions onto mHAP bionanocomposites are favourable.

Table 5

Langmuir and Freundlich isotherm constants for the copper and zinc adsorption onto mHAP nanocomposite from fish scale

 Langmuir isotherm
Freundlich isotherm
qmax (mg/g)KL (L/mg)R2RL1/nFKF (mg/g)R2
Cu2+ 13.46 0.37 0.96 0.06 0.359 4.44 0.62 
Zn2+ 15.08 0.23 0.94 0.1 0.442 3.60 0.72 
 Langmuir isotherm
Freundlich isotherm
qmax (mg/g)KL (L/mg)R2RL1/nFKF (mg/g)R2
Cu2+ 13.46 0.37 0.96 0.06 0.359 4.44 0.62 
Zn2+ 15.08 0.23 0.94 0.1 0.442 3.60 0.72 

CONCLUSIONS

This research demonstrated that biowaste fish scale, produced in fisheries, is a suitable material precursor for magnetic hydroxyapatite. This could be attributed to its abundant supply, free cost, and inherent composition. In this study, nHAP was synthesized from fish scale. Through a convenient synthetic procedure, the synthetic HAP was impregnated into the magnetic nanoparticles through the sol–gel method, which resulted in mHAP bionanocomposite. This bionanocomposite can be applied as an efficient and fast tool for removing copper and zinc ions from water and wastewater resources.

This study also strove to investigate the effect of different parameters on the elimination of Cu2+ and Zn2+ from simulated groundwater. The copper and zinc sorption on the mHAP nano-adsorbent was augmented at pH = 5 ± 0.1, with the initial metal ion concentration of 40 mgL−1, and the sorbent dosage of 0.1 g. The adsorption process was very fast, and equilibrium was reached within 30 min of contact.

The interaction between mHAP and the heavy metals was reversible, and such heavy metals could be highly released from the nanocomposite under basic conditions. The adsorption isotherm correlated well with the Langmuir adsorption model, and the kinetics confirmed a pseudo-second-order model. The thermodynamic parameters indicated that it was a spontaneously endothermic reaction. All in all, the prepared mHAP is an excellent biomaterial, used as a recyclable tool for wastewater treatment.

ACKNOWLEDGEMENTS

This work was supported by the Department of Marine Chemistry, Faculty of Marine and Oceanic Sciences, University of Mazandaran, Babolsar, Iran.

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