Hydroxyapatite conjugated graphene oxide nanocomposite: a new sight for significant applications in adsorption

Abstract

Great efforts have been made to develop efficient adsorbents in recent years. In this study, graphene oxide (GO) was synthesized and the surface chemistry of GO was modified by means of hydroxyapatite (HAP) conjugation in order to increase the number of active sites responsible for cationic dye adsorption. The obtained HAP conjugated GO nanocomposites were characterized using various techniques such as FTIR, RAMAN, XRD, TGA, FESEM, BET surface area and AFM. Herein, malachite green (MG) was used as a model dye and the effect of several parameters, including time, pH, temperature, adsorbent dose, and MG concentration, on the adsorption were investigated. The isotherm, kinetic and thermodynamic parameters were measured. The adsorption of MG was best described by the Freundlich isotherm with a pseudo second order kinetic model. In addition, the removal efficiency was maintained at 82% even in the fourth cycle, which supported the reusability and stability of the fabricated nanocomposite for cationic dye adsorption from an aqueous medium.

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1. Introduction

The adsorption process has been recognized as one of the most straightforward, effective and efficient strategies in removing a variety of pollutants from contaminated water.^1,2 With rapid advancements in nanotechnology and nanoscience, graphene oxide and its composites as an adsorbent have been extensively studied for the removal of a variety of pollutants.^3,4 Although other adsorbents, such as mesoporous silica,⁵ activated carbon,⁶ polymers¹ and magnetic nanocomposites,² have been widely investigated over the last few decades with varying levels of success, GO sets a new dimension in the field of environmentology. GO is a single-layered sheet of carbon atoms with a honeycomb-like structure and this unique structure is enriched with a wide range of O-functionality⁷ such as hydroxyl and epoxy groups on its basal plane and carboxyl and carbonyl groups at the edges of the nanosheets.⁸ The O-functionalities act as anchoring sites for the nucleation and growth of different nanoparticles.⁹ However, blocking the O-functionalities of GO after nucleation and growth of nanoparticles might lead to a reduction in its applications in adsorption technology. Therefore, the selection of nanoparticles for nucleation and growth onto the surface of GO is an important issue in the strategies used to prepare GO-based nanocomposites made for adsorption as well as photocatalysis. Herein, we have chosen hydroxyapatite (HAP) because it has great value and significance due to its biocompatibility,¹⁰ binding affinity¹¹ and low cost¹² and has been investigated for applications in tissue engineering, bone repair as well as protein,¹³ drug and gene delivery.¹⁴ Despite its wide variety of potential applications in biological fields, HAP has emerged as one of the most reliable adsorbent biomaterials because it displays good cytocompatibility, which has been experimentally proven in many research reports.^15,16 Different pollutants, including cyanide,¹⁷ phenol,¹⁸ congo red¹⁹ and heavy metal ions,²⁰ have been removed from the environment using HAP-based composites. HAP is also a suitable candidate for establishing bonds with organic molecules of different size. Recently, a few research works have been devoted to the fabrication of GO and HAP composites for potential applications. For instance Li et al. reported that a Ti substrate coated with GO/HA composites not only showed superior in vitro biocompatibility but also higher corrosion resistance when compared with the HA coated and uncoated Ti substrate.²¹ Gao et al. coated AZ91 magnesium alloy with a HA/GO hybrid and improved corrosion resistance of the Mg alloys was achieved.²² However, there are no reports on dye adsorption using GO/HAP nanocomposites published to date. Therefore, the target of the present study was to fabricate HAP conjugated GO (GO/HAP) nanocomposites for dye adsorption. Herein, malachite green (MG) was chosen as a model dye because it is a tinctorially strong synthetic dye with the most controversy.²³ Its arrival in the environment causes fertility reduction in living beings, carcinogenicity and mutagenicity.

2. Experimental

2.1. Materials

Graphite powder (APS 7–11 micron, 99%) and calcium nitrate tetrahydrate (99%) were purchased from Alfa Aesar (A Johnson Matthey Company). KMnO₄, H₂SO₄, FeCl₃ (anhydrous), FeSO₄·7H₂O, ammonia solution, ammonium dihydrogen phosphate and malachite green were acquired from Merck India. These chemicals were used as received. Millipore distilled water was used throughout the experiments.

2.2. Synthesis of GO

The oxidation of graphite powder to synthesize GO was performed according to the improved Hummers method with a little modification to the reported method.^24,25 Briefly, 1 g of graphite powder was dispersed into 23 mL of concentrated H₂SO₄ and stirred for half an hour to make a homogeneous mixture. Then, 3 g of KMnO₄ was gradually added to the homogeneous mixture under continuous stirring and the system was then transferred to an oil bath maintained at 50 °C. After 2 h of stirring, 70 mL of water was slowly added. Consequently, the solution temperature increased and was maintained at 98 °C for 15 minutes. The reaction was terminated by adding another 70 mL of water and 5 mL of H₂O₂ (30%). The solid product was collected and initially washed with 5% HCl. Finally, the product was repeatedly washed with water till the supernatant pH reached neutral and dried for further modification.

2.3. Synthesis of HAP

HAP nanograins were synthesized under an inert atmosphere using a wet precipitation method with some modification to the previous reports.^26,27 Calcium nitrate tetrahydrate and ammonium dihydrogen phosphate were used as the only sources of calcium ions and phosphate, respectively. In brief, separate stock solutions (50 mL) of calcium nitrate tetrahydrate (0.01 M) and ammonium dihydrogen phosphate (0.006 M) were prepared and stirred at 300 RPM for two hours. Thereafter, in an argon atmosphere, the phosphate solution was added to the nitrate solution with continuous stirring at 35 °C. At this stage, the solution turned milky white. When the solution temperature reached to 60 °C, aqueous ammonia was added and the pH maintained at 11. After 2 h of vigorous stirring at 90 °C, the thick milky white precipitate was cooled to room temperature and aged overnight. Finally, the precipitate was collected, washed and dried for further application.

2.4. Synthesis of GO/HAP nanocomposites

A stock solution (50 mL) of calcium nitrate tetrahydrate was added dropwise to the GO suspension over 30 minute with vigorous stirring. After 1 h, a stock solution (50 mL) of ammonium dihydrogen phosphate was added dropwise under an argon atmosphere and stirred for another 1 h at 35 °C. When the temperature reached 60 °C, aqueous ammonia was added and the pH maintained at 11. The resulting dark brown dispersion was heated to 90 °C for 2 h, and then the dark brown thick precipitate was cooled at room temperature and aged overnight. Finally, the precipitate was filtered, washed and dried at 90 °C for further applications. In this study, a series of nanocomposites were prepared by varying the HAP precursor and these are labeled as GO/HAP-1, GO/HAP-2, and GO/HAP-3. The different amounts of precursor used for the preparation of the GO/HAP nanocomposites are listed in Table 1.

Table 1 Experimental parameters used for the GO/HAP nanocomposites synthesis^a

Sample	Molarity of S1 (M)	Amount of S1 (g)	Molarity of S2 (M)	Amount of S2 (g)	Amount of GO (g)	pH
a Note: [S1: calcium nitrate tetrahydrate, S2: ammonium dihydrogen phosphate].
GO/HAP-1	0.01	0.2361	0.006	0.0792	0.01	11
GO/HAP-2	0.02	0.4723	0.012	0.1584	0.01	11
GO/HAP-3	0.03	0.7084	0.018	0.2369	0.01	11

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2.5. Characterization techniques

The FTIR spectra were measured using a Thermo Nicolet Nexux (model 870) FTIR spectrophotometer. Powder X-ray diffraction was recorded on a Philips PW 1710 X-ray diffractometer with the scanning angle (2θ) ranging from 10° to 80°. The morphology was obtained by scanning electron microscopy (SEM) using a JEOL JEM1010 electron microscope. Raman spectra were obtained with a Nicolet Almega XR dispersive Raman spectrometer using a Nd:YAG laser source with a wavelength of 532 nm. Thermal decomposition analysis was carried out with Perkin Elemer 1 DTA-TGA instrument under a flow of nitrogen. The surface area was investigated by the BET method using a Quantachrome NOVA 3200e instrument. Atomic force microscopy images were obtained using a Perkin Bruker Icon with scan analyst. The change in absorbance of MG was measured using a UV-1800 spectrophotometer with an operating voltage of 220–240 V/50–60 Hz (Shimadzu corporation).

2.6. MG adsorption analysis

Batch experiments were performed using the GO/HAP-2 nanocomposites for the removal of MG from a supplied aqueous solution. When necessary, the desired concentration of MG was prepared by diluting a stock solution of dye (100 mg L⁻¹) and used as soon as possible. For the adsorption studies, typically 0.01 g of the GO/HAP-2 adsorbent was immersed in 20 mL of the desired MG solution and shaken at 110 rpm with a Rivotek incubator shaker. The effect of influencing factors, including time (0–90 minute), pH (2–10), temperature (303–323 K), dye concentration and adsorbent dose, were investigated. After adsorption, the clear supernatant solution was collected and the concentration of MG remaining in the solution was determined using UV spectroscopy at a wavelength of 617 nm. The extent of MG adsorption on GO/HAP-2, in terms of percentage dye removal efficiency (R_e) and adsorption efficiency (q_e), was calculated using the following equation.¹where C₀ and C_e are the initial and equilibrium liquid concentration in mg L⁻¹, respectively, V is the volume of MG solution taken and W is the weight of GO/HAP-2 taken (g).

2.7. Dye desorption and reuse study

For desorption, the dye adsorbed GO/HAP-2 nanocomposites were dispersed in methanol solution containing 4% acetic acid. After desorption, the clear supernatant solution was collected and percentage dye desorption (D_e%) was calculated using the following equation.

Similarly, after desorption, the GO/HAP-2 nanocomposites were collected, rinsed with water and then reused in the next cycle of adsorption. The percentage dye removal efficiency (R_e%), in this case, was calculated using eqn (1).

3. Results and discussion

The various oxygen containing functionalities of GO act as anchor sites and thus GO strongly absorbs Ca²⁺ ions through electrostatic interactions and consequently, the rate of nucleation of HAP on the surface of GO increases.²⁸ Herein, we synthesized three types of GO/HAP nanocomposites with varying HAP precursors. The synthetic progress of the GO/HAP nanocomposites starting from GO can be monitored by UV-VIS spectroscopy, as shown in Fig. 1a. The UV-VIS absorption peak of GO at 232 nm shifted to 263 nm with poor absorbance and the colour of the GO solution changed from light yellow to grey (Fig. 1b), demonstrating that conjugation of HAP onto GO sheets took place and the GO was slightly reduced. Among GO, HAP and the various GO/HAP nanocomposites, GO/HAP-2 showed the highest adsorption efficiency towards the adsorption of MG (Fig. S1, ESI†). Therefore, our detailed study focused on the GO/HAP-2 nanocomposite only. The prime driving force of the adsorption of dye onto the GO/HAP-2 nanocomposite was the electrostatic interactions found between the considerable quantities of negative sites localized over the GO/HAP-2 nanocomposite and the cationic MG.^29,30 Herein, the π–π stacking interactions between the dye and adsorbent also play major role in the adsorption of MG.

Fig. 1 UV-VIS absorption spectra of GO, HAP and the GO/HAP nanocomposite (a) and their corresponding images (b). FTIR spectra of HAP, GO and GO/HAP-2 (c) and the Raman spectra of HAP, GO and GO/HAP-2 (d).

3.1. FTIR studies

Fig. 1c depicts the FTIR spectra of GO, HAP and GO/HAP-2. Apart from the aromatic C=C skeletal vibration at 1625 cm⁻¹ for the sp² domains, as expected, the FTIR spectrum of GO shows the presence of different oxygenated functional groups such as the C–O stretching mode at 1075 cm⁻¹, C–OH stretching vibration at 1250 cm⁻¹,³¹ C=O stretching vibration at 1730 cm⁻¹ and O–H stretching vibration at 3415 cm⁻¹. HAP and GO/HAP-2 show a doublet with one peak at 608 cm⁻¹ and another at 567 cm⁻¹, which was attributed to the bending vibration of PO₄³⁻.³² The sharp peaks at 1039 cm⁻¹ and 1092 cm⁻¹ were considered to be the breathing modes of the P–O stretching vibration. The broad peak at 3458 cm⁻¹ was ascribed to the presence of –OH, possibly due to adsorbed water. The emergence of the characteristic peaks of HAP in the spectra of GO/HAP-2 suggested the successful conjugation of HAP onto the surface of GO. However, when compared to GO, GO/HAP-2 contains no vibrations attributed to C–O stretching at 1075 cm⁻¹ and C–OH at 1250 cm⁻¹. In addition, the relative intensity of the C=O stretching vibration was decreased compared to that observed for neat GO. Such phenomena observed in the spectrum of GO/HAP-2 suggest that the GO was partially reduced. The FTIR spectra of MG adsorbed GO/HAP-2 were analyzed to identify the adsorption process, as shown in Fig. S2 (ESI†). The appearance of bands at 1576 and 1401 cm⁻¹ confirms the adsorption of MG onto the GO/HAP-2 nanocomposite.

3.2. Raman studies

The Raman spectrum of HAP is shown in Fig. 1d and shows a narrow band at around 957–961 cm⁻¹, confirming the symmetric, non-degenerated and stretching mode of PO₄³⁻ ions in the stoichiometric HAP with a Ca/P molar ratio of 1.66 and/or carbonate apatite.³³ The systematic bending, ν₂, mode of PO₄³⁻ is present in the range 400–450 cm⁻¹. Similarly, the characteristic peaks of GO at 1348 cm⁻¹ and 1591 cm⁻¹ were assigned to the sp³ D band and sp² G band, respectively,³⁴ and the value of I_D/I_G was calculated to be 1.02. However, after conjugation, the intensity and position of the bands of GO/HAP-2 were found to be different from the native HAP as well as GO and the value of I_D/I_G was calculated to be 0.96. The decrease in the I_D/I_G value indicates that the GO was slightly reduced. The reduction of GO was also discussed by Zhuo et al.³⁴ using Raman spectroscopy. Moreover, the slight change in the position of the Raman bands in GO/HAP-2 shows the interactions between GO and HAP.

3.3. XRD studies

The phase structure and purity of GO, HAP and GO/HAP-2 were investigated using wide angle XRD, as demonstrated in Fig. 2a. The sharp diffraction peak at around 2θ = 12.0° corresponds to the 002 reflection for stacked GO sheets. However, the diffractogram obtained for HAP presents several peaks in a wide range of the spectrum. The characteristic peaks of HAP reveal the presence of calcium HAP.³⁵ However, no diffraction bands for GO were seen in the XRD pattern of GO/HAP-2, indicating the destruction of the regular layer stacking of the GO sheets³⁶ by the incorporation of HAP nanograins or that the peak of GO may be covered by the strong peaks of HAP.

Fig. 2 XRD spectra of HAP, GO and GO/HAP-2 (a) and TGA analysis of HAP, GO and GO/HAP-2 (b).

3.4. TGA analysis

HAP, GO and GO/HAP-2 were subjected to TGA analysis to verify the FESEM observations and the TGA curves obtained depicted in Fig. 2b. Due to its high thermal stability, HAP shows a slow and overall 5.5% weight loss during heating under a nitrogen atmosphere. The first phase weight loss (4.2%), in the temperature range up to 395 °C corresponds to the loss of surface adsorbed water molecules. However, the second phase weight loss (1.3%) in the temperature range above 395 °C may be attributed to the loss of intra-crystalline water along with the decomposition of PO₄³⁻ ions.³⁷ In contrast, it is observed that GO starts to lose weight at an early stage (∼100 °C) and fast weight loss (37.3%) occurs at 190 °C, which corresponds to the volatilization of adsorbed water in its π-stacked structure. Next, the weight loss reached to 45.4% at 350 °C, which was attributed to the decomposition of the epoxy and hydroxyl groups. These results demonstrate that graphite was successfully converted to thin layered GO structures. For GO/HAP-2, the first weight loss (12.2%) up to 350 °C was observed, which may be assigned to the decomposition and vaporization of various oxygen containing functional groups.³⁸ The further weight loss (3.4%) may be due to the loss of intra-crystalline water along with the decomposition of PO₄³⁻ ions.

3.5. FESEM studies

The morphology of HAP, GO and the GO/HAP composites was examined by FESEM and the obtained images are shown in Fig. 3. It can be seen that the shapes of GO look like flakes of leaves,³⁹ as shown in Fig. 3a. This type of morphology is also seen in the AFM image of GO (Fig. S3, ESI†). Fig. 3b depicts the FESEM image of HAP, which indicates the rice grain morphology. It is visualized that GO possesses ample amount of cavities formed between GO leaves, which permits the free entry of HAP into the inner layers. The free entry of HAP results in the strong conjugation between HAP and GO, as shown in Fig. 3c–e. The high magnification images of GO/HAP-2 and GO/HAP-3 are shown in Fig. S4 (ESI†). In GO/HAP-1, the graphene leaves are visible with partial coverage with HAP. Upon increasing the HAP content as in the GO/HAP-2 nanocomposite, the cavities of GO become filled but in GO/HAP-3, GO was completely covered with HAP. GO/HAP-2 shows better adsorption activity when compared to GO/HAP-1 and GO/HAP-3 towards the adsorption of MG.

Fig. 3 FESEM image of GO (a), HAP (b), GO/HAP-1 (c), GO/HAP-2 (d) and GO/HAP-3 (e).

3.6. Surface area analyses

The textural properties of GO, HAP and the GO/HAP-2 nanocomposite were investigated from their N₂ adsorption/desorption isotherms (Fig. S5, ESI†) and the relative parameters are displayed in Table T1 (ESI†). When compared to GO (119.13 m² g⁻¹), the BET surface area of GO/HAP-2 (91.189 m² g⁻¹) decreased. This is due to the smaller specific surface area of HAP (39.028 m² g⁻¹).

3.7. Effect of adsorbent dose on the adsorption of MG

The effect of adsorbent dose on MG adsorption by GO/HAP-2 was investigated and the results shown in Fig. 4a. The experimental data reveal that MG adsorption was found to be at a minimum (90.2%) with an adsorbent dose of 0.004 g and increased to 99.49% at an adsorbent dose of 0.01 g. As we further increased the GO/HAP-2 dose up to 0.016 g, the adsorption becomes almost constant with a maximum adsorption of 99.69%. The increase in adsorption of MG with increasing adsorbent dose was attributed to the availability of a sufficient surface area, and consequently, the availability of the large number of active sites available for MG adsorption. Furthermore, at the higher adsorption dose (>0.01 g), the active sites are much more than the threshold saturated adsorption point, which leads to aggregation of the active sites. Therefore, the optimum adsorbent dose was chosen as 0.01 g.

Fig. 4 Effect of the mass of adsorbent (a); pH (b); time (c); and initial MG concentration (d) on the adsorption process. {Temperature: 303 K for (a–c), initial MG concentration: 40 mg L⁻¹ (20 mL) for (a), (b) and (d), GO/HAP-2 nanocomposite: 0.01 g for (b–d)}.

3.8. Effect of pH

Different solution pH values were used to investigate the effect of pH on the adsorption of MG on the GO/HAP-2 nanocomposite and the obtained graph is presented in Fig. 4b. It can be seen that the adsorption efficiency of MG for the GO/HAP-2 nanocomposite experienced a steady increase from 89.73% to 98.93% upon increasing the solution pH. The effects of pH on MG adsorption can be explained on the basis of the pH_ZPC value (7.0) of GO/HAP-2. At pH < pH_ZPC, the surface of the nanocomposites is positively charged⁴⁰ and it becomes more positively charged with a decrease in pH. This positive environment, in an acidic medium, is less favorable for the adsorption of a cationic dye such as MG. On the contrary, at pH > pH_ZPC, the surface of the nanocomposite is negatively charged and so the adsorption of MG was increased, owing to the strong electrostatic interactions found between the negatively charged nanocomposite and MG.

3.9. Effect of contact time: kinetic analysis

The adsorption of MG on the GO/HAP-2 nanocomposite was investigated as a function of contact time to determine the suitable time at which the adsorption process reaches equilibrium.^41,42 As we can see, the adsorption reached 97.5%, 94.0% and 93.6% within 5 minutes for an initial MG concentration of 30, 40 and 50 mg L⁻¹, respectively. Thereafter, slow adsorption rates were observed till 60 minutes and after 60 minutes almost static adsorption (%) occurred for all the MG concentrations (30, 40 and 50 mg L⁻¹) studied. The static behavior of adsorption after a particular time has been previously reported.^17,18,42 Therefore, the optimum time for adsorption of MG was set at 60 minutes in our further studies. Furthermore, the rate that controls the adsorption of MG was investigated in terms of pseudo first order, pseudo second order and intraparticle diffusion models.⁴³ The first two models are expressed by eqn (4) and (5).

ln(q_e − q_t) = ln q_e − K₁t (4)

where K₁ (min⁻¹) and K₂ (g mg⁻¹ min⁻¹) are the adsorption rate constants for pseudo first order and pseudo second order, respectively. q_t and q_e are the adsorption capacity at time t and equilibrium, respectively. The related parameters obtained from these models are presented in Table 2. As per the R² values, the data exhibit a better fit to the pseudo second order model than that of the pseudo first order model (Fig. 5) for all the adsorption data. Moreover, the better fit to the pseudo second order model can be further justified by the point that the calculated q_e for the pseudo second order is very close to the expected q_e. However, these models were not applicable to identify the diffusion mechanism. Therefore, the intraparticle mechanism was employed to elucidate the diffusion mechanism.⁴⁴ According to this model, adsorption at any time (q_t) is supposed to be proportional to t^1/2 rather than the contact time t. The model parameters of intraparticle diffusion were calculated by eqn (6) and are listed in Table 2.

q_t = k_idt^1/2 + c (6)

where k_id (mg g⁻¹ t^1/2) is the intraparticle diffusion rate constant and c (mg g⁻¹) is the intercept of the intraparticle plot. If the value of c is zero, the adsorption till equilibrium is controlled by intraparticle diffusion. The values of c for all adsorption data (30 mg L⁻¹, 40 mg L⁻¹ and 50 mg L⁻¹) confirmed that the intraparticle diffusion was involved as a part of diffusion process but not the sole rate limiting step.⁴⁵

Table 2 The adsorption kinetic parameters for MG adsorption on GO/HAP-2^a

MG	Pseudo first order				Pseudo second order			Intraparticle diffusion
	qe, exp (mg g^−1)	qe, cal (mg g^−1)	R^2	K1 (min^−1)	qe, cal (mg g^−1)	R^2	K2 (g mg^−1 min^−1)	C (mg g^−1)	R^2	Kid (mg g^−1 t^1/2)
a cal: calculated; exp: expected.
30 mg L^−1	59.62	1.52	0.90	0.0414	59.70	0.999	0.092	58.14	0.97	0.19
40 mg L^−1	78.63	4.48	0.83	0.0417	78.98	0.999	0.026	73.97	0.96	0.59
50 mg L^−1	95.57	3.64	0.98	0.0378	95.78	0.999	0.032	91.42	0.98	0.52

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Fig. 5 Pseudo first order (a), pseudo second order (b) and intraparticle diffusion (c) kinetic models for MG adsorption. {Temperature: 303 K, GO/HAP-2: 0.01 g}.

3.10. The effect of MG dose: isotherm analysis

Adsorption isotherm experiments were performed with MG solutions ranging from 30 mg L⁻¹ to 90 mg L⁻¹ using a fixed amount of the GO/HAP-2 nanocomposite (0.01 g). The results indicate that upon increasing the initial MG concentration from 30 mg L⁻¹ to 90 mg L⁻¹, adsorption declined at all the tested temperatures (Fig. 4d). This was attributed to the saturation of the active sites of the nanocomposite.⁴⁶ However, the relative increase in the adsorption efficiency with temperature was attributed to the increase in kinetic energy of the MG molecules.⁴⁷ Freundlich, Hansley and Langmuir isotherm models were exploited to elucidate the adsorption behavior at each temperature studied. The Freundlich and Langmuir isotherms can be best described by eqn (7) and (8), respectively.where K_f (mg g⁻¹) is the Frendluich constant, which implies the amount of dye adsorbed, n is the heterogeneity factor, q₀ is the maximum adsorption capacity and K₁ (L mg⁻¹) represents the Langmuir constant related to the heat of adsorption.^1,48 The linear fit for these models and the relative observations are displayed in Fig. 6 and Table 3, respectively. As per the R² values, the adsorption isotherm was best fitted to the Freundlich isotherm. Moreover, the values of K_f were 84.54, 93.40 and 104.44 mg L⁻¹ at 303, 313 and 323 K, respectively. The value of K_f increased with increasing temperature, revealing that the adsorption efficiency of MG onto the GO/HAP-2 nanocomposite increased with an increase in temperature. On the contrary, the value of 1/n decreased with increasing temperature but lies between 0 and 1. The value of 1/n favors the assumption that the type of adsorption was favorable and also reveals a higher likelihood of multi-layer adsorption of MG.⁴⁸

Fig. 6 Freundlich (a) and Langmuir (b) adsorption isotherms for MG adsorption on GO/HAP-2. {Time: 60 minute, temperature: 303 K, initial MG concentration: 30–90 mg L⁻¹ (20 mL) and GO/HAP-2: 0.01 g}.

Table 3 Adsorption isotherm parameters for MG adsorption on GO/HAP-2

Temperature	Freundlich			Langmuir
	Kf (mg g^−1)	R^2	1/n	q0 (mg g^−1)	R^2	K1 (L mg^−1)	RL
303 K	84.54	0.978	0.31	178.5	0.965	0.87	0.03
313 K	93.40	0.977	0.28	180.5	0.960	1.08	0.02
323 K	104.44	0.993	0.25	176.0	0.978	1.77	0.01

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The dimensionless separation factor R_L of the Langmuir isotherm was used to assess the feasibility of MG adsorption.¹ R_L is expressed as follows.

The value of R_L indicates the type of isotherm to be favorable (0 < R_L < 1), unfavorable (R_L > 1), linear (R_L = 1) and irreversible (R_L = 0). The results demonstrate that the adsorption of MG at every temperature studied was favorable. In order to get more insight in the MG adsorption process, the Hansley model was also fitted to the experimental data to find any possibility of multi-layer adsorption. The linearised Hansley equation⁴⁸ is expressed by the following equation,

ln q_e = [(1/n)ln K] + [ln C_e/n] (10)

The correlation coefficient for the Hansley isotherm also fitted well (data not shown here) to the adsorption data implying that multi-layer adsorption takes place on the GO/HAP-2 nanocomposite. This finding supports the observations of the Freundlich model.

3.11. Effect of temperature: thermodynamic studies

The effect of different temperatures on the adsorption of MG by the GO/HAP-2 nanocomposite was investigated and the obtained graph is presented in Fig. S6 (ESI†). It was observed that as the temperature increased from 303 K to 323 K, the adsorption increased. The thermodynamics of adsorption of MG on GO/HAP-2 in terms of the change in Gibbs energy (ΔG: kJ mol⁻¹), enthalpy (ΔH: kJ mol⁻¹) and entropy (ΔS: kJ mol⁻¹ K⁻¹) were investigated using the known equations.⁴⁷

ΔG = −RT ln K_c (11)

where R is the gas constant (8.314 J mol K⁻¹), T is temperature (K) and K_c is the distribution coefficient calculated from q_e/C_e. The values of ΔS and ΔH were calculated from the intercepts and slopes of the Van't Hoff plot of ln K_c versus 1/T (Fig. 7a). The results of the thermodynamic calculations are listed in Table 4. The negative value of ΔG at all the temperatures studied showed that the adsorption process was spontaneous and feasible in nature, and the degree of spontaneity increased with increasing temperature. In general, the value of ΔG gives information on the nature of the adsorption process.⁴⁹ If ΔG is in between −20 and 0 kJ mol⁻¹, the adsorption is physi-sorption, while chemi-sorption has a range of −80 to −400 kJ mol⁻¹. Our results, as shown in Table 4, suggest that the adsorption was via physi-sorption. In addition, the positive value of ΔH confirmed the endothermic nature, whereas the positive value of ΔS implied more randomness at the solid solute interface during the adsorption process. Furthermore, the low value of ΔS revealed that during adsorption, there is no remarkable change in entropy.⁵⁰

Fig. 7 The Van't Hoff plot of the adsorption of MG (a) and the adsorption–desorption cycle of MG (b). {Temperature: 303 K for (b), GO/HAP-2: 0.01 g for (a) and (b), initial MG concentration: 40 mg L⁻¹ for (b)}.

Table 4 Adsorption thermodynamic parameters for MG adsorption on GO/HAP-2

MG	Temperature (K)	ΔH (kJ mol^−1)	ΔS (kJ mol^−1 K^−1)	Kc	ΔG (kJ mol^−1)
30 mg L^−1	303	47.99	0.2	163.92	−12.84
	313			329.85	−15.08
	323			532.28	−16.85
40 mg L^−1	303	27.67	0.13	125.91	−12.18
	313			140.37	−12.86
	323			249.88	−14.82
50 mg L^−1	303	22.25	0.1	59.64	−10.29
	313			65.98	−10.90
	323			103.47	−12.45

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3.12. Recycling experiments

The adsorption performance of GO/HAP-2 in the recycling experiments was investigated and the time profile of four repeated experiments in the adsorption of MG (20 mL of 40 mg L⁻¹) is shown in Fig. 7b. It can be seen that GO/HAP-2 can be reused without a drastic decrease in its adsorption efficiency and an 82% adsorption can be obtained in the fourth run, which proves its feasibility in practical applications.

4. Conclusion

Hydroxyapatite was conjugated to GO and used to prepare the GO/HAP-2 nanocomposite, which demonstrated an enhanced adsorption efficiency for the removal of MG from an aqueous phase through electrostatic interactions. The FTIR studies proved the adsorption of MG into the GO/HAP-2 nanocomposite. Alkaline conditions were favorable for MG adsorption because the PZC of the adsorbent was calculated to be 7. The thermodynamic studies revealed that adsorption was a spontaneous and endothermic process.

Acknowledgements

The contents of this research were developed after research bursaries from the Indian School of Mines, Dhanbad during the doctoral studies of the first author. Therefore, the authors would like to express their gratitude to the Indian School of Mines, Dhanbad, Jharkhand.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16022g

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