Kinetic approach for cadmium sorption using microwave synthesized nano-hydroxyapatite

Journal of Non-Crystalline Solids 357 (2011) 1118–1129

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Kinetic approach for cadmium sorption using microwave synthesized nano-hydroxyapatite M.F. Elkady ⁎, M.M. Mahmoud, H.M. Abd-El-Rahman Fabrication Technology Department, Advanced Technology and New Materials Research Institute (ATNMRI), Mubarak City for Scientific Research and Technology Applications (MuCSAT), Alexandria, Egypt

a r t i c l e

i n f o

Article history: Received 10 January 2010 Received in revised form 14 October 2010 Available online 27 November 2010 Keywords: Cadmium sorption kinetics; Characterization of hydroxyapatite; Thermodynamics; Nano-hydroxyapatite; Waste water treatment

a b s t r a c t Nano-crystalline hydroxyapatite adsorbent that was prepared by microwave processing was utilized for cadmium removal from aqueous solutions using the batch technique. Cadmium sorption on the prepared adsorbent was studied as a function of initial cadmium concentration in the aqueous solution, adsorbent dosage, agitation speed, hydrogen ion concentration of the aqueous solution (pH), and the solution temperature. The highest Cd2+ sorption was achieved at agitation rate of 500 rpm. The sorption process was relatively fast and equilibrium was achieved after about 240 min of contact. The optimum sorption of cadmium occurred at pH range 4–7. The kinetic process of Cd2+ sorption onto the synthesized nanohydroxyapatite was tested by applying the pseudo-first order, the pseudo-second order, the simple Elovich and intraparticle diffusion rate models. The sorption process follows a pseudo second-order kinetics with a contribution of intraparticle diffusion. Thermodynamic parameters ΔH, ΔS and ΔG have been calculated. Positive value of ΔH and negative value of ΔG show endothermic and spontaneous nature of sorption respectively. The relatively small value of the activation energy that equal to 8.61 kJ/mol confirms that cadmium sorption process is diffusion controlled. The main mechanism for cadmium ions removal using the synthesized nano-hydroxyapatite was suggested to be ion-exchange and diffusion controlled. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Nowadays heavy metals are one of the most important pollutants in source and treated water and are becoming a severe public health problem. Heavy metal contamination exists in aqueous waste streams of many industries, such as metal plating facilities, mining operations and tanneries. The soils surrounding many military bases are also contaminated and pose a risk of metals ground water and surface water contamination. Some metals associated with these activities are cadmium, chromium, lead and mercury [1]. Heavy metals are not biodegradable and tend to accumulate in living organisms, causing various diseases and disorders [1,2]. The toxic elements discharged in the effluents will be absorbed and accumulated by microorganisms. Eventually, the toxic element will get transferred to humans via the food chain. Cadmium is a highly toxic element and considered as a carcinogen. It can enter the human body by eating food, drinking water, breathing or smoking. Most of the cadmium that enters the body goes to kidney, liver and can remain there for many years and can cause serious damage to kidney and bones [3,4]. Human beings have reported nausea and vomiting at levels of 15 mg/L cadmium, with no adverse effects at 0.05 mg/L [5].

⁎ Corresponding author. Tel./fax: + 20 3 4593414. E-mail address: [email protected] (M.F. Elkady). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.10.021

There has been a sharp rise in the global use of cadmium for batteries and other applications, such as pigments, metal plating, plastic, nonferrous mining, and smelting industries. Cadmium is usually regulated to concentrations of less than 0.1 mg/L. Cadmium cannot be precipitated to the regulatory levels by pH adjustment alone. However, it can be removed very effectively in chelating ion exchange resins. Nevertheless, it cannot be removed by reduction, since its oxidation potential is too high [6,7]. The treatment of cadmium contaminated water is similar to that of many metal contaminated effluents. There are several methods to treat the metal contaminated effluent such as precipitation, ion exchange chemical oxidation, reduction, precipitation, electrolytic recovery, coagulation and adsorption etc, but the selection of the treatment methods is based on the concentration of waste and the cost of treatment [8,9]. In the last few years, adsorption has been shown to be an economically feasible alternative method for removing trace metal from waste water and water supplies [10]. It is the only method that has availability for removal, recovery and recycling of toxic heavy metals from waste water. Many kinds of sorbents have been reported for Cd2+removal, such as activated carbon, mesoporous materials, clay, zeolite, chitosan and apatite [4,11]. The toxicity of solid substances containing heavy metals is closely related to their solubility such as the organic resins. For this reason, phosphate stabilization resulting in formation of highly insoluble phosphates which are stable over almost the entire pH

M.F. Elkady et al. / Journal of Non-Crystalline Solids 357 (2011) 1118–1129

range found in the natural environment represents an efficient strategy for reducing heavy metals toxicity by decrease of their mobility and bioavailability [12]. Apatite is the name given to a group of crystals of the general chemical formula M10(RO4)X2, where R is most commonly phosphorus, M could be one of several metals, although it is usually calcium, and X is commonly hydroxide or a halogen such as fluorine or chlorine. A large number of apatite based materials (mineral phosphates [13–16], synthetic apatite [17–19], bone meal [20,21] and bone char [22]) have been considered as matrixes for remediation of metal contaminated water and soil. Generally, calcium-hydroxyapatite (HAP) Ca10(PO4)6(OH)2, has demonstrated the best removal efficiency due to its moderate solubility–between highly insoluble and highly soluble phosphate bearing materials such as phosphate rock and phosphate fertilizers, respectively [23]. It is an ideal material for long term containments because of its high sorption capacity for heavy metals, low water solubility, high stability under reducing and oxidizing conditions, availability and low cost [24]. Reported data indicate that divalent metal sorption capacities on HAP, as well as the sorption mechanisms strongly depend on: (a) type of divalent metal, (b) HAP physico-chemical properties and (c) other factors, such as metal concentration, solution pH, contact time, presence of other ionic species, etc. [25,26]. Microwave processing of materials is a technology that can provide a new, powerful, and significantly different tool to process materials or to improve the performance characteristics of existing materials. In many cases, materials processing using microwave technology have numerous advantages compared to traditional materials processing techniques [27–30]. These anticipated benefits include more precise and controlled volumetric heating, faster rampup to temperature, lower energy consumption, and enhanced quality and properties of the processed materials. In general, hydroxyapatite (HAP) is one of the most important bio-materials [31]. HAP adsorbent powders were successfully prepared using multimode home model microwave oven. Detailed information about the conditions and the method of the HAP preparation using microwave energy is described elsewhere and will be published [32]. The objectives of the present study were: (i) to explore the feasibility of the microwave synthesized nano-hydroxyapatite as an adsorbent for the removal of cadmium from aqueous solution, (ii) to investigate the effect of pH, amount of adsorbent material, contact time, initial metal concentration, solution temperature and agitation speed on the sorption of cadmium using the synthesized HAP, (iii) to propose the theoretical model for describing the kinetic data. 2. Experimental procedure 2.1. Stock solution preparation Stock solution of cadmium (Pangalore, India) of 1000 mg/L was prepared by dissolving cadmium chloride in distilled water. The concentration range of cadmium prepared from the stock solution varied between 100 and 1000 mg/L using the synthesized HAP. Before mixing the adsorbent, the pH of each last solution was adjusted to the required value with dilute solutions of 0.1 N HCl and 0.1 N NaOH. All the chemicals used were of analytical reagent grade. 2.2. Preparation of nano-hydroxyapatite HAP powders were successfully prepared in a 2.45 GHz–900 W multimode home model microwave oven. The HAP powder was prepared by microwave heating of a mixture that contain both phosphoric acid (H3PO4, Riedel-dehaen, 85%) and calcium nitrate tetrahydrate solutions (Ca(NO3)2.4H2O, Riedel-dehaen, 99–103%) at pH 10 by the addition of sodium hydroxide solution during the mixing process. The mixture was heated in the microwave oven for 8 min until complete dryness of the mixture.

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2.3. Characterization of nano-hydroxyapatite Different characterization techniques have been used to identify the prepared HAP. X-ray powder diffraction (XRD) was carried out using X-ray diffractometer (Schimadzu-7000, U.S.A.) with CuKα radiation beam (λ = 0.154060 nm). The finely powdered sample was packed into a flat aluminum sample holder. Data were collected between 10° and 80° in 2θ. The chemical composition of the synthesized HAP was determined by dissolving 0.5 g of the sample in 50 ml of aqua-regia. The material was analyzed for calcium and phosphorus using inductive coupled plasma mass spectrophotometer (ICP-AES). Further characterization for the dried powder as KBr discs (sample/KBr mass ratio 1:100) was involved Fourier-transform infrared FTIR by (Shimadzu FTIR-8400 S, Japan) in order to determine the I.R. spectrum of the prepared HAP in the frequency range 600– 4000 cm− 1 at room temperature. In order to determine the morphological features and surface characteristics of the adsorbent materials, scanning electron microscope (JEOL JSM 6360LA, Japan) was used. Powdered HAP stocked over the holder. Then it was goldsputtered before examination. The samples were scanned to identify the structure of prepared samples and estimate the particle diameter at different magnifications 10,000× and 50,000×. The confirmation from the determined mean particle size of the prepared HAP takes place by the particle size analyzer. The particle size Analyzer (N5 submicron particle size analyzer Beckman Coulter, Miami-Florida, USA) utilizes the Photon Correlation Spectroscopy technique and is based on the principles of Dynamic Light Scattering. Where, as the HAP particles diffuse through the sample cell due to Brownian motion, an incident beam of laser light illuminates the particles. The particles scatter the light producing fluctuations in the scattering intensity as a function of time. The scattered light is collected through optical fibers at different angles, and is measured by a highly sensitive detector. 2.4. Adsorption procedure The batch experiments were carried out using a digital heating controlled magnetic stirrer (J.P. Selecta, Spain). Batch mode adsorption studies were carried out to determine the cadmium sorption. A known volume (25 ml) of metal solutions of varying initial concentrations (100–1000 ppm), taken in 100 ml stoppered glass conical, was shaken with a fixed dose of adsorbent (0.25 g) for a specified period of contact time in a digital heating controlled magnetic stirrer for various agitation speed (0–1000 rpm) and different solution temperatures (25–80 °C). The residual cadmium was analyzed through selective ion electrode (Denver Instrument, USA). All the experiments were carried out in duplicate and mean values are presented. The cadmium ion measurements were repeated three times to obtain an accurate cadmium concentration. The percentage error in the measurement of cadmium ion concentration is ± 0.1. The average values were obtained and used for calculation the percentage ions removal by the adsorbent from the following Eq. (1): %R = ððCo −CÞ = Co Þ  100

ð1Þ

Where R is the ions removal, Co is the initial concentration of the metal ions in solution, and C is the final metal ion concentration in aqueous solution after phase separation. In order to elucidate the uptake capacity of the metal ion, the up take amounts per gram of HAP was evaluated from the change in solution concentration using Eq. (2): Q ðmg = gÞ = VðCo −CÞ = M

ð2Þ

Where Q is the uptake capacity (mg/g), V is the volume of the solution (ml) and M is the mass of the solid material (g).

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Fig. 3. Scanning electron microphotographs (SEM) of of the microwave synthesized HAP powder with measured mean diameter 40 nm. Fig. 1. XRD pattern of the microwave synthesized HAP powder.

Kinetic studies of adsorption were also carried out at different concentrations of adsorbent (0.1–1.5 g) where the extent of adsorption was investigated as a function of time.

step. Since there is no significant increase on Cd2+ sorption when the time is 120 min, the equilibrium time could be considered to be taking place at 240 min to ensure the complete sorption.

3. Result

3.3. Effect of hydrogen ion concentration

3.1. Characterization of the microwave synthesized HAP powder

The uptake of Cd2+as a function of hydrogen ion concentration was examined over a pH range of 2–11 and the revealed data are shown in Fig. 6. The sorption of Cd2+onto synthesized apatites increases with increasing the initial pH up to 7 and approaches a plateau at pH range 9–11 reflecting the presence a second sorption mechanism.

The X-ray diffraction (XRD) pattern of the microwave synthesized HAP powder was shown in Fig. 1. The XRD pattern was successfully indexed and identified as crystalline HAP phase when compared with the literature and the JCPDS card No.(86-0740) [33,34]. The Ca/P ratio of the microwave prepared HAP was measured using ICP analysis and was found to be 1.67. Fig. 2 shows the FTIR pattern of the microwave synthesized HAP powder. It was obvious from this figure that there are three different characteristics regions represent the formation of HAP. Fig. 3 represents the size and morphology of the HAP particles. It was observed that the produced HAP particles are relatively uniform in diameter and have spherical shape with a mean diameter equal to 40 nm. The particle sizes micrographs of the prepared HAP powder (Fig. 4) confirm that the microwave prepared HAP have a nano-scale particle size with a mean value of 68 nm. 3.2. Effect of contact time Sorption of Cd2+ by HAP as a function of contact time has been illustrated in Fig. 5. The result revealed that a removal takes place in two different steps; the first involves a rapid Cd2+ removal (first 60 min), the second one exhibits a subsequent removal until equilibrium is reached, which is slow and quantitatively insignificant

Fig. 2. FTIR pattern of the microwave synthesized HAP powder.

3.4. Effect of nano-HAP amount The relationship between HAP amount over a studied range (0.1– 1.5 g) and the removal efficiency of Cd2+ was shown in Fig. 7. In our experiments, the cadmium removal efficiency increased rapidly with the increasing HAP amount from 0.1 to 0.5 g and then slight increase was observed above 0.5 g. However the case was inverted in Fig. 8, where the sorbet amount of Cd2+ was decreased with the increase in the HAP amount. Also, it was observed from this figure that the effect of time on cadmium removal tend to be negligible for all studied HAP amount above 0.25 g, thus 0.25 g of HAP was chosen as optimum dosage for earlier studied factors.

Fig. 4. Particle size analysis of the microwave synthesized HAP powder with mean diameter of 68 nm.

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opposite trend (Fig. 10), where the amount of cadmium ions sorbet increased at time 300 min from 9.8 mg/g to 88.5 mg/g as cadmium concentration was increased from 100 ppm to 1000 ppm. 3.6. Effect of agitation speed The effect of agitation speed (from 0–1000 rpm) on the %removal of the initial cadmium concentration was investigated in Fig. 11. The percentage removal seemed to be affected by the agitation speed for values between 0 and 500 rpm, thus confirming that the influence of external diffusion on the sorption kinetic control plays a significant role. Also it is clear that while increasing mixing rate above 500 rpm, the percentage removal decreased from the time 180 min to 300 min. This gives indication that a 500 rpm shaking rate is sufficient to assure that all the surface binding sites is made readily available for cadmium ions uptake. Fig. 5. Effect of contact time on cadmium amount removed per gram of HAP (cadmium concentration = 500 ppm, HAP dosage = 10 g/L, agitation speed = 500 rpm, pH = 7, temperature = 25 °C).

3.5. Effect of initial concentration of cadmium solution The effect of initial cadmium concentration (100 ppm–1000 ppm) on the removal efficiency using fixed amount of HAP sorbent (0.25 g) was showed in Fig. 9. As seen from results, the sorption yield of cadmium was decreased with increasing cadmium concentration over the studied range especially for the higher studied cadmium concentrations 750 & 1000 ppm. Where the removal efficiency at 300 min decreased from 99.5% to 88.5% as the cadmium concentration increased from 100 ppm to 1000 ppm. While the sorption capacities of the sorbents showed the

3.7. Effect of solution temperature The effect of temperature on the cadmium sorption was studied over temperature range (25–80 °C) using initial cadmium concentration 500 ppm. Fig. 12 depicts that the percentage cadmium sorption was increased at time 120 min from 93% to 98% with an increase in temperature from 25 °C to 80 °C. 3.8. Sorption kinetics In order to investigate the rate of Cd2+ sorption, the kinetic data was analyzed using four kinetics models, the pseudo-first-order, the pseudosecond-order, the simple Elivoch and the intraparticle diffusion models.

Fig. 6. Effect of pH on cadmium removal (cadmium concentration=500 ppm, HAP dosage =10 g/L, agitation speed =500 rpm, cadmium volume= 25 ml, temperature =25 °C.

Fig. 7. Effect of HAP amount on cadmium removal (cadmium concentration = 500 ppm, pH = 7, agitation speed = 500 rpm, cadmium volume = 25 ml, temperature = 25 °C).

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Fig. 8. Effect of HAP amount on amount cadmium removed per gram of HAP (cadmium concentration= 500 ppm, pH= 7, agitation speed= 500 rpm, cadmium volume = 25 ml, temperature= 25 °C ).

Fig. 9. Effect of cadmium initial concentration on cadmium removal (HAP amount = 0.25 g, pH = 7, agitation speed = 500 rpm, cadmium volume = 25 ml, temperature = 25 °C).

3.8.1. Pseudo first-order rate model The first order rate equation of Lagergren is one of the most widely used for the sorption of a solute from liquid solution [35] and is represented as: ln ðqe −qt Þ = ln qe –k1 t

Where qe and qt are amounts of ions sorbed (mg/g) at equilibrium and at time t (min), respectively. k1 (min− 1) is the first-order reaction rate constant. The first-order-rate constant k1, can be obtained from the slope of the plot ln(qe − qt) vs. time Fig. 13. The estimated reaction rate and the correlation coefficient (R2) values of fitting the first-order rate model are reported in Table 1. It was indicated from

Fig. 10. Effect of cadmium initial concentration on amount of cadmium removed (HAP amount = 0.25 g, pH = 7, agitation speed = 500 rpm, cadmium volume = 25 ml, temperature = 25 °C).

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Fig. 11. Effect of agitation speed on cadmium removal (HAP amount = 0.25 g, pH = 7, cadmium concentration = 500 ppm, cadmium volume = 25 ml, temperature = 25 °C).

this table the correlation coefficients are not high for the different cadmium concentrations. Also, the estimated values of qe calculated from the equation differed from the experimental values. 3.8.2. Pseudo second-order rate model This model based on sorption equilibrium capacity and may be expressed in the form [36]:   2 t = qt = 1 = k2 qe + t = qe Where k2 is the second-order reaction rate equilibrium constant (g/mg min). A plot of t/qt against t should give a linear relationship for the applicability of the second-order kinetic. Fig. 14 shows the lineralized form of the pseudo second-order model for the sorption of different initial concentrations of cadmium ions on HAP. The correlation coefficients, R2, the pseudo second-order rate parameters and the estimated values of qe calculated from the equation are shown in Table 2. Based on linear regression (R2 N 0.999) values from this

table, the kinetics of cadmium sorption on to HAP can be described well by second-order equation. 3.8.3. The simple Elovich model This model may be expressed in the form [37]: qt = α + β ln t Where α represents the rate of chemisorption at zero coverage (mg/g min) and β is related to the extent of surface coverage and activation energy for chemisorption (g/mg). The plot of qt vs. ln t should give a linear relationship for the applicability of the simple Elovich kinetic. Fig. 15 illustrates the plot of qt against lnt for the sorption of different initial concentrations of cadmium ions on HAP. From the slope and intercept of the linearization of the simple Elovich equation, the Elovich equation parameters could be estimated and tabulated in Table 3. This table declared that the Elovich equation also fit with the experimental data well with high correlation coefficients.

Fig. 12. Effect of solution temperature on cadmium removal (HAP amount = 0.25 g, pH = 7, cadmium concentration = 500 ppm, cadmium volume = 25 ml, agitation speed = 500 rpm).

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Fig. 13. First order plots for different cadmium concentration removal using HAP (HAP amount = 0.25 g, pH = 7, solution temperature = 25 °C, cadmium volume = 25 ml, agitation speed = 500 rpm).

Table 1 Estimated kinetic parameter of the first order rate model and comparison between the experimental and calculated qe values for different cadmium concentrations. Cadmium concentration (ppm)

R2

K1 (min–1)

qecal.

qeexp.

100 250 500 750 1000

0.84 0.95 0.88 0.65 0.99

0.0135 ± 0.002 0.0186 ± 0.004 0.0182 ± 0.003 0.0068 ± 0.0025 0.0217 ± 0.0025

0.1697 ± 0.08 0.4178 ± 0.1 1.756 ± 0.7 5.24 ± 0.8 8.99 ± 0.6

9.93 ± 0.5 24.11 ± 0.9 48.28 ± 1.2 67.35 ± 1 87.2 ± 0.8

3.8.4. Intraparticle diffusion model The possibility of intra-particle diffusion resistance affecting the sorption was explored by Weber and Morris [38] using the intraparticle diffusion model as follows: qt = kid t

1=2

+I

Where, kid is the intra-particle diffusion rate constant. Values of I give an idea about the thickness of the boundary layer [39], i.e., the larger intercept the greater is the boundary layer effect [39]. The plot of qt vs. t1/2 was presented in Fig. 16 for the different cadmium concentrations using HAP. From Fig. 16, it can be seen that there are two separate regions–the first portion is attributed to the film

diffusion and the second portion to intraparticle diffusion [40]. The values of kid,1 and kid,2 as obtained from the slopes of the two straight lines are listed in Table 4. Also the values of I1, I2 that obtained from the intercept of the two straight lines, which give an idea about the thickness of the boundary layers due to both the film diffusion and the intraparticle diffusion respectively were tabulated in the same table. It is clear from this table that the thickness of the boundary layer in the second portion that corresponding to the intraparticle diffusion (I2) is larger than that of the first portion that concerned to the film diffusion (I1). Consequently the values of the intraparticle diffusion rate kid,2 are smaller than the film diffusion rate kid,1. 3.9. Sorption thermodynamics The thermodynamic parameters, such as enthalpy ΔH, entropy ΔS and Gibbs free energy ΔG were examined in the range 298–353 K for the sorption systems of cadmium removal using the synthesized hydroxyapatite, under optimized conditions mentioned earlier. The activation energy for the sorption systems was determined from the Arrhenius equation: ln k2 = ln A−

Ea Rg T

Where A is the temperature independent factor (frequency factor) (g/mg min); k2 is the second-order rate constant value for the ions

Fig. 14. Second order plots for different cadmium concentration removal using HAP (HAP amount = 0.25 g, pH = 7, solution temperature = 25 °C, cadmium volume = 25 ml, agitation speed = 500 rpm).

M.F. Elkady et al. / Journal of Non-Crystalline Solids 357 (2011) 1118–1129 Table 2 Estimated kinetic parameter of the second order rate model and comparison between the experimental and calculated qe values for different cadmium concentrations. Cadmium concentration (ppm)

R2

100 250 500 750 1000

1 0.2 ± 0.002 9.96 ± 0.8 9.93 ± 1.2 1 0.132 ± 0.0015 24.15 ± 0.5 24.11 ± 1.4 1 0.027 ± 0.001 48.54 ± 0.9 48.28 ± 1.4 0.9999 0.0027 ± 0.001 68 ± 1.2 67.35 ± 1.5 0.9999 0.0048 ± 0.002 88.5 ± 1.3 87.2 ± 1.2

K2 (g/mg min)

qecal.

qeexp.

sorption, Ea the activation energy in kJ/mol, T the temperature in Kelvin and Rg is the gas constant that equal to 8.314 J/mol K. As seen in Fig. 17, a plot of lnk2 vs. 1/T was found to be linear with acceptable correlation coefficient values R2 for cadmium ions removal for all studied temperatures. The activation energy for the studied sorption system was derived from the slope of this plot and calculated to be equal 8.61 kJ/mol. Additionally the change in enthalpy (ΔH) and entropy (ΔS°) could be calculated using the van't Hoff [41]: ln kc =

ΔS ΔH − R RT

Where, kc = Fe/(1 − Fe), and Fe = (Co − Ce)/Co; is the fraction adsorbed at equilibrium, while T is the temperature in Kelvin. The plot of lnkc vs. 1/T gives a straight line with acceptable coefficient of determination (R2) as shown in Fig. 18. From the slope and the intercept of van't Hoff plot, the values of ΔH and ΔS have been calculated to be equal 14.44 kJ/mol and 76.75 kJ/mol respectively. While the Gibbs free energy changes ΔG was calculated using the following equation [42]: ðΔGÞ = −RT ln kc Table 5 illustrates the Gibbs free energy change ΔG for the studied range of solution temperature. It was noticed from this table that the value of ΔG was decreased as the temperature increased.

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Table 3 Parameters obtained from the simple Elovich model for different cadmium concentrations. Cadmium concentration (ppm)

R2

α (mg/gmin)

β (g/mg)

100 250 500 750 1000

0.82 0.94 0.81 0.96 0.97

9.5457 ± 0.7 23.383 ± 0.5 44.551 ± 0.8 54.865 ± 0.8 74.438 ± 0.5

0.0726 ± 0.01 0.1363 ± 0.05 0.7079 ± 0.1 2.2848 ± 0.14 2.4303 ± 0.19

because the microwave synthesis was done in ambient air [44]. It was confirmed from the morphology study and particle size analysis that the produced HAP produced in a nano-scale with diameter range of 40–70 nm. The adsorption of cadmium in aqueous solution on HAP was examined by optimizing various physicochemical parameters such as contact time, pH, and amount of adsorbent, solution concentration and temperature. It was obvious from the investigation of the effect of contact time (Fig. 5) that cadmium sorption process is firstly (first 60 min)so fast due to a larger surface area of the sorbent being available for the sorption of Cd2+ then tend to be slow due to a quick exhaustion of the sorption sites with cadmium ions. This is in accordance with the expected results and with the results reported by Ajmal et al. [45] and Namasivayam and Panganathan [46]. The solution pH has a great effect on the ion exchange processes. It considered one of the most important environmental factors which influence not only the ion exchanger behavior, but also the solution chemistry of the metal ions. So cadmium sorption process was examined over a pH range of 2–11 (Fig. 6). Firstly, it is worthy to note that sorption of Cd2+on the synthesized apatites decreases the final pH to values around 6. Such decrease, in the final pH, was also observed by other investigators in sorption of Pb and Zn on hydroxyapatite [47,48]. They reported that the total quantities of the displaced H+ are comparable to the concentrations of the metal ions present initially in the solution. This finding indicates that sorption of Cd2+ results in proton liberation from the surface ≡ POH sites of hydroxyapatite into the aqueous solution according the following reactions:

4. Discussion 2þ

The microwave synthesized technique produce hydroxyapatite that has high crystallinity with different plane of orientations, which indicated from the peaks width of the XRD patterned. The chemical analysis of the prepared HAP illustrate that The Ca/P ratio matches the theoretical value of HAP. The FTIR Fig. 2 illustrates that the presence of the triple peak at 550, 590, and 625 cm− 1, in addition to the broad vibration bands at 1020 & 1080 cm− 1 are characteristics for the formation of HAP[43]. The 1403 cm− 1 peak is due to the carbonate ion

þ

þ

HAP–OH þ Cdaq ↔HAP–O–Cd þ Haq ; 2þ

þ

2HAP–OH þ Cdaq ↔ðHAP–OÞ2 Cd þ 2Haq ; and suggests the contribution of surface complexes formation, to some extent, in the overall sorption mechanism. It was found that synthetic HAP exhibits excellent buffering properties. If no specific sorption from aqueous solution occurs, acidic as well as basic aqueous

Fig. 15. Simple Elovich plots for different cadmium concentration removal using HAP (HAP amount = 0.25 g, pH = 7, solution temperature = 25 °C, cadmium volume = 25 ml, agitation speed = 500 rpm).

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Fig. 16. Intraparticle diffusion plots for different cadmium concentration removal using HAP (HAP amount = 0.25 g, pH = 7, solution temperature = 25 °C, cadmium volume = 25 ml, agitation speed = 500 rpm).

Table 4 Parameters obtained from the interparticle diffusion model for different cadmium concentrations. Cadmium concentration (ppm)

kid,1

I1

R21

kid,2

I2

R22

100 250 500 750 1000

0.049 ± 0.01 0.0763 ± 0.01 0.5212 ± 0.2 1.1569 ± 0.2 0.9181 ± 0.2

9.5239 ± 0.5 23.418 ± 0.8 44.086 ± 1.2 56.206 ± 2 77.434 ± 2.2

0.785 0.982 0.896 0.94 0.97

0.0079 ± 0.001 0.007 ± 0.005 0.034 ± 0.005 0.295 ± 0.008 0.2565 ± 0.008

9.8094 ± 1.2 24.001 ± 1 47.745 ± 2 62.638 ± 2.5 83.683 ± 2.5

0.957 0.973 0.992 0.915 0.785

solutions, having initial pH range 2–9, are buffered after reaction with HAP to their pHPZC values. The buffering characteristics of HAP are the result of acid–base reactions of the reactive surface sites [49]. Wu et al. [50] mentioned that the reactions responsible for the surface properties of HAP in aqueous solutions are: –

þ

0

≡PO þ H ↔P–OH ; þ

0

þ

≡CaðOHÞ2 ↔≡CaðOHÞ þ H ; In the lower range of initial pH value (5.01), protons in the solution were consumed by protonation of the surface ≡ P–O− and ≡ Ca–OH result in final pH value increase. The positively charged ≡ Ca–OH+ 2 and neutral ≡ P–OH sites prevail on HAP surface in acidic solutions, making surface charge of HAP in this pH region positive. On the other hand, final

Fig. 17. Arrhenius plot for different solution temperatures for cadmium removal using HAP (HAP amount = 0.25 g, pH = 7, cadmium concentration = 500 ppm, cadmium volume = 25 ml, agitation speed = 500 rpm).

pH decrease takes place in the range of higher initial pH (6.04–8) due to OH– consumption via deprotonation of surface ≡ Ca–OH+ 2 and ≡ P–OH sites. Thus, neutral ≡ Ca–OH and negatively charged ≡ P–O− species predominate in alkaline solutions, causing HAP surface to become negatively charged in solutions with high pH value [47]. As previously reported in literature, cadmium present in aqueous solution is mainly in the form of Cd2+ in the pH range of 5–8 [51]. Therefore, the sorption amount increased with the increasing pH value can be ascribed to the electrostatic forces. Also, it has been reported that precipitation of cadmium starts at pH 8.3 [51,52], that explain the removal behavior of cadmium ion at each pH9 & 11. However, at low pH value (5.01), the significant amount of cadmium sorption occurred suggesting other sorption mechanisms may exist. It is well known that the Ca2+ ions in HAP can be easily ion exchanged with many other metal ions [52]. The ionic radius of Cd2+

Fig. 18. van't Hoff plot for different solution temperatures for cadmium removal using HAP (HAP amount = 0.25 g, pH = 7, cadmium concentration = 500 ppm, cadmium volume = 25 ml, agitation speed = 500 rpm).

M.F. Elkady et al. / Journal of Non-Crystalline Solids 357 (2011) 1118–1129 Table 5 The Gibbs free energy change ΔG for different solution temperature of cadmium removal using HAP. Solution temperature

ΔG (kJ/mol)

25 °C 40 °C 60 °C 80 °C

− 8.2619±0.6 − 9.38808±0.3 − 11.07209±0.8 − 12.66164±0.6

(0.095 nm) is smaller than that of Ca2+ (0.099 nm), so we think that Ca2+ can be easily substituted in the HAP crystal lattice. Sorbent dosage is an important parameter because it determines the sorbent capacity for a given initial concentration of the solute. As observed from Fig. 7, the increment in cadmium removal efficiency as increasing the HAP amount is to be expected, for a fixed initial cadmium concentration, as increasing HAP amount provides greater sorption sites. Although the sorbet amount of Cd2+ was decreased with the increase in the HAP amount (Fig. 8), due to the adsorbent sites on HAP that remained unreacted during the sorption process [53]. The examination of the effect of initial cadmium concentration on cadmium removal efficiency using fixed amoumt of HAP (Fig. 9) illustrates that there was a reduction in immediate solute sorption, owing to the lack of available active sites required for the high initial concentration of cadmium ions [54], where the removal efficiency of cadmium decrease as cadmium concentration increased. However, the sorption capacities of HAP increased as cadmium concentration increased due to increasing the mass transfer driving force and therefore the rate at which cadmium ions pass from the bulk solution to the particle surface [47,53,55]. Agitation is an important parameter in sorption phenomena, influencing the distribution of the solute in the bulk solution and the formation of the external boundary film. The percentage cadmium removal seemed to be affected by the agitation speed for values between 0 and 500 rpm (Fig. 10), thus confirming that the influence of external diffusion on the sorption kinetic control plays a significant role. Where, the increase in the agitation speed decrease the boundary layer resistance to mass transfer in bulk solution and an increase in the kinetic energy of hydrated ions [56]. While the decrease in the percentage cadmium removal as agitation speed increased above 500 rpm (Fig. 11), may be attributed to an increase desorption tendency of cadmium ions and/or having similar speed of adsorbent particles and adsorbate ions (i.e. the formation of a more stable film around the adsorbent particles). These results were in agreement with Batzias F.A., and D.K. Sidiras [57]. The effect of temperature on the sorption process is important not only because it affects the rate and extent of sorption but also due to the fact that temperature dependence of sorption provides information about possible sorbate–sorbent interaction [58]. It was indicated from Fig. 12 that the percentage cadmium sorption was increased as temperature increased. It can be suggested that the temperature stimulates Cd2+ sorption and enables Cd2+ to diffuse further and quickly through HAP particles and the sorption process could be endothermic. Similar results have been dedicated by Xiao Wang and Byung Gil Min [59]. The study of the sorption kinetics is the main factor for designing an appropriate sorption system and quantifying the changes in sorption with time requires that an appropriate kinetic model is used. The plotting of cadmium sorption using HAP that investigated previously in Fig. 5, shows that kinetic of cadmium sorption consisted of two phases; an initial rapid phase where sorption was fast and a second slower phase where equilibrium uptake was achieved. The first phase is related to external surface sorption and sorption occurs instantaneously. The second phase is the gradual sorption stage before the metal uptake reaches equilibrium. The pseudo-first

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order considers the rate of occupation of adsorption sites to be proportional to the number of unoccupied sites. The results obtained from applying the first order kinetic model indicated that the correlation coefficients (R2) values of fitting the first-order rate model are not high for the different cadmium concentrations; furthermore, the estimated values of qe calculated from the equation differed from the experimental values (Table 1). Which show that the model is not appropriate to describe the cadmium sorption process. However, the linear regression for applying the pseudo second-order rate model (Table 2) their values near to 1 (R2 N 0.999), which indicate that the kinetics of cadmium sorption on to HAP can be described well by second-order equation. This suggests that the rate limiting step in these sorption processes may be chemisorption involving valent forces through the sharing or exchange of electrons between sorbent and sorbate, as also reported by Ho and McKay [36]. Additionally, comparing the values of qe,calculated that resulted from the intersection points of the second degree reaction kinetic curves (Table 2) with that obtained from the experimental data for the different studied cadmium concentrations. It was confirmed that cadmium sorption using the synthesized hydroxyapatite obeys the second order kinetic model. Furthermore, a widely used equation to describe the kinetics of chemisorption of gas on solids was proposed by Elovich [60]. It was declared that the Elovich model applicable for describing cadmium sorption using HAP, where the Elovich equation fit with the experimental data well with high correlation coefficients. This suggests that the sorption systems studied may be chemisorption involving valence forces through sharing or exchange of electrons between sorbent and sorbate [61]. From these results that collected from the three different kinetics models studied. It was confirmed that the ion exchange mechanism plays a significant roles in all studied cadmium sorption systems. The sorption of any metal ions from aqueous phase onto solid phase is a multi-step process involving transport of metal ions from aqueous phase to the surface of the solid particles (bulk diffusion) and then, diffusion of metal ions via the boundary layer to the surface of the solid particles (film diffusion) followed by transport of metal ions from the solid particles surface to its interior pores (pore diffusion or intraparticle diffusion), which is likely to be a slow process, therefore, it may be the rate-determining step. In addition, sorption of metal ion at an active site on the solid phase surface could also be occurred and called chemical reaction such as ion-exchange, complexation and chelation. The metal ion sorption is controlled usually by either the intraparticle (pore diffusion) or the liquid-phase mass transport rates (film diffusion) [62]. If the experiment is a batch system with rapid stirring, there is a possibility that intraparticle diffusion is the ratedetermining step [63]. If the intraparticle diffusion is involved in the sorption process, then the plot of qt vs. t1/2 would result in a linear relationship, and the intraparticle diffusion would be the controlling step if this line passed through the origin [62]. The shape of Fig. 16 confirms straight lines not passed through the origin for all studied cadmium sorption processes. The deviation of straight lines from the origin, as shown in the figure, may be because of the difference between the rate of mass transfer in the initial and final steps of the sorption process. Further, such deviation of straight line from the origin indicates that the pore diffusion is not the sole rate-controlling step [64]. Furthermore, it was observed from Table 4 that the values of thickness of the boundary layer for intraparticle diffusion (I2) were larger than that for the film diffusion (I1). Accordingly the values of the intraparticle diffusion rate kid,2 were smaller than the film diffusion rate kid,1. This gives prediction that the sorption systems are governed mainly by intraparticle diffusion. The temperature variations influence the distribution of adsorbate between solid and liquid phases. So, it was obvious from studying the sorption thermodynamic parameters at different temperatures that the calculated activation energy for the studied sorption system was equal to 8.61 kJ/mol. The magnitude of the activation energy may give

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an idea about the type of sorption. The relatively small value of the activation energy below 42 kJ/mol confirms the fact that the process of cadmium ions removal using the synthesized hydroxyapatite is diffusion controlled, i.e. the chemical step is much faster compared to mass transfer of ions inside the ion exchanger pores [65,66]. Moreover the increase in cadmium ions sorption with a rise in temperature can be explained on the basis of thermodynamic parameters such as change in enthalpy (ΔH), entropy (ΔS°) and Gibbs free energy (ΔG). The positive values of ΔH indicate that the studied sorption processes are endothermic in nature [67]. Furthermore the negative values of ΔG demonstrate the spontaneous behavior of the sorption processes [42]. The decrease in the value of ΔG with the increase of temperature shows that the reaction is more spontaneous at temperature high which indicates that the sorption processes are favored by the increase in temperature [68]. Finally, the positive values of ΔS suggest that the increased randomness at the solid-solution interface during the sorption process. The adsorbed solvent molecules which are displaced by the adsorbate species gain more translational entropy than is ions lost by adsorbate thus allowing for prevalence of randomness in the system [69]. 5. Conclusion The present studies clearly reveal that the microwave synthesized nano-hydroxyapatite produced has high crystallinity with particle diameter ranged between 40 and 70 nm. This prepared adsorbent can be fruitfully employed in treating industrial effluents containing toxic metal ions. It was indicated that HAP has rapid cadmium sorption rate and good sorption capacity. The sorption performances were considerably affected by parameters such as: solution temperature, amount of HAP, pH and initial cadmium concentration. The amount of cadmium removed by this material was increased with the increase of these parameters at a specific time. Nevertheless, there was slight dependence of cadmium sorption on agitation rate. The sorption process was confirmed to be endothermic and Cd2+ sorption increased with temperature. Sorption in our investigated system follows pseudo-second order kinetics. The rate of uptake of the Cd2+ by the HAP is very high initially, followed by a low rate. The ion exchange mechanism plays a significant role in studied cadmium sorption systems. Moreover, the sorption systems are governed mainly by intraparticle diffusion. The relatively small value of the activation energy that equal to 8.61 kJ/mol confirms that cadmium sorption process is diffusion controlled. i.e. the chemical step is much faster compared to mass transfer of ions inside the ion exchanger pores. The calculated thermodynamic parameters ΔS, ΔG and ΔH indicate that cadmium sorption process is thermodynamically favorable, spontaneous and endothermic in nature.

Nomenclature Co Initial concentration of the metal ions in solution (mg/l) C The final metal ion concentration in aqueous solution (mg/l) V Volume of the solution (ml) M Mass of the solid material (g) R The ions removal Q The amount adsorbed (mg/g) qe Amounts of ions sorbed (mg/g) at equilibrium q Amounts of ions sorbed (mg/g) at time t k1 First-order reaction rate constant (min-1) k2 Second-order reaction rate equilibrium constant (g/mg min) α The rate of chemisorption at zero coverage (mg/g min) β Related to the extent of surface coverage and activation energy for chemisorption (g/mg) kid The intra-particle diffusion rate constant I Give an idea about the thickness of the boundary layer

Rg Ea ΔH Δs A T Fe ΔG

The gas constant Activation energy (kJ/mol) Change in enthalpy Change in entropy The temperature independent factor (frequency factor) (g/mg min) Solution temperature (K) The fraction adsorbed at equilibrium Change in Gibbs free energy

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