The Sorption of Divalent Metal Ions on AlPO4

Journal of Colloid and Interface Science 252, 6–14 (2002) doi:10.1006/jcis.2002.8425

The Sorption of Divalent Metal Ions on AlPO4 A. Naeem, S. Mustafa,1 N. Rehana, B. Dilara, and S. Murtaza National Centre for Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan Received July 17, 2001; accepted April 8, 2002; published online July 2, 2002

Adsorption/ion exchange is one of the versatile techniques, compared to others (2, 3). Adsorption is by definition a general term describing the attachment of species from a solution to its coexisting solid surfaces. Three types of processes are identified for the sorption phenomena: (a) surface adsorption, which is limited to the accumulation of sorbate onto the external surface, (b) absorption, ion exchange, or diffusion into the solid, and (c) precipitation or co-precipitation. These processes often act together, and the dominance of one specific process is often hard to be distinguished without careful chemical measurements and advanced analytical techniques. The sorption properties of metal phosphates have been studied in great detail, and there are several overviews of this subject in the literature (4–6). One of the most studied metal(IV) phosphate is ZrP, and the process responsible for the uptake of the metal cations is believed to be ion exchange; i.e., metal cations like Na+ , K+ , Li+ , Zn2+ , Ni2+ , Co2+ , and Cu2+ ions from the aqueous solution are exchanged for protons from the solid. Recently, a detailed study has investigated metal(II) phosphates like calcium phosphates (4, 7, 8) and barium hydroxyapatite (9) for this purpose. As far as the metal(III) phosphates are concerned, very little is known about their sorption properties toward the metal cations, although they are also highly insoluble materials playing an important role in the corrosion of metals and alloys and in metal fixation in soils and sediments. It was shown by us (10–14) recently that metal(III) phosphates like FePO4 , AlPO4 , and CrPO4 behave as weak acid cation exchangers and possess appreciable exchange capacities for alkali, alkaline earth, and divalent transition metal cations. The purpose of the present study is to investigate in detail the sorption properties of AlPO4 toward Pb2+ , Cu2+ , Ni2+ , and Cd2+ ions under different experimental conditions of temperature, concentration, and pH of the system.

The sorption of Cu2+ , Pb2+ , Ni2+ , and Cd2+ ions on the aluminum(III) phosphate was observed to increase with increases in the concentration, temperature, and pH of the system. The apparent dissociation (pKa ), binding (pKb ) and exchange (pKex ) constants of aluminum(III) phosphate were evaluated and found to be dependent upon the temperature and nature of the metal cations. The values of the dissociation constants (pKa ) followed the order Pb2+ < Cu2+ < Cd2+ < Ni2+ < K+ , opposite to the values of the binding constants and selectivity shown by the solid. Ion exchange between protons from the surface and metal cations from aqueous solution was found to be responsible for the metal ions sorption by AlPO4 . The values of the dissociation (pKa ) and exchange (pKex ) constants were used to estimate the corresponding apparent thermodynamic parameters (H◦(diss) , S◦(diss) and H◦(exch) , S◦(exch) ). The values of H◦(diss) and H◦(exch) were found to be positive, which showed that both the dissociation and exchange processes were endothermic in nature. The S◦(diss) and S◦(exch) were also found to be positive, which showed that both the deprotonation and exchange processes on the AlPO4 were entropy driven and spontaneous. C 2002 Elsevier Science (USA) Key Words: ion exchange; sorption; AlPO4 ; metal phosphates; divalent metal cations.

INTRODUCTION

The presence of metals in potable waters and industrial effluents above certain limits have devastating effect on both human and aquatic life. The toxic elements are discharged by the industries into the water, air, and soil from where they get into the human food chain. The entrance of these metal ions into the blood causes (1, 2) anemia, kidney dysfunctions, brain damage, bone disease, skin and lung cancer, etc. It is, therefore, necessary to develop schemes for the removal of heavy metals from the wastewaters. Various processes for the purification of water are in progress and efforts are also being made to improve and apply already existing and well-established technologies to solve these problems. However, several problems still exist, both economic and technical, concerning the removal of metal ions from the wastewaters.

EXPERIMENTAL

Materials. All solutions were made with ACS reagent grade chemicals and doubly distilled deionized water. All the experiments were conducted in the presence of a N2 atmosphere to bubble out the CO2 gas. Almost all the potentiometric titrations and sorption experiments were conducted in duplicate to ensure the validity of the results. Only those results where on the

1 To whom correspondence should be addressed. E-mail: naeeem64@yahoo. com.

0021-9797/02 $35.00

 C 2002 Elsevier Science (USA)

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THE SORPTION OF DIVALENT METAL IONS ON AlPO4

average percent error between the duplicates was less than ±2 were taken into account. Preparation and Characterization of Aluminum(III) Phosphate AlPO4 was prepared by the reaction of Na3 PO4 with the solution of Al(NO3 )3 , according to the reaction Na3 PO4 + Al(NO3 )3 → AlPO4 + 3NaNO3 .

[1]

The AlPO4 was filtered, dried, and subjected to X-ray diffraction, FTIR spectroscopy, surface area, wet chemical analysis, scanning electron microscopy (SEM), and electron microprobe analysis (EPM). An air-dried sample of AlPO4 was subjected to the X-ray analyzer. The X-ray patterns of the powder sample were determined using a JEOL X-ray diffractometer, model JDX-7E with Mn-filtered CuK α radiation. The infrared spectra of the AlPO4 was recorded on a Perkin–Elmer 16PC FTIR spectrophotometer. The KBr wafers were prepared by mixing KBr crystals with the sample, the product of which was dried at 373 K for 24 h, ground to fine powder, again heated at 373 K for 1 h, and finally pressed into KBr wafers in vacuum. Scan time was 3 min for the spectrum. The SEM and Al/P ratio in the sample surface were determined with an electron probe X-ray micro analyzer, model JXA 733 (Jeol CO, Japan). The instrument with its own computer program system was standardized with British Standard fluorapatite with a concentration of CaO = 55.35% and P2 O5 = 40.02%. The sample was viewed through the microscope as well as on the video screen. The surface area was measured by the nitrogen adsorption method (15) using a surface area and pore size Analyzer, model ST-O3. The Murphy and Riley (16) method was employed to determine the concentration of phosphate in the solutions at a wavelength of 882 nm by using a spectronic 20D spectrophotometer model (Bauch and Lomb, USA). The concentrations of the metal ions in the filtrates were determined with the help of a Perkin–Elmer model 3100 atomic absorption spectrometer. Dissolution of AlPO4 In this method, 0.2 g sample and 30 ml of 0.1 M KCl were mixed in different flasks. The initial pH of each suspension was adjusted in the range of 2 to 11 with the help of 0.1 M HCl or 0.1 M KOH. The suspensions were placed in a thermostat at 303 K for 24 h. The suspensions were then centrifuged and the concentration of phosphate in the supernatants was measured colorometrically by using a spectronic 20D spectrophotometer by the method reported in the literature (16). Potentiometric Titration of Aluminum(III) Phosphate Sixty milliliter of divalent metal ions of different concentrations, 20–250 ppm, along with background electrolyte (0.1 M

7

KCl) containing 0.2 g solid sample were placed in a thermostated, double-walled Pyrex cell. The suspension was initially equilibrated for 30 min at pH 3 with constant stirring by means of a magnetic stirrer to attain the desired temperature. After equilibration, the pH of the suspension was measured and adjusted again to the initial value 3 with a pH meter, model Orion SA 520. The standardized solution of metal hydroxide (KOH) was added by means of a microburette in steps of 0.2 ml. After each addition of base, the suspension pH was equilibrated for 2 min with constant stirring, at the end of which the pH changes were observed to be less than 0.01 units/min. The blank titrations were also performed with similar solutions in the absence of divalent metal cations. Sorption of Divalent Metal Cations on AlPO4 To determine the sorption of divalent metal ions on AlPO4 , 0.2 g sample was placed in 40 ml of metal ions solution. The initial pH’s of the metal ions solutions were adjusted in the range of 4 to 7 with the help of KOH/HCl before and again after the addition of the sample. The suspensions were filtered after equilibration for 24 h at different temperatures with an accuracy of ±1◦ C. Preliminary experiments revealed that 1 h was sufficient for the system to reach equilibrium; yet the system was left intact for 24 h to ensure that the system had attained the status of true equilibrium. The uptake of the metal cation along with the changes in the pH determined after 1 and 24 h are almost coincident with each other. The concentrations of the metal ions in the filtrates were determined with the help of the Perkin– Elmer model 3100 atomic absorption spectrometer. Blank runs were performed with similar solutions in the absence of an exchanger. In all the sorption experiments, precautions were taken to keep the initial concentration of the divalent metal cations below the solubility limit of the metal hydroxide [M(OH)(s)] at the desired pH. RESULTS AND DISCUSSION

Characterization of AlPO4 The XRD showed the AlPO4 to be amorphous in nature. The surface area of the sample determined by the nitrogen adsorption method was found to be 60 ± 1 m2 /g. Both the wet chemical analysis and electron probe microanalysis gave an Al/P molar ratio of 1.10 and 1.06, respectively, which are close to the ideal ratio of 1.00. The SEM photograph given in Fig. 1a shows that the sample consists of large particles with an irregular shape and size. The dissolution of AlPO4 was undertaken in order to examine the hydrolytic stability of the exchanger. The plot of phosphate released from AlPO4 as a function of pH is given in Fig. 1b. It can be observed from the figure that the dissolution of AlPO4 is at minimum in the pH range of 3.50 to 7.80, which is similar to the results reported elsewhere (3). The FTIR spectrum of AlPO4 shown in Fig. 1c indicates the sample to be hydrated metal phosphate. Phosphate is evident from the P–O stretching and bending vibrations at 1040 and

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NAEEM ET AL.

FIG. 1.

(a) Scanning electron micrograph (SEM) of AlPO4 , magnification (3600×). (b) pH effect on AlPO4 dissolution at 303 K. (c) FTIR spectrum of AlPO4 .

THE SORPTION OF DIVALENT METAL IONS ON AlPO4

9

FIG. 2. Potentiometric titration curves of AlPO4 in the presence of (a) Ni2+ at 303 K, (b) 100 ppm Pb2+ and (c) 100 ppm Cd2+ at different temperatures, and (d) 100 ppm M 2+ ions at 303 K.

550 cm−1 , respectively. The spectrum also shows a weak absorption band at 1630 cm−1 and a broad band in between 3000 and 3500 cm−1 , which can be assigned to the OH bending and stretching, respectively. The vibration bands of the FTIR spectrum of AlPO4 in the present study were observed to be similar to those reported in the literature (13, 15). Potentiometric Titration Studies of the AlPO4 The potentiometric titration curves of AlPO4 in the presence of Cd2+ , Pb2+ , Cu2+ , Ni2+ , and K+ ions are shown in Figs. 2a– 2d and Fig. 3a. These figures show an increase in pH from the very beginning, indicating that the solid is of weak monobasic acid type. Further, the curves in Fig. 2d show a shift toward the lower pH values in the presence of divalent metal cations, indicating that the exchange of divalent metal ions is accompanied by the release of protons from the solid into the aqueous solution. This increase in shift toward low pH values with the increase in concentration of the divalent metal ions (Fig. 2a) reveals that more of the H+ ions are released by the metal ions from the exchanger into the aqueous solutions, as suggested by the

reaction, z+ ⇔ Rn M z−n + nH+ n RH + Maq aq ,

[2]

where R stands for solid. The temperature has also positive effect on the pH shift; i.e., the shift of the titration curves toward the low pH values tends to increase with increase in the temperature of the system, indicating that the exchange of protons from the solid by the divalent and alkali metal cations is favored at higher temperatures (Figs. 2b, 2c and Fig. 3a). A similar increase in the shift of the titration curves toward the low pH values with rise in temperature was reported in the literature (6, 15). As suggested by Anderson and Rubin (17) the pH shifts in the curves (Fig. 2d) may be taken as a measure of the selectivity of the exchanger. The selectivity of AlPO4 is, thus, found to be in the order Pb2+ > Cu2+ > Ni2+ > Cd2+ , which remains the same at all the temperatures. This sequence shows that the adsorption affinity of the metal ions toward the solid surface is generally related to their first hydrolysis constant (pK h1 ); i.e., the

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NAEEM ET AL.

other metal ions. Further, the values of pK a generally decrease with increasing temperature in the presence of both alkali and divalent metal ions, indicating a greater tendency of protons to dissociate from the solid. The present values of pK a are also in agreement with the values reported in the literature (20, 21) for the metal(III) phosphates. Sorption of Metal Ions on the AlPO4

FIG. 3. (a) Potentiometric titration curves of AlPO4 in the presence of 0.01 M K+ , and (b) plots of pH–log(α/1 − α) vs α in 0.01 M K+ at different temperatures.

most readily hydrolyzable metal ions in solution are considered to have greater affinity for adsorbent surfaces (18). Similar effects of the pH of hydrolysis were also observed in the literature (15, 19).

The sorption isotherms for Cu2+ , Pb2+ , Ni2+ , and Cd2+ ions sorption on AlPO4 are presented in Figs. 4a, 4o, and 4b. The sorption of metal ions is observed to increase with the increase in temperature and concentration of metal ions in solution. The increase in the sorption with temperature may be correlated with the increase in surface dissociation of aluminum phosphate with the rise in temperature. It is interesting to note that there is almost complete removal of Pb2+ from the aqueous solution at pH 4 (Fig. 4o), which shows that AlPO4 has a special affinity toward Pb2+ similar to that of the metal(II) phosphates (9, 22). The special affinity of AlPO4 toward Pb2+ can be explained on account of the lowest pK a values of the solid in its presence. That the values of pK a in the presence of Pb2+ (Table 1) are lower than those of the other metal cations shows that more of the hydrogen ions are released into the solution, leading to the increased sorption of the Pb2+ ions. The sorption of Cu2+ , Cd2+ , and Ni2+ on AlPO4 was also studied as a function of pH. The representative sorption isotherms at different pH’s for Cu2+ on AlPO4 are presented in Fig. 4b, which demonstrate that the sorption of the metal cations increases as the initial pH of the suspension is increased. A similar increase in the sorption with pH was also observed during the present investigations in the cases of Ni2+ and Cd2+ . Further, with the increase in pH the shape of the sorption isotherms changes from nonfavorable to favorable, which is quite obvious in the case of Cu2+ (Fig. 4b). It is also interesting to note the complete removal

Determination of the Dissociation Constants ( pK a ) The dissociation constants (pK a ) for AlPO4 were determined (17) by employing the Simpson’s rule to the area under the curves of the plots of pH–log(α/1 − α) vs α (Fig. 3b) from the potentiometric titration data via the equation pK a = pH − log(α/1 − α),

[3]

where α represents the degree of dissociation. To determine α, the number of moles of base added at a given pH was divided by the maximum number of moles of base consumed to reach the constant pH values of each curve in the presence of the metal cations. The pK a values reported in Table 1 are in the same range as those reported elsewhere (1) for other weak acid cation exchangers. The data in the table also reveal that the pK a values depend upon the nature of the metal cation present in the aqueous phase and follow the trend Pb2+ < Cu2+ < Cd2+ < Ni2+ < K+ , which demonstrates a greater dissociation of protons from the exchanger in the presence of Pb2+ than in the presence of the

TABLE 1 Values of the Dissociation (pK a ), Binding (pK b ), and Exchange (pK ex ) Constants of Aluminum(III) Phosphate Sample

Temp. (K)

pK a (diss)

pK b (bind)

pK ex (exch)

Cu2+

303 313 323 303 313 323 303 313 323 303 313 323 303 313 323

7.76 6.85 6.55 7.43 7.01 6.65 8.55 8.21 7.71 6.79 6.58 6.27 9.33 8.90 8.46

−3.58 −3.61 −3.51 −4.40 −4.42 −4.49 −4.52 −4.59 −4.56 — — — — — —

4.18 3.24 3.04 2.96 2.59 2.06 4.03 3.62 3.15 — — — — — —

Cd2+

Ni2+

Pb2+ K+

THE SORPTION OF DIVALENT METAL IONS ON AlPO4

11

divalent metal ions and protons for the surface. However, at and above the initial pH 5, a decrease in the equilibrium pH is observed, which further decreases with the increase in sorption of metal cations. The pH changes are thus in agreement with the change in the shape of the isotherms shown in Fig. 4b. The drop in equilibrium pH suggests that H+ ions are liberated from the solid surface into the aqueous phase as a result of the exchange with the metal cation. As such the mechanism of metal ion sorption is presumed to occur at the surface of AlPO4 as follows, n P–OH + M z+ ⇔ (P–O)n M z−n + nH+ ,

[4]

where P stands for the exchanger. The equilibrium constant for the reaction [4] can be written as K = [(P–O)n M z−n ][H+ ]n /[P–OH]n [M z+ ].

[5]

∼ constant due to the low adsorption denTaking [P–OH] = sities and [(P–O)n M z−n ]/[M z+ ] = K d , the expression for equilibrium constant transforms to the well-known Kurbatov equation (17), which is given in the form Log K d = npHeq + C,

FIG. 4. Sorption isotherms of AlPO4 in the presence of (a) Ni2+ and (o) Pb2+ at different temperatures at pH 4, and (b) Cu2+ at different pH’s at 303 K.

[6]

where K d (L · g−1 ) represents the distribution coefficient, C is a constant, and n is the slope of the straight line giving an indication of the H+ /M z+ stiochiometry of the exchange reaction. The plots of log K d vs equilibrium pH for the divalent ion exchange on aluminum(III) phosphate are shown in Fig. 5, which show that the equation is applicable to the data with R 2 values >0.99. The values of n determined from the slopes of the straight lines at all the temperatures are close to 1 ± 0.15, showing that the temperature has little effect on the nature of the

of Pb2+ from the solution at pH > 4. The increase in the uptake of metal cations with the increase in pH also indicates that the exchanger is a weak acid type. A similar increase in sorption with pH has been reported by a number of authors (3, 9, 23). The changes in pH accompanying the sorption of metal cations are given in the Table 2. The data in the table show an increase in the equilibrium pH of the suspension when the initial pH is lower than 5, indicating a competition between the TABLE 2 Sorption of Cu2+ on AlPO4 as a Function of pH at 303 K pHi

pHeq

Cu2+ sorbed (X ) × 105 (mol/g)

4.02 5.00 6.03 7.06

4.30 4.65 5.01 5.27

2.84 4.36 5.78 6.42

FIG. 5. 303 K.

Plots of log K d vs pH for the sorption of Cu2+ ions on AlPO4 at

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NAEEM ET AL.

metal ions binding with the surface of the solid. These values of n show a one-to-one correspondence between cation loading and the hydrogen ion release from the solid, indicating that M 2+ is adsorbed in its hydrolyzed (MOH+ ) form. These findings are in accordance with the results reported elsewhere (13, 17, 18) that the singly charged MOH+ are more easily exchanged than the doubly charged M 2+ ions. The dependence of the extent of adsorption of different cations on their first hydrolysis constant discussed earlier also points toward the uptake of the cations in their hydrolyzed form, i.e., MOH+ . The mechanism of metal ion sorption on aluminum(III) phosphate, thus, may be described by the reactions M 2+ + H2 O ⇔ MOH+ + H+

[7]

P–OH + MOH+ ⇔ P–O–MOH + H+ .

[8]

A similar mechanism for the sorption of metal ions on the surfaces of titanium and tin antimonates, metal(IV) phosphates, and oxides/hydroxides was also proposed elsewhere (5, 18, 24). The Langmuir equation, which gives an indication of the sorption maxima and binding energy constants, has been used extensively for adsorption and ion exchange isotherms (9, 15, 25). Typical Langmuir isotherms for Cd2+ and Ni2+ are presented in Figs. 6a and 6b. The values of the correlation coefficients (R 2 ) are found to be greater than 0.99 in all the cases, which show that the present experimental data are best fitted to the linear form of the Langmuir equation, given in the form Ce / X = 1/(K b X m ) + Ce / X m ,

[9]

where Ce is the equilibrium concentration of metal ions in the solution, X is the amount of metal ions sorbed per unit weight of the exchanger, K b is the binding constant where pK b = −log K b , and X m is the maximum sorption capacity of the exchanger. The values of the binding constants (K b ) and sorption maxima (X m ) computed from the intercepts and slopes of the straight lines are compiled in Tables 1 and 3, respectively. The K b values for the different metal cations are found to be in the order Ni2+ > Cd2+ > Cu2+ , which indicates the greater stability of Ni2+ complexes with the solid surface while the converse is true for the complexation of Cu2+ with the AlPO4 surface. Comparison of the X m values with the values of the binding and dissociation constants of the solid reveales that the X m valTABLE 3 Langmuir Parameter Xm (mol · g−1 ) for the Exchange of Metal Ions on Aluminum(III) Phosphate Temp. (K)

Cu2+ (X m ) × 105

Cd2+ (X m ) × 105

Ni2+ (X m ) × 105

303 313 323

10.06 12.84 20.12

7.26 — 10.86

8.01 9.50 11.67

FIG. 6. at pH 4.

Langmuir plots for (a) Ni2+ and (b) Cd2+ ion sorption on AlPO4

ues are directly related to the values of the dissociation constants of the solid, showing that the deprotonation plays a major role in the overall uptake of the metal cations by AlPO4 . The plots of the dissociation constants (pK a ) vs the sorption maxima (X m ) of AlPO4 at different temperatures are given in Fig. 7a. One can observe from Fig. 7a that the sorption maxima (X m ) increases with the decrease in the pK a values, i.e., with the increase in the proton dissociation from the P–OH groups of the solid. The results presented here are also consistent with those reported recently by Bortun et al. (26) for titanium phosphate. Further, both the sorption maxima (X m ) and the corresponding pH shifts in the titration curves are showing that the exchange of metal cations increases with the increase in temperature. The sorption studies, thus, also augment the conclusions drawn earlier from the potentiometric titration data of aluminum(III) phosphate. The values of both the sorption maxima (X m ) and the binding constant (K b ) are compatible with the values cited in the literature (13, 15).

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THE SORPTION OF DIVALENT METAL IONS ON AlPO4

Using reactions [10] and [12], one can obtain the general mechanism for the overall exchange reaction as RH + MOH+ ⇔ R MOH + H+

[14]

K ex = [R MOH][H+ ]/[RH][MOH+ ],

[15]

where K ex represents the ion exchange constant. On comparison of Eqs. [11], [13], and [15], the following relationship between these constants can be obtained pK ex = pK a + pK b .

[16]

The values of the dissociation (pK a ) and binding (pK b ) constants were obtained from Eqs. [3] and [9], respectively. Thus, knowing the values of the pK a and pK b , one can easily compute the values of the overall exchange constant (pK ex ) according to relation [16], which are given in Table 1. The standard enthalpy (H ◦ ) and entropy (S ◦ ) changes for the surface deprotonation and metal exchange processes are computed by plotting pK a and pK ex vs T −1 (Figs. 7b, 7c), according to pK =

FIG. 7. AlPO4 .

Plots of (a) pK a vs X m , (b) pK a vs T −1 , and (c) pK ex vs T −1 for

Thermodynamic Parameters of the Ion Exchange The values of the apparent dissociation (pK a ), binding (pK b ), and exchange constants (pK ex ) can be determined from the following expressions. Generally, the dissociation of the exchanger RH is given by RH ⇔ R − + H+ .

[10]

Applying the law of mass action to reaction [10], the apparent dissociation constant can be written as K a = [R − ][H+ ]/[RH],

[11]

where K a refers to dissociation constant. Further, the binding mechanism may be represented by −

+

R + MOH ⇔ R MOH −

+

K b = [R MOH]/[R ][MOH ], where K b denotes the binding constant.

[12] [13]

−S ◦ H ◦ + . 2.303R 2.303RT

[17]

The values of the apparent thermodynamic parameters are ◦ listed in Table 4. As can be seen both the enthalpy H(diss) and ◦ entropy S(diss) of dissociation are found to be positive. The ◦ point to an endothermic nature of positive values of H(diss) the dissociation process, as energy is required to dissociate the ◦ protons from the exchanger. The observed values of H(diss) for the deprotonation are reasonable in magnitude since AlPO4 is considered to be weakly acidic and is therefore reluctant to dissociate the protons from the P–OH groups to the incoming di◦ valent metal ions. Further, the overall heat of exchange H(exch) are also found to be positive, suggesting that large amount of heat is consumed to transfer the metal ions from aqueous solution into the solid phase. The endothermic nature of the process can be accounted for by the partial dehydration of the metal cations before their sorption on the AlPO4 . Both the surface dissociation and ion exchange mechanisms ◦ ◦ are also supported by the positive values of S(diss) and S(exch) , TABLE 4 Values of Standard Enthalpy and Entropy Changes of the Dissociation and Exchange of Metal Ions with Aluminum(III) Phosphate H ◦ (kJ mol−1 )

S ◦ (JK−1 mol−1 )

Metal ions

◦ H(diss)

◦ H(exch)

◦ S(diss)

◦(exch)

Cd2+ Cu2+ Ni2+ Pb2+

82.4 113.9 78.5 48.6

32.5 46.5 35.8 —

129.4 229.2 94.8 30.0

83.1 119.1 84.6 —

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NAEEM ET AL.

respectively. As was suggested by Clearfield and Kalnins (27) and Nunes and Airoldi (28) the transition metal ions would have to divest themselves of most or part of their water of hydration before they could enter the exchanger. Such a release of water from the divalent cations would then result in positive values of ◦ S(exch) . ◦ ◦ and S(exch) indicate Thus, the positive values of both S(diss) the increased disorder in the system and can be correlated with changes in the hydration of the exchanging cations, i.e., H+ and ◦ divalent metal ions. Moreover, the values of S(exch) in the pres2+ ence of various metal cations follow the trend Cu > Ni2+ > Cd2+ , which is parallel to the selectivity shown by AlPO4 toward ◦ ◦ these metal ions. The positive values of S(diss) and S(exch) indicate that both the deprotonation and exchange processes on AlPO4 are entropy driven and spontaneous in nature (27–30). CONCLUSIONS

The sorption of metal ions is found to increase significantly with the increase in pH, temperature, and concentration of the metal ions. The sorption affinity of the exchanger AlPO4 is observed to be in the order Pb2+ > Cu2+ > Ni2+ > Cd2+ , similar to that reported for its well-known counterparts, metal(IV) phosphates. The mechanism of sorption is found to be an exchange between the protons from the solid and metal ions from the solution. The enthalpy H ◦ and entropy S ◦ of both the dissociation and exchange reactions are found to be positive for AlPO4 , which shows the deprotonation and the ion exchange mechanisms to be endothermic and entropy driven in nature. Finally, it can be concluded that in the neutral or slightly alkaline solution, the aluminum(III) phosphate is reliable exchange material and can act as an efficient sorbent for metal cations without suffering any extensive hydrolysis. The exchanger is particularly selective toward the Pb2+ ions, which are well-known pollutants in the environment of our planet. ACKNOWLEDGMENTS The valuable comments by the three anonymous reviewers are highly appreciated.

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