Sorption Mechanism of Some Divalent Metal Ions Onto Low-Cost

External-diffusion and film-diffusion models were tested to evaluate ..... cations and Ca2+ released by the adsorbent using atomic absorption spectrop...
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Ind. Eng. Chem. Res. 2004, 43, 1512-1522

SEPARATIONS Sorption Mechanism of Some Divalent Metal Ions Onto Low-Cost Mineral Adsorbent Murari Prasad*,† and Sona Saxena‡ Regional Research Laboratory (CSIR), Hoshangabad Road, Habibganj Naka, Bhopal 462 026, India, and BARC, Trombay, Mumbai 400 085, India

This paper investigates the underlying mechanism of uptake from aqueous solution of divalent metal cations (Pb2+, Cu2+, and Zn2+) by a low-cost mineral adsorbent. Batch adsorption studies were carried out for the concentration ranges of 24.1-2410 µmol/L for lead, 78.65-7865 µmol/L for copper, and 76.45-7645 µmol/L for zinc solutions under natural conditions. Two simple kinetic models, that is, pseudo-first-order and pseudo-second-order models, were tested to investigate the adsorption mechanism. All of the parameters of these models were calculated and are discussed. Rate constants were found to be nearly constant at all metal concentrations for the first-order model, whereas they gradually decreased with increasing metal concentration in the order Pb2+ > Cu2+ > Zn2+ for the second-order model. The sorption kinetics appears to be mainly controlled by liquid-film diffusion. External-diffusion and film-diffusion models were tested to evaluate mass-transfer coefficients and film-transfer constants at different initial concentrations of solute (metal cation). Both mass-transfer and film-transfer constants were noticed to be affected by the initial metal concentration. They gradually decreased with increasing initial concentration of metal cation in the order Pb2+ > Cu2+ > Zn2+. The film-diffusion model was found not to be applicable for the adsorption of lower concentrations of divalent cations. The equilibrium data were described to a lesser extent by Freundlich model, and the Langmuir model seemed to be more appropriate, giving maximum fixation capacities for Pb2+, Cu2+, and Zn2+ of 90.9, 270.2, and 250.0 µmol/g, respectively. The crystallographic dimensions of the treated mineral adsorbent samples were measured by X-ray diffraction (XRD), which revealed appreciable expansion of the crystal size as a result of the incorporation of divalent cations in the place of exchangeable cations of the adsorbent. The affinity order for the adsorption of cations by the mineral adsorbent is validated by ionic radius theory. Cation exchange along with complexation were found to be the most probable mechanisms for the sorption of divalent cations. An attempt was made to quantify the ion-exchange and complexation mechanisms. Introduction Contamination of water with heavy metals from the aqueous waste streams of industries such as metal plating, mining operations, tanneries, etc., remains a serious environmental and public problem. Increased concern about the environment and tighter national and international regulations on water pollution and the discharge of heavy metals make it necessary to develop efficient and cost-effective technologies for their removal. The main techniques that have been used to reduce the heavy-metal content in wastewaters include (i) chemical precipitation in the form of insoluble species, (ii) ion exchange, (iii) foam flotation, (iv) reverse osmosis, (v) solvent extraction, (vi) electrolytic methods,1 and (vii) adsorption onto activated carbon.2-6 The conventionally followed chemical methods for heavy-metal removal involving precipitation/filtration lead to the generation of hazardous solids or wet sludge, posing disposal * To whom correspondence should be addressed. E-mail: [email protected]. † Regional Research Laboratory (C.S.I.R.). ‡ BARC.

problems, and increased levels of total dissolved solids (TDS) in the treated water. Methods ii-vi have been found to be limited, because they often involve high operational costs, especially for the treatment of highvolume/high-concentration effluents, and might also be insufficient to meet strict regulatory requirements. Sorption onto solid substrate materials is considered as the most suitable process for the removal of heavy-metal ions from solutions at low and high concentrations. To date, the only sorbent typically used for low concentrations of heavy-metal ions is active carbon, which separates the ions from the wastewater by adsorbing relatively low amounts of metal ions and is very unselective to the type of metal under consideration. The severe limitation of process vii, however, lies in the high cost of the substrate material and the difficulty of its regeneration/recycling. Therefore, numerous approaches have been studied for the development of cheaper metal sorbents, such as fly ash,7 peat,8,9 microbial biomass,10 clays and related minerals,11-13 phosphate minerals,14-16 and biosorbents.17 However, the most practical type of heavy-metal adsorbent would need to have a strong affinity toward the target metals, essentially irrevers-

10.1021/ie030152d CCC: $27.50 © 2004 American Chemical Society Published on Web 02/14/2004

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1513

ibly binding them under natural ambient conditions. It is also desirable that the metal ion adsorbent should have the ability to desorb the ions from its structure so that the adsorbent could be regenerated for cyclic use. A majority of the reserves of low-grade ( Zn2+. The mean values of the mass-transfer coefficients obtained for lead, copper,

Co (mg/L)

Pb

Cu

Zn

50.0 100.0 500.0

15.09 5.72 3.52

5.45 4.04 1.03

3.638 3.211 0.665

and zinc solutions (10 mg/L) were 0.52, 0.21, and 0.125 m/s, respectively. The film-diffusion model (eq xiii) was used to evaluate the fim-transfer constant at different initial concentrations of solute (metal cation). The film-transfer constants were found to be affected by the initial concentration as shown in Table 3. They gradually decreased as the initial concentration of metal cation increased and fell in the order Pb2+ > Cu2+ > Zn2+. This was coincident with the fact that the equlibrium stage was reached more slowly at increasing cation concentration. The sorptivity of the mineral adsorbent is constant. Thus, the cation removal rate on the surface of the adsorbent becomes relatively restricted at increasing cation concentration, which causes a decrease of the film-transfer constant. It is noteworthy that this film-diffusion model was not found to be applicable for adsorption at lower concentrations (10 mg/L) of divalent cations. Thus, the concept of a film-transfer constant does not appear to be valid for lower concentrations of cations. Metal Adsorption Isotherms. The distribution of divalent cations between the liquid phase and the adsorbent is a measure of the equilibrium condition in the adsorption process and can generally be expressed in terms of the two most popular isotherm theories, viz., the Freundlich42 and Langmuir43 isotherms. Equilibrium sorption studies were performed to determine the maximum metal adsorption capacities of the adsorbent in the concentration ranges of 24.1-2410 µmol/L for lead, 78.65-7865 µmol/L for copper, and 76.45-7645 µmol/L for zinc. The adsorption data were analyzed by a regression analysis to fit the Freundlich and Langmuir isotherm models. Both of these isotherms plots (Figures 4 and 5, respectively) represented and were in good agreement with the experimental data, suggesting that the sorbed metal ions formed a monolayer on the adsorbent surface. The coefficients of these two models were computed using linear least-squares fittings, as reported in Table 4. The Langmuir parameters qe and

1518 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 Table 5. pH’s of Lead Ion Solutions with Francolite Mineral Pb2+ concentration (µmol/L)

pH1a

pH2b

Ca2+ concentration (µmol/L)

24.1 48.3 241.5 483.0

6.45 5.82 5.20 4.10

7.17 6.63 5.89 5.34

199.44 274.23 750.39 1495.80

a pH refers to the initial pH of the test solution without 1 sorption. b pH2 refers to the final pH of the test solution after sorption.

subsequent precipitation (complexation); ion exchange; and absorption have been reported for the separation of aqueous heavy metals, including Pb, Cu, Zn, and Cd, among others, by many investigators. “Sorption” is a generalized term involving these processes of species attachment from a solution to a coexisting solid surface. These processes of sorption often act together, and the dominance of one specific process is difficult to distinguish. Processes such as adsorbent dissolution; cation exchange; and metal complexation on an adsorbent (apatite/rock phosphate) surface, followed by precipitation of a new metal phase, have been proposed to describe the uptake of different metals from aqueous solution by synthetic hydroxyapatite (HA) and rock phosphate.44-47 Here, it is proposed that the dissolution of phosphate rocks and precipitation of a carbonated fluoropyromorphite-like mineral is the primary mechanism for Pb removal by rock phosphate. Assuming the presence of equal amounts of PO43-, CO32-, F-, and OH- as an example, this mechanism can be expressed by following equations

Ca10(PO4)3(CO3)3FOH(c) + 6H+ 9 8 dissolution 10Ca2+ + 3H2PO4- + 3CO32- + F- + OH- (xvi) 8 10Pb2+ + 3H2PO4- + F- + OH- 9 precipitation PB10(PO4)3(CO3)3FOH(c) + 6H+ (xvii)

Figure 5. Langmuir isotherms for the adsorption of Pb2+, Cu2+, and Zn2+. Table 4. Langmuir and Freundlich Parameters for Metal Adsorption onto the Mineral Adsorbent metal ion Pb2+ Cu2+ Zn2+

Freundlich model

Langmuir model

R2

qe (µmol/g) b (L/µmol)

K

1/n

42.26 0.461 0.971 21.47 0.445 0.991 12.64 0.427 0.981

90.90 270.27 250.0

0.363 26.83 47.25

R2 1.00 0.937 0.962

b and the Freundlich parameters K and 1/n were computed with the respective determination of correlation coefficients, R2. On comparison, it appears that the equilibrium data were less well described by the Freundlich model and that the Langmuir model seems to be more appropriate. As expected with the kinetic studies, the maximum fixation capacity (qm) values obtained were 90.9 µmol/g for Pb2+, 270.2 µmol/g for Cu2+, and 250.0 µmol/g for Zn2+. Mechanism of Adsorption. A number of governing mechanisms such as surface adsorption, dissolution, and

The above mechanism can be justified by evidence from X-ray diffraction studies performed on the adsorbed solid phase, as well as analysis of the solution for Ca2+ and observation of pH changes occurring during Pb2+ uptake. To understand the appropriate adsorption mechanism of metal cations, the study of pH behavior is important, and as such, the solution pH was an important parameter in this study. The present study was carried out under natural conditions, i.e., no effort was made to control the solution pH during the experiments and no ionic strength was imposed. The initial pH (pH1) of the metal solutions and final pH (pH2) of the filtrate at adsorption equilibrium were measured for each metal concentration. The pH values for all test solutions, as reported in Tables 5-7, decreased with increasing metal concentration and increased gradually after equilibration with the adsorbent. The decrease of the pH as the initial metal concentration increases might be due to the release of H+ ions and might indicate an adsorption mechanism by ion exchange. If cation substitution were responsible for heavy-metal removal, then the decrease in pH with increasing metal concentration would result in less heavy-metal adsorption because more rock phosphate would dissolve and less rock phosphate would

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1519 Table 6. pH’s of Copper Ion Solutions with Francolite Mineral Cu2+ concentration (µmol/L)

pH1a

pH2b

Ca2+ concentration (µmol/L)

78.65 157.30 786.50 1573.00

7.70 6.73 6.47 5.91

7.58 7.35 6.96 6.45

336.55 387.66 1530.70 3265.83

a pH refers to the initial pH of the test solution without 1 sorption. b pH2 refers to the final pH of the test solution after sorption.

Table 7. pH’s of Zinc Ion Solutions with Francolite Mineral. Zn2+ concentration (µmol/L)

pH1a

pH2b

Ca2+ concentration (µmol/L)

76.4 152.8 764.0 1528.0

7.60 6.60 6.17 5.66

7.42 7.29 6.42 6.20

398.88 498.66 2096.61 3739.50

a pH refers to the initial pH of the test solution without 1 sorption. b pH2 refers to the final pH of the test solution after sorption.

be available for cation substitution at lower pH. Similar results were also reported by Takuchi and Arabi.48 Therefore, it appears that cation substitution (ion exchange) is taking place in the present studies of adsorption but that it is more predominant at lower concentration, i.e., at considerably higher pH’s of the metal solution where greater amounts of adsorbent (rock phosphate) are available. This finding is well supported by the results of Sinha et al.,49 who found the removal of thorium from dilute solutions (lower concentration) taking place mostly through ion exchange. However, at higher concentration, they found that the hydrolysis of thorium ions occurs, so that thorium hydroxide precipitation becomes more dominant. These results can be explained alternatively by a precipitation mechanism. More rock phosphate dissolved at lower pH, i.e., at considerably higher metal concentrations, and thus, more unoccupied active sites on the adsorbent (denoted by P in eq v) were available to react with Pb, Cu, and Zn to form fluoropyromorphite, copper fluoride phosphate, and hopeite, respectively, as shown by X-ray diffraction peaks in Figures 6-8. In general, the final Ca concentration increases with increasing initial metal concentration for a constant equilibration time and amount of rock phosphate. It is probable that more precipitation occurred, e.g. more Pb was precipitated as fluoropyromorphite; more unoccupied active sites (P) were consumed; and more H+ ions were released into solution, resulting in a lower filtrate pH. Furthermore, the lower pH caused more dissolution ofthe phosphate rock, thus releasing more Ca2+ into solution. In our present studies, the Ca2+ concentrations of the metal solutions were measured, as reported in Tables 5-7. It is evident that, for all of the metal solutions investigated, the Ca2+ concentration increased with increasing initial metal concentration, being more pronounced for Zn, then Cu, and then Pb. The gradual increase of the Ca2+ concentration with increasing metal concentration in the order Zn > Cu >Pb indicates more dissolution with precipitation and with less ion exchange in the order Pb > Cu > Zn. It can be summarized that, for Pb the solution, the dissolution-with-precipitation mechanism is more predominant than ion-exchange, and for Cu and Zn solutions, ion exchange is more predominant than dissolution with precipitation.

Figure 6. X-ray diffraction patterns for the sorption of Pb2+ onto a mineral adsorbent. (A) Rock phosphate (adsorbent) and (B) 48.2, (C) 241.0, (D) 482.0, and (E) 2410.0 µmol L-1 Pb2+ solutions.

However, the extents of these mechanisms depend on the adsorbent porosity and metal-ion chemical properties (molecular weight, ionic radius, and electronegativity).52 In the present investigation, an attempt was made to quantify these two mechanisms in each case of sorption of the divalent metal cations. In Table 8, qm denotes the maximum fixation capacity (mequiv/g) of a metal ion in the concentration range selected for study, and qCa is the maximum calcium desorption capacity. CEC (cation-exchange capacity) values were obtained by summing the values for all four

1520 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 Table 8. Effect of Metal Adsorption on Calcium Release from the Mineral Adsorbent and Adsorption Mechanism metal qm qCa ion (mequiv/g) (mequiv/g) qm/CEC Pb2+

0.792

0.255

0.211

Cu2+

0.629

0.401

0.168

Zn2+

0.550

0.411

0.147

adsorption mechanism 32% ion exchange + 68% complexation 63.7% ion exchange + 36.3% complexation 74.7% ion exchange + 25.3% complexation

Table 9. Unit Cell Dimensions of the Mineral Adsorbent for the Sorption of Lead, Copper, and Zinc cell constant sample no.

description

a (Å)

c (Å)

1 2 3 4 5

original treated with 48.2 mmol of Pb treated with 241.0 mmol of Pb treated with 482.0 mmol of Pb treated with 2410.0 mmol of Pb

9.3378 9.3723 9.3853 9.4125 9.5292

6.8506 6.8924 6.9006 6.9345 7.0534

1 2 3 4 5

original treated with 157.3 mmol of Cu treated with 786.2 mmol of Cu treated with 1573.0 mmol of Cu treated with 7865.0 mmol of Cu

9.3378 9.3690 9.3732 9.3890 9.4218

6.8506 6.8711 6.8820 6.8940 6.9550

1 2 3 4 5

original treated with 152.9 mmol of Zn treated with 764.5 mmol of Zn treated with 1529.0 mmol of Zn treated with 7645.0 mmol of Zn

9.3378 9.3700 9.3788 9.3804 9.4179

6.8506 6.8728 6.8736 6.8821 6.9171

cations listed in Table 1b. The ratio of qm to CEC decreases (from 0.211 to 0.147) from Pb2+ Cu2+ to Zn2+, indicating the dominance of the cation-exchange mechanism for the adsorption of Cu2+ and Zn2+ by the mineral adsorbent. The higher value of the ratio provides information about the complexation mechanism of sorption in addition to cation exchange. Assuming that cation exchange occurs only with calcium, the percentage of ion exchange was calculated by dividing qCa by qm. The rest is contributed by complexation, given that ion-exchange together with complexation were observed to be the most favorable mechanisms of removal of heavy metals. Now, the affinity sequence of metal cations is to be investigated and the proposed mechanisms of adsorption validated. Therefore, crystallographic studies of the adsorbed and unadsorbed adsorbent mineral phases were carried out using the X-ray diffraction technique. Measurement of Lattice Parameters. Cell Constants of Adsorbent Treated At Different Concentrations. A change in lattice parameters can also be an indication that ion exchange has taken place. The unit cell dimensions (lattice parameters) as calculated for the hexagonal structure of the crystal of the asreceived rock phosphate ore and the -100 + 75 µm size sample were a ) 9.3378 Å, c ) 6.8506 Å, and a ) 9.3607 Å, c) 6.8875 Å, respectively, which are very similar to the unit cell dimensions of sedimentary rock phosphates reported elsewhere.51,52 The crystal dimensions of the hydroxyapatite before and after treatment with heavymetal solutions were determined by Suzuki et al.,53 and a change in lattice parameters was observed. In the present investigation, the adsorption of lead, copper, and zinc on the francolite surface appears to be a result of the exchange of the lead, copper, and zinc cations for Ca2+. The values of the cell constants (a and c) of the treated francolite samples were observed to increase, as reported in Table 9, indicating an expansion

Figure 7. X-ray diffraction patterns for the sorption of Cu2+ onto mineral adsorbent. (A) Rock phosphate (adsorbent) and (B) 157.3, (C) 786.25, (D) 1573.0, and (E) 7865.0 µmol L-1 Cu2+ solutions.

of the lattice due to the incorporation of Pb2+, Cu2+, and Zn2+. For the case of the adsorption of Pb2+, the cell constants (a ) 9.3378 Å, c ) 6.8506 Å) for the untreated adsorbent increased to a ) 9. 5292 Å, c ) 7.0534 Å, for the 500 mg/L lead-treated adsorbent. It is noteworthy that the cell constants continue to increase with increasing concentration of cations and that the change is more pronounced in the case of leadtreated adsorbent than copper- and zinc-treated adsorbent. This behavior is also observed from the diffractograms (Figures 6-8) of adsorbent treated with solutions of lead, copper, and zinc at different concentrations. In the case of the lead-treated adsorbent (Figure 6), the original mineral phases of the adsorbent start disappearing and new phases of compounds such as fluropyromorphite and hydrocerruserite emerge at higher (>241.5 µmol/L) concentrations, indicating the forma-

Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1521

(apatite) lattice to a much lesser extent than cations with larger ionic radii. It has been reported54 that, in the ion-exchange process, larger multivalent ions are more effectively removed than smaller ones. The hydrated radius of an ion is a function of the charge and ionic radius, which dictates the removal phenomenon. The lower removals of Cu2+ (ionic radius ) 0.96 Å) and Zn2+ (0.74 Å) in comparison to Pb2+ can be attributed to the fact that the ionic radii of both Cu2+ and Zn2+ are smaller than the ionic radii of both Ca2+ (0.99 Å) and Pb2+ (1.20 Å). This corroborates the observations of earlier investigators.28,46,55 It can be observed from Table 1 that the ionic radii of the ECs (except for potassium) are smaller than that of lead and that the amount of divalent ECs (calcium and magnesium), 2.809 mequiv/g, is greater than the amount of monovalent ECs (sodium and potassium), 0.929 mequiv/g, in the adsorbent. Because the ionic radius of lead is greater than the ionic radius Ca2+ (0.99 Å), Pb2+ can be easily exchanged with Ca2+. This might be the reason for the lower removals of Cu2+ and Zn2+ in comparison to Pb2+. Conclusions

Figure 8. X-ray diffraction patterns for the sorption of Zn2+ onto mineral adsorbent. (A) Rock phosphate (adsorbent) and (B) 152.9, (C) 764.5, (D) 1529.0, and (E) 7645.0 µmol L-1 Zn2+ solutions.

tion of complex compounds. A similar emergence of new phases of compound was also observed for the coppertreated adsorbent (Figure 7) but was least found in the case of the zinc-treated adsorbent (Figure 8). This validates the proposed mechanism of cation exchange together with complexation for the sorption of Pb2+, Cu2+, and Zn2+. Ionic Radius Theory. Different theories (such as free energy, rates of complex formation, ionic radius) for the binding of metal ions onto a given adsorbent that provide insights into the factors governing complex formation and ion exchange are available. In our case, ionic radius theory appears to hold and to justify the order of affinity of the adsorbent for the metal cations investigated. This is just contrary to the findings of an investigator17 who reported that the ionic radius was not helpful in reflecting the adsorption capacities. For cation exchange to occur effectively, it is believed that cations whose ionic radii are smaller than that of Ca2+ (0.99 Å) can be incorporated into the adsorbent

The objective of this work was to study the mechanism of metal uptake by a mineral adsorbent. 1. Ion exchange coupled with complexation was found to be the most probable mechanism responsible for metal uptake by the sorption process. 2. The metal uptake affinity order was found to be Pb2+ > Cu2+ > Zn2+. 3. A study of the kinetics of the adsorption process revealed the following: (i) Both the mass-transfer coefficient and the filmtransfer constant are affected by the initial metal concentration and contact time. They gradually decrease as the initial concentration of metal cation increases and fall in the order Pb2+ > Cu2+ > Zn2+. Thus ,the cation removal rate on the surface of the adsorbent is relatively restricted with increasing cation concentration, which causes a decrease of the film-transfer constant or masstransfer coefficient. (ii) The rate of adsorption appears to be first-order. (iii) Liquid-film diffusion controls the mass- transfer process for the metal ions. (iv) The equilibrium data are less well described by the Freundlich model, and the Langmuir model seems to be more appropriate in describing the adsorption process. 4. Expansion of the fluorapatite crystal lattice is clearly observed to occur as a result of the incorporation of Pb2+, Cu2+, and Zn2+. 5. Ionic radius theory appears to hold and to justify the order of affinity of the metal cations, namely, Pb2+>Cu2+ > Zn2+, for their removal by the mineral adsorbent investigated. Acknowledgment The authors are grateful to Dr. N. Ramakrishnan, Director R.R.L, Bhopal, India, for his encouragement for the present research work and kind permission to publish this paper. The authors are also thankful to M.P. State Mining Corporation, Bhopal, India, for providing low-grade rock phosphate samples. Literature Cited (1) Atkinson, B. W.; Bux, F.; Kasan H. C. Water SA 1998, 24 (2), 129.

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Received for review February 18, 2003 Revised manuscript received September 5, 2003 Accepted November 18, 2003 IE030152D