Modeling the Competitive Adsorption of Pb, Cu, Cd, and Ni onto a

The use of Italian “Red Soil” as an alternative low-cost sorbent material has been investigated for heavy metal removal in multicomponent systems...
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Ind. Eng. Chem. Res. 2004, 43, 5032-5041

Modeling the Competitive Adsorption of Pb, Cu, Cd, and Ni onto a Natural Heterogeneous Sorbent Material (Italian “Red Soil”) Marco Petrangeli Papini,* Teresa Saurini, Annalisa Bianchi, Mauro Majone, and Mario Beccari Department of Chemistry, University “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy

The use of Italian “Red Soil” as an alternative low-cost sorbent material has been investigated for heavy metal removal in multicomponent systems. Pb, Cu, Cd, and Ni adsorption was studied at fixed pH (6.0) and constant ionic strength (0.1 mol L-1 by NaNO3) by a flowthrough reactor setup performing monocomponent and binary adsorption tests and sequential extraction. The whole set of experimental data was represented by a competitive model based on the surface complexation concept. High Pb and Cu adsorption was largely due to a surface complexation mechanism, whereas lower Cd and Ni adsorption was due to cation-exchange reactions. Sequential extraction results independently substantiated the different adsorption behavior. Pb and Cu adsorption was not significantly affected by the presence of the other metals, whereas Cd and Ni strongly compete with each other and were displaced in the presence of Pb and Cu. “Red Soil” was effective in the removal of heavy metals from contaminated aqueous streams in competitive conditions if compared with other low-cost sorbents. Introduction Heavy metals are recognized as significant pollutants in soil and groundwater where they are released due to different anthropogenic activities. Metal electroplating facilities, mining operations, tanneries, petrochemical, painting, and battery industries produce aqueous waste streams strongly contaminated by heavy metals that are discharged into the environment often without adequate purification. Differently from organic contaminants, heavy metals are not biodegradable and can persist and accumulate in the environment and in living organisms, posing a serious threat to plants, animals, and humans. Consolidated heavy metal removal technologies include chemical precipitation, membrane filtration, carbon adsorption, ion-exchange, and reverse osmosis. Among the different processes, sorbent-based technologies and chemical precipitation are the most commonly applied. Chemical precipitation is sometimes limited by the solubility of precipitated metals, which do not allow the achievement of low dissolved metal concentrations in the treated effluent. In this regard, sorbent-based processes are attractive when they take advantage of the use of the large sorptive capacity of high surface area solids, such as hydrous iron and aluminum oxide,1 the main limitation being the cost of sorbents and regeneration phase. In recent years, a lot of work has been done about the possible industrial use of low-cost materials, which are abundant and easily available sorbents in nature or byproducts or waste materials from another industry.2 Tested sorbents include tannin rich materials,3 dead biomass,4 chitosan,5 zeolite and clay,6 fly ash,7 activated carbon from food industries,8 and biopolymer adsorbents.9 Testing and characterizing heavy metal sorption ability of sorbents for technological application are usually achieved by isotherm analysis, which provides * To whom correspondence should be addressed. Tel.: ++ 39 06 49913646. Fax: ++ 39 06 490631. E-mail: [email protected].

useful information about the sorbent retention capacity and the strength of the interaction at the solid-liquid interface. Langmuir- and Freundlich-based (semi)empirical adsorption models are widely used due to their simplicity and ease of parameter optimization and because comparison between different sorbents can be done with few parameters (usually maximum adsorption capacity and affinity constant).10 The limitation of this approach is that adsorption behavior is well described only in simplified and constant conditions but cannot be used to predict the behavior for different solution compositions. This aspect becomes relevant when the sorbent material is used in metal removal from complex solutions, as it is often the case for a waste stream, where complexation and competition effects might strongly affect sorbing capacity. In recent years, the surface complexation (SC) concept, which derives from the application of the SC model developed by Stumm and Schindler and largely applied for describing metal adsorption onto pure solid phases,11-13 has been adopted in the description of the adsorption properties of heterogeneous sorbent materials.14-16 Davis et al.14 described two types of modeling approaches based on SC concepts for describing metal adsorption onto complex mineral assemblage: the component additivity approach is proposed to predict adsorption when the sorbent is composed of a well-identified mixture of mineral phases whose surface properties are available from independent studies; on the other hand, the general composite (GC) approach is usually adopted when the composition of the mineral assemblage is too complex to be quantified in terms of the contribution of individual well-known sorbent phases. In a previous paper,17 we have applied the GC approach for the characterization of the adsorption properties of an Italian “Red Soil” that, for its large availability and large attenuation capacity with respect to heavy metals, is under evaluation as alternative natural low-cost sorbent. Lead ion adsorption was investigated in different background electrolytes and at

10.1021/ie0341247 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/23/2004

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different pHs. The optimized adsorption model was able to represent lead removal in a large range of experimental conditions and “Red Soil” showed an interesting overall adsorption capacity as compared with other potentially low-cost sorbents presented in the literature. The aim of the present work was to investigate the adsorption behavior of “Red Soil” in multicomponent heavy metal system (lead, copper, cadmium, and nickel). Adsorption experiments were carried out in monocomponent and binary conditions for a deeper insight into competitive effects. A model based on the GC approach was proposed for describing the overall set of adsorption data. Materials and Methods Soil Material. The natural sorbent material was an Italian “Red Soil”, already used in a previous paper,17 where lead adsorption was studied as a function of liquid phase composition. All the adsorption tests were performed on the soil previously air-dried and sieved down to a size fraction less than 590 µm. Clays (mainly illite and kaolinite), (hydr)oxides (goethite), and calcite were revealed by X-ray diffraction analysis. The specific surface area was 56.8 m2 g-1 as measured by the BET method. The cation-exchange capacity (CEC) was 9 ( 2 mequiv/100 g as measured by the BaCl2 method (three replicates). Soil pH was measured according to standard methods and resulted in 8.53, 7.41, and 7.99 for H2O, 0.1 M KCl, and 0.01 M CaCl2‚2H2O solutions, respectively. The total carbon was measured by an automatic analyzer (C-mat 550, Strho¨hlein Instrument) and resulted in 1.3 ( 0.2% (made up of 0.3 ( 0.1% and 1.0 ( 0.1% of total inorganic and organic carbon, respectively). The total metal content was determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES Varian Liberty 150) with ultrasonic nebulization (Cetac U-5000 AT+), after microwave mineralization (Milestone MLS 1200 mega), and the results were as follows: 13% Al, 5% Fe, 1.5% Ca, 0.5% Mg, 0.2% K, and Mn, Na, Zn, Cu, Ni, and Pb at trace level (between 10-3 and 10-4%). Adsorption Tests. The adsorption tests were performed by the flowthrough reactor setup.18-19 All tests were carried out at pH ) 6.0 and constant ionic strength (I ) 0.1 M) by NaNO3. 2-(N-Morpholino)ethanesulfonic acid of 5 × 10-3 M was used as the pH buffer. Theoretical speciation and blank tests demonstrated the absence of precipitate formation at all the adopted experimental conditions. The experimental system was composed of eight reactors (Bio-Rad Econo Column, 10cm length, 1.5-cm internal diameter) fed by a multihead peristaltic pump (Masterflex L/S) with the chosen solution stored in eight polyethylene bottles maintained under magnetic stirring and constant temperature. Two types of adsorption tests were performed: monocomponent adsorption tests (single metal isotherms) and binary tests (metal adsorption on soil already “loaded” with another metal). Soil pre-equilibration steps were similar for both types of adsorption tests. (a) Soil preequilibration: A known amount of soil (0.500 ( 0.005 g) was placed in each reactor, and the liquid phase (0.1 M NaNO3) was flushed in an “open-loop” configuration for an optimized time. At the end of this phase, the solution was quantitatively removed from the column by gravity (residual humidity < 2%). This step was adopted in order to remove all the natural presorbed ions and to equilibrate the soil with the background

Table 1. Experimental Conditions for All the Adsorption Experiments (pH ) 6.0, 0.1 mol L-1 NaNO3) experiment

conditions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Pb isotherm Pb isotherm onto Cd soil (0.008 mmol g-1) Pb isotherm onto Cd soil (0.023 mmol g-1) Pb isotherm onto Cd soil (0.028 mmol g-1) Pb isotherm onto Cd soil (0.047 mmol g-1) Cd isotherm Cd isotherm onto Pb soil (0.029 mmol g-1) Cd isotherm onto Pb soil (0.047 mmol g-1) Cd isotherm onto Pb soil (0.054 mmol g-1) Cd isotherm onto Ni soil (0.030 mmol g-1) Cu isotherm Cu isotherm onto Pb soil (0.029 mmol g-1) Cu isotherm onto Pb soil (0.055 mmol g-1) Ni isotherm Ni isotherm onto Cd soil (0.027 mmol g-1) Cd isotherm onto Ni soil (0.009 mmol g-1) Cd isotherm onto Cd soil (0.018 mmol g-1) Cd isotherm onto Cd soil (0.043 mmol g-1) Ni isotherm onto Cd soil (0.013 mmol g-1)

electrolyte. (b) pH equilibration: The background electrolyte added with the pH buffer was recycled through the columns in a “closed-loop” configuration, and the pH was corrected to the chosen value until the system reached equilibrium (pH ) 6.0). The total solution volume for each column was 60 mL. In the case of monocomponent tests, the procedure was completed as follows. (c) Metal adsorption: A total of 50 mL of the solution in each of eight reactors was substituted with metal-containing solutions with a prechosen concentration in order to obtain the whole isotherm. The solutions were recycled through the columns (“closed-loop” configuration) for 24 h, and the pH was periodically measured and corrected if necessary. After the system had reached the equilibrium, the total metal concentration was determined by ICP-AES directly on the liquid phase. Sorbed metal was calculated by the difference between the initial and equilibrium total dissolved concentration. When binary experiments were performed, step c was substituted by the adsorption of one metal at the same concentration for all eight reactors. When the system reached equilibrium, the pump was stopped, and the second metal was spiked in each reactor to obtain the same range of initial concentrations as those used in the monocomponent experiments; under magnetic stirring, 1 mL of solution was collected and the concentration of both metals was measured by ICP-AES. By this procedure, adsorption of the first metal was calculated together with the initial concentration of the second metal. Then, the solutions were recycled through the columns (“closed-loop” configuration) for 24 h, and the pH was periodically measured and corrected if necessary. After the system reached equilibrium, the total metal concentrations were determined by ICP-AES on the liquid phase and metal distribution among the liquid and solid phases was calculated on the basis of the previous analysis. Table 1 reports the list of all the performed experiments. Sequential Extraction Procedure. A simplified sequential extraction based on the Tessier20 procedure was used to obtain independent evaluation of metal distribution on the solid phase after adsorption in monocomponent systems to be compared with speciation at the solid-liquid interface as calculated by the optimized adsorption model. The procedure was applied on soil after metal adsorption from single-metal solutions at two equilibrium concentrations for each metal,

5034 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 2. Formation Reactions and Equilibrium Constants Used for Metal Speciation in the Liquid Phase from MINEQL+ Database (Ionic Strength ) 0 mol L-1, T ) 25 °C) log K Pb2+ + H2O S Pb(OH)+ + H+ Pb2+ + 2H2O S Pb(OH)2(aq) + 2H+ 2Pb2+ + H2O S Pb2(OH)3+ + H+ Pb2+ + 3H2O S Pb(OH)3- + 3H+ Pb2+ + 4H2O S Pb(OH)42- + 4H+ 3Pb2+ + 4H2O S Pb3(OH)42+ + 4H+ Pb2+ + NO3- S PbNO3+ Cd2+ + H2O S Cd(OH)+ + H+ Cd2+ + 2H2O S Cd(OH)2(aq) + 2H+ Cd2+ + 3H2O S Cd(OH)3- + 3H+ Cd2+ + 4H2O S Cd(OH)42- + 4H+ Cd2+ + NO3- S CdNO3+ Cu2+ + H2O S Cu(OH)+ + H+ Cu2+ + 2H2O S Cu(OH)2(aq) + 2H+ Cu2+ + 3H2O S Cu(OH)3- + 3H+ Cu2+ + 4H2O S Cu(OH)42- + 4H+ 2Cu2+ + H2O S Cu2(OH)3+ + H+ Ni2+ + H2O S Ni(OH)+ + H+ Ni2+ + 2H2O S Ni(OH)2(aq) + 2H+ Ni2+ + 3H2O S Ni(OH)3- + 3H+

-7.710 -17.120 -6.360 -28.060 -39.699 -23.880 1.170 -10.080 -20.350 -33.300 -47.350 0.3990 -8.000 -13.680 -26.899 -39.600 -10.359 -9.860 -19.000 -30.000

indicated as low and high, and chosen in the initial and final ranges of the experimental isotherms (adopted equilibrium concentrations: Cdlow ) 0.03 mmol L-1, Cdhigh ) 0.2 mmol L-1, Nilow ) 0.05 mmol L-1, Nihigh ) 0.2 mmol L-1, Pblow ) 0.004 mmol L-1, Pbhigh ) 0.03 mmol L-1, Culow ) 0.007 mmol L-1, Cuhigh ) 0.03 mmol L-1). After the adsorption phase, the solution was discharged from the columns and sorbed metal was calculated as reported in the adsorption test description. Then, 50 mL of a 0.1 M Mg(NO3)2 solution at pH ) 7 was recirculated in the closed-loop configuration. After 2 h (time estimated by preliminary tests), the metal concentration in the solution was measured and the extraction percentage calculated from the quantity of metal previously adsorbed on the soil. Nitrate salt was used instead of the chloride one as indicated in the Tessier procedure because it has a lower ability to form complexes with the selected heavy metals. After discharge of the Mg(NO3)2 solution, 50 mL of a 0.1 M sodium acetate solution at pH ) 5.0 was recirculated in the closed-loop configuration for 4 h. The metal concentrations were measured, and the relative extraction was calculated. Theoretical Speciation. Metal speciation in the liquid phase was calculated by the MINEQL+ speciation code.21 Table 2 reports all the formation reactions used for the calculations with the correspondent equilibrium constant (25 °C, ionic strength ) 0 mol L-1). Equilibrium constants were corrected by the speciation code for the actual ionic strength at the experimental conditions (0.1 mol L-1) used. Input values were the total component concentrations and adopted pH (6.0). Considering only species more abundant than 1%, total dissolved Pb was 65% and 35% as free ion (Pb2+) and charged complex [Pb(NO3)+], respectively; Cu and Ni occurred only as free ion species, whereas Cd was mainly in the free form (92%) with the remaining part as Cd(NO3)+ charged complex. No solid precipitation was predicted under the adopted experimental conditions. Metal speciation resulted independently from total metal concentration in the adopted experimental range. Data Regression. Metal adsorption at the different conditions was represented by the adsorption model presented in the Results and Discussion section. Model

Figure 1. Experimental monocomponent adsorption isotherms for Pb, Cd, Cu, and Ni (pH ) 6.0 and constant ionic strength ) 0.1 M).

adjustable parameters were optimized by simultaneous multivariate nonlinear least-squares fitting of all the experimental results by means of commercial software.22 The minimization procedure was based on the Levenberg-Marquardt algorithm and the goodness of fitting was evaluated by the coefficient of determination, which measures the fraction of the total variance accounted for by the model. Comparison of models with different number of parameters was based on the calculation of the model selection criterion, which relates the coefficient of determination to the number of parameters (or the number of degrees of freedom).22 Results and Discussion Monocomponent Adsorption Tests. The adsorption behavior of Pb, Cu, Cd, and Ni on the “Red Soil” was initially investigated by monocomponent adsorption tests. Figure 1 reports the sorbed metal at equilibrium as a function of the total dissolved equilibrium metal in the solution. Monocomponent isotherms are also reported as calculated according to the LangmuirFreundlich equation:23

Q)

MKCn 1 + KCn

where Q is the metal concentration in the solid phase (mmol g-1) and C the equilibrium metal concentration in the liquid phase (mmol L-1), whereas M, K, and n are the maximum adsorption capacity, the affinity constant, and the heterogeneity index, respectively. However, calculated isotherms are reported here only for the purpose of a better comparison of the experimental behavior, and no physical significance is attributed to their parameters. From the isotherm profiles, it derives that Pb and Cu exhibit a similar and high affinity for the solid phase, whereas Cd and Ni are sorbed at a significantly lower extent. It is noteworthy that Pb and Cu are adsorbed, at the highest concentration in the liquid phase, above the measured CEC (0.045 mmol g-1). On the other hand,

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Figure 2. Sequential extraction results on soil samples where Cd, Pb, Cu, and Ni had been previously sorbed onto in monocomponent conditions.

Cd and Ni adsorption is always significantly below the CEC, even if they are close to their plateau range. “Red Soil” is made up of different phases ranging from clays (mainly illite and kaolinite) to metal oxides (Al, Fe, and Mn hydrous oxides) and small percentages of organic matter. Thus, metal adsorption behavior can be explained by the contribution of different adsorption mechanisms: cation exchange at the permanent negative charge due to isomorphic substitution on the clay fraction (outer-sphere complexes) and SC at specific functional groups due to Fe, Al, and Mn hydrous oxide and organic matter (inner-sphere complexes). In recent years X-ray absorption spectroscopy and its derivatives (XAFS and XANES) have made a large contribution to the comprehension of the structure and composition of metal sorption complexes and surface binding sites. Most of the XAFS studies on metal sorption have been applied on single metal ions and pure solid phases. Several studies have demonstrated that Pb2+ adsorption onto Fe and Al oxides occurs by formation of innersphere complexes.24-27 On the other hand, Pb2+ sorption mechanism onto clay minerals, such as montmorillonite, varies from predominantly outer-sphere complexation to a mixture of outer- and inner-sphere complexation depending on solution pH and ionic strength.28 In our system, due to high ionic strength (I ) 0.1 M) innersphere complexation should play a significant role as outer-sphere adsorption should be partially inhibited by the high concentration of Na+ ions. Moreover, the experimental adsorption affinity order (Pb = Cu . Cd = Ni) agrees well with first hydrolysis metal pK’s as reported in Table 2 (7.71, 8.00, 10.08, and 9.86 respectively for Pb, Cu, Cd, and Ni). This agreement has been reported as an indication of the presence of specific adsorption because of the analogy between hydrolysis reactions and SC adsorption mechanisms.29 To better understand the relative contribution of the different adsorption mechanisms for each metal, sequential extractions were carried out on soil samples onto which each metal had been previously sorbed (at two levels indicated as low and high in the x axis of Figure 2). In this regard, it is important to recall the mechanisms acting during the extraction procedure. The Mg(NO3)2 solution extracts metals mainly sorbed by the cation-exchange mechanism. The high Mg2+ concentration with respect to heavy metals moves heavy metals sorbed onto cation-exchange sites by mass action to the

liquid phase, whereas it does not compete with metals sorbed more specifically onto SC sites. Extraction with CH3COONa solution is based on several mechanisms: acetate anion is able to complex heavy metals in solution, the lower pH (5.0) implies competition of protons on SC sites where metals can be specifically sorbed, and finally, the high Na+ concentration extracts metals from cation-exchange sites. However, the latter mechanism is minimized when previous extraction is performed. Although sequential extraction procedures have to be considered as operationally defined, the experimental results substantiated that Pb and Cu are mainly adsorbed onto specific sites as indicated by the very low recovery by 0.1 M Mg(NO3)2 solution. Moreover, a significant sorbed fraction is still present on the surface after extraction by a 0.1 M CH3COONa solution. On the contrary, sorbed Cd and Ni are significantly extracted by a 0.1 M Mg(NO3)2 solution, indicating a major role for the cation-exchange mechanism. Moreover, the CH3COONa solution extracts a lower amount of sorbed metals and the residual sorbed fraction is at a much lower extent. Binary Adsorption Tests. Competitive sorption experiments were performed by determining the adsorption isotherms of each metal on soil samples previously “loaded” with a different metal (see Table 1 and the Materials and Methods section). Each equilibrium condition is now represented by the concentration of the two metals in the liquid phase and their correspondent concentrations at the solid surface. In this way, the competitive effect was investigated by comparing metal adsorption in binary conditions with respect to the adsorption in a monocomponent system and by the possible desorption of the second metal previously adsorbed on the solid phase. Pb adsorption has been studied in the presence of Cd or Cu in order to investigate the effect of competition due to a weakly or strongly adsorbed metal, respectively. Figure 3 reports Pb adsorption in a monocomponent system (esp1) in comparison with Pb adsorption obtained with soil onto which Cd (esp 2 and 3) or Cu (esp 4 and 5) had been previously adsorbed. Pb adsorption is not significantly affected by the presence of a second metal previously adsorbed. The different Pb adsorption isotherms superimpose in the range of experimental errors (i.e., no systematic trend can be observed). On

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Figure 3. Experimental Pb isotherms in monocomponent system (esp1) compared with Pb isotherms obtained with soil onto Cd (esp 2 and 3) and Cu (esp 4 and 5) that had been previously adsorbed.

the other hand, Cu was not desorbed during Pb adsorption whereas Cd desorption was measured as function of Pb adsorption and initial Cd in the soil (desorption data not reported). These results suggest that Pb and Cu do not compete significantly for the adsorption sites, whereas Pb and Cd interact with each other although at a small extent due to the low Cd concentration in the soil and the different affinity for the surface. The interaction between Cd and Pb is confirmed by Figure 4a, where monocomponent Cd adsorption is compared with Cd adsorption obtained in experiments where an increasing Pb amount had been previously sorbed onto the soil (esp 7-9). Cd adsorption is significantly affected by the presence of Pb sorbed onto the soil, decreasing as Pb concentration onto the soil increases. On the other hand, during Cd adsorption no detectable Pb was desorbed from the soil.

Figure 4b reports the effect of Ni or Cu on Cd adsorption. Cd adsorption was depressed at the highest Ni concentration in the soil (esp 10), whereas it was not significantly affected when Ni was at the lowest concentration (esp 16). Accordingly, a significant Ni release was observed only at the highest Ni concentration in the soil. The presence of Cu on the soil decreased Cd adsorption as function of initial Cu concentration in the soil (esp 17 and 18). However, as in the case of Pb, no significant Cu desorption was measured during Cd adsorption. Figure 5 reports the comparison between monocomponent Ni adsorption (esp 14) and Ni adsorption in the presence of two Cd concentrations in the soil (esp 15 and 19). The interaction between Ni and Cd is again confirmed: Ni adsorption clearly decreases as Cd concentration in the soil increases and a correspondingly increasing amount of Cd is desorbed from the soil. The presence of Pb on the soil seems to slightly affect Cu adsorption. From Figure 6, Cu adsorption decreases with respect to the monocomponent system (esp 11) when Pb is previously adsorbed onto the soil (esp 12 and 13). Competitive Model. The adsorption model able to represent metal adsorption behavior at the different experimental conditions was developed on the basis of the SC concept following the GC approach. The GC approach is generally used when dealing with heterogeneous surfaces, as it is in the case of the “Red Soil”. When the composition of the solid phase is too complex to be represented quantitatively as the sum of individual contributions of well-recognized phases, the surface reactivity can be described in terms of generic surface functional groups, chosen on the basis of the solid characterization and experimental adsorption behavior, the number and stoichiometry of which is determined only by fitting experimental data. Modeling the competitive adsorption of Pb, Cu, Cd, and Ni has been done starting with the model developed in a previous paper17 where Pb adsorption onto the “Red Soil” was described as function of liquid-phase composition. Based on the composition of the solid phase and

Figure 4. Monocomponent Cd isotherm (esp 6) compared with Cd isotherms obtained with soil onto Pb (esp 7-9, 4a) and Ni (esp 10, 16, 4b) or Cu (esp 17 and 18, 4b) that had been previously adsorbed.

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be sorbed according to the following equilibrium:

tS + Me2+ a tSMe

KtSMe )

{tSMe} {tS}[Me2+]y2

(2)

Proton adsorption was allowed onto both tS and t S1 SC sites, whereas conditional constants at pH ) 6.0 were calculated for site X. In all expressions y1 and y2 are the activity coefficients for a monovalent and divalent dissolved cation, respectively, as calculated by the extended Davies equation:

[

log yi ) -AZi2

Figure 5. Monocomponent Ni isotherm (esp 14) compared with Ni isotherms obtained with soil onto Cd (esp 15 and 19) had been previously adsorbed.

Figure 6. Monocomponent Cu isotherm (esp 11) compared with Cu isotherms obtained with soil onto Pb (esp 12 and 13) had been previously adsorbed.

Pb experimental behavior, a set of reasonable adsorption reactions was proposed. In the final model formulation, the surface was represented by the presence of two types of sites: a cation-exchange site X (outer-sphere adsorption mechanism) where free metal undergoes ion exchange with Na+ according to the following general equilibrium:

2XNa + Me2+ a X2Me + 2Na+ KMe Na )

{X2Me}[Na+]2y12 {XNa}2[Me2+]y2

(1)

and two SC sites (tS and tS1) (inner-sphere adsorption mechanism) onto which metals (free or complexed) can

]

I2 - 0.3I 1 + xI

(3)

where A ) 1.82 × 106(DT)-3/2, D ) dielectric constant for water, T ) temperature (K), Zi ) ion charge, and I ) solution ionic strength. Considering the constant high ionic strength, no explicit correction was adopted for electrostatic interactions.15,17 Pb monocomponent adsorption onto “Red Soil” was satisfactory described in a large experimental range (pH ) 4-7; the presence of different organic and inorganic complexing anions) assuming the adsorption of free Pb2+ and charged complexes (Pb(NO3)+ and [Pb(CH3COO-)]+ ions) onto SC sites, and Pb2+/Na+ cation exchange onto site X. A complete description of the procedure can be found in the already cited paper of Petrangeli Papini et al.17 The competitive model was developed assuming the same surface representation and taking into account the new experimental evidences obtained in the monocomponent and binary adsorption experiments carried out with Pb, Cu, Cd, and Ni. The adsorption onto all sites was initially considered as possible for all charged species (free metal and positively charged complexes) occurring in the liquid phase, as calculated by theoretical speciation. The final formulation of the model, based on the minimum number of reactions needed to represent the experimental behavior, was chosen on the basis of statistical analyses by comparing the goodness of fit with the number of adjustable parameters (model selection criterion).22 In the final model formulation, free lead (Pb2+) was sorbed onto site tS, tS1, and site X whereas Pb(NO3)+ was only sorbed onto site tS. It is noteworthy that the total concentration of site tS and the correspondent equilibrium constants were assumed from the previous paper17 without any further adjustment. All the other metals were sorbed only as free ion (Me2+). Cd was sorbed onto all three sites, Ni onto site tS and X, whereas Cu only onto complexation sites tS and tS1. The total concentration of the X site was assumed equivalent to the measured CEC. Optimization of adjustable parameters was achieved by simultaneous nonlinear least-squares regression of all experimental data from monocomponent and binary experiments, and their adjusted values are reported in Table 3. Independent variables were the concentration of all the possible sorbing species at the different experimental conditions (Pb2+, PbNO3+, Cu2+, Cd2+, Ni2+, and H+), whereas dependent variables were the equilibrium sorbed metal concentrations (in mmol g-1) as obtained from mass balance calculations (total dissolved metal before and after the contact with the soil). The good ability of the model to represent the adsorption of the four metals in monocomponent and competi-

5038 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 3. Adjustable Parameters of the Adsorption Model As Calculated by Means of Nonlinear Multivariate Regression of All Adsorption Data adjustable parameters Stota S1tot CECa KtSPba KtSPbNO3a KtSCd KtSCu KtSNi KtS1Pb KtS1Cd KtS1Cu KtSHa a KPb Na

a

units

estimates

mmol g-1 mmol g-1 mmol g-1 L mmol-1 L mmol-1 L mmol-1 L mmol-1 L mmol-1 L mmol-1 L mmol-1 L mmol-1 L mmol-1 g L-1 -1 KCd Nag L -1 KNi g L Na

0.044 0.059 0.045 1.37 × 104 3.32 × 104 86.2 8.21 × 102 2.37 × 102 25.3 2.14 348.4 2.41 × 103 2.09 × 107 3.51 × 106 4.15 × 105

standard deviation 0.022

12.8 1.48 × 102 0.31 × 102 2.53 0.36 28.5 4.94 × 105 2.06 × 105

Fixed value from a previous paper.17

tive binary conditions is indicated by the high overall coefficient of determination (0.9907). The good correlation can be confirmed by comparing the adsorption of the single metal as calculated by the model with the experimental adsorption (Figure 7). Cd and Ni show a good correlation without any significant systematic deviation (coefficient of determination 0.9878 and 0.9860,

respectively); some deviation can be observed for Pb and Cu even though their adsorption is still satisfactorily predicted by the model (coefficient of determination 0.9808 and 0.9693, respectively). It has to be stressed that metal equilibrium adsorption data derive both from monocomponent experiments and competitive binary ones (adsorption of one metal and desorption of the metal previously adsorbed onto the soil), in a quite large range of experimental conditions. To better evaluate the performance of the model in representing the competitive behavior Figure 8 reports the comparison between the experimental and calculated data for four selected binary experiments (out of the total of 19 experiments). Each experiment is characterized by the adsorption of one metal along with the corresponding desorption of the other metal previously adsorbed onto the “Red Soil”, both as a function of the equilibrium concentration in the liquid phase of the metal being sorbed. The model is able to represent the features already underlined in the description of the binary experiments. Cd and Ni (Figure 8a) strongly interact with each other as indicated by the desorption of Ni from the surface due to the adsorption of Cd. In this case the model perfectly predicts the experimental behavior without any systematic deviations. The same behavior has been obtained and modeled for the reversed experiments where Ni is adsorbed onto a soil previously loaded with Cd.

Figure 7. Correlation between experimental and calculated Pb, Cd, Ni, and Cu adsorption according to the adsorption model (after multivariate nonlinear regression of all experimental data).

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Figure 8. Comparison between experimental and calculated adsorption data for selected binary experiments according to the adsorption model.

The behavior is quite different when Cd adsorption is carried out onto a soil with an initial Cu content (Figure 8b). Cu is not desorbed by Cd adsorption and the experimental behavior is satisfactory predicted by the model, although with some small deviations. Adsorption of Pb gives rise to Cd desorption (Figure 8c), whereas Cu is sorbed without any significant desorption of previously sorbed Pb (Figure 8d) as well predicted by the model. It is important to stress that the model was able to represent the entire data set (19 experiments); the only observable deviations were systematic, whereas all the specific interactions between couples of metals were well predicted. The speciation at the liquid-solid interface as calculated by the adsorption model can be used to evaluate the relative contribution of the different mechanisms to the overall adsorption of the four metals and to understand the differences in their adsorption properties. Moreover, the comparison with sequential extraction experimental results can be used as an independent substantiation of the proposed model. Figure 9 reports the calculated speciations for the different metals in the monocomponent conditions along with the experimental isotherms. Pb adsorption has been already discussed in the previous paper:17 cation exchange is small with respect to SC of free ions and Pb(NO3)+. Cu exhibits a different behavior as

cation exchange can be neglected and the adsorption is mainly due to one SC site (tS1). Considering that organic matter was found to be an efficient adsorbent in removing Cu from the liquid phase,30 this behavior can be explained by Cu adsorption on the organic coating of “Red Soil”. In the case of Cd and Ni, the relative contribution of cation exchange is more evident but SC is still necessary to well represent the experimental behavior in all of the experimental range. The contribution of adsorption mechanisms on different metal interaction with the “Red Soil” can be also interpreted on the basis of the adopted pH. In the literature it is largely reported that pH, besides the ionic strength, significantly affects metals interactions with hydroxide and clay component of soils.31-33 In particular, metal adsorption via inner-sphere complexation appears to be significantly affected by pH changes, whereas cation exchange appears to be a less pHdependent mechanism.28 In their large review on hydrous ferric oxide, Dzombak and Morel12 observed that pH sorption edges for Cd and Ni do not begin to reach maximum sorption until pH > 6 in most cases, whereas Pb and Cu maximum sorption occurs around pH ) 5-6. In our system, in the presence of a heterogeneous solid phase and relatively high ionic strength, it derives that the relative contribution of the cation-exchange mech-

5040 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004

Figure 9. Comparison between experimental and calculated isotherms in monocomponent systems along with calculated speciations for the different metals according to the adsorption model.

anism at pH ) 6 becomes more relevant for Cd and Ni rather than for Pb and Cu. The calculated speciation is in good agreement with the sequential extraction results; the small Pb and Cu fraction extracted by Mg(NO3)2 well compare with the low contribution of cation exchange as calculated by the optimized model. The SC mechanism as predicted by the model is also experimentally confirmed by the relevant fraction still remaining on the soil after the Mg(NO3)2 extraction step. A good agreement is also observed in the case of Cd where cation exchange appears to be the main mechanism, from both the extraction results and modeling prediction. On the contrary, in the case of the Ni model, prediction seems to overestimate the SC mechanism with respect to sequential extraction results. Conclusions In this work we have studied the adsorption properties of a heterogeneous natural sorbent material (Italian “Red Soil”) with respect to Pb, Cu, Cd, and Ni at fixed pH (6.0) and constant ionic strength (0.1 mol L-1) in mono- and multicomponent conditions. The whole set of experimental data was well represented by a competitive model developed on the basis of the SC concept. Based on experimental and modeling results, metal

affinities for the sorbent material were Pb = Cu . Cd = Ni due to different mechanisms involved in each metal adsorption at the solid phase. Pb and Cu adsorption were largely due to interaction with specific sorption sites (inner-sphere complexes) due to the presence of different oxides and organic matter, whereas Cd and Ni were adsorbed mainly by nonspecific cation-exchange reactions (outer-sphere complexes) due to the presence of a significant clay fraction on the soil surface. Sequential extractions performed with 0.1 M Mg(NO3)2 and 0.1 M CH3COONa solutions independently substantiated the different sorption interaction of metals with the solid phase as calculated by the adsorption model. As a consequence, Pb and Cu adsorption were not significantly affected by the presence of the other metals, whereas Cd and Ni strongly compete with each other and were displaced in the presence of Pb and Cu. From the applicative point of view, the ability of the “Red Soil” in the removal of heavy metals from contaminated aqueous streams was also confirmed in competitive conditions if compared with other potentially low-cost sorbent presented in the literature.2 Moreover, the presented modeling approach was demonstrated to be appropriate for characterizing the adsorption behavior of heterogeneous natural sorbents in a large range of experimental conditions.

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Received for review September 12, 2003 Revised manuscript received March 19, 2004 Accepted May 23, 2004 IE0341247