- Email: [email protected]
Sorption of heavy metals on blast furnace sludge
PII: S0043-1354(97)00304-7
Wat. Res. Vol. 32, No. 4, pp. 989±996, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
SORPTION OF HEAVY METALS ON BLAST FURNACE SLUDGE A. LOÂPEZ-DELGADO, C. PEÂREZ and F. A. LOÂPEZ* Department of Materials Recycling, National Center of Metallurgical Research (CSIC), Avda. Gregorio del amo 8, E-28040 Madrid, Spain (First received December 1996; accepted in revised form August 1997) AbstractÐAn investigation into the use of sludge, a by-product of the steel industry, as an adsorbent for the removal of heavy metals from liquid euents was carried out. Gases produced in the blast furnace were washed and led towards a Dorr thickener where the sludge was obtained as a suspension. The sorption of Pb2+, Zn2+, Cd2+, Cu2+ and Cr3+ on the sludge was investigated by determination of adsorption isotherms. The eects of time, equilibrium temperature and concentration of metal solution on sludge adsorption eciency was evaluated. The adsorption process was analyzed using the theories of Freundlich and Langmuir and the thermodynamic values of DG, DH and DS corresponding to each adsorption process were calculated. Blast furnace sludge was found to be an eective sorbent for Pb, Zn, Cd, Cu and Cr±ions within the range of ion concentrations employed. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐadsorption, heavy metals, industrial euents, blast furnace sludge, waste
NOMENCLATURE b= Langmuir isotherm constant (l.mgÿ1) C= Equilibrium metal concentration (mg.lÿ1) X= Equilibrium solid-liquid metal concentration (gm.gÿ1) Xm= Langmuir molecular layer capacity (gm.gÿ1) a= Freundlich isotherm constant (l.gÿ1) n= Freundlich isotherm constant Ci= Initial concentration (mg.lÿ1) Kc= Apparent equilibrium constant DG= Free energy (KJ.molÿ1) DH= Enthalpy (KJ.molÿ1) DS= Entropy (KJ.Kÿ1.molÿ1) T= Temperature (K) R= 8.315 J.Kÿ1.molÿ1
INTRODUCTION
The use of activated carbons, low cost carbons (lignite, bitumen carbons etc.), zeolites and other silicates, iron and aluminium oxides in adsorption/ desorption processes has been, and is still one of the most widely used methods for the elimination of heavy metals from industrial wastewater. Recently, research eorts have been directed towards the use of industrial waste as an adsorbent material in an attempt to minimize processing costs. Blast furnace slags, generated by the ferrous and non-ferrous metal industries, have pozzolanic properties and have been tested as metal sorbents for aqueous solutions (Dimitrova, 1996). *Author to whom correspondence should be addressed. Email: ¯[email protected] 989
Blast furnace sludge is a by-product of the steelmaking industry. Gases generated during the manufacture of pig iron are cleaned before their atmospheric emission. Several systems may be employed to do this. When a wet process is used, the euent consists of a sludge which is led towards a Dorr thickener to increase its concentration. The concentrated sludge obtained from this thickener was used in this study. In 1995 the total production of pig iron in Europe was 93.636 million tons. Production during the same period in Spain reached 5.221 million ton. Since the production of blast furnace sludge is around 6 kg per ton of pig iron, only in Spain, the steel industry produced 31,300 t of sludge in that year. These vast quantities of sludge are generally dumped in land®lls. Indoor recycling poses problems due to small particle-size (<0.1 mm) and to the presence of lead, zinc and alkaline metals (LoÂpez et al., 1991). To date, no alternative uses for this sludge have been developed. The dried blast furnace sludge consists of iron oxides (45±50% w/ w) and coke (30±35% w/w) as the major phases. Its high iron oxide and coke content may enable its use as a metal sorbent in industrial wastewater puri®cation processes. Preliminary investigations on the capacity of this sludge to retain lead and copper ions in aqueous solution have been previously reported (LoÂpez et al., 1995, 1996; LoÂpez-Delgado et al., 1996). The aim of this study was to investigate the adsorption of other heavy metals such as chromium, zinc and cadmium to add to the work already published.
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The measurement of surface area and pore size distribution of solids may be performed in several ways. Gas adsorption is generally accepted as the most widely applicable and accurate method for total surface area measurements. The method may be de®ned as the physical characterization of material structures using a process where gas molecules of known size are condensed or adsorbed onto an unknown sample surface. The quantity of gas condensed and the resultant sample pressure are recorded and used for subsequent calculation. This data, when measured at a constant temperature, allows the construction of an isotherm. Isotherm data are then subjected to a variety of calculation models to obtain surface areas. The BET (Brunauer et al., 1938) method is the most widely used for the characterization of speci®c surface area. Gas adsorption isotherms have been classi®ed into ®ve types (Brunauer et al., 1940). Type I isotherms are encountered when adsorption is limited to, at most, only a few molecular layers. Physical adsorption that produces the type I isotherm indicates that the pores are microporous and that the exposed surface resides, almost exclusively, within the micropores which, once ®lled with the adsorbate, leave little or no external surface for additional adsorption. Type II isotherms (S-shaped or sigmoid isotherms) are most frequently encountered when adsorption occurs on nonporous powders or on powders with pore diameters larger than micropores. The in¯ection point or knee of the isotherm usually occurs near the completion of the ®rst adsorbed monolayer and with increasing relative pressure, second and higher layers are completed until at saturation point the number of adsorbed layers becomes in®nite. Type III isotherms are characterized principally by heats of adsorption which are less than the adsorbate heat of liquefaction. Thus, as adsorption proceeds, additional adsorption is facilitated since adsorbate interaction with an adsorbed layer is greater than interaction with the adsorbent surface. Types IV and V isotherms are closely related to Types II and III. In the former cases maximum adsorption is attained at a pressure lower than the vapour pressure of the gas, whereas in the latter cases adsorption increases as the vapour pressure of the adsorbed gas is approached. Isotherms may also be divided into four main classes (Giles et al., 1960) according to the nature of the initial portion of the curve and thereafter into subgroups. S curves indicate a vertical orientation of adsorbed molecules at the sorbent surface. L curves are indicative of molecules adsorbed ¯at on the surface, or of vertically oriented adsorbed ions with particularly strong intermolecular attraction. H curves are given by high-anity ions exchanging with low-anity ions and C curves pro-
duced by solutes which penetrate into the solid more readily than the solvent. When solution adsorption experiments of complex composition involve solids of heterogeneous structure as adsorbents, the use of the theoretical models proposed to describe the adsorption processes is inappropriate. In these cases, the adsorption process takes place along with other processes of transportation of the sorbent from the solution to the solid surface (Farley et al., 1985). MATERIALS AND METHODS
The original sludge, provided by The Spanish National Steel Company, Ensidesa S.A. (AvileÂs, Spain), was a 57% w/w suspension of solids in water. The suspension was vacuum-®ltered and the resulting solid dried at 808C for 24 h (this temperature does not aect composition). The solid was then crushed to yield a powder of a particle size under 40 mm. This powder is hereafter referred to as BFS (blast furnace sludge). For chemical analysis, a representative sample of sludge obtained by a quartering technique, was dissolved in a 40:60 HNO3/HClO4 mixture and ®ltered. The solution was then subjected to atomic absorption spectroscopy using a Philips Pye Unicam SP9 spectrophotometer. Estimation of each cation was performed using their principal wavelengths. For the determination of Ca, a 0.2% w/v Cl3La solution was added to prevent reduction of the signal by aluminium. For Na and K, a 0.1%w/v CsCl solution was added as an ionization buer. Mg and Al were measured under more oxidative conditions using N2O/acetilene mixed ¯ue gases and adding a 0.1 or 0.2% w/v KCl solution respectively as the ionization buer. The silicon retained in the solid was determined by gravimetry (oxidation with HClO4). Determination of sulphur and total carbon was carried out using a Leco CS-244 inductive furnace analyzer. Mineral phase composition was determined by X-ray powder diraction (XRD) using a Philips PW 1710 diffractometer and monochromatized CuKa radiation (generator tension = 40 kV, current = 30 mA). The XRD pattern was recorded from 5 to 708 (2y) with a step size of 0.028 and a counting time of 0.4s per step. Morphological analysis of the sludge was performed by scanning electron microscopy (SEM) using a Zeiss DSM 960 microscope. The working tension was 15 kV. Samples were prepared in two ways: (a) sample powders deposited on the support and metallized with gold and (b) sample powders inlaid within a polimeric resin. The surface to be observed was ®nely burnished. The speci®c surface area (SBET) and pore volume of BFS were determined by N2 volumetric adsorption using a Coulter SA 3100 volumetric gas adsorption analyzer. Nitrogen adsorption isotherms were determined at 77 K up to a P/Po value of 0.98 (where P is the equilibrium pressure and Po is the saturation pressure at the adsorbate at the measurement temperature). The sample was degassed at 473 K for 180 min at a pressure <10ÿ4 torr to clean the surface. The isoelectric point and zeta potential were determined by measuring electrophoretic mobilities of an aqueous dispersion as a function of pH, employing a Coulter Hialeah FL (Delsa 440). For the adsorption experiments, Pb, Cu, Cr, Zn and Cd metal solutions were prepared by dissolving the corresponding nitrate and NaNO3 (0.01 M) in distilled water. Adsorption assays of the dierent cations on the BFS were performed in 250 ml Erlenmeyer ¯asks. Dierent concentrations (0.005±10 g/l) of the relevant metal solution
Sorption of heavy metals (100 ml) were added to dierent amounts (1±7.5 g) of sludge in the ¯ask. Assays were performed in a thermostatic agitator at a constant temperature of 20 20.28C. The reaction time was varied (1±24 h) in order to determine the time of equilibrium. Once this was established, assays were performed with each cation at dierent temperatures (20, 40, 60 and 808C). The resulting suspensions were ®ltered and the solutions analyzed by atomic absorption spectroscopy. The quantity of adsorbed metal ion on the sludge was calculated as the dierence between initial concentration and concentration at equilibrium. Analysis of the relationship between sludge adsorption capacity and metal cation concentration at equilibrium was performed using the equations of Freundlich [equation 1] and Langmuir [equation 2]: log X log a
1 log C n
c 1 C X Xm b Xm
1
according to Brunauer. This suggests that gas adsorption on the material may take place in a multilayer fashion. The completion of the ®rst adsorbed monolayer occurs at a very low relative pressure. The desorption branch is also depicted in the ®gure. Results yielded a speci®c surface area SBET=27.43 m2.gÿ1. Total pore volume was calculated at the relative pressure close to saturation in the adsorption branch yielding Vp=8.1 10ÿ2 cm3.gÿ1. The average pore diameter, Dp, can be calculated based on the assumption that no surface other than the inner walls of the pores exits and that pores are of cylindrical geometry using equation 3:
2
Vp Dp SBET 4
where X is the quantity of metal ion adsorbed per unit mass of sludge, a and Xm indicate the adsorption capacity of the sludge, n and b are constants relating to adsorption intensity and C is the cation concentration at equilibrium.
RESULTS AND DISCUSSION
Sludge characterization The chemical composition of the BFS is shown in Table 1. Values are expressed as %w/w. BFS is a complex heterogeneous material composed mainly of hematite (a-Fe2O3) and coke with minor quantities of wustite (a-FeO), magnetite (Fe3O4), maghemite (g-Fe2O3), calcium ferrite (CaO.Fe2O3), quartz (SiO2) and calcium and aluminium silicates. Due to its chemical and mineralogical composition, it is not thought to possess pozzolanic activity. The morphological appearance of the sludge is shown in Fig. 1. Figure 1(a) shows the heterogeneity in the shape and size of the dierent particles. In Fig. 1(b) the detailed appearance of the carbonous phase (magni®cation 2 104) is shown. Dierent pore sizes may be observed. The appearance of the ferritic phase (on the burnished surface) is shown in Fig. 1(c) where the typical crystalline structure may be observed. The large pores correspond to ferric oxide. The nitrogen adsorption isotherm of the BFS is shown in Fig. 2 and may be classi®ed as Type II
Table 1. Chemical analysis of sludge (%w/w) Element Fe Si Al Ca Mg K Na Zn Pb S CTOTAL
%w/w 33.00 3.65 1.70 2.30 0.70 0.18 0.06 1.20 0.75 1.15 34.05
991
3
This equation yields a Dp value of 12 nm for the BFS which may be accordingly considered a mesoporous material. The isoelectric point of 3.31 and Zeta potential of ÿ21.28 mv were obtained at the working pH (7.2). Sorption experiments Equilibrium time. In order to determine the equilibrium time of the sorption process, adsorption runs were performed at several sorbate concentrations, at a ®xed temperature of 208C and sludge concentration of 50 g.lÿ1. In Fig. 3, the variation of the weight percent of ions adsorbed during contact time is shown. Only curves obtained using a pre-determined initial ion concentration are included in this ®gure. Results show that equilibrium is reached in the sludge-solution system after 5 h. Sorption isotherms. Figure 4 shows the relationship between the dierent quantities of metal ions adsorbed per unit mass of sludge and the equilibrium concentration of the relevant ions at 208C for a residence time of 5 h and a sludge concentration of 50 g.lÿ1. All the sorption isotherms exhibit a similar shape. From the initial slope of the curves, the isotherms may be considered as type H according to Giles classi®cation. This indicates that the sludge samples have a high anity for the metals studied and that these are completely adsorbed from dilute solutions. It may be observed that, in each case, the mass concentration adsorbed is greater at the lower metal ion concentrations. First degree surface saturation is always attained. After the in¯exion points, curves tend towards a horizontal expression indicating that solutes have a higher anity for the solvent than for the new layer formed by the solute molecules already adsorbed. From experimental data, the proportion of adsorbed mass varies in the order Pb>Cu>Cr>Cd>Zn. Although this relationship may be attributed to the dierent electronegativity of the metallic elements (Allred, 1961) along with the hydrated ionic radii in the solutions (Nightingale, 1959), it is thought that surface complex for-
Fig. 1. SEM photomicrographs of blast furnace sludge. (a) General appearance, magni®cation 1 104. (b) Carbonaceous phase, magni®cation 2 104 (c) Ferritic phase (polished surface), magni®cation 1.5 103.
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993
Fig. 4. Mass of metal ions adsorbed per unit mass of sludge with respect to dissolved cation concentration. (Temperature = 208C, residence t = 5 h and sludge concentration = 50 g.lÿ1). Fig. 2. Nitrogen adsorption isotherm of blast furnace sludge at 77 K.
mation and surface precipitation, especially on the hematitic phase (Farley et al., 1985), may be involved in the sorption process. Langmuir isotherms. The Langmuir and Freunlich models, applicable to solution adsorption processes, were used to determine the adsorption capacity of the relevant cations on the BFS. Generally, experimental data were better ®tted to the Langmuir equation (R2>0.98) than to the Freundlich equation. However, satisfactory correlation coe-
Fig. 3. Weight percent of ions adsorbed at several initial ion concentrations according to the residence time. (T = 208C and sludge concentration = 50 g.lÿ1).
cients were obtained using the latter equation for Cd and Zn (R2>0.94, R2>0.96 respectively). The results reported hereafter are based on the parameters obtained from the Langmuir equation 2. Figure 5 shows Langmuir isotherms obtained for the adsorption of Pb2+, Cu2+, Zn2+, Cd2+ and Cr3+ in aqueous solution on BFS. The metal solution concentration was varied and the residence time and temperature ®xed at 5 h and 208C respectively. The Langmuir values Xm and b, were calculated respectively as the slope and ordinate at the origin of each isotherm line at dierent temperatures and are shown in Table 2. Since the sludge adsorption capacity and anity for the metals under study improved with temperature, the adsorption process may be clearly de®ned as endothermic.
Fig. 5. Langmuir isotherms.
A. LoÂpez-Delgado et al.
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Table 2. Langmuir values according to temperature Ions
T (8C)
Pb2+
20 40 60 80 20 40 60 80 20 40 60 80 20 40 60 80 20 40 60 80
Cu2+
Cr3+
Cd2+
Zn2+
R
2
0.99 0.99 0.99 0.99 0.98 0.99 0.99 0.99 0.99 0.97 0.98 0.98 0.97 0.98 0.97 0.99 0.98 0.99 0.99 0.99
Xm varied in the order Pb>>Cu>Cr>Cd>Zn. Budinova et al. (1994) and Allen and Brown (1995) reported a similar relationship when active carbon and lignite were used as the adsorbent materials. As the temperature increases, the adsorption capacity for all the cations under test increases, as shown in Fig. 6, although the eect was more pronounced in the Pb2+ adsorption process than in those of the other metal cations. Sludge concentration. Table 3 shows the quantity of each cation adsorbed on the sludge according to sludge concentration. Quantities are expressed as percent weight with respect to initial dissolved cation weight. It may be seen that, at low initial dissolved cation concentrations (which dier according to the particular cation), almost total adsorption was achieved even at the lower sludge concentrations. As the metal ion concentration increased, 100% adsorption was reached by increasing the sludge content of the suspension. These results suggest that the use of BFS for the adsorption of heavy metals may be eective for solutions of high metal ion concentration (particularly Pb and Cu solutions).
b (l.mgÿ1)
cm (mg.gÿ1)
0.0170 0.0177 0.0302 0.0331 0.0407 0.0644 0.0769 0.1548 0.0500 0.0280 0.0297 0.0377 0.0299 0.0209 0.0273 0.0406 0.0505 0.0293 0.0310 0.0316
64.17 68.23 73.65 79.87 16.07 18.43 22.58 23.66 9.55 9.46 12.06 16.05 6.74 7.39 9.20 10.15 4.26 6.72 8.76 9.65
of the metals. These values were calculated from the Kc values using equation 5 and equation 6. DG ÿR T ln Kc ln Kc
ÿDH DS RT R
5 6
The DG values of all the cations decreased with temperature. Negative DG values, consistent with spontaneous reactions, were obtained for the most highly adsorbed cations Pb2+ and Cu2+, at each of the temperatures used. Negative DG values were obtained only for the Zn2+, Cd2+ and Cr3+ adsorption processes at temperatures exceeding 408C. DH values were positive for each metal. Consequently, the adsorption of the cations on BFS may be considered an endothermic process. This result is in agreement with the variation of Xm with temperature shown in Table 2. Values for DS were
Thermodynamic values The values for the apparent equilibrium constant (Kc) of the adsorption process of the dierent metals in aqueous solution on BFS were calculated with respect to temperature (LoÂpez-Delgado et al., 1996) at dierent initial concentration values and constant sludge concentration (m/v) and residence time (50 g.lÿ1, 5 h): Kc
% adsorption 100 ÿ % adsorption
4
Kc values (Table 4) increased with temperature and were highest for the Pb and Cu adsorption processes. Table 4 also shows the thermodynamic values DG, DH and DS for the adsorption processes
Fig. 6. Variation of Xm with temperature for each cation (t = 5 h and sludge concentration = 50 g.lÿ1).
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995
Table 3. Percentage of metal adsorbed with respect to dry sludge concentration (m/v) and initial metal ion solution concentration (Ci)
Ion
Ci (g.lÿ1)
10
48 100 260 480 730 1450 2100 2950 4000 4800 7100 3.6 7.5 34.3 63.3 198 353 555 773 5.1 9.6 49 98 280 480 750 970 4.5 50 100 300 490 720 950 1400 1900 2300 3000
100 99.5 94.6 82.7 61.1 33.3 25.0 21.9 14.6 11.8 8.1 90.6 88.0 65.9 58.9 39.4 ÿÿ ÿÿ ÿÿ 98.0 96.7 65.3 42.9 21.4 16.7 10.0 7.2 100 99.6 99.7 56.7 35.5 23.6 12.1 7.1 2.6 ÿÿ ÿÿ
Pb2+
Zn2+
Cd2+
Cu2+
percentage of metal adsorbed (%) sludge concentration (g.lÿ1) 25 50 100 99.9 99.8 99.7 99.7 78.3 55.0 47.6 35.7 35.9 23.6 88.8 85.3 84.5 70.5 40.4 31.2 18.0 ÿÿ 100 99.0 89.8 76.0 42.9 32.3 26.7 24.7 100 99.6 99.7 99.3 79.6 42.4 36.8 28.6 18.8 ÿÿ ÿÿ
positive for each metal and showed no signi®cant variation with temperature.
CONCLUSIONS
The present investigation evaluates the use of blast furnace sludge as an adsorbent for the elimination of heavy metal ions from aqueous euents. The theories of Freundlich and Langmuir were used to describe the adsorption process of Pb2+, Cu2+, Zn2+, Cd2+ and Cr3+ on BFS. However,
100 100 100 99.9 99.9 99.7 97.6 83.1 63.8 60.4 42.5 95.7 94.7 94.2 87.4 64.6 51.6 47.1 27.0 100 98.4 96.1 90.3 64.3 50.0 40.0 35.6 100 99.6 99.7 99.7 98.3 86.8 69.7 49.3 36.8 28.3 26.7
75 100 100 100 100 100 100 99.8 99.3 87.8 70.6 50.4 97.0 94.0 93.3 89.4 80.8 51.0 37.8 34.4 100 98.8 98.0 93.7 73.2 56.3 51.7 49.5 100 99.8 99.8 99.9 99.9 99.9 97.4 76.4 58.9 43.5 36.7
Langmuir isotherms were better ®tted to the process with the exception of the adsorption of Cd and Zn which also showed satisfactory correlations with Freundlich isotherms. The data obtained shows that blast furnace sludge is an eective sorbent of these ions and performs well over a wide range of concentrations. The employment of the sludge as a heavy metal sorbent provides a potential use for this industrial waste product. Its capacity to remove heavy metals from multi-cationic aqueous solutions and its action on
Table 4. Thermodynamic values of the adsorption process Ion Cd
2+
ÿ1
Ci (mg.l )
T (8C)
ln kc
DG (KJ.molÿ1)
DH (KJ.molÿ1)
DS (KJ.K.molÿ1)
750
20 40 80 20 40 80 20 40 80 20 40 80 20 40 80
ÿ0.42 ÿ0.27 0.41 ÿ0.69 ÿ0.20 0.83 0.26 0.41 1.34 ÿ0.78 0.34 1.31 1.59 1.74 3.41
1.01 0.71 ÿ1.19 1.68 0.50 ÿ2.44 ÿ0.63 ÿ1.03 ÿ3.93 1.90 ÿ0.86 ÿ3.85 ÿ3.88 ÿ4.53 ÿ10.01
12.04 12.04 12.04 21.59 21.59 21.59 16.40 16.40 16.40 30.87 30.87 30.87 28.26 28.26 28.26
0.04 0.04 0.04 0.07 0.07 0.07 0.06 0.06 0.06 0.10 0.10 0.10 0.11 0.11 0.11
Cr3+
1500
Cu2+
1400
Zn2+
520
Pb2+
2950
A. LoÂpez-Delgado et al.
996
real wastewater samples require further investigation. AcknowledgementsÐThe authors would like to thank the ``ComisioÂn Interministerial de Ciencia y TecnologõÂ a'' for funding of the project MAT93-0676 (Programa Nacional de Materiales). REFERENCES
Allen S. P. and Brown P. A. (1995) Isotherm analyses for single component and multi-component metal sorption onto lignite. J. Chem. Tech. Biotechnol. 62, 17±24. Allred A. L. (1961) Electronegativity values from thermochemical data. J. Inorg. Nucl. Chem. 17, 215±219. Brunauer S., Emmett P. H. and Teller E. (1938) J. Amer. Chem. Soc. 60, 309. Brunauer S., Deming L. S., Deming W. E. and Teller E. (1940) On the theory of the van der Waals adsorption of gases. J. Amer. Chem. Soc. 62, 1723±1732. Budinova T. K., Gergova M., Petrov N. V. and Minkova V. N. (1994) Removal of metal ions from aqueous solution by activated carbons obtained from dierent raw materials. J. Chem. Tech. Biotechnol. 60, 177±182.
Dimitrova S. V. (1996) Metal sorption on blast furnace slag. Wat. Res. 30, 228±232. Farley K. J., Dzombak D. A. and Morel M. M. (1985) A surface precipitation model for the sorption of cations on metal oxides. J. Colloid Interf. Sci. 106, 226±242. Giles C. H., MacEvan T. H., Nakhwa S. N. and Smith D. (1960) A system of classi®cation of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurements of speci®c surface areas of solids. J. Chem. Soc. 4, 3973±3994. LoÂpez F. A., Medina J., Sainz E. and GarcõÂ a Garcedo F. (1991) Recovery of carbon from blast furnace dusts. A®nidad 433, 159±166. LoÂpez F. A., PeÂrez C., Sainz E. and Alonso M. (1995) Adsorption of Pb2+ on blast furnace sludge. J. Chem. Tech. Biotechnol. 62, 200±206. LoÂpez F. A., PeÂrez C. and LoÂpez-Delgado A. (1996) The adsorption of copper (II) ions from aqueous solution on blast furnace sludge. J. Mat. Sci. Letters 15, 1310±1312. LoÂpez-Delgado A., PeÂrez C. and LoÂpez F. A. (1996) The in¯uence of carbon content of blast furnace sludge and coke on the adsorption of lead ions from aqueous solution. Carbon 34, 423±426. Nightingale E. R. (1959) Phenomenological theory of ion solvation. Eective radii of hydrated ions. J. Phys. Chem. 63, 1381±1387.
Wat. Res. Vol. 32, No. 4, pp. 989±996, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
SORPTION OF HEAVY METALS ON BLAST FURNACE SLUDGE A. LOÂPEZ-DELGADO, C. PEÂREZ and F. A. LOÂPEZ* Department of Materials Recycling, National Center of Metallurgical Research (CSIC), Avda. Gregorio del amo 8, E-28040 Madrid, Spain (First received December 1996; accepted in revised form August 1997) AbstractÐAn investigation into the use of sludge, a by-product of the steel industry, as an adsorbent for the removal of heavy metals from liquid euents was carried out. Gases produced in the blast furnace were washed and led towards a Dorr thickener where the sludge was obtained as a suspension. The sorption of Pb2+, Zn2+, Cd2+, Cu2+ and Cr3+ on the sludge was investigated by determination of adsorption isotherms. The eects of time, equilibrium temperature and concentration of metal solution on sludge adsorption eciency was evaluated. The adsorption process was analyzed using the theories of Freundlich and Langmuir and the thermodynamic values of DG, DH and DS corresponding to each adsorption process were calculated. Blast furnace sludge was found to be an eective sorbent for Pb, Zn, Cd, Cu and Cr±ions within the range of ion concentrations employed. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐadsorption, heavy metals, industrial euents, blast furnace sludge, waste
NOMENCLATURE b= Langmuir isotherm constant (l.mgÿ1) C= Equilibrium metal concentration (mg.lÿ1) X= Equilibrium solid-liquid metal concentration (gm.gÿ1) Xm= Langmuir molecular layer capacity (gm.gÿ1) a= Freundlich isotherm constant (l.gÿ1) n= Freundlich isotherm constant Ci= Initial concentration (mg.lÿ1) Kc= Apparent equilibrium constant DG= Free energy (KJ.molÿ1) DH= Enthalpy (KJ.molÿ1) DS= Entropy (KJ.Kÿ1.molÿ1) T= Temperature (K) R= 8.315 J.Kÿ1.molÿ1
INTRODUCTION
The use of activated carbons, low cost carbons (lignite, bitumen carbons etc.), zeolites and other silicates, iron and aluminium oxides in adsorption/ desorption processes has been, and is still one of the most widely used methods for the elimination of heavy metals from industrial wastewater. Recently, research eorts have been directed towards the use of industrial waste as an adsorbent material in an attempt to minimize processing costs. Blast furnace slags, generated by the ferrous and non-ferrous metal industries, have pozzolanic properties and have been tested as metal sorbents for aqueous solutions (Dimitrova, 1996). *Author to whom correspondence should be addressed. Email: ¯[email protected] 989
Blast furnace sludge is a by-product of the steelmaking industry. Gases generated during the manufacture of pig iron are cleaned before their atmospheric emission. Several systems may be employed to do this. When a wet process is used, the euent consists of a sludge which is led towards a Dorr thickener to increase its concentration. The concentrated sludge obtained from this thickener was used in this study. In 1995 the total production of pig iron in Europe was 93.636 million tons. Production during the same period in Spain reached 5.221 million ton. Since the production of blast furnace sludge is around 6 kg per ton of pig iron, only in Spain, the steel industry produced 31,300 t of sludge in that year. These vast quantities of sludge are generally dumped in land®lls. Indoor recycling poses problems due to small particle-size (<0.1 mm) and to the presence of lead, zinc and alkaline metals (LoÂpez et al., 1991). To date, no alternative uses for this sludge have been developed. The dried blast furnace sludge consists of iron oxides (45±50% w/ w) and coke (30±35% w/w) as the major phases. Its high iron oxide and coke content may enable its use as a metal sorbent in industrial wastewater puri®cation processes. Preliminary investigations on the capacity of this sludge to retain lead and copper ions in aqueous solution have been previously reported (LoÂpez et al., 1995, 1996; LoÂpez-Delgado et al., 1996). The aim of this study was to investigate the adsorption of other heavy metals such as chromium, zinc and cadmium to add to the work already published.
990
A. LoÂpez-Delgado et al.
The measurement of surface area and pore size distribution of solids may be performed in several ways. Gas adsorption is generally accepted as the most widely applicable and accurate method for total surface area measurements. The method may be de®ned as the physical characterization of material structures using a process where gas molecules of known size are condensed or adsorbed onto an unknown sample surface. The quantity of gas condensed and the resultant sample pressure are recorded and used for subsequent calculation. This data, when measured at a constant temperature, allows the construction of an isotherm. Isotherm data are then subjected to a variety of calculation models to obtain surface areas. The BET (Brunauer et al., 1938) method is the most widely used for the characterization of speci®c surface area. Gas adsorption isotherms have been classi®ed into ®ve types (Brunauer et al., 1940). Type I isotherms are encountered when adsorption is limited to, at most, only a few molecular layers. Physical adsorption that produces the type I isotherm indicates that the pores are microporous and that the exposed surface resides, almost exclusively, within the micropores which, once ®lled with the adsorbate, leave little or no external surface for additional adsorption. Type II isotherms (S-shaped or sigmoid isotherms) are most frequently encountered when adsorption occurs on nonporous powders or on powders with pore diameters larger than micropores. The in¯ection point or knee of the isotherm usually occurs near the completion of the ®rst adsorbed monolayer and with increasing relative pressure, second and higher layers are completed until at saturation point the number of adsorbed layers becomes in®nite. Type III isotherms are characterized principally by heats of adsorption which are less than the adsorbate heat of liquefaction. Thus, as adsorption proceeds, additional adsorption is facilitated since adsorbate interaction with an adsorbed layer is greater than interaction with the adsorbent surface. Types IV and V isotherms are closely related to Types II and III. In the former cases maximum adsorption is attained at a pressure lower than the vapour pressure of the gas, whereas in the latter cases adsorption increases as the vapour pressure of the adsorbed gas is approached. Isotherms may also be divided into four main classes (Giles et al., 1960) according to the nature of the initial portion of the curve and thereafter into subgroups. S curves indicate a vertical orientation of adsorbed molecules at the sorbent surface. L curves are indicative of molecules adsorbed ¯at on the surface, or of vertically oriented adsorbed ions with particularly strong intermolecular attraction. H curves are given by high-anity ions exchanging with low-anity ions and C curves pro-
duced by solutes which penetrate into the solid more readily than the solvent. When solution adsorption experiments of complex composition involve solids of heterogeneous structure as adsorbents, the use of the theoretical models proposed to describe the adsorption processes is inappropriate. In these cases, the adsorption process takes place along with other processes of transportation of the sorbent from the solution to the solid surface (Farley et al., 1985). MATERIALS AND METHODS
The original sludge, provided by The Spanish National Steel Company, Ensidesa S.A. (AvileÂs, Spain), was a 57% w/w suspension of solids in water. The suspension was vacuum-®ltered and the resulting solid dried at 808C for 24 h (this temperature does not aect composition). The solid was then crushed to yield a powder of a particle size under 40 mm. This powder is hereafter referred to as BFS (blast furnace sludge). For chemical analysis, a representative sample of sludge obtained by a quartering technique, was dissolved in a 40:60 HNO3/HClO4 mixture and ®ltered. The solution was then subjected to atomic absorption spectroscopy using a Philips Pye Unicam SP9 spectrophotometer. Estimation of each cation was performed using their principal wavelengths. For the determination of Ca, a 0.2% w/v Cl3La solution was added to prevent reduction of the signal by aluminium. For Na and K, a 0.1%w/v CsCl solution was added as an ionization buer. Mg and Al were measured under more oxidative conditions using N2O/acetilene mixed ¯ue gases and adding a 0.1 or 0.2% w/v KCl solution respectively as the ionization buer. The silicon retained in the solid was determined by gravimetry (oxidation with HClO4). Determination of sulphur and total carbon was carried out using a Leco CS-244 inductive furnace analyzer. Mineral phase composition was determined by X-ray powder diraction (XRD) using a Philips PW 1710 diffractometer and monochromatized CuKa radiation (generator tension = 40 kV, current = 30 mA). The XRD pattern was recorded from 5 to 708 (2y) with a step size of 0.028 and a counting time of 0.4s per step. Morphological analysis of the sludge was performed by scanning electron microscopy (SEM) using a Zeiss DSM 960 microscope. The working tension was 15 kV. Samples were prepared in two ways: (a) sample powders deposited on the support and metallized with gold and (b) sample powders inlaid within a polimeric resin. The surface to be observed was ®nely burnished. The speci®c surface area (SBET) and pore volume of BFS were determined by N2 volumetric adsorption using a Coulter SA 3100 volumetric gas adsorption analyzer. Nitrogen adsorption isotherms were determined at 77 K up to a P/Po value of 0.98 (where P is the equilibrium pressure and Po is the saturation pressure at the adsorbate at the measurement temperature). The sample was degassed at 473 K for 180 min at a pressure <10ÿ4 torr to clean the surface. The isoelectric point and zeta potential were determined by measuring electrophoretic mobilities of an aqueous dispersion as a function of pH, employing a Coulter Hialeah FL (Delsa 440). For the adsorption experiments, Pb, Cu, Cr, Zn and Cd metal solutions were prepared by dissolving the corresponding nitrate and NaNO3 (0.01 M) in distilled water. Adsorption assays of the dierent cations on the BFS were performed in 250 ml Erlenmeyer ¯asks. Dierent concentrations (0.005±10 g/l) of the relevant metal solution
Sorption of heavy metals (100 ml) were added to dierent amounts (1±7.5 g) of sludge in the ¯ask. Assays were performed in a thermostatic agitator at a constant temperature of 20 20.28C. The reaction time was varied (1±24 h) in order to determine the time of equilibrium. Once this was established, assays were performed with each cation at dierent temperatures (20, 40, 60 and 808C). The resulting suspensions were ®ltered and the solutions analyzed by atomic absorption spectroscopy. The quantity of adsorbed metal ion on the sludge was calculated as the dierence between initial concentration and concentration at equilibrium. Analysis of the relationship between sludge adsorption capacity and metal cation concentration at equilibrium was performed using the equations of Freundlich [equation 1] and Langmuir [equation 2]: log X log a
1 log C n
c 1 C X Xm b Xm
1
according to Brunauer. This suggests that gas adsorption on the material may take place in a multilayer fashion. The completion of the ®rst adsorbed monolayer occurs at a very low relative pressure. The desorption branch is also depicted in the ®gure. Results yielded a speci®c surface area SBET=27.43 m2.gÿ1. Total pore volume was calculated at the relative pressure close to saturation in the adsorption branch yielding Vp=8.1 10ÿ2 cm3.gÿ1. The average pore diameter, Dp, can be calculated based on the assumption that no surface other than the inner walls of the pores exits and that pores are of cylindrical geometry using equation 3:
2
Vp Dp SBET 4
where X is the quantity of metal ion adsorbed per unit mass of sludge, a and Xm indicate the adsorption capacity of the sludge, n and b are constants relating to adsorption intensity and C is the cation concentration at equilibrium.
RESULTS AND DISCUSSION
Sludge characterization The chemical composition of the BFS is shown in Table 1. Values are expressed as %w/w. BFS is a complex heterogeneous material composed mainly of hematite (a-Fe2O3) and coke with minor quantities of wustite (a-FeO), magnetite (Fe3O4), maghemite (g-Fe2O3), calcium ferrite (CaO.Fe2O3), quartz (SiO2) and calcium and aluminium silicates. Due to its chemical and mineralogical composition, it is not thought to possess pozzolanic activity. The morphological appearance of the sludge is shown in Fig. 1. Figure 1(a) shows the heterogeneity in the shape and size of the dierent particles. In Fig. 1(b) the detailed appearance of the carbonous phase (magni®cation 2 104) is shown. Dierent pore sizes may be observed. The appearance of the ferritic phase (on the burnished surface) is shown in Fig. 1(c) where the typical crystalline structure may be observed. The large pores correspond to ferric oxide. The nitrogen adsorption isotherm of the BFS is shown in Fig. 2 and may be classi®ed as Type II
Table 1. Chemical analysis of sludge (%w/w) Element Fe Si Al Ca Mg K Na Zn Pb S CTOTAL
%w/w 33.00 3.65 1.70 2.30 0.70 0.18 0.06 1.20 0.75 1.15 34.05
991
3
This equation yields a Dp value of 12 nm for the BFS which may be accordingly considered a mesoporous material. The isoelectric point of 3.31 and Zeta potential of ÿ21.28 mv were obtained at the working pH (7.2). Sorption experiments Equilibrium time. In order to determine the equilibrium time of the sorption process, adsorption runs were performed at several sorbate concentrations, at a ®xed temperature of 208C and sludge concentration of 50 g.lÿ1. In Fig. 3, the variation of the weight percent of ions adsorbed during contact time is shown. Only curves obtained using a pre-determined initial ion concentration are included in this ®gure. Results show that equilibrium is reached in the sludge-solution system after 5 h. Sorption isotherms. Figure 4 shows the relationship between the dierent quantities of metal ions adsorbed per unit mass of sludge and the equilibrium concentration of the relevant ions at 208C for a residence time of 5 h and a sludge concentration of 50 g.lÿ1. All the sorption isotherms exhibit a similar shape. From the initial slope of the curves, the isotherms may be considered as type H according to Giles classi®cation. This indicates that the sludge samples have a high anity for the metals studied and that these are completely adsorbed from dilute solutions. It may be observed that, in each case, the mass concentration adsorbed is greater at the lower metal ion concentrations. First degree surface saturation is always attained. After the in¯exion points, curves tend towards a horizontal expression indicating that solutes have a higher anity for the solvent than for the new layer formed by the solute molecules already adsorbed. From experimental data, the proportion of adsorbed mass varies in the order Pb>Cu>Cr>Cd>Zn. Although this relationship may be attributed to the dierent electronegativity of the metallic elements (Allred, 1961) along with the hydrated ionic radii in the solutions (Nightingale, 1959), it is thought that surface complex for-
Fig. 1. SEM photomicrographs of blast furnace sludge. (a) General appearance, magni®cation 1 104. (b) Carbonaceous phase, magni®cation 2 104 (c) Ferritic phase (polished surface), magni®cation 1.5 103.
Sorption of heavy metals
993
Fig. 4. Mass of metal ions adsorbed per unit mass of sludge with respect to dissolved cation concentration. (Temperature = 208C, residence t = 5 h and sludge concentration = 50 g.lÿ1). Fig. 2. Nitrogen adsorption isotherm of blast furnace sludge at 77 K.
mation and surface precipitation, especially on the hematitic phase (Farley et al., 1985), may be involved in the sorption process. Langmuir isotherms. The Langmuir and Freunlich models, applicable to solution adsorption processes, were used to determine the adsorption capacity of the relevant cations on the BFS. Generally, experimental data were better ®tted to the Langmuir equation (R2>0.98) than to the Freundlich equation. However, satisfactory correlation coe-
Fig. 3. Weight percent of ions adsorbed at several initial ion concentrations according to the residence time. (T = 208C and sludge concentration = 50 g.lÿ1).
cients were obtained using the latter equation for Cd and Zn (R2>0.94, R2>0.96 respectively). The results reported hereafter are based on the parameters obtained from the Langmuir equation 2. Figure 5 shows Langmuir isotherms obtained for the adsorption of Pb2+, Cu2+, Zn2+, Cd2+ and Cr3+ in aqueous solution on BFS. The metal solution concentration was varied and the residence time and temperature ®xed at 5 h and 208C respectively. The Langmuir values Xm and b, were calculated respectively as the slope and ordinate at the origin of each isotherm line at dierent temperatures and are shown in Table 2. Since the sludge adsorption capacity and anity for the metals under study improved with temperature, the adsorption process may be clearly de®ned as endothermic.
Fig. 5. Langmuir isotherms.
A. LoÂpez-Delgado et al.
994
Table 2. Langmuir values according to temperature Ions
T (8C)
Pb2+
20 40 60 80 20 40 60 80 20 40 60 80 20 40 60 80 20 40 60 80
Cu2+
Cr3+
Cd2+
Zn2+
R
2
0.99 0.99 0.99 0.99 0.98 0.99 0.99 0.99 0.99 0.97 0.98 0.98 0.97 0.98 0.97 0.99 0.98 0.99 0.99 0.99
Xm varied in the order Pb>>Cu>Cr>Cd>Zn. Budinova et al. (1994) and Allen and Brown (1995) reported a similar relationship when active carbon and lignite were used as the adsorbent materials. As the temperature increases, the adsorption capacity for all the cations under test increases, as shown in Fig. 6, although the eect was more pronounced in the Pb2+ adsorption process than in those of the other metal cations. Sludge concentration. Table 3 shows the quantity of each cation adsorbed on the sludge according to sludge concentration. Quantities are expressed as percent weight with respect to initial dissolved cation weight. It may be seen that, at low initial dissolved cation concentrations (which dier according to the particular cation), almost total adsorption was achieved even at the lower sludge concentrations. As the metal ion concentration increased, 100% adsorption was reached by increasing the sludge content of the suspension. These results suggest that the use of BFS for the adsorption of heavy metals may be eective for solutions of high metal ion concentration (particularly Pb and Cu solutions).
b (l.mgÿ1)
cm (mg.gÿ1)
0.0170 0.0177 0.0302 0.0331 0.0407 0.0644 0.0769 0.1548 0.0500 0.0280 0.0297 0.0377 0.0299 0.0209 0.0273 0.0406 0.0505 0.0293 0.0310 0.0316
64.17 68.23 73.65 79.87 16.07 18.43 22.58 23.66 9.55 9.46 12.06 16.05 6.74 7.39 9.20 10.15 4.26 6.72 8.76 9.65
of the metals. These values were calculated from the Kc values using equation 5 and equation 6. DG ÿR T ln Kc ln Kc
ÿDH DS RT R
5 6
The DG values of all the cations decreased with temperature. Negative DG values, consistent with spontaneous reactions, were obtained for the most highly adsorbed cations Pb2+ and Cu2+, at each of the temperatures used. Negative DG values were obtained only for the Zn2+, Cd2+ and Cr3+ adsorption processes at temperatures exceeding 408C. DH values were positive for each metal. Consequently, the adsorption of the cations on BFS may be considered an endothermic process. This result is in agreement with the variation of Xm with temperature shown in Table 2. Values for DS were
Thermodynamic values The values for the apparent equilibrium constant (Kc) of the adsorption process of the dierent metals in aqueous solution on BFS were calculated with respect to temperature (LoÂpez-Delgado et al., 1996) at dierent initial concentration values and constant sludge concentration (m/v) and residence time (50 g.lÿ1, 5 h): Kc
% adsorption 100 ÿ % adsorption
4
Kc values (Table 4) increased with temperature and were highest for the Pb and Cu adsorption processes. Table 4 also shows the thermodynamic values DG, DH and DS for the adsorption processes
Fig. 6. Variation of Xm with temperature for each cation (t = 5 h and sludge concentration = 50 g.lÿ1).
Sorption of heavy metals
995
Table 3. Percentage of metal adsorbed with respect to dry sludge concentration (m/v) and initial metal ion solution concentration (Ci)
Ion
Ci (g.lÿ1)
10
48 100 260 480 730 1450 2100 2950 4000 4800 7100 3.6 7.5 34.3 63.3 198 353 555 773 5.1 9.6 49 98 280 480 750 970 4.5 50 100 300 490 720 950 1400 1900 2300 3000
100 99.5 94.6 82.7 61.1 33.3 25.0 21.9 14.6 11.8 8.1 90.6 88.0 65.9 58.9 39.4 ÿÿ ÿÿ ÿÿ 98.0 96.7 65.3 42.9 21.4 16.7 10.0 7.2 100 99.6 99.7 56.7 35.5 23.6 12.1 7.1 2.6 ÿÿ ÿÿ
Pb2+
Zn2+
Cd2+
Cu2+
percentage of metal adsorbed (%) sludge concentration (g.lÿ1) 25 50 100 99.9 99.8 99.7 99.7 78.3 55.0 47.6 35.7 35.9 23.6 88.8 85.3 84.5 70.5 40.4 31.2 18.0 ÿÿ 100 99.0 89.8 76.0 42.9 32.3 26.7 24.7 100 99.6 99.7 99.3 79.6 42.4 36.8 28.6 18.8 ÿÿ ÿÿ
positive for each metal and showed no signi®cant variation with temperature.
CONCLUSIONS
The present investigation evaluates the use of blast furnace sludge as an adsorbent for the elimination of heavy metal ions from aqueous euents. The theories of Freundlich and Langmuir were used to describe the adsorption process of Pb2+, Cu2+, Zn2+, Cd2+ and Cr3+ on BFS. However,
100 100 100 99.9 99.9 99.7 97.6 83.1 63.8 60.4 42.5 95.7 94.7 94.2 87.4 64.6 51.6 47.1 27.0 100 98.4 96.1 90.3 64.3 50.0 40.0 35.6 100 99.6 99.7 99.7 98.3 86.8 69.7 49.3 36.8 28.3 26.7
75 100 100 100 100 100 100 99.8 99.3 87.8 70.6 50.4 97.0 94.0 93.3 89.4 80.8 51.0 37.8 34.4 100 98.8 98.0 93.7 73.2 56.3 51.7 49.5 100 99.8 99.8 99.9 99.9 99.9 97.4 76.4 58.9 43.5 36.7
Langmuir isotherms were better ®tted to the process with the exception of the adsorption of Cd and Zn which also showed satisfactory correlations with Freundlich isotherms. The data obtained shows that blast furnace sludge is an eective sorbent of these ions and performs well over a wide range of concentrations. The employment of the sludge as a heavy metal sorbent provides a potential use for this industrial waste product. Its capacity to remove heavy metals from multi-cationic aqueous solutions and its action on
Table 4. Thermodynamic values of the adsorption process Ion Cd
2+
ÿ1
Ci (mg.l )
T (8C)
ln kc
DG (KJ.molÿ1)
DH (KJ.molÿ1)
DS (KJ.K.molÿ1)
750
20 40 80 20 40 80 20 40 80 20 40 80 20 40 80
ÿ0.42 ÿ0.27 0.41 ÿ0.69 ÿ0.20 0.83 0.26 0.41 1.34 ÿ0.78 0.34 1.31 1.59 1.74 3.41
1.01 0.71 ÿ1.19 1.68 0.50 ÿ2.44 ÿ0.63 ÿ1.03 ÿ3.93 1.90 ÿ0.86 ÿ3.85 ÿ3.88 ÿ4.53 ÿ10.01
12.04 12.04 12.04 21.59 21.59 21.59 16.40 16.40 16.40 30.87 30.87 30.87 28.26 28.26 28.26
0.04 0.04 0.04 0.07 0.07 0.07 0.06 0.06 0.06 0.10 0.10 0.10 0.11 0.11 0.11
Cr3+
1500
Cu2+
1400
Zn2+
520
Pb2+
2950
A. LoÂpez-Delgado et al.
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real wastewater samples require further investigation. AcknowledgementsÐThe authors would like to thank the ``ComisioÂn Interministerial de Ciencia y TecnologõÂ a'' for funding of the project MAT93-0676 (Programa Nacional de Materiales). REFERENCES
Allen S. P. and Brown P. A. (1995) Isotherm analyses for single component and multi-component metal sorption onto lignite. J. Chem. Tech. Biotechnol. 62, 17±24. Allred A. L. (1961) Electronegativity values from thermochemical data. J. Inorg. Nucl. Chem. 17, 215±219. Brunauer S., Emmett P. H. and Teller E. (1938) J. Amer. Chem. Soc. 60, 309. Brunauer S., Deming L. S., Deming W. E. and Teller E. (1940) On the theory of the van der Waals adsorption of gases. J. Amer. Chem. Soc. 62, 1723±1732. Budinova T. K., Gergova M., Petrov N. V. and Minkova V. N. (1994) Removal of metal ions from aqueous solution by activated carbons obtained from dierent raw materials. J. Chem. Tech. Biotechnol. 60, 177±182.
Dimitrova S. V. (1996) Metal sorption on blast furnace slag. Wat. Res. 30, 228±232. Farley K. J., Dzombak D. A. and Morel M. M. (1985) A surface precipitation model for the sorption of cations on metal oxides. J. Colloid Interf. Sci. 106, 226±242. Giles C. H., MacEvan T. H., Nakhwa S. N. and Smith D. (1960) A system of classi®cation of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurements of speci®c surface areas of solids. J. Chem. Soc. 4, 3973±3994. LoÂpez F. A., Medina J., Sainz E. and GarcõÂ a Garcedo F. (1991) Recovery of carbon from blast furnace dusts. A®nidad 433, 159±166. LoÂpez F. A., PeÂrez C., Sainz E. and Alonso M. (1995) Adsorption of Pb2+ on blast furnace sludge. J. Chem. Tech. Biotechnol. 62, 200±206. LoÂpez F. A., PeÂrez C. and LoÂpez-Delgado A. (1996) The adsorption of copper (II) ions from aqueous solution on blast furnace sludge. J. Mat. Sci. Letters 15, 1310±1312. LoÂpez-Delgado A., PeÂrez C. and LoÂpez F. A. (1996) The in¯uence of carbon content of blast furnace sludge and coke on the adsorption of lead ions from aqueous solution. Carbon 34, 423±426. Nightingale E. R. (1959) Phenomenological theory of ion solvation. Eective radii of hydrated ions. J. Phys. Chem. 63, 1381±1387.