Desalination 167 (2004) 165-174
Adsorption of heavy metal ions on composite materials prepared by modification of natural silica H. Hadjal~*, B. Hamdi b, Z. Kessaissia b aC.R.A.P, BP248, Alger RP 16004Alger, Algdrie TeL +213 (21) 24 79 50/60 Poste 975; Fax +213 (21) 24 73 11; email: [email protected]
~Laboratoire d'Etude Physico-Chimique des Mat&iaux et Application ~ l'Environnement, Facult~ de Chimie U.S.T.H.B, BP 32 Bab Ezzouar, 16111, Alger, Alg~rie
Received 16 March 2004; accepted 26 March 2004
Recently because of increasing of environmental consciousness and demands, several discussions about the preservation of natural resources have led to more efforts concerning materials of good selectivity and high sorption capacity [ 1]. We have developed new composite materials with improved adsorptive properties thanks to the presence of Algerian silica (Kieselgtihr) , after undergoing suitable treatments. We have investigated the effect of different amounts of silica on the performance and adsorptive behaviour of the resulting composites. Thus, the work herein describes our investigations on the adsorption of heavy metal ions onto the composite materials from aqueous solutions in relation to different variables, such as shaking time, concentration of metal ions and pH. The adsorption behaviour of the concerned material is explained on the basis of its chemical nature and porous texture. Keywords." Lead nitrates; Heavy metal; Adsorption; KieselgOhr; Charcoal
I. Introduction Consumption of both industrial water and that of domestic use has increased. Despite the considerable number o f studies and advances focused on the comprehension o f chemistry, biochemistry and microbiology o f processes o f purification, much effort has to be put into the performance of *Corresponding author.
the treatment methods because o f the complexity of the poured effluents . The determination of the metal traces contained especially in polluted water is very important in any study relating to the environmental applications [4,5]. The strict environmental regulations with respect to heavy metal loadings make it necessary to develop the processes their extraction from liquid effluents . Several procedures o f separation are used for this
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May--2June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi; 10.1016/j.desal.2004.06.126
H. Hadjar et al. / Desalination 167 (2004) 165-174
end [4,7]. Adsorption seems to be one of the most nonspecific processes adopted for the extraction of metals from water. Several porous materials are suggested [8,9]. The present work describes our investigations on testing the efficiency of new inorganic composite materials (ICM) prepared from Kieselgithr and charcoal in the adsorption of lead. The two initial materials are being widely used as they are known for their different properties. Indeed, kieselgtihr is responsible for good adsorbent properties towards polar molecules. On the other side, charcoal that has a large surface area, is essentially microporous and hydrophobic, this being a good combination for the adsorption of non-polar molecules such as hydrocarbons. Since the adsorptive properties of both materials are complementary, a mixture of both could be of high interest for some specific applications such as separation and purification purposes.
Kieselgtihr/charcoal systems have undergone two procedures of treatments. First, during pyrolysis, each mixture was exposed to an inert gas flow up to 850°C with heating speed equal to 5°C/min and a landing of 1 h at the final temperature. After chemical treatment with hydrochloric acid and subsequent washing with distilled water, the samples obtained were dried overnight at 110°C and then preserved in hermetically closed bottles before their use. These treatments make the ICM strong enough to be used in adsorption. We kept three different ICMs for this study. The characteristics of the starting and final materials are listed in Table 1. 2.2. Preparation of lead solutions The chemical used for preparing lead-containing solutions was lead nitrate Pb(NO3) 2 Riedel-Ha~n, the metal has been taken in the divalent cation form with the oxidation degree +2 since it presents serious dangers of pollution [ 10].
2. Experimental 2.1. Preparation of adsorbents
2.3. Analysis o f lCM
The charcoal used in this work has been prepared from pine and carbonised at 400°C in order to eliminate some volatile organic compounds and then mixed in different percentages with natural KieselgUhr obtained from Sig in the west of Algeria. The mixtures were crushed and sieved to a particle size of less than 630 ~tm. Then,
ICM surfaces were photographed using scanning electron microscopy (SEM). The micrographs were taken by Philips XL30 apparatus. Specific surface area and textural characteristics were obtained from N 2adsorption-desorption data at 77 K with BET method. ASAP2010 analyser was used.
Table 1 Characteristics of materials used Reference C400 C850 KNT KT KC2 KC4 KC6
Proportions, % 100 100 100 100 90/10 80/20 70/30
Starting compositions C400 C850 KNT KNT KNT/C400 KNT/CA00 KNT/C400
Preparation Charcoal carbonisedat 400°C Treatedcharcoal UntreatedKieselgtihr Treated Kieselgiihr ICM treated mixture
H. Hadjar et al. ~Desalination 167 (2004) 165-174 2.4. Application of lCM 2.4.1. Kinetics measurements For each experiment, approximately 0.1 g of ICM was dissolved in 30 ml of metallic solution in a glass bottle for a given time period, pbE+concentration being fixed. The analyses were carried out using a bath technique with MEMMERT WB/ OB 7-45 WBU 45 agitator at free pH and room temperature (30+2°C). This agitation makes it possible to eliminate the phenomena of diffusion from the solution towards the material. The stirring speed was fixed at 120 rpm. This value is considered to be suitable since it is sufficient for the homogeneity of the mixture. The solutions were separated thereafter from the solid by filtration through Thomapor BA83 membrane filters (47 mm diameter) for analysis. The first 3 ml were rejected because of a possible adsorption of lead on the filter paper. The fraction F(t) of adsorbed ions at each time was calculated according to relation (1):
where F (t) is the fraction of adsorbed metal ions at time t on the solid; C--initial concentration of lead ions in the solution (mg/l); Ct - final concentration of lead ions in the solution (rag/l). The concentrations of the solutions before and after adsorption were measured by atomic adsorption. The measurements were made using in Perkin Elmer apparatus.
2.4.2. Effect of p H on the adsorption We have studied the effect ofpH on the adsorption capacity of KC4 towards lead ions using the same method as above. The concentration of Pb 2+ ions was fixed at 50 mg/l. The pH measurement of the initial solutions was carried out using a pHmeter pH 211: Hanna. The pH was adjusted by addition ofHCl andNaOH until the required value . From preliminary kinetic studies, it was
found that an equilibration time of a few minutes was presumed to be sufficient for adsorption to reach equilibrium. For consistency, a time of 2 h was chosen to stop the experiment. So in all cases the equilibrium was reached within 2 h.
2.4.3. Effect of lead concentration The study of the adsorption capacities of lead in various ICM solids was also done according to the remaining concentration C of Pb 2+ ions at equilibrium by contacting 0.1 g of ICM with 30 ml of lead solutions containing known different concentrations that were increased in the range 50-315 mg/1. During equilibration time, all solutions were shaken under the previous conditions. 3. Results and discussion
3.1. Analyses of lCM 3.1.1. Scanning electron microscopy (SEM) Measurements with scanning electron microscopy (SEM) enabled us to notice the surface and pore modifications probably favoured by the pyrolysis of the mixture at high temperature. The investigation has also shown the adhesion of silica (Kieselgtihr) with the carbonaceous product and given evidence of a coal deposition as well as a uniform distribution on the porous matrix surface [11,12] (Fig. 1).
3.1.2. Gas adsorption at 77K Fig. 2 that includes the adsorption desorpfion isotherms ofN 2 at 77 K on the starting materials (Kieselg~ihr and charcoal) treated separately, indicates clearly the difference in structure between the two adsorbents. Indeed, the charcoal, being a microporous solid, has an adsorption isotherm of type I in the BET classification, in contrast with that of the Kieselgiihr which is of type II characterizing a multilayer adsorption that concerns macroporous materials. Comparing these two curves with those of the graphs illustrated in
H. Hadjar et al. / Desalination 167 (2004) 165-174 KC~ ; 10% eharcc~l
KC4 = 20% c h ~ c o a l
KC6 ; 3&/. cha~oal
Fig. 1. SEM images of Kieselgiihr and ICM.
0.8 0.6 0.4 0.2 0
E 4E d 2-
- n - - C850
Figs. 3, 4 and 5 we can notice that new materials were obtained having an intermediate mesoporous structure. Indeed, N 2 adsorption gives a typical type IV o f the BET classification for all the three ICM adsorption-desorption isotherms which present hysteresis loops. A more detailed quantitative comparison o f surface area values is possible through the data of Table 2 which also includes some values o f the experimental parameters computed from the conventional BET equation. Thus, according to these results, the surface area increases favoured by the rate o f the added charcoal solid, until it
Fig. 2. N 2 (77 K) adsorption-desorption isotherms of KieselgCihrand charcoal.
Fig. 3. N2(77 K) adsorption-desorptionisotherms of KC2.
- - O - adsorption
E ~ 4 a 2 1 0
Fig. 4. N2(77 K) adsorption-desorptionisotherms of KCA.
tl. Hadjar et al. / Desalination 167 (2004) 165-174
Table 2 Experimental parameters of BET and specific surface of materials used Sample
Correlation coefficient ao,mmol/g
0.00886 0.0024 0.00166 0.01469
99.98 99.99 99.97 99.98
0.9978 2.0052 3.4525 1.2097
KC2 KC4 KC6 C850 8 6 "5 E E
0.9933 0.4963 0.2913 0.84135
113.11 207.79 48.07 56.27
98 196 336 118
II KC4 & KC6
Fig. 5. N 2(77 K) adsorption-desorption isotherms of KC6.
Fig. 6. Adsorption of lead on the various ICM as a function of shaking time at 30°C.
reaches the top. Thereafter, this surface undergoes a fall until arriving to that of coal alone. This appears thanks to the mixture of two materials. So this means that impregnation of Kieselgtihr and charcoal produces an increase in the specific surface, rendering a more developed texture.
the process gradually slows down until it reaches a stage (after about 45 min) indicating the establishment of the equilibrium of adsorption when the concentration in aqueous solution becomes constant. Beyond this time, the total capacity of adsorption does not change indicating that all the active sites were saturated. These observations resemble those given in previous works [1,1315]. In addition, the speed of fixing is related to the contact surface between the solid and the liquid phase. The ICM, having fine particles, support a fast adsorption of the metal ions and thus, a short residence time and a more significant flow. It can be noticed however that the level of saturation stage rises while passing from KC2 to KC6, which means that the adsorption capacity is favoured by the rise in the rate of present charcoal. By approximation, we can consider according to curves obtained that the retention of Pb 2+ ions consists of two distinct stages. The adsorption is fast in
3.2. Application of adsorption on ICM 3.2.1. Effect of shaking time We have traced for each solid a curve representing the variation in the adsorbed fraction F(t) of lead ions in aqueous solution against shaking time t as shown in Fig. 6. In spite of some differences between the three ICM solids, the results reveal that their behaviours are rather similar towards Pb 2+ ions. The plotted curves indicate that this kinetics is considered fast. So in the beginning and under the chosen conditions, the F(t) fraction increases quickly. Then
H. Hadjar et al. / Desalination 167 (2004) 165-174
the 1st stage. It becomes then slow in the 2nd stage and finally stabilizes when the surface of adsorption becomes practically saturated. The slowness of the second stage can be explained by the phenomenon of diffusion of the ions in the pores of ICM. Such assumption was already reported in other systems of adsorption [1,15]. The literature reveals the existence of different opinions concerning the diffusion giving place to various models for the prediction and the description of the process. However, these approaches converge at an identical total sight concerning the adsorption mechanisms. Such mechanisms could be the subject of a thorough study. However, we will be satisfied in our case to bring back the following mechanism which proposes 4 phases : • Transfer of matter of the solution towards the boundary layer surrounding the particle. • Transfer of the boundary layer towards adsorbent surface: external diffusion. • Transfer of surface towards the adsorbent sites: intraparticular diffusion (in the solid and the various pores). • Adsorption, complexation and precipitation of metal species. The adsorbing solid generally consists of highly porous particles characterized by a broad surface and a highly developed internal structure. This structure contains a complex network of pores and channels through the particles. This makes the diffusion dependent on the internal structure of adsorbate . Thus, if it is supposed that the solutions are perfectly shaken, the dispersion of the aqueous solution in the liquid phase will be considered homogeneous. This makes it possible to neglect the step of the aqueous solution transfer towards the boundary layer. On the other hand, the mechanisms of adsorption and complexation being quasi-instantaneous, the phase of precipitation can present a slower kinetics. This reasoning led to consider the diffusion between the boundary layer and the surface and that between the external surface and active intra-
particular sites as the principal steps controlling the kinetics of adsorption. This conclusion is corroborated by those of authors [ 16] who deduced the existence of a relation between the speed of adsorption and the diffusion coefficient. So, the mechanism ofintraparticular diffusion appears to be the dominating mechanism confirming the assumption suggested above. 3.2.2. Effect o f the solution p H
In order to check the contribution ofpH to the variation of the lead adsorption level, the variations of Q, expressed in milligrams of adsorbed solution per gram of adsorbent are schematically represented for the value range ofpH as shown in Fig. 7. According to this figure, the adsorption of Pb2+ ions seems to be strongly affected by the pH of the initial solution. The quantity Qe increases with the rise in the solution pH. It then starts to slow down to reach a constant value corresponding to the maximum of adsorption. These results were already reported in other studies [9,13]. The influence of the pH has also been deduced by a number of researchers [1,17] that allotted the phenomenon to oxydoreduction reactions. In fact, the behaviour of the metal ions in the aqueous solutions is a rather complex phenomenon since these ions can be present as species having various
16141210 8 6 4 2 0 0
pH Fig. 7. Adsorptionof lead on KC4 as a functionofpH at 30°C.
H. Hadjar et al. / Desalination 167 (2004) 165-I 74
compositions and various degrees of activity [ 13]. Several metals tend to form complexes with various ligands. These complexes are able to participate in reactions of oxydoreduction. The metal ions can thus be probably adsorbed according to various types of mechanisms such as ion exchange, surface complexation facilitated by the dissociation of the functional acid groups and others [14,18,15]. None of these types can be excluded. According to the literature, the variation of the pH depends on various factors and phenomena supposed to take place, in particular the formation of various metal species, the ionization degree, distribution of the surface sites , the load of the adsorbent surface, as well as a number of complexing agents. 3.2.3. Concentration in acid and formation o f hydroxides
In the beginning, the exchanges with the metal cations involve acidification in the medium which is unfavourable to the adsorption in the majority of cases. The degree of dissociation of the reactive functional groups resulting from surface is related to the presence of H ÷ and OH- ions or, in other words, to the acid or basic solution character [6,15]. The inhibiting effect of the acid conditions results in a competition between the metal species Pb z* and H30 ÷ ions, present in excess, for the attribution of the adsorption sites. During that, desorption can be favoured by this competition, which improves the instability of equilibrium. When the environment becomes less acid, the competition between protons and metal ions decreases, the fixing of the metal ions is thus favoured. The maximum capacity of adsorption is reached when the hydroxyl forms appear and become prevalent thanks to a very weak concentration in acid [ 14]. Indeed, chemistry of lead in solution leads to the formation of partially soluble hydroxides. Though the ion in this form can present a good affinity in term of adsorption with the solid supports, its diffusion is likely to be disturbed by phenomena of steric bulk related to the state
of hydration of the aqueous solution [ 16]. Beyond pH 8, precipitation appears . In brief, we can distribute the evolution of the curve obtained in our experiment as follows. Initially, Pb 2+ions exist in aqueous solution in such forms. The competitive adsorption of ions H30 + and that of Pb 2÷ varies in relation to the acidity of the solution. When the pH of the solution is below 4, the adsorption of H30 ÷ ions decreases favouring the fixing of the metal ions. Beyond pH 4, the metal ions start to be hydrolyzed leading to the formation of hydroxides such Pb(OH)2 that are slightly adsorbed, compared with Pb 2÷ ions. At this moment the adsorption starts slowing down to take again a light rise. After that, the process of adsorption cannot continue because of the formation of insoluble complexes in aqueous solutions . During the process of ion exchange, a substitution of ions held on the surface can lead to a rupture of bonds on the solid surface and the dissociation of the protons supported by the increase of pH. Besides, a possible charge distribution is created on the heterogeneous surface . Corapcioglu and Huang  reported that with low pH values, acid sites are produced by the reaction between the hydroxyl groups of surface and H ÷ ions in the solution. The surface of the ICM becomes thus positively charged. That involves an electrostatic repulsion between the metal ions and the surface sites which prevents their adsorption. By the increase of pH, the acid groups present on the surface liberate H ÷ ions by ionization in the aqueous solution. The surface thus acquires a negative charge which increases relatively to the rise of the acidic group rate on the surface . 3.2.4. Effect o f the initial lead concentration
The isotherms of adsorption of lead on the ICM are shown in Fig. 8. The curves of the adsorption isotherms obtained present a very fast rise of the capacity holding (in mg/g) at weak concentrations. Adsorption capacity continuous to increase with
H. Hadjar et al. / Desalination 167 (2004) 165-174
172 120 IO0
8o E ~" 60 £t 40
larger its value, the more significant is the capacity. n is the parameter of the Freundlich isotherm describing the distribution of the sites. It is considered as a factor of heterogeneity and its values lie between 0 and 1.
-.....I-. KC2 --I1-- KC4 KC6
Ce= 1 +Ce Qe Q , . b Q , .
Ce (mg/I) Fig. 8. Adsorption isothermsof lead on the studied ICM at 30°C. the rise in the initial concentration indicating that strong interactions take place between the exchange surface and the added cation . A saturation landing is obtained thereafter. This plate can be explained by the saturation of the active sites implied in the process of adsorption. By comparing the three solids, we can attest the beneficial contribution of Kieselgiihr and coal impregnation. When the process of adsorption starts, it continues until obtaining equilibrium between the adsorbate concentrations on the solid phase and the concentrations in the solution. Equilibrium that represents the end of the process, reflects the adsorbent capacity or affinity for a given aqueous solution . The experimental data for lead adsorption were adjusted to the isotherm models of Langmuir and Freundlich that are represented mathematically as follows :
By applying Eq. (4) that is a linear form of Eq. (2), a straight line is obtained by tracing C/Oe against C as shown in Fig. 9, indicating the conformity of experimental data of the various ICM with the model of Langmuir . Constants Qm and b obtained from the slopes and intercepts of curves are shown in Table 3. The constants of Freundlich can be evaluated by linearization of Eq. (3). Another equation [Eq. (5)] is thus obtained giving (lnQe) according to (lnC). The values of constants n and K were found 8 7 6 v
KC4. .~. KC6
Fig. 9. Linear form of the Langmuir isotherms for the adsorption of lead on differentICM.
where Q,, is the parameter of the Langmuir isotherm (mg/g); b - - the parameter of the Langmuir isotherm (1/mg); Q, - - mass of lead adsorbed at equilibrium (rag/g).
Table 3 Langmuirisothermparametersfor adsorptionof lead from aqueous solution on different ICM
ae = ICCJ
where K is the parameter of the Freundlich isotherm expressing the adsorption capacity. The
ICM KC2 KC4 KC6
Q=, mg/g 47.62 95.47 114.94
b, l/rag 0.0575 0.0105 0.0827
H. Hadjar et al. / Desalination 167 (2004) 165-174
3.5 3 C~ 2.5 2 1.5 1
• KC2 • KC4 & KC6
0.5 0 0
Fig. 10. Linearization of the Freundlich isotherms for the adsorption of lead on the used ICM. Table 4 Freundlich isotherm parameters for adsorption of lead from aqueous solution on different ICM ICM KC2 KC4 KC6
n 0.205 0.078 0.158
K, 1/mg 14.53 61.76 48.42
from the slope and the intersection of the righthand side in Fig. 10. In Qe = In K + n In C e
Parameter values for the Langmuir isotherms as well as those of the Freundlich isotherms for lead adsorption on the different ICM fitted with each corresponding model (Table 4). Moreover, the high values of K confirm the great affinity of the ions of lead towards the solid surface . 4. Conclusion The goal of this work was to determine the sorption behaviour of new composite materials ICM towards lead that belongs to the most harmful pollutants in order to test the efficiency of these solids as adsorbing materials. The undertaken study focused essentially on the examination of different factors treated separately: pH, shaking
time and metal concentration. So, it was observed that kinetics of adsorpion of metallic ions of lead was reasonably fast. Its study enables us to suppose that the adsorption of this metal on the particles of the ICM obey to the law of diffusional mechanisms in which the intraparticular diffusion plays a dominating role in the time necessary for equilibrium. The pH of initial solutions has also an influence on the adsorption of the metal ions studied. Indeed, the rate of the ions adsorbed on all the invested solids was considerably improved by the increase ofpH. Adsorption is also improved by the increase of the initial concentration of Pb 2+ in aqueous solution. The isotherms of adsorption obtained are in agreement with the models of Langmuir and Freundlich in the whole range of the concentrations studied. In what precedes, various aspects affecting adsorption were underlined, in particular, the formation of metal hydroxides, the ionization level, the ionic ray, the distribution of the surface functional sites, as well as the charge of the surface of adsorbent. After using scanning electron microscopy (SEM) and the gas adsorption techniques, it can be noticed that the prepared ICM are characterised by a very developed porous texture and high specific surfaces as well as by the adherence of charcoal on the silicic matrix and the obtaining of a mesostructure onto the different solids. The results presented here and their analyses indicate that the two applied treatments lead to a considerable enhancement of the properties on the prepared ICM with a good adsorption capacity. Hence, the different ICM show an ability to adsorb metal ions, and so can be successfully used in the environmental applications.
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