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Adsorption of zinc (Zn2+) from aqueous solution on natural bentonite
Desalination 267 (2011) 286–294
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Adsorption of zinc (Zn2+) from aqueous solution on natural bentonite Tushar Kanti Sen ⁎, Dustin Gomez Department of Chemical Engineering, Curtin University of Technology, Perth, GPO Box U1987, 6845 Western Australia, Australia
a r t i c l e
i n f o
Article history: Received 7 January 2010 Received in revised form 17 August 2010 Accepted 24 September 2010 Available online 25 October 2010 Keywords: Zinc adsorption Bentonite Kinetic model Adsorption isotherm
a b s t r a c t The adsorptive properties of natural bentonite in the removal of zinc (Zn2+) from aqueous solution were studied. The results show that the amount of adsorption of zinc metal ion increases with initial metal ion concentration, contact time, and solution pH but decreases with the amount of adsorbent and temperature of the system. Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn2+) on bentonite is a two step process: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow decreasing intraparticle diffusion in the interior of the adsorbent which has also been confirmed by intraparticle diffusion model. Overall the kinetic studies showed that the zinc adsorption process followed pseudo-second-order kinetic model. The different kinetic parameters including rate constant, half adsorption time, and diffusion coefficient are determined at different physicochemical conditions. The equilibrium adsorption results are fitted better with Langmuir isotherm compared to Freundlich models. The value of separation factor, RL from Langmuir equation and Freundlich constant, n both give an indication of favorable adsorption. Finally in thermodynamic studies, it has been found that the adsorption process is exothermic due to negative ΔH0 accompanied by a decrease in entropy change and Gibbs free energy change (ΔG0). © 2010 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ion pollution is currently of great concern due to its increased awareness of the potentially hazardous effects of elevated levels of these materials in the environment [1–3]. Presently, it is appearing as an increasing and alarming challenge to the researchers and environmental control agencies for polluting the water and soil resources severely through the indiscriminate disposal of metals in the environment. Major sources of zinc in the environment are the manufacturing of brass and bronze alloys and galvanization [4,5]. It is also utilized in paints, rubber, plastics, cosmetics and pharmaceuticals [4]. Zinc is an essential element for life and acts as a micronutrient when present in trace amounts [5]. The WHO recommended the maximum acceptable concentration of zinc in drinking water as 5.0 mg/L [6]. Beyond the permissible limits, Zn2+ is toxic [7]. The availability of micronutrients and toxins in soils also depends on the interaction of these materials with oxide or oxyhydroxide particles, clay minerals and organic matter. Clay minerals such as kaolinite, illite and montmorillonite are abundant in natural soil systems. Usually soils serve as important sinks for contaminant metals, however clay minerals and organic contents are important components that control the sorption/retention capacity of soils towards trace metals. Moreover, the fate and transport of metal ions including Zn2+ in natural water as well as in water treatment/
⁎ Corresponding author. Tel.: + 61 8 92669052. E-mail address: [email protected] (T.K. Sen). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.041
industrial waste water treatment processes are often controlled by their reactions with adsorbents under different environmental conditions [8]. Many technologies have been developed to remove heavy metals from contaminated waters, such as chemical precipitation, ion exchange, filtration, solvent extraction and membrane technology and adsorption [5]. Some of these methods have disadvantages and limitations. Precipitation, for example produces large amount of sludge in solution [9] but membrane filtration, ion exchange, electro-deposition and filtration are costly [9]. Alternatively, adsorption can remove these metals efficiently at low cost and also very simple in operation. Several solid materials can be employed as adsorbents. Adsorption on activated carbon is the conventional methods for the removal of heavy metal ions from aqueous solutions but its high cost limits its use [5]. Therefore, the adsorption is used especially in the water treatment field and the investigation has to be made to determine inexpensive and good adsorbent. Clay is one of potential good adsorbent alternatives to activated carbon because of its large surface area, high cation exchange capacity, chemical and mechanical stability and layered structure. The presence of both bronsted and Lewis types of acidity in clays [3,10] boosts the adsorption capacity of clays. Aluminum oxides and clay minerals such as kaolin, bentonite are the most wide-spread minerals of the earth crust which are known to be good adsorbent of various metal ions, inorganic anions and organic ligands [11–16]. The abundance of bentonite and its low cost are likely to make it a strong candidate as an adsorbent for the removal of heavy metal from wastewater [17]. Bentonite is a 2:1 mineral with one octahedral sheet and two silica sheets, which forms a layer [18]. Layers are held together by van der
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Waals forces. Because of these weak forces and some charge deficiencies in the structure, water can easily penetrate between layers and cations balance the deficiencies [18]. Bentonite attracted the researcher's concern for a long time and has been studied extensively. Most of these studies considered the adsorption characteristics of bentonite for some toxic elements. It is well documented that bentonite is an efficient adsorbent for some heavy metals, especially for lead [19,20], copper [21], cadmium [19,22] and zinc [18]. Therefore, there are a number of studies that have been reported using clays, mainly montmorillonite to show their effectiveness for removal of metal ions such as Zn2+, Pb2+ and Al3+ from aqueous solution [22–25]. However, systematic studies on zinc (Zn2+) adsorption characteristics on bentonite under various physicochemical parameters are limited and also are very scarce. For this reason, a detailed study was conducted in order to determine the influence of initial solution pH, initial metal ion concentration, adsorbent doses and temperature changes on adsorption characteristics of natural bentonite. Another reason for this study is the importance of adsorption on solid surfaces in many industrial applications in order to improve efficiency and economy. Therefore, it is essential to understand the mechanism and kinetics of adsorption, because the studies of adsorption kinetics are ultimately a prerequisite for designing an adsorption column [11]. This study will also help to design the effective composite reactive barrier in contaminant leaching and also in the field of cost-effective water filter design. Equilibrium adsorption isotherms or capacity studies are of fundamental importance in the design of adsorption system. This study also gives new data set within this small concentration range of metal ion. It has been found that the amount of adsorbed zinc metal ion increases with initial metal ion concentration, contact time and with the solution pH but decreases with adsorbent doses and temperature respectively. The kinetic adsorption results have been analyzed using pseudo-first-order, pseudo-2nd-order reactions and intraparticle diffusion model respectively. The isotherm equilibrium results have been fitted more with Langmuir and less with Freundlich isotherm model respectively.
2. Materials and methods 2.1. Materials 2.1.1. Adsorbent and characterization The adsorbent being used is laboratory grade reagent bentoniteK10 clay obtained from ACROS ORGANICS, Germany. The particle size distribution was determined by using the Malvern MasterSizer 2000S with the Hydro 2000S (A). Spectrum 100 FT-IR Spectrometer with a Universal ATR Sampling Accessory with MIR detector from Perkin Elmer was used to determine the functional groups of bentonite. Bentonite sample was also analyzed by an X-ray diffractometer.
2.1.2. Adsorbate (Zn2+) and other chemicals All chemicals used were of analytical grade. Stock standard solution of Zn2+ has been prepared by dissolving the appropriate amount of its nitrate in deionized water, acidified with small amount of nitric acid. This stock solution was then diluted to specified concentrations. The pH of the system was adjusted using reagent grade NaOH and HNO3 respectively. All plastic sample bottles and glassware were cleaned, then rinsed with deionized water and dried at 60 °C in a temperature controlled oven. All measurements were conducted at a room temperature of 30 °C. The concentration of Zn2+ was measured using Varian 110 Atomic Absorption Spectrophotometer (AAS). The pH was measured by Orion pH meter.
287
2.2. Adsorption experiments 2.2.1. Adsorption procedure Adsorption measurements were determined by batch experiments. For this purpose, 0.01 g of bentonite and 40 mL of aqueous Zn2+ solutions at specified concentration were put on a shaker using Thermo line Scientific Orbital Shaker Incubator at 80 rpm at 30 °C for a given time. The suspensions were then filtered through a micro filter of pore size 0.47 μm and the filtrates were analyzed using flame atomic absorption spectrophotometer with air-acetylene flame. The pH of the solutions was initially adjusted by addition of small amount of either 0.1 M HCl or 0.1 M NaOH solutions. The experiments were carried out by varying concentrations of initial Zn2+ solution, contact time, amount of adsorbent, temperature and pH of initial suspension. The Zn2+ concentration retained in the adsorbent phase, qt (mg/g) was calculated according to following relation qt =
ðC0 −Ct ÞV m
ð1Þ
where C0 (mg/L) and Ct (mg/L) are the concentration in the solution at time t = 0 and at time t, V is the volume of solution (L) and m is the amount of adsorbent (g) added. 2.2.2. Effect of initial solution pH on metal ion adsorption kinetics In this study the sorbent (0.01 g) and 40 mL of 30 ppm (mg/L) Zn2+ solution were mixed in plastic bottle. The pH of the mixture was adjusted either by 0.1 M HCl or 0.1 M NaOH solution until the initial pH was close to the target value ranged from 3.81 to 7.69. The whole mixture was taken in a series of 50 mL plastic bottles and put on a Thermo line Scientific Orbital Shaker Incubator at 80 rpm and at 30 °C for a period of 180 min. Speed was such that it maintains the contents completely mixed and the adsorbents were suspended throughout the plastic bottle. The samples were then collected in different time intervals throughout equilibrium time period and filtered each time through a micro filter. The left out concentrations in the filtrate solution was analyzed using flame atomic absorption spectrophotometer. The quantity of adsorbed metal ion on bentonite was calculated as the difference between initial concentration and concentration at any time, t as per (Eq. (1)). Each experiment was repeated in twice to check the reproducibility. Measurements are, in general, reproducible within ± 10%. 2.2.3. Effect of adsorbent dose on metal ion adsorption kinetics The adsorbent dosage in the aqueous solution was increased from 0.01 g to 0.03 g in 40 mL of metal ion. The effect of adsorbent mass on the amount of metal ion adsorbed was obtained by contacting 40 mL of Zn (II) ion solution of initial concentration of 30 ppm (mg/L) with different weight amounts of 10 mg, 20 mg and 30 mg of adsorbent using Thermo line Scientific Orbital Shaker Incubator. The shaker was running at a temperature of 30 °C and a constant speed of 80 rpm for specified time. The experiments were carried out at initial solution pH of 6.76. At each time intervals, the samples were then filtered and the supernatant solution was analyzed as per Section 2.2.1. 2.2.4. Effect of temperature on metal ion kinetic adsorption kinetics The batch adsorption experiments were carried out with 40 mL Zn(II) metal ion solution of 30 ppm at 30, 50 and 65 °C separately by contacting with 0.01 g of adsorbent using Thermo line Scientific Orbital Shaker Incubator. The speed of incubator was 80 rpm for a period of 180 min. The solution pH was 6.76. 2.2.5. Equilibrium isotherm experiments For isotherm studies, a series of 50 mL plastic bottles containing 40 mL of Zn2+ metal ions solutions of known concentrations, varying from 10 to 90 mg/L were prepared. Identical amounts (0.01 g) of
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bentonite were added to the each bottle and the resulting suspensions were agitated on a Thermo line Scientific Orbital Shaker Incubator at a speed of 80 rpm and 30 °C for 4 h at a constant pH of 6.64. After equilibrium time, the suspensions were then filtered through a micro filter of pore size 0.47 μm and the filtrates were analyzed using flame atomic absorption spectrophotometer with air-acetylene flame as per Section 2.2.1. 3. Results and discussions 3.1. Characterization of adsorbent The characterization of the structure and surface chemistry of the adsorbent is of considerable interest for the development of adsorption and separation processes. One important characteristics of an adsorbent is the surface functional groups present which are largely characterized by the FT-IR spectroscopy method. FT-IR spectroscopy of bentonite (Fig. 1) indicated the presence of hydroxyl, carboxyl, and Si–O which are important sorption sites. The peaks at 3411.6 and 3625 cm− 1 are due to lattice OH and bound water stretching vibrations. A strong and sharp band is detected at 1022 cm− 1 which is related to Si–OH stretching vibrations [9]. The X-ray diffraction analysis (Fig. 2) identifies four different mineral phases present: mainly quartz (SiO2), and muscovite ((K,Na)(Al,Mg,Fe)2(Si3.1Al0.9)010(0H)2) and trace amount of kaolinite (Al2Si205(0H)4) and albite respectively. The particle size distribution of bentonite which is not shown here for which specific surface area was 0.208 m2/g. By taking the average surface weighted mean of each of the trials, the mean particle size for the bentonite used was 5.04 μm. 3.2. Effect of initial solution pH on metal ion kinetic experiment The pH of the adsorbate solutions is an important parameter governing adsorption on different adsorbents [5,8,12,26–28]. In principle, the dependence of metal uptake on pH can be associated with both the surface functional groups on the adsorbent as well as the metal chemistry of the solution [35]. The effect of pH on the sorption of Zn2+ ions onto bentonite was studied by changing initial solution pH values in the range of 2–8 and results are presented in Fig. 3. It has been
found from Fig. 3 that the adsorption of zinc (Zn2+) on bentonite increased with time and increased with pH up to 6.76 and then declined with a further increase in pH. In general, most metal sorption is enhanced with pH, increasing to a certain value followed by a reduction on further pH increase. The increase in zinc (Zn2+) adsorption with increasing pH (to pHmax = 6.76) is similar to the adsorption pattern of other hydrolyzable metal cations and maximum adsorption takes place at pH 6.76. It is known that metal species (M (II) = Zn2+) are present in deionized water in the forms of M2+, M(OH)+, M(OH)2(S), etc [18]. At pH~ 5.0, the solubility of the M(OH)2(S) is high and therefore, the M2+ is the main species in the solution [5,28–30]. With the increase of the pH value, the solubility of M(OH)2(S) decreases and at pH~ 10.0, the solubility of M(OH)2(s) is very small [5]. At this time, the main species in the solution is M(OH)2(S). Therefore in the alkaline range, the metal ion precipitation plays the main role in the removal of the M(II) ions attributed to the formation of precipitate of M(OH)2(S). To avoid precipitation of the metal ions, all the experiments were carried out at a maximum initial solution pH of 7.7. It has also been found from Fig. 3 that the amount of zinc metal ion adsorbed per gram of bentonite is very fast from the beginning up to 10 min and it increases with time as well as with increase in pH or alkalinity. With a further increase in time, the amount of adsorption decreased progressively and finally reached an equilibrium stage within 80 min in all cases. Maximum adsorption takes place within 10 min at each pH. The fast adsorption at the initial stage is probably due to the increased concentration gradient between the adsorbate in solution and adsorbate in adsorbent as there must be increased number of available surface sites in the beginning [5]. Adsorption of heavy metals onto various clay minerals occurs in two or more stages [31–34]. Fig. 3 also shows that zinc metal ion adsorption on bentonite occurs more or less in two stages: a very rapid fast reaction followed by decreasing equilibrium plateau. The oxygen atoms present on the clay surface interact with water in an acidic medium forming some aqua complexes [3], which result in positive charge formation as follows: þ
−
– MO þ H–OH → M–OH2 þ OH :
ð2Þ
The surface charge is responsible for preventing Zn2+ ions from approaching the surface and explains the smaller extent of adsorption
Fig. 1. FT-IR of bentonite sample.
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Fig. 2. XRD diffractogram of the analyzed bentonite sample.
The results of the kinetic experiments with varying adsorbent concentrations are presented in Fig. 4. It has been found that maximum zinc uptake took place within 10 min as initial speed of reaction is very fast and the amount of Zn2+ adsorbed per unit mass of
adsorbent decreases as the adsorbent mass increases. Several other investigators have also reported the same trend of other adsorbent concentration effect on metal ion adsorption which has been mentioned in Sen et al. [5,14,26]. Although the number of adsorption sites per unit mass of an adsorbent should remain constant, independent of the total adsorbent mass, increasing the adsorbent amount in a fixed volume reduces the number of available sites as the effective surface area is likely to decrease [3]. Various investigators have also offered different explanations for the observed dependency. These explanations can be categorized into two groups: (1) ‘real’ physical/chemical processes and (2) experimental artifacts. One possible explanation is that the particle concentration effect is thought to be caused by particle–particle interactions. In system with higher solid content, these interactions are perhaps physically blocking some adsorption sites from the adsorbing solutes and thus, causing decreased adsorption, or creating electrostatic interferences such that the electrical surface charges on the closely packed particles diminish attractions between the adsorbing solutes and surfaces of individual grains. In general, pH of final solution increases gradually with the increase in adsorbent dosage [36]. It may be attributed to an evident
Fig. 3. Effect of initial solution pH on Zn2+ adsorption. Initial Zn2+ = 30 ppm, amount of bentonite added = 10 mg; temperature = 30 °C; shaker speed = 80 rpm.
Fig. 4. Effect of amount adsorbent bentonite on zinc metal ion adsorption kinetics. Initial Zn2+ = 30 ppm; initial solution pH= 6.76, temperature= 30 °C; shaker speed = 80 rpm.
at low pH [3]. Basically, at low pH, due to high positive charge density, electrostatic repulsion will be high during uptake of metal ions resulting in lower adsorption. In an alkaline medium, the clay surface becomes negatively charged favoring Zn2+ uptake as per the following: −
– MOH þ OH
2þ
– MO þ Zn
−
¼ –MO þ H2 O
ð3Þ
2þ
¼ – M–O Zn –
ð4Þ
The previously mentioned fact related to the effect of pH on adsorption also supported by many earlier workers [2,3,14,15,35]. 3.3. Effect of adsorbent dosage on metal ion kinetic adsorption
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increase of the amount of negatively charged sites which can induce more H+ ions adsorb on the bentonite surface and results in an increase in pH of the final solution.
3.4. Kinetics of zinc metal ion adsorption 3.4.1. Effect of contact time and kinetics of adsorption Fig. 5 represents a plot of the amount of zinc metal ion adsorbed (mg/g) versus contact time for different initial metal ion concentration of 30, 40 and 50 ppm (mg/L) respectively. From these plots, it is found that the amount of adsorption i.e. mg of adsorbate per gram of adsorbent increases with increasing contact time at all initial metal ion concentrations and equilibrium is attained within 80 min. Further it is observed that the amount of metal ion uptake, qt (mg/g) is increased with an increase in initial metal ion concentration. This kinetic experiment clearly indicates that adsorption of zinc metal ion (Zn2+) on bentonite clay is a more or less two step process similar to experiments by Sen et al. [31–34] and Arias and Sen [5] kaolin and oxides: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow intraparticle diffusion in the interior of the adsorbent. This two stage metal ion uptake can also be explained as adsorption occurring onto two different types of binding sites on the adsorbent particles. The rapid kinetics has significant practical importance, as it facilitates smaller reactor volumes, ensuring high efficiency and economy [37]. In order to investigate the mechanism of adsorption, particularly potential rate-controlling step, the transient behavior of the zinc metal ion (Zn2+) adsorption process was analyzed using the pseudofirst-order, pseudo-second-order and intraparticle diffusion model. As per Lagergren pseudo-first-order model, plot of log (qe − qt) versus t gives a straight line which allow computation of the rate constant K1 [26,27] where qt and qe represent the amount of Zn(II) metal ion adsorbed (mg/g) at any time t and at equilibrium time, respectively. Fig. 6 shows plot of log (qe − qt) versus t gives a straight line as with very poor linear regression coefficient (R2) of 0.39 to 0.94. The rate constant K1 for three different initial metal ion concentration of 30, 40 and 50 ppm are 0.0713, 0.0552 and 0.0739 min− 1 respectively. Moreover pseudo-first-order kinetic model predicts a much lower value of the equilibrium adsorption capacity, (qe) than the experimental value which is not shown here and hence it gives the inapplicability of this model. The same pseudo-first-order model are fitted with kinetic adsorption experimental results in case of initial solution pH effect and amount of adsorbent respectively but it gives very poor linear regression coefficient, R2 value which are not presented here.
Fig. 5. Effect of contact time on zinc metal ion adsorption. Amount of bentonite = 10 mg; temperature= 30 °C; initial solution pH= 6.76; shaker speed= 80 rpm.
Fig. 6. Pseudo-first-order kinetic model for zinc adsorption by bentonite at different initial metal ion concentrations. Amount of adsorbent = 10 mg. Initial solution pH= 6.76, shaker speed = 80 rpm, and temperature = 30 °C.
The adsorption data were then analyzed using the pseudo-secondorder kinetic model. The pseudo-second-order model based on equilibrium adsorption is expressed as [26,27] t 1 1 = + t: qt qe K2 q2e
ð5Þ
A plot between t/qt versus t gives the value of the constants K2 (g/ mg h) and also qe (mg/g) can be calculated. The constant K2 is used to calculate the initial sorption rate h, at t → 0, as follows 2
h = K2 qe :
ð6Þ
Thus the rate constant K2, initial adsorption rate h and predicted qe can be calculated from the plot of t/q versus time t using Eq. (5). Figs. 7–9 represents the pseudo-second-order kinetic plots between t/qt versus time t for zinc adsorption at different initial
Fig. 7. Pseudo-second-order kinetic model for zinc (Zn2+) at different initial metal ion concentrations. Amount of bentonite added = 10 mg; initial solution pH = 6.76; temperature = 30 °C; shaker speed = 80 rpm.
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Table 1 Pseudo-second-order kinetic parameters. Parameters
Fig. 8. Pseudo-second-order kinetic model for zinc (Zn2+) adsorption at different initial solution pH. Initial Zn2+ = 30 ppm; amount of bentonite= 10 mg; temperature= 30 °C; shaker speed = 80 rpm.
metal concentrations, solution pH effect and amount of adsorbents respectively. Similar plot at different temperatures is not shown here. High linear regression coefficients (R2) suggest that the Zn2+ adsorption experiment follows pseudo-second-order kinetics. The initial sorption rate is the h = K2 q2e . The equilibrium adsorption capacity (qe) and second order constant (K2) can be determined from the slope and intercept of plot t/q versus t (Figs. 7–9). All these parameters including the linear regression coefficients, R2, the pseudo-second-order rate constant, K2 and equilibrium sorption capacity, qe were calculated and tabulated in Table 1. Higher linear regression coefficients (R2) with respect to fitted pseudo 1st-order model suggest that adsorption of zinc metal ion on kaolin follows pseudo-second-order kinetics. Moreover, calculated, qe values from pseudo-second-order fitting model is very close to the experimental qe values (Table 1) also suggesting the suitability of this model. Also this suggests the assumption behind the pseudo-second-order model that the metal ion uptake process is due to chemisorptions [26] and more than one-step may be involved in sorption processes. Also from Table 1 the adsorption capacity increases with an increase in initial metal ion concentration, in initial solution pH but decreases with amount of adsorbent and temperature respectively. Similar type model parameters are also obtained by various researchers for different systems [26,38]. 3.4.2. Intraparticle diffusion and adsorption mechanism For the process design and control of adsorption systems, it is important to understand the underlying mechanism that results in
Fig. 9. Pseudo-second-order kinetic model for zinc (Zn2+) adsorption at different amounts of bentonite added. Initial Zn2+ = 30 ppm; initial solution pH = 6.76; temperature = 30 °C; shaker speed = 80 rpm.
qe,
exp
(mg/g)
K2 (g/mg min)
qe,
cal
(mg/g)
Initial metal concentration (ppm) 30 7.5 0.1284 40 10.0 0.01452 50 47.2 0.03942
7.9744 9.8911 47.1698
Solution pH 3.81 6.76 7.69
0.02030 0.049856 0.02439
11.933174 46.948 28.4900
Adsorbent dose (mg) 10 47.4 20 20.7 30 16.6
0.047757 0.01613 7.2240
46.948 20.661 16.6389
Temperature 30 50 65
0.03511 0.01504 0.04255
47.169 36.0628 34.3642
11.8 47.4 28.8
(°C) 47.4 37.2 34.2
R2
h 8.16935 1.42054 87.709
0.9852 0.9765 0.9958
2.8907 109.888 19.79687
0.9847 0.9954 0.9978
105.2619 6.8855 1999.98
78.1249 19.5690 50.24722
0.9954 0.9931 0.9963
0.9957 0.9931 0.9965
the apparent dynamic behavior of the system. The removal of Zn(II) metal ion by adsorption on bentonite was found to be rapid at the initial period of contact time and then become slow and stagnate with increase in contact time. For a solid/liquid sorption process, the solute transfer is usually characterized by either external mass transfer (boundary layer diffusion) or intraparticle diffusion or both. The mechanism for the removal of Zn2+ by adsorption may be considered into the following steps [39] (a) Migration of metal ion from the bulk of the solution to the surface of the adsorbent (b) Diffusion of metal ion through the boundary layer to the surface of adsorbent (c) Adsorption of metal ion at an active site on the surface of adsorbent (d) Intraparticle diffusion of metal ion into the interior pore structure of adsorbent. The overall rate of sorption will be controlled by the slowest step, which would be either film diffusion or pore diffusion. However, the controlling step might be distributed between intraparticle and external transport mechanisms. Whatever the case, external diffusion will be involved in the sorption process. The sorption of Zn2+ metal ion onto bentonite particles may be controlled due to film diffusion at earlier stages and as the adsorbent particles are loaded with metal ions, the sorption process may be controlled due to intraparticle diffusion. The most commonly used technique for identifying the mechanism involved in the sorption process is by fitting the experimental data in an intraparticle diffusion plot. The intraparticle diffusion plot is the plot of amount sorbed per unit weight of sorbent, qt (mg/g) versus square root of time, √t is shown in Fig. 10 for initial Zn2+ metal ion concentration of 30, 40 and 50 ppm. The other intraparticle diffusion model fitting plot at different pH, different adsorbent dosages and at different temperatures are not shown here but the nature of plot was similar to Fig. 10. Fig. 10 shows that the adsorption plots are not linear over the whole time range and can be separated into two–three linear regions which confirms the multi stages of adsorption. This plot represented the two different stages viz. external mass transfer followed by intraparticle diffusion, signified that the metal ions were transported to the external surface of the bentonite particles through film diffusion and its rate was very fast. After that, metal ions were entered into bentonite particles by intraparticle diffusion through pores. Generally, when adsorption steps are not dependent of one another, the plot of qt against t0.5 should give two or more intercepting lines depending on the actual mechanism [40]. Moreover from Fig. 10,
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Fig. 11. Freundlich adsorption isotherm.
Fig. 10. Effect of contact time on amount of Zn2+ adsorption as per diffusion model.
conclusion can be made that none of plot gives linear straight line segment passing through the origin, I ≠ 0 which is not shown here. This indicates that the intraparticle diffusion is involved in the adsorption process but not the only rate-controlling step. As lines did not pass through the origin indicating that film diffusion and intraparticle diffusion occurred simultaneously. Some other mechanisms such as complexation or ion exchange may also control the rate of adsorption [41]. The diffusion coefficient, D, largely depends on the surface properties of adsorbents. The diffusion coefficient for the intraparticle transport of different initial concentrations of metal ions were also calculated using the following relationships [39] 2
t1 = 0:03 r0 = D
ð7Þ
2
and t1 = 2
1 ðK2 qe Þ
ð8Þ
where t1/2 is the half life of adsorption in seconds, K2 is the pseudosecond-order rate constant and r0 is the radius of the adsorbent particle in centimeters and D is the diffusion coefficient value in cm2/s. The value of r0 was calculated as 5.04 × 10− 4 cm. The behavior of the concentration dependent diffusivity agrees with literature works [39,42]. The diffusion coefficients, D values were found to be 7.33 × 10− 9 cm2/s, 1.10 × 10− 9 cm2/s and 1.41 × 10− 8 cm2/s for an initial methylene blue concentration of 30, 40 and 50 ppm respectively.
Fig. 11 gives results on Freundlich isotherm fittings for bentonite with linear regression coefficient of 0.90098. Freundlich constants i.e. adsorption capacity, Kf and rate of adsorption, n, are calculated from this plot which are 1.4845 mg/g and 1.23 L/mg respectively. The value of ‘n’ is larger than 1 which indicates the favorable nature of adsorption [43,44]. Similar type Freundlich parameters are obtained by many researchers [5,7,46] for zinc adsorption on different inorganic adsorbents. Fig. 12 gives results on Langmuir-2 isotherm fittings for bentonite adsorbent. The maximum adsorption capacity of Zn2+, qm, and constant related to the binding energy of the sorption system, Ka is calculated which are 68.4931 mg/g and 0.019850 respectively for Znbentonite system. The reported maximum zinc adsorption capacity of various inorganic adsorbents such as phosphatic clay is 25.10 mg/g [46], natural zeolite is 3.66 mg/g [45], granular activated carbon is 10.70 mg/g [45], bentonite is 21.1 mg/g [18], activated alumina is 13.29 mg/g [7], activated carbon 31.11 mg/g [6] and kaolin is 250 mg/g [5] respectively. The separation factor, RL has been calculated from Langmuir plot as per the following relation [43]:
RL =
1 : 1 + Ka C0
ð11Þ
It has been found that the range of RL is 0.83437 to 0.358873 for this initial metal ion concentration range of 10 to 90 ppm which gives favorable adsorption as it lies in b0 b RL b 1. From Figs. 11 and 12, it is also seen that Langmuir isotherms usually fitted better with the
3.5. Comparison of adsorption equilibrium isotherm Adsorption isotherms of bentonite for Zn2+ metal ion were expressed mathematically in terms of the Langmuir and Freundlich isotherm models. The obtained experimental data are commonly well fitted with the Freundlich (Eq. (9)) and Langmuir (Eq. (10)) models: ln qe = ln Kf + 1 = qe
1 ðln Ce Þ n
1 1 1 + Ka qm Ce qm
ð9Þ
ð10Þ
where Kf, n and Ka, qm are the constants for Freundlich and Langmuir models respectively.
Fig. 12. Langmuir adsorption isotherm.
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experimental data rather than Freundlich isotherms which is also reported by Kaya and Oren [18]. 3.6. Thermodynamics for adsorption and effect of temperature To observe the effect of temperature on the adsorption capacity, experiments are carried out in three different temperatures of 30, 50 and 65 °C for a fixed initial metal ion concentration of 30 ppm. It has been found from Fig. 13 that with an increase in temperature, adsorption capacity decreases. This is mainly because of decreased surface activity suggesting that adsorption between Zn2+ metal ion and bentonite was an exothermic process. With increasing temperature, the attractive forces between the bentonite surface and metal ion are weakened and then sorption decreases. Similar type's results are also obtained by earlier investigators for different adsorbent systems [3,26,31,47,48] Thermodynamic parameters such as Gibb's free energy (ΔG0), enthalpy change (ΔH0) and change in entropy (ΔS0) for the adsorption of zinc on bentonite have been determined by using the following equations [38]:
Fig. 14. Van't Hoff plot for adsorption of zinc metal ions.
3.7. Desorption studies 0
0
0
ΔG = ΔH −TΔS
log
qe ΔS0 −ΔH 0 + = Ce 2:303 R 2:303 RT
ð12Þ
ð13Þ
where qe is the amount of zinc adsorbed per unit mass of bentonite (mg/g), Ce is equilibrium concentration (mg/L) and T is temperature in K. qe/Ce is called the adsorption affinity. The previously mentioned equation is for unit mass of adsorbent dose. The values of Gibbs free energy (ΔG0) have been calculated by knowing the value of enthalpy of adsorption (ΔH0) and the entropy of adsorption (ΔS0) which are obtained from slope and intercept of a plot of log (qe/Ce) versus 1/T shown in Fig. 14. All these thermodynamic parameters are presented in Table 2. The exothermic nature is also indicated by the decrease in the amount of adsorption with temperature (Fig. 13). The negative value of ΔH0 and ΔG0 indicates (Table 2) that the adsorption processes are spontaneous and exothermic in nature. The negative value of ΔS0 suggests decreased randomness during adsorption [38,49]. Similar results for thermodynamic parameters were also reported by the earlier workers for the adsorption of Zn2+ as well as other heavy metals from aqueous solution [3,4,50,51].
Desorption studies of Zn (II) metal ions were conducted to explore the possibility of recycling of bentonite and recovery of the metal ions. Approximately 0.02 g of metal ion loaded bentonite from the previous adsorption experiment was stirred with 40 mL of 0.1 M HCl solution for 24 h. The filtrate was analyzed for desorbed metal ions using Atomic Absorption Spectrophotometer. The same adsorbents were washed with distilled water for several times and the concentration of released metal ions from washing process was again analyzed. The total desorption for Zn (II) metal ion was around 42%. The acid solution (0.1MHCl) used for desorption has a pH of 2.05. As such, the amount of metal ion (58% for Zn (II)) retained in the bentonite after acid treatment and partial elution of metal represent the equilibrium adsorption at pH 2.05. A complexing agent such as EDTA, ethylene diamine may be much more effective for recovery of metals to compensate for the additional cost of the chemicals. A systematic work is under progress. 4. Conclusions The removal of zinc (Zn2+) metal ions from aqueous solution by bentonite was investigated. The following conclusions can be drawn based on this investigation: • The adsorption characteristics of zinc metal ion (Zn2+) are strongly affected by initial solution pH, initial metal ion concentration, amount of adsorbent and temperature respectively. It has been found that the amount of metal ion (Zn2+) adsorption on natural bentonite increases with initial metal ion concentration, contact time, and solution pH respectively but decreases with the amount of adsorbent and system temperature respectively. • It is also been found that the amount of adsorption i.e. mg of adsorbate per gram of adsorbent increases with increasing contact time at all initial metal ion concentrations and equilibrium is attained within 80 min for Zn-bentonite systems at a fixed solution pH. The adsorption process is strongly dependent on the pH of the medium with enhanced adsorption as the pH turns from acidic to Table 2 Thermodynamic parameters for zinc adsorption at different temperatures.
Fig. 13. Effect of temperature on kinetic adsorption of Zn2+ on bentonite.
Temperature (°C)
ΔG0 (kJ mol− 1)
ΔH0 (kJ mol− 1)
ΔS0 (j mol− 1 K− 1)
30 50 65
− 2.3586 − 1.7014 − 1.211
− 12.3143 − 12.3143 − 12.3143
− 0.03285 − 0.03285 − 0.03285
294
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alkaline side till precipitation sets in. The maximum adsorption takes place at an optimum pH of 6.76. The process is very fast initially and maximum adsorption is obtained within 20 min. • Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn2+) on bentonite is more or less a two step process: a rapid adsorption of zinc metal ion to the external surface followed by intraparticle diffusion into the interior of adsorbent which has also been confirmed by intraparticle diffusion model. Overall the kinetic studies showed that the zinc adsorption process followed pseudosecond-order kinetics among pseudo-first-order, pseudo-secondorder and intraparticle diffusion model respectively. The various pseudo-second-order kinetic parameters including rate constant are determined at different initial metal ion concentrations, pH, amount of adsorbent and temperature respectively. • Langmuir isotherm is more applicable compared to Freundlich isotherm to describe the adsorption of zinc (Zn2+) on bentonite within this initial metal ion concentration range. The constant value, RL (low separation factor) in Langmuir isotherm gives an indication of favorable adsorption. • Finally thermodynamic parameters are determined at three different temperatures and it has been found that the adsorption process is exothermic because of negative ΔH0 accompanied by a decrease in entropy change and Gibbs free energy change (ΔG0) respectively.
Nomenclature Final metal ion concentration, ppm (mg/L) Cf Initial metal ion concentration, ppm (mg/L) C0 Ct Metal ion concentration at time t, ppm (mg/L) ΔG0 Gibbs free energy change, (kJ/mol) ΔH0 Enthalpy change, (kJ/mol) ΔS0 Entropy change, (J/k mol) K1 Pseudo-first-order rate constant (min− 1) K2 Pseudo-second-order rate constant, (mg/g min) Kf Freundlich adsorption constant, (mg/g) Kid Intraparticle rate constant [(mg/g) min0.5] M Mass of adsorbent per unit volume (g L− 1) m Amount of adsorbent added in gm n Freundlich constant q Amount of adsorbate per gram of adsorbent (mg/g) qe Amount of adsorbate per gram of adsorbent at equilibrium qt Amount of adsorbate per gram of adsorbent at any time, t qm Equilibrium adsorption capacity using model qmax Maximum adsorption capacity (mg/g) R2 Linear correlation coefficient RL Separation factor t time (min) V Volume of the solution, (mL)
Acknowledgements Chemical Engineering Department of Curtin University of Technology, Perth for financial support through internal funding project entitled “Metal Ion Adsorption”; Prof. Rob D. Hart for help in taking
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Adsorption of zinc (Zn2+) from aqueous solution on natural bentonite Tushar Kanti Sen ⁎, Dustin Gomez Department of Chemical Engineering, Curtin University of Technology, Perth, GPO Box U1987, 6845 Western Australia, Australia
a r t i c l e
i n f o
Article history: Received 7 January 2010 Received in revised form 17 August 2010 Accepted 24 September 2010 Available online 25 October 2010 Keywords: Zinc adsorption Bentonite Kinetic model Adsorption isotherm
a b s t r a c t The adsorptive properties of natural bentonite in the removal of zinc (Zn2+) from aqueous solution were studied. The results show that the amount of adsorption of zinc metal ion increases with initial metal ion concentration, contact time, and solution pH but decreases with the amount of adsorbent and temperature of the system. Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn2+) on bentonite is a two step process: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow decreasing intraparticle diffusion in the interior of the adsorbent which has also been confirmed by intraparticle diffusion model. Overall the kinetic studies showed that the zinc adsorption process followed pseudo-second-order kinetic model. The different kinetic parameters including rate constant, half adsorption time, and diffusion coefficient are determined at different physicochemical conditions. The equilibrium adsorption results are fitted better with Langmuir isotherm compared to Freundlich models. The value of separation factor, RL from Langmuir equation and Freundlich constant, n both give an indication of favorable adsorption. Finally in thermodynamic studies, it has been found that the adsorption process is exothermic due to negative ΔH0 accompanied by a decrease in entropy change and Gibbs free energy change (ΔG0). © 2010 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ion pollution is currently of great concern due to its increased awareness of the potentially hazardous effects of elevated levels of these materials in the environment [1–3]. Presently, it is appearing as an increasing and alarming challenge to the researchers and environmental control agencies for polluting the water and soil resources severely through the indiscriminate disposal of metals in the environment. Major sources of zinc in the environment are the manufacturing of brass and bronze alloys and galvanization [4,5]. It is also utilized in paints, rubber, plastics, cosmetics and pharmaceuticals [4]. Zinc is an essential element for life and acts as a micronutrient when present in trace amounts [5]. The WHO recommended the maximum acceptable concentration of zinc in drinking water as 5.0 mg/L [6]. Beyond the permissible limits, Zn2+ is toxic [7]. The availability of micronutrients and toxins in soils also depends on the interaction of these materials with oxide or oxyhydroxide particles, clay minerals and organic matter. Clay minerals such as kaolinite, illite and montmorillonite are abundant in natural soil systems. Usually soils serve as important sinks for contaminant metals, however clay minerals and organic contents are important components that control the sorption/retention capacity of soils towards trace metals. Moreover, the fate and transport of metal ions including Zn2+ in natural water as well as in water treatment/
⁎ Corresponding author. Tel.: + 61 8 92669052. E-mail address: [email protected] (T.K. Sen). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.041
industrial waste water treatment processes are often controlled by their reactions with adsorbents under different environmental conditions [8]. Many technologies have been developed to remove heavy metals from contaminated waters, such as chemical precipitation, ion exchange, filtration, solvent extraction and membrane technology and adsorption [5]. Some of these methods have disadvantages and limitations. Precipitation, for example produces large amount of sludge in solution [9] but membrane filtration, ion exchange, electro-deposition and filtration are costly [9]. Alternatively, adsorption can remove these metals efficiently at low cost and also very simple in operation. Several solid materials can be employed as adsorbents. Adsorption on activated carbon is the conventional methods for the removal of heavy metal ions from aqueous solutions but its high cost limits its use [5]. Therefore, the adsorption is used especially in the water treatment field and the investigation has to be made to determine inexpensive and good adsorbent. Clay is one of potential good adsorbent alternatives to activated carbon because of its large surface area, high cation exchange capacity, chemical and mechanical stability and layered structure. The presence of both bronsted and Lewis types of acidity in clays [3,10] boosts the adsorption capacity of clays. Aluminum oxides and clay minerals such as kaolin, bentonite are the most wide-spread minerals of the earth crust which are known to be good adsorbent of various metal ions, inorganic anions and organic ligands [11–16]. The abundance of bentonite and its low cost are likely to make it a strong candidate as an adsorbent for the removal of heavy metal from wastewater [17]. Bentonite is a 2:1 mineral with one octahedral sheet and two silica sheets, which forms a layer [18]. Layers are held together by van der
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Waals forces. Because of these weak forces and some charge deficiencies in the structure, water can easily penetrate between layers and cations balance the deficiencies [18]. Bentonite attracted the researcher's concern for a long time and has been studied extensively. Most of these studies considered the adsorption characteristics of bentonite for some toxic elements. It is well documented that bentonite is an efficient adsorbent for some heavy metals, especially for lead [19,20], copper [21], cadmium [19,22] and zinc [18]. Therefore, there are a number of studies that have been reported using clays, mainly montmorillonite to show their effectiveness for removal of metal ions such as Zn2+, Pb2+ and Al3+ from aqueous solution [22–25]. However, systematic studies on zinc (Zn2+) adsorption characteristics on bentonite under various physicochemical parameters are limited and also are very scarce. For this reason, a detailed study was conducted in order to determine the influence of initial solution pH, initial metal ion concentration, adsorbent doses and temperature changes on adsorption characteristics of natural bentonite. Another reason for this study is the importance of adsorption on solid surfaces in many industrial applications in order to improve efficiency and economy. Therefore, it is essential to understand the mechanism and kinetics of adsorption, because the studies of adsorption kinetics are ultimately a prerequisite for designing an adsorption column [11]. This study will also help to design the effective composite reactive barrier in contaminant leaching and also in the field of cost-effective water filter design. Equilibrium adsorption isotherms or capacity studies are of fundamental importance in the design of adsorption system. This study also gives new data set within this small concentration range of metal ion. It has been found that the amount of adsorbed zinc metal ion increases with initial metal ion concentration, contact time and with the solution pH but decreases with adsorbent doses and temperature respectively. The kinetic adsorption results have been analyzed using pseudo-first-order, pseudo-2nd-order reactions and intraparticle diffusion model respectively. The isotherm equilibrium results have been fitted more with Langmuir and less with Freundlich isotherm model respectively.
2. Materials and methods 2.1. Materials 2.1.1. Adsorbent and characterization The adsorbent being used is laboratory grade reagent bentoniteK10 clay obtained from ACROS ORGANICS, Germany. The particle size distribution was determined by using the Malvern MasterSizer 2000S with the Hydro 2000S (A). Spectrum 100 FT-IR Spectrometer with a Universal ATR Sampling Accessory with MIR detector from Perkin Elmer was used to determine the functional groups of bentonite. Bentonite sample was also analyzed by an X-ray diffractometer.
2.1.2. Adsorbate (Zn2+) and other chemicals All chemicals used were of analytical grade. Stock standard solution of Zn2+ has been prepared by dissolving the appropriate amount of its nitrate in deionized water, acidified with small amount of nitric acid. This stock solution was then diluted to specified concentrations. The pH of the system was adjusted using reagent grade NaOH and HNO3 respectively. All plastic sample bottles and glassware were cleaned, then rinsed with deionized water and dried at 60 °C in a temperature controlled oven. All measurements were conducted at a room temperature of 30 °C. The concentration of Zn2+ was measured using Varian 110 Atomic Absorption Spectrophotometer (AAS). The pH was measured by Orion pH meter.
287
2.2. Adsorption experiments 2.2.1. Adsorption procedure Adsorption measurements were determined by batch experiments. For this purpose, 0.01 g of bentonite and 40 mL of aqueous Zn2+ solutions at specified concentration were put on a shaker using Thermo line Scientific Orbital Shaker Incubator at 80 rpm at 30 °C for a given time. The suspensions were then filtered through a micro filter of pore size 0.47 μm and the filtrates were analyzed using flame atomic absorption spectrophotometer with air-acetylene flame. The pH of the solutions was initially adjusted by addition of small amount of either 0.1 M HCl or 0.1 M NaOH solutions. The experiments were carried out by varying concentrations of initial Zn2+ solution, contact time, amount of adsorbent, temperature and pH of initial suspension. The Zn2+ concentration retained in the adsorbent phase, qt (mg/g) was calculated according to following relation qt =
ðC0 −Ct ÞV m
ð1Þ
where C0 (mg/L) and Ct (mg/L) are the concentration in the solution at time t = 0 and at time t, V is the volume of solution (L) and m is the amount of adsorbent (g) added. 2.2.2. Effect of initial solution pH on metal ion adsorption kinetics In this study the sorbent (0.01 g) and 40 mL of 30 ppm (mg/L) Zn2+ solution were mixed in plastic bottle. The pH of the mixture was adjusted either by 0.1 M HCl or 0.1 M NaOH solution until the initial pH was close to the target value ranged from 3.81 to 7.69. The whole mixture was taken in a series of 50 mL plastic bottles and put on a Thermo line Scientific Orbital Shaker Incubator at 80 rpm and at 30 °C for a period of 180 min. Speed was such that it maintains the contents completely mixed and the adsorbents were suspended throughout the plastic bottle. The samples were then collected in different time intervals throughout equilibrium time period and filtered each time through a micro filter. The left out concentrations in the filtrate solution was analyzed using flame atomic absorption spectrophotometer. The quantity of adsorbed metal ion on bentonite was calculated as the difference between initial concentration and concentration at any time, t as per (Eq. (1)). Each experiment was repeated in twice to check the reproducibility. Measurements are, in general, reproducible within ± 10%. 2.2.3. Effect of adsorbent dose on metal ion adsorption kinetics The adsorbent dosage in the aqueous solution was increased from 0.01 g to 0.03 g in 40 mL of metal ion. The effect of adsorbent mass on the amount of metal ion adsorbed was obtained by contacting 40 mL of Zn (II) ion solution of initial concentration of 30 ppm (mg/L) with different weight amounts of 10 mg, 20 mg and 30 mg of adsorbent using Thermo line Scientific Orbital Shaker Incubator. The shaker was running at a temperature of 30 °C and a constant speed of 80 rpm for specified time. The experiments were carried out at initial solution pH of 6.76. At each time intervals, the samples were then filtered and the supernatant solution was analyzed as per Section 2.2.1. 2.2.4. Effect of temperature on metal ion kinetic adsorption kinetics The batch adsorption experiments were carried out with 40 mL Zn(II) metal ion solution of 30 ppm at 30, 50 and 65 °C separately by contacting with 0.01 g of adsorbent using Thermo line Scientific Orbital Shaker Incubator. The speed of incubator was 80 rpm for a period of 180 min. The solution pH was 6.76. 2.2.5. Equilibrium isotherm experiments For isotherm studies, a series of 50 mL plastic bottles containing 40 mL of Zn2+ metal ions solutions of known concentrations, varying from 10 to 90 mg/L were prepared. Identical amounts (0.01 g) of
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bentonite were added to the each bottle and the resulting suspensions were agitated on a Thermo line Scientific Orbital Shaker Incubator at a speed of 80 rpm and 30 °C for 4 h at a constant pH of 6.64. After equilibrium time, the suspensions were then filtered through a micro filter of pore size 0.47 μm and the filtrates were analyzed using flame atomic absorption spectrophotometer with air-acetylene flame as per Section 2.2.1. 3. Results and discussions 3.1. Characterization of adsorbent The characterization of the structure and surface chemistry of the adsorbent is of considerable interest for the development of adsorption and separation processes. One important characteristics of an adsorbent is the surface functional groups present which are largely characterized by the FT-IR spectroscopy method. FT-IR spectroscopy of bentonite (Fig. 1) indicated the presence of hydroxyl, carboxyl, and Si–O which are important sorption sites. The peaks at 3411.6 and 3625 cm− 1 are due to lattice OH and bound water stretching vibrations. A strong and sharp band is detected at 1022 cm− 1 which is related to Si–OH stretching vibrations [9]. The X-ray diffraction analysis (Fig. 2) identifies four different mineral phases present: mainly quartz (SiO2), and muscovite ((K,Na)(Al,Mg,Fe)2(Si3.1Al0.9)010(0H)2) and trace amount of kaolinite (Al2Si205(0H)4) and albite respectively. The particle size distribution of bentonite which is not shown here for which specific surface area was 0.208 m2/g. By taking the average surface weighted mean of each of the trials, the mean particle size for the bentonite used was 5.04 μm. 3.2. Effect of initial solution pH on metal ion kinetic experiment The pH of the adsorbate solutions is an important parameter governing adsorption on different adsorbents [5,8,12,26–28]. In principle, the dependence of metal uptake on pH can be associated with both the surface functional groups on the adsorbent as well as the metal chemistry of the solution [35]. The effect of pH on the sorption of Zn2+ ions onto bentonite was studied by changing initial solution pH values in the range of 2–8 and results are presented in Fig. 3. It has been
found from Fig. 3 that the adsorption of zinc (Zn2+) on bentonite increased with time and increased with pH up to 6.76 and then declined with a further increase in pH. In general, most metal sorption is enhanced with pH, increasing to a certain value followed by a reduction on further pH increase. The increase in zinc (Zn2+) adsorption with increasing pH (to pHmax = 6.76) is similar to the adsorption pattern of other hydrolyzable metal cations and maximum adsorption takes place at pH 6.76. It is known that metal species (M (II) = Zn2+) are present in deionized water in the forms of M2+, M(OH)+, M(OH)2(S), etc [18]. At pH~ 5.0, the solubility of the M(OH)2(S) is high and therefore, the M2+ is the main species in the solution [5,28–30]. With the increase of the pH value, the solubility of M(OH)2(S) decreases and at pH~ 10.0, the solubility of M(OH)2(s) is very small [5]. At this time, the main species in the solution is M(OH)2(S). Therefore in the alkaline range, the metal ion precipitation plays the main role in the removal of the M(II) ions attributed to the formation of precipitate of M(OH)2(S). To avoid precipitation of the metal ions, all the experiments were carried out at a maximum initial solution pH of 7.7. It has also been found from Fig. 3 that the amount of zinc metal ion adsorbed per gram of bentonite is very fast from the beginning up to 10 min and it increases with time as well as with increase in pH or alkalinity. With a further increase in time, the amount of adsorption decreased progressively and finally reached an equilibrium stage within 80 min in all cases. Maximum adsorption takes place within 10 min at each pH. The fast adsorption at the initial stage is probably due to the increased concentration gradient between the adsorbate in solution and adsorbate in adsorbent as there must be increased number of available surface sites in the beginning [5]. Adsorption of heavy metals onto various clay minerals occurs in two or more stages [31–34]. Fig. 3 also shows that zinc metal ion adsorption on bentonite occurs more or less in two stages: a very rapid fast reaction followed by decreasing equilibrium plateau. The oxygen atoms present on the clay surface interact with water in an acidic medium forming some aqua complexes [3], which result in positive charge formation as follows: þ
−
– MO þ H–OH → M–OH2 þ OH :
ð2Þ
The surface charge is responsible for preventing Zn2+ ions from approaching the surface and explains the smaller extent of adsorption
Fig. 1. FT-IR of bentonite sample.
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289
Fig. 2. XRD diffractogram of the analyzed bentonite sample.
The results of the kinetic experiments with varying adsorbent concentrations are presented in Fig. 4. It has been found that maximum zinc uptake took place within 10 min as initial speed of reaction is very fast and the amount of Zn2+ adsorbed per unit mass of
adsorbent decreases as the adsorbent mass increases. Several other investigators have also reported the same trend of other adsorbent concentration effect on metal ion adsorption which has been mentioned in Sen et al. [5,14,26]. Although the number of adsorption sites per unit mass of an adsorbent should remain constant, independent of the total adsorbent mass, increasing the adsorbent amount in a fixed volume reduces the number of available sites as the effective surface area is likely to decrease [3]. Various investigators have also offered different explanations for the observed dependency. These explanations can be categorized into two groups: (1) ‘real’ physical/chemical processes and (2) experimental artifacts. One possible explanation is that the particle concentration effect is thought to be caused by particle–particle interactions. In system with higher solid content, these interactions are perhaps physically blocking some adsorption sites from the adsorbing solutes and thus, causing decreased adsorption, or creating electrostatic interferences such that the electrical surface charges on the closely packed particles diminish attractions between the adsorbing solutes and surfaces of individual grains. In general, pH of final solution increases gradually with the increase in adsorbent dosage [36]. It may be attributed to an evident
Fig. 3. Effect of initial solution pH on Zn2+ adsorption. Initial Zn2+ = 30 ppm, amount of bentonite added = 10 mg; temperature = 30 °C; shaker speed = 80 rpm.
Fig. 4. Effect of amount adsorbent bentonite on zinc metal ion adsorption kinetics. Initial Zn2+ = 30 ppm; initial solution pH= 6.76, temperature= 30 °C; shaker speed = 80 rpm.
at low pH [3]. Basically, at low pH, due to high positive charge density, electrostatic repulsion will be high during uptake of metal ions resulting in lower adsorption. In an alkaline medium, the clay surface becomes negatively charged favoring Zn2+ uptake as per the following: −
– MOH þ OH
2þ
– MO þ Zn
−
¼ –MO þ H2 O
ð3Þ
2þ
¼ – M–O Zn –
ð4Þ
The previously mentioned fact related to the effect of pH on adsorption also supported by many earlier workers [2,3,14,15,35]. 3.3. Effect of adsorbent dosage on metal ion kinetic adsorption
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increase of the amount of negatively charged sites which can induce more H+ ions adsorb on the bentonite surface and results in an increase in pH of the final solution.
3.4. Kinetics of zinc metal ion adsorption 3.4.1. Effect of contact time and kinetics of adsorption Fig. 5 represents a plot of the amount of zinc metal ion adsorbed (mg/g) versus contact time for different initial metal ion concentration of 30, 40 and 50 ppm (mg/L) respectively. From these plots, it is found that the amount of adsorption i.e. mg of adsorbate per gram of adsorbent increases with increasing contact time at all initial metal ion concentrations and equilibrium is attained within 80 min. Further it is observed that the amount of metal ion uptake, qt (mg/g) is increased with an increase in initial metal ion concentration. This kinetic experiment clearly indicates that adsorption of zinc metal ion (Zn2+) on bentonite clay is a more or less two step process similar to experiments by Sen et al. [31–34] and Arias and Sen [5] kaolin and oxides: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow intraparticle diffusion in the interior of the adsorbent. This two stage metal ion uptake can also be explained as adsorption occurring onto two different types of binding sites on the adsorbent particles. The rapid kinetics has significant practical importance, as it facilitates smaller reactor volumes, ensuring high efficiency and economy [37]. In order to investigate the mechanism of adsorption, particularly potential rate-controlling step, the transient behavior of the zinc metal ion (Zn2+) adsorption process was analyzed using the pseudofirst-order, pseudo-second-order and intraparticle diffusion model. As per Lagergren pseudo-first-order model, plot of log (qe − qt) versus t gives a straight line which allow computation of the rate constant K1 [26,27] where qt and qe represent the amount of Zn(II) metal ion adsorbed (mg/g) at any time t and at equilibrium time, respectively. Fig. 6 shows plot of log (qe − qt) versus t gives a straight line as with very poor linear regression coefficient (R2) of 0.39 to 0.94. The rate constant K1 for three different initial metal ion concentration of 30, 40 and 50 ppm are 0.0713, 0.0552 and 0.0739 min− 1 respectively. Moreover pseudo-first-order kinetic model predicts a much lower value of the equilibrium adsorption capacity, (qe) than the experimental value which is not shown here and hence it gives the inapplicability of this model. The same pseudo-first-order model are fitted with kinetic adsorption experimental results in case of initial solution pH effect and amount of adsorbent respectively but it gives very poor linear regression coefficient, R2 value which are not presented here.
Fig. 5. Effect of contact time on zinc metal ion adsorption. Amount of bentonite = 10 mg; temperature= 30 °C; initial solution pH= 6.76; shaker speed= 80 rpm.
Fig. 6. Pseudo-first-order kinetic model for zinc adsorption by bentonite at different initial metal ion concentrations. Amount of adsorbent = 10 mg. Initial solution pH= 6.76, shaker speed = 80 rpm, and temperature = 30 °C.
The adsorption data were then analyzed using the pseudo-secondorder kinetic model. The pseudo-second-order model based on equilibrium adsorption is expressed as [26,27] t 1 1 = + t: qt qe K2 q2e
ð5Þ
A plot between t/qt versus t gives the value of the constants K2 (g/ mg h) and also qe (mg/g) can be calculated. The constant K2 is used to calculate the initial sorption rate h, at t → 0, as follows 2
h = K2 qe :
ð6Þ
Thus the rate constant K2, initial adsorption rate h and predicted qe can be calculated from the plot of t/q versus time t using Eq. (5). Figs. 7–9 represents the pseudo-second-order kinetic plots between t/qt versus time t for zinc adsorption at different initial
Fig. 7. Pseudo-second-order kinetic model for zinc (Zn2+) at different initial metal ion concentrations. Amount of bentonite added = 10 mg; initial solution pH = 6.76; temperature = 30 °C; shaker speed = 80 rpm.
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Table 1 Pseudo-second-order kinetic parameters. Parameters
Fig. 8. Pseudo-second-order kinetic model for zinc (Zn2+) adsorption at different initial solution pH. Initial Zn2+ = 30 ppm; amount of bentonite= 10 mg; temperature= 30 °C; shaker speed = 80 rpm.
metal concentrations, solution pH effect and amount of adsorbents respectively. Similar plot at different temperatures is not shown here. High linear regression coefficients (R2) suggest that the Zn2+ adsorption experiment follows pseudo-second-order kinetics. The initial sorption rate is the h = K2 q2e . The equilibrium adsorption capacity (qe) and second order constant (K2) can be determined from the slope and intercept of plot t/q versus t (Figs. 7–9). All these parameters including the linear regression coefficients, R2, the pseudo-second-order rate constant, K2 and equilibrium sorption capacity, qe were calculated and tabulated in Table 1. Higher linear regression coefficients (R2) with respect to fitted pseudo 1st-order model suggest that adsorption of zinc metal ion on kaolin follows pseudo-second-order kinetics. Moreover, calculated, qe values from pseudo-second-order fitting model is very close to the experimental qe values (Table 1) also suggesting the suitability of this model. Also this suggests the assumption behind the pseudo-second-order model that the metal ion uptake process is due to chemisorptions [26] and more than one-step may be involved in sorption processes. Also from Table 1 the adsorption capacity increases with an increase in initial metal ion concentration, in initial solution pH but decreases with amount of adsorbent and temperature respectively. Similar type model parameters are also obtained by various researchers for different systems [26,38]. 3.4.2. Intraparticle diffusion and adsorption mechanism For the process design and control of adsorption systems, it is important to understand the underlying mechanism that results in
Fig. 9. Pseudo-second-order kinetic model for zinc (Zn2+) adsorption at different amounts of bentonite added. Initial Zn2+ = 30 ppm; initial solution pH = 6.76; temperature = 30 °C; shaker speed = 80 rpm.
qe,
exp
(mg/g)
K2 (g/mg min)
qe,
cal
(mg/g)
Initial metal concentration (ppm) 30 7.5 0.1284 40 10.0 0.01452 50 47.2 0.03942
7.9744 9.8911 47.1698
Solution pH 3.81 6.76 7.69
0.02030 0.049856 0.02439
11.933174 46.948 28.4900
Adsorbent dose (mg) 10 47.4 20 20.7 30 16.6
0.047757 0.01613 7.2240
46.948 20.661 16.6389
Temperature 30 50 65
0.03511 0.01504 0.04255
47.169 36.0628 34.3642
11.8 47.4 28.8
(°C) 47.4 37.2 34.2
R2
h 8.16935 1.42054 87.709
0.9852 0.9765 0.9958
2.8907 109.888 19.79687
0.9847 0.9954 0.9978
105.2619 6.8855 1999.98
78.1249 19.5690 50.24722
0.9954 0.9931 0.9963
0.9957 0.9931 0.9965
the apparent dynamic behavior of the system. The removal of Zn(II) metal ion by adsorption on bentonite was found to be rapid at the initial period of contact time and then become slow and stagnate with increase in contact time. For a solid/liquid sorption process, the solute transfer is usually characterized by either external mass transfer (boundary layer diffusion) or intraparticle diffusion or both. The mechanism for the removal of Zn2+ by adsorption may be considered into the following steps [39] (a) Migration of metal ion from the bulk of the solution to the surface of the adsorbent (b) Diffusion of metal ion through the boundary layer to the surface of adsorbent (c) Adsorption of metal ion at an active site on the surface of adsorbent (d) Intraparticle diffusion of metal ion into the interior pore structure of adsorbent. The overall rate of sorption will be controlled by the slowest step, which would be either film diffusion or pore diffusion. However, the controlling step might be distributed between intraparticle and external transport mechanisms. Whatever the case, external diffusion will be involved in the sorption process. The sorption of Zn2+ metal ion onto bentonite particles may be controlled due to film diffusion at earlier stages and as the adsorbent particles are loaded with metal ions, the sorption process may be controlled due to intraparticle diffusion. The most commonly used technique for identifying the mechanism involved in the sorption process is by fitting the experimental data in an intraparticle diffusion plot. The intraparticle diffusion plot is the plot of amount sorbed per unit weight of sorbent, qt (mg/g) versus square root of time, √t is shown in Fig. 10 for initial Zn2+ metal ion concentration of 30, 40 and 50 ppm. The other intraparticle diffusion model fitting plot at different pH, different adsorbent dosages and at different temperatures are not shown here but the nature of plot was similar to Fig. 10. Fig. 10 shows that the adsorption plots are not linear over the whole time range and can be separated into two–three linear regions which confirms the multi stages of adsorption. This plot represented the two different stages viz. external mass transfer followed by intraparticle diffusion, signified that the metal ions were transported to the external surface of the bentonite particles through film diffusion and its rate was very fast. After that, metal ions were entered into bentonite particles by intraparticle diffusion through pores. Generally, when adsorption steps are not dependent of one another, the plot of qt against t0.5 should give two or more intercepting lines depending on the actual mechanism [40]. Moreover from Fig. 10,
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Fig. 11. Freundlich adsorption isotherm.
Fig. 10. Effect of contact time on amount of Zn2+ adsorption as per diffusion model.
conclusion can be made that none of plot gives linear straight line segment passing through the origin, I ≠ 0 which is not shown here. This indicates that the intraparticle diffusion is involved in the adsorption process but not the only rate-controlling step. As lines did not pass through the origin indicating that film diffusion and intraparticle diffusion occurred simultaneously. Some other mechanisms such as complexation or ion exchange may also control the rate of adsorption [41]. The diffusion coefficient, D, largely depends on the surface properties of adsorbents. The diffusion coefficient for the intraparticle transport of different initial concentrations of metal ions were also calculated using the following relationships [39] 2
t1 = 0:03 r0 = D
ð7Þ
2
and t1 = 2
1 ðK2 qe Þ
ð8Þ
where t1/2 is the half life of adsorption in seconds, K2 is the pseudosecond-order rate constant and r0 is the radius of the adsorbent particle in centimeters and D is the diffusion coefficient value in cm2/s. The value of r0 was calculated as 5.04 × 10− 4 cm. The behavior of the concentration dependent diffusivity agrees with literature works [39,42]. The diffusion coefficients, D values were found to be 7.33 × 10− 9 cm2/s, 1.10 × 10− 9 cm2/s and 1.41 × 10− 8 cm2/s for an initial methylene blue concentration of 30, 40 and 50 ppm respectively.
Fig. 11 gives results on Freundlich isotherm fittings for bentonite with linear regression coefficient of 0.90098. Freundlich constants i.e. adsorption capacity, Kf and rate of adsorption, n, are calculated from this plot which are 1.4845 mg/g and 1.23 L/mg respectively. The value of ‘n’ is larger than 1 which indicates the favorable nature of adsorption [43,44]. Similar type Freundlich parameters are obtained by many researchers [5,7,46] for zinc adsorption on different inorganic adsorbents. Fig. 12 gives results on Langmuir-2 isotherm fittings for bentonite adsorbent. The maximum adsorption capacity of Zn2+, qm, and constant related to the binding energy of the sorption system, Ka is calculated which are 68.4931 mg/g and 0.019850 respectively for Znbentonite system. The reported maximum zinc adsorption capacity of various inorganic adsorbents such as phosphatic clay is 25.10 mg/g [46], natural zeolite is 3.66 mg/g [45], granular activated carbon is 10.70 mg/g [45], bentonite is 21.1 mg/g [18], activated alumina is 13.29 mg/g [7], activated carbon 31.11 mg/g [6] and kaolin is 250 mg/g [5] respectively. The separation factor, RL has been calculated from Langmuir plot as per the following relation [43]:
RL =
1 : 1 + Ka C0
ð11Þ
It has been found that the range of RL is 0.83437 to 0.358873 for this initial metal ion concentration range of 10 to 90 ppm which gives favorable adsorption as it lies in b0 b RL b 1. From Figs. 11 and 12, it is also seen that Langmuir isotherms usually fitted better with the
3.5. Comparison of adsorption equilibrium isotherm Adsorption isotherms of bentonite for Zn2+ metal ion were expressed mathematically in terms of the Langmuir and Freundlich isotherm models. The obtained experimental data are commonly well fitted with the Freundlich (Eq. (9)) and Langmuir (Eq. (10)) models: ln qe = ln Kf + 1 = qe
1 ðln Ce Þ n
1 1 1 + Ka qm Ce qm
ð9Þ
ð10Þ
where Kf, n and Ka, qm are the constants for Freundlich and Langmuir models respectively.
Fig. 12. Langmuir adsorption isotherm.
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experimental data rather than Freundlich isotherms which is also reported by Kaya and Oren [18]. 3.6. Thermodynamics for adsorption and effect of temperature To observe the effect of temperature on the adsorption capacity, experiments are carried out in three different temperatures of 30, 50 and 65 °C for a fixed initial metal ion concentration of 30 ppm. It has been found from Fig. 13 that with an increase in temperature, adsorption capacity decreases. This is mainly because of decreased surface activity suggesting that adsorption between Zn2+ metal ion and bentonite was an exothermic process. With increasing temperature, the attractive forces between the bentonite surface and metal ion are weakened and then sorption decreases. Similar type's results are also obtained by earlier investigators for different adsorbent systems [3,26,31,47,48] Thermodynamic parameters such as Gibb's free energy (ΔG0), enthalpy change (ΔH0) and change in entropy (ΔS0) for the adsorption of zinc on bentonite have been determined by using the following equations [38]:
Fig. 14. Van't Hoff plot for adsorption of zinc metal ions.
3.7. Desorption studies 0
0
0
ΔG = ΔH −TΔS
log
qe ΔS0 −ΔH 0 + = Ce 2:303 R 2:303 RT
ð12Þ
ð13Þ
where qe is the amount of zinc adsorbed per unit mass of bentonite (mg/g), Ce is equilibrium concentration (mg/L) and T is temperature in K. qe/Ce is called the adsorption affinity. The previously mentioned equation is for unit mass of adsorbent dose. The values of Gibbs free energy (ΔG0) have been calculated by knowing the value of enthalpy of adsorption (ΔH0) and the entropy of adsorption (ΔS0) which are obtained from slope and intercept of a plot of log (qe/Ce) versus 1/T shown in Fig. 14. All these thermodynamic parameters are presented in Table 2. The exothermic nature is also indicated by the decrease in the amount of adsorption with temperature (Fig. 13). The negative value of ΔH0 and ΔG0 indicates (Table 2) that the adsorption processes are spontaneous and exothermic in nature. The negative value of ΔS0 suggests decreased randomness during adsorption [38,49]. Similar results for thermodynamic parameters were also reported by the earlier workers for the adsorption of Zn2+ as well as other heavy metals from aqueous solution [3,4,50,51].
Desorption studies of Zn (II) metal ions were conducted to explore the possibility of recycling of bentonite and recovery of the metal ions. Approximately 0.02 g of metal ion loaded bentonite from the previous adsorption experiment was stirred with 40 mL of 0.1 M HCl solution for 24 h. The filtrate was analyzed for desorbed metal ions using Atomic Absorption Spectrophotometer. The same adsorbents were washed with distilled water for several times and the concentration of released metal ions from washing process was again analyzed. The total desorption for Zn (II) metal ion was around 42%. The acid solution (0.1MHCl) used for desorption has a pH of 2.05. As such, the amount of metal ion (58% for Zn (II)) retained in the bentonite after acid treatment and partial elution of metal represent the equilibrium adsorption at pH 2.05. A complexing agent such as EDTA, ethylene diamine may be much more effective for recovery of metals to compensate for the additional cost of the chemicals. A systematic work is under progress. 4. Conclusions The removal of zinc (Zn2+) metal ions from aqueous solution by bentonite was investigated. The following conclusions can be drawn based on this investigation: • The adsorption characteristics of zinc metal ion (Zn2+) are strongly affected by initial solution pH, initial metal ion concentration, amount of adsorbent and temperature respectively. It has been found that the amount of metal ion (Zn2+) adsorption on natural bentonite increases with initial metal ion concentration, contact time, and solution pH respectively but decreases with the amount of adsorbent and system temperature respectively. • It is also been found that the amount of adsorption i.e. mg of adsorbate per gram of adsorbent increases with increasing contact time at all initial metal ion concentrations and equilibrium is attained within 80 min for Zn-bentonite systems at a fixed solution pH. The adsorption process is strongly dependent on the pH of the medium with enhanced adsorption as the pH turns from acidic to Table 2 Thermodynamic parameters for zinc adsorption at different temperatures.
Fig. 13. Effect of temperature on kinetic adsorption of Zn2+ on bentonite.
Temperature (°C)
ΔG0 (kJ mol− 1)
ΔH0 (kJ mol− 1)
ΔS0 (j mol− 1 K− 1)
30 50 65
− 2.3586 − 1.7014 − 1.211
− 12.3143 − 12.3143 − 12.3143
− 0.03285 − 0.03285 − 0.03285
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alkaline side till precipitation sets in. The maximum adsorption takes place at an optimum pH of 6.76. The process is very fast initially and maximum adsorption is obtained within 20 min. • Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn2+) on bentonite is more or less a two step process: a rapid adsorption of zinc metal ion to the external surface followed by intraparticle diffusion into the interior of adsorbent which has also been confirmed by intraparticle diffusion model. Overall the kinetic studies showed that the zinc adsorption process followed pseudosecond-order kinetics among pseudo-first-order, pseudo-secondorder and intraparticle diffusion model respectively. The various pseudo-second-order kinetic parameters including rate constant are determined at different initial metal ion concentrations, pH, amount of adsorbent and temperature respectively. • Langmuir isotherm is more applicable compared to Freundlich isotherm to describe the adsorption of zinc (Zn2+) on bentonite within this initial metal ion concentration range. The constant value, RL (low separation factor) in Langmuir isotherm gives an indication of favorable adsorption. • Finally thermodynamic parameters are determined at three different temperatures and it has been found that the adsorption process is exothermic because of negative ΔH0 accompanied by a decrease in entropy change and Gibbs free energy change (ΔG0) respectively.
Nomenclature Final metal ion concentration, ppm (mg/L) Cf Initial metal ion concentration, ppm (mg/L) C0 Ct Metal ion concentration at time t, ppm (mg/L) ΔG0 Gibbs free energy change, (kJ/mol) ΔH0 Enthalpy change, (kJ/mol) ΔS0 Entropy change, (J/k mol) K1 Pseudo-first-order rate constant (min− 1) K2 Pseudo-second-order rate constant, (mg/g min) Kf Freundlich adsorption constant, (mg/g) Kid Intraparticle rate constant [(mg/g) min0.5] M Mass of adsorbent per unit volume (g L− 1) m Amount of adsorbent added in gm n Freundlich constant q Amount of adsorbate per gram of adsorbent (mg/g) qe Amount of adsorbate per gram of adsorbent at equilibrium qt Amount of adsorbate per gram of adsorbent at any time, t qm Equilibrium adsorption capacity using model qmax Maximum adsorption capacity (mg/g) R2 Linear correlation coefficient RL Separation factor t time (min) V Volume of the solution, (mL)
Acknowledgements Chemical Engineering Department of Curtin University of Technology, Perth for financial support through internal funding project entitled “Metal Ion Adsorption”; Prof. Rob D. Hart for help in taking
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