Journal of Industrial and Engineering Chemistry 20 (2014) 841–847
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Kinetic and thermodynamic studies on the removal of Cu(II) ions from aqueous solutions by adsorption on modiﬁed sand Deepak Gusain, Varsha Srivastava, Yogesh Chandra Sharma * Department of Chemistry, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221 005, India
A R T I C L E I N F O
Article history: Received 12 January 2013 Accepted 9 June 2013 Available online 19 June 2013 Keywords: Adsorption Batch adsorption Copper Kinetic study Sand
A B S T R A C T
Studies on the removal of copper by adsorption on modiﬁed sand have been investigated. The adsorbent was characterized by XRD, FTIR and SEM. Removal of Cu was carried out in batch mode. The values of thermodynamic parameters namely DG0, DH0 and DS0 at 25 8C were found to be 0.230 kcal1 mol1, +4.73 kcal1 mol1 and +16.646 cal K1 mol1, respectively. The process of removal was governed by pseudo second order rate equation and value of k2 was found to be 0.122 g mg1 min1 at 25 8C. The resultant data can serve as baseline data for designing treatment plants at industrial scale. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Water is essential for survival of living organisms on aquatic as well as at terrestrial ecosystem. Water has been used for a wide range of purposes including domestic, agriculture and industrial uses. Its increasing sphere of utility has laden it with number of pollutants, one among them are metallic pollutants. Metallic pollutants are non biodegradable due to which water contaminated with metallic pollutants ﬁnds its way into streams as wastewater. Among various metals, Cu(II) and copper containing materials have number of applications such as a supplement in cattle feed, for manufacturing of copper water pipes and brass radiators, as a constituent of fertilizers, pesticides, ﬁreworks and antifouling paints applied on ship hulls . Copper is one of the elements which is essential for human beings, but at higher concentration it affects the health of fauna, ﬂora and humans adversely. Excessive intake of copper is associated with Indian childhood cirrhosis , Wilson disease , Alzhiemer’s disease, Tyrolean infantile cirrhosis , familial amyotrophic lateral scelerosis (FALS) . WHO has set a guideline value for copper in drinking water at 2 mg L1  but various industries are releasing higher amount of Cu(II) in to water streams. Due to its toxic effects industrial efﬂuents must be treated before being discharged in to water bodies. There are innumerable numbers of technique such as precipitation, reverse osmosis, membrane ﬁltration, ﬂocculation, ion exchange, nanoﬁltration, cementation,
* Corresponding author. Tel.: +91 542 6701865; fax: +91 542 2368428. E-mail address: [email protected]
bio sorption, adsorption and electro coagulation [7–15]. Among these techniques, adsorption on naturally available materials is superior to them in terms of cost effectiveness, better removal efﬁciency even from water containing trace amount of metallic concentration. Currently there are multitudinous adsorbents developed and currently being reported for the removal of metallic pollutants from aqueous solutions and industrial efﬂuents. Numerous adsorbents like carbon nanotube, activated carbon, modiﬁed zeolite, nano adsorbent (B2O3, TiO2), clay (illite), chitosan, rice hull ash, and cashew nut shells [14,16–22] have been reported for the removal of copper from aqueous solutions. But high cost of material and energy intensive process for their synthesis limit their application economically. On the other hand, sand is abundant on earth and supplant the currently used adsorbent due to its economical feasibility .In present study, modiﬁed sand is used for removal of Cu(II) from there aqueous solutions. Effect of various important parameters viz. contact time, pH, adsorbent dose and temperature were studied for the optimization of removal process. Kinetic and thermodynamic studies were also carried out for better illustration of Cu(II) adsorption on modiﬁed sand.
2. Materials and methods Materials employed in the experiment were copper sulphate pentahydrate (CuSO45H2O), concentrated nitric acid (HNO3), ferric chloride (FeCl3), ferrous chloride (FeCl2), ammonium hydroxide (25% NH4OH) were all of AR grade and were supplied by Merck, Mumbai, India. Sand was collected from the bank of river Ganga in Varanasi.
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.06.014
D. Gusain et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 841–847
2.1. Preparation of adsorbent The sand was collected from river bed of Ganges in Varanasi. Sand samples were sieved and washed to remove all earthen impurities. Sieved samples were then dried in oven at 105 8C. Afterwards, solution of ferric and ferrous chlorides in a molar ratio of 2:1 was prepared. Dehydrated sand particles were then added to 200 mL homogenous mixture of iron ion solution. Chemical solution precipitation was achieved at 25 8C under vigorous stirring by adding NH4OH (25%). pH was maintained around 10 using a pH metre (IKON, India). The precipitates were washed several times with distilled water and then ﬁnally dehydrated in an oven at 70 8C for 24 h. Prepared adsorbent was characterized by XRD (X-ray Diffractometer) (RIGAKU, MINIFLEX II, Desktop X-ray Diffractometer, Japan), FTIR (Varian 1000 FT-IR, Scimitar Series) and SEM (INSPECT S50, FEI) to investigate its speciﬁc characteristics of adsorbent material. 2.2. Determination of point of zero charge (pHZPC) Determination of pHZPC of modiﬁed sand has been carried out to investigate the surface charge properties of adsorbent material. To determine the pHZPC, a solution of 0.01 M NaCl has been prepared with its pH adjusted in 2–12 with addition of HCl/NaOH. Afterwards, 50 mL of various samples of 0.01 M NaCl were prepared with distinct pH arranged in reagent bottles. Thereafter, 0.20 g of adsorbent is added in the solutions. After a period of 48 h, pH of each solution was measured. A graph was then plotted with two y-axis variants: one is initial pH of the solution (pH intial) and other one is the ﬁnal pH of the solution (pH ﬁnal). The point of interaction of pHintial and pHﬁnal represents the pHzpc of adsorbent. 2.3. Batch adsorption studies Batch adsorption studies were conducted to determine the optimum conditions for the removal of copper ions from aqueous
solutions. Stock solution of copper was prepared from copper sulphate pentahydrate (Merck, India) by dissolving 3.93 g of copper sulphate in 1000 mL distilled water . This solution was used for making working solutions. pH of the solutions was adjusted with 0.1 M HCl and 0.1 M NaOH. Batch experiments were conducted by taking 50 mL Cu(II) solution in 125 mL reagent bottles at desired experimental conditions. Equilibrium time and dose were ensconced at 30 min and 10 g L1, respectively. The residual concentrations of copper after batch adsorption experiment were determined by atomic absorption spectrophometer (Szhimadzu, AA 7000). Copper was analyzed at 324.53 nm with a slit width of 0.7 nm with an air acetylene ﬂame having acetylene ﬂow rate at 1.8 L min1.The amount of Cu(II) ions adsorbed on per unit mass of the adsorbent (mg g1) was determined by the following expression : Ci Ce qe ¼ (1) V W where qe is the amount adsorbed on per unit mass of the adsorbent (mg g1), Ci and Ce (both in mg L1) are the initial and the equilibrium concentration, respectively, W is the weight of adsorbent (g). Percentage removal of Cu(II) was calculated by applying following equation : %Removal of metallic ions ¼
Ci Ce 100 Ci
3. Results and discussion 3.1. Characterization of adsorbent Modiﬁed sand was characterized by XRD. The sample is matched with cmdf ﬁle (CRYSTAL MAKER 1.2) of Quartz (alpha) and goethite (Fig. 1). Peaks at 26.6468 (0 1 1), 20.8588 (0 1 0), 50.1388 (1 1 2) represent the peak of quartz. Whereas peaks were matched at 21.258 (0 1 0) and 26.288 (2 0 1) and 35.698 (0 1 1) represent the peak of goethite.
Fig. 1. X-ray diffractogram of modiﬁed sand, goethite and quartz.
D. Gusain et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 841–847
3620 2927 30
778 459 10 5 4000
Bare adsorbent Cu(II) loaded adsorbent 3500
signiﬁcant characteristics that determines the pH at which the adsorbent surface has net electrical neutrality. At this pH value, the acidic or basic functional groups are completely neutralized and there is no movement of adsorbent under the inﬂuence of external electric ﬁeld, subsequently any functional group present on the adsorbent surface does not contribute to the pH of the solution. Cations adsorption is more favourable at pH value higher than pHzpc due to rise of negatively charged sites. It was found to be 8.2 which shows basic nature of adsorbent surface and offers adsorption of cationic pollutants better but, in the present study Cu(II) is precipitated at this pH as Cu(OH)2 and therefore, this or even higher pH could not be taken into consideration for adsorption of Cu(II) in present studies.
457 1082 1500
3.2. Effect of contact time on the removal of Cu(II) 500
Wave number(cm ) Fig. 2. FTIR of modiﬁed sand.
The surface groups’ analysis of the modiﬁed sand was carried out by FTIR. FTIR of adsorbent materials is shown in Fig. 2. Absorption peaks in the range of 400–1000 cm1 is related with the metal–oxygen vibration. The absorption peaks at 3442 cm1 is associated with the stretching vibration of hydroxyl groups. On comparison of FTIR of bare sand  and modiﬁed sand, it was observed that various peaks of iron oxide are also present with the peak of Si–O. It shows that, loading of iron oxide on to sand particles. FTIR of modiﬁed sand before and after adsorption was also investigated to observe the changes on adsorbent materials during adsorption process. Results indicate that there is not any signiﬁcant difference in the adsorbent material before and after adsorption. It shows physical adsorption is involved in Cu(II) adsorption. It also gives an idea about the possibility of regeneration of adsorbent Peak at 1626 cm1 is due to bending of water molecules. Coating of iron oxide on sand was analyzed by SEM. It is clear from the SEM of modiﬁed sand that during the precipitation reaction, iron oxide particles cover the sand surface (Fig. 3) which can enhance the adsorption efﬁciency of sand. SEM of the adsorbent was taken (Fig. 3) and it shows that the surface of modiﬁed sand is quite rough with many humps and cavities. The surface roughness supports adsorption of the species. pHzpc of adsorbent was determined from the plot between pHintial and pHﬁnal (ﬁgure not given) .The pHzpc of an adsorbent is a
Contact time is an essential parameter to explicit the perspective about economical viability of contaminant removal from wastewater . The inﬂuence of contact time on adsorption of Cu(II) on modiﬁed sand was investigated by conducting batch experiments at various time up to 90 min. The experimental conditions were maintained at initial concentration of 10 ppm, pH 6.0, agitation speed 200 rpm, and adsorbent dose 10 g L1. The experimental results showed that the removal of Cu(II) increased from 43.43% to 59.59%. Copper removal rises very sharply during the ﬁrst 5 min which is attributed to boundary layer diffusion. Equilibrium reached in 30 min and after that no signiﬁcant removal of Cu was observed. The rate of removal of copper is rapid during the initial period, but it decreases later on until it reaches equilibrium. The rapid rate of Cu(II) removal by modiﬁed sand initially can be attributed to availability of plentiful surface area for the adsorption of Cu(II) ions. Later on adsorption sites had been exhausted during the adsorption. So, under these circumstances rate of transport of Cu(II) from external to internal sites of the adsorbent governed rate of uptake rather than that of external surface diffusion. Hence the rate is decreased later due to slow process governing the adsorption . In addition to this, active sorption sites available on the surface of adsorbent are ﬁxed, and each active site adsorbs only one ion in its monolayer, which was evident by sharp rise of adsorption rate initially, slowing down with progressing time period due to the intensiﬁed competition of Cu(II) ions remained in the solution for the progressively decreasing availability of active sites on adsorbent surface . 3.3. Effect of adsorbent dose on the removal of Cu(II) The inﬂuence of adsorbent dose on Cu(II) via modiﬁed sand was probed by varying the dose from 10 g L1 to 40 g L1 while keeping other variables constant. Study on adsorbent dose determines the adsorption capacity at a particular concentration . It was observed that percentage of adsorption increases from 43.43% to 63.63% via raising the dose from 10 g L1 to 20 g L1.When the adsorbent dose increases, the number of active surface for adsorption will increase and this ultimately results in increase of the percentage of Cu(II) removal from the solution. Increasing trend of removal of Cu(II) is due to availability of greater active surface for adsorption. It was observed that on further raising the dose, removal did not increase which was due to higher number of unsaturated adsorption sites . 3.4. Effect of pH on the removal of Cu(II)
Fig. 3. SEM of modiﬁed sand.
A change in pH not only moulds the surface charge on the adsorbent but also alters solution chemistry of heavy metals: hydrolysis, redox reaction, degree of ionization, precipitation and
D. Gusain et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 841–847
298K 308K 318K
58 56 54 52
50 48 46 44 42 40 38 36 34 32 Fig. 4. Chemical equilibrium diagram.
Contact time(min) speciation of heavy metals, which apparently affect the adsorption process . This parameter is intimately associated with contention skill of hydronium ions with metallic ions for active sites on adsorbent surface. Also, charge density variation with pH is a strong factor regulating the adsorption of Cu(II) onto adsorbent surface. At higher pH, copper ions is hydrolyzed and precipitated  so in order to make adsorption process remain unaffected by hydrolysis and precipitation, experiments were not performed at higher pH. Hence, to examine the impact of hydronium ions on adsorption of Cu(II) by modiﬁed sand, experiments were conducted in a pH range of 2.0–6.0. Chemical equilibrium diagram (Fig. 4) has been derived using a chemical equilibrium software: medusa (having input range from 1 ppm to 20 ppm with a pH range of 1–12). It depicts that up to 6.5, copper remains in the form of Cu2+ whereas up to pH 6.8 it co existed with CuO(cr). After pH 6.8 it remains in the form of CuO(cr). This adsorption of Cu(II) does not vary signiﬁcantly with change in pH and overall percentage removal varies in the purview of 27.27–59.59%. These results conﬁrm the relevance of pH for the removal of Cu(II) from aqueous solutions by modiﬁed sand. The increase in adsorption with rise of pH can be described by rise in uncovering of more charged functional groups, which resulted in slight rise in attraction of adsorption sites to positively charged copper ions in the solution. Secondly copper adsorption acted upon by protonation of binding sites, resulting in competition between hydronium ions and Cu(II) ions for occupancy of the active sites. As pH is raised, more surface of the adsorbent carried a negative charge and this resulted in increased attraction of Cu(II) ions by modiﬁed sand surface at this pH(viz. 6.0). This was accomplished due to superior ability of Cu(II) ions to compete with hydronium ions for the active sites present on adsorbent surface . 3.5. Effect of temperature on the removal of Cu(II) Temperature has signiﬁcant effect on adsorption process. It determines whether the adsorption process is favourable at higher temperature or at lower temperature. A temperature supplies energy to the system in the form of heat and affects the adsorption capacity depending on the exothermic or endothermic nature of the system. Experiments were conducted at three different temperatures (Fig. 5). Removal of Cu increased with increase in temperature which means that the present adsorbate–adsorbent system follows endothermic process. The rise of adsorption capability with rise of temperature was attributed to due to increase of surface active sites available for adsorption of Cu(II) ions, boundary layer
Fig. 5. Effect of temperature on the removal of Cu(II) by adsorption on modiﬁed sand.
thickness around the adsorbent decreased leading to raised adsorption capability due to greater dissociation of functional groups present on the surface [30,31]. In addition to this, enlargement of pore size and activation of the adsorbent surface is responsible for the rise in adsorption capacity . Solubility of copper increases with rise of temperature, hence the interactive forces between ‘solute and solvent’ become slightly stronger than the forces between ‘solute and adsorbent’ but as in case of present system, the removal is higher at elevated temperature, it seems that the difference between the two forces is very small and hence even after increment in solubility of Cu, a higher removal was obtained. However, the removal increases at higher temperature because of dissociation of functional groups present on the adsorbent surface. 3.6. Thermodynamic study Thermodynamic parameters viz. change in standard free energy (DG0), standard enthalpy (DH0) and standard entropy (DS0) is taken into consideration to determine the spontaneity and other aspects of a given adsorption process. The experiments were carried out at three different temperature 25 8C, 35 8C and 45 8C, respectively. Thermodynamic parameters were estimated by using the following well known expressions [28–31]: Kc ¼
C ad Ce
DG0 ¼ RT lnK c
T 2T 1 Kc2 ln DH 0 ¼ R T2 T1 Kc1
DH0 DG0 T
where Kc is the equilibrium constant; Cad and Ce are the equilibrium concentrations of metal ions adsorbed and in aqueous phase, respectively. R is the gas constant (1.987 cal mol1 K1). The values of DG0, DH0 and DS0 calculated at three different temperature (25, 35 and 45 8C) are given in Table 1. The positive values of enthalpy change (DH0 = +4.73 kcal mol1) indicate the endothermic nature of the adsorption process. Enthalpy
D. Gusain et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 841–847 Table 1 Values of different thermodynamic parameters.
(cal K1 mol1)
298 308 318
0.230 0.396 0.526
change due to physiosorption occurs at value lower than 40 kJ mol1 (i.e. 9.556 kcal mol1) whereas process having chemisorption as the chief way for adsorption have value more than 40 kJ mol1 (i.e. 9.556 kcal mol1) . The low value of enthalpy change availed in the present study ensure a weak interaction between adsorbent and adsorbate, which was evident for physiosorption of Cu(II) ion onto modiﬁed sand. Negative values of DG0 indicate that the process is spontaneous in nature and can occur on its own without requirement of any external source of energy. Further, decrease in values of DG0 with rise of temperature indicates that the process becomes more feasible at higher temperatures than at lower temperature. The positive values of DS0 indicate the increase of disorderness at adsorbate adsorbent interface during adsorption of copper on the surface. With rise of temperature DS0 decreased which depicts that the disorderness decreases with a rise of temperature. Hence, mobility of copper ions to escape from the adsorbent surface to the aqueous solution decreases with rise of temperature, which is authenticated by rise of adsorption at higher temperature than at lower temperature. The positive value of DS0 is indicative of increased randomness in the system and at solid solution interface during the process of removal of Cu by adsorption onto modiﬁed sand. At the same time, positive value of DH0 indicates endothermic nature of the removal. The positive value of DS0 also reﬂects afﬁnity of the adsorbent for Cu(II) ions in the solution and suggest that some structural changes in Cu(II) and in the modiﬁed sand might have taken place during the process of adsorption [29,32]. This can also be expected because of the mass transfer of copper(II) ions from the solution phase to the adsorbent which, consequently might have resulted in increase of relative disorderness in the system and at the solid–liquid interface. It can also be understood that spontaneity of the process might have also resulted due to a rise in the value of DS0 as the system shifts to a more uniform and stable state [33,34]. 3.7. Kinetic modelling for removal of copper from aqueous solution Entire adsorption process is governed by either one or more steps (e.g. ﬁlm diffusion, intraparticle diffusion, further pore diffusion) and also inﬂuenced by structural properties of the adsorbent, porosity, speciﬁc area, particle size, surface complexation, hydrolysis, and speciation. In order to gain a better holistic comprehension of the adsorption process, kinetic models should be used to test the experimental data. Several kinetic models have been developed to portray the kinetics of heavy metal adsorption, partaking pseudo ﬁrst order and pseudo second order kinetic model and intraparticle diffusion model . In the present study pseudo ﬁrst order, pseudo second order kinetic model and intraparticle diffusion model is used to investigate kinetics of
298 K 308 K 318 K
Contact time(min) Fig. 6. Pseudo-ﬁrst order kinetic plots for the adsorption of Cu(II).
adsorption of Cu(II) by modiﬁed sand by implementing them on resultant data. 3.8. Pseudo ﬁrst order kinetic model Pseudo ﬁrst order kinetic model can be expressed by the following equation : dq ¼ k1 ðqe qt Þ dt
The integrated form of above equation is expressed as follows : k1 logðqe qÞ ¼ log qe (8) t 2:303 where k1 (min1) is the ﬁrst order rate constant, qe and q are the amount of adsorbate species adsorbed on adsorbent at equilibrium and at any time, respectively. The values of k1 at different temperatures were calculated by retrieving slope via plotting a graph between ‘log(qe q) vs t’ at different temperatures (Fig. 6). The values of the rate constants of pseudo ﬁrst and pseudo second order kinetic models at different temperatures are tabulated in Table 2. Value of pseudo ﬁrst order rate constant at different temperatures are 0.098, 0.058 and 0.037 min1. 3.9. Psuedo second order kinetic model Resultant data were also tested for pseudo second order kinetic model. Pseudo second order model is founded on the assumption that rate limiting step is chemisorption in nature. This model is mathematically represented as follows : dq ¼ k2 ðqe qt Þ2 dt
where k2 (g mg1 min1) is the rate constant for pseudo second order model equation. Above mentioned equation can be
Table 2 Values of pseudo-ﬁrst order and second order rate constant for the removal of Cu(II) by adsorption on modiﬁed sand. Values of pseudo ﬁrst order constants
Values of pseudo second order constants
qe (mg g1)
k2 (g mg1 min1)
qe (mg g1)
298 308 318
0.596 0.657 0.697
0.098 0.058 0.037
3.763 4.104 4.442
0.99971 0.99998 0.99994
0.122 0.128 0.189
0.611 0.678 0.715
0.99984 0.99965 0.99981
D. Gusain et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 841–847 0.72
represented in integrated form as follows [18,21]: t 1 1 ¼ þ t qt k2 q2e qt
h ¼ k2 q2e
here h (mg g1 min1) is the initial sorption rate. The values of k2 and qe are acquired from the slope and intercept of the plot between ‘t/qt vs t’ (Fig. 7). The values of the rate constants of pseudo second order kinetic model are 0.122, 0.128 and 0.189 g mg1 min1 at different temperatures (Table 2). On comparing the values of correlation coefﬁcients (R2) it is proximate that the kinetic data is better ﬁtted in second order model as compared with the ﬁrst order model. Theoretical values of qe obtained from the graph (Table 2) have been compared with experimental values of qe. Experimental values of qe have been better concorded with theoretical values obtained from pseudo second order model. Values of qe calculated from the ﬁrst order kinetic model by plotting log (qe qt) vs t were however not comparable to the experimental value of qe, hence it pared down the validity of this model for the present system. Eventually aforementioned write up deduced the cogency of pseudo second order model than pseudo ﬁrst order kinetic model for adsorption of Cu(II) ions on modiﬁed sand. 3.10. Intraparticle diffusion model To determine legitimacy of intraparticle diffusion as the rate limiting step in the adsorption of Cu(II) onto modiﬁed sand, intraparticle diffusion model proposed by Weber and Morris was used to examine the experimental results. Adsorption of substances from the solution is covered under the following major steps: the external surface diffusion followed by pore diffusion or intraparticle diffusion and ﬁnally adsorption through the interior surface bounding the pore and capillary spaces of the adsorbent. Adsorption systems have a prospect for intraparticle diffusion being the rate limiting step. The rate constant value of intraparticle diffusion (kid) were clinched graphically from the slopes of graph executed between amount adsorbed and square root of time at different temperature by using the following equation : q ¼ kid t 1=2 þ C
160 140 120
100 80 60
298 K 308 K 318 K
Contact time(min) Fig. 7. Pseudo-second order kinetic plots for the adsorption of Cu(II).
0.66 0.64 0.62 0.60 0.58 0.56 0.54 0.52
298 K 308 K 318 K
0.50 0.48 0.46 0.44 0.42 2
Fig. 8. Intraparticle diffusion plot for the removal of copper.
here kid is the intraparticle diffusion rate constant value, q is the amount adsorbed at time t (mg g1), t1/2 is square root of time (min1/2) and C is the intercept (mg g1). The value of intercept C gives information concerned to thickness of boundary layer, and larger intercept intimate towards major role of external diffusion as rate limiting step. If the intraparticle diffusion is involved in the adsorption process, the plot of ‘q vs t1/2’ (Fig. 8) will be linear and if slope passes through the origin then intraparticle diffusion is the only rate limiting process. In addition, if the data shows multi linear plots then a combination of diffusion inﬂuences the adsorption process. Values of intraparticle diffusion constants at different temperature were calculated from (Fig. 8) and values of kid estimated from the slopes of the curves at different temperatures are presented in Table 3. The plot is bifurcated into two parts. Maiden part was sharper in nature and subsequent part is linear in nature. This duality behaviour of curve is attributed to varying quantity of adsorbate adsorbed at different time period. Secondly, the linear plot did not pass through origin which further testiﬁes that rate of mass transfer varies during the initial and ﬁnal stages of adsorption. During the initial stages, adsorption is governed by external surface diffusion and subsequently during the period of straight portion adsorption is ceased due to reduction in number of available adsorption sites and it is a badge of weak intraparticle diffusion due to absence of linearity. Here, a linear plot was absent in the graph, which is a badge of intraparticle diffusion model. Hence, intraparticle diffusion was not the rate limiting step in adsorption of Cu(II) by modiﬁed sand. The intercept values increased with rise in temperature, which depicts that the external diffusion is increased with rise in temperature. So, external surface diffusion is predominantly raised with subsequent rise in temperature. So, adsorption of Cu(II) by modiﬁed sand is predominately governed by external surface diffusion.
Table 3 Intraparticle diffusion rate constant for the removal of Cu(II) by adsorption on modiﬁed sand at different temperatures.
kid (mg g1 min1/2)
298 308 318
0.020 0.025 0.024
0.433 0.448 0.505
0.86 0.88 0.87
D. Gusain et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 841–847
4. Conclusions On the basis of present study, following conclusions may be drawn: Adsorbent material used in present study is not very costly and hence makes the treatment process cost effective. Modiﬁcation process is simple and it enhances the adsorption efﬁciency. Fast removal is achieved in 5 min and equilibrium time was 30 min for adsorption of Cu(II) on modiﬁed sand. pH study indicated that higher removal is achieved at 6.5 pH, thereafter, the removal is due to precipitation of ions. Adsorption of copper rises with rise of temperature which conﬁrms endothermic nature of removal process. Value of DG0 was found to be negative at all the temperature range studied which shows feasibility of adsorption process. Adsorption of Cu(II) followed pseudo second order kinetics. As in present investigation, the raw materials used for the preparation of adsorbent is naturally available material and modiﬁcation process is not very costly it can be proved a economically viable alternate of costly adsorbents for the Cu(II) removal from their aqueous solutions. Acknowledgements One of the authors, Mr Deepak Gusain is thankful to Council of Scientiﬁc and Industrial Research(CSIR), Govt of India for ﬁnancial support(JRF). Authors also acknowledge Department of Ceramic Engineering, Indian Institute of Technology(Banaras Hindu University) Varanasi for extending XRD analysis of the samples. Authors gratefully thank the anonymous reviewers for their comments on the manuscript. References  R.J. Lifset, M.J. Eckelman, E.M. Harper, Z. Hausfather, G. Urbina, Sci. Total Environ. 417/418 (2012) 138.
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