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Use of Rice Husk Ash for Adsorptive Removal of Cobalt (II) ions along with Kinetic, Isothermal and Thermodynamical Modelling.

Byline: UZMA ZAFAR, SUMRA NAEEM, RABIA REHMAN, MUHAMMAD ZIA-UL-HAQ, AND NAZIR AHMAD

Summary: Rice husk ash (RHA) has been employed in this research work for Co (II) ions adsorptive amputation from water in batch mode. Optimized conditions for Co (II) adsorption came out as: pH [?] 7.5, adsorbent dose [?] 20 gl -1 of metal ion solution and equilibrium time [?] 120 minutes for the initial Co (II) concentration range of 50-200 mgl -1 , following pseudo-second order kinetics. Langmuir isothermal model was more applicable as compared to Freundlich and Dubnin- Radushkevich (D-R), within the concentration range studied; indicating that monolayer chemisorption of Co (II) ions occurred on homogenously distributed binding sites on RHA. Thermodynamic parameters value of (delta)H 0 =-18.448KJ/mole and (delta)G 0 =-5.195KJ/mol at 283K suggests that the adsorption of Co (II) on RHA is an exothermic and spontaneously feasible process. (delta)G 0 becomes less negative at higher temperature and therefore less cobalt is adsorbed at higher temperatures. So, it is concluded from this study that RHA can be effectively used for adsorption of Co (II) ions.

Keywords: Cobalt (II), Rice husk ash, adsorption.

Introduction

Increasing environmental pollution is a serious challenge for scientists. Heavy metal ions is of great concern because of their carcinogenity, non- biodegradability and bio-accumulative properties [1]. Cobalt as a natural element occurred in certain earth crust having several radio-isotopes. Due to its colouring and ferromagnetic nature, its compounds are widely employed in enamels, nuclear medicine, grinding wheels, semiconductors, painting on glass and porcelain, in electroplating, in vitamin B 12 manufacture, and as a catalyst for organic syntheses. Cobalt salts solubility varied from highly soluble to insoluble range. Its salts are highly persistent in water usually with a half-life greater than 200 days.

It has both beneficial and harmful effects on health. Its excessive concentration stored in liver, kidney and pancreas in human beings. 5-8 (mu)g is tolerable ingestion limit, but its excess leads to thyroid gland enlargement, diarrhoea, numbness in fingers and polycythenia (increased number of RBCs in blood) [2, 3]. The maximum permissible concentration cobalt in fresh waters and ground water are 2.8 and 3.2 (mu)gl -1 respectively [4]. Therefore it is necessary to remove Co (II) ions from industrial effluents before their discharge into main water streams. Various physiochemical ways are adopted for its removal like: chemical precipitation or flocculation, reverse osmosis, cation exchange resins and adsorption. Among all these methods adsorption on activated carbon is more effective. But high cost of activated carbon urged scientists to search more adsorbents using indigenous resources [5, 6].

Various scientists employed different adsorbing materials like agrowaste materials, clays, zeolites, plant leaves etc [7-10]. Rice husk ash has potential to adsorb low concentrations of contaminants from aqueous solutions [11]. More than 100 million tonnes rice husk is generated from rice milling industry annually as a by-product [12], 96% of which came from Asian and African countries. Carbon and silica are main constituents of rice husk. When it is burnt, nearly 20 wt% remains as ash. RHA has mostly contained [greater than or equal to] 95 wt% of silica having more porosity and greater surface area. It has already been reported as a useful adsorbent for lead, cadmium, chromium, zinc, nickel, gold, phenols and dyes removal from water [13, 14].

This study is focussing to explore the possibility of rice husk ash use as adsorbent for Co (II) ions amputation from water. Initially adsorption parameters were optimized followed by kinetics, isothermal and thermodynamic studies of adsorption using optimized conditions of all parameters.

Results and Discussion

Adsorbent Characterization

Physio-chemical characteristics of RHA are enlisted in Table-1. It is clear from Table-1 that RHA largely consisted of SiO 2 . The chemical composition %age varied in different RHA samples depending on sowing and geo-tropical conditions along with fertilizer and soil chemistry [12, 13]. X-ray diffraction pattern and SEM micrograph of adsorbent are depicted in Figs. 1 and 2. Fig. 1 confirms the presence of excessive silica in the sample as obtained by ashing the rice husk below 700 o C (Table-1). Silica occurs in several forms (polymorphs) within the husks. In nature, the polymorphs of silica are quartz; Cristobalite; Tridymite, Coesite; Stishovite; lechatelerite and opal [15]. Whereas SEM micrograph indicated that RHA has porous heterogeneous surface, which is helpful in physio- sorption.

Table-1: Physiochemical characteristics of the rice husk ash.

###Chemical Composition (%)###

Loss on ignition###12.2

SiO2###84.3

Fe2O3###0.6

Al2O3###0.3

MgO###0.5

CaO###1.4

Na2O###0.4

K2O###0.2###

Physical Characteristics

Surface area (m2.g-1)###57.5

Bulk density (g.cm-3)###0.96

Mean Diameter (m)###3.02 x 10-4

Contact Time Effect

It was investigated at various contact time intervals ranging between 10-180 min for the initial Co (II) ions concentration varying in between 50-200 mgl -1 range and results are graphically depicted in Fig. 3. It indicated that Co (II) adsorption on RHA was rapid in first 10 min at 306 K, and equilibrium reached after 120 min. After that it only increased more 0.93 % further till 180 min interval and becomes almost constant after equilibrium establishment. Hence 120 min contact time was used for all further studies.

Conditions: C i = 50-200 ppm, temp.= 306K; Adsorbent conc.=20g/l, pH= 7.5

Effect of pH

The metal ion solution pH affects the surface chemistry of adsorbent and solubility of the adsorbate species [16]. The influence of pH on adsorptive removal of Co (II) ions by RHA was studied in 2-9 pH range and result obtained is shown in Fig. 4. The Ksp value for Co(OH) 2 is 3 x 10 -16 and pOH comes out is 5, while pH is 9. While in this case, maximum adsorptive removal occurred at 7 pH, afterward adsorption decreases, because solubility of Co(II) ions decreases to a greater extent in basic pH conditions. It was found that greater adsorptive capacity of RHA for Co (II) ions was observed in solution of pH 7.0 and 8.0. When the pH is reduced, surface charge of RHA becomes increasingly positive and because the competition of the hydrogen ions for the binding sites, metal ions tend to desorb at low pH region as well as small decrease in Co (II) adsorption was observed at pH higher than 9.0.

This behaviour is attributed to the formation of soluble Co (II) complexes, which remain in solution as dissolved species and hindered in chemisorption [17]. Experimental conditions: C i = 50 ppm, temp.=306K, Adsorbent conc.=20g/l Initial Concentration of Co (II) Ions Effect

The metal concentration range is varied from 50-200 mgl -1 for this purpose and results are given in Figs. 5 and 6. It is obvious from these graphs that the adsorption capacities 'q e '(mgg -1 ) of RHA enhanced with increasing cobalt concentration because it facilitates to overcome the resistances in mass transfer of adsorbate between aqueous phase and solid phase by enhancing interaction between Co (II) ions and RHA [12].

Adsorbent Amount Effect

The dependence of Co (II) adsorption onto RHA amount (m) was studied at 306 K at the 50 mgl - 1 initial concentration of Co (II) ions, by varying the adsorbent amount and volume of the metal solution constant and the result is given in Fig. 7. It is obvious from this graph that adsorption of Co (II) ions increased first and then approximately remained constant afterward. This enhancement in adsorption is due to larger surface area and more adsorption sites availability. At m less than 15 gl -1 , RHA surface becomes covered with Co (II) and the remaining concentration in the solution is more. With increase in RHA amount, the Co (II) removal enhances. At m greater than 20gl -1 , Co (II) adsorptive removal becomes very low because surface Co (II) concentration and its solution concentration equilibrate with each other. After that, it becomes nearly constant.

Here 'q e ' and 'q t ' are adsorbed amounts of Co (II) ions on RHA at equilibrium and at time 't'(mgg -1 ), 'k f ' (min -1 ) is Lagergren rate constant, 'k s ' (gmg -1 min -1 ) is pseudo-second-order rate constant, ' ' (mgg -1 min -1 ) is the initial adsorption rate, ' ' (gmg -1 ) is the desorption constant in Elovich model, 'V'(ml) is volume of Co (II) solution, and k 0 are Bangham's model constants, 'I' is intercept and 'k id ' (mgg 1 min -1/2 ) is intra-particle diffusion rate constant [18-21].

For finding the values of these parameters, regression analysis of respective model graphs were carried out using straight-line plots of 'ln(q e -q t )' vs. 't' for Lagergren model (Fig. 8), 't/q t ' against 't' for the pseudo-second-order model (Fig. 9), the plots of 'q t ' vs. ln(t) (Fig. 10) for Elovich model and the plot vs. 'log(t)' for Bangham's model (Fig. 11). These parameters values are given in Table-2.

Table 2: Kinetic parameters for the removal of Co (II) ions by RHA from aqueous solution (T=306K, t=120 min, =50-200 mgl -1 , solution to adsorbent

ratio=50:1, m=20 gl -1

###Pseudo-first-order

Ci###qe,exp###qe.calc###kf###

(mgl-1) (mgg-1)###(mgg-1)###(min)-1###R2

50###2.173###0.147###0.020###0.916

100###3.525###0.179###0.034###0.959

130###4.218###0.490###0.011###0.952

150###4.443###0.507###0.016###0.909

200###5.500###0.474###0.021###0.821

###Pseudo-Second-order

Ci###qe.calc###h###kf

(mgl-1) (mgg-1)###(mgg-1min-1) (gmg-1min-1)###

50###2.197###2.13###0.207###0.999

100###3.732###6.02###0.072###1

130###4.400###1.32###0.053###0.998

150###4.608###1.54###0.047###0.998

200###5.980###0.52###0.028###0.985

###Elovich

Ci###A

(mgl-1)###(mgg-1min-1) (beta)(gmg-1)###R2

50###1.203###16.129###0.744

100###1.079###18.52###0.938

130###6.060###0.629

150###1.192###6.800###0.654

200###65.29###1.845###0.659

###Bangham###

(mgl-1)###(alpha)B###Ka (g)###R2

50###0.117###1.650###0.710

100###0.023###1.364###0.972

130###0.069###0.909###0.660

150###0.061###0.837###0.690

###Intra-particle diffusion

(mgl-1) (mgg-1min-1/2)###I (mgg-1)###R2

50###0.026###1.957###0.819

100###0.026###3.310###0.953

130###0.065###3.529###0.727

150###0.056###3.865###0.765

200###0.202###3.246###0.793

Temperature Effect

Fig. 13 shows adsorption isotherms graphs, 'q e ' vs 'C e, at these temperatures, i.e. 283, 306, 343 K. It is clear from this graph that with increase in temperature, adsorption of Co (II) ions decreases, and at lower Co (II) initial concentration 'C i ' adsorption capacity 'q e ' rises markedly. The adsorption decreased significantly for the initial Co (II) concentration from 283 to 343K suggesting that adsorption between Co (II) ions and RHA is an exothermic process [22].

Adsorption Isotherms

Isothermal investigations were carried out for determining mechanism of adsorptive removal of Co (II) ions by RHA. Freundlich [23], Langmuir [24] and Dubinin-Radushkevich [25], have been used for this purpose. The related parameters of these models are summarized in Table 3.

Freundlich adsorption isotherm gives an empirical expression, which was used for the study of Co (II) ions adsorption onto RHA in the following linearized form.

where 'q e ' and 'C e ' has the same meanings as described in previous section and K F and 1/n are Freundlich constants. Fig. 14 is showing the Freundlich plot of 'logq e ' against 'logC e '. The intercepts and slope of the straight lines give the values of 'K F ' and '1/n' (Table 3). 1/n values less than 1 show the favourable adsorption nature.

Table 3: Isothermal parameters for adsorption of Co (II) ions by RHA, (t = 120 min, C i = 50-200 mgl -1 , solution to adsorbent ratio=50:1 therefore m=20 gl -1 ).

The Langmuir isotherm represents monolayer adsorption on discrete localized adsorption sites having the same adsorption energies and no side ways interaction between adsorbate species. The Langmuir equation is:

where 'q m ' (mgg -1 ) is maximum adsorption capacity and 'K L ' represents the binding constant [24]. In Fig. 15, Langmuir isotherm is plotted between 'C e /q e ' and 'C e '. The values of 'q m ' and 'K L ' are calculated by regression analysis of this graph and enlisted in Table-3. It shows that maximum adsorption capacity of RHA is greater at lower temperatures, i.e.: 6.28 mgg -1 at 283 K and decreased at higher temperatures, i.e.: 3.63 mgg -1 at 343 K. It is also further confirmed by thermodynamic investigations.

In Fig. 16, Dubinin-Radushkevich isotherm for the adsorption is given. Its equation is:

where 'q D-R' represent maximum adsorption capacity of adsorbent (mgg -1 ), is constant with dimension of energy and '(epsilon)' is the Polanyi adsorption potential, which is equal to

where 'R' is a gas constant (KJmol -1 k -1 ), 'T' is the temperature (Kelvin) and 'C e ' is the concentration of adsorbate in solution (mgl -1 ). The plots of 'lnq e ' vs ' ' give the values of and 'q D-R ', which were calculated from the slopes and intercepts of respective plots are shown in Table-3. Adsorption energy, 'E' is determined by eq.10 [26]: If E value is in the range of 8-16 KJmol -1 , the adsorption type is ion exchange and if E less than 8 the adsorption type is of physical nature. The value of 'E' found to be in between 7.46 and 11.18 KJmol -1 showing physical as well as ion-exchange type of Co (II) ions adsorption on RHA.

Estimation of Thermodynamic Parameters

Thermodynamic parameters are calculated by following equations: where ' 'is equilibrium constant, ' ' is equilibrium concentration of Co(II) ions in solution (mgl -1 ) and 'C i A e ' is the solid phase concentration (mgl -1 ) at equilibrium. The values of (delta)H o and (delta)S o were determined from slope and the intercept of the plot of ' ' versus '1/T' shown in Fig. 17 [11]. Adsorption of cobalt (II) onto RHA decreased when the temperature was increased from 283 to 343K. The process was thus exothermic in nature. Table-4 shows the values of the enthalpy change ((delta)H o ), the entropy change ((delta)S o ) and free energy ((delta)G o ) respectively, for a Co (II) concentration of 50 mgl -1 . The negative (delta)H o value indicates the exothermic nature of adsorption and (delta)S o values show the change in the randomness at the RHA-Co(II) solution interface during the adsorption. The negative ((delta)G o ) value shows that this process is feasible and spontaneous.

Materials and Methods

Rice husk was obtained from Darogh-e-wala Rice Mill, Lahore (Pakistan). It was washed and oven dried at 105 o C for 3 hours. It was crushed and sieved with 80 mesh (ASTM). Rice husk ash (RHA) was prepared from it by incineration below 700 o C in a Muffle furnace. BET method is applied for specific surface area calculations. Characterization analysis of RHA was done by classical chemical methods [12]. X-ray powder diffraction (XRD) (Siemen D5000 Diffractometer) analysis was done by employing CuK (alpha) radiation. To gain additional insight micrographs were recorded by employing Scanning Electron Microscope (SEM) S2700 Hitachi-JAPAN. Stock solution of Co (II) ions (1000 mgl -1 ) containing 4.037 g CoCl 2 . 6H 2 O per litre distilled water was prepared. The concentration of Co (II) ions was determined by AAS (Perkin-Elmer AAnalyst 100).

Adsorption Studies

Adsorption studies were carried out in batch mode. A known weight (i.e. 0.5g of RHA) was added in Co (II) ions solution (25 ml) of known concentrations in flasks and shake in Gallen Kamp thermo state water bath shaker (Model BKS No. 305- 166, UK). After equilibrium the suspension was centrifuge (Wirowka-Type WE-1 centrifuge) for 10 min at 4500 rpm, and then filtered. pH meter (Model HI8417-Hanna) was used to check pH. Blank experiment with RHA using distilled water carried out simultaneously for zero setting of instrumentation. To study the effect of pH, in one set of experiments the pH of the suspensions was adjusted by using 0.01 M NaOH / HNO 3 . Adsorption of cobalt on RHA in terms of percentage extraction and the amount of Co (II) ions adsorbed per unit weight of the RHA ' ' were determined by these equations.

Adsorption capacity for the adsorption of cobalt species has been evaluated from Freundlich, Langmuir and Dubinin-Raduskevich (D-R) isotherm models, at three different temperatures (i.e. 283 K, 306 K and 343 K). The cobalt concentration studied was in the range of 50-200 mgl -1 for 50:1 solution to adsorbent ratio.

Conclusions

This research work indicated that rice husk ash has a good potential for adsorptive removal of Co (II) ions. This process is favourably regulated by decrease in temperature with maximum adsorption capacity 6.28 mgg -1 at 283 K. The optimum adsorbent dose was [?] 20 gl -1 and equilibrium time was 120 minutes.

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Author:Zafar, Uzma; Naeem, Sumra; Rehman, Rabia; Zia-Ul-Haq, Muhammad; Ahmad, Nazir
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Geographic Code:9PAKI
Date:Jun 30, 2013
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