CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.10, pp 329-337, 2017
Abstract : The present study was undertaken to evaluate the feasibility of citrus lemonium peel waste for the removal of Cr(VI) ions from aqueous solutions. Batch experiments were performed to study the biosorption of Cr(VI) on c.lemonium peel biosorbent. The maximum
biosorption capacity of c.lemonium peel biosorbent for Cr(VI) removal was ca. 22mgg−1.
Two simplified kinetic models viz. pseudo-first-order, pseudo-second-order models were tested to describe the biosorption process. Kinetic parameters, rate constants, equilibrium sorption capacities, and related correlation coefficients for kinetic models were determined. It was found that the present system of Cr(VI) biosorption on c.lemonium peel biosorbent could be described more favorably by the pseudo-second-order kinetic model. The biosorption process has been found to be exothermic. The results of the present study suggest that c.lemonium peel waste can be used beneficially in treating industrial effluents containing heavy metal ions. Keywords : Biosorption,Cr(VI), C.limonium, isotherm models, kinetic models, Thermodynamic studies.
Heavy metal pollution is considered as one of the prime candidates for causing environmental problems. Chromium (VI) is one of the heavy metals having catastrophic health effect on human body which includes nasal irritation, skin ulceration, lung cancer and respiratory diseases like bronchial asthma. The toxicity of Cr (VI) imposes two major health hazards i.e. genotoxicity andcarcinogenicity mainly due to the interaction of chromate ionswith genetic material i.e. deoxy-ribose nucleic acid (DNA) and freeradical (e.g. thiyl, hydroxyl) formation respectively. Anthropogenic activitylike leachate from landfill disposal, fly ash disposal and domestic or industrial sewage have increased concentration of Cr (VI) inatmosphere. Cr (VI) has been used widely in a variety of industriessuch as photography, tannery, ceramic, glass industries, pigments,paints, fungicides chrome alloy and metallurgic industries.Wastewater generated from these industries has been found tocontain significant amount of chromium that exists in twooxidation states as Cr (III) and Cr (VI). The maximum permissiblelimit for hexavalent chromium in inland is 0.1 mg/L whereas forpotable water is 0.05 mg/L, respectively.There are severalmethods available for Cr removalwhich includechemical precipitation, ion exchange, reduction, electrochemicalprecipitation, solvent extraction, membrane separation, cementation, evaporation and foam formation.However, high energy andchemical requirement, incomplete removal, generation of toxicsludge are the limiting factors of these treatment procedures .
Many studies have been reported using different natural biosorbents like activated carbon from sugarcane bagasse, Nymphaearubra, dried water hyacinth roots, sulphuric acid treated cashew nut shell, modified corn stalk, Echorniacrassipes and activated carbon fromTamarind wood. Thereare four mechanisms of biosorption for Cr (VI) viz., anionicadsorption, adsorption coupled reduction, anionic and cationicadsorption and cationic adsorption[1].
One of the important agricultural wastes is peel of different fruits, which can serve as potential adsorbents for the removal of diverse types of pollutants especially metal ions. Different types of fruit peels have been investigated so far for wastewater treatment. The aim behind using fruit peels as biosorbents is that it will provide a two-fold advantage to environmental pollution. Firstly, the volume of wastes could be partly reduced and secondly, prepared biosorbent can treat toxic industrial effluents at a reasonably cost. The aim of this study was to examine the efficiency of citrus limonium peel waste as biosorbent for the removal of arsenic from aqueous solutions. Equilibrium and kinetic studies were performed to describe the biosorption process[2].
Citrus limonum was collected from juice shops and washed with de-ionised water several times to remove impurities. Then completely dried in sunlight for about 3-4 weeks. It was crushed into small pieces and was powdered using domestic mixer. The test powder was graded using BSS sieve set to get the required average size for using as biosorbent. In the present study the powdered materials in the range of 75-212 µm average particle size were then directly used as biosorbent without any pre-treatment. This biosorbent powder was stored in moisture free glass bottles.
A stock solution of 1000 mg/l of Cr(VI) was acquired by dissolving potassium dichromate (Merck Company) in distilled water. The test solutions of various concentration 20,40,60,80,100 mg/l were prepared from the stock solution. The solution pH was balanced utilizing 0.1M HNO3 and 0.1M NaOH toward the start of the trial and not controlled subsequently. The conical flasks (250 ml) were shaken at 180 rpm in a temperature controlled rotatory shaker.
The batch biosorption experiments were performed at room temperature in 250 ml reagent bottles that contained 30 ml solution of a particular arsenic ion concentration at required pH and relevant amount of .climonium peel powder. The flasks were sealed with wax paper and shaken in a shaking incubator (Lab Companion, SI-300R, India) at 180 rpm with appropriate time and temperature. After shaking for a particular time period, the solution of the flasks was filtered using Whatman 42 filter paper (Sigma–Aldrich, UK) for estimation of metal concentration by atomic absorption spectrophotometer(Shimadzu AA-6300). The influence of pH of the solution on biosorption equilibrium was studied after changing the pH of the solution in a range of 2–10. The effect of contact time between solution and the c.limonium peel powder were monitored by varying it from 0 to 60 min at optimum pH. For equilibrium studies five different metal ion concentrations between 20 and 100 mg/l were used, while, for optimum biosorption study, the c.limonium peel biomass was varied between 0.1 and 0.5g. The percent Cr(V) removal was calculated for each run by following expression[3]:
%Biosorption =
(1)
where Ci and Ce were the initial and final concentration of Cr(VI) in the solution in mg/L. The amount of chromium ions biosorbed on the c. limonium peel powder was estimated from the differences between metal quantity added to the biomass and metal content of the supernatant using the following equation.
q = (C0-Ce)(2)
!
= = = =
Fig2: Effect of contact time on Cr(VI) removal by c.limonium peel. for 0,40,60,80,100mg/l metal and 0.1g/30ml of biosorbent concentration.
The sorption potential of the c. limonium peelover time was observed from 0.5 min to 10,15, 20,25,30,35, 40, 45,50,55,60 min by using 30 ml of 20 mg/l Cr(VI) at pH 5 (Fig. 2). The experimental results
–
Fig4: Effect of biosorbent dosage on Cr(VI) removal by c.limonium peel. for 20, 40, 60, 80, 100mg/l metal and 0.1 to 0.5g/30ml of biosorbent concentration.
The sorption capacity of Cr(VI) onto c. limonium peel by varying biosorbent dosage from 0.1g to 0.5g is shown in Fig.4. From the results it was found that the biosorption of Cr(VI) increased with an increase in biosorbent dosage and is highly dependent on biosorbent concentration. Increase in biosorption by increase in biosorbent dosage was because of increase of ion exchange site ability, surface areas and the number of available biosorption sites. It was observed that as the biosorbent concentration increased % biosorption decreased
4. Biosorption Equilibrium
Freundlich, Langmuir, were used to describe the equilibrium between biosorbed ions on the biomass cell (qe, q) and ions in the solution (Ce, q).
Freundlich Isotherm
Fig5: Freundlich biosorption isotherm at 0.1g/30ml of biosorbent concentration
The empirical Freundlich equation is used for modeling the sorption on heterogeneous surface. Thelinearized Freundlich equation is represented as [6,7]
Kf is an indication of the biosorption capacity of the biosorbent; n indicates the effect of concentration on the biosorption capacity and represents the biosorption intensity.These values of n and Kf can be acquired from the slopes and the intercepts of the linear plots individually. The logarithmic plot of sorbed and equilibrium concentration gives a straight line with a coefficient of determination is 0.994 indicating that sorption data fitted well with the Freundlich model (Fig.5). The value of 1/n for Cr(VI) is 0.355 g/l and Kf value is 4.627 mg/g.4.2
Langmuir isotherm
Fig. 6: Langmuir biosorption isotherm at 0.1g/30ml of biosorbent concentration.
Linear form of Langmuir isotherm equation is represented as[9]
(4)
These values qmax and Kl (where Kl, is the biosorption equilibrium constant) can be acquired from the slopes and the intercept of the linear plot. In Fig.6 Ce/qe is plotted against Ce yielding a straight line with R2 (0.984). The value of qmax (19.6 mg/g) was calculated from the slope of the linear plot, whereas the value of Kl (0.1478 L/mg) was derived from the intercept.
5.Kinetic Studies
The kinetics of the biosorption data was analyzed using two kinetic models, pseudo-first order and pseudo-second order. These models correlate solute uptake, which are important in predicting the reactor volume. These models are explained as follows.
5.1The Pseudo First-Order Equation
Fig. 7: pseudo first order for 20 mg/l of metal and 0.1 g/ 30 ml of biomass concentration.
The kinetics equation proposed by Lagergren[12] has been used to describe the biosorption of biosorbate from an aqueous solution. The pseudo first-order model is described by Eq.5
(5)
where qe is the amount of metal ion biosorbed on biosorbent at equilibrium (mg/g), qt is the amount of metal ion biosorbed on biosorbent at time ‘t’ (mg/g) and kl is Lagergren constant
(min-1). Integrating the above equation and transforming to log scale.
Linear plot of ln(qe -qt) against time indicates whether this kinetic model is applicable or not for biosorption process. The results of kinetic parameters are shown in table1.
5.2The Pseudo Second-Order Equation
Fig. 8: pseudo second order for 20 mg/l of metal and 0.1 g/ 30 ml of biomass concentration.
The pseudo second-order kinetic model is given as[12]
(6)
Where qe = amount of adsorbed metal ion on biosorbent at equilibrium (mg/g), qt = amount of adsorbed metal ion (mg/g) on biosorbent at time‘t’, k2 = second order rate constant (g/mg min). A linear plot of t/q vs t indicates whether this model of biosorption is applicable for this case or not.The values of constants of kinetic models obtained from the plots for biosorption of Cr(VI) onto c. limonium peel at 3030K are shown in Table 1 .The data showed good agreement with the pseudo second-order kinetic model (R2 = 0.999). However, the value of the determination coefficient (R2) indicates the applicability of the pseudo second-order model for describing the experimental results to a higher degree of accuracy. In addition, Fig.8 and Table 1, show that the q values (qe,cal) determined from the pseudo second-order model were closer to the experimental q values (qe, exp) than those determined from the pseudo first-order model.
Table.1: Biosorption rate constants, qe estimated and coefficient of correlation associated to the pseudo-first and second-order biosorption for the c. limonium peel biomass
Metal | qe Exp | Pseudo first order | Pseudo second order | ||||
---|---|---|---|---|---|---|---|
(mg/g) | K1(min -1) | qe cal(mg/g) | R2 | K2 | qe cal(mg/g) | R2 | |
Chromiu m | 5.51 | 0.084 | 2.048 | 0.867 | 0.124 | 5.617 | 0.999 |
The results of kinetic parameters are shown in Table 1. The validity of each model was checked by the fitness of the straight lines (R2 values).The values of correlation coefficients (R2) of pseudo-first-order model were less than pseudo-second-order model indicating that the pseudo-second order is better obeyed than the pseudo-first-order model.
Thermodynamic Studies
Fig.9 : Plot of ln(KD) vs. 1/T for the estimation of thermodynamic parameters for biosorption of Cr(VI) onto c.limonium peel biomass.
In any field of engineering, thermodynamic analysis is carried out to assess the feasibility of the process. Energy functions such as enthalpy change (ΔH0), entropy change (ΔS0) and free energy change (ΔG0) are used to explore nature and feasibility of the process. Thus, (ΔH0) and (ΔS0) were obtained from slope and intercept of plot of the following equation[5,10].
Whereas KD is calculated from where, qe and Ce are meta uptake and residual metal concentrations at equilibrium. Then free energy change is calculated from Eq
(8)
Where, T (K) is temperature, R (8.3145 J/mol K) is gas constant, and ΔGo is standard free energy change.The thermodynamic energy functions ΔS0 and ΔH0 were calculated from the linear plot of log KD versus 1/T shown in Fig9. and ΔGo was calculated from equilibrium constants (KD). In this study the negative values of ΔGo confirm the natural possibility of process with high preference of arsenic at low temperatures. The value of ΔHo for biosorption of arsenic onto c.limonium peel was obtained as −33.006 kJ/mol. The negative value of ΔHo reflects that the sorption process is exothermic and spontaneous.
The batch experiments were conducted with the biomass of c. limonium peel and it exhibited the potential of Cr(VI) removal from an aqueous solution. Optimum pH, contact time, initial metal ion concentration, dosage for biosorption in this study were 5,75 min, 20mg/l, 0.1g respectively. With an increase in the initial metal ion concentration percentage of biosorption decreased and metal ion uptake capacity increased. It was observed that percentage of biosorption increased and metal ion uptake capacity decreased by increase in the amount of biomass. The biosorption process followed the Freundlich isotherm model. Kinetic studies on the biosorption of Cr(VI) onto c. limonium peel revealed that experimental data were fitted with the pseudo second-order kinetic model and that film diffusion initially controls the sorption process. Thermodynamic parameters (ΔH0, ΔS0 and ΔG0) of the Cr(VI) ions indicate that the process is exothermic and proceeds spontaneously for the c.limonium peel. Based on the experimental data it was notified that c. limonium peel is an appropriate biosorbent for the removal of chromium from an aqueous solution.
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