Competitive adsorption of malachite green (MG) in single and binary system on microwave activated epicarp of Ricinus communis (MRC) and microwave assisted zinc chloride activated epicarp of Ricinus communis (ZRC) were analyzed. The preparation of ZRC from Ricinus communis was investigated in this paper. Orthogonal array experimental design method was used to optimize the preparation of ZRC. Optimized parameters were radiation power of 100 W, radiation time of 4 min, concentration of zinc chloride of 30% by volume and impregnation time of 16 h, respectively. The MRC and ZRC were characterized by pHzpc, SEM-EDAX and FTIR analysis. The effect of the presence of one dye solution on the adsorption of the other dye solution was investigated in terms of equilibrium isotherm and adsorption yield. Experimental results indicated that the uptake capacities of one dye were reduced by the presence of the other dye. The adsorption equilibrium data fits the Langmuir model well and follows pseudo second-order kinetics for the bio-sorption process. Among MRC and ZRC, ZRC shows most adsorption ability than MRC in single and binary system.
Dyes are widely used in industries such as textiles, leather, paper, plastics, etc. to color their final products. The textile industries are the greatest generators of liquid effluent, due to high quantity of water used in the dyeing processes [
Dye removal from wastewater effluent is a major environmental problem because of the difficulty of treating such streams by conventional physical, chemical, physico-chemical and biological treatment methods. Many physical and chemical treatment methods including adsorption, coagulation, precipitation filtration, electrodialysis, membrane separation and oxidation have been used for the treatment of dye-containing effluents [
Activated carbon was widely used in removal dyes from textile effluent, which had relatively high sorption capacity for a wide variety of dyes. Commercially available activated carbons are usually derived from natural materials such as wood or coal, therefore, are still considered expensive [
Two methods are used for the preparation of AC via, physical and chemical activation. During physical activation, the raw material is carbonized first at high temperature and then it is activated by CO2 or steam under pressure to increase porosity and surface area of AC. In chemical activation both carbonization and activation takes place simultaneously, in which raw material is first impregnated with activating chemical and then carbonized at desired temperature that varies according to activating chemical used [
The application of microwave (MW) heating technology for regenerating industrial waste activated carbon has been investigated with very promising results [16, 17]. The main difference between MW devices and conventional heating systems is in the way of t heating. In the MW device, the microwaves supply energy directly to the carbon bed. Energy transfer is not by conduction or convection as in conventional heating, but microwave energy is readily transformed heat into inside the particles by dipole rotation and ionic conduction [16-18]. Recently microwave energy has been widely using in several fields of applications on both research and industrial processes. Although the use of microwave energy changes the properties of carbonaceous materials very much, there are relatively few publications that describe the use of microwaves for producing and regenerating activated carbons. Nabasis et al., studied on the surface chemistry modification of activated carbon fibres by means of microwave heating which was found to be very effective. Microwave is now being used in various fields in order to heat dielectric materials because it can considerably shorten the treatment time and reduce energy consumption.
The aim of the present work was to optimize the MRC and ZRC preparation conditions using orthogonal array experimental design method, investigate the ability of a malachite green dye sorbent prepared from epicarp of Ricinus communis (RC) is an agricultural waste material by microwave assisted chemical activation using zinc chloride as activating agent for the adsorption of malachite green dye from aqueous solution. The adsorption ability of classical activated Ricinus communis was previously investigated for the adsorption of MG dye from aqueous solution [
The objective of the present study is to evaluate the possibility of using the dried epicarp of Ricinus communis to develop a new low-cost activated carbon and to study the effect of several parameters (pH, contact time, initial metal concentration, pHzpc, adsorption isotherms and kinetics) on the adsorption efficiency of malachite green (MG) dye from aqueous solution in single and binary system and compare the performance of MRC and ZRC for the adsorption of MG in single (MG) and binary system (MG + Methylene blue).
Samples of Malachite green (MG) and Methylene blue (MB) were obtained from Aluva, Edayar (specrum reagents and chemicals pvt. Ltd). All other chemicals used in this study were analytical grade and Purchased from Aluva, Edayar (specrum reagents and chemicals pvt. Ltd) and was used without any further purification. The chemical structure of malachite green is shown in
A Stock solution of 500 mg/L was prepared by dissolving accurately weighed amounts of MG in doses of 1000 mL distilled water. The desirable experimental concentrations of solutions were prepared by diluting the stock solution with distilled water when necessary.
The epicarp of Ricinus communis (RC) was obtained from an agricultural form in Tirupur district (TamilNadu). It was air-dried and powdered in a grinder. Dried Ricinus communis with the mass of 6 g were placed in a MW heating apparatus (MW71E) which produced by SAMSUNG, Malaysia. After a certain heating power of 100 W and microwave radiation time of 4 min, and finally dried at 150˚C in a hot air oven. The MRC was then stored in an air-tight container for later experimental use.
The epicarp of Ricinus communis was obtained from the
agricultural form in Tirupur district (Tamil Nadu). It was air-dried and powdered in a grinder. Dried Ricinus communis with the mass of 6 g were mixed with 30 mL of ZnCl2 to vary concentrations in the range of 30% - 60% by volume. The slurry was kept at room temperature for various time spans in the range of 16 - 28 h to ensure the access of the ZnCl2 to the Ricinus communis. After mixing, the slurry was placed in a MW heating apparatus (MW71E, SAMSUNG). After a heating power of 100 W - 600 W and microwave radiation time of 10 min - 4 min, the carbonized sample were washed with 0.5 M HCl, hot water and cold distilled water until the pH of the washing solution reached 6 - 7, filtered and finally dried at 150˚C in hot air oven for 6 h. The ZRC was then stored in an air-tight container for later experimental use.
In order to optimize the preparation conditions of the ZRC, Taguchi experimental design method was used [
where M is the weight of MRC and ZRC and M0 is the weight of air dried Ricinus communis leaves.
According to the L16 array designed by Taguchi method
16 different ZRC samples were prepared. Iodine number and yield of each sample were determined and shown in
Effect of microwave radiation power (Parameter A) on adsorption capacity and the yield of ZRC were evaluated under the concentration of ZnCl2 (XZn) of 30 ml and microwave radiation time of 4 min.
Effects of microwave radiation time (parameter B) on the
yield and iodine number of ZRC were evaluated, under the conditions of XZn of 30 ml and microwave power of 100 W (
Under the microwave power of 100 W, radiation time of 8 min, the effects of the impregnation ratio (parameter C) of ZnCl2 on the yield and iodine number of ZRC was studied (
The results presented in
During impregnation stage the base attacked the cellular structure of Ricinus communis leaves, forming cleavage to the linkages between the lignin and cellulose. It was followed by recombination reactions, where larger structural units and strong cross linked solids were formed. This base worked, principally, in early stage during impregnation and might extended to have a slight effect in the carbonization stage [
In the production of AC, relatively high product yield and adsorption capacity were expected. Therefore, more attention should be paid to improve the carbon yield and enhance its adsorption capacity for economical viability. However, it was difficult to optimize both these responses under the same condition, for the different interest in different region. From the discussions mentioned above, the microwave radiation power, microwave radiation time, impregnation ratio and the impregnation time of ZnCl2 had significant effects on the yield and the adsorption capacity of the activated carbon from Ricinus communis leaves with ZnCl2 activation by microwave radiation. Therefore the optimum conditions were obtained as following: the microwave power of 100 W, microwave radiation time of 4 min, XZn of 30 ml and impregnation time of 16 h. Iodine number and the yield of activated carbon prepared under optimum conditions were 69.68% and 783.16 (mg/g), respectively. The ZRC was used in the characterization analysis and adsorption experiments which were prepared under optimum conditions.
The surface morphology of MRC and ZRC were identified by using SEM technique (Jeol jsm-6390). A Fourier transforms infrared spectroscopy (SHIMADZU, IR Affinity-1) with KBr pellet was used to study surface functional groups of the MRC and ZRC, with a scanning range of 4000 - 400 cm−1. The zero surface charges (pHZPC) of MRC and ZRC were determined by using the solid addition method [
To study the effect of parameters such as adsorbent dose, dye concentration and solution pH for the removal of adsorbate on MRC and ZRC, batch experiments were performed. Stock solutions of MG in single and binary system were prepared by dissolving MG (S) and MG + MB (B) in deionized water and further diluted to the 50 - 200 mg/L concentrations for the experiments. pH was adjusted by adding 0.1 M HCl or 0.1 M NaOH into the solutions with known initial MG concentrations. Batch adsorption experiments were conducted in asset of 250 mL stoppered flasks containing 0.2 g of MRC and ZRC and 50 mL of dye solutions with different concentrations (50, 100, 150 and 200 mg/L) at pH 5. The flasks were agitated using a mechanical orbital shaker, and maintained at room temperature for 2 h until the equilibrium was reached. The suspensions were filtered and dye concentrations in the supernatant solutions were measured using a UV-vis spectrophotometer at 617 nm. The amounts of uptake of MG by MRC and ZRC in the equilibrium (qe) were calculated by the following mass-balance relationship
where Co and Ce (mg/L) are the liquid phase concentrations of dye at initial and equilibrium, respectively. V (L) is the volume of the solution, and W (g) is the mass of adsorbent used.
Adsorption isotherm is the most important information which indicates how the adsorbate molecules distribute between the liquid phase and the solid phase when adsorption process reaches on equilibrium state. When the system is at equilibrium is of importance in determining the maximum sorption capacity of MRC and ZRC towards dyes.
To observe the effect of adsorbent dose on dye adsorption, different amounts of adsorbent varying from 0.2 g/ 50 ml, 0.4 g/50 ml, 0.6 g/50 ml, 0.8 g/50 ml and 1 g/50 ml were added into initial concentration of 100 mg/L MG in single and binary solution. The mixtures were shaken in 250 mL stoppered flasks at room temperature at pH 5 until the equilibrium time was reached.
To study the effect of solution pH on MG adsorption, 100 mg/L initial concentration at different pH values (2 - 9) was agitated with 0.2 g of MRC and ZRC in a mechanical orbital shaker at room temperature. The effect of pH on MG adsorption was studied by varying the pH from 2.0 to 9.0. The concentration of MG solution used for this study was 100 mg/L and the adsorbent dose was 0.2 g. The initial pH was written as pHi and the solution pH after adsorption was also measured and written as pHf. The pH was adjusted with 0.1 M NaOH and 0.1 M HCl solutions.
In order to study the effect of initial dye concentration on the adsorption uptake, MG solutions with initial concentrations of 50 - 200 mg/L with varying the adsorbent dose of 0.2 g/50 mL of MRC and ZRC respectively. In this case, the solution pH was kept as 5.
Surface acidity was estimated by mixing 0.2 g of MRC and ZRC with 25 mL of 0.5 M NaOH in a closed flask, the flask was agitated for 48 h at room temperature (28˚C). The Suspension was decanted and the remaining NaOH was titrated with 0.5 M HCl. The surface basicity was measured by titration with 0.5 M NaOH after agitation of 0.2 g of MRC and ZRC with 0.5 M HCl. MRC has the surface acidity of 1.803 mmol/g and 4.06 mmol/g surface basicity and ZRC has the surface acidity of 2.73 mmol/g and 2.53 mmol/g surface basicity. Acidity and basicity were confirmed by Boehm titration method. Boehm titrations quantify the basic and oxygenated acidic surface groups on activated carbons [
The influence on the solution pH on the dye uptake can be explained on the basis of the pH zero point charge or isoelectric point of the adsorbent. The value of the pH necessary to affect a net zero charge on a solid surface in the absence of specific sorption is called the point of zero charge, pHZPC.
The zero surface charge of MRC and ZRC were determined by using the solid addition method [
Figures 3(a) and (b) shows that the plot between ΔpH, i.e. (pH0 − pHf) and pH0 for pHZPC measurement for MRC and ZRC. The point of zero charge for MRC is found to be 3.14 and for ZRC is 4. This result indicated that the pHZPC of MRC and ZRC were depended on the raw material and the activated carbon. The zero point charge (pHZPC 3.14 for MRC and pHZPC 4 for ZRC) is below the solution pH (pH 5) and hence the negative charge density on the surface of MRC and ZRC increased which favours the adsorption of cationic dye [
The aim of using FTIR analysis is to determine the existence of functional groups and identification of characteristic peaks is based on the studies reported in the literature [23-25]. The FTIR spectrum of MRC and ZRC were shown in Figures 4(a) and (b). The absorption bands identify in the spectra and it revealed corresponding functional groups.
The broad band at about 3406.29 cm−1 was observed, which was assigned to the O-H stretching vibration of the hydroxyl functional groups including hydrogen bonding. The intense bent at about 2927.94 cm−1 for the precursor was attributed to the C-H stretching vibration. The peak at 1639.49 cm−1 was characteristics of the C=O stretching vibration of lactonic and carbonyl groups. The peaks occurring at 1396.46 cm−1, 1185.00 cm−1 were all as-
cribed to oxygen functionalities such as highly conjugated C-O stretching, C-O stretching in carboxylic groups, and carboxylate moieties. The band located at 2352.6 cm−1 and 2332.6 cm−1 are attributed to the C≡C stretching is due to the ZnCl2 activation [
Scanning electron microscopy was used to study the surface morphology and the pore size of the samples. Samples of MRC and ZRC were subjected to SEM studies and SEM micrograph (Figures 5(a) and (b)) shows many orderly and developed pores.
It can be seen from the micrographs that the external surface of ZRC is full of cavities compared with MRC, and quite irregular as a result of activation and the pores were different sizes and different shapes. According to the micrograph, it seems that the cavities resulted from the evaporation of ZnCl2 during carbonization, leaving the space previously occupied by the ZnCl2. It is clear that the adsorbent has considerable number of heterogeneous pores where there is a good possibility for dye to be trapped and adsorbed [
Based on the EDAX results, the elementary analysis of
the MRC and ZRC are presented in the Figures 6(a) and (b). The elements, percentage mass of elements presented in MRC and ZRC are summarized in Tables 3 and 4.
The effect of solution pH is very important when the adsorbing molecules are capable of ionizing in response to the current pH [
The optimum pH value for the adsorption of MG onto MRC and ZRC (pH = 5) was observed. That may be attributed to the hydrophobic nature of the developed carbon which led to absorb hydrogen ions (H+) onto the surface of the carbon when immersed in water and make
it positively charged. Low pH value (1.0 to 4.0) leads to an increase in H+ ion concentration in the system and the surface of the activated carbon acquires positive charge by absorbing H+ ions. On the other hand, increase of the pH value led to increase of the number of negatively charged sites. As the adsorbent surface is negatively charged at high pH, a significantly strong electrostatic attraction appears between the negatively charged carbon surface and cationic dye molecule leading to maximum adsorption of MG from waste water [
The adsorbent dose is an important parameter in adsorption studies because it determines the capacity of adsorbent for a given initial dye concentration of dye solution. The effect of adsorbent dose on MG dye removal percentage is shown in
librium times for each adsorption process. It was observed that the percentage of adsorption increases with increase in adsorbent dose from 0.2 g to 1 g in MG with the concentration of dye solution of 100 mg/L. The increase in % dye removal was due to the increase of the available sorption surface and availability of adsorption sites [
The data for the uptake of MG onto MRC and ZRC as a function of initial dye concentration is presented in
decreased from 79.10% to 35.04% (MG onto MRC), 88.24% to 55.60% (MG onto ZRC) for the dye concentration of 50 to 200 mg/L.
This may be attributed to an increase in the driving force of the concentration gradient with increase in the initial dye concentration [33,34].
The Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models were used to describe the relationship between the amount of MRC, ZRC adsorbed and its equilibrium concentration in solutions.
Langmuir [
In Equation (3), Ce and qe are defined as before in Equation (2), Qm is a constant and reflect a complete monolayer coverage (mg/g), KL is adsorption equilibrium constant (L/mg) that is related to the apparent energy of sorption. Langmuir isotherm [
A plot of Ce/qe verses Ce should indicate a straight line slope of 1/Qm and an intercept of 1/KLQm.
The value of Qm obtained was equal to 12.65 mg/g and 24.39 mg/g for MRC (S) and ZRC (S), indicating a very strong monolayer adsorption of the adsorbate on the surface.
The Freundlich isotherm is an empirical equation assuming that the adsorption process takes place on a heterogeneous surface through a multilayer adsorption mechanism and adsorption capacity is related to the concentration of dye at equilibrium [
where qe is the amount of adsorbate at equilibrium (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), Kf is the Freundlich adsorption constant
related to adsorption capacity of the adsorbent and 1/n is the adsorption intensity. A linear form of the Freundlich equation is generally expressed as follows:
The values of Kf and 1/n were calculated from the intercept and slope of the plot of ln qe versus ln Ce.
The Dubinin-Radushkevich equation can be expressed [
where ε (Polanyi potential) is equal to RT ln(1 + 1/Ce), qe is the amount of the dye adsorbed per unit activated carbon (mol/g), qm the theoretical monolayer saturation capacity (mol/g), Ce the equilibrium concentration of the dye solution (mol/L), K' is the constant of the adsorption energy (mol2/kJ2), R is the gas constant (8.314 KJ/mol K), and T is the temperature (K). The linear form of the D-R isotherm is:
K' is related to mean adsorption energy E (kJ/mol) as [
The calculated D-R adsorption isotherm parameters should be summarized in
Temkin and Pyzhev considered the effects of some indirect sorbate/adsorbate interactions on adsorption isotherms, and suggest that the heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to these interactions [
where KT is the equilibrium binding constant (L/g), b is related to heat of adsorption (J/mol), R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (K). Equation (10) can be written as the following form:
The Temkin adsorption isotherm parameters are calculated and the values are summarized in
Adsorption isotherms describe the interaction of adsorbates with the adsorbent materials, and thus are critical for optimization of the adsorption mechanism pathways [
The kinetics describes the rate of adsorbate uptake on activated carbon. In order to identify the potential rate controlling steps involved in the process of adsorption, four kinetic models were studied and used to fit the experimental data from the adsorption of MG dye onto MRC and ZRC. These models are the pseudo-first-order, pseudo-second-order, Elovich and intra-particle kinetic models.
The pseudo first-first-order equation of Lagergren is generally expressed as follows [46,47]:
After integration and applying boundary conditions, t = 0 to t = t and qt = 0 to qt = qt; the integrated form of the above equation becomes:
However, Equation (13) is transformed into its linear form for use in the kinetic analyses of data can be expressed as:
where qe (mg/g) and qt (mg/g) are the amount of adsorbed adsorbate at equilibrium and at time t, respectively, and k1 (1/min) is the rate constant of pseudo firstorder adsorption. The straight line plots of log (qe − qt) against t of Equation (14) were made.
The data for the pseudo-first-order kinetic model of MG onto MRC (S) and ZRC (S) are summarized in
The rate of sorption is a second-order mechanism, the pseudo-second-order chemisorptions kinetic rate equation is expressed as:
where qe and qt are the sorption capacities at equilibrium and at time t, respectively (mg/g) and k is the rate constant of pseudo-second-order sorption (g/mg/min). Where h can be regarded as the initial sorption rate as qt/t tents to zero, hence:
Equation (16) can be written as:
Equation (16) does not have the disadvantage of the problem with assigning an effective qe. If Pseudo-second-order kinetics are applicable, the plot of t/qt against t of Equation (17) should give a linear relationship, from which qe, k and h can be determined from the slope and intercept of the plot (
know any parameter. The qe and k2 values were estimated from the slope (1/qe) and intercept (1/k2) of linear plot of t/qt verses t.
The data for the pseudo-second-order kinetic model of MG onto MRC (S) and ZRC (S) are summarized in
The adsorption of MG dye onto MRC (S) and ZRC (S) may be controlled by via external flim diffusion at earlier stages and later by the particle diffusion. The possibility of intra particle diffusion resistance was identified by using the following intra particle diffusion model as [
where Kdif is the intra-particle diffusion rate constant (mg/(g∙min1/2), C is the intercept. The values of qt correlated linearly with the values of t1/2 and the rate constant Kdif directly evaluated from the slope of regression line. The data for the intra-particle kinetic model of MG onto MRC (S) and ZRC (S) are summarized in
The linearity of the plots demonstrated that intra-particle diffusion played a significant role in the uptake of the MG onto MRC and ZRC. However, as still there is no sufficient indication about it, Ho [
The Elovich kinetic is another rate equation based on the adsorption capacity generally expressed as follows:
where BE is the initial adsorption rate constant (mg (g/min) and AE is the desorption constant (g/mg) during any experiment.
It is simplified by assuming AEBEt t and by applying the boundary conditions qt = 0 at t = 0 and qt = t at t = t above Equation (19) becomes:
If MG adsorption by MRC and ZRC fits the Elovich model, a plot of qt versus ln (t) should yield a linear relationship with a slope of (1/AE) and an intercept of (1/AE) ln (AEBE). Thus the constants can be obtained from the slope and the intercept of the straight line.
Thus, the constants can be obtained from the slope and the intercept are shown in
The correlation coeffecients obtained for the pseudosecond-order kinetic model are greater than 0.93 for MRC (S) and ZRC (S). The theoretical qt values of the pseudo-second-order kinetic model for MRC (S) and ZRC (S) are close to the experimental values than those of the other models. The pseudo-second-order kinetic model fits the experimental data better than the other kinetic models in this study.
The Langmuir isotherm and pseudo-second-order kinetic model provide best correlation with the experimental data for the adsorption of MG onto MRC and ZRC for different initial dye concentrations over the whole range studied. Both Langmuir isotherm and pseudo-secondorder kinetic model assumes that the MRC and ZRC surface is homogenous and the operating adsorption mechanism is chemisorptions process.
Effects of the presence of MRC and ZRC on the adsorption of MG were investigated in terms of equilibrium isotherm and adsorption kinetics. A comparison of the adsorbed quantity of MG onto MRC and ZRC in single system at equilibrium between the solutions with MG present in the binary system (B) was given in
All the correlation coefficient, R2 values and the constants obtained from the four isotherm models and four kinetic models are summarized in Tables 5 and 6. The Langmuir isotherm model gave the highest R2 values. Among the four kinetic models pseudo-second-order model fits well for MG adsorption onto MRC and ZRC in single and binary system. In the single dye solution, the maximum uptake obtained at initial concentrations of MG 100 mg/L, pH 5 was found to be 12.65 mg/g for MRC (S) and 24.39 mg/g for ZRC (S), while the uptake obtained in the binary solutions at the same initial dye concentration of MG and adsorption conditions, was found to be 11.764 mg/g for MRC (B) and 20.4081 mg/g for ZRC (B), respectively.
A fixed quantity of MG onto MRC (S) and ZRC (S) could only offer a finite number of surface binding sites, some of which would be expected to be saturated by the competing dye solutions. The decrease in sorption capacity of same activated carbon in target dye solution than that of single (S) dye may be ascribed to the less availability of binding sites. In case of binary dye (B) solution,
the binding site is competitively divided among the various dye solutions.
Among MRC and ZRC, ZRC shows most adsorption ability than MRC in single system only onto MG adsorption from waste water using epicarp of Ricinus communis.
It had indicated that ZnCl2 was a suitable activating agent for the preparation of activated carbon from epicarp of Ricinus communis by microwave radiation. The effects of the impregnation ratio of ZnCl2, microwave radiation power and microwave radiation time and impregnation time on the yield and iodine number of ZRC were investigated systematically. The optimum conditions were microwave power of 100 W, microwave radiation time of 4 min, concentration of zinc chloride of 30% by volume and impregnation time of 16 h. SEM micrographs showed that the external surface of the chemically activated carbon was full of cavities compared with untreated Ricinus communis. The activated carbon prepared could effectively used as adsorbent for the removal of basic dye from aqueous solutions. Adsorption was found to be maximum in the pH of 5. Langmuir isotherm models given were fitting better than Freundlich, Temkin and Dubinin-Radushkevich isotherms interpreting the adsorption phenomenon of MG. MG adsorption system follows pseudo-second-order kinetic model, based on the assumption that the rate-limiting step may be chemisorptions process. MG adsorption rate onto MRC and ZRC was greater in single system (S) than in binary system (B) due to the competitive adsorption of dye onto the active site of the activated carbon. Among MRC and ZRC, ZRC shows most adsorption ability than MRC in single and binary system.