This study includes the adsorption capability of Acid activated vegetable kitchen waste (AVKW) to remove aqueous basic dye solutions. The experiment was carried out in a batch system to optimize operation. Adsorption kinetic and equilibrium isotherm of the AVKW was studied using pseudo-first order and second order kinetic equations and Freundlich and Langmuir models. The adsorption kinetic followed the pseudo-first order equation for the adsorbent .The equilibrium data was found to best fit to the Langmuir model. The values of Langmuir & Freundlich constants suggested favorable sorption of Novasol Blue GF (NBGF) on AVKW. In comparison to other biosorbents, AVKW was appeared to be an excellent low cost sorbent and might have significant potential for the removal of color from wastewater.
Key words: AVKW; NBGF sorption; sorption kinetics; sorption isotherms.
Pollution caused by textile wastewater is a common problem faced by many industrial countries. Dyes have long been used in the dyeing, paper and pulp, textiles, plastics, leather, cosmetics and food industries. Many types of dye are used in the textile industry such as direct, reactive, disperse, acid and basic dyes, which can be toxic to some microorganisms and may cause direct destruction or inhibition of their metabolism . Due to technological and economical limitations, developing countries discharge the effluent to surface water mostly without any treatment. These discharged dyes are hazardous, toxic in nature, impede light penetration, retard photosynthetic activity, inhibit the growth of biota, and have a tendency to chelate metal ions. It may also lead to adverse effect on human health, domestic animals, aquatic life, wildlife, soil quality and the environment .
Before the waste is disposed into waters, color removal is an important consideration. Because of their complex molecular structures, dyes present in the textile wastewater are not removed easily by conventional wastewater treatment processes. Several methods such as – physiochemical pretreatment, biological treatment, physiochemical post treatment, wet air oxidation, ozone treatment, anaerobic treatment, coagulation process, membrane filtration, microfiltration (MF), ultrafitration (UF), nanofiltration (NF), reverseosmosis (RO) .
Adsorption is a physiochemical process which has gained a lot of attention since it creates a high quality treated effluent. It has been found to be superior to other techniques for water reuse in terms of initial cost, flexibility and simplicity of design, ease of operation and insensitivity to toxic pollutants without resulting in the formation of harmful substances. In sorption process dissolved molecules are attracted to the surface of the sorbent .
Although adsorption technology using activated carbon has been considered to be an effective and proven technology, it has its limitations as it is very expensive and necessitates regeneration . As cost is an important consideration in most developing countries, efforts have been made to explore the possibility of using various low-cost alternatives that are biodegradable, abundant, readily available, and are derived from waste materials . Many studies have been undertaken to investigate the use of low-cost adsorbents such as peat , bentonite, steel-plant slag, fly ash, china clay, maize cob, wood shaving, sand silica for color removal [7,8]. But these low-cost adsorbents have generally low adsorption capacities and are thus required in large amounts. Therefore, there is a need to find new, economical, easily available and highly effective adsorbents. However, they are fairly stable to light, as well as heat, and resist biodegradation, thus posing a challenge to conventional physio-chemical and biological treatment methods .
Recently an outstanding low cost adsorbent named AVKW (acid activated vegetable kitchen waste) has been proposed by some researchers to remove reactive dyes from water . It is the most low cost sorbent and abundantly available substance in Bangladesh. The performance of AVKW as a sorbent is remarkable. AVKW has been used to treat anionic (reactive) dyes contaminated waste water and the result is quite promising. As basic dyes are applied widely in industries such as textiles and carpets, cationic Novasol Blue GF (basic) dye has been treated with AVKW in the present study. The experimental data were analyzed with Langmuir and Freundlich isotherm models. The kinetic of sorption of Novasol Blue GF onto AVKW were investigated by pseudo first-order and pseudo second-order. The results were analyzed and compared with other low cost sorbents and suitability of the use of vegetable kitchen waste as a sorbent.
2. Methods and materials
The basic dye used in this study was Novasol Blue GF (NBGF), chemical structure: C21H15BrN2O2, molecular weight: 407.26. It was collected from Huntsman in Bangladesh. The maximum absorbance for Novasol Blue GF basic dye was found at the wavelength of around 650 nm. Extinction coefficient value (ε=9.379) was calculated.
2.2 Preparation of sorbent
For a several days solid vegetable kitchen waste was collected from a kitchen. The vegetable kitchen waste includes residues of arum (Colocasia Esculenta) (50%), potato (Solanum Tuberosum) (30%), and onion (Allium Cepa) (20%). Kitchen waste was chopped into very small pieces and dried under sunlight. These dried small pieces of kitchen waste were then converted into powder and large particles were removed with the help of a sieve of size 1 mm× 1 mm. 5 g of kitchen waste was suspended in 200 ml water and 5 ml of HCl was poured to the suspension. The mixture was stirred for 30 minutes at 70ºC ± 5 ºC on a hot plate with magnetic stirrer. The kitchen waste was then filtered out and dried in air and termed as acid activated vegetable kitchen waste (AVKW).
2.3 Batch sorption experiments
The experiment was performed at room temperature (302K). The kinetic study was carried out at four different concentrations of NBGF. At first initial concentration (Co) of 50 mg/L was made and 200 ml of that solution was taken in 250ml beaker. Then 2 g of AVKW was added to the beaker. This mixture was agitated at room temperature on a magnetic stirrer and the liquor was sampled at the time intervals of 1, 2, 5, 10, 20, 30, 45, 60, 90 and 120 minutes. After desired time the solution was filtered and absorbance of sampled solutions were measured in a spectrophotometer (Datacolor 650, NJ, USA). Similar procedure was followed to determine absorbance value for other three initial concentrations (Co) such as 100 mg/L, 150 mg/L and 200 mg/L. The concentrations of dye solution for each time interval were determined using the calibration curves and Beer Lambert equation established earlier.
The amount of NBGF adsorbed onto AVKW was calculated by following Eq. (1)
RGY, respectively; V (L) is volume of dye solution and W (g) is the weight of sorbent .
The percentage removal of dye (R) was calculated following the Eq. (2):
A detailed kinetic study has been carried out using pseudo first-order, pseudo second-order to calculate the sorption mechanism of NBGF onto AVKW.
The pseudo first-order kinetic equation given by Lagergren (Lagergren, 1898)  is expressed as Eq. (3):
In some cases a pseudo-second-order kinetic model (Ho and Mckay, 2000)  provides a better fit given by Eq. (2)
2.5 Isotherm models
Adsorptions isotherms are important for the description of how molecules of adsorbate interact with adsorbent surface and also, are critical in optimizing the use of AVKW as a low cost adsorbent. Hence, the correlation of equilibrium data using either a theoretical or empirical equation is essential for the adsorption interpretation and prediction of the extent of adsorption. Three well known isotherm equations, the Langmuir, Freundlich and Temkin have been applied for deeper interpretation of the adsorption data obtained as expressed in equation 5-8 respectively.
plot, for example, the value of KL were obtained from the linear best fit plot of log10 (qe-qt) against agitation time, t. The origin 8 software package was used to linear regression analysis for all models.
3. Result and discussion
3.1 Effect of contact time and initial concentrations:
The sorption of Novasol Blue GF onto AVKW with the variation of agitation time has been shown in Figure 2. It was observed from the figure that the removal of dye is rapid in the initial stages and finally reaches the equilibrium. The equilibrium time, required time for the maximum sorption of dye onto the sorption surface, was observed to be 60 minutes for initial dye concentrations of 50 mg/L, 100 mg/L, 150 mg/L and more than 60 minutes for 200 mg/L. It was also observed that the sorption of Novasol Blue GF depends on its initial concentration. According to McKay some sorbents may be more efficacious for waste streams that contain smaller concentrations of dyes because of more rapid kinetics. The decrease of sorption kinetics with the increase of concentration might be the results of steric interactions between cationic molecules in the solution.
3.2 Adsorption Kinetic
In order to investigate the mechanism of adsorption and potential rate controlling steps such as mass transfer and chemical reaction, processes are used to test experimental data. Sorption kinetics largely depends on the physical and chemical characteristics of the sorbent material and in turn influences the sorption mechanism . In order to design an effective sorber having the knowledge of the rate at which the sorption takes place by the sorbent, is an important factor . In order to explore the sorption of NBGF on AVKW and analyze the rate controlling step of the sorption process, pseudo first-order, pseudo second-order and models were tested to fit the kinetic data. Figures 2 – 3 show the fitted curves of these models. The values of rate constants, amount of the dye adsorbed at equilibrium and correlation coefficients calculated from these curves have been presented in Table 1. The pseudo first-order model assumes that the rate of sorption is proportional to the number of free sorption sites (Hanafiah et al., 2012). In the present investigation, the best fit plots for pseudo first-order model showed good linearity (Table 1) up to 45 min. contact time (Figure 2) but the calculated amount of dye adsorbed at equilibrium, qe,cal showed a considerable difference with the experimental values, qe(exp) (Figure 4). The pseudo second-order model is built based on the assumption that the rate controlling step is a chemical sorption involving valence force by sharing or exchange of electrons between sorbent and sorbate. Therefore, the sorption process did not fit very well to the pseudo-second order model (Figure 3). The correlation coefficient (r2) for pseudo first-order equation was found to be 0.9975, 0.996, 0.995 and 0.99618 at the initial concentrations of 50, 100, 150 and 200 mg/L respectively (Table 1) and the difference between qe (cal) and qe (exp) is reasonably small (Figure 4) which indicates that sorption of NBGF on AVKW follows first order kinetics mechanism. However, the value of r2 was found to be 0.9875 in pseudo second-order equation for initial concentration of 200 mg/L and the difference between qe(cal) and qe(exp) is also relatively high. It seems that at higher concentration other sort of mechanism plays important role in sorption process.
Table 1: Pseudo first-order and pseudo second-order models rate constants and calculated amount of dye adsorbed at equilibrium from experimental data.
To analyze the sorption process main three isotherm models Langmuir, Freundilch and Temkin were studied respectively presented in the figure 5-7. The model parameters along with the correlation coefficient (r2) calculated from these plots have been shown in Table 3. The values of r2 for Langmuir, Freundlich and Temkin plots were found to be 0.999, 0.938 and 0.9386 respectively (Table 3), which indicate that sorption data fitted very well with Langmuir isotherm model. The best fitting of Langmuir model indicates that sorption sites on the surface of AVKW were homogeneous. The essential characteristics of Langmuir isotherm is the dimensionless constant separation factor for equilibrium parameter, RL .
In present investigation for different concentrations we found favorable sorption of NBGF by AVKW. The Langmuir constant obtained in the present investigation was Q0 = 16.667 mg/g. The value of Langmuir constant was reported to be 19.88 mg/g for the sorption of Acid violet 17 on orange peel , 3.797 gm/g for Luganil green Non sawdust . The greater value of Langmuir constant indicates the better capacities of kitchen waste for the removal of Novasol Blue GF from wastewater .
The Freundlich isotherm is applicable to heterogeneous systems and reversible adsorption . The n value in this equation suggests deviation from linearity. If n = 1, the adsorption is homogeneous and there is no interaction between the adsorbed species. If 1/n < 1, the adsorption is favored and new adsorption sites are generated. If 1/n > 1, the adsorption is unfavorable, bonds become weak and adsorption capacity decreases. In our studies, we have found that for all the three dyes 1/n < 1, which indicates that adsorption is favorable; though it is more favorable in the case of Novasol Blue GF (1/n = 0.37) as compared to other dyes. In the present investigation the value of n was found to be 2.73 indicating the beneficial sorption of NBGF onto AVKW.
Figure 7: Temkin isotherm for the adsorption of Novasol Blue GF on AVKW.
Table 2: Langmuir,Freundlich and Temkin Isotherm model constants and parameters
Removal of Basic dye (Novasol Blue GF) from aqueous solution onto AVKW was carried out at room temperature. The conditions of adsorption of NBGF were optimized. It was seen that under these conditions, a maximum of 75–90% dye could be removed from the solution. Sorption data fitted well to Langmuir isotherm and sorption capacity was found to be 16.665 mg/g. The value of RL between 0 and 1 obtained from Langmuir isotherm and n between 1 and 10 from Freundlich isotherm indicate that sorption of NBGF onto AVKW is favorable and beneficial. The pseudo first order kinetic model agrees very well for NBGF sorption onto AVKW. The present investigation shows the potentiality of AVKW to serve as a low cost sorbent for the treatment of wastewater generating from dyeing process using Basic dyes especially Novasol Blue GF.
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