Modified Cenospheres as an Adsorbent for the Removal of Disperse Dyes

1Department of Civil Engineering, Institute of Engineering and Technology, Lucknow 226021, India 2Department of Civil Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India 3Department of Environmental Engineering, IIT Roorkee and AIT, Bangkok, Bhargava Lane, Devpura, Haridwar 249401, India 4Environmental Monitoring Division, CSIR-Indian Institute of Toxicology Research, M.G. Marg, Lucknow 226001, India


Introduction
According to Confederation of Indian Textile Industry, textile mills are discharging approximately 1.2 × 10 3 million liters per day (MLD) colored wastewater into the natural water bodies without proper treatment [1].There are 3441 textile mills in India out of which 3244 are spinning mills and 197 are composite mills.The annual consumption of synthetic and natural fibers/filaments is 6800 million kilos and 2601 million kilos, respectively.Total productions of Khadi (handloom) and others are 62625 million kilos.India's export business of textiles and clothing including of silk, jute, coir, and handicrafts is about $300 billion (2013-2014) [1].
Presently, more than 1.0 × 10 5 commercially available dyes are in the world whose production was >7.0 × 10 5 tons annually [2,3].It has been anticipated that about 2% dye is lost during manufacturing process while 10% is lost from textile and related industries [4,5].Among various dyes, disperse blue 79:1 (DB) and disperse orange 25 (DO) are commonly used in textile industry in India [6].These are used for coloring of a wide range of synthetic as well as natural fibers.Disperse dyes are preferred to acrylic black O, red GTL, and others dyes due to its high tendency to bind the fibers and remaining persistent over the years.However, it becomes toxic in the water bodies because of its complex molecular structure with fused aromatic groups [6].
Ions of disperse dyes in the water streams either reflect back the solar radiation or scatter in the water bodies.Solar radiation cannot penetrate beyond littoral zone and therefore the deep water zone (i.e., aphotic zone) develops anaerobic condition.The entire ecological cycle including self-purification system of water stream is disturbed due to lack of dissolved oxygen [7].The indiscriminate discharge of untreated wastewater is more conspicuous in case of developing countries rather than developed one, where the shortage of modern technology cum advanced knowledge and insufficient funds are aggravating the problems [8].
The discharge of industrial effluents (textile mill, pulp and paper, tannery, distillery, carpet, and pharmaceutical industries) containing toxic components when entering the surface water bodies causes severe environmental problems as dyes damage the aesthetic nature of the environment.Most of the industries use dyes which are stable to light and oxidizing in nature.Bacteria are partially effective in color degradation as dyes are resistant to aerobic digestion [9].According to various researchers, the elimination of dyes from wastewater before their release into the natural environment is an absolute necessity [10][11][12].
Coal fly ash (CFA) is a solid waste by-product of the coal fired power plant which is not only encroaching on the agricultural land but also creating environmental problems.The lightweight portion of CFA is usually known as cenospheres; because they are hollow (empty sphere), they form a large portion of the lightweight fraction.However, because they are collected in a sink through wet process, all particles are less dense than water [13,14].There are several reports which demonstrate the potential use of CFA for the removal of organic synthetic dyes and toxic compounds from aqueous solutions via surface-adsorption mechanism [15].Researchers studied adsorbent efficiency of coagulants (ferric chloride, ferrous sulfate, and alum) with CFA for the removal of dyes and pigments from the wastewater.The individual removal efficiencies of ferric chloride, ferrous sulfate, alum, and CFA were 57%, 20%, 63%, and 58%, respectively; but the removal capacity of hybrid processes of ferric chloride-CFA, ferrous sulfate-CFA and alum-CFA had markedly increased and removed the pollutants up to 73%, 60%, and 68%, respectively [16].
Keeping in view, the toxicity and adverse environmental impact are likely to crop up due to discharged color wastes; the present study has been designed to develop suitable cheaper adsorbent material by modifying cenospheres to increase the free surface binding sites and porosity of CFA.The main objective of this investigation is to determine the dye removal capacity of modified cenospheres from aqueous solutions of disperse blue 79:1 and disperse orange 25.

Materials and Methods
2.1.Chemicals, Adsorbent, and Other Reagents.All the reagents used were of analytical grade (AR-grade).Disperse dyes (disperse blue 79:1 and disperse orange 25) with 99.9% purity were purchased from M/s Siddheshwari Industries, GIDC, Gujarat, India.Chemical structures and molecular formulae of DB and DO dyes are shown in Figure 1.Stock dye solutions were prepared by dissolving 10 to 100 mg of DB/DO in 1 L double distilled water.The CFA was collected from the bottom of hopper of electrostatic precipitator installed at M/s Panki Thermal Power Station, Kanpur, Uttar Pradesh, India.

Preparation of Modified Cenospheres.
Cenospheres were modified to increase its adsorption capacity.First of all, cenospheres (low density CFA separated through wet method) were segregated and dried and finally metals were leached out using the toxicity characteristic leaching procedure (TCLP) method [17] to get modified cenospheres.Initially, extraction fluid was prepared by mixing 5.7 mL glacial acetic acid first with 500 mL reagent water and then with 64.3 mL of 1 N NaOH and finally volume was make up to 1 L by adding reagent water.10.0 g of cenospheres was taken in flasks with extraction fluid in 1 : 20 w/v ratio.The mixture was then shaken on orbital incubator shaker (G.G. Technologies, New Delhi, Model: GGT 1201) at a speed of 30 rpm for 20 h at room temperature.The leached metals will be dissolved in supernatant which is decanted and disposed of.The settled portion is dried at 100 ± 5 ∘ C for 24 h in hot air oven (York Scientific Industries, New Delhi, Model: YSI 431) and finally we get modified cenospheres from CFA which has been used as adsorbent in the present study.

Adsorption Studies.
The batch experiments were performed to study the effects of pH, adsorbent dose, adsorbate concentration, agitation speed, contact time, and temperature.For this, 100 mL of DB/DO dyes solution (concentration varying from 10 to 100 mg/L) was taken in 250 mL conical flask having adsorbent dose varying from 0.1 to 01.0 g.The pH (pH-meter Horiba Scientific, F74 BW) of mixture was adjusted by adding 0.1 N NaOH and 0.1 N H 2 SO 4 as per requirements.The flasks were then subjected to agitation (speed varying from 80 to 240 rpm) using orbital incubator shaker for proper adsorption.The contact time was varied from 5 to 300 min.After adsorption, solution was separated using Whatman filter paper number 42 and residual concentration of dyes was measured spectroscopically at wavelength of 545 nm for DB and 456 nm for DO using Dynamica UV-Vis single beam spectrophotometer (Model: Halo SB-10).All experiments were performed at three different temperatures (25 ∘ C, 35 ∘ C, and 45 ∘ C) based on Indian climatic condition.
The percent removal of the dye was calculated using the following equation: where  0 is the initial dye concentration (mg/L) and   is the dye concentration (mg/L) after time  (min).
In order to optimize five parameters (such as pH, adsorbent dose, adsorbate concentration, agitation speed, and contact time at three different temperatures), initially trial runs were conducted by fixing four parameters at a time and varying fifth one.The optimum values of the parameters thus obtained from trial runs were used in final experiments where one parameter was varied and other four parameters were fixed.

Adsorption Isotherms and Kinetics.
Adsorption capacity of adsorbent is defined as mass of dye adsorbed per unit mass of adsorbent and nature of the adsorption can be described by relating the adsorption capacity to equilibrium concentration of the solute remaining in the solution using various isotherms [18].The data obtained from batch experiments were tested for suitability of isotherms proposed by Langmuir [19], Freundlich [20], and Temkin and Pyzhev [21].The Langmuir isotherm assumes that the adsorption takes place at specific homogeneous sites within the adsorbent.The logarithmic form of the Langmuir isotherm is expressed by The Freundlich isotherm is derived by assuming heterogeneous surface with a nondistribution of heat of adsorption over the surface.The logarithmic form of the Freundlich isotherm is expressed by Temkin and Pyzhev [21] suggested that the heat of adsorption of all molecules in layer decreases linearly with coverage due to adsorbent-adsorbate interactions and the adsorption is characterized by a uniform distribution of the bonding energies, up to maximum binding energy.The Temkin isotherm is represented by where   is the concentration of adsorbate in solution at equilibrium (mg/L),   is the amount of dye adsorbed on adsorbent at equilibrium (mg/g),   is the maximum quantity of dye required to form a single monolayer on unit mass of adsorbent,  is a parameter for apparent energy of adsorption, defined as  = (  /  ),  is the Freundlich exponent constant that represents the parameter characterizing Quasi-Gaussian energetic heterogeneity of the adsorption surface,   is the Freundlich constant indicative of the relative adsorption capacity of the adsorbents (L/g),   is the equilibrium binding constant (L/mg), and   is the variation of adsorption energy (kJ/mol).
The kinetics of adsorption of dyes on modified cenospheres had been studied by the Lagergren pseudo-first order and pseudo-second order [22,23].The Lagergren rate equation is one of the widely used adsorption rate equations for the adsorption of solute from a liquid solution.The pseudo-first-order kinetics of Lagergren may be expressed by The pseudo-second-order kinetics of Lagergren is expressed in [12,22,24] where   is the amount of dye adsorbed on adsorbent (mg/g) at time ,  1 is the rate constant of pseudo-first-order kinetics, and  2 is the rate constant of the pseudo-second-order kinetics.

Statistical Analysis. Linear regression (SPSS 16
) was considered to understand the relationship between two variables on percent removal of DB and DO dyes.The rate of change in adsorption between two groups of dyes was calculated by  coefficient.in temperature, and is higher in DO as compared to DB.At all temperatures, both dyes DO and DB showed maximum removal at pH 6. Mohan et al. [25] also found maximum removal of Rosaniline Hydrochloride dye onto fly ash at pH 6. DO dye removal was 1.4, 1.1, and 1.0 times compared to DB at 25 ∘ C, 35 ∘ C, and 45 ∘ C, respectively, when the pH was varied from 2 to 6.However, the net removal (i.e., % change from pH 2 to 6) of DO at 25 ∘ C, 35 ∘ C, and 45 ∘ C was 15%, 14%, and 14%, respectively, whereas for DB, it was 13%, 13%, and 13%, respectively.The net removal of DO was 2%, 1%, and 1% higher as compared to DB.

The Effect of Adsorbent Dose.
At various temperatures, the effects of adsorbent dose on dye removal are presented in Figure 3.The removal of dyes increased with an increase of adsorbent dose particularly from 0.1 to 0.3 g.The adsorbent dose and temperature showed similar trend of dye removal (regression  coefficient) as in case of pH.The DO showed maximum removal at 0.2 g while DB showed maximum removal at 0.3 g at all temperatures.The regression analysis revealed 2.0-, 1.6-, and 2.1-fold more removal in DO (from 0.1 to 0.2 g) at 25 ∘ C, 35 ∘ C, and 45 ∘ C, respectively, as compared to DB (from 0.1 to 0.3 g).However, the net removal of DO (i.e., % change from 0.1 to 0.2 g) at 25 ∘ C, 35 ∘ C, and 45 ∘ C was 23%, 19%, and 17%, respectively, and for DB (from 0.1 to 0.3 g), it was 12%, 12%, and 8%, respectively.The net removal of DO was 11%, 7%, and 9%, respectively, higher than DB.The increase of dyes adsorption might be due to unbalanced attractive forces with the increase of surface area per unit mass of the adsorbent [26].

The Effect of Adsorbate Concentration.
The effects of adsorbate concentration at various temperatures on dye removal are summarized in Figure 4. Similar to effects of pH and adsorbent dose, in this case also, the rate of removal (regression  coefficient) of DO is higher than DB and decreases with increase in temperature for both dyes.At all temperatures, both dyes DO and DB showed maximum removal at 40 mg/L of dye concentration.The regression analysis revealed that there is the same percentage removal of DO and DB dyes at all three temperatures.However, the net removal (i.e., % change from 10 to 40 mg/L) of DO at 25 ∘ C, 35 ∘ C, and 45 ∘ C was 31.0%,30.0%, and 28.6%, respectively, whereas for DB, it was 30%, 29%, and 28%, respectively.The net removal of DO was 1.0%, 1.0%, and 0.8% higher as compared to DB. Observations of present study are similar to the findings of Doulati Ardejani et al. [27], in which the adsorption of Direct Red 80 dye from aqueous solution onto almond shells was maximum at 40 mg/L.

The Effect of Agitation Speed.
The effects of agitation speed at various temperatures on dye removal are summarized in Figure 5.The contact time was kept as found in initial trial runs.According to agitation speed and temperatures, the rate of removal (regression  coefficient) of both dyes follows similar trend as for pH, adsorbent dose, and adsorbate concentration.At all temperatures, both the DO and DB showed maximum removal at 140 rpm.The results are in

The Effect of Contact Time.
The effects of contact time at various temperatures on dye removal are presented in Figure 6.In both dyes, the rate of removal (regression  coefficient) was also temperature dependent and decreases with increase in temperature.However in case of DO, the rate of dyes removal was more conspicuous than to DB.At all temperatures, maximum removal of DO and DB was observed at 100 min and 120 min respectively.Our findings are in accordance with Namasivayam et al. [28], in which maximum removal of Congo red dye onto orange peel was observed between 90 to 120 min.The regression analysis revealed 1.2-, 1.1-, and 1.1-fold more removal (5 to 100 min) in DO at 25 ∘ C, 35 ∘ C, and 45 ∘ C, respectively, as compared to DB.However, the net removal (i.e., % change from 5 to 100 min) of DO at 25 ∘ C, 35 ∘ C, and 45 ∘ C was 27%, 27%, and 26%, respectively, and for DB (5 to 120 min), it was 23%, 22%, and 21%, respectively.The net removal of DO was 4.0%, 5.0%, and 5.0%, respectively, higher as compared to DB.It can be concluded that the maximum removal of DB dye is obtained at pH 6, adsorbent dose 0.3 g/100 mL, adsorbate concentration 40 mg/L, agitation speed 140 rpm, and contact time 120 min.The optimum parameters for maximum removal of DO dye are pH 6, adsorbent dose 0.2 g/100 mL, adsorbate concentration 40 mg/L, agitation speed 140 rpm, and contact time 100 min.The experiments were concluded at three temperatures to observe the effect of temperature on optimized parameters and maximum removal of dyes.The removal of dyes increased with increase in temperature.At 45 ∘ C, maximum removal of dyes was observed which approximately is 10% higher than 25 ∘ C temperature.

Adsorption Isotherms.
For both DB and DO dyes, experimental isotherms were plotted using the data obtained from the batch experiments.Their constants and coefficient of determination ( 2 ) are shown in Table 1.
The higher value of coefficient of determination ( 2 > 0.98) close to 1 for Langmuir isotherm showed that both dyes are more applicable and appropriate in describing the data for modified cenospheres adsorbent.The adsorption sites will be independent of the neighboring sites which accommodate only one dye molecule on each adsorption site in the form of complexes of reactive functional groups present on the surface of adsorbent.The calculated constants of Langmuir, Freundlich, and Temkin isotherm for DO and DB dyes are shown in Table 1.Single layer adsorption of ions on modified cenospheres surface was reported by Langmuir isotherm.The low values in this study indicate a weak interaction between adsorbate and adsorbent for ion-exchange mechanism [29].The adsorption capacity of cenospheres adsorbent (33.33 for DO and 32.26 for DB) is comparable with those of commercial adsorbents such as biopolymer chitosan (12.70 mg/g) [30], banana pith (20.29 mg/g) [31], and waste slurry (9.50 mg/g) [32].

Adsorption Kinetics.
The kinetics of adsorption has been studied to explain the dye uptake mechanism onto the modified cenospheres.It was observed that the adsorption of dyes increases with increase of contact time.However, the adsorption of DB was quick in the first 120 min and for DO was 100 min after which the rate of adsorption slowed down and stabilized as the equilibrium approaches.Adsorption kinetics was studied using the Lagergren pseudo-first-order model and pseudo-second-order model [12,24,33] and is presented in Table 2 and Figure 7.
The mean values of kinetic rate coefficients  1 (pseudofirst-order kinetic model) are 0.034 ± 0.005 and 0.038 ± 0.004 whereas  2 (pseudo-second-order kinetic model) are 0.020 ± 0.007 and 0.041 ± 0.009 for DO and DB dyes, respectively.The higher values of coefficient of determination ( 2 > 0.99) for pseudo-second-order kinetics show that both dyes are more applicable and appropriate in describing the data.From Table 2, it is concluded that removal of dye from modified cenospheres was well represented by pseudo-second-order model for both dyes.
The scatter plots between experimentally observed ( ,exp ) and model calculated ( ,cal ) values for pseudo-first andsecond order are shown in Figure 8.
Based on slope/intercept and  2 values of trend lines, we are getting the similar conclusion that removal of dye from modified cenospheres was well represented by pseudosecond-order model for both dyes.

Conclusions
It can be concluded that the optimum determined parameters were pH 6, dye concentration was 40 mg/L, and agitation  q 1 (mg/g) q 2 (mg/g) q 3 (mg/g) q 4 (mg/g) q 5 (mg/g) q 6 (mg/g) q 7 (mg/g) q 8 (mg/g) q 9 (mg/g) q 10 (mg/g) q 1 (mg/g) q 2 (mg/g) q 3 (mg/g) q 4 (mg/g) q 5 (mg/g) q 6 (mg/g) q 7 (mg/g) q 8 (mg/g) q 9 (mg/g) q 10 (mg/g) t/q t (d) DO  speed was 140 rpm for both dyes while the adsorbent dose was 0.3 g for DB and 0.2 g for DO.The equilibrium contact time was 120 min for DB and 100 min for DO and the best temperature was 45 ∘ C. The maximum dyes removal capacity of modified cenospheres was found to be 78% for disperse blue 79:1 and 81% for disperse orange 25.The effect of pH revealed that modified cenospheres may produce good results at pH 6 that means it does not require any especial acidic or basic chemical for dye removal.This pH 6 also falls within the prescribed limit of pH (5.5 to 9.0) of treated industrial effluent allowed to be discharged into the inland surface water bodies in India.The adsorption process was explained by Langmuir isotherm having monolayer adsorption capacity 32.26 mg/g and 33.33 mg/g for DB and DO, respectively.The investigated adsorption mechanisms for disperse blue and disperse orange dyes followed the pseudo-second-order kinetics model.Our results suggest that modified cenospheres could be suitably used as an alternative and effective resource materials as compared to the expensive commercial adsorbents for the removal of disperse blue 79:1 and disperse orange 25 from colored wastewater.The used modified cenospheres could easily be dumped into landfill with lining or in concrete pits or can also be used in brick making to minimize the environmental risk.

Figure 2 :
Figure 2: The effects of pH and temperature on % dye removal of DB and DO.

Figure 4 :Figure 5 :Figure 6 :
Figure 4: The effect of adsorbate concentration and temperatures on % dye removal of DB and DO.

Figure 7 :
Figure 7: Pseudo-first-order kinetics for (a) DB and (b) DO and pseudo-second-order kinetics for (c) DB and (d) DO dyes.

Figure 8 :
Figure 8: Plot between experimentally observed ( ,exp ) and model calculated ( ,cal ) values of pseudo-first-order and pseudo-second-order kinetic model for (a) DO and (b) DB dyes.

Table 1 :
Isotherms parameters for adsorption of DO and DB dyes on the modified cenospheres.

Table 2 :
Comparison of pseudo-first-order and pseudo-second-order kinetics at different concentrations of DO and DB dyes. 1 and  2 are rate constants of pseudo-first-order and pseudo-second-order kinetics.