Textile industry effluents shows large amounts of dye chemicals which are known
as severe water pollution. Thus, it is important to reduce the dye concentration
in the wastewater before its discharge into the environment. Discharging large
amounts of dyes into water bodies, accompanied by organics can affect the physical
and chemical characteristic of fresh water (Erdem et
al., 2004). Dyes in wastewater can obstruct light penetration. Dyes
are also stable to light irradiation and heat and are known toxic to microorganisms.
The removal of dyes is stringent due to their complex structure and synthetic
origins (Hu et al., 2006). Reactive blue 29 is
an anthraquionone dye and due to the complex aromatic structure its considered
stable in the environment and toxic to aquatic life (Crini,
2006). One of the effective treatment processes for the removal of dyes
from water at low cost is adsorption. The successful removal of dyes have been
demonstrated on lowering dye by using different adsorbents such as activated
carbon, peat, chitin, clay and others (Tahir and Rauf, 2005).
Adsorption techniques have gained attention in recent years due to their proven
efficiency in the removal of pollutants from effluents. Besides the high quality
product obtained, the processes have proved economically feasible (Jumasiah
et al., 2005).
Montmorillonite is a very soft phyllosilicate mineral that typically forms
in microscopic crystals, forming clay. The particles made this mineral are plate-shaped
with an average diameter of approximately 1 micrometre. The particle thickness
is extremely small (~1 nm). It is the main constituent of the volcanic ash weathering
product, bentonite. Montmorillonite's water content is variable and it increases
greatly in volume when it absorbs water. Chemically it is hydrated sodium calcium
aluminium magnesium silicate hydroxide (Na, Ca)x(Al, Mg)2(Si4O10)(OH)2•nH2O.
Potassium, iron and other cations are common substitutes, the exact ratio of
cations varies with source (Jaafar, 2006). Like other
clays, montmorillonite swells with the addition of water. Montmorillonite is
widely used in various science fields because of the large cation exchange capacity,
high specific surface area, good swelling capacity and high platelet aspect
ratio. Moreover, its surface can be modified easily (Ahmad
et al., 2009). Presence of inorganic cations such as Na+
and Ca+2 on the basal planar surface of montmorillite layers makes
it hydrophilic in nature and hence, shows the clay ineffective for absorption
of aliphatic and relatively anionic compounds. Thus, the modification of the
surface is of great importance.The properties of MMTcan be changed greatly by
replacing the natural inorganic cations with other cation such as metal ions
(Low et al., 2000) and quaternary amines.
In this study, MMT modified with diethylentriamine (DETA) and evaluated as
the possible sorbent for the removal of a reactive anthraquionone dye. The aim
of this study is to investigate the effects of adsorbent dosage, temperature,
contact time and pH on the adsorption process. Thermodynamic parameters, such
as ΔG°, ΔH° and ΔS° were also calculated.
MATERIALS AND METHODS
Materials: Montmorillonite (sodium form) was purchased from F.C.C. (China).
Reactive blue dye stock solution was prepared by dissolving RB29 in deionized
(DI) water. Standards samples at a required concentration range were prepared
by appropriate dilution of the stock solution with DI water. The formula, molecular
weight and maximum wavelength of light absorbed by RB29 were C31
H19 O9 N5 S2 Cl2 Na2,
788 g mol-1 and 589 nm, respectively. All reagents used were of analytical
reagent grade. The method of detecting RB29 in aqueous solution had been described
before (Nadafi et al., 2011).
Preparation of adsorbent: The montmorillonite (5 g) was dissolved in 500 mL of deionized water. Then, the solution was stirred using nonmagnetic stirrer for 24 h at room temperature. The 5 mL diethylentriamine (DETA) was added to the obtained material and the pH value of the mixture was adjusted 3.5 by addition of hydrochloric acid (0.1 M). The suspension was stirred for 2 h in 50°C. The white precipitation was filtered, washed several times with distilled water and dried in a vacuum oven at 50°C.
General procedure: Adsorption of RB29 onto DETA-MMT was studied by batch experiments. A stock RB29 solution (1000 mg L-1) was used in adsorption experiments. The required concentration of the dye solution was prepared by serial dilution of stock solution. A fixed amount of the adsorbent was added to dye solution. All batch adsorption experiments were carried out in 50 mL sealed plastic tubes with the working volume of 25 mL. The solutions were stirred continuously at room temperature (25±1°C) to achieve equilibrium. After equilibrium, the solid was separated by centrifugation (3000 rpm) and analyzed spectrophotometerically. The percentages of RB29 removal is calculated based on the following Eq:
In this equation, C0 is the initial dye concentration and Ce is the equilibrium concentration. The amount of nitrate adsorbed (qe in mg g-1) was calculated as follows:
where, C0 and Ce are the initial and equilibrium concentrations of nitrate in solution (mg L-1), V is the volume of solution (L) and m is mass of the adsorbent (g).
pH studies: In order to investigate the effect of pH on RB29 adsorption, the pH of the solutions were adjusted from 3-12. The initial pH of the solution was adjusted by using 0.1 M HCl or 0.1 M NaOH and DETA-MMT was added to 25 mL solution. The mixture was shaken using a temperature-controlled shaker. After adsorption, the final pH of all solutions were measured and the value providing the maximum nitrate removal was determined.
RESULTS AND DISCUSSION
Adsorbent characterization: The surface morphologies of the MMT and
DETA-MMT were studied using scanning electron microscopy. Micrographs of the
surface of each material are shown in Fig. 1(a-b).
Figure 1(a) corresponds to the raw MMT. It is clearly shown
that MMT consists of small particles and has a nonporous surface (Fig.
1a); however, it was not compact. After DETA-MMT composite fabrication,
the sharp sheet is not observable anymore, due to the DETA coating onto the
surface (Fig. 1b). This figure revealed the round surface
texture and heterogeneous porosity. The chemically modified nanoclay developed
more pores than that of the unmodified one. Figure 2 shows
the FT-IR spectra of MMT and DETA-MMT. The FT-IR spectrum exhibited the characteristic
bands of clay lattice -OH stretching vibrations (3630 cm-1) while
those at 3448 and 1639 cm-1 are attributed to the adsorbed H2O
deformation. The band at 1639 (Fig. 2a) was shifted to 1632
(Fig. 2b). Spectra of modifeid MMT showed all the characteristic
bands of the MMTgroups with additional peaks at 3033 cm-1 which is
due to the stretching vibration of N-H. These results confirmed that the MMT
was modified by DETA adsorption isotherms.
|| SEM images of (a) MMT and (b) DETA-MMT
|| FT-IR spectrum of (a) MMT and (b) DETA-MMT
|| Adsorption isotherm of RB29 onto DETA-MMT
The adsorption isotherm is of great importance for describing how the adsorbate molecules distribute between the liquid and the solid phases. The adsorption isotherm of RB29 on DETA-MMT is shown in Fig. 3. As shown in this figure, RB29 uptake increased with the increasing of dye concentrations which is due to the increase in the driving force from the concentration gradiant. From Fig. 3, the experimental adsorption capacity of RB29 on DETA-MMT was 23.44 (mg g-1) at 293 K (25°C).
|| Isotherm parameters for removal of RB29 by DETA-MMT
In this study, Langmuir and Freundlich isotherms were employed for the study
of the adsorption of RB29 dye onto modified montmorillonite. Langmuir model
is represented mathematically as follows (Adamson et al.,
Another important parameter, RL, the separation factor or equilibrium parameter is determined from the relation:
where, b is the Langmuir Constant and Co (mg L-1) is
the highest nitrate concentration. The value of RL shows this type
of isotherm to be either favourable (0<RL<1) or unfavourable
(RL>1), linear (RL = 1) or irreversible (RL
= 0) (Zheng et al., 2008). RL values
for the present study was less than 1 and greater than zero indicationg favourable
adsorption (Table 1).
The freundlich isotherm model is an empirical relationship which assumes different
sites with distinct adsorption energies are involved and its linear form is
where, Ce (mg L-1) is the equilibrium concentration, qe (mg g-1) is the amount of adsorbate adsorbed per unit mass of adsorbate and qm and b are the Langmuir constants related to adsorption capacity and rate of adsorption, respectively. Kf and n are Freundlich constants.
The freundlich model assumes a heterogeneous adsorption surface with sites
that have different adsorption energies which are not equally available. This
isotherm is achieved through Eq. 5. In this equation, Kf
and n are the empirical constants and their values were obtained from the intercept
(Ln Kf) and slopes (1/n) of linear plots of Ln qe versus
Ln Ce. The Kf and n values in Freundlich equation were
found to be 2.33 and 1.29, respectively (Table 1). It has
been reported that values of n in the 2-10 range represent good, 1-2 moderately
difficult and <1 poor adsorption characteristics. In the present research,
the n obtained was 1.98, indicating that the adsorption is moderately difficult.
|| Effect of pH on RB29 dye removal
|| Reported adsorption capacities of some adsorbents for reactive
The validity of models was determined by calculating the standard deviation
where, the exp and cal are the experimental and calculated data and n is the number of data points. As seen in Table 1, SD values are smaller than 12% for both models indicating that both isotherms fitted the experimental results obtained. On comparing the freundlich and langmuir isothermal models, it is evident that Freundlich isotherm is more favorable from the R2 values presented. By comparing the results obtained from the present study with other reported works on adsorption capacities of other adsorbents for reactive blue dye removal it can be stated that our findings are acceptable due to the the natural character of montmorillonite and its low cost. (Table 2).
Effect of various parameters effect of pH: The adsorption of RB29 onto the composite as a function of pH was investigated at the initial dye concentration of 100 mg L-1 and the contact time of 24 h. The effect of pH on the adsorption of RB29 onto composite is shown in Fig. 4. The removal of dye was maximum in the pH of 3. In the acidic solution, the adsorption process of the RB29 by DETA-MMT is an electrostatic interaction, where the amine groups of DETA-MMT interact with the anionic groups of the dye. While at high pH, more OH¯ ions present and compete with the anionic groups of RB29 for the adsorption sites of adsorbent, thus the available adsorption sites decrease.
Effect of adsorbent dosage: The influence of adsorbent dose on RB29 removal was studied by varying the adsorbent dosage from 1.0 to 7.0 g L-1 at an initial RB29 concentration of 100 mg L-1. Increased adsorbent dosage implied a greater surface area and a greater number of binding sites available for the constant amount of RB29. The results showed (Fig. 5) that 2 g L-1 of DETA-MMT is required for 80% removal of RB29 for initial concentrations of 100 mg L-1.
|| Effect of adsorbent dosage on RB29 dye removal
|| Effect of contact time on adsorption of RB29 dye
Effect of contact time and agitation rate: The adsorption of RB29 on DETA-MMT was investigated as a function of contact time (2-60 min) at initial dye concentrations (100 mg L-1) with an initial solution pH of 3.5. It was noticed that dye removal increased with time (Fig. 6). The trend of the plot in Fig. 6 exhibit that dye uptake was rapid in the beginning followed by a slower removal that gradually reached a plateau. Maximum removal of dye was achieved within the first 2 min of contact time and equilibrium was attained in 45 min. It was also found that about 84% removal of dye occurs within 2 min. The agitation speed ranging between 80 and 120 rpm was maintained (Figures not shown). For all speeds, the removal was not varied significantly. An agitation speed of 110 rpm was chosen as an optimum value. The small effect of agitation showed that external mass transfer was not the only rate limiting step.
Thermodynamic parameters: In adsorption processes the temperature plays
an important role in determining thermodynamic dependency. The nature and thermodynamic
feasibility of the sorption process were analyzed by standard free energy (ΔG°),
satndard enthalpy (ΔH°) and standard enthropy (ΔS°) using
the following equations (Liu et al., 2009):
where, b is the Langmuir constant (L mol-1), R is the gas constant and T is the temperature (K).
On the basis of Eq. 7, the values of Gibbs free energy were calculated as -11.14, -11.43 and -15.3 at temperatures of 298, 303 and 323 K, respectively. The decrease in the negative value of ΔG° with an increase with temperature shows that the adsorption is favorable at higher temperature. The values of ΔH° and ΔS° were -46.37 kJ mol-1 and -0.11 J mol-1 suggesting that the adsorption is exothermic. The negative value of ΔS° suggest the probability of favorable adsorption.
The present investigation evaluated the effect of the chemically modified montmorillonite for the adsorption of RB29 dye. The adsorption of RB29 was found to be pH dependent with maximum removal was obtained at pH 3. The maximum sorption capacity of DETA-MMT was found to be 23.4 mg g-1 at 25°C. The equilibrium data were analyzed using the Langmuir and Freundlich isotherm models. Freundlich isotherm fitted the data well. The thermodynamic calculations indicate that the adsorption of RB29 is a feasible process which undergoes an exothermic process.
The authors would like to thank the staff of medical science research center, Islamic Azad University, Tehran, Iran for their collaboration in this study.