Adsorption of Direct Dyes Using Rice Husk: A Case Study of Direct F. Scarlet

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Use of rice husk for adsorption of direct dyes from aqueous solution: A case study of direct F. Scarlet

Conference Paper · May 2005

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USE OF RICE HUSK FOR ADSORPTION OF DIRECT DYES FROM AQUEOUS SOLUTION: A CASE STUDY OF

DIRECT F. SCARLET

O^LAA^BDELWAHAB,A^HMEDE^LN^EMR*,A^MANYE^LS^IKAILY,A^ZZAK^HALED Environmental Division, National Institute of Oceanography and Fisheries, Kayet Bay, El-Anfushy, Alexandria, Egypt

Key words: Adsorption, Rice Husk, Direct Red-23, Isotherms, Kinetics ABSTRACT

Dyes are widely used for coloring in textile industries, and significant losses occur during the manufacture and processing of dyes, and these lost chemicals are discharged in the effluent. Adsorption of dyes is a new technology for treatment of wastewater containing different types of dyes. The goal of this research is to develop a new and efficient adsorbent of direct dyes. Thus, rice husk, a commonly available agriculture waste in Egypt, was investigated as viable materials for treatment of synthetic Direct F. Scarlet (Direct Red 23) containing industrial wastewater. The results obtained from the batch experiments revealed the ability of the rice husk in removing the Direct Red 23 and are dependent on the dye and rice husk concentrations. Maximum dye colour removal was observed after 72 hours for all the system conditions. Kinetics of adsorption was investigated as well as equilibrium isotherm of Direct Red 23 using Langmuir model, which represented a correlation coefficient > 0.96. Activation of rice husk was studied in order to increase its capacity to remove the dye. The maximum adsorption capacity was ~13 mg of dye per one gram of dry rice husk. This study showed that the rice husk could be employed as low-cost and effective sorbent for the removal of Direct F. Scarlet from aqueous solution.

INTRODUCTION

The release of the colored dyes into the ecosystem is a dramatic source of esthetic pollution, of eutrophication, and of perturbations in aquatic life. Some azo dyes and their degradation products such as aromatic amines are highly carcinogenic (Mohamed 2004). Proper treatment of the dye plant effluent is thus, a matter of concern before discharge. This led to an intensive search for the best available technology, which can be used for the removal and remediation of dyes. In addition, it makes the treatment of industrial effluent to be an important target for industry and environment protection. Different treatment methods are described in the literature, including filtration, flocculation, chemical precipitation, ion exchange, membrane separation, and adsorption (Tarley and Arruda 2003). Practically, dye removal

process requires the following potential advantages for the adsorbent: (1) a large accessible pore volume, (2) hydrophobicity, (3) high thermal and hydrothermal stability, (4) no catalytic activity, and (5) easy regeneration. New approaches based on the use of natural, inexpensive sorbent materials for effluent treatment have been reported (Marshall et al. 1999; Wafwoyo et al. 1999;

Vaughan et al. 2001). However, the use of these materials is still limited, although they show good adsorption capacity relative to that of the other expensive treatment processes (Dushenkov et al. 1995).

The adsorption process is one of the efficient methods to remove contaminant from effluent (Nigarn et al. 1996; Imagawa et al. 2000; Mohan et al. 2001; Malik 2003).

The process of adsorption has an edge over the other methods due to its sludge free clean operation and complete removal of dye even

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from dilute solutions. Activated carbon is the most widely used adsorbent for this purpose because of its extended surface area, microporous structure, high adsorption capacity and high degree of surface reactivity. However, commercially available activated carbons are very expensive. In addition, the laboratory preparation of activated carbons have been accompanied by a number of problems such as combustion at high temperature, pore blocking, and hygroscopicity. This has led to search for cheaper and simplest substituents.

Rice husk, an undesirable agriculture mass residue in Egypt, is a by- product of the rice milling industry. It is one of the most important agricultural residues in quantity. It represents about 20% of the whole rice produced, on weight basis of the whole rice (Daifullah et al. 2003). The estimated annual rice production of 500 million tones in developing countries, approximately 100 million tones of rice husk is available annually for utilization in these countries aloe. Traditionally, rice husks have been used in manufacturing block employed in civil construction as panels and was used by the rice industry itself as a source of energy for boilers (Della et al. 2001).

However, the amounts of rice husk available are so far in excess of any local uses and have posed disposal problems. It was chosen because of its granular structure, chemical stability and its local availability at very low cost and there is no need to regenerate them due to their low production costs.

The main constituents of rice husk are: 64-74% volatile matter and 12-16% fixed carbon and 15-20% ash (Armesto et al. 2002;

Daifullah et al. 2003). The rice husk composition are: 32.24% cellulose, 21.34%

hemicellulose, 21.44% lignin, 1.82%

extractives, 8.11% water and 15.05% mineral ash (Govindarao 1980; Rhman et al. 1997;

Nakbanpote et al. 2000). The mineral ash is 94.5-96.34% SiO2.

The purpose of this work is to investigate the adsorption capacity of the untreated and activated rice husk on

adsorption of direct dyes from aqueous solution. Direct F. Scarlet (Direct red 23) dye was selected for the adsorption experiment due to its presence in several industrial effluents such as textile, tannery, paper, soap, cosmetics, polishes, wax etc. As Direct F. Scarlet has diazo group, it has toxicity and carcinogenic nature. The dye (DR-23), dose will not allow sunlight to pass through and thereby affects the photosynthesis of aquatic plants. Modern dyes are resistant to conventional biological treatment but these azo dyes are reduced to colorless primary amines by this treatment.

These primary amines are even more toxic than the original dye. The equilibrium and kinetics of DR-23 adsorption on rice husk were investigated. The Langmuir model was used to fit the equilibrium isotherm. The batch contact time method was used to measure the adsorption rate and kinetic parameters were then evaluated.

MATERIALS AND METHODS Preparation of Sorbents

Rice husk was obtained from local rice mills and was washed several times with water followed by filtration. The cleaned rice husk was oven dried completely at 105^°C, then cooled and sieved to 250-500 µm size, which was used without further treatment.

Another part of same size fraction of rice husk was exposed to activation using citric acid (Marshall et al., 1999; Wartelle and Marshall 2000; Marshall and Wartelle 2003) which was reported as follows: 100 g of rice husk were soaked in 0.6M citric acid for 2 h at 20^°C. The acid-husk slurry was dried overnight at 50^°C and the dried husk was then heated to 120^°C. The reacted product was washed repeatedly with distilled water (200 mL per g of husk) to remove any excess of citric acid followed by oven drying overnight at 100^°C.

Preparation of Dye Solution

Direct F. scarlet (Direct Red 23) was obtained from ASMA Company and was used without further purification. Its

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molecular formula is C35H25N7Na2O10S2 and its colour index C.I. 29160 (Fig. 1). A stock solution of the dye was prepared by dissolving 1 gram of dye in 1000 mL distilled water to make a stock solution of 1000 mg/l.

The experimental solution was prepared by diluting definite volume of the stock solution to get the desired concentration. For absorhance measurements a UV-VIS spectrophotometer (Milton Roy, Spectronic 21D) was employed using silica cells of path length 1 cm. The maximum wavelength max

for the DR-23 was measured at 507 nm.

Concentrations during experimental work were determined from a standard calibration curve.

Adsorption Studies

Equilibrium adsorption isotherms for the untreated and activated rice husk were determined at agitation rate of 150-rpm at room temperature and different concentrations of dye and rice husk. 10 g/l of adsorbent (based on dry weight of rice husk) was placed in a set of 250-mL Erlenmeyer flasks and different initial dye concentrations of (5, 10, 15, 20, 25 mg/l) were added to each flask. The contents were shaken for 2 hours, small amount was withdrawing and centrifuged at 4000 rpm for 5 min. The supernatant was analyzed for dye contents.

All experiments were carried out in duplicate and the relative deviation was less than 5%.

The adsorption behaviours of the samples were studied by evaluating the

percentage removal efficiency of DR-23, from the relation

Removal efficiency = [(C0 – C) ÷ C0] × 100 (1) where C0 is the initial concentration of DR- 23, C is the solution concentration after adsorption at any time.

Effect of time on the dye removal at various predetermined intervals was monitored by shaking the reaction mixture, centrifuged and analyzed for the dye content at the end of each contact time. Each flask represents an interval, i.e. one point on the curve. Time experiments were studied for different sorbent concentrations to study the effect of sorbent dosage on the adsorption of colour. Experiments were performed using sorbent concentrations of 2.5-10.0 g/l, in a dye solution of 10.0 mg/l for both the untreated and activated rice husk.

Kinetics of adsorption were determined by analyzing adsorptive uptake of the dye colour from aqueous solution at different time intervals. The amount of dye adsorbed onto the rice husk qe (mg/l) was calculated according to the following mass balance equation

qe = (Co – Ce) V ÷ W (2) where Co and Ce are the initial and equilibrium concentrations mg/l of dye, respectively, V is the volume of the solution (l), and W is the weight (g) of the rice husk used (adsorbent).

Fig.1. Structure of Direct F. Scarlet (Direct Red 23 or Direct F. Orange).

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RESULTS AND DISCUSSION The adsorption of DR-23 on rice husk and citric acid treated rice husk were studied for its possible importance in the treatment of containing industrial effluents.

The influence of agitation time, sorbent and dye concentrations on removal of DR-23 were studied in Figs 2a and 2b. The amount of adsorbed dye (mg/g) increased with increase in agitation time and concentration and remained nearly constant after equilibrium time. The rate of removal of colour was rapid initially. The rate levelled off gradually and then attained a more or less constant value beyond which there was no significant increase in colour removal. The time required to attain equilibrium was 120 minutes for untreated and 90 minutes for activated rice husk. The activated rice husk possesses the highest sorption capacity, especially for 2.5 and 5 g/l of sorbent. The removal curves are single, smooth and continuous leading to saturation.

However, from Figs 2a and 2b, the dye colour removal pattern showed that increase in the biomass concentration indicated increase dye removal capacity, which may be attributed to the increase of biomass of rice husk which gives more surface area for sorption of the dye molecule on the surface (Mohan et al. 2001). The incremental increase of 30% was observed for increase of biomass from 2.5 to 5 g/l and another 30% for increase of biomass from 5 to 10 g/l for untreated rice husk. While for citric acid treated rice husk, an incremental increase of 10% was observed for every 2.5 g/l increase of biomass. It can also follow from the above discussion that the rice husk biomass uptake was dependent on both the dye concentration and rice husk biomass.

Adsorption isotherm

Adsorption data were analyzed according to Langmuir model for both untreated and citric acid treated rice husk.

The Langmuir equation, which has been successfully applied to many adsorption systems (Malik 2003; Namasivayam and

Kavitha 2003; Waranusantigul 2003) is given by the equation:

qe= (KL × Sm × Ce) ÷ (1 + KL × Ce) (3) Where KL is the equilibrium adsorption coefficient (l/mg), Sm is the maximum adsorption capacity (mg/g), Ce the adsorbate equilibrium concentration. Eq. (3) can be written in the following linear form:

Ce ÷ (X÷M) = [1 ÷ (KL × Sm)] + (Ce ÷ Sm) (4) where X = amount of solute adsorbed per unit weight of adsorbent and M = the adsorbent dosage required to reduce the initial concentration C0. The dye adsorption data were plotted according to linear Langmuir isotherms. A linearized plot of Ce/(X/M) versus Ce was obtained for the untreated and activated rice husk as shown in Fig. 3.

Regression data and adsorption parameters obtained are indicated in Table 1. One advantage of the Langmuir equation in its linear form is that the adsorption maximum can be calculated from the regression. This parameter and the equilibrium dye concentrations are useful in describing the adsorbent capacity (Tokunaga and Uthium 1997). The data for biosorption of DR-23 onto untreated and citric acid treated rice husk were fitted to the Langmuir model (r = 0.99 and 0.97, respectively). The high fitness of the Langmuir model for this biosorption system indicates the monolayer coverage of dye on the outer surface of biosorbent, in which the adsorption of dye ions occurs uniformly on the active part of the surface.

The fits are quite well for both sorbents, to suggest the applicability of the Langmuir model for the investigated system. Sm and KL, Langmuir constants, are the measures of maximum adsorption capacity and energy of adsorption, respectively. The values of Langmuir parameters along with the correlation coefficients were computed from the intercept and the slope of the fitted Langmuir equation. Activated rice husk is observed to have higher adsorption capacity than the untreated rice husk in the removal of DR-23.

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0 10 20 30 40 50 60 70 80 90

0 20 40 60 80 100 120

Time (min)

% Removal

2.5 g/l 5.0 g/l 10.0 g/l

0 10 20 30 40 50 60 70 80 90

0 20 40 60 80 100 120

Time (min)

% Removal

2.5 g/l 5.0 g/l 10.0 g/l

Fig. 2a. Influence of contact time and weight of untreated rice husk on removal of DR-23 using dye concentration of 10 mg/l.

Fig. 2b. Influence of contact time and weight of activated rice husk on removal of DR-23 using dye concentration of 10 mg/l.

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0 2 4 6 8 10 12

0 5 10 15 20 25

Ce (mg/l) Ce/(X/M)

activated rice husk Untreated rice husk

Fig. 3. Fitting adsorption data with Langmuir using sorbent concentration of 10 g/l.

Table 1. Langmuir constants for the adsorption of DR-23 using untreated and modified rice husk.

Adsorbent Sm (mg/g) KL (l/mg) RL r- value

Untreated rice husk 2.415 0.17 0 < RL <1 0.99

Activated rice husk 4.35 0.14 0 < RL <1 0.97

The essential characteristic of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor, RL, which is defined by Mohan et al. (1974) as follows

RL= 1 ÷ [1 + (KL × C0)] (5) where C0 is the initial concentration and RL

indicates the shape of the isotherm. The types of equilibrium isotherm are related with the RL values as RL > 1 for unfavourable, RL = 1 for linear, 0< RL < 1 for favourable and RL >

0 for irreversible. In this work, the values of R L were found to be less than 1 and greater than 0 indicating the favourable adsorption of DR-23 on rice husk.

Kinetics of Adsorption

The kinetics of adsorption of DR-23, on both untreated and activated rice husk, was studied by applying the Lagergeren first order rate equation (Lee et al. 1996; Trivedi et al. 1973).

log (qe-q) = log qe – (Kt ÷ 2.303) (6) where q (mg/g) is the amount of dye adsorbed at time t, qe (mg/g) is the amount of dye adsorbed at equilibrium, and K is the equilibrium rate constant of sorption. The straight line plots of log (qe – q) versus t for dye at different biomasses under the used conditions indicated the validity of Eq. (6) and the process follows first order kinetics

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(Figs. 4a and 4b). Rate constants were calculated and the first order rate constants and correlation coefficient (r-value), for different dye concentrations with untreated and citric acid treated rice husk are given in Table (2).

In batch stirring reactors, the transport of adsorbate from bulk solution to the interior surface of the pores is often the rate limiting step of the adsorption process (Weber 1972; Annadurai 1999). The rate parameters controlling intra-particle diffusion for DR-23 with both types of adsorbents are determined using the equation of Waranusantigul 2003:

qt = Kp × t^0.5 (7)

where qt is the amount of dye adsorbed (mg/g) at time t; and Kp is the intra-particle diffusion rate constant (mg/g.Min^0.5).

Plotting the amount sorbed per unit weight of sorbent (qt) vs t^0.5 indicates the intra-particle diffusion rate, Kp, values, obtained from the slopes of the linear plots for each initial concentration for both types of adsorbents. Intra-particle diffusion plots are shown in Figs. 5a and 5b. It was found that the plots of different dye concentrations were linear and pass through the origin for both untreated and citric acid treated rice husk.

The data assigned that the rate determining step may be controlled by intra-particle diffusion.

Table 2. Rate constants for the removal of DR-23 by untreated and activated rice husk.

Type of adsorbent Concentration of

adsorbent (g/l) Dye Concentration

(mg/l) Rate constant

K (min^-1) r- value Untreated rice husk

Activated rice husk

5.0

10

5.0

10

5 10 15 20 25 10 5 15 20 25

0.039 0.035 0.037 0.037 0.035 0.051 0.055 0.091 0.049 0.053 0.014 0.018 0.018 0.036 0.013 0.083 0.077 0.039 0.024 0.026

0.98 0.97 0.96 0.98 0.99 0.96 0.98 0.99 0.93 0.93 0.89 0.99 0.92 0.96 0.98 0.98 0.97 0.98 0.96 0.96

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-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

0 20 40 60 80 100

Time (min)

log(qe-q)

5 mg/l 10 mg/l 15 mg/l 20 mg/l 25 mg/l

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

0 20 40 60 80 100

Time (min)

log(qe-q)

5 mg/l 10 mg/l 15 mg/l 20 mg/l 25 mg/l

Fig. 4a. Lagergren plots for the removal of DR-23 by adsorption on untreated rice husk and sorbent concentration of 5 g/l.

Fig. 4b. Lagergren plots for the removal of DR-23 by adsorption on activated rice husk and adsorbent concentration of 5 g/l.

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0 2 4 6 8 10 12

t

^0.5

q

t

5 mg/l 15 mg/l 20 mg/l

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0 2 4 6 8 10 12

t

^0.5

q

t

5 mg/l 15 mg/l 20 mg/l

Fig. 5a. Intra-particle diffusion plots for the removal of DR-23 using untreated rice husk.

Fig. 5b. Intra-particle diffusion for the removal of DR-23 on activated rice husk.

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CONCLUSION

The results obtained from the present investigation revealed the ability of rice husk in treating azo dye effluents, e.g.

Direct Red-23 dye, using rice husk activated with citric acid, which was capable of removing Direct Red-23 dye. Adsorption is highly dependent on the contact time, adsorbent dose and dye concentration. The adsorption isotherm of DR-23 onto rice husk biomass is described by the Langmuir isotherm model. Kinetics of adsorption follows Lagergren first order kinetic model with film diffusion being the constitutive rate-controlling step. The monolayer adsorption capacity obtained from Langmuir isotherms for DR-23 was relatively higher for the citric acid treated rice husk compared to that obtained without chemical treatment.

Consequently, safely can point to the use of this natural material due to abundance and very cheap biomass. This leads to its superiority as a potential sorbent in removal of some coloured dyes from waste waters.

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