Adsorption of Acid blue 25 dye on diatomite in aqueous solutions
Khashayar Badii1*, Faramarz Doulati Ardejani2, Masoud Aziz Saberi2, Narges Yousefi Limaee1 &
Seyed Zia-e-din Shafaei2
1Department of Environmental Researches, Institute for Colorants, Paints, and Coatings, Tehran, 1668814811, Iran
2Faculty of Mining, Petroleum and Geophysics, Shahrood University of Technology; Shahrood, P.O. Box: 36155-316, Iran Email: badii@icrc.ac.ir
Received 17 April 2009; revised 14 October 2009
The adsorption of Acid blue 25 (AB 25) dye from aqueous medium on diatomite was studied. The effects of pH, contact time, initial dye concentration, calcinations and sorbent dosage on the adsorption process were examined and optimum experimental conditions were identified. The maximum removal of dye was obtained under acidic conditions, in particular, at pH 2, but pH 8 was more suitable for industrial purposes. The percentage removal of dye decreased with an increase in initial concentration. Also, the results indicated that an increase in temperature raised the percentage removal of dye. The maximum percentage removal of AB 25 dye from aqueous solution using raw diatomite at pH 2, temperature 25±1°C, agitation speed of 200 rpm, initial dye concentration of 50 mg/L, and for a mixing time period of 300 min was 72.81%
(64.22% at pH 8). Furthermore, under same conditions, the maximum adsorption of dye on calcined diatomite was 54.5%.
The experimental data showed that the adsorption of dye on raw diatomite follows the Langmuir model, but its adsorption on calcined diatomite followed the Freundlich model. The kinetics effect of the adsorbent was pseudo-second-order.
Keywords: Dye removal, Adsorption, Diatomite, Isotherms, Acid blue 25
Contamination of surface and groundwater with the textile industry effluents is a major concern to public health. Synthetic dyes, suspended solids and dissolved organics are the main hazardous materials found in textile effluents1. These materials can affect the physical and chemical properties of fresh water. In addition to the undesirable colours of textile effluents, some dyes may degrade to produce carcinogens and toxic products2. Furthermore, the coloured effluents reduce light penetration and potentially prevent photosynthesis3,4.
Many treatment systems have been proposed for the removal of synthetic dyes from aqueous solutions. Coagulation5, flocculation6, photocatalytic degradation7-9, membrane filtration10, microbiological decomposition11, electrochemical oxidation12, fungus biosorbent13 and adsorption2-5,14-18 are the most commonly used methods for removing dyes from waste effluents systems. Adsorption is considered to be particularly competitive, economically cost effective and efficient process for the removal of dyes, heavy metals and other organic and inorganic hazardous impurities from aqueous solutions19-21. Besides, the microbiological, photocatalytic and electrochemical decomposition procedures are not efficient because many dyes can not be easily decomposed12. Although
activated carbon is the most efficient and popular adsorbent and has been used with great success, the high cost of activated carbon sometimes restricts its applicability for dye removal17,20,22. Therefore, in recent years, considerable attention has been devoted to the study of different types of low-cost and efficient materials as sorbent for the removal of dyes from aqueous solutions, which included wood and saw dust23,24, fly ash25, wheat straw26, apple pomace26, orange peel27-29, banana peel30, peanut hull3, leaf31, soy meal hull32, egg shell membrane33 etc.
AB 25 dye is used in huge quantities in Iran and produces many environmental problems. So, to search for an appropriate and low-cost adsorbent is an important consideration for designing a suitable treatment plant for minimising pollution load.
Diatomite (SiO2.nH2O) is a pale-coloured, soft, lightweight siliceous sedimentary rock made up principally from the skeletons of aquatic plants called diatoms. Diatomite contains a wide variety of shape and sized diatoms, typically 10-200 µm, in a structure including up to 80-90% pore spaces34. Diatomite’s extremely porous structure, low density and high surface area make it suitable as an adsorbent for organic and inorganic chemicals. Diatomite is found in abundance in Iran. Several studies have been
carried out on the use of diatomite as an adsorbent for removing some contaminants such as heavy metals34, basic dye (Methylene blue)2, basic and reactive dyes (Methylene blue, reactive black, reactive yellow)4,35 and some textile dyes (Sif Blau BRF, Everzol Brill Red 3BS, Int Yellow 5GF)36. Furthermore, the unique properties of diatomite caused its applications as filtration media in a number of industries34,36. Diatomite is approximately 500 times cheaper than commercial activated carbon36 and has the potential of being successfully used as a cost-effective alternative to activated carbon.
In the present paper, the possibility of utilization of the diatomite in the original or calcined form as an adsorbent for removal of AB 25 dye from an aqueous medium has been studied. The equilibrium and kinetic study are investigated to observe the effects of various process parameters such as pH, contact time, initial dye concentration, temperature, calcinations and the sorbent dosage on the adsorption process. Equilibrium data are attempted by various adsorption isotherms including Langmuir, Freundlich and Brunauer- Emmett-Teller (BET) isotherms in order to select an appropriate isotherm model. Moreover, a kinetics study of the adsorption process is also considered to describe the rate of sorption.
Experimental Procedure
Preparation of adsorbent
Diatomite sample was obtained from Tabriz, Iran.
The sample was washed several times with distilled water to remove fines and other adhered impurities and to achieve neutralization. The sample was finally filtered, dried at 40°C and stored in closed containers for further use. The calcination process was carried out by placing diatomite sample in the furnace at 980°C for 4 h. The sample was then allowed to cool in a desiccator. The calcined sample was used to examine the effect of silanol groups and the role of pore size distribution on the adsorption process.
Reagents and solutions
AB 25 dye was obtained from Ciba Ltd. and was used without further purification. The chemical structure of this dye is shown in Fig. 1. Distilled water was throughout employed as solvent. For adsorption experiments, various concentrations of dye solutions (50, 100 and 150 mg/L) were prepared. The pH measurements were made using Hach pH meter. The pH adjustments of the solution were made by adding a
small amount of HCl or NaOH (1 M). These chemicals were of analar grade and purchased from Merck, Germany.
Adsorption procedure
The adsorption expriments were performed by mixing various amounts of diatomite (0.2 –1.1 g) in jars containing 250 mL of dye solutions with varying concentrations (ranging from 50-150 mg/L) and 5 g NaCl at various pH (2-12). The pH studies were carried out to determine the optimum pH at which maximum dye removal could be achieved with diatomite. Adsorption experiments were conducted at various concentrations of dye solutions (50, 100 and 150 mg/L) using optimum amount of diatomite (0.9 g) at pH 2, an agitation speed of 200 rpm and temperature 25 ±1°C for 5 h to attain equilibrium conditions. An FC6S-VELP (Scientifica) jar test was used for agitating purpose. The changes of absorbance were determined at certain time intervals (2, 4, 6, 8, 10, 20, 30, 60, 120, 180, 240, 300 and 1440 min) during the adsorption process. After adsorption experiments, the dye solutions were centrifuged for 12 min in a Hettich EBA20 centrifuge at 4000 rpm in order to separate the sorbent from the solution and dye concentration was then determined.
Analysis
The residual dye concentrations in aqueous medium were determined using a CECIL 2021 spectro-photometer corresponding to maximum wavelength (λmax) of AB 25 dye. The XRD analysis was performed on raw and calcined diatomite samples using a Philips Xpert x-ray diffractometer. The samples were scanned from 10º to 70º. Scanning electron microscopic (SEM) of both raw and calcined diatomite were carried out using LEO 1455VP scanning electron microscope before and after adsorption process.
Fig. 1 — Chemical structure of AB 25 dye.
Results and Discussion
Surface characterization
In order to explore the surface characteristics of diatomite, an FTIR analysis was performed in the range of 450 to 4000 cm-1. Figure 2 shows the infra- red spectra of raw and calcined diatomite samples.
In the first spectrum (‘Raw’ curve in Figure 2), the peak positions showing major adsorption bands were observed at 3846, 3736, 3615, 3590, 3564, 3444, 2929, 2867, 1638, 1087, 794, 696, 523 and 470 cm-1. The bands at 3846 and 3736 cm-1 illustrate the H atom that is attached to heteroatoms (Si-H). The peaks at 3615, 3590, 3564, 3444, 2929 and 2867 cm-1 are due to the free silanol group (Si-O-H), the band at 1638 cm-1 represents H-O-H bending vibration of water, the band at 1087 cm-1 reflects the siloxane (-Si-O-Si-) group stretching, the bands at 794 and 696 cm-1 correspond to SiO-H vibration. The peak positions of 523 and 470 cm-1 are attributed to the Si- O-Si bending vibration. The peak positions of the major bands in the spectrum of calcined diatomite (mentioned as ‘Calcined’ in Figure 2), is seen more or less at the same position as in the spectrum of raw diatomite. In addition, there is a small peak at 2364 cm-1. It can be a trace of ammonia ions because of calcinations at high temperature and existence of nitrogen in air.
Comparison of these two spectra shows that there is only slight difference between the band positions of these two adsorbents, especially at 2364 cm-1 position that it is not very important on adsorption process.
The most important difference of these spectra is in the intensity of the free silanol group. It means that the amount of this group in raw diatomite is more than the calcined one. This group is responsible for the adsorption process.
Scanning electron micrographs of raw and calcined diatomite are shown in Figs. 3 and 4 respectively. As evident from Fig. 3, raw diatomite has considerable numbers of pore spaces where dyes can be adsorbed into these relatively large pores. An important change in the surface characteristics and the size of the pore spaces of the diatomite is seen after calcination process at 980°C, as evident from Fig. 4, the thermal treatment of the diatomite reduced the volume of the pore spaces and decreased the surface functional groups from the raw diatomite (FTIR spectra).
Moreover, the solid structure of diatomite becomes more visible. As a result, the adsorption of dye by calcined diatomite is decreased.
Fig. 2 — FT-IR of raw and calcined diatomite.
Fig. 3 — Scanning electron micrographs of raw diatomite.
Aadsorbent dosage = 0.9 g, Temperature= 25 ± 1°C, equilibrium time=5 h, agitation speed = 200 rpm.
Fig. 4 — Scanning electron micrographs of calcined diatomite.
Adsorbent dosage = 0.9 g, Temperature= 25 ± 1°C, equilibrium time=5 h, agitation speed = 200 rpm.
XRD analysis results of the raw and calcined diatomite are shown in Fig. 5. It can be seen from Fig. 5 that the x-ray pattern of the raw diatomite is different from the pattern of the calcined diatomite, suggesting that a phase transformation probably occurred during the calcinations process. The main composition of raw diatomite is quartz, anorthite and muscovite. It is evident that sanidine was appeared;
while anorthite and muscovite were completely removed as the diatomite was calcined at 980°C. In fact, some peaks in the diatomite disappeared and some peaks were created by calcination process.
Similar behaviour was previously investigated by other researchers4.
The surface area of the diatomite was determined by BET method. In this investigation, the values 129.4 and 7.5 m2/g were calculated for raw and calcined diatomite respectively.
A particle size analysis was carried out to determine the distribution of particles of the
adsorbent. The maximum distribution of particles is varied from 10 to 40 µm.
Effect of adsorbent dosage
The adsorption of AB 25 dye on raw and calcined diatomite dosage was investigated at 25 ± 1°C by varying the adsorbent amount from 0.2 to 1.1 g while keeping the volume of dye solution constant equal to 250 mL, with an initial dye concentration of 50 mg/L. Figure 6 shows the percentage removal of AB 25 dye versus adsorbent amount. As it is clear from the figure, the percentage removal of dye increased with an increase in the adsorbent amount. The main reason for this fact is due to the greater availability of the adsorption sites at higher concentrations of the adsorbent36. Based on the results shown in Fig. 6, 0.9 g of the raw and calcined diatomite was used for further experiments.
Effect of initial dye concentration
A change in the initial dye concentration can considerably affect the adsorption process. Figure 7
Fig. 5 — XRD patterns of (a) raw and (b) calcined diatomite. Q: Quartz, A: Anorthite, M: Muscovite and S: Sanidine.
depicts the effect of dye concentration on the percentage removal of AB 25 dye by diatomite. As it is evident from the figure, when the dye concentration was increased from 50 to 150 mg/L, the percentage removal of AB 25 dye decreased from 72.81 to 47.3%. As expected, when the concentration of dye is increased, the limited capacity of the adsorbent checks any further adsorption of dye and hence the overall removal percentage decreases. The equilibrium controls the maximum adsorption and decreases the final removal percentage due to increasing amount of dye.
Effect of contact time
The adsorption of AB 25 dye onto diatomite was evaluated as a function of contact time. Figure 8 shows the effect of contact time on the percentage removal of AB 25 dye in the aqueous phase by raw (Fig. 8a) and calcined diatomite (Fig. 8b). The initial
dye concentration was varied from 50 to 150 mg/L.
At all initial dye concentrations investigated, the adsorption occurs very fast initially. After 2 min of adsorption process, the amount of adsorption by raw diatomite reaches to 84, 89 and 73% of the ultimate adsorption of AB 25 dye for initial dye concentrations of 50, 100 and 150 mg/L respectively.
As illustrated in Fig. 8b, the adsorption is also fast at early stage of the adsorption process for calcined diatomite. Typically about 95% of the ultimate adsorption of AB 25 dye with an initial concentration of 50 mg/L takes place within the first 2 min of contact and it almost remains constant thereafter.
It means that the most of mass transfer resistance is in bulk of fluid and high rate agitation would decrease this resistant. In addition, these results show that most of the dye molecules are adsorbed on the outside surface of the adsorbent, and transferred to the pores and internal surfaces layer. More experiments are necessary to be carried out to prove this investigation.
Fig. 6—Effect of adsorbent dosage on the percentage removal of AB 25 dye by raw and calcined diatomite. Temperature = 25 ± 1°C, initial dye concentration = 50 mg/L, pH = 2, agitation speed = 200 rpm.
Fig. 7 — Effect of initial dye concentration on adsorption of AB 25 dye by raw diatomite. Contact time = 300 min, Temperature=
25 ± 1°C, pH = 2, agitation speed = 200 rpm.
Fig. 8 — Effect of contact time on adsorption of AB 25 dye on (a) raw and (b) calcined diatomite. Temperature = 25 ± 1°C, pH = 2, agitation speed = 200 rpm, adsorbent dosage = 0.9 g.
Adsorption isotherms
The distribution of dye between the adsorbent and the dye solution at equilibrium is important in establishing the capacity of the adsorbent for dye removal from aqueous systems. The adsorption isotherms of AB 25 dye on both raw and calcined diatomite are shown in Fig. 9. It is clearly seen from Fig. 9 that the amount of adsorbed AB 25 dye on calcined diatomite was much lower than that of raw diatomite. Results of FTIR, SEM, XRD and BET show that the important functional groups, such as free silanol, and size of pores decreased and the structure of diatomite changed. The BET test illustrated that the surface area of diatomite decreased 17.3 times after calcination process. Therefore, the amount of the existing surface area is the most important difference between raw and calcined diatomite.
The experimental data obtained were evaluated by various isotherm models incorporating Langmuir, Freundlich5,13,17 and Brunauer-Emmett-Teller (BET)13,37 isotherms.
Langmuir isotherm is applicable for monolayer adsorption on a surface containing a finite number of identical adsorption sites17. A linear expression for the Langmuir isotherm is as follows:
( )
00
/ 1 / 1 1 /
1 C Q
Q
q K e
L
e +
= … (1)
where Ce is the concentration of dye under equilibrium condition (mg/L), qe denotes the amount of dye adsorbed at equilibrium (mg/g), Q0 indicates the maximum adsorption capacity and KL is the Langmuir isotherm constant (l/mg). The values of KL
and Q0 were calculated from the slope and intercept of the linear plot of 1/qe versus 1/ Ce.
Freundlich equation was also applied for the adsorption of AB 25 dye on diatomite as given below:
en F
e K C
q
1
= …(2)
where Ce is the equilibrium dye concentration in aqueous system (mg/L), qe is the amount of dye adsorbed per weight of the adsorbent used (mg/g), KF
and n are Freundlich isotherm constants incorporating all factors affecting the adsorption process.
Taking log10 from both sides of the Eq. (2) yields the following equation:
e F
e C
K n
q 10 10
10 1 log
log
log = + … (3)
Although not shown here, linear plot of log10 qe versus log10Ce gives the values of KF and n.
Brunauer-Emmett-Teller (BET) model was also used to fit the adsorption data according to the linear form of its rearranged adsorption isotherm model, which may be expressed as:
( )
−
+
− = s
e m b b m
b e e s
e
C C q K K q
K q C C
C 1 1
… (4) where Ce is the concentration of dye in solution (mg/L), Cs denotes the saturation concentration of dye (mg/L), qe is the amount of dye adsorbed per weight of the diatomite used (mg/g), qm is the amount of dye adsorbed in forming a complete monolayer (mg/g), Kb
indicates a constant explaining the energy of interaction with the surface. The values of Kb and qm
were calculated from the slope and intercept of the linear plot of
e e S
e
q C C
C 1
− versus S
e
C C .
The determined constants and the correlation coefficients of the Langmuir, Freundlich and BET isotherms are included in Table 1. The negative values of Kb related to the BET isotherm model describe that the adsorption process did not follow the BET isotherm model. It is evident from Table 1 that the isotherm data for the adsorption of AB 25 dye by raw diatomite were best-fitted using Langmuir model with a correlation coefficient of 0.9609. Furthermore, the Freundlich model is most appropriate for the adsorption of AB 25 dye on calcined diatomite with a correlation coefficient of 0.945. In addition, it is clear from Table 1 that the adsorption capacity of raw
Fig. 9 — Adsorption isotherms of AB 25 dye onto raw and calcined diatomite.
diatomite is more than calcined diatomite. The Langmuir isotherm (or equilibrium) constant (KF) and the maximum adsorption capacity (Q0) for raw diatomite are 2.2 and 2.3 times more than calcined one, respectively.
Adsorption kinetics
The prediction of the adsorption kinetics of dye from aqueous system is important in order to design a suitable treatment system. The kinetics of adsorption of AB 25 dye on diatomite may be described by the pseudo-first-order Lagergren rate equation3,13,17 and the pseudo-second-order rate expression developed by Ho and McKay38. The Lagergren equation is:
( )
K tq q
qe t e ad 303 . log 2
log − = − 1, … (5)
where qe and qt are the amounts of dye (mg/m) adsorbed at equilibrium and at time t (min) and K1,ad
is the pseudo-first-order rate constant (1/min).
The Ho and McKay equation is given below:
e e
t ad q
t q q K
t = +
2 , 2
1 … (6)
where qe and qt are the amounts of dye (mg/m) adsorbed at equilibrium and at time t (min) and K2,ad
is the rate constant of the pseudo-second-order model (g/mg min).
Linear plot of log10(qe−qt)against t gives the rate constant of K1,ad. Moreover, the value of K2,ad is obtained from the intercept of the linear plot of t/qt versus t.
Adsorption kinetics constants of the pseudo-first- order and pseudo-second-order models at pH 2, temperature 25 ± 1°C, an agitation speed of 200 rpm, an initial concentration of 50 mg/L and for a time period of 300 min are given in Table 2. As Table 2 shows, the high values of correlation coefficients of the pseudo-second-order model for both raw and calcined diatomite showed that the adsorption data conformed well to the Ho and McKay kinetics model [Eq. (6)].
Effect of pH
The pH is the most important factor affecting the adsorption process. The pH studies were conducted to determine the optimum pH at which maximum colour removal could be achieved with diatomite for AB 25 dye. The effect of pH was observed by studying the adsorption of dye over a broad pH range of 2-12. The results are shown in Fig. 10. As depicted in figure, for both raw and calcined diatomite, the amount of AB 25 dye adsorbed was maximum at pH 2. The dye removal decreased as the solution pH was increased from 2 to 4. The amount of dye sorbed increased slightly when pH was raised from 4 to 6. From pH 6 to 8, the dye removal increased again by raw diatomite and reached to 10.69 mg/g at pH 8. But, in this range of pH (6-8), the amount of dye sorbed on calcined diatomite remained constant, equal to 2.65 mg/g. The adsorption of AB 25 dye decreased from 10.69 to 9.04 mg/g with increasing pH of dye solution from 8 to 12 when raw diatomite was used as the adsorption medium, whereas, the quantity of dye sorbed increased from 2.63 to 7.08 mg/g in the pH range of 8 to 12 using calcined diatomite as adsorbent. Because the maximum dye adsorption occurred at a pH of 2 and the corresponding sorption capacities were 12.12 and 8.08 mg/g for raw and calcined diatomite respectively, so the effective pH was 2 and it was used in all adsorption experiments.
The Ho and McKay linear plots were obtained for various pH ranged from 2 to 12. The results are shown in Fig. 11 for raw diatomite. The kinetics constants of the pseudo-first-order and pseudo-second-order models at different pH of the dye solution are given in Table 3 for raw and calcined diatomite. These parameters were achieved from the Lagergren and Ho and McKay plots.
At any pH and for both raw and calcined diatomite, the high values of correlation coefficients mean that the adsorption process follows the pseudo-second-order kinetic model.
Effect of temperature
The temperature has a significant effect on the adsorption process. Increasing the temperature will change the equilibrium capacity of the adsorbent for a
Table 1 — Parameters of various isotherms for adsorption of AB 25 dye onto raw and calcined diatomite.
Langmuir Freundlich BET
Adsorbent Q0 KL R2 KF 1/n R2 Kb qm R2
Raw diatomite 21.41 0.15 0.9609 4.52 0.45 0.944 -1.71 1.32 0.8191
Calcined diatomite 9.41 0.068 0.9140 1.9 0.351 0.945 -1.17 0.88 0.881
particular adsorbate. Furthermore, an increase in temperature can raise the rate of diffusion of the dye molecules in the internal pores of the adsorbent15. In this study, the removal of AB 25 dye from aqueous solution using diatomite has been investigated at four different temperatures ranged from 25 to 55°C. The Ho and McKay linear plots are shown in Fig. 12 for different temperatures and for raw diatomite.
The kinetics parameters of adsorption at different temperatures are also given in Table 4 for raw and calcined diatomite. From Table 4, it can be observed
that at any temperature and for both raw and calcined diatomite, the adsorption process follows the pseudo- second-order kinetic model. However, the adsorption of AB 25 dye on calcined diatomite at temperature 55°C may follow the pseudo-first-order kinetics model. It is also evident from Fig. 12 that adsorption increases with the increase in temperature. At 55°C, the percentage removal of AB 25 dye from aqueous solution is 89.75% after a process time of 300 min, whereas at 25°C this value is 72.81%. Hence, increase in temperature favours the adsorption process.
Fig. 10 — Effect of pH on the adsorption of AB 25 dye by raw and calcined diatomite. Agitation speed = 200 rpm, Temperature=25 ± 1°C, equilibrium time = 300 min.
Fig. 11 — Pseudo-second-order sorption kinetics of AB 25 dye onto raw diatomite at various pH. Agitation speed = 200 rpm, initial dye concentration = 50 mg/L, Temperature=25 ± 1°C.
Table 2 — Kinetic constants for AB 25 dye adsorption by raw and calcined diatomite.
Pseudo-first-order Pseudo-second-order
Adsorbent qe (mg/g) K1,ad(1/min) R2 qe (mg/g) K2,ad(g/mg min) R2
Raw diatomite 2.952 0.00161 0.6808 10.10 0.963 0.9993
Calcined diatomite 2.473 0.0046 0.8687 7.33 0.0276 0.9947
Table 3—Kinetics constants of AB 25 dye adsorption onto raw and calcined diatomite at different pH.
Pseudo-first-order kinetic constants Pseudo-second-order kinetic constants
Adsorbent pH K1,ad qe R2 K2,ad qe R2
2 0.00161 2.952 0.6808 0.963 10.10 0.9993
4 0.0094 2.04 0.9833 0.019 9.27 0.9979
Raw 6 0.0101 3.479 0.9299 0.0165 9.625 0.9978
diatomite 8 0.0076 4.49 0.962 0.0123 10.246 0.9971
10 0.0074 3.53 0.9846 0.0138 8.97 0.9935
12 0.0094 5.98 0.9813 0.00582 9.025 0.9868
2 0.0046 2.473 0.8687 0.0276 7.33 0.9947
4 0.0035 0.722 0.3163 0.425 1.71 0.971
Calcined 6 0.0032 1.0957 0.0998 1.21 1.767 0.9582
diatomite 8 0.0081 1.033 0.4758 0.0765 2.32 0.9691
10 0.0074 1.608 0.3758 0.0456 2.75 0.9312
12 0.009 5.47 0.6427 0.0032 7.037 0.9085
Conclusions
Diatomite has been studied for removal of AB 25 dye from aqueous solution. Thermal treatment of the adsorbent at 980°C adversely affects its adsorption capacity due to the reduction of the volume of the pore spaces and removal of the surface functional groups from the raw diatomite. The adsorption process was also influenced by solution pH and temperature. The maximum sorption capacity occurred at pH 2, but there is no big difference between pH 2 (qe =12.12mg/g at 25°C) and 8 (qe =10.69mg/g at 25°C). The adsorption of AB 25 dye by diatomite increased with an increase in temperature. Equilibrium data for the adsorption of AB 25 dye by raw diatomite fit well to the Langmuir isotherm model. Furthermore, the Freundlich model is most appropriate for the adsorption of AB 25 dye on calcined diatomite. In addition, the rate of adsorption process obeys the pseudo-second-order kinetics model.
It was found that in order to obtain the highest possible removal of AB 25 dye, the experiments can be carried out at pH 2, temperature 25°C, an agitation
speed of 200 rpm, an initial dye concentration of 50 mg/L, a centrifugal rate of 4000 rpm and a process time of 300 min. Under these conditions, a maximum percentage removal of 72.81 (with a mass AB 25 dye removed/mass diatomite of 12.12 mg/g) and 54.5%
(with a mass AB 25 dye removed/mass diatomite of 8.08 mg/g) can therefore be obtained for AB 25 dye from aqueous solution using raw and calcined diatomite respectively; but the high acidic solution (pH 2) would cause a high corrosion effects and it is suggested that the pilot and industrial cases would be carried out at pH 8, temperature 25°C, an agitation speed of 200 rpm, an initial dye concentration of 50 mg/L, a centrifugal rate of 4000 rpm and a process time of 300 min by raw diatomite. In such case, the removal percentage is 64.22. The results presented here can help to design an appropriate remediation plan to minimise the unfavourable impacts caused by industrial effluents containing AB 25 dye.
Acknowledgement
The authors are thankful to Institute for Colorants, Paints & Coatings (ICPC) for providing the financial help and to Shahrood University of Technology, for supporting this research.
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Table 4—Kinetic constants of AB 25 dye adsorption onto raw and calcined diatomite at different temperatures.
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Adsorbent T(°C) K1,ad qe R2 K2,ad qe R2
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Calcined 35 0.00161 8.262 0.962 0.0092 4.892 0.9642
diatomite 45 0.0035 8.00 0.9371 0.0043 6.812 0.9449
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Fig. 12 — Pseudo-second-order sorption kinetics of AB 25 dye onto raw diatomite at different temperatures. Agitation speed = 200 rpm, initial dye concentration = 50 mg/L, pH = 2.
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