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Assessment of hydrothermally modified fly ash for the treatment of methylene blue dye in the textile industry wastewater

Suman Mor1,2 • Manchanda K. Chhavi1 • Kansal K. Sushil3 •

Khaiwal Ravindra4

Received: 20 August 2016 / Accepted: 19 December 2016 / Published online: 21 February 2017 � Springer Science+Business Media Dordrecht 2017

Abstract Dyes and pigments are one of the major water pollutants and if not discharged properly cause ecological disturbance. Considering this, the current study investigates the

application of thermal power plant by-product, i.e., fly ash for the elimination of a haz-

ardous methylene blue dye from its synthetic aqueous solution. Experiments were con-

ducted in batch mode to study the effect of pH, temperature, adsorbent dose and contact

time. Highest dye removal (94.3%) was achieved at pH 10 using adsorbent dose of 10 g/L

in 90 min of contact time at 40 �C. However, for cost-effective operation at neutral pH and room temperature (30 �C), it yields 89.3% dye removal having similar dose and contact time. Equilibrium isotherms for adsorption were analyzed by Langmuir and Freundlich,

Temkin and Dubinin–Radushkevich isotherm equations. The results revealed that the best

fit model of adsorption closely followed Langmuir adsorption. Based on adsorption

Electronic supplementary material The online version of this article (doi:10.1007/s10668-016-9902-8) contains supplementary material, which is available to authorized users.

& Suman Mor [email protected]; http://publichealth.puchd.ac.in/show-biodata.php?qstrempid=5417&qstrempdesigcode=138; http://scholar.google.co.in/citations?hl=en&user=oQTvFS4AAAAJ

Manchanda K. Chhavi [email protected]

Kansal K. Sushil [email protected]

Khaiwal Ravindra [email protected]

1 Department of Environment Studies, Panjab University, Chandigarh 160014, India

2 Centre for Public Health, Panjab University (PU), Chandigarh 160025, India

3 Dr. S.S. Bhatanagar University Institute of Chemical Engineering and Technology, Panjab University, Chandigarh 160014, India

4 School of Public Health, Postgraduate Institute of Medical Education and Research (PGIMER), Chandigarh 160012, India

123

Environ Dev Sustain (2018) 20:625–639 https://doi.org/10.1007/s10668-016-9902-8

isotherm models, thermodynamics parameters DG, DH and DS were calculated. The negative value of DG and DH revealed that adsorption process was exothermic, sponta- neous and physical. The present work suggests that through simple process hydrothermally

modified fly ash has the potential to be used as cost-effective and efficient adsorbent for the

treatment of wastewater from textile industries.

Keywords Hydrothermal activation � Waste material � Fly ash � Methylene blue � Textile industry � Radushkevich isotherm model

1 Introduction

The widespread use of synthetic dyes in various industries such as textile, leather, food

processing, dyeing, cosmetics, paper and dye manufacturing industries results in enormous

amount of wastewater, which is of serious environment concern (Banerjee et al. 2014; Hor

et al. 2016; Khodam et al. 2015). Wastewater generated from textile industries contains

high amount of suspended organic compounds, inorganic salts, azo dyes, non-azo dyes and

heavy metals such as chromium, nickel and lead and volatile and nonvolatile recalcitrant

organic compounds (Visa and Chelaru 2014). Synthetic dyes used for coloration are highly

visible, may be toxic and mutagenic and are sometimes also resistant to biological

breakdown (Doğan et al. 2007; Salleh et al. 2011). Most of the synthetic dyes reduce

sunlight penetration and photosynthetic activity, therefore imparting adverse threat to

aquatic life (Pengthamkeerati et al. 2010). Potential toxicity and high color index of dyes

are the major concerns for textile effluents, as even a trace amount of dye degrades esthetic

value (Tavlieva et al. 2013). As reviewed by Rafatullah et al. 2010 and other studies,

methylene blue is one of the most commonly used basic azo dyes in textile industry and

printing industry (Hor et al. 2016). The dye exhibits toxic effects on inhalation shortness of

breath, and if ingested it may cause burning sensation, nausea, vomiting, dizziness,

headache, discoloration of urine and methemoglobinemia (Rafatullah et al. 2010).

Therefore, these harmful effects of the dye make its removal from the effluents imperative.

There are many techniques proposed for wastewater treatment such as electrocoagu-

lation, chemical oxidation, ozonation, photo-degradation and membrane filtration (Arel-

lano-Cárdenas et al. 2013; Chowdhury et al. 2011). However, adsorption is most preferred

method due to simple and easy operation, low cost and efficient removal of target com-

pounds (Matheswaran and Karunanithi 2007; Mittal 2006; Vargas et al. 2012). Rani et al.

(2016) highlight that activated carbon has high dye removal capacity; however, relatively

high cost and regeneration limit its application in adsorption studies. Hence, most of the

studies focus on locally availability of raw material as adsorbents for the treatment of

textile industry wastewater.

Various low-cost adsorbents such as canola residues (Balarak et al. 2015), cocoa pod

husk (Pua et al. 2013), mango seed kernel powder (Kumar and Kumaran 2005), sludge ash

(Weng and Pan 2006), potato waste (Gupta et al. 2011), cashew nut shell (Ahmad and

Kumar 2010; Subramaniam and Kumar Ponnusamy 2015), natural zeolites (Hor et al.

2016), fly ash and red mud (Wang et al. 2005), fly ash (Wang and Zhu 2005) have been

used for the removal of methylene blue dye from wastewater. The use of fly ash for dye

removal has been reported in number of investigations, but due to (Balarak et al. 2015)

versatility of the fly ash, it still needs to be explored (Pengthamkeerati et al. 2008).

626 S. Mor et al.

123

Different classes of fly ash show different adsorption behaviors for several categories of

dyes (acidic, basic, azo and non-azo, direct and non-direct dyes). Further various treat-

ments like sonochemical (Wang and Zhu 2005), microwave and ultrasound (Bukhari et al.

2015), hydrothermal treatment (Abdalqader et al. 2015; Pengthamkeerati et al. 2010; Wang

et al. 2006) and chemical acid treatment of fly ash (Bai et al. 2011) change the porous

structure and surface area of the adsorbent on which adsorption capacity is highly

dependent (Lin et al. 2008).

Fly ash is a by-product obtained from combustion of coal in coal-powered thermal

power plants. It is estimated that around 160 million tons of fly ash is generated every year

in India. Disposal of fly ash has become increasing economic and environmental burdens,

which demand its effective utilization. Hence, this current study explores the application of

hydrothermally treated fly ash for the treatment of methylene blue dye using synthetic

wastewater.

2 Materials and methods

2.1 Batch sorption experiments and characterization

Fly ash was procured from a coal-fired thermal power plant situated in Panipat, Haryana,

India. It was sieved through BSS -40-mesh particle size and washed several times with

distilled water to remove dust and other impurities. After washing, fly ash was dried in

oven at 100 �C for 24 h and stored in vacuum desiccators. The activation of fly ash was done by immersing it in 3 N NaOH solution. The ratio of sodium hydroxide to fly ash was

kept 1.2 by weight. The solution was heated at 90 �C and stirred mechanically for 3–4 h. The slurry prepared was aged for 3–4 days before washing and filtering it and drying at

100 �C for 24 h to have the activated fly ash. Methylene blue dye (Fig. 1) was procured from Merck chemicals. Stock solution of dye was prepared by dissolving 1 g of dye in

1000 ml of double-distilled water as reported by (Sadaf et al. 2015). The further working

dye solutions of standard were prepared from serial dilution of the stock solution.

The X-ray diffraction of the raw fly ash and processed fly ash was analyzed on Pana-

lytical D/Max-2500 X-ray diffractometer equipped with Cu-k radiation (1.5406 Å) oper-

ating at 40 kV, 50 mA with scanning rate of 0.02 s -1

to examine the crystalline phases of

the fly ash. The chemical composition of raw and activated fly ash was analyzed using

WD-XRF (wavelength-dispersive X-ray fluorescence), Model: S8 TIGER, Make-Bruker,

Germany. Pellet was prepared using homogenized fly ash (8 gm) powder of particle size of

up to 5 micron. For spectrophotometric analysis, samples were centrifuged at 10,000 rpm

for 10 min at room temperature, and supernatant was analyzed using UV spectropho-

tometer (SHIMADZU, Germany) at 665 nm for determination of residual dye

Fig. 1 Chemical structure of the methylene blue

Assessment of hydrothermally modified fly ash… 627

123

concentration. The study was conducted using batch method and by varying pH, tem-

perature, dose, and adsorbent contact time using 250-ml flask and maintaining shaking

speed of 200 rpm. The removal efficiency was calculated using the following equation:

Percentage Removal %ð Þ ¼ Ci � CO

Ci � 100 ð1Þ

where Ci is the initial concentration of methylene blue in solution and Co is the final

concentration of methylene blue in solution.

2.2 Adsorption isotherms

The following adsorption isotherms were plotted to evaluate the adsorption process of dye

on the adsorbent.

2.2.1 Langmuir adsorption isotherm

The Langmuir adsorption isotherm model works on the principle that adsorption takes

place at specific homogenous sites within the adsorbent (Anirudhan and Ramachandran

2015). The equation can be represented as follows:

Ce

qe ¼

1

Qob þ

Ce

Qo ð2Þ

where Ce denotes the equilibrium concentration of the dye molecules in mg/L, whereas qe is the amount dye uptake in mg/g at equilibrium). Further, Qo represent maximum

adsorbent dose capacity, whereas b is known as constants related to the energy of

adsorption as mentioned by Matheswaran and Karunanithi (2007). The graphs of Ce/Qe versus Ce were plotted for best fit model of Langmuir adsorption isotherm.

2.2.2 Freundlich adsorption isotherm

This model explains adsorption process working on heterogeneous system. The equation

can be represented as follows:

q ¼ K � Cl=ne ð3Þ

In this equation, K and n are the constants where k indicates adsorbent capacity and n is

representative of degree of affinity between adsorption and adsorption density (Liu et al.

2015). The equation in the linear form can be depicted as follows (Ahmaruzzaman 2009)

Log q ¼ log K þ 1=n log Ce ð4Þ

For Freundlich isotherm, graphs of log Qe versus log Ce were plotted. The value of

1/n between 0 and 1 indicates favorable adsorption (Tan et al. 2015). The model that shows

R-square values closest to unity is selected.

2.2.3 Temkin isotherm

It is based on the assumption that heat of adsorption of the molecules decreases linearly

with coverage of adsorbent surface (Mall et al. 2005). It can be represented by the fol-

lowing linear equation.

628 S. Mor et al.

123

qe ¼ B ln k2 þ B ln Ce ð5Þ

where qe is the amount of the dye adsorbed at equilibrium (mg/g) and B is Temkin

isotherm energy constant. The isotherm constants were obtained from linear graph of qe and ln Ce.

2.2.4 Dubinin–Radushkevich isotherm model

The equation of Dubinin–Radushkevich isotherm in liner form is defined as below:

ln Cads ¼ ln Xm � be2 ð6Þ

where Cads is the dye adsorbed at equilibrium (mg/g) and Xm and b values are obtained from intercept and slope of linear plot of ln Cads versus e

2 (Mittal et al. 2009a, b). Polanyi

potential is given as follows:

e ¼ RT ln þ 1

Ce ð7Þ

Dubinin–Radushkevich constant gives an account of mean free energy of adsorption by

the following relation:

E ¼ 1

2B 1 2

ð8Þ

The mean free energy value gives an idea about the controlling mechanism that is

physical or chemical process (Chaudhary et al. 2014). The values of mean energy ranging

between 1 and 8 kJ/mol signify physisorption, and values ranging from 8 to 16 kJ/mol

indicate chemisorption process (Mittal et al. 2010a, b)

2.3 Adsorption kinetics

As detailed below, pseudo-first-order and pseudo-second-order rate models were fitted to

adsorption data for better understanding the controlling factors that govern the adsorption

and dynamics of the adsorption process.

2.3.1 Pseudo-first-order model

Pseudo-first-order kinetics equation is given below:

log qe � qtð Þ ¼ log qe � K1

2:303 t ð9Þ

where qe is the amount of dye adsorbed at equilibrium, whereas qt is the dye adsorbed at

time t. The pseudo-first-order rate constant is marked as K1 (Lin et al. 2013)

2.3.2 Pseudo-second-order model

The current work also evaluated the adsorption kinetics using the following equation:

Assessment of hydrothermally modified fly ash… 629

123

t

qt ¼

1

k2q 2 e

þ 1

qe t ð10Þ

where qe is the equilibrium adsorption amount, qt is the adsorption amount at time t, K1 and

K2 are pseudo-first-order and pseudo-second-order rate constants, respectively (Chaudhary

et al. 2013a, b)

3 Results and discussion

3.1 Characterization of the adsorbent

X-ray diffraction study was conducted to understand about the crystalline phases of fly ash.

The main crystalline phases identified in fly ash and activated fly ash are quartz and mullite

as shown in Fig. 2 and Visa and Chelaru (2014) also report similar findings. The various

peaks in Fig. 2 also confirm the presence of various minerals like hematite and magnetite.

The prominent peaks of quartz were identified at 2 theta value of 20.8776 and 26.6370, and

mullite was identified at 16.4747 and 25.905 (Banerjee et al. 2014). Interestingly, there was

no major change or shift of peaks was observed after hydrothermal modification of raw fly

ash. Elemental composition of raw and activated fly ash was studied using X-ray fluo-

rescence technique. Important elements and their percentage values in raw and activated

fly ash are listed in Table 1. The major elements in fly ash are silicon and aluminum, and

their percentage varies depending on the type of coal and combustion method used in a

thermal power plants. Table 1 suggests that activation of fly ash with base decreases the

silica content and makes the residual ash more aluminum enriched (Mor et al. 2016);

however, percentage values of other elements remain more or less similar. Scanning

electron microscopy was done to understand the internal morphology of the processed fly

ash. SEM images (Fig. 3) show several hollow and compact spheres adhered to small

particles, which is similar to (Sun et al. 2010) and Pengthamkeerati et al. (2010). Plate b in

Fig. 3 depicts magnified view of a big microsphere and smaller particles adhered to it.

Hydrothermal activation of fly ash affected the ball-shaped surfaces into rough surfaces

Fig. 2 X-ray diffraction images of raw fly ash (a) and (b) hydrothermally modified fly ash

630 S. Mor et al.

123

and agglomeration of small particles as shown in micrograph. Further SEM micrographs

also indicates change in internal surface and increase in roughness of surface area.

3.2 Effect of adsorbent dosage

The effect of various doses of activated fly ash ranging from 1 to 20 g/L was studied using

synthetic wastewater of known dye solution (5 mg/L). The maximum dye removal (89.3%)

was observed after a contact time of 90 min using adsorbent dose of 1 g/L. Similar to Mor

et al. (2016a, b), adsorbent dose and contact time were optimized on the basis of maximum

percentage removal in minimum time. Initially, the percentage removal of the dye

Table 1 Elemental composition of raw and hydrothermally mod- ified fly ash

Elements Hydrothermally modified fly ash Raw fly ash

SiO2 47.04 62.28

Al2O3 30.34 25.32

Na2O 10.62 0.10

Fe2O3 7.42 6.44

TiO2 1.90 1.52

CaO 0.94 0.69

MgO 0.085 0.62

K2O 0.33 2.02

BaO 0.09 0.08

MnO 0.09 0.08

Fig. 3 Scanning electron micrograph of raw a, b and c, d hydrothermally modified fly ash

Assessment of hydrothermally modified fly ash… 631

123

increased with increase in adsorbent dose and for particular period of contact time and

started declining after attaining equilibrium (Fig. 4). This trend is attributed to the fact that

with increase in the amount of adsorbent, the availability of active sites for the adsorption

also increases, leading to higher percentage removal of dye until equilibrium is achieved

and then decline is observed (Chaudhary et al. 2013a, b). Further decline in adsorption

capacity could be due to the formation of monolayer of the adsorbate molecules on

adsorbent surface (Mor et al. 2007) including the diffusion through pores to inner surface

of the adsorbent (Yadav et al. 2006) Similar trend was also reported for chrysoidine dye

using fly ash in batch process (Matheswaran and Karunanithi 2007) and hazelnut shell for

the removal of methylene blue by Doǧan et al. (2009).

3.3 Effect of pH

As highlighted by Kaur et al. (2016), pH is very crucial factor to understand the mechanism

of adsorption of dye on adsorbent surface. The pH influences chemistry of dye solution and

charge on the surface of the adsorbent (Chaudhary et al. 2013a, b; Doğan et al. 2007). In

the current study, pH of the synthetic wastewater was varied from pH 2 to pH 12, and it

shows that there is gradual increase in percentage removal of the dye as pH of the medium

increases (Fig. 5). This could be due to the fact that methylene blue is basic dye which

releases positively charged ions in the solution. The shift from acidic medium to basic

medium affects the charge on the surface of the adsorbent; thus, electrostatic forces of

attraction between negatively charged adsorbent and positively charged dye molecules

favor the adsorption process (Lin et al. 2008; Singh et al. 2016; Tolba et al. 2015).

3.4 Effect of temperature

To understand the application and efficiency of fly ash as an adsorbent in practical sense,

the study also evaluates the effect of temperature on dye removal. As shown in Fig. 5, dye

removal increases significantly when the temperature was raised from 20 to 40 �C having 10 �C temperature grading. However, further increase in temperature removal efficiency

Fig. 4 Effect of adsorbent dose and contact time on dye removal using hydrothermally modified fly ash as an adsorbent

632 S. Mor et al.

123

starts declining. It can be explained by the fact that with increase in temperature there may

be swelling effect in the internal structure of the adsorbent leading to greater adsorption of

large size dye molecules into the active sites of the adsorbent. The kinetic energy of the

adsorbate molecules is low at lower temperature, which may decrease the adsorption.

Further, at high temperature the mobility of dye ions increases and due to high kinetic

energy of the dye ions than attractive potential of active sites of the adsorbent, resulting in

decline in dye removal as reported by Mor et al. (2007). The maximum dye removal

(94.3%) was observed at 40 �C, but looking at field and practical consideration it is suggested to use at room temperature (30 �C), which yield 89.3% removal from the synthetic wastewater.

3.5 Adsorption isotherms

The adsorption data obtained were appraised using standard isotherms models, e.g.,

Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (Figs. 6 and 7). Both fig-

ures show that all the models in general were in good agreement with the experimental data

and showed good R 2 values. However, Langmuir model was found to be best fit adsorption

model at temperature 40 �C. The energy constant calculated from Dubinin–Radushkevich isotherm model also supports physical adsorption model. The calculated energy constant

from the graph of Dubinin–Radushkevich is 1.49 kJ/mol, which indicates physisorption as

if value of constant E lies between 1 and 8 kJ/mol (Mor et al. 2016a, b). The value of

E [ 8 kJ/mol suggests chemisorption (Mittal et al. 2010a, b). The values of correlation coefficients R

2 and constants of all the isotherm models studied are listed in Table 2.

3.6 Adsorption kinetics

The adsorption data were also analyzed using pseudo-first-order and pseudo-second-order

kinetic model to understand the mechanism of adsorption. As shown in Fig. 8, pseudo-

2 4 6 8 10 12 65

70

75

80

85

90

95

20 3 0 40 50

80

82

84

86

88

90

92

94

96

P er

ce nt

ag e

R em

ov al

(% )

pH

P er

ce nt

ag e

R em

ov al

(% )

Temp.(Degree Celcius)

Fig. 5 Effect of pH (left) and temperature (right) on dye removal using hydrothermally modified fly ash as an adsorbent

Assessment of hydrothermally modified fly ash… 633

123

second-order kinetic model shows the best fit to the experimental data. The value of

various constants of pseudo-first order and pseudo-second order is also listed in Table 2.

3.7 Adsorption thermodynamics

Using the data results of the adsorption kinetics, thermodynamic analysis was conducted to

understand the adsorption mechanism. The value of various thermodynamic coefficients,

i.e., DG (free energy), DS (measure of entropy) and DH (enthalpy), was also estimated using the following equations as defined in detail by Mor et al. (2016a, b)

-0.345

-0.340

-0.335

-0.330

-0.325

-0.320

-0.7 -0.6 -0.5 -0.4 -0.3 0.3 0.4 0.5

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Log Ce

L og

q e C e/

Q e

Ce

Fig. 6 Freundlich (left) and Langmuir isotherm (right) curve for the removal of dye using hydrothermally modified fly ash as an adsorbent

-1.4 -1.2 -1.0 -0.8 0.450

0.455

0.460

0.465

0.470

0.475

0.480

0.49 0.56 0.63 0.70 -0.80

-0.79

-0.78

-0.77

-0.76

-0.75

-0.74

Q e

ln Ce

ln Q

e

Log(1+1/Ce)

Fig. 7 Temkin (left) and Dubinin–Radushkevich (right) isotherm model for the removal of dye using hydrothermally modified fly ash as an adsorbent

634 S. Mor et al.

123

DG ¼ �RT ln bð Þ ð11Þ

DG � ¼ DH

� � TDS ð12Þ

Table 2 Adsorption isotherms and kinetics models for the adsorption of methylene blue

Equations Parameters Hydrothermally modified fly ash

Langmuir Qmax (mg/g) 0.045

B (l/mg) 49.61

R 2

0.9999

Freundlich K (mg/g) 0.43196

1/n 0.0713

R 2

0.97518

Temkin B (J/mol) 0.03392

KT 4.455

R 2

0.98055

Dubinin–Radushkevich qDR 0.900

E (KJ/mol) 1.49

R 2

0.9658

Pseudo-first order Qe (mg/g) 1.235

Kt 0.016

R 2

0.868

Pseudo-second order K2 (g/mg) 0.850

Qe (mg/g) 0.482

R 2

0.998

-2.2

-2.1

-2.0

-1.9

-1.8

-1.7

-1.6

60 90 120 150 60 90 120 150 120

140

160

180

200

220

240

260

Lo g(

q e -q

t)

Time(min)

t/q t

Time(min)

Fig. 8 Pseudo-first-order (left) and pseudo-second-order (right) kinetics for the removal of dye using hydrothermally modified fly ash as an adsorbent

Assessment of hydrothermally modified fly ash… 635

123

The value of thermodynamic constants is listed in Table 3, whereas Fig. 9 shows the

trend of DG versus temperature. The negative value of DG indicates that adsorption process is spontaneous and favorable, whereas negative value of DH shows that process is exothermic in nature. Further, the positive value of DS suggests increase in randomness with increase in temperature (Mittal et al. 2009a, b).

4 Conclusions

The present study assesses the feasibility of hydrothermally processed fly ash for the

removal of hazardous methylene blue dye from its synthetic aqueous solution of textile

industry. Scanning electron micrographs revealed change in surface of the adsorbent which

enhanced its adsorption capacity for the removal of cationic dye. Dye removal efficiency

increased with increase in adsorbent dose and contact time and pH of the synthetic

wastewater. Various isotherm models including Temkin and Dubinin–Radushkevich were

fitted at different temperatures, and it was noticed that Langmuir isotherm best fitted to the

experimental data, which depicts monolayer physical adsorption process. Maximum per-

centage removal of (94.3%) was achieved in 90-min time duration by using adsorbent dose

of 10 g/L at pH 10. The negative value of DG indicates that adsorption is spontaneous, whereas negative value of DH depicts exothermic adsorption. Hence, it could be suggested that hydrothermally modified fly ash, a thermal industry by-product, offers easy and

economical option for the treatment of textile industry wastewater.

Table 3 Thermodynamic parameters for the removal of dye using hydrothermally modified fly ash as an adsorbent

Temperature (K) DG� (kJ/mol) DH� (kJ/mol) DS� (kJ/mol)

303 -438,271.66 -6.0314 18,594.8

313 -129,105.68

323 -66,372

300 305 310 315 320 325

-400000

-300000

-200000

-100000

G (k

J/ m

ol )

Temp.(k)

Fig. 9 Thermodynamic plot for the removal of dye using hydrothermally modified fly ash at different temperature

636 S. Mor et al.

123

Acknowledgements The author would like to thank the DSTPURSE grant and Department of Health Research (DHR), Indian Council of Medical Research (ICMR), Ministry of Health and Family Welfare, for providing the Fellowship Training Programme in Environmental Health under Human Resource Develop- ment Health Research Scheme. The author is also thankful to CIL laboratory, Panjab University Chandigarh for their help in instrumentation.

Compliance with ethical standards

Conflict of interest Authors declare that they have no conflict of interest.

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