Fabrication of Inexpensive Polyethylenimine-Functionalized Fly Ash

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Fabrication of Inexpensive PolyethyleneimineFunctionalized Fly Ash for Highly Enhanced Adsorption of both Cationic and Anionic Toxic Dyes from Water Subhajit Dash, Haribandhu Chaudhuri, Udayabhanu G. Nair, and Ashis Sarkar Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00900 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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Graphic for manuscript

Design and fabrication of branched polyamine functionalized fly ash towards effective removal of toxic dye stuffs from aqueous solution.

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Fabrication of Inexpensive Polyethyleneimine-Functionalized Fly Ash for Highly Enhanced Adsorption of both Cationic and Anionic Toxic Dyes from Water Subhajit Dash, Haribandhu Chaudhuri, G Udayabhanu, Ashis Sarkar* Organic Materials Research Laboratory, Department of Applied Chemistry, Indian School of Mines, Dhanbad, Jharkhand-826004, India. Subhajit Dash, E-mail: [email protected] Haribandhu Chaudhuri, E-mail: [email protected] G Udayabhanu, E-mail: [email protected] *Corresponding author: Ashis Sarkar, E-mail: [email protected]; Tel. +91 9430335255 Fax: +91 326-2307772 ABSTRACT: Polyethyleneimine based modified coal fly ash (CFA-PEI) is synthesized by wet impregnation method. The material was found to be an excellent adsorbent for removal of cationic (Malachite green; MG) and anionic (Reactive red 2; RR2) dyes from aqueous solution. CFA-PEI was characterized by various sophisticated instruments such as FTIR spectroscopy,

13

C MAS

NMR spectroscopy, XRD, Elemental analysis, Brunauer-Emmett-Teller (BET) isotherm, TGA, FESEM and HRTEM. The FTIR spectra reveals that N-containing functional group is introduced on the surface of CFA-PEI which plays a vital role in the removal of these dyes. The adsorption behavior of the CFA-PEI toward toxic dyes is evaluated by investigating the effect of solution pH, contact time, solution temperature, adsorbent dose, and concentration of dye solution. The obtained adsorption isotherm and the kinetics data of both MG and RR2 are in compliance with Langmuir adsorption isotherm and pseudo-second-order kinetics respectively. Moreover, excellent adsorption capacity on CFA-PEI of dyes (𝑞𝑚 : 174.83 mg/g for MG and 316.75 mg/g for RR2) is 1 ACS Paragon Plus Environment

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attributed to the various factors (H-bonding, electrostatic interaction). CFA-PEI also has high regeneration capacity towards these dyes from aqueous solution. Finally, the advantage of the adsorbent lies in its low cost. KEYWORDS: Fly ash; Polyethyleneimine; adsorption; kinetics; isotherm; thermodynamics INTRODUCTION: Pollution of water bodies due to discharge of effluents from different industries is of major concern worldwide. Tanneries, refineries, textile manufacturing units, chemical manufacturing plants are the major culprits in this respect. Apart from these, one of the major contributors to such water pollution is industries directly dealing with dye stuff. Units involved in paper manufacturing, cloth dyeing, treatment of leather and printing directly discharge effluents containing dye stuff.1 These dyes are either nonbiodegradable or give rise degradation products which are toxic. Hence, removal of these dyes from effluents is of utmost importance from the point of view of environmental pollution.2 The decontamination of toxic dyes via reliable and environmental friendly techniques has attracted huge interest recently. Now a day, amongst various treatment procedures of wastewater containing due stuff, the adsorption process has become one of the most useful technique.3, 4 Various adsorbents like silica,2 clay,5 activated carbon,6 mesoporous silica7 have been used for such treatment. Coal fly ash (CFA) a coal combustion by product (CCB) appears to be a useful material as adsorbent for treatment of wastewater. CFA is mainly generated by thermal power plants to the tune of 750 million tons world over.8 The generation is likely to increase manifold over the next few decades as the demand for power increases. However, the current utilization of CFA is approximately 25% of the generated amount.9 Although there are various utilization channels of CFA like landfill,10 cement production,11 2 ACS Paragon Plus Environment

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construction of roads,11 brick manufacture,11 adsorbent,12 catalysis,13 synthesis of zeolites and MCM 41- a mesoporous silica,14, 13 new channels for utilization have to be explored. Recently, Polyethyleneimine (PEI) has attracted tremendous attention as absorbent as it has a high amine density and amine groups are at the end of the chain. Many researchers have reported15, 16 PEI/silica composite materials which have a high capacity for CO2 adsorption. In the present work we report a novel utilization of CFA in which CFA functionalized with PEI has been used as an adsorbent for two toxic dyes namely Reactive Red 2 (RR2, anionic) and Malachite Green (MG, cationic). MG can cause various environmental problems along with reduction in sunlight penetration. RR2 can also prevent reoxygenation in receiving waters by cutting off sunlight penetration. Generally, it is used for knot dyeing cotton, viscose, wool, silk etc. Hence, in this experiment RR2 and MG are selected as target pollutants.17-20 Fine CFA (average porosity 0.016 g/cc) collected from a super thermal power plant was first converted to nanosized particles with increased surface silanol groups and subsequently functionalized with PEI (CFA-PEI). The CFA-PEI material, pre and post adsorption process, was thoroughly characterized using various instrumental techniques such as fourier transform infrared spectra (FTIR), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and surface area porosity analyzer. Such characterization revealed that there is not much change in various properties after adsorption of the dyes. Further, the regeneration capacity of the adsorbent is also quite good. EXPERIMENTAL SECTION: Chemicals: Coal fly ash (CFA) was collected from a super thermal power plant (Rihand, Uttar Pradesh, India). Sodium hydroxide (NaOH, Merck, India), hydrochloric acid (HCl, Merck, India), 3 ACS Paragon Plus Environment

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Polyethyleneimine (Acros) were of analytical grade. The selected dyes MG (Loba Chemie, India, λmax, 615 nm) and RR2 (Loba Chemie, India, λmax, 538 nm) were used as adsorbate. All other chemicals used were of analytical grade. For all experiments, all the solutions were prepared using double distilled water. The chemical structures of two dyes are presented in Fig. S1 (See Supporting Information, SI). Synthesis of surface modified fly ash: Surface modification of fly ash was carried out in several steps. At first, the fine fly ash received was separated into two components, iron enriched and iron depleted, by using a Davies tube. The iron enriched component which is enriched in Fe-bearing crystallites21 was rejected. This is because such Fe-bearing particles when heated with alkali give rise to ferrous and ferric hydroxides. The presence of these hydroxides of iron is undesirable in this experiment. Iron depleted part of fly ash was surface modified in multiple steps. First of all it was calcined with sodium hydroxide (1:2 parts by weight) in the temperature range of 550-6000C for 3 hours. After calcination the mixture was cooled to ambient temperature (25-300C) and washed with distilled water several times to free the adhered alkali. The residue thus obtained was mixed with 100 ml of conc. HCl and refluxed for 12 h. The mixture was filtered and the residue washed several times with distilled water till free from acid. It was washed with acetone and dried in hot air oven for 24 h. The heat and alkali treated fly ash was denoted as HATF. The chemical compositions of nonmagnetic fly ash before and after HCl treatment are presented in Table 1. [Insert Table 1] Synthesis of Polyethyleneimine-functionalized coal fly ash (CFA-PEI):

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CFA was functionalized with PEI by the wet impregnation method. Typically, 1 g of PEI was dissolved in 6 ml of methanol. After 30 min of stirring, 1.5 g of HATF was added to the solution which was stirred for 1 h at room temperature.15Then the mixture was dried at 373K for 24 h. Characterization: The functionalized material was characterized using XRD, FTIR, FESEM, HRTEM,

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C NMR,

BET surface area, TGA/DTA, zeta potential and CHN analyzer. The instrumental details are given in SI. Batch adsorption measurements: The adsorption study was carried out by using an orbital shaker (Rivotek, Kolkata, India).2 The required dyes (RR2, MG) were dried in vacuum oven at 1050C overnight to remove moisture.7 In this experiment, we examined the effect of varying pH, temperature, adsorbate dose, time and dye concentration.2 The initial pH values of all the solution was adjusted using 0.1 M NaOH and 0.1 M HCl solutions. The adsorbent was separated through centrifugation (Model- R24, Remi) at 2500 rpm and concentration of the dye solution was measured by UV-VIS spectrophotometer (Shimadzu Japan, UV 1800). The percent dye uptake was calculated by equation 1.22 % 𝐴𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 =

(C0 − Ce ) ⨯ 100 Ce

(1)

The dye adsorption at equilibrium condition was calculated by equation 2.7, 22 𝑞𝑒 = (𝐶0 − 𝐶𝑒 ) ×

𝑉 𝑊

(2)

Where 𝑞𝑒 is the capacity of dye (mg g-1) adsorption at equilibrium of CFA-PEI. 𝐶0 ,𝐶𝑒 , V and W are the initial, equilibrium dye concentration (mg L-1) in solution, volume of dye solution (L) and the weight of the adsorbent (g) taken respectively. The arithmetic mean of three readings has been reported here.22


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Desorption measurements: For desorption experiment, first of all dye loaded material at the optimized adsorption condition was separated by centrifugation and dried completely. Two different dye solution at pH 3 and pH 8 were taken to estimate the desorption efficiency on CFA-PEI. Desorption study was repeated three times and percentage regeneration capacity was calculated by the following equation 3.23 𝐷𝑒𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 =

𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑒𝑠𝑜𝑟𝑏𝑒𝑑 (𝑚𝑔/ 𝐿) ⨯ 100 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑 (𝑚𝑔/𝐿)

(3)

[Insert scheme 1] RESULTS AND DISCUSSION: The heat and alkali treated fly ash (HATF) prepared, was found to be superior than the coal fly ash (CFA) used. The surface area of HATF increased to 102.89 m2/g from a meagre 66.78 m2/g for CFA (iron depleted). Total pore volume and average pore size also improved significantly (Table 2). A perusal of Fig. 1 reveals that compared to CFA the particles of HATF are much smaller. The spherical nature of CFA has been lost on conversion to HATF (Fig. 1b). TEM of HATF (Fig. 1f) clearly indicates the presence of nanopores on the surface of HATF. FTIR spectra of HATF (Fig. S2) shows the presence of the following: (i) A broad band at 3433 cm-1 (Si-OH. Str.). (ii) A sharp and intense band at 1647 cm-1 (Si- OH. bending vib.). (iii) A sharp, intense band at 1097 cm-1 (SiO-Si str. vib.). (iv) A weak band at 717 cm-1 (sym. Si-O-Si str. vib.). [Insert Table 2] [Insert Fig. 1] A comparison of the FTIR spectra of HATF (Fig. 2b) and that of CFA clearly reveals that the silanol density has improved in case of HATF. This conclusion is made on the basis of the increased intensities of the bands at 3465 cm-1 and 1640 cm-1 (Si-OH. OH str., Si-OH. OH 6 ACS Paragon Plus Environment

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bending). The increased silanol density makes HATF a better potential candidate for functionalization using PEI as compared to CFA. [Insert Fig. 2] Phase analysis using XRD does not show development of any new phase in the chemically activated ash (Fig. 2c). Prolonged treatment of fly ash often leads to the formation of zeolites.24 However, in this case there is no evidence of formation of zeolites. The intensities of the peaks due to various phases have decreased. Thus the peaks due to quartz, mullite and other phases are very strong in the non-magnetic component of fly ash (Fig. 2c). The intensities of these peaks have decreased in HATF. This has happened most probably because of dissolution of the amorphous silica and alumina phases during fusion with sodium hydroxide. [Insert Fig. 2] The characteristics of CFA-PEI material is completely different from that of HATF. Various observations about CFA-PEI reveals that a homogeneous combination of PEI and HATF has taken place. The impregnation of PEI onto the fly ash was identified by elemental analysis, which demonstrated the higher percentage of N (Table S1, SI). The surface properties of HATF, CFA-PEI were characterized by nitrogen adsorption-desorption measurement (Fig. 2a). The nitrogen adsorption-desorption isotherm of HATF gives a BET specific surface area of 102.89 m2/g for HATF, which suggests the formation of small particles (HRTEM observation). Moreover, a perusal of pore size distribution reveals that the particles have pore radius mostly in the range of 25-55 nm. Some particles have pore radius below 25 nm and a few have pore radius greater than 50 nm (inset in Fig. 2a). After impregnation of PEI, the surface area decreased to 57.47 m2/g due to significant pore blockage by PEI on HATF. In addition, there are sharp decrease in surface area of MG/CFA-PEI and RR2/CFA-PEI. So, we can conclude that


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after dye adsorption surface area is decreased, although there is no linear relationship between adsorption and surface area (Table 2). [Insert Fig. 2] Additionally, FTIR spectroscopic analysis gives a strong evidence not only for PEI impregnation on fly ash, but also adsorption of the dyes (Fig. 2b). FTIR spectra of HATF gives a broad band at 3465 cm-1 for silanol groups, asymmetric Si-O-Si stretching at 1085 cm-1, symmetric Si-O-Si stretching at 792 cm-1, bending mode of -OH at 1640 cm-1. After PEI functionalization, we found the two peaks of -CH2 group at 2954 cm-1 and 2842 cm-1 for asymmetric and symmetric stretching respectively. Besides this, the appearance of the two new peaks at 1572 cm-1 (N-H bending vibration) and 1475 cm-1 (C-H stretching vibration) give strong evidence of CFA-PEI.25 The FTIR spectra of CFA-PEI with MG and RR2 explain the probable interaction between the adsorbate and adsorbent molecules. After dye adsorption, the peaks were shifted for silanol groups, but C-H stretching, bending vibration and C-N stretching vibration are not observed. This can be explained the significant H-bonding interaction between the organic functional groups of dyes and the silanol groups as well as -NH2 groups of CFA-PEI. [Insert Fig. 1] The XRD patterns were taken before and after adsorption in (Fig. 2c) indicates that there is no appreciable change in the spectra and no other impurity peaks were detected. This reveals that MG and RR2 dye adsorbed CFA-PEI did not change the chemical structure of the adsorbent i.e., adsorption is physical in nature.3 The TGA analysis (Fig. 2d) for both HATF and CFA-PEI gives some useful information. For HATF, a band lies in between 1100C to 1900C which is due to loss of the H2O molecule. TGA results of CFA-PEI have two parts: first one occurs between 1050C to 1850C due to loss of moisture


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present in CFA-PEI. The second one occurs in between 4000C to 6930C due to decomposition of PEI in two step.25 The solid state 13C NMR of CFA-PEI shows (Fig. S3, SI) the characteristic peak for -CH2 groups of PEI at 39.7 ppm and 50.179 ppm. The intensity of these peaks is higher due to the presence of many -CH2 groups. The additional peak at 164.4 ppm is due to presence of CO2, which is not present in CFA-PEI. This is because PEI functionalized materials are good CO2 absorbent.15 Morphological analysis of CFA-PEI was carried out. FESEM analysis of CFA-PEI clearly reveals that these are having nano rod like structure (Fig. 1c). HRTEM analysis indicates that CFA-PEI are agglomerated particles (Fig. 1g). Adsorption of the dyes onto CFA-PEI is indicated by dark patches on the CFA-PEI surfaces (Fig. 1h & 1i). [Insert Fig. 1] Adsorption characteristics: Toxic dye removal from aqueous solution depends on various factors like pH of the solution, temperature, adsorbent dose, initial dye concentration, adsorption equilibrium time etc. To understand the effect of pH, the zero point charge (zpc) of the CFA-PEI has been determined using zeta potential measurement in the pH range 2-9 (Fig. 3a). It is known that adsorption of cation predominates at pH < zpc, whereas for anion the adsorption predominates at pH > zpc. The isoelectric point of CFA-PEI was found to be 4.39. This indicates that CFA-PEI carries a positive surface charge. In RR2, negative charge predominates because of the presence of anionic sulphonated groups in it. On the other hand, MG dye molecules have only positive charges because of the cationic -NH2 groups. Hence, in lower pH, the increased surface excess of H+ ions on the adsorbent indicates competition of H+ with cationic dye (MG) molecules, which reduces the maximum adsorption of MG onto CFA-PEI. Additionally, at higher pH, the concentration of OH-


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ions increases on the adsorbent implying competition of OH- with the anionic dye (RR2) molecules, which reduces the maximum adsorption of RR2 onto CFA-PEI. [Insert Fig. 3] Effect of pH: Fig. 3b reveals that for both MG and RR2, the maximum percentage of adsorption was observed at pH 8 and pH 3 respectively. For MG, the % adsorption is decreased with decrements in pH, while for RR2, with a decrease in pH, the dye uptake capacity is increased. At lower pH, there is a high electrostatic attraction between protonated -NH2 group and -SO3H groups of RR2 but at higher pH, electrostatic repulsive forces are enhanced among the ionized groups resulting in the decrement of dye adsorption. Apart from this, increased surface area is another factor for adsorption of dye from aqueous solution. [Insert Fig. 3] The effect of adsorbent dose, dye concentration, contact time and temperature were explained in supplementary information (SI, Fig. S4). Adsorption isotherms: Adsorption isotherm is a mathematical tool that is used to understand the interactive behavior between the adsorbent and adsorbate molecules in solid and liquid phase.2 Fig. 4a, 4b and S5, SI shows that the equilibrium isotherm for dye uptake tendency onto CFA-PEI and the experimental data were analyzed using Langmuir,26 Freundlich27 and Temkin

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isotherm models. From the

obtained data, it was found that Langmuir isotherm (Fig. 4a and 4b) shows the best fit in comparison with Freundlich (Fig. S5a and S5c, SI) and Temkin isotherms (Fig. S5b and S5d, SI). It indicates that monolayer adsorption takes place on the homogeneous surface of the adsorbent with some binding sites of adsorbate. The mathematical expression of Langmuir26 adsorption isotherm is:


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𝑐𝑒 1 𝑐𝑒 = + 𝑞𝑒 𝑞𝑚𝑎𝑥 𝑏 𝑞𝑚

(4)

Where 𝑐𝑒 (mg L-1) is the equilibrium concentration of the adsorbate and 𝑞𝑒 (mg g-1) is the amount of dye adsorbed at equilibrium. 𝑞𝑚 (mg g-1) is the maximum adsorption capacity and Langmuir constant b (L mg

-1

) indicates the free energy and affinity of adsorption.26,

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The important

characteristics of the Langmuir adsorption isotherm model can be explained in terms of separation factor (RL) which shows the feasibility of the adsorption process. The mathematical expression of RL is defined by following equation: 𝑅𝐿 =

1 1 + 𝑏𝐶0

(5)

Where b is the Langmuir adsorption constant and 𝐶0 is the initial dye concentration (mg L-1). Ho and McKay30 demonstrated that (i) if 𝑅𝐿 > 1, then isotherm is unfavorable; (ii) if 𝑅𝐿 = 1, then the isotherm is linear; (iii) if 0 < 𝑅𝐿 < 1, then the isotherm is favorable; (iv) if 𝑅𝐿 = 0, then the isotherm is irreversible. For MG, at 308 K: 𝐶0 = 100 mg L-1, 𝑅𝐿 = 0.0373 and for RR2, at 313 K: 𝐶0 = 100 mg L-1, 𝑅𝐿 = 0.0182 which supports the view that Langmuir adsorption isotherm is favorable. It can also be observed that the Langmuir adsorption isotherm model gives a higher correlation coefficient (R2) and lower χ2 value. The Langmuir constant ‘b’ is directly proportional to temperature, which suggests a stronger attraction between the surface active groups of adsorbent and adsorbate, and also indicates that, at higher temperature the attraction between adsorbate and adsorbent is higher than at lower temperature. From Langmuir adsorption isotherm model, maximum adsorption capacity (𝑞𝑚 ) was found to be 316.75 mg g-1 for RR2 at 313 K and 174.83 mg g

-1

for MG at 308 K. The results prove that CFA-PEI is highly effective for anionic dye

removal in comparison to the cationic dye from aqueous solution. Adsorption kinetics: 11 ACS Paragon Plus Environment

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Adsorption is a physicochemical process which associates the exchange of mass transfer of solutes between the liquid phase and surface of the solid phase.3 This exchange process of the adsorption process was demonstrated by using various adsorption kinetic models like pseudo-first- order (see SI),31 pseudo-second order,32 second-order (see SI)33 and intra particle diffusion model34 Fig. 4c and 4d exhibits that the pseudo-second-order adsorption model has higher R2 and lower χ2 value compared with other models (Fig. S6 and Table S3, SI). This observation indicates that the adsorption kinetics depend on the amount of solute adsorbed on the surface of the adsorbent at equilibrium. In intraparticle diffusion, the plots of 𝑞𝑡 vs 𝑡1/2 have two parts (Fig. 4e and 4f). The first part indicates the boundary layer diffusion due to the mass transfer between the solid phase and liquid phase and the second part shows a continuous adsorption step which indicates the intraparticle diffusion between the dye molecules and the adsorbents.35 In addition, if the plot of 𝑞𝑡 vs 𝑡1/2 is linear and passes through the origin, then intraparticle diffusion takes place in the rate determining step36 and mass transfer from the surface of the adsorbent may also occur in the rate determining step due to the large intercepts of the linear plots.3 So, the whole adsorption process may be controlled by intraparticle diffusion and external mass transfer.3 Thus, it can be conceived that the adsorption process onto the surface along with the intraparticle diffusion occur simultaneously. Adsorption thermodynamics: Van’t Hoff equation suggests the spontaneity of the dye adsorption process on the CFA-PEI surface and gives the value of various adsorption parameters. ∆𝑆 0 ∆𝐻 0 𝑙𝑛 𝑏 = − 𝑅 𝑅𝑇

(6)

∆𝐺 0 = ∆𝐻 0 − 𝑇∆𝑆 0

(7)


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Where ∆𝐺 0 is the change in Gibbs free energy (J. mol-1), ∆𝐻 0 is change in enthalpy (J. mol‑1), ∆𝑆 0 is the change in entropy (J. mol‑1 K‑1), R is the universal gas constant (8.314 J. K‑1 mol‑1) and b is Langmuir constant at temperature T (K). The values of ∆𝐻 0and ∆𝑆 0 were calculated from the slope and intercept of the plot of ln 𝑏 vs 1⁄𝑇 (Fig. 5) and (Table S6 and S7, SI) gives the values of the parameters. The negative values of ∆𝐺 𝑂 confirms the spontaneity of the reaction. It also shows that ∆𝐺 𝑂 values gradually decreases with the increase of temperature. The positive value of ∆𝐻 0 indicates the endothermic nature of the reaction37 and the positive value of ∆𝑆 0 suggests that during adsorption of RR2 and MG dye on the surface of the active sites there is increase in randomness at the solid solution interface.38, 39 [Insert Fig. 5] Desorption study: The regeneration capacity of an adsorbent plays a key role in its potential application. A useful adsorbent must have good adsorption as well as good regeneration capacity to make the adsorbent economically viable. The % regeneration capacity is the maximum at pH 8 and pH 3 for MG and RR2 and minimum at pH 2 and pH 9 for MG and RR2 respectively. This trend is opposite to adsorption phenomenon. This result also indicates that the dye adsorption capacity may follow ion exchange mechanism. 40, 41 The regeneration capacity of an adsorbent was studied by three successive adsorption-desorption cycles (Fig. S7; Table S4 and S5, SI) which indicates that CFA-PEI showed the excellent regenerating ability for the treatment of both MG and RR2 dyes from aqueous solution. Adsorption mechanism: It is known that the dye adsorption efficiency is influenced by various factors such as the structure of the adsorbate, functional behavior and interaction between the adsorbate and the surface of the adsorbent. It is presumed that adsorption of both MG and RR2 by -NH2 group containing CFA/PEI


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took place through the formation of hydrogen bonding between the lone pair of nitrogen, sulphur, and oxygen atom, Π electron cloud of benzene ring of those dyes. Beside this, in an acidic environment, there is existence of electrostatic interaction between the positively charged surface of CFA-PEI and negatively charged groups RR2 dye molecules, which results in higher adsorption efficiency for RR2. Moreover, in basic environment, electrostatic repulsion exists between positively charged surface of CFA-PEI and positively charged groups of MG and RR2 dye molecules, leading to diminished adsorption capacity with respect to RR2. In addition, the adsorption mechanism can be explained by dye diffusion, intraparticle diffusion and surface adsorption.25 It has been observed that initially rapid dye removal takes place and after that it slowly reaches the equilibrium. The hydrophilic nature of the CFA-PEI is the key factor for fast adsorption process due to the mass transport from the surface of the adsorbent to adsorbate2. Beside this, the polymeric chain of CFA-PEI promotes the diffusion process due to the hydrophilic nature of -NH2 groups. Thus, intraparticle diffusion process may also take place in the adsorption process for both cationic and anionic dyes and may influence the adsorption mechanism. CONCLUSION: Thus, it can be concluded that PEI was successfully impregnated on HATF having surface area 57.47 m2/g. The resulting CFA-PEI was applied as an effective adsorbent for removal of both cationic and anionic dyes from aqueous solution due to the multiple adsorption interaction mechanism (for e.g., H-bonding, Π-Π interaction and electrostatic interaction) between the adsorbent and dyes. The adsorption capacity of CFA-PEI is appreciably higher compared to the various reported adsorbents (Table S9, SI). Thus, CFA-PEI can act as a complete, versatile adsorbent for removal of both cationic and anionic dyes and opens up a new vista for researchers working in this area. 14 ACS Paragon Plus Environment

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ACKNOWLEDGEMENT: SD earnestly acknowledges the Director, Indian School of Mines, Dhanbad for providing research fellowship. The authors would like to thank CRF at Indian School of Mines, Dhanbad for providing FE-SEM facility. Additionally, SAIF, IISC, Bangalore, India as well as SAIF, Punjab University, Chandigarh, India are acknowledged for the help in analysis purpose. REFERENCES: 1. Parker, L. H.; Hunt, J. A.; Budarin, L.V.; Shuttleworth, S. P.; Miller, L.K.; Clark, H. J. The Importance of Being Porous: Polysaccharide-Derived Mesoporous Materials for Use in Dye Adsorption. RSC Adv. 2012, 2, 8992-8997. 2. Ghorai, S.; Sarkar, A.; Roufi, M.; Panda, B. A.; Schönherr, H. Enhanced Removal of Methylene Blue and Methyl Violet Dyes from Aqueous Solution Using a Nanocomposite of Hydrolyzed Polyacrylamide Grafted Xanthan Gum and Incorporated Nanosilica. ACS Appl. Mater. Interfaces 2014, 6, 4766- 4777. 3. Ma, J.; Yu, F.; Zhou, L.; Jin, L.; Yang, M.; Luan, J.; Tang, Y.; Fan, H.; Yuan, Z.; Chen, J. Enhanced Adsorptive Removal of Methyl Orange and Methylene Blue from Aqueous Solution by Alkali-Activated Multiwalled Carbon Nanotubes. ACS Appl. Mater. Interfaces 2012, 4, 5749−5760. 4. Gupta, V. K.; Ali, I. Removal of Endosulfan and Methoxychlor from Water on Carbon Slurry. Environ. Sci. Technol. 2008, 42, 766−770. 5. Ozcan, A.S.; Ozcan, A. Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite. J. Colloid Interface Sci. 2004, 276, 39–46.


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6. Walker, G.M.; Weatherley, L.R. Kinetics of acid dye adsorption on GAC. Water Res. 1999, 33, 1895–1899. 7. Chaudhuri, H.; Dash, S.; Sarkar, A. Synthesis and Use of SBA-15 Adsorbent for Dye-loaded Wastewater Treatment. J. Environ. Chem. Eng. 2015, 3, 2866–2874. 8. Ciccu, R.; Ghiani, M.; Muntoni, A.; Serci, A. The Italian approach to the problem of fly ash. In: Proceedings of the international ash utilization symposium, Lexington, Kentucky, USA. 1999, 18– 20. 9. Ahmaruzzaman, M. Role of Fly Ash in the Removal of Organic Pollutants from Wastewater. Energ. Fuel 2009, 23, 1494-1511. 10. Ahmaruzzaman, M. A review on the utilization fly ash, Prog. Energy Combust. Sci. 2010, 36, 327-363. 11. Bhattacharya, U.; Kandpal, T.C. Potential of fly ash utilization of fly ash. Energy. 2002, 27, 151-166. 12. Kelesog˘lu, S,; Kes, M,; Sutcu, L,; Polat, H, Adsorption of Methylene Blue from Aqueous Solution on High Lime Fly Ash: Kinetic, Equilibrium, and Thermodynamic Studies. J. Disper. Sci. 2012, 33, 15-23.

13. Banichul, H.; Sarkar, A.; Mesoporous Material from Fly Ash as a Catalyst for Stereoselective Reduction of Substituted Cyclohexanones. Energ. Source. 2013, 35, 352-63. 14. Querol, X.; Monreno. N.; Umana, J.C.; Alastuey, A.; Hernandez, E.; Lopez- Soler, A.; Plana, F. Synthesis of zeolites from coal fly ash: an overview. Int. J. Coal Geol. 2002, 50, 413-423. 15. Gargiulo, N.; Peluso, A.; Aprea, P.; Pepe, F.; Caputo, D. CO2 Adsorption on Polyethyleneimine-Functionalized SBA-15 Mesoporous Silica: Isotherms and Modeling. J. Chem. Eng. Data 2014, 59, 896-902. 16. Li, K.; Jiang, J.; Tian, S.; Yan, F.; Chen, X. Polyethyleneimine-nano silica composites: a lowcost and promising adsorbent for CO2 capture. J. Mater. Chem., DOI: 10.1039/c4ta04275a. 16 ACS Paragon Plus Environment

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17. Senthilkumaar, S.; Kalaamani, P.; Porkodi, K.; Varadarajan, R.P.; Subburaam, C. V. Adsorption of dissolved Reactive red dye from aqueous phase onto activated carbon prepared from agricultural waste. Bioresour. Technol.2006, 97, 1618-25. 18. Santhy, K.; Selvapathy, P. Removal of reactive dyes from wastewater by adsorption on coir pith activated carbon. Bioresour. Technol.2006, 97, 1329-36. 19. Paul, M.; Pal, N,; Bhaumik, A. Selective Adsorption and Release of Cationic Organic Dye Molecules on Mesoporous Borosilicates. Mater. Sci. Eng. C 2012, 32, 1461-1468. 20. Dubey, S.; Sujarittanonta,L,; Sharma, C. Y. Application of fly ash for adsorptive removal of

malachite green from aqueous solutions. Desalin. Water Treat., 2015, 53, 91-98. 21. Kumar, P.; Mal, N.; Oumi, Y.; Yamana, K.; Sano, T.; Mesoporous materials prepared using coal fly ash as the silicon and aluminium source. J. Mater. Chem.2001, 11, 3285-3290. 22. Chaudhuri, H.; Dash, S.; Ghorai, S.; Pal, S.; Sarkar, A. SBA-16: Application for the removal

of neutral, cationic, and anionic dyes from aqueous medium. J. Environ. Chem. Eng. 2016, 4, 157-166. 23. Sajab, M. S.; Chia, C. H.; S. Zakaria, S.; Jani, S. M.; Ayob, M. K.; Chee, K. L.; P.S. Khiew, P.S.; W.S. Chiu, W. S. Citric acid modified kenaf core fibres for removal of methylene blue from aqueous solution. Bioresour. Technol. 2011, 102, 7237–7243. 24. Ojha, K.; Pradhan, N. C.; Samanta, A. Zeolite from fly ash: synthesis and characterization. Bull. Mater. Science, 2004, 27(6), 555-564. 25. Sui, Y. Z.; Cui, Y.; Zhu, H. J.; Han, B. H. Facile Preparation of Three-Dimensional Graphene Oxide–Polyethylenimine Porous Materials as Versatile Adsorbents. ACS Appl. Mater. Interfaces 2013. DOI: 10.1021/am402661t


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Table 1. Mass fraction of fly ash and HATF Scheme 1. Schematic representation for the Synthesis of PEI-CFA Fig. 1. FESEM and TEM micrograph of (a) fly ash (b) HATF (c) CFA-HATF (d) MG (PEI-CFA) (e) RR 2 (PEI-CFA) (f) HATF (g) CFA-PEI (h) MG (PEI-CFA) (i) RR 2 (PEI-CFA) respectively.

Fig. 2. (a) Nitrogen adsorption–desorption isotherms (b) FTIR spectra (c) HR-XRD patterns of HATF, PEI-CFA, MG/ PEI-CFA and RR 2/ PEI-CFA (d) TGA curves of HATF and PEI-CFA Fig. 3. (a) Effect of pH on zeta potential of PEI-CFA and effect of (b) pH on adsorption characteristics of MG and RR 2 from aqueous solution using PEI-CFA.


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Fig. 4: Modeling of adsorption isotherm of (a) MG (b) RR 2 using Langmuir model, adsorption kinetics of (c) MG (d) RR 2 using pseudo second order model and intraparticle diffusion models of (e) MG (f) RR 2 onto PEI-CFA as an adsorbent. Fig. 5. Thermodynamic study for adsorption of MG and RR 2 dyes using PEI-CFA as adsorbent;

from top to bottom: (a) RR2 and (b) MG.

Sample

% SiO2

% Al2O3

% Fe2O3

% TiO2

% Na2O

Non-magnetic ash

62.79

30.42

2.4

1.62

0.18

64.12

24.25

1.45

1.8

2.92

HATF Table 1

Scheme 1


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Solid

BET surface area (m2/g)

Average pore size (nm)

Total pore volume (c.c/g)

Non-magnetic CFA

66.782

2.93

0.215

HATF

102.89

3.28

0.313

PEI-CFA

57.47

1.95

0.131

MG (PEI-CFA)

5.08

0.68

0.059

RR 2 (PEI-CFA)

3.82

0.42

0.034

Table 2


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Fig. 1


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Fig. 2


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Fig. 3

Fig. 4


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Fig. 5


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Fabrication of Inexpensive Polyethylenimine-Functionalized Fly Ash

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