New crosslinked hydrazide–based polymers as Cr(VI) ions adsorbents

Accepted Manuscript Title: New crosslinked hydrazide–based polymers as Cr(VI) ions adsorbents Authors: Umesh K. Dautoo, Yashwant Shandil, Ghanshyam S. Chauhan PII: DOI: Reference:

S2213-3437(17)30542-0 https://doi.org/10.1016/j.jece.2017.10.041 JECE 1951

To appear in: Received date: Revised date: Accepted date:

13-7-2017 15-10-2017 17-10-2017

Please cite this article as: Umesh K.Dautoo, Yashwant Shandil, Ghanshyam S.Chauhan, New crosslinked hydrazide–based polymers as Cr(VI) ions adsorbents, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2017.10.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

New crosslinked hydrazide–based polymers as Cr(VI) ions adsorbents Umesh K. Dautoo, Yashwant Shandil, Ghanshyam S. Chauhan* Department of Chemistry, Himachal Pradesh University, Shimla, India – 171005 e–mail: [email protected]; [email protected]: 0911772830944, FAX: 0911772830775

GRAPHICAL ABSTRACT CONHNH2 H2NHN

CH3 C=O CH3

CONHNH2

H3C H3C CONHNH2

After Adsorption Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr

n

Adsorbent

Cr(VI) solution

Cr(VI) ion Adsorption by Adsorbent

Clear Solution

100 90

q (mg/g)

80 70 60 50

25 35 45 55

40

100 ppm of Cr(VI) Poly(MAAcH)-cl-DVB

30 0

50

100

150

200

250

300

350

400

Time (min)

Two new crosslinked polymers were synthesized by simple and green protocol and evaluated as Cr(VI) ions adsorbents with high efficiency even at very low concentration of 1.0 ppm with rapid adsorption are reported herein.

1

450

°C °C °C °C

500

HIGHLIGHTS    

New hydrazide-based nanopolymers, differing only in the nature of crosslinker, were synthesized These nanopolymers were evaluated as Cr(VI) adsorbents These exhibited high rapid removal with high adsorption capacity of the targeted ions Adsorption capacity is very high even at 1.0 ppm

ABSTRACT Cr(VI) salts are abundantly used in the industrial processes, but they have become serious water pollutants and their removal from the wastewaters is a huge challenge. In the present study, two new crosslinked polymers having hydrazide functional group, differing in the nature of crosslinking agents ethyleneglycol dimethacrylate or divinyl benzene, were synthesized for use as Cr(VI) ion adsorbents. These were analysed by FTIR, SEM, EDS, TEM, AFM and XRD to confirm their structure. The adsorption parameters such as contact time, temperature, pH and Cr(VI) ions concentration were investigated. Adsorption of Cr(VI) ions was rapid, pH dependent, and up to 100% adsorption from 100 ppm solution was observed within 1 min at pH = 2.0 and 25 °C. Langmuir isotherm best fitted the experimental values. These adsorbents, synthesized by a simple green protocol, show 100% adsorption efficiency even at a very low concentration of 1.0 ppm.

Adsorption kinetics,

mechanism and thermodynamics of the adsorption process were also investigated. Keywords: Crosslinked polymers; Hydrazide; Crosslinking agent; Adsorption capacity 1. Introduction Heavy metal ions contamination is a big threat to the health of aquatic living systems [1]. Cr metal which finds applications in many industries such as leather tanning, steel fabrication, metal finishing, electroplating and paint manufacturing [2], is one of the most toxic water contaminants. In wastewaters Cr exists in Cr(VI) and Cr(III) oxidation states [3]. Toxicity of former is around 100 times more than the latter and due to their water soluble nature these can easily enter the living cells through food chain [4,5]. The presence of excess Cr(VI) ions in the human body leads to a lot of health issues such as hepatocirrhosis, skin dermatisis, pulmonary congestion, severe

2

diarrhea, kidney damage, carcinogenicity, etc. [6–8]. Recommended limit by World Health Organization (WHO) for Cr(VI) ions in drinking water is 0.05 mg L–1 for potable water and 0.1 mg L–1 for discharge to the inland surface water [9]. Hence, to remove these ions from the wastewater from such a low level is an enormous technological challenge. Cr(VI) ions exists as soluble oxyanion in most aquatic bodies. In industrial wastewater Cr(VI) ions exist in different ionic form such as chromate (CrO42−), dichromate (Cr2O72−) or hydrochromate (HCrO4−). Chromate (CrO42−) ions are dominating species in the neutral and basic pH while dichromate (Cr2O72−) ions are the dominant species at lower pH [10,11]. Cr(VI) ions have strong oxidizing action and on reaction with organic substrates present in wastewater they are reduced to Cr (III) ions. Various methods reported for their removal include ion exchange, filtration, electrochemical precipitation and reverse osmosis produce large amount of harmful material that create problem of disposal [12–14]. Adsorption is economically attractive and effective method to reduce Cr(VI) ions concentration below the permissible limit [6,15]. A number of adsorbents including activated alumina, activated carbon fibers, oxide or hydroxide of iron and manganese, different types of biowaste have been reported in the past. The modified biopolymers and the biowaste as such have been widely reported adsorbents for Cr(VI) ions [1–7,11,13,16–21]. Nanomaterials have also been reported as Cr(VI) ions adsorbents [22–25]. Another class of adsorbents includes synthetic polymers with different forms of the amino groups including their quaternary forms as the active sites [26–28]. Some examples of the latter include modified chitosan [29,30], imidazole derivatives [31–33] and poly(amidoamine)–grafted cellulose nanofibril aerogels [34]. From the preceding discussion it is evident that there are many reports in literature where adsorbents with N containing Cr(VI) ions specific functional groups have been reported. However, new cost–effective, easily available, thermally and chemically stable adsorbents need to be developed for this purpose. In view of the afore–said, in the present study, we report synthesis of hydrazide–based polymers as efficient Cr(VI) ions adsorbents. At low pH, –CONHNH2 group of the synthesized polymers get protonated and the efficient adsorption of Cr(VI) ions can be expected by the electrostatic attraction between the negatively charged dichromate (Cr2O72−) anion and the protonated parts of the polymer chain [7,18,19,28]. Use of hydrazine compositions has been reported to reduce Cr(VI) [35]. In earlier reports we reported a hydrazionodeoxy cellulose graft copolymers as Cr(VI) ions adsorbents [36,37], but this 3

is the first report where hydrazine groups has been covalently linked onto a pre– crosslinked polymer to generate a new Cr(VI) ions adsorbent by polymer analogous reactions of the crosslinked polymers with hydrazine. Adsorption study was executed for the removal of Cr(VI) ions from the simulated water samples with optimum operating conditions being contact time (1–540 min), temperature (25 °C–55 °C), pH (2.0–9.0), and Cr(VI) ions concentration (1–350 ppm). Adsorption isotherm, kinetic, and thermodynamic models were used to understand the mechanism of adsorption process. 2. Materials and methods Methacrylic acid (MAAc) (HiMedia Laboratories Pvt. Ltd.), ammonium persulphate (APS) (SISCO research laboratories Pvt. Ltd. Mumbai, India), hydrazine hydrate (Fischer Scientific), methanol, divinylbenzene (DVB), ethyleneglycol dimethacrylate (EGDMA), potassium dichromate (K2Cr2O7) [E. Merck, Ltd., India], Cr (VI) reagent (Merck, Schuchardt, Germany), were used as received.

2.1. Synthesis of poly(MAAc )–cl–EGDMA and poly(MAAc )–cl–DVB and their functional modification Poly(MAAc)–cl–EGDMA and poly(MAAc)–cl–DVB were synthesized by mixing methacrylic acid with crosslinker EGDMA or DVB (1% of the total weight of the monomer), along with 0.5% APS used as initiator. The weight ratio of the components were taken as 5: 0.05: 0.025. The resulting reaction mixture was stirred for 5 min. in a chemical reactor and was kept as such for polymerization at 70 °C for 30 min in water bath. The crosslinked networks of poly(MAAc)–cl–EGDMA and poly(MAAc)–cl– DVB thus obtained were washed with the double distilled water and dried in oven for 48 h at 45 °C to remove any adsorbed water without affecting polymer structure. Washing and drying cycle were repeated until constant weights were obtained. [Scheme 1(I) and Scheme 2 (IV)]. Poly(MAAc)–cl–EGDMA and poly(MAAc)–cl–DVB were functionalized by following an earlier reported route [38] and described here briefly. Polymer samples (1.0 g) were separately taken in a round bottom flask alogwith 15.0 mL of methanol. To the reaction mixture was added 4–5 drops of concentrated H2SO4. The mixture was refluxed on water bath for 1 h and to the hot solution (50 °C) was added 15.0 mL of the distilled water. The contents were placed in an ice bath at near 0 °C, stirred well for 30 4

min. and then washed with 5% Na2CO3. The esters obtained were filtered [Scheme 1(II) and Scheme 2(V)]. To the ester formed was added 15.0 mL of methanol and 3.0 mL of hydrazine hydrate. Mixture was further refluxed at 50 °C in water bath for 1.5 h. The hydrazide crystallized on cooling near about 0 °C in an ice bath, which was recrystallized with methanol, filtered and separated. The synthesized polymers were dried in vacuum oven at 45 °C. [ Scheme 1 ( III ) and Scheme 2 ( VI ) ].

COOCH 3

HOOC * 2 C CH

CH2 C

H3 C

O

C-H 2C

CH3

O

O

CONHNH2 CONHNH2 C

CH2

C-CH2

CH3

H3C

O

H CH 3OH / Ref lux

30 min/70 °C HO O

CH2 CH

CH3

O

O

APS

CONHNH2

HOOC HOOC

O

NH2NH2

I

CH3

O O

O

CH3OH / Ref lux

O O

O

O O MAAc EGDM A

HOOC CH2 C

O HOOC

CH2 C

CH3

O

HOOC

CONHNH2

O

O

CONHNH2 C-H2 C

C-H2C

CH3

CH3

CH2 C

n

n

I

H3C

O

CONHNH2

CH 2C

CH3

CH3

n

III

II

Scheme 1. Synthesis of (I) poly(MAAc)–cl–EGDMA, (II) methyl ester of (I), and (III) poly( MAAcH )–cl–EGDMA. COOH

CONHNH2

COOCH 3

CH NHNH2 3 C=O

H3C O

COOH OH

APS 30 min/70°C

CH3

H COOH

NH2NH2

IV

CH3OH / Ref lux

CH3OH / Reflux

H3C Methacrylic acid DVB

CH3

NHNH2 C=O

H3C

COOH CH3

n

n

V

IV

CH3 CONHNH2

VI

Scheme 2. Synthesis of (IV) poly( MAAc)–cl–DVB, (V) methyl ester of (IV), and (VI) poly(MAAcH)–cl–DVB.

2.2. Characterization studies Synthesized polymers were characterized by various techniques to get evidence of their synthesis and structure. Fourier Transform Infrared (FTIR) spectra were 5

n

recorded on Nicolet 5700 in transmittance mode in KBr. Scanning Electron Microscope (SEM) images and EDS spectra were recorded on HITACHI SEM 8010. Atomic force microscopy (AFM) images, transmission electron microscopy (TEM), and X‒rays diffraction (XRD) analysis were recorded respectively,

on NT–MDT AFM,

FP5022/22–Tecnai G220 S–TWIN and PANalytic XRD with XPERT‒PRO diffractrometer system using a typical wavelength of 1.54060 Å (Cu–Kα radiation). The diffraction angle 2θ was varied from 10 to 60 degrees. 2.3. Adsorption studies of Cr( VI )ions Adsorption capacity (q) of poly(MAAcH)–cl–EGDMA and poly(MAAcH)–cl–DVB was studied as a function of contact time (1, 3, 5, 10, 15, 30, 60, 120, 180, 240, 300, 360, 420, 480 and 540 min), temperature (25 °C, 35 °C , 45 °C and 55 °C), concentration (1, 10, 30, 50, 75, 100, 200, 250, 300 and 350) ppm and pH (2.0, 3.0, 4.0, 5.0, 7.0 and 9.0 at 35 °C). Cr(VI) ions solutions of different concentrations i.e. (1.0 to 350) ppm, were prepared in distilled water using potassium dichromate (K2Cr2O7). Consequently, polymer (0.01 g) was added to the sample tube containing solutions of different pH or concentrations. There after the sample tube was put into water bath for different time intervals, as stated above, at separately at 25 °C, 35 °C, 45 °C and 55 °C and maintaining pH = 4.7 and using Cr(VI) ions solution of 100 mg/L. There after pH of the solution was varied in the above stated range at different time intervals of 1 to 180 min and 35 °

C and using Cr(VI) ions solution of 100 mg/L. Effect of the initial concentration of the

adsorbate was studied for 120 min and under pH of 4.0 and different temperatures. The concentration of the unadsorbed Cr(VI) ions in the filtrate solution was determined by using Cr(VI) reagents on UV‒Vis spectrophotometer at 𝜆 = 540 nm [7]. The adsorption capacity was calculated as reported elsewhere [39] as shown in equation 1: 𝑞=

(𝐶𝑖 − 𝐶𝑡 ) ×𝑉 𝑤

(1)

Where q is the adsorption capacity (mg g‒1); Ci and Ct, respectively, are the initial and final concentrations of Cr(VI) ions solution (mg L‒1), V is the volume of the solution in litters and w is weight of the polymer in g. 2.4. Adsorption isotherms Adsorption isotherms explain the equilibrium relationship between adsorbent and adsorbate. The maximum adsorption capacity of adsorbent can also be determined by 6

adsorption isotherm. There are various isotherm models for analyzing experimental data and to describe equilibrium of adsorption. Different equilibrium isotherm models, namely, Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (DR) and SIP were applied to test the experimental data. The Langmuir isotherm model is based on the assumption that a monolayer of Cr(VI) ions is adsorbed over a uniform adsorbent surface. It is assumed that once the adsorbent sites are covered with these ions no further adsorption occurs at those sites. It also suggests that all of the adsorption sites have equivalent energy. The linear form of the Langmuir equation is presented in equation 2 as presented in [40]: 1 1 1 1 = . + 𝑞𝑒 𝐾𝐿 𝑞𝑚 𝐶𝑒 𝑞𝑚

(2)

Where, KL = Langmuir equilibrium constant for adsorption (L/mg), qm = maximum adsorption capacity (mg g‒1), qe = amount adsorbed at equilibrium (mg g‒1), and Ce = equilibrium concentration (mg L–1). The slope and intercept of the plot 1/qe versus 1/Ce yield the values of KL and qm. 𝑅𝐿 =

1 1 + 𝐾𝐿 𝐶𝑖

(3)

RL in eq. 3 is equilibriumfactor that represents essential characteristics of Langmuir isotherm model and is related to the shape and favourability of the isotherm model according to the following characteristics: unfavourable (RL > 1), favourable (0 < RL < 1) and irreversible (RL = 1). Freundlich isotherm model assumed that the adsorption process takes place on a heterogeneous surface. In linear form it can be represented by eq. 4, obtained from qe = KF.Ce [41]: 1

log 𝑞𝑒 = log 𝐾𝐹 + 𝑛 log 𝐶𝑒

(4)

Where, qe is the amount adsorbed at equilibrium (mgg‒1) and Ce is the equilibrium concentration ( mg L–1). KF and n are Freundlich isotherm constants and were calculated from the slope and intercept of the plot of log qe vs log Ce. The Temkin isotherm is usually used for non-uniform distribution of sorption heat. The linear form of the Temkin isotherm equation is presented in eq. 5.

7

𝑞𝑒 = Where, 𝐵 =

𝑅𝑇 𝑏

𝑅𝑇 𝑅𝑇 ln𝐴 + ln 𝐶𝑒 𝑏 𝑏

(5)

constant related to heat of sorption (J/mol) obtained from the Temkin

plot (qe versus lnCe); A (slope) = Temkin isotherm equilibrium binding constant (L/g); b (intercept) = Temkin isotherm constant; R = universal gas constant (8.314 J.mol−1.K−1); T = Temperature at 308 K, and listed in Table 1. Dubinin-Radushkevich (DR) isotherm is used to calculate the amount of gas adsorbed in a microporous sorbent. The linear form of the (DRK) isotherm equation can be represented by eqs. 6 and 7. 𝑙𝑛 𝑞𝑒 = 𝑙𝑛𝑞𝑚 − 𝐵𝜀 2 1 ) 𝐶𝑒

𝜀 = 𝑅𝑇𝑙𝑛 (1 +

(6) (7)

Where, qm = maximum sorption capacity (mg/g); β = A activity coefficient constant related to sorption energy; ɛ = Polanyi potential. The DR parameters are calculated from the slop of the plot of lnqe versus ε2 gives β (mol2/J2) and exp (intercept) gives qm (mg/g) in Fig. 8 and listed in Table 1. The SIP isotherm is a combination of the Langmuir and Freundlich isotherms, which represent systems for which one adsorbed molecule could occupy more than one adsorption site represented by the variable n in the isotherm eq. 8 given below: 𝑛

𝑞𝑒 =

𝑄𝑆 𝐾𝑆 𝐶𝑒 𝑠

(8)

𝑛

1 + 𝐾𝑆 𝐶𝑒 𝑠

SIP has three parameters, Qs, Ks and ns, that can be expressed as a function of temperature which allows easy interpolation and extrapolation of the experimental isotherms to other temperatures and compositions. If the value of nS is equal to 1 then this equation will become a Langmuir equation. Alternatively, as either Ce or KS approaches zero, this isotherm reduces to the Freundlich isotherm. 2.5. Kinetic studies Several models have been applied to evaluate the order of the adsorbent–adsorbate interactions and the rate of adsorption of the Cr(VI) ions. In this study, pseudo–first order and pseudo–second order kinetic models were applied to the experimental data.

8

The former related to the adsorption capacity of the adsorbent and linear form of the pseudo–first order equation can be generally expressed as in eq. 9 [42]:

log( 𝑞𝑒 − 𝑞𝑡 ) = log 𝑞𝑒 −

𝑘1 𝑡 2.303

(9)

Where, qt and qe are the adsorption capacity at time t (min) and at equilibrium, respectively, k1 is the rate constant of pseudo–first order adsorption (L.min–1). From the plot of log(qe–qt) versus t, the values of K1 and qe were calculated. The linear form of pseudo–second order kinetic model is based on the adsorption capacity of the adsorbent and is expressed in eq. 10 [12]:

𝑡 1 1 = + 𝑡 𝑞𝑡 𝑘2 𝑞𝑒2 𝑞𝑒

(10)

Where, qt and qe are the adsorption capacity at time t (min) and at equilibrium respectively (mg/g) and k1 is the rate constant of pseudo–second order adsorption (g/mg. min) and is determined from the linear plot of t/qt versus t. 2.6. Thermodynamic studies Thermodynamic factors are important parameters to evaluate the feasibility of adsorption process. Gibbs free energy change (∆G°), indicates whether chemical reaction is feasible or not. The equilibrium constant Kc is given by eq. 11 [7]: 𝐾𝑐 =

𝐶𝑒 𝐶𝑜

(11)

Where, Ce and Co (mg L−1) are the equilibrium concentration and concentration in the solution, respectively. ∆G°, ∆H° and ∆S° are related to the adsorption equilibrium constant Kc and ∆G° can be calculated using eq. 12 and 13 [20]: ∆𝐺 𝑜 = −𝑅𝑇 ln 𝐾𝑐

(12 )

Where, ∆G° = standard free energy change (J/moL), R = universal gas constant (8.314 J/moL K), and T = absolute temperature (Kelvin). ∆H° and ∆S° can be calculated from the slope and intercept of a plot of lnKc versus 1/T from eq. 13 given below [29,43]: 9

lnK 𝐶 = −

∆H° ∆S° + RT R

(13)

3. Results and discussion 3.1. Characterization of synthesized polymers by different techniques Poly(MAAc)–cl–EGDMA and poly(MAAc)–cl–DVB were prepared by mixing methacrylic acid with crosslinker EGDMA or DVB by APS initiated polymerization. The result is formation of the crosslinked polymers which were functionalized with hydrazine hydrate to obtain two new hydrazide–based polymers, poly(MAAcH)–cl– EGDMA and poly(MAAcH)–cl–DVB, respectively. The polymers synthesized as above were characterized by FTIR, SEM, TEM, AFM, EDS and XRD to get evidence of synthesis. .

90

(C)

80

% Transmittance

70

1685 1532

(B)

1187

1107

60 759

50 40 30

849

1637 1524

(A)

1187

1107

3191 3337

20 10

520

1257 1165

1697

0 3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Fig. 1. FTIR spectra of (A) poly(MAAc)–cl–EGDMA (B) poly(MAAcH)–cl– EGDMA (C) Cr(VI) loaded poly(MAAcH)–cl– GDMA. The FTIR spectra of differently crosslinked poly(MAAc)–cl–EGDMA and their hydrazide forms are shown in Fig. 1. The successful synthesis of poly(MAAc)–cl– EGDMA copolymer (Fig. 1 A) was confirmed by the characteristic sharp adsorption band at 1697 cm−1 ( C

O stretching of the carbonyl ) and 1165cm−1 (antisymmetric

stretching of the C–O–C of ester groups ) [44]. After hydrazine hydrate treatment, new peaks appeared at 1524 and 1637 cm−1 which may be due to –NH– and –CO– vibrations 10

and 3191 cm−1, 3337 cm−1 ( N–H stretching ), indicating the introduction of –CO–NH– NH2 groups in the copolymer chain [45–48]. (Fig. 1 B) Similarly, FTIR spectrum of poly(MAAc)–cl–DVB and its corresponding hydrazide form is shown in Fig. 2. The successful synthesis of poly(MAAc)–cl–DVB copolymer was confirmed by the characteristic sharp peak at 1694 cm−1 (C

O stretching of the carbonyl ) and the alkyl

C–H stretching vibrations of methylene and methyl groups are observed at 2933 and 2998 cm–1 and at 826 cm−1 (C–C stretching of para substituted benzene ring). After hydrazine hydrate treatment, new peaks appeared at 1528 and 1678 cm−1 which may be due to –NH– and –CO– vibrations and 3185 cm−1, 3335 cm−1 (N – H stretching), indicating the introduction of CO–NH–NH2 groups in the copolymer chains [49,50].

60

(C)

55 50

(B)

% Transmittance

45 40

1678 1334

35 30

3335

3185

1106

1528

(A)

25 826

20

2998

1252

2933

15

1162 1694

10 3500

3000

2500

2000

-1

1500

1000

500

Wavenumber (cm )

Fig. 2. FTIR spectra of (A) poly( MAAc )–cl–DVB) (B) poly(MAAcH)–cl–DVB (C) Cr(VI)-loaded poly(MAAcH)–cl–DVB. Scanning electron micrography (SEM) images of the crosslinked polymers were recorded to study the surface morphology of polymeric structures. It provides evidence of changes affected in the surface morphology after polymerization and functional modification. Hence, comparison of SEM of the precursor and there derivatives support modification of the former.

11

Fig. 3. SEM images of (A) poly(MAAc)–cl–EGDMA (B) poly(MAAcH)–cl– EGDMA, (C) Cr(VI)-loaded poly(MAAcH)–cl–EGDMA (D) poly(MAAc)–cl–DVB and (E) poly(MAAcH)–cl–DVB (F) Cr(VI) ions-loaded poly( MAAcH)–cl–DVB. Surface of poly(MAAc)–cl–EGDMA and poly(MAAc)–cl–DVB is smooth and less porous than there functionally modified polymer (Fig. 3). Surface of functionally modified polymer looks porous and the changes in the surface morphologies of the polymers provide evidence of the functionalization of the precursors. Also, the surface of the EGDMA crosslinked polymer is more intense than the other polymer both before and after reaction with hydrazine. DVB is hydrophobic in nature, hydrophobic nature of the latter results in lower interactions of various groups. AFM images supports the formation of poly(MAAcH )–cl–DVB in nanometric scale. The average grain size and root mean square roughness of poly(MAAcH)–cl–DVB was found to be 8.77 nm and 1.103 nm, respectively (Fig. 4 D). The size of the other crosslinked polymer was not studied. TEM images of poly(MAAcH )–cl–DVB indicated its spherical-shaped morphology having dimensions in nanoscale with diameter in the range of 1–100 nm (Fig. 4A,B,C). Nanosize of the poly(MAAcH)–cl–DVB was further affirmed by the particle size analysis which showed that the diameter of the poly(MAAcH)–cl–DVB ranging from 1– 100 nm (Supplementary materials Fig. S1). 12

Fig. 4. Analysis of poly(MAAcH)–cl–DVB by TEM (A) 100 nm (B) 20 nm (C) 10 nm, and AFM (D). Energy–dispersive X–ray spectroscopy (EDS) provides the elemental composition or chemical characteristics of a polymer. EDS spectrum of poly(MAAc)–cl–EGDMA shows basic elemental structure of the polymer (Supplementary materials Fig. S 2 A). Addition of one extra peak in the spectrum due to nitrogen element shows hydrazide formation (Supplementary materials Fig. S 2 B), and after adsorption of Cr(VI) ions in the spectrum of the hydrazide form of the polymer an additional peak corresponding to Cr(VI) ions was observed (Supplementary materials Fig. S2 C), which confirms the adsorption property of the polymer. Similar characteristics has been shown by poly(MAAc)–cl–DVB and their functionally modified derivatives (Supplementary materials Fig. S 3). The XRD pattern of crosslinked and functionally modified polymers are shown in (Fig. 5). The important peaks were observed for poly(MAAc)–cl–EGDMA at 2θ = 15.92° and 31.62° with relative intensities of 751 and 352, respectively. Similarly, poly (MAAcH)–cl–EGDMA shows peaks at 2θ = 15.30°, 26.91°, 31.59°, 42.25° and 45.33° with relative intensities of 541, 305, 373, 291 and 257, respectively. Poly(MAAcH)– cl– EGDMA shows some semi–crystalline nature with a dominant amorphous phase. It 13

is interesting to note that intensity of the poly(MAAc)–cl–EGDMA peaks decreased after functional modification [46,47]. This observation can be related to the loss of some intermolecular association. Similarly, in poly(MAAc)–cl–DVB peaks were observed at 2θ

= 15.37°, 26.93° and 32.21° with relative intensities of 789, 674 and 354,

respectively. Decrease in the intensities of all the above peaks in the XRD spectrum of poly(MAAc)–cl–DVB confirm its functional modification to poly(MAAcH)–cl–DVB. Here again, there is a decrease in the intensities of peaks and increase in the amorphous nature of polymer after crosslinking and functional modification [51].

(A)

800

poly(MAAc)-cl-EGDMA poly(MAAcH)-cl-EGDMA

700

700

600

600

500

500

Intensity

Intensity

800

400 300

(B)

poly(MAAc)-cl-DVB poly(MAAcH)-cl-DVB

400 300

200

200

100

100 0

10

20

30

40

0

50

Degree 2

10

20

30

Degree 2

40

50

Fig. 5. Comparative XRD patterns of (A) poly(MAAc)–cl–EGDMA and poly(MAAcH )–cl–EGDMA (B) poly(MAAc)–cl–DVB and poly( MAAcH)–cl–DVB. 3.2. Effect of contact time, temperature, pH and Cr(VI) ions concentration on adsorption capacity Effect of contact time on the adsorption capacity was studied at 25 °C and 100 ppm concentration in the distilled water solution (pH = 4.7). The amount of Cr(VI) ions adsorbed (mg/g) increased with increase in time and after 300 min high q values of 98.0 mg/g and 95.0 mg/g were recorded, respectively, for poly(MAAcH)–cl–DVB and poly(MAAcH)–cl–EGDMA. Above 300 min there was slow increase in the q values. Similar trends in change of q values with contact time increase are reported in literature with equilibrium time of 40 min though the equilibrium q value reported is comparable yet obtained from 400 mgL-1 initial concentration [52]. The adsorption of Cr(VI) ions was rapid in the initial stage due to the availability of active binding sites on the adsorbent. With gradual decrease in these active sites, the adsorption process becomes slower in the later stages and took more time to adsorb Cr(VI) ions. The adsorption 14

studies were carried out up to 540 min. The effect of temperature was studied at four different temperatures (25–55 °C). The adsorption capacity (mgg−1) increased in the range from ~ 42 mgg−1 to ~ 79 mgg−1 and ~41 mgg−1 to ~76 mgg−1 in case of poly(MAAcH)–cl–DVB and poly(MAAcH)–cl–EGDMA), respectively,

when

temperature was increased from 25 °C to 55 °C (Fig. 6 A–B). Effect of pH on adsorption capacity is presented in (Fig. 6 C–D). The adsorption of Cr(VI) ions reached maximum at pH 2.0 with a high adsorption capacity of 100 mg g−1 corresponding to 100% ion removal for both the adsorbents within one min at 35 C. At low pH, –CONHNH2 groups of hydrazide–based polymer get protonated and the

°

high adsorption capacity of Cr(VI) ions can be expected by the electrostatic attraction between the negatively charged dichromate (Cr2O72− ) anions and the protonated group of the polymer [34,53]. But with an increase in pH, up to 9.0, protonation of the surface get reduced gradually and so was decrease adsorption capacity. At pH 9.0 the adsorption capacity ( mg g−1 ) for poly(MAAcH)–cl–DVB was ~ 72 mg g−1 and for poly (MAAcH)–cl–EGDMA) was ~70 mgg−1 after 180 min. This indicates that the adsorption process is pH – dependent. Almost similar trends with decrease of q values with pH increase have been reported in literature though the present study show more efficient adsorption [54]. This decrease in adsorption capacity with an increase in pH results from the poor electrostatic interactions and dual competition of both the anions ( CrO42– and OH– ) to be adsorbed on the surface of the adsorbent, of which OH– predominates [13]. Variation in concentration of Cr(VI) ions was studied from 1 ppm to 350 ppm (Fig. 6 E – F). Adsorption of Cr(VI) ions decreases with the increase in initial Cr(VI) ions concentration, because there is an increase in the number of Cr(VI) ions competing for available binding sites present on the adsorbents. Though the %adsorption decreased with increase in Cr(VI) ion concentration for both adsorbents but the actual amount of Cr(VI) ions adsorbed per unit mass of the adsorbent increased. With increase in contact time % adsorption increased at lower concentration, 100 ppm or lesser, but decreased when

concentration

was

increased

above

100

ppm.

Similar trends in the equilibrium adsorption capacities with variation of initial concentrations has been reported in literature [55]. The results reported at lower initial concentration in the present studies are higher than the cited literature report.

15

(B) 100

90

90

80

80

70

70

q (mg/g)

q (mg/g)

(A) 100

60

60 50

50

25 °C 35 °C 45 °C 55 °C

40 30

Poly(MAAcH)-cl-DVB 0

50

100

150

200

250

300

350

400

450

30

Poly(MAAcH)-cl-EGDMA 0

500

50

100

150

200

110

100

100

90

90

80

80

70

q (mg/g)

q (mg/g)

(C)

100 ppm Poly(MAAcH)-cl-DVB

60

1 (min) 5 (min) 7 (min) 10 (min) 30 (min) 90 (min) 180 (min)

50 40 30 2

6

7

8

9

2

pH

450

500

550

(D)

3

4

5

6

7

8

9

pH

(E)

120

(F)

120

100

100

80

q (mg/g)

80

q (mg/g)

400

1 (min) 5 (min) 7 (min) 10 (min) 30 (min) 90 (min) 180 (min)

60

30 5

350

70

40

4

300

100 ppm Poly(MAAcH)-cl-EGDMA

50

3

250

Time (min)

Time (min)

110

25 °C 35 °C 45 °C 55 °C

40

60

40

20

0 0

50

100

150

200

250

300

40

20

25 °C 35 °C 45 °C 55 °C

Poly(MAAcH)-cl-DVB

60

0

Poly(MAAcH)-cl-EGDMA 0

350

25 °C 35 °C 45 °C 55 °C

50

100

150

200

250

300

350

concentration (ppm)

concentration (ppm)

Fig. 6. Effect of (A–B) contact time and temperature (time = 5–480 min; temp. = 25– 45 ˚C; pH = 4. 7); (C–D) (pH = 2–9; Temp. = 35 °C) and (E–F) concentration (time = 120 min; temp. = 25 – 45 ˚C ; pH = 4.0 ) on adsorption of Cr(VI) ions.

3.3. Variation of Dose Rate on Adsorption and Reusability Studies Effect of the dose rate (mg), of both the polymers, on q values was studied at 100 ppm concentration, contact time of 1, 3, 5, 10, 15 min and pH 4.0 at 35 ˚C. q values increased with complete uptake of Cr(VI) ions with adsorbent dose of up to 50 mg 16

within 5 min. (Fig. 7 A and B). Reusability studies were carried out by leaching out the adsorbed Cr(VI) ions with 10.0 mL solution of eluent 1M NaOH separately for 5 h at 35 °C under centrifugation. After regeneration, the adsorbent was washed thrice with the double distilled water and dried at 40 °C before using for the next cycle. After every repeated cycle, q values decreased and reached 21.66 and 21.22 in fifth cycle for poly(MAAcH)–cl–DVB and poly(MAAcH)–cl–EGDMA respectively (Fig. 7C). Among Na2CO3, Na3PO4 and NaOH the latter has been reported to be the most effective regeneration agent for Cr(VI) ions desorption from maghemite [56]. Authors reported 87.7% efficiency in five cycles. In the present study regeneration efficiency decreased after first cycle as Cr(VI) ions are strongly adsorbed on the adsorbents. Poly(MAAcH)– cl–EGDMA exhibited somewhat low q value than poly(MAAcH)–cl–DVB because presence of aromatic rings in case of poly(MAAcH)–cl–DVB prevent self–summation of polymeric chains and ensures easy admittance of Cr(VI) ions to the active groups present on the polymer. (A)

1 min 3 min 5 min 10 min 15 min

100

80

40

40

20

20

0 10

20

30

40

50

Dose rate (mg)

poly(MAcH)-cl-DVB poly(MAcH)-cl-EGDMA

80

60

60

60

(C)

100

q (mg/g) e

q (mg/g) e

80

(B)

1 min 3 min 5 min 10 min 15 min

q (mg/g) e

100

40

20

0

0 10

20

30

40

50

1

Dose rate (mg)

2

3

Number of cycle

Fig. 7. Effect of adsorption of Cr(VI) by (A) poly(MAAcH)–cl–DVB (B) poly(MAAcH)–cl–EGDMA by variation of dose rate and (C) reusability cycle of both the hydrazide based polymers. The adsorption capacities of the two polymers reported in this study was compared with other adsorbents reported in literature taking into consideration the nature of the adsorbents, adsorbate dose used, equilibrium contact time, initial concentration and pH of the media for the attainment maximum adsorption capacity. Results are tabulated and presented in Table 1. Table 1. Comparison of the reported adsorbents with the literature values

17

4

5

Adsorbents

Initial Cr(VI) Time Concentration (min. mg/L )

pH Adsorption

Polyacrylamide-Grafted Sawdust

150

m-Poly(DVB-VIm) microbeads

100.0

Dose Ref.

capacity (mg/g)

(g)

3.0

55.2

2.0

[4]

300

3.0

35.21

0.05

[32]

Polyacrylamide modified 400.0 magnetic nanoparticles (PMMNs)

40

3.0

35.186

Amino-functionalized 100.0 polyacrylamide-grafted lignocellulosics poly(N,N-dimethylamino 150.0 ethyl methacrylate)-cl-N, N-ethylenebisacrylamide

120

4.0

49.75

2.0

[54]

200

2.0

143.0

0.02

[55]

Maghemite

15

-

2.5

-

[56]

60

5.0

189.3

0.1

[57]

Protonated chitosan

crosslinked 580.0

rate

[52]

CMS-g-PDMAEMA

100

60

3.0

78.0

0.4

[58]

Poly(MAAcH)–cl–DVB

100

120

4.0

98.00

0.01

Present Study

Poly(MAAcH)–cl– EGDMA

100

120

4.0

93.66

0.01

Present Study

From the foregone discussion the comparative advantage of the reported adsorbents is evident from the presented results. 3.4. Mechanism of adsorption The adsorption behaviour of the polymer was studied and interpreted using Langmuir, Freundlich, Dubinin-Radushkevich (DR), Temkin and SIP isotherm models. The parameters calculated from above models are presented in Table 2. Table 2. Adsorption isotherm constants for the adsorption of Cr(VI) ions on poly(MAAcH)–cl–EGDMA and poly(MAAcH)–cl–DVB Adsorption isotherm Poly(MAAcH)–cl– models and parameters EGDMA 18

Poly(MAAcH)–cl– DVB

qm(mg g-1) R2

Langmuir

104.49

109.17 0.9945

0.9969

N Freundlich KF R2 B (J/mol) Dubininqm(L/g) Radushkevich E (J/mol) (DR) R2 A (L/g) Temkin B R2

8.173 56.57 0.8296 1.9×10−4 86.56 50.82 0.9421 1.51 199.29 0.9683

9.997 67.16 0.7610 4.5×10−5 108.74 104.51 0.9728 1.24 562.87 0.8592

Sip

0.046 0.682 0.9945

0.192 1.912 0.9969

ns Ks R2

The value of correlation coefficient (R2 ) was found to be nearly 1 for the Langmuir isotherm (R2 = 0.96 and 0.99) for poly(MAAcH)–cl–EGDMA and poly(MAAcH)–cl– DVB compared to 0.81 and 0.77 for the Freundlich at 25 °C, Table 1. Correlation coefficient (R2) value was also studied at higher temperatures. The values of qe as calculated from the Langmuir isotherm model shows best fit with the experimental values (Fig. 8). 120 100

100 80

qe (mg/g)

qe (mg/g)

80

60

60

40

40 20

20

0

35 °C 0

50

Experimental Langmuir Freundlich

poly(MAcH)-cl-DVB 100

150

200

0 0

250

Experimental Langmuir Freundlich

35 °C poly(MAAcH)-cl-EGDMA 50

100

150

200

250

3

Ce (mg/dm )

3

Ce (mg/dm )

Fig. 8. Comparison of Langmuir and Freundlich isotherm models with experimental values for Cr(VI) ions adsorption by (A) poly(MAAcH)–cl– DVB and (B) poly(MAAcH)–cl–EGDMA. 3.5. Mechanism of kinetic studies Two kinetic models, pseudo–first order and pseudo–second order were applied to examine the adsorption kinetics (Fig. 9). The value of rate constant estimated from the 19

pseudo–first–order model did not follow any particular pattern with increase in reaction temperature. The values of equilibrium adsorption capacity, qe calculated at different temperatures are presented in Table 3. These values were quite small and not in agreement with experimental data. The correlation coefficients (R2), obtained also had low values between 0.8765–0.9368 for poly(MAAcH)–cl–EGDMA and 0.9588 – 0.9972 for poly(MAAcH)–cl–DVB). On the other hand pseudo–second order model explains the adsorption in better way and best fits the experimental data, compared to the pseudo–first order model, with correlation coefficients values (R2) quite close to 1 and calculated equilibrium adsorption capacities (qt) are closer to the experimental values in case of the pseudo–second order model. The latter assumes that the rate determining step in the adsorption process may be chemical adsorption involving electronic interactions between adsorbent and adsorbate [59]. Thus, it is suggested that the adsorption kinetics of Cr(VI) ions can be best described by the pseudo–second order model for both the polymers. (E)

100

(F)

100

80

80

60

60

Experimental Data Pseudo-first-order Pseudo-second-order

40

35 °C poly(MAAcH)-cl-EGDMA

qt

qt

35 °C poly(MAAcH)-cl-DVB 40

20

20

0

0 0

100

200

300

400

500

Experimental Data Pseudo-first-order Pseudo-second-order

0

time (min)

100

200

300

400

500

time (min)

Fig. 9. Pseudo–first order and pseudo–second order kinetic models. Table 3. Pseudo–first order and pseudo–second order kinetic constants for the adsorption of Cr(VI) ions on poly(MAAcH)–cl–EGDMA and poly(MAAcH)–cl–DVB. Pseudo–first order

Poly(MAAcH) –cl–EGDMA

Poly(MAAcH)– cl–DVB

Pseudo–second order

Temperature 25°C 35°C 45°C 55°C

qe (mg g–1) 36.174 27.492 20.692 20.555

k1 (min–1) 0.006 0.008 0.011 0.022

R2 0.937 0.943 0.912 0.876

qe (mg g–1) 102.0 101.7 101.1 100.6

k2 (g mg–1 min–1) 59167 98121 194625 427591

R2 0.999 0.999 0.999 0.999

25°C 35°C 45°C 55°C

56.645 25.466 30.410 33.318

0.012 0.010 0.027 0.049

0.959 0.946 0.969 0.997

102.9 100.9 100.8 100.5

75067 172701 349568 563029

0.999 0.999 0.999 0.999

20

RL value calculated from Langmuir constant for both the polymers was found to be in between 0.04 to 0.002 for initial concentration values in the range 1–350 mg/L. The obtained RL values indicate a favourable adsorption process (Fig. 10). (A)

25 °C 35 °C 45 °C 55 °C

0.35 0.30

(B)

0.6

25 °C 35 °C 45 °C 55 °C

0.5

0.25

0.4

RL

RL

0.20 0.15

0.3

0.2

0.10 0.05

0.1

0.00

0.0 0

50

100

150

200

250

300

350

0

50

100

150

200

250

300

Ci

Ci

Fig. 10. RL values for (A) poly(MAAcH)–cl–DVB and (B) poly(MAAcH)–cl– EGDMA at different temperature plotted against Ci. 3.6. Thermodynamic studies Values of enthalpy (∆H°) and entropy (∆S°) can be calculated from the slope and intercept of a plot of lnKc versus 1/T. Plot of lnKc versus 1/T for the calculation of ∆G°, ∆H° and ∆S° is presented in Fig. 11, both for poly(MAAcH)–cl–EGDMA and poly (MAAcH)–cl–DVB. The R2 values are given in the inset of the figures.

(A) 100 ppm

5

5

2

10 (min), R = 0.797

(B) 100 ppm

2

10 (min), R = 0.717

2

2

30 (min), R = 0.988

30 (min), R = 0.961

2

60 (min), R = 0.970 4

4

2

120 (min), R = 0.843

3

ln Kc

ln Kc

2

120 (min), R = 0.832

3

2

2

1

1

0

0

0.00300

2

60 (min), R = 0.977

0.00306

0.00312

0.00318

0.00324

0.00330

0.00300

0.00336

1/T (Kelvin)

0.00306

0.00312

0.00318

0.00324

0.00330

0.00336

1/T (Kelvin)

Fig. 11. van’t Hoff plot for Cr(VI) adsorption by (A) poly(MAAcH)–cl–DVB and (B) poly(MAAcH)–cl–EGDMA at different time intervals. Thermodynamic parameters ΔG° , ΔH° and ΔS° values were determined from van’t Hoff plots are presented in Table 4. Magnitude of free energy change (ΔG° ) shifted to a higher –ve values with rise in temperature from 25 °C to 55 °C suggesting rapid and spontaneous adsorption process. Hence, rise in temperature resulted in increasing 21

Cr(VI) ions accessibility to the polymers. The implication of positive ΔH° values is that the adsorption process is of endothermic nature, while high +ve entropy factor (ΔS°) suggests increase in randomness during the adsorption process [60]. Table 4: Thermodynamic parameters for adsorption at different temperatures

Polymer

Temperature (K)

poly(MAAcH)–cl–DVB

poly(MAAcH)–cl– EGDMA

298 308 318 328 298 308 318 323

∆G°

∆H°

∆S°

(kJ/moL)

(kJ/moL)

(JmoL−1 K−1)

6.16 7.72 11.85 25.11 5.47 6.29 7.91 11.30

117.29

394.08

98.95

330.90

3. Conclusions Two new hydrazide functional groups bearing crosslinked polymers derived from methacrylic acid were synthesized via a simple green protocol. There was neither any toxic substance generated at the synthetic stage nor will their decomposition, after use, generate any toxic materials. These are efficient adsorbent for Cr(VI) ions using dichromate ions as the candidate anions. Though –CONHNH2 is the major functional group on these polymers, yet there are other active sites which respond effectively to the adsorption process. At low pH 100% removal of ions was observed within one min from a solution of 100 mg/L. The adsorption capacities of 98.0 mg/g and 93.66 mg/g were obtained, respectively, for poly(MAAcH)–cl–DVB and poly(MAAcH)–cl– EGDMA from 100 mg/L of the adsorbate solution. The former is comparatively more efficient, less costly and more effective adsorbent. Absorbents can be regenerated and are reusable. All the experimental data show better match with Langmuir and followed pseudo–second order kinetics. Adsorption processes are endothermic in nature. These new polymers have significant potential for use in chromium ions removal from wastewaters. References 22

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