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Kinetic and equilibrium modeling for adsorption of textile dyes in aqueous solutions by Carboxymethyl cellulose/poly(acrylamide-co-hydroxyethyl methacrylate) semi-interpenetrating network hydrogel.

INTRODUCTION

Synthetic dyes are of major concerns for our environment. More than 100,000 types of dyes are used in industries like plastics, paints, paper, textile, cosmetics etc. to color various products (1). It is reported that 2% of total dyes produced in its manufacturing units and 10-20% of dyes used for coloring different products are discharged in effluent water (2). However, most of the dyes are toxic and carcinogenic. Because of very high tinctorial values (<1 mg/L) discharge of very small quantity of dye in water impart intense color which inhibits penetration of sunlight. As a result photosynthesis of aquatic plants are also disturbed (3). Most of the textile dyes are made from bio-recalcitrant synthetic aromatic compounds with low biological oxygen demand to chemical oxygen demand ratio (~20%) (4). Conventional methods like coagulation, chemical precipitation, membrane extraction, complexation, solvent extraction, ozonation etc. can not effectively remove dye from waste water (5). However, adsorption is a better candidate for dye--water treatment because of its low cost, easy operation with simple design and insensitivity to toxic dye molecules (6). Adsorbents like activated carbon, fly ash, orange peel, jute etc. may effectively remove low concentration of dyes from water (5). In recent years, various polymeric hydrogels based on acrylic polymer/copolymers (7), semi--and full-interpenetrating network (IPN) (8) and natural polymers like chitosan (9), modified cellulose (2), alginates (6), (10) were tried for removal of dye from water. Natural polymers are abundant, renewable and biodegradable. However, structural integrity of synthetic hydrogels is better (10). Hence, hydrogels based on both natural/semi-synthetic polymer and synthetic polymers would be very effective. Carboxy methyl cellulose (CMC) is water soluble ionic ether of cellulose with wide spread commercial applications (8), (11). Hydrogel obtained by crosslinking this cellulose ether would be of poor gel strength 1 1 and because of its inherent crystallinity the polymer can not absorb much of water. Thus CMC was chemically modified with other synthetic polymer to produce several hydrogels (8), (11), (12). Interpenetration of two polymers followed by crosslinking of at least one of the constituent polymers results in formation of IPN type polymer with strong network structure (13). IPN formation is an effective way of enhancing mechanical properties and toughness of a hydrogel. Further, polymers with reactive functional groups can be combined into a stable IPN blend to form a strong adsorbent. Thus, in recent years CMC based IPN hydrogels have been widely used for various applications. Bajpai and Misra (14) synthesized IPN of acrylic acid and CMC and used it for delivery of tetracycline drug. Xiao et al. synthesized pH responsive IPN of CMC and polyvinyl alcohol (15). In this hydrogel CMC was crosslinked with ferric chloride in aqueous solution of polyvinyl alcohol. Ma et al. prepared (16) clay loaded semi-IPN of CMC and N'isopropyl acrylamide with improved response rate and mechanical properties. Polyacrylamide is extensively used as hydrogel materials. Metal and dye sorption of polyacrylamide is increased by copolymerizing AM with maleic acid, itaconic acid, hydroxyethyl methacrylate (HEMA) etc. monomers (17). In this work, semi-IPN type hydrogels were synthesized by free radical copolymerization of AM and HEMA in aqueous solution of CMC. These hydrogels were used for adsorption of two important synthetic dyes, i.e., basic fuchsin and methyl violet from water. These two dyes are extensively used in Indian textile industries. Removal of these dyes from water with a suitable adsorbent is industrially very significant since both of these dyes are of high tinctorial values and even a concentration as low as 1 mg/L of these dyes produce color in water (3). Thus, in this work, the IPN hydrogels were used for adsorption of both low (2.5-20 mg/L) and high range (100-1000 mg/L) of feed concentration of these dyes. The effect of feed concentration, contact time, dosage of hydrogel, solution pH, and ionic strength on adsorption of these dyes was studied.

EXPERIMENTAL

Materials

Monomers i.e., AM, HEMA, and N,N'-methylene bisacrylamide (NMBA; from Fluka), redox initiator pair, i.e., potassium peroxodisulphate (from Fluka) and sodium metabisulfite (Merck), were of analytical grade and used without further purification. CMC (degree of substitution 1.8 and molecular mass 20,000) was obtained from S.d. fine chemicals, Mumbai and used as obtained. Basic fuchsin (molecular mass 324, [[lambda].sub.max] = 550 nm) and methyl violet (molecular mass 408, [[lambda].sub.max] = 585 nm) dye used in sorption studies, were purchased from SRL Chemical, India.

Preparation of IPN Hydrogels

Three semi-IPN type hydrogels were synthesized in aqueous solution of CMC by free radical crosslink copolymerization of AM, HEMA, and NMBA (comonomer cross-linker) in a three-necked reactor at 65[degrees]C for 3 h using potassium peroxodisulfate and sodium metabisulfite (each, 0.5 mass% of the total monomer mass) as redox pair of initiators. For this copolymerization reaction, AM:HEMA comonomer ratio was fixed at 10:1 while the amount of NMBA was 0.5% (mass% of total monomer AM and HEMA).The amount of CMC was 5, 7.5 and 10% (mass% of total comonomer) for these hydrogels and these were designated as IPN1, IPN2, and IPN3, respectively. The gelled mass resulting from this free radical crosslink copolymerization was immersed in cold deionized water and kept for three days to remove water soluble oligomer, uncrosslink polymer and unreacted monomers from the gel. The gel obtained was dried in a vacuum oven at 70[degrees]C to a constant weight. The dried gel was then disintegrated in a blender.

Characterization of the Hydrogel

Fourier Transforms Infrared Spectroscopy (FTIR). Various functional groups of the IPN hydrogels were characterized by FTIR spectroscopy (Perkin Elmer model-Spectrum-2, Singapore) using KBr pellet made by mixing KBr with fine powder of the polymer gel samples. (10:1 mass ratio of KBr to polymer).

X-Ray Diffraction (XRD). The change of crystallinity of the copolymer and CMC by IPN formation was characterized by XRD. Wide angle XRD profile of the hydrogel samples were studied at 25'C with a diffractometer (model: X'Pert PRO, made by PANalytical B.V., The Netherlands) using Ni-filtered Cu [K.sub.[alpha]], radiation ([lambda] = 1.5418 [angstrom]) and a scanning rate of 0.005 deg(2[theta])/s). The angle of diffraction was varied from 2-72 degree.

Scanning Electron Microscopy (SEM). The morphology of the dry and swollen hydrogels were characterized by scanning electron microscopy (SEM, model no. S3400N, VP SEM, Type-H, made by Hitachi, Japan) with the accelerating voltage set to 15 kV. Hydrogels swollen in dye solution were frozen in liquid nitrogen and then freeze dried for SEM analysis.

Mechanical Properties. Mechanical properties of the IPN hydrogels were also characterized with measurement of tensile strength (TS) and elongation at break (EAB) by an Instron-Tensile tester (Lloyd instruments, England). The experiment was performed by a method reported elsewhere (13). In this work, cubic sample of 2 mm X 2 mm X 80 mm size was used. The crosshead speed of 100 mm [min.sup.-1] was maintained. The cubic samples were elongated at a strain rate of 5% [min.sup.-1]. TS and EAB were calculated on the basis of initial cross section area of the sample.

Equilibrium Swelling (ES%). The water uptake of the hydrogels ([W.sub.C] was determined by using the following Eq. l.

[W.sub.C] = [W.sub.t] - [W.sub.d] / [W.sub.d] (1)

where W1 is the mass of swollen hydrogel polymer at time "t" and Wd is the mass of dry polymers. The amount of water absorbed by the hydrogels under equilibrium conditions, also called equilibrium swelling (ES) was obtained when [W.sub.t] did not change any more ([W.sub.[alpha]]) with time.

Study of Dye Adsorption of the Hydrogels

Lower (2.5-20 mg/L) and higher (100-1000 mg/L) range of feed concentration of basic fuchsin and methyl violet dyes were prepared in distilled water at different pH and also in distilled water with varied molar concentration of sodium chloride and calcium chloride. Fifty milligrams of hydrogel was taken in 50 mL of the dye solution with continuous stirring on a magnetic stirrer until equilibrium was reached. After equilibrium was reached, the dye solution was separated by decantation from the hydrogel. The concentration of dye solutions before and after addition of hydrogel were determined by spectrophotometric measurement from a precalibrated curve of absorbance versus concentrations using Perkin Elmer lamda 2 5 UV--visible double beam Spectrophotometer. The absorbance of the dye solutions were measured at wavelength of 550 nm for basic fuchsin and 585 nm for methyl violet dye. The structure of basic fuchsin and methyl violet dyes are shown in Fig. la and b, respectively. The amount of dye uptake ([Q.sub.e], mg/g) by unit mass (in g) of the hydrogel at equilibrium was calculated using the following [E.sub.q]. la

[Q.sub.e] = ([C.sub.0][V.sub.0] - [C.sub.3]V)/[W.sub.d] (1a)

Here [C.sub.0] and [C.sub.e] are initial and final equilibrium (after contact time t) concentration of dye solution (mg/L) while [V.sub.0] and V is volume (L) of the initial and final dye solution containing the hydrogel and [W.sub.d] is mass (g) of the dry hydrogel polymer used for the experiment. The removal% of dye by the hydrogel polymers were determined by using the following Eq. 2

Removal% = ([C.sub.0][V.sub.0] - [C.sub.e]V)/[C.sub.0] x 100 (2)

The results for dye uptake experiments were reproducible and the errors inherent in the measurements were less than [+ or -]3%.

Synthesis of Hydrogels

The IPN hdrogels were synthesized by free radical copolymerization of AM and HEMA in presence of CMC. In this case NMBA, a comonomer crosslinker also takes part in the polymerization reaction. The reaction occurs through free radical mechanism where primary radicals are formed on all of these monomers, i.e. AM, HEMA and NMBA. AM and HEMA radicals copolymerize while NMBA being bifunctional (Fig. 1c) copolymerize with both HEMA and AM resulting in formation of crosslink copolymer. Hydroxy(-0H) and carboxyl(-CO[O.sup.-])functional groups of CMC also interact with the copolymer through electrostatic and hydrogen bonding interaction and thus a double network of copolymer and CMC is formed (18). The resulting polymer will be semi-IPN since only one polymer of these double networks, i.e., the copolymer is crosslinked. The possible structure of the IPN type hydrogel and its interaction with dye molecule is shown in Fig. lc.

Characterization of the Hydrogel

FTIR Analysis. The FTIR of CMC and the three IPN hydrogels are shown in Fig. Id. The stretching vibration of carboxylic group of pure CMC is observed at 1580 c[m.sup.-1] while its C[H.sub.2] scissoring and OH bending vibration is observed at 1419 and 1326 c[m.sup.-1]. 1, 4-[beta]-D-Glucoside stretching vibration of CMC is observed at 1036 c[m.sup.-1]. The broad band from 1203 c[m.sup.-1] to 1036 c[m.sup.-1] are due to absorption of sugar ring of CMC (19), (20). The band at 2950 c[m.sup.-1] corresponds to C--H stretching of alkane of the hydrogels (21). The N--H stretching of AM is observed at around 3320 c[m.sup.-1] in the IPNs. The carbonyl stretching of AM, NMBA, and HEMA are shifted at around 1670 c[m.sup.-1]. The carbonyl stretching of CMC is also shifted to around 1650 c[m.sup.-1] in the IPN hydrogels. C[H.sub.2] scissoring of CMC is shifted in between 1423 and 1452 c[m.sup.-1] in the IPNs (21). The C--O stretching band of 1216 c[m.sup.-1] of HEMA comonomer is shifted to 1192 c[m.sup.-1] in the IPNs. The O--H bending vibration of HEMA is also shifted to 1123 cm-1 in the IPN. All of these shifting clearly indicate interaction of CMC and PAMHEMA in the double network of IPN.

SEM Analysis. The SEM of the dry IPN2 hydrogel is shown in Fig. 2a. The globular morphology of CMC (22), (23) dispersed in continuous phase of the copolymer is evident from this figure. Other IPNs show similar type of SEM. The SEM of IPN2 hydrogel swollen in dye solution is shown in Fig. 2b. The swollen internal structure of the hydrogel is clearly evident from its SEM. It also confirms the three dimensional network structure of the hydrogel (20).

XRD Analysis. The XRD of CMC and the three IPNs are shown in Fig. 3. The crystallinity of CMC arises from intramolecular hydrogen bonding between hydroxy and carboxylic functional groups of its structure. In-situ copolymerization of AM and HEMA reduces intramolecular hydrogen bonding. Hence crystallinity of CMC is also reduced. Thus, from Fig. 3, it is observed that the crystalline peak of virgin CMC at 2[theta] of 2[theta] degree (24) is shifted to 2[theta] of 23 degree in the IPNs with much reduction in peak intensity. In fact, polyacrylamide shows a low XRD peak at 2[theta] of around 21-32 degree (25). Hence, the XRD peak of the three IPNs at 2[theta] of 23 degree may be due to modified CMC and AM moiety of the copolymer.

Mechanical Properties. The TS and EAB of the hydrogel samples are given in Table 1. It is observed that with increasing amount of CMC, TS of the hydrogel increases while EAB decreases. IPN is formed by polymerization of PAMHEMA copolymer in CMC with formation of double networks. Entangled double networks of PAM-HEMA and CMC increases its stiffness. Hence, with increasing amount of CMC and interpenetration of the IPN, IS increases while EAB decreases from PAM-HEMA (0 wt% CMC) to IPN2. IPN3 shows slightly lower TS which may be due to decrease in compatibility of the copolymer and CMC in this IPN.

TABLE 1. Composition, mechanical properties and equilibrium
swelling% (ES%) of the Hydrogels.

Name of the  Composition            Tensile  Elongation   ES%
polymer                            strength    at break
hytlrogel                             (MPa)         (%)

PAMHEMA      10:1 copolymer ml AM     35.13       55.21  1623
             and HEMA

IPN1         10:0.5 mass ratio of     44.13       47.12  1693
             PAMHEMA and CMC

IPN2         10:0.75 mass ratio       49.23       39.11  1755
             of PAMHEMA and CMC

IPN3         10:1 mass ratio of       48.23       35.26  1731
             PAMHEMA and CMC


Equilibrium Swelling. The water uptake of initially dry hydrogels was measured gravimetrically for a period of 48 h at 30[degrees]C and pH 7 using Eq. 1. It was observed that there was no further change in mass of the hydrogel after 48 h of swelling. Equilibrium swelling% (ES%) were determined from the swelling curve (swelling % vs. time, not shown). The ES% of the hydrogels is shown in Table 1. It is observed that the IPN hydrogels show much higher ES% than the copolymer hydrogel. The presence of CMC increases hydrophilicity as well as ES% of the IPN hydrogels. In fact, the carboxylic groups of CMC ionizes (pKa of CMC is 4.6 which is less than solution pH 7) and repel one another. Thus, the network structure expands to absorb more water. From Table 1. it is observed that ES% increases with increase in amount of CMC from IPN1 to IPN2. However, IPN3 containing higher amount of CMC than IPN2 shows slightly lower ES%. This may be due to increased interaction and entanglement between CMC and the copolymer in this IPN (13).

Study of Dye Removal Capacity of the Hydrogels

Effect of Dosage of the Hydrogel. The dye adsorption was studied in a batch experiment with 50 mL aqueous solution of 5 mg/L basic fuchsin and methyl violet dyes for 48 h at 25[degrees]C. The dosage of hydrogel (IPN2) was varied from 0.25 to 3 g/L. The experiment was carried out for 48 h to ensure equilibrium of dye adsorption. It is observed from Fig. 4a that removal% (R%) and equilibrium dye adsorption ([Q.sub.e]) increases with increasing dosage of hydrogel. However, above 1 g/L of hydrogel, [Q.sub.e] decreases though R% further increases to reach saturation at around 2 g/L of hydrogel. Thus, at a hydrogel dosage of 1 g/L, the IPN2 polymer is observed to show 62% BF dye adsorption (R%) and adsorption capacity ([Q.sub.e]) of 3.1 mg/g while at hydrogel dosage of 2 g/L, [Q.sub.e] decreases to 2.4 mg/g and R% increases to 95%. Similar kind of trend is also observed for methyl violet dye adsorption. The increase in % of dye adsorption (R%) with hydrogel dosage may be attributed to increase in its surface area which adsorbs more of the dye molecules. However, the decrease in [Q.sub.e] at higher dosage of hydrogel (above 1 g/L) may be due to competition among adsorbents and also split in the concentration gradient (26). In all of the subsequent experiments, hydrogel dosage was fixed at I g/L since at this dosage the hydrogel showed optimum performance in terms of [Q.sub.e] and R%.

Effect of pH. The pH of the aqueous solution of dye plays an important role for dye--hydrogel interaction. Dye adsorption for IPN2 hydrogel was studied at different pH of the dye solutions with dye concentration of 5 mg/L at 25[degrees]C. Dilute aqueous solution of NaOH and HC1 was added to adjust the pH of the dye solutions. From Fig. 4b, it is observed that over the pH range of 2-9 the variation of dye adsorption or removal% ([Q.sub.e] or R%) for IPN2 hydrogel is marginal. Similar kind of trend lines was also obtained with IPN1 and IPN3. Both [Q.sub.e] and R% decreases above a pH of 8 which may be due to deprotonation of the cationic dye (3). In the subsequent experiments, solution pH was maintained at pH of 7.

Effect of Ionic Strength. Dye adsorption was also studied with similar experiments in presence of varied concentration of monovalent and bivalent salts i.e. sodium chloride and calcium chloride, respectively. For dying textile fiber, sodium chloride is extensively used as it promotes adsorption of dye (3), (27). From Fig. 5a and b, it is observed that with increase in concentration of both sodium chloride and calcium chloride both [Q.sub.e] and R% decreases for basic fuchsin (Fig. 5a) and methyl violet (Fig. 5b) dye. Ionic strength of the solution increases with increase in salt concentration. As a result electrical double layer surrounding the functional groups of the hydrogels becomes compressed resulting in decreased adsorption of dye. Because of higher ionic strength bivalent calcium chloride is also observed to show lower adsorption than monovalent sodium chloride for both basic fuchsin and methyl violet dye.

Effect of Contact Time.

Two Distinct Stages of Adsorption. The variation of adsorption of basic fuchsin dye with contact time in the low and high concentration range is shown in Fig. 6a and b, respectively. Similar kind of trendlines was also observed for adsorption of methyl violet dye. From Fig. 6a and b, it is observed that for both concentration ranges initially the rate of adsorption is very high. As the contact time is further increased dye uptake rate becomes slower and reaches almost a constant value. Initially all the functional groups of the hydrogels are available for interacting with dye molecules. Thus, initial rate of adsorption is very high. As these functional groups exhaust by dye adsorption, rate of adsorption becomes slower with time and at a point of time it reaches a constant value. This time is defined as equilibrium time when a dynamic equilibrium is formed between the hydrogel and the dye solution, i.e., at this time the rate of desorption from the hydrogel equals the rate of adsorption by the hydrogels and dye adsorption reaches its maximum value.

The Different Equilibrium Time for Hydrogels. From Fig. 6a, equilibrium time for dye uptake is observed to increase in the following order: IPN1 (1315 min) < IPN2 (1380 min) < IPN3 (1521 min). The different equilibrium times for the hydrogels may be ascribed to its structure. Due to mutual interpenetration and network formation of the two constituent polymers (copolymer and CMC) the IPN hydrogels needed higher contact time to reach saturation in the dye solution. With increasing amount of CMC in the IPN hydrogel, interpenetration increases from IPN1 to IPN3 and thus IPN3 with the highest level of mutual interpenetration showed the longest equilibrium time.

IPN Type and Adsorption. From Fig. 6a and b, it is observed that for the same contact time dye adsorption increases in the following order: PAMHEMA < IPN1 <IPN2 > IPN3. In this case as the amount of CMC increases from 0% (PAMHEMA) to 7.5 % (IPN2), hydrophilicity of the resulting hydrogel increases because of carboxylic and hydroxy groups of CMC in the IPN. Thus, dye adsorption increases due to increased interaction of dye molecules with hydrogel (Fig. 1c). IPN3 showed slightly lower adsorption than IPN2 which may be due to increased interpenetration and interactions between the two networks in the hydrogel which reduces hydrogel-dye interaction (13).

Low and High Concentration. In comparison to low feed dye concentration (5 mg/L, Fig. 6a) saturation of dye adsorption occurs much earlier for high feed dye concentration (500 mg/L, Fig. 6b). In this case, within 180 min all of the hydrogels reach equilibrium time. Dye adsorption by hydrogel is governed by film diffusion of dye molecules from solution to surface of the hydrogels followed by pore diffusion into the interior of the hydrogel (27). At higher concentration range, mass transfer resistance for transport of dye molecules is reduced and thus equilibrium time is reached much faster. However, for all of the experiments an equilibrium time of 48 h was given to ensure equilibrium for both low and high concentration range of dye solution.

Effect of Initial Concentration of Dye. The variation of dye up take properties of the hydrogels with feed concentration is shown for low feed concentration range of 2.5-20 mg/L and high concentration range of 100-1000 mg/L of basic fuchsin dye in Fig. 7a and b, respectively. Similar type of isotherms was also obtained for methyl violet dye. From these figures, it is observed that with increase in equilibrium dye concentration in feed adsorption of dye molecules by the hydrogels increases. In fact, dye adsorption by hydrogels is concentration dependant. Mass transfer resistance of dye molecules between solid (hydrogel) and liquid (dye solution) decreases with increase in feed dye concentration. Thus, dye adsorption increases with feed concentration. It is also observed that removal% decreases with increase in feed concentration of dye. A given amount of hydrogel can adsorb a fixed amount of dye molecules. As the feed concentration increases, the % of this fixed amount decreases with respect to increased feed concentration. Basic fuchsin and methyl violet dye adsorption by IPN2 hydrogel for both high and low feed dye concentrations is compared in Fig. 8a and b, respectively. Similar kind of isotherms was also observed for the other hydrogels. From these figures, it is observed that for the same feed concentration the hydrogels show much higher adsorption of basic fuchsin dye than methyl violet dye. Both of these dyes are cationic. However, basic fuchsin contains primary amine groups while methyl violet contains tertiary amine groups. Primary amine is more basic ([pK.sub.b] = 3.36) than tertiary amine ([pK.sub.b] = 4.23) in water. The methyl substituents of methyl violet dye may cause some steric hindrance for approaching carboxylate anion of the hydrogel. The carboxylate anion of hydrogels comes from its CMC and HEMA moieties. Further, the primary amine of basic fuchsin dye forms hydrogen bonding with hydroxy groups present in the IPN and also shows strong electrostatic interaction with carboxylate anion of CMC (18) (Fig. lc). This may be the reason for higher adsorption of basic fuchsin dye than methyl violet dye by the hydrogels at any feed concentration of dye.

Adsorption Kinetics

The different rates of dye adsorption may be evaluated by the following kinetic equations.

Lagergren Pseudo First Order Kinetics. Lagergren pseudo first order kinetic equation is given by

d[Q.sub.t]/dt = [K.sub.1]([Q.sub.e] - [Q.sub.t]) (3a)

Integrating the above Eq. 3a with boundary condition of Q = 0 at t = 0 and Q = [Q.sub.t] at t = t, the following linear Eq. 3h is obtained

ln([Q.sub.e] - [Q.sub.t]) = In[Q.sub.e] - [K.sub.1]t (3b)

The equation may also be expressed as

[Q.sub.t] = [Q.sub.e][1 - exp(-[K.sub.1]t)] (3c)

where [Q.sub.e] and [Q.sub.t] are dye adsorption (mg/g) at equilibrium time and time t (min), respectively. The linear plotting of In([Q.sub.e] - [Q.sub.t]) against t using Eq. 3b or by non-linear fitting of [Q.sub.t] against t using Eq. 3c yields the rate constant k ([min.sup.-1]) and theoretical equilibrium adsorption ([Q.sub.e]) from slope and intercept (from coefficients for non linear fittings) respectively.

Pseudo Second Order Kinetics. Pseudo second order kinetic equation for equilibrium dye adsorption as given by Ho and McKay (28), (29) is

d[Q.sub.t]/dt = [K.sub.2][([Q.sub.e] - [Q.sub.t]).sup.2] (4a)

Integrating the above equation with boundary condition of Q = 0 at t = 0 and Q = [Q.sub.t] at t= t yields

t/[Q.sub.t] = 1/[K.sub.2][[Q.sub.e].sup.2] + 1/[Q.sub.e] t (4b)

On further simplification, Eq. 4b becomes

[Q.sub.t] = [[Q.sub.e].sup.2][K.sub.2]t/1 + [K.sub.2][Q.sub.e]t (4c)

where [K.sub.2] is second order rate constant (g/mg min) for dye adsorption. The values of [Q.sub.e] and [K.sub.2] is obtained from slope and intercept of the linear trendlines of t/[Q.sub.t] against t using Eq. 4b or by non-linear fitting of [Q.sub.t] against t using Eq. 4c.

Intra Particle Diffusion Model. Intra particle diffusion model as proposed by Weber and Morris (29) was tested for the present system to understand diffusion mechanism. According to this theory

[Q.sub.t] = [K.sub.P][t.sup.1/2] + c (5)

where c is intercept, intra particle rate constant [K.sub.p](mg/g [h.sup.1/2]) is obtained from linear plotting of [Q.sub.t] vs. [t.sup.1/2]. The diffusion of dye molecules are only by intra particle diffusion if the trend lines passes through origin (i.e., c = 0). For some values of c (i.e. c [not equal to] 0), diffusion is controlled by some other mechanisms apart from intra particle diffusion. In fact, the curves following intra particle diffusion have three different stages, i.e., initial very fast surface adsorption (external mass transfer) followed by a linear intra particle diffusion and finally a plateau showing equilibrium sorption where intra particle diffusion is very slow due to low concentration of dye (solute) in solution (30).

Elovich Kinetic Model. This model assumes heterogeneous active sites of adsorbent and also different activation energies for sorption of organics like dye molecules.

It is given by the following Eq. 6a.

d[Q.sub.t]/dt = [alpha] exp(-[beta][Q.sub.t]) (6a)

Integrating the above Eq. 6a with boundary condition of [Q.sub.t] = 0 at r =0 and Q = [Q.sub.t] at t = t, the above equation becomes (31)

[Q.sub.t] = 1/[beta] In([alpha].[beta]) + 1/[beta] Int (6b)

where [alpha] is initial rate of adsorption (mg [g.sup.1] [min.sup.1]) and [beta] is desorption rate constant for this adsorption. The values of [alpha] and [beta] are obtained from the slope and intercept of linear trendlines of [Q.sub.t] against Int. (6a)

Bangham Kinetic Model. This kinetic equation is given by

[Q.sub.t] = [K.sub.t] [t.sup.1/m] (7a)

The linear form of this model is given by

In([Q.sub.t]) = In[K.sub.t] + 1/m In(t) (7b)

where [K.sub.t] is rate constant for sorption and 1/m measures the intensity of sorption. The non-linear fitting of [Q.sub.t] against t using Eq. 7a or linear plot of In([Q.sub.t]) against In(t) gives the values of rate constant [K.sub.t] and m. For Lagergren pseudo first order and Ho and Mccay pseudo second order both linear and nonlinear regression were carried out with experimental adsorption data. For intra particle, Elovich and Bangham kinetic models only linear regression was carried out since non-linear regression would give the same statistical parameters.

Adsorption Isotherms

For absorption isotherms equilibrium dye absorption values ([Q.sub.e]) at different feed dye concentrations ([C.sub.e]) were fitted to seven adsorption isotherms, i.e., two-parameter model equations like Langmuir non-linear (Eq. 8a), and linear (Eq. 8b), Dubinin--Radushkevich linear (Eq. 9a) and non-linear (Eq. 9b), Freundlich non-linear (Eq. 10), Tempkin non-linear (Eq. 11). Redlich-Peterson non-linear (Eq. 12), Sips non-linear (Eq. 3a) and linear (Eq. 13b) and Fritz--Schliinder nonlinear (Eq. 14) models (32), (33) as given below.

Langmuir Isotherm. The non-linear and linear form of this isotherm is given by Eqs. 8a and 8b, respectively.

[Q.sub.e] = [Q.sub.max][K.sub.L][C.sub.e]/1 + [K.sub.L][C.sub.e] (8a)

1/[Q.sub.e] = 1/[Q.sub.max][K.sub.L][C.sub.e] + 1/[Q.sub.max] (8b)

The characteristic of Langmuir isotherm is expressed in terms of dimensionless separation factor [R.sub.L] defined as

[R.sub.L] = 1/[K.sub.L] + [C.sub.O] (8c)

where [C.sub.O] is the maximal dye concentration. The value of [R.sub.L] indicates if the Langmuir process is unfavorable ([R.sub.L] > 1), favorable (0 < [R.sub.L] < 1), linear ([R.sub.L] = 1) or irreversible ([R.sub.L] = 0).

Dubinin-Radushkevich Isotherm. For heterogeneous surface non-linear and linear form of this isotherm is given by Eqs. 9a and 9b, respectively.

[Q.sub.e] = [Q.sub.max]exp(-[beta][[epsilon].sup.2]) (9a)

In[Q.sub.e] = In[Q.sub.max]exp - [beta][[epsilon].sup.2] (9b)

where

[epsilon] = RTIn (1 + 1/[C.sub.e]) (9c)

The constant [beta] is related to the energy of sorption E as

E = 1/[square root of ([beta])] (9d)

Freundlich Isotherm. The non-linear form of this equation is given by

[Q.sub.e] = [K.sub.F][C.sub.e.sup.1/n] (10)

where [K.sub.F] is Freundlich constant and "1/n" signifies nature of the isotherm. For linear adsorption n is unity. When the adsorption is dominated by chemical sorption, the value of n becomes less than unity. A value of n > 1 indicates physical sorption.

Tempkin Isotherm. In this model, it is assumed that heat of sorption of the molecules on the adsorbent surface reduces linearly due to adsorbate-adsorbate interaction. The non-linear form of this model is given by

[Q.sub.e] = RT/[b.sub.T] In([A.sub.T][C.sub.e]) (11)

where constant RT/[b.sub.T] = [Q.sub.max], [Q.sub.max] is maximum adsorption capacity, R is universal gas constant (8.314 J [mol.sup.-1] [K.sup.-1]), T is absolute temperature (298 k). AT is TI constant (L/mg) signifying maximum binding energy.

Redlich-Peterson Isotherm. The non-linear form of this three-parameter model equation is given by Eq. 12,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (12)

Here [K.sub.RP] (L/mg) and ARP (L/g) are Redlich-Peterson isotherm constants. The value of [[beta].sub.RP] lies between 0 and 1. For [beta] = 1, the Redlich-Peterson isotherm becomes identical with Langmuir isotherm while for [beta] = 0 the R-PI becomes Henry's law form.

Sips Isotherm Model. Like Redlich-Peterson isotherm this model also combines Langmuir and Freundlich model in one equation. The non-linear and linear form of this model is given by Eqs. 13a and 13b

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13a)

- In([K.sub.s]/[Q.sub.e]) = [[beta].sub.s]In([C.sub.e]) - In([A.sub.s]) (13b)

where [K.sub.s] and [A.sub.s] are Sips constant. This model is applicable for adsorbent with heterogeneous surfaces. At low concentration of dye, it becomes Freundlich isotherm while at higher concentration it shows a mono layer adsorption similar to Langmuir isotherm.

Fritz--Schlunder Model. Most of the above model equations are combined in the following generalized five parameter Fritz--Schlunder model Eq. 14

[Q.sub.e] = [A.sub.FS][C.sub.e.sup.[alpha]]/c + [B.sub.FS][C.sub.e.sup.[beta]] (14)

In most of the cases, c = 1 and the above model reduces to

[Q.sub.e] = [A.sub.FS][C.sub.e.sup.[alpha]]/1 + [B.sub.FS][C.sub.e.sup.[beta]] (15)

where [A.sub.FS] and [B.sub.FS] are Fritz--Schlunder constants, while [alpha] and [beta] are equation exponent. This model equation is reduced to Sip model when [alpha] = [beta] and c = 1, Redlich-Peterson model equation when [alpha] = c = 1, Langmuir model when [alpha]= c = [beta] = 1 and Freundlich model when c = 0.

Data Fitting to Model Equations. In most of the reported works, linear regression is widely used for fitting experimental data to linearized form of various model equations. In linear regression a Gaussian distribution is assumed for the trend lines where error distribution is same for each experimental value. However, after linearization of a model equation the error distribution gets altered. In non-linear fitting experimental data are directly fitted to the model equations and regression analysis is carried out by adjusting parameter values through interaction till convergence. For the present system, non-linear regression was carried out for all of the five kinetic models and seven adsorption isotherms by directly fitting the experimental [Q.sub.t], t, [Q.sub.e], and [C.sub.e] values to the non-linear model equations since it gives better fitting of experimental data (33), (34). For comparison of these two types of regression, both linear and non-linear fitting was carried out for basic fuchsin dye in the low concentration range for pseudo first order and pseudo second order kinetics. For adsorption isotherms, both linear and non-linear fittings were applied for Langmuir, Dubinin--Radushkevich, and Sips adsorption isotherms. Both of these linear and non-linear fittings were carried out by Origin-8 software. The non-linear fitting with this software is based on Levenberg-Marquardt (L-M) algorithm where parameter values of a model are adjusted in an iterative process using chi-square ([x.sub.2]). The validity of the kinetic and adsorption isotherm models were evaluated in terms of regression coefficient ([R.sub.2]), non-linear [x.sub.2] and F values (obtained from Anova analysis in Origin). For a good fitting, [R.sub.2] should be close to unity, [x.sub.2] should be low while F value should be high (35). The various kinetic and model parameters along with regression coefficient, chi-square, and F values are shown in Tables 2 and 3 for adsorption of low and high concentration of basic fuchsin dye by the four hydrogels. From Table 2, it is observed that basic fuchsin dye shows good fitting for all of the kinetic models in high concentration range as evident from respective [R.sub.2], [x.sub.2], and F values. In the low concentration range, it also shows good fitting except intra particle diffusion model. Methyl violet dye was also observed to show similar fitting (not shown). From Table 3. it is observed that basic fuchsin dye also shows good fitting to all of the two-, three-, and four-parameter adsorption models. Similar kind of fitting was also observed for methyl violet dye. Favorable Langmuir adsorption is quite evident from [R.sub.L] values as shown for Langmuir parameters in Table 3. Similarly, all the hydrogels show chemical adsorption for low concentration of dye and physical adsorption for high concentration dye as observed from n values of Freundlich isotherm for basic fuchsin dye given in Table 3. Methyl violet dye was also observed to show similar trend (not shown). Linear and non-linear regression for two kinetic models and three adsorption models are shown in Table 4 for low concentration of basic fuchsin dye. These linear (shown in inset) and non-linear fittings are also shown in Fig. 9a and b (pseudo first and second order), Fig. 10a and b (Langmuir and Dubinin--Radushkevich) and Fig. 11a (Sip isotherm). From these figures and Table 4, it is observed that non-linear regression gives better fitting of the experimental dye sorption data to the various models used in this study. Accordingly, values of [R.sub.2] for fitting to pseudo first order are observed to be less than 0.8 for all the hydrogels while non-linear fitting give [R.sub.2] > 0.98 with the same experimental data. Similar results are observed to the other models as seen in Table 3. Among the entire adsorption models four parameter Fritz--Schkin-der model is observed to show the best fitting in terms of statistical parameters as observed in Table 3 and Fig. 11b. This model is very flexible to incorporate all of the other models at various process conditions (34) which may be responsible for its good fitting. In Fig. 12a and b, predicted [Q.sub.e] based on various models are plotted against experimental [Q.sub.e] for basic fuchsin and methyl violet dyes at low (Fig. 12a) and high (Fig. 12b) concentration range with IPN2 hydrogel. Similar results were obtained with the other three hydrogels. From these figures, it is observed that both of these dyes show very good fitting at high concentration range. At low concentration range, methyl violet dye is observed to show better fitting than basic fuchsin dye.

TABLE 2.Kinetic parameters of the hydrogels for low (2.5--20 mg/L)
and high concentration (100-1000 mg/L) range of BF dye.

                           PAMHEMA             IPN1                IPN2

Model                     Low/high         Low/high            Low/high

Pseudo fist order

[Q.sub.eth](mg/g)         1.99/293         2.72/340        3.067/374.48

[K.sub.1]            0.0026/0.0221    0.0026/0.0227      0.00279/-0.023
[min.sup.-1]

[R.sup.2]            0.9977/0.9963    0.9888/0.9965       0.9947/0.9953

[x.sup.2]             0.0016/33.48     0.0147/42.42        0.0088/67.52

F value                 9737/10150       2005/10897           4301/8340

Pseudo second
order

[Q.sub.eth](mg/g)         .236/368      3.05/425.09         .449/466.07

[K.sub.2](g/mg     0.0014/5.98E-05    0.0010/5.4E05    0.00097/5.02E-05
min)

[R.sup.2]            0.9812/0.9898    0.9829/0.9909     0.99023/0.99116

[x.sup.2]             0.0136/91.62       0.0225/108          0.0163/127

F value                  1160/3705        1308/4251           2328/4404
                   1160.61/3705.87  1308.26/4251.88  2328.50128/4404.85

Intra particle

[K.sub.p](mg/g         0.023/21.99     0.034/25.392        0.038/27.853
[min.sup.1/2])

c                     0.456/18.063     0.631/23.594        0.728/27.817

[R.sup.2]            0.6615/0.9257    0.6903/0.9251       0.6984/0.9257

([x.sup.2])             0.2191/669       0.2191/900         0.3684/1073

F value                  65.83/502        65.83/507           74/518.71

elovich

[alpha]               0.0183/16.17     0.0257/19.43       0.02997/21.91

[beta]                2.479/0.0118     1.82/0.01029        1.620/0.0094

[R.sup.2]            0.9226/0.9867    0.9345/0.9875       0.9426/0.9877

([x.sup.2])             0.0558/119       0.0866/149         0.09561/178

F value                   277/2840         335/3085            391/3159

Bangham

[K.sub.b]            0.05751/37.77     0.0869/45.58         0.103/51.33

m                       2.23/2.487       2.30/2.532          2.33/2.560

[R.sup.2]            0.9026/0.9432    0.9086/0.9449       0.9086/0.9469

([x.sup.2])              0.079/512        0.069/662          0.0285/768

F value                  60.96/658        73.32/692           83.55/727

                               IPN3

Model                      Low/high

Pseudo fist order

[Q.sub.eth](mg/g)      2.985/364.93

[K.sub.1]           0.0027/--0.0235
[min.sup.-1]

[R.sup.2]             0.9919/0.9950

[x.sup.2]              0.0128/68.45

F value                   2802/7899

Pseudo second
order

[Q.sub.eth](mg/g)       3.36/452.21

[K.sub.2](g/mg     0.00098/5.34E-05
min)

[R.sup.2]             0.9908/0.9889

[x.sup.2]                0.0163/127

F value                   2488/3632

Intra particle

[K.sub.p](mg/g        0.0372/27.094
[min.sup.1/2])

c                     0.70101/28.82

[R.sup.2]             0.7109/0.9167

([x.sup.2])             0.4532/1151

F value                   77.39/463

elovich

[alpha]                0.0291/21.69

[beta]                 1.662/0.0096

[R.sup.2]             0.9472/0.9845

([x.sup.2])              0.0832/214

F value                    426/2516

Bangham

[K.sub.b]               0.102/51.53

m                        2.34/2.596

[R.sup.2]             0.9122/0.9402

([x.sup.2])               0.064/826

F value                   85.26/647

TABLE 3. Adsorption parameters of the hydrogels for adsorption
of low (2.5-20 mg/L) and high concentration (l00-1000 mg/L)
range of BF dye.

                                         PAMHEMA             IPNl

Model                                   Low/High         Low/High

Langmuir

[K.sub.L] (L/mg)                   0.172/0.00120    0.247/0.00124

[Q.sub.max] (mg/g)                      5.08/705         5.27/847

[R.sub.L]                            0.225/0.454      0.168/0.446

[R.sup.2]                          0.9852/0.9571    0.9853/0.9597

[x.sup.2]                              0.025/776      0.0303/1018

F value                                 1639/367         1797/389

Freundlich

n                                     0.087/1.59      0.4143/1.57

[K.sub.F] (L/mg)                       2.23/5.24        2.66/5.94

[R.sup.2]                            0.941/0.938    0.9088/0.9421

[x.sup.2]                            0.0061/1114      0.0068/1463

F value                                  747/254          695/269

Temkin

a                                       1.17/143        1.152/169

b                                       14/0.014       530/0.0139

[R.sup.2]                          0.9467/0.9443    0.9210/0.9459

[x.sup.2]                            0.0376/1009       0.055/1367

F value                                  978/281          974/288

DR

[Q.sub.max] (mg/g)                      4.07/384         4.57/454

[beta] ([mol.sup.2]/[kJ.sup.2])  -5.5E-06/0.0101   -4.6E-06/0.010

[R.sup.2]                          0.9790/0.8309    0.9852/0.8327

[x.sup.2]                             0.036/2200       0.033/3046

F value                                  919/127         1404/128

Sip

[K.sup.S] (L/mg)                     0.736/0.094      0.953/0.122

[beta]                                 1.15/1.41         1.29/1.4

[A.sup.S]                         0.157/1.91E-04   0.199/2.05E-04

[R.sup.2]                          0.9836/0.9579   0. 9856/0.9601

[X.sup.2] [S.sup.q]                   0.0274/763       0.029/1007

F value                                  988/262         1226/249

RP

[K.sup.RP] (L/mg)                    0.005/0.062     0.0162/0.072

[A.sub.RP]                        -1.0007/-1.131    -1.005/-1.133

B                                 -0.008/-0.0419  -0.0243/-0.0410

[R.sup.2]                          0.8897/0.9068    0.8293/0.9126

[x.sup.2]                            0.1856/1688      0.3526/2208

F value                                  144/111          101/118

FS, [A.sub.FS]                     34283/5.7E+09    4266/2.69E+09

A                                    -2.62/-2.27      -1.95/-2.14

[B.sub.FS]                        39459/1.11E+10    3683/4.37E+09

B                                    -3.22/-3.31      -2.52/-3.16

[R.sup.2]                          0.9932/0.9755    0.9942/0.9752

[x.sup.2]                             0.0113/442       0.0119/626
F value                                 1798/323         2293/317

                                            IPN2             IPN3

Model                                   Low/High         Low/High

Langmuir

[K.sub.L] (L/mg)                   0.248/0.00127    0.241/0.00123

[Q.sub.max] (mg/g)                      5.96/920        5.842/899

[R.sub.L]                            0.167/0.440      0.171/0.448

[R.sup.2]                          0.9852/0.9597    0.9853/0.9592

[x.sup.2]                             0.039/1204       0.037/1148

F value                                 1790/388         1772/387

Freundlich

n                                   0.4143/1.573       0.538/1.57

[K.sub.F] (L/mg)                       2.67/6.43        2.66/6.28

[R.sup.2]                          0.9083/0.9420    0.9216/0.9419

[x.sup.2]                            0.0069/1736      0.0058/1655

F value                                  692/268          760/268

Temkin

a                                      1.303/184        1.278/180

b                                    29.14/0.013      34.14/0.014

[R.sup.2]                          0.9205/0.9463    0.9272/0.9463

[x.sup.2]                            0.0716/1606      0.0716/1606

F value                                 1043/290         1080/290

DR

[Q.sub.max] (mg/g)                      5.17/494         5.03/483

[beta] ([mol.sup.2]/[kJ.sup.2])  4.6E-06/-0.0103  4.6E-06/-0.0102

[R.sup.2]                          0.9851/0.8346    0.9817/0.8346

[x.sup.2]                            0.0419/3046       0.041/3404

F value                                 1399/129         1134/129

Sip

[K.sup.S] (L/mg)                      1.07/0.126       1.15/0.123

[beta]                                 1.30/1.40        1.19/1.40

[A.sup.S]                          0.199/1.9E-04   0.2129/1.9E-04

[R.sup.2]                          0.9856/0.9605    0.9838/0.9604

[X.sup.2] [S.sup.q]                  0.0380/1183      0.0405/1128

F value                                 1223/264         1087/264

RP

[K.sup.RP] (L/mg)                   0.0185/0.077      0.016/0.076

[A.sub.RP]                        -1.0053/-1.132    -1.004/-1.131

B                                  -0.024/0.0407  -0.0215/-0.0408

[R.sup.2]                            0.8287/9124    0.8438/0.9128

[x.sup.2]                            0.4532/2620      0.3923/2499

F value                                  100/118          110/119

FS, [A.sub.FS]                     4652/2.88E+09    18127/2.8E+09

A                                    -1.94/-2.13      -2.35/-2.13

[B.sub.FS]                          3551/4.4E+09    13683/4.3E+09

B                                    -2.51/-3.16      -2.88/-3.16

[R.sup.2]                          0.9942/0.9755    0.9932/0.9755

[x.sup.2]                             0.0153/731       0.0171/697

F value                                 2279/321         1934/321

DR: Dubinin-Radushkevich; R P: Redlich-Peterson; FS: Fritz-Schlunder.

TABLE 4. Comparison of linear and non-linear regression for
adsorption of low concentration of BF dye.

                                      PAMHEMA              IPNl

Model                       Linear/Non-linear  Linear/Non-linear

Pseudo first order

[Q.sub.eth] (mg/g)                0.603/1.994         1.138/2.72

[k.sub.1] (L/mg)                0.0011/0.0026      0.0014/0.0023

[R.sub.2]                       0.6351/0.9977      0.7304/0.9888

F value                            21.73/9737         24.82/1089

Pseudo second order

[Q.sub.eth] (mg/g)                2.101/2.236         2.923/3.05

[R.sub.2] (g/mg.min)            0.0018/0.0014     0.0013/0,00106

[k.sub.2] (L/mg)                0.9933/0.9812      0.9947/0.9829

F value                             2696/1160          3462/1308

[Q.sub.eth] (mg/g) for Low              1.988              2.772
concentration BF

Langmuir

[K.sub.L] (L/mg)                  0.183/0.172        0.233/0.247

[Q.sub.max] (mg/g)                 4.934/5.08         5.346/5.27

[R.sup.2]                       0.9699/0.9852      0.9808/0.9853

F value                             1064/1639          2212/1797

DR

[Q.sub.m] (mg/g)                    3.57/4.07          4.12/4.57

[beta]                        1.3E-06/5.5E-06   1.12E-06/4.6E-06
([mol.sup.2]/[kJ.sup.2]

[R.sup.2]                       0.8302/0.9790      0.8881/0.9852

F value                             35.23/919         56.55/1404

Sip

[K.sub.S] (L/mg)                   1.01/0.736         1.03/0.953

B                                  0.446/1.15         0.374/1.29

[A.sup.S]                         1.026/0.157        0.760/0.199

[R.sup.2]                       0.9414/0.9836      0.9088/0.9856

F value                               682/988           758/1226

                                         1PN2               IPN3

Model                        Linear/Nonlinear  Linear/Non-linear

Pseudo first order

[Q.sub.eth] (mg/g)              1.498/3.06716        1.606/2.985

[k.sub.1] (L/mg)               9.6E-4/0.00279     8.8E-4/0.00277

[R.sub.2]                      0.8007/0.99533     0.7557/0.99186

F value                         31.67/4301.05      23.35/2802.24

Pseudo second order

[Q.sub.eth] (mg/g)                3.309/3.449       3.235/3.3606

[R.sub.2] (g/mg.min)           0.0011/0.00097     0.0012/0.00098

[k.sub.2] (L/mg)                0.9968/0.9902      0.9966/0.9908

F value                             5677/2328          5564/2488

[Q.sub.eth] (mg/g) for Low              3.151              3.108
concentration BF

Langmuir

[K.sub.L] (L/mg)                  0.234/0.248        0.240/0.248

[Q.sub.max] (mg/g)                 6.049/5.96         5.830/5.96

[R.sup.2]                       0.9807/0.9852      0.9803/0.9852

F value                             2203/1790          2224/1790

DR

[Q.sub.m] (mg/g)                    4.66/5.17          4.52/5.03

[beta]                       1.11E-06/4.6E-06   1.09E-06/4.6E-06
([mol.sup.2]/[kJ.sup.2]

[R.sup.2]                       0.8883/0.9851      0.8697/0.9817

F value                            56.70/1399         47.75/1134

Sip

[K.sub.S] (L/mg)                    1.02/1.07          1.05/1.15

B                                  0.376/1.30         0.371/1.19

[A.sup.S]                         0.765/0.199       0.761/0.2129

[R.sup.2]                       0.9083/0.9856      0.9216/0.9838

F value                              746/1223           890/1087


Regeneration and Reusability of the Hydrogels

For regeneration of the hydrogels, desorption experiments similar to adsorption experiments were carried out with dye loaded hydrogels at varied pH. No significant desorption was observed at pH 7, while desorption was maximum (up to 98.7%) at pH 3 indicating strong electrostatic interactions between dyes and hydrogels (36). The regenerated hydrogel was used for five numbers of repeated adsorption /desorption cycles without any significant change of adsorption% indicating efficient regeneration and reusability of the hydrogels.

Comparison With Reported Work

The dye adsorption and removal% for basic fuchsin and methyl violet dye are compared with reported works with different hydrogels in Table 5. From the data given in Table 5, it is observed that adsorption capacity of different reported hydrogels (mg/g of hydrogels) varies with feed concentration range (40-1000 mg/L) of dye. Thus, poly(HEMA--g--GMA) was reported (3) to show adsorption of 121.5 mg methyl violet dye/g of hydrogel for feed concentration of 700 mg/L dye while poly(AM-co-AA) hydrogel shows adsorption of only 6.38 mg methyl violet dye/g of the gel for feed concentration of 50 mg/L dye (41). This kind of different dye adsorption for widely varied feed concentration range was also obtained with other reported works as shown in Table 5. In this work, very low (2.5-20 mg/L) and very high range (100-1000 mg/L) of feed dye concentration was used. For the high range of feed concentration, the present hydrogel is observed to show much higher adsorption and removal% than most of the reported work with similar feed dye concentration. The low range of concentration also shows high adsorption and removal% though adsorption by other hydrogels with similar feed dye concentration is yet to be reported.

TABLE 5. Comparison of present work with reported data.

Name of hydrogel        Dye used in             Adsorption  Reference
                        water,            Performance mg/g
                        concentration,               resin
                        pH

PolylHEMA-g-GMA         700 mg/L of MV,              121.5        [3]
                        pH 5,
                        [Q.sub.max] =
                        0.189

                        700 mg/L of BF,               68.7
                        pH 5

                        [Q.sub.max] =
                        0029

Jute stick              50 mg/L of                4.6 mg/g        [5]
                        Rhodamin B at pH
                        7

Poly                    200-1000 mg/L of      917 for 1000       [18]
(AA-co-AM)/attapulgite  MV at pH 7               mg/L feed

Soya ash                pH 9, at 25.9                4.209       [38]
                        mg/L

                        [Q.sub.m] =
                        5.76

Composite Poly          40 mg/L of                     4.1       [39]
(AA-co-VP)              Crystal Violet
                        at pH 7

Supramolecular and      1000 mg/L of         Removals 95.1       [40]
                        methyl violate       and 95.7% for
                        at pH 7             supramolecular

composite gel of                            gel and hybrid
agarose                                               gel.
                                             respectively.

Poly (AM-co-AA)         50 mg/L of                    6.38       [41]
                        methyl violet at
                        pH7

Poly (VP-ff-MA)         500 mg/L of                   4.22       [42]
                        methyl violet at
                        pH 7

IPN2                    For 2.5 mg/L         2.249 for BF,  This work
                        feed dye               [Q.sub.m] =
                        ([C.sub.e]) at                5.96
                        pH 7,
                        Temperature
                        25[degrees]C

                                             1.723 for MV,
                                               [Q.sub.m] =
                                                      3.93

IPN2                    For 500 mg/L        368.70 for BF,  This work
                        feed dye               [Q.sub.m] =
                        ([C.sub.e]) at                 920
                        pH 7,
                        Temperature
                        25[degrees]C

                                            283.76 for MV,
                                               [Q.sub.m] =
                                                     613.8


CONCLUSION

IPN type hydrogels were synthesized from PAM-HEMA copolymer and CMC by free radical polymerizetion. The effect of dosage of hydrogel in water, solution pH, initial feed dye concentration, and contact time on dye adsorption and removal% for both low and high range of two industrially important textile dyes, i.e., basic fuchsin and methyl violet dye was studied. Mechanical properties, equilibrium swelling, dye adsorption, and removal% were observed to increase with increase in amount of CMC in the hydrogel. The experimental dye adsorption data were fitted to five kinetic models and seven adsorption isotherm models by non-linear analysis. The adsorption data of these hydrogels were found to show best fitting for Fritz--Schlunder model. Non-linear fitting was also compared with linear fitting using same experimental data. Non-linear fittings were found to be better than linear fitting in terms of values of various statistical parameters.

ACKNOWLEDGMENTS

The authors thank Council of Scientific and Industrial Research (CSIR-EMR-22(0547)/11/EMR-II) and Department of Science and Technology (DST-SERC-SR/S3/CE/056/2009), Government of India for supporting the work.

Abbreviations

AM Acrylamide

CMC Carboxy methyl cellulose

EAB Elongation at break

FTIR Fourier transforms infrared

HEMA Hydroxyethyl methacrylate

IPN Interpenetrating network

NMBA N, N'-methylene bisacrylamide

SEM Scanning electron microscopy

TS Tensile strength

XRD X-ray diffraction

Correspondence to: Samit Kumar Ray: e-mail: samitcu2@yahoo.co.in

DOI 10.1002/pen.23501

Published online in Wiley Online Library (wileyonlinelibrary.com).

@ 2013 Society of Plastics Engineers

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Ruma Bhattacharyya, Samit Kumar Ray

Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
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Author:Bhattacharyya, Ruma; Ray, Samit Kumar
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Date:Nov 1, 2013
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