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Adsorption of Phenazine Dyes Using Poly(hydroxamic acid) Hydrogels from Aqueous Solutions.

INTRODUCTION

Hydrogels are a unique class of macromolecular networks that may contain a large fraction of aqueous solvent within their structure. The hydrophilicity of the network is due to the presence of chemical residues such as hydroxylic (--OH), carboxylic (--COOH), amidic (--CONH--), primary amidic (--CON[H.sub.2]), sulfonyl (--S[O.sub.3]H), and others that can be found within the polymer backbone or as lateral chains. They are three-dimensional crosslinked polymeric structures that are able to swell in the aqueous environment. Crosslinked polymers capable of imbibing large volumes of water have found widespread applications in bioengineering, biomedicine and food industry, and water purification and separation process [1-5].

Recently, it was determined that crosslinked polymeric materials having functional groups could be used as complexing agents for removal of dyes or other pollutants from aqueous solutions [6-10], Polyacrylamide (PAAm) based hydrogels find many applications such as purification of wastewater and metal extraction [11, 12].

The hydroxamic acid group (--NH --OH) is well known for its ability to form a stable chelates with various heavy metal ions. A number of chelating polymers, containing hydroxamic acid group, have been prepared from various starting materials using different methods. Several workers attempted to the prepare poly(hydroxamic acid) (PHA) resins from several polymers [13-16].

In our previous study, we were reported that PHA hydrogels were sorbed heavy metal ions and dyes [17].

This article is aimed to study a convenient method for removing some water soluble phenazine dyes from aqueous solutions by adsorption on the polymeric adsorbent such as crosslinked PHA hydrogels prepared from PAAm hydrogels. Watersoluble phenazine dyes such as Neutral Red, Safranin T, and Janus Green were selected for this investigation. Phenazine dyes are the derivatives of pyrazine. Phenazine also called azophenylene, dibenzo-p-diazine, dibenzopyrazine, and acridizine, is a dibenzo annulated pyrazine and the parent substance of many dyestuffs, such as the toluylene red, indulines, and safranines. Phenazine dyes are polar, readily water-soluble compounds, and weak bases. They are used for dyeing tannin, cotton, bast fibers, wool, silk, leather, and paper [18]. Also, phenazine dye molecules, having redox property as well as light sensitivity, have been used in comprehensive solar cell as a redox couple [19].

MATERIALS AND METHODS

Materials

Adsorbents used in this study are PHA hydrogels. PHA hydrogels were prepared from PAAm containing a crosslinker such as N, N' methylenebisacrylamide or ethylene glycol dimethacrylate by the modification with hydroxylamine hydrochloride. Preparation and characterization of PHA hydrogels were reported in our previous study [17]. The samples were taken the abbreviations, PHA-NNMBA'and PHA-EGDMA, for crosslinker N, N' methylenebisacrylamide and ethylene glycol dimethacrylate, respectively.

Phenazine dyes; Neutral Red, Safranin T, and Janus Green were obtained from Merck (Darmstadt, Germany). The dyes were analytical grade and were used without further purification. Distilled water was used for all the experiments. Some properties of these dyes are listed in Table 1.

Phenazine dye stock solutions of 250 mg [L.sup.-1] were prepared in double distilled water and the experimental solutions of the desired concentration were obtained by successive dilutions.

Swelling

To measure the parameters of swelling and diffusion, PHA hydrogels was accurately weighed, and transferred into aqueous phenazine dyes solutions of 20 mg [L.sup.-1] in a beaker. Solution uptake with respect to time was obtained by periodically removing a sample from the solution quickly blot drying, and reweighing. The measurements were conducted at 25 [+ or -] 0.1[degrees]C in a water bath.

The swelling (S) of PHA hydrogel in the aqueous phenazine dye solutions of 20 mg [L.sup.-1] were calculated from the following relation.

S = [[[m.sub.t] - [m.sub.o]]/[m.sub.o]] x 100 (1)

where [m.sub.t] is the mass of the swollen gel at time t and [m.sub.o] is the mass of the dry gel at time 0.

Sorption

The synthetic aqueous solutions of the phenazine dyes were prepared in the concentration ranges; 8-80 mg [L.sup.-1]. 100 mg of dry gel were transferred into 50 mL of the aqueous solutions of the phenazine dyes, and allowed the equilibrate for 24 h at 25[degrees]C. These aqueous phenazine dye solutions were separated by decantation from the hydrogels. Spectrophotometric measurements were carried out using a Shimadzu 160 model UV-Vis spectrophotometer at ambient temperature. The absorbancies of these solutions were read at the wavelengths given in Table 1. The equilibrium concentrations of the phenazine dyes solutions were determined by means of precalibrated scales.

Removal Efficiency and Partition Coefficient

For the influence of crosslinker type to the removal efficiency and partition coefficient, 100 mg of the hydrogels were put into 50 mL of concentration of 80 mg [L.sup.-1] dye solutions and left for 48 h at 25[degrees]C. Spectrophotometric methods were used to follow the concentrations of these dyes solutions.

RESULTS AND DISCUSSION

PHA hydrogels, obtained from the crosslinked poly(acrylamide) hydrogels, have been prepared and their heavy metal ion and dye binding properties were investigated [17]. Here, in this study, the influence of hydrogel ionogenity and hydrophilicity on the swelling behavior, adsorption and decolorization of the phenazine dyes such as Neutral Red, Safranin T, and Janus Green were investigated, too.

Swelling

The swelling of the PHA hydrogels occurs because of the osmotic pressure difference caused by the presence of the ionic repeat units in the three-dimensional crosslinked network [20]. Phenazine dye molecules in aqueous solutions intake of initially dry hydrogels were followed for a period of time, gravimetrically. Swelling isotherms were constructed and the curves are shown in Fig. 1.

Figure 1 shows the variation in the swelling capacity of the PHA hydrogels with time. All the PHA hydrogels absorbed the fluids and swelled at a higher rate in the beginning. After a certain period of time, the fluid uptake became constant, and the PHA hydrogels achieved their equilibrium swelling capacity ([S.sub.eq]). At equilibrium, the polymer chains attained the elongated configurations, and an elastic retractive force developed preventing, further expansion of the network. [S.sub.eq] of PHA hydrogels is given Table 2.

Table 2 shows that the values of [S.sub.eq] are changed 2.16-2.45 g [g.sup.-1] for PHA-NNMBA and 23.21-33.25 g [g.sup.-1] for PHAEGDMA.

Hydrophilicity of PHA-EGDMA copolymers becomes greater than that of PHA-NNMBA, so, the swelling of PHA-EGDMA is greater than the swelling of PHA-NNMBA. The more hydrophilic groups in the PHA-EGDMA get the more the swelling of the PHA-NNMBA.

These results show that the PHA hydrogel rapidly swollen, and the PHA hydrogels containing EGDMA swollen about 10-14 times more than those PHA hydrogels containing NNMBA Moreover, while Safranin T swelled PHA-EGDMA hydrogel 1.33-1.43 times more than other dyes, it swelled PHA-NNMBA hydrogel 1.02-1.13 times more than the others.

The values of [S.sub.eq] of PHA hydrogels are different in the same family dyes. The reason of this difference is the hydrophilic character of crosslinker and dye molecules. Dye molecule has got more hydrophilic sites, as nitrogen atoms. When, dye molecules have interacted with much water, so, there has been much swelling than swelling values in water, also. That is way, dye molecules have got hydrophilic groups, more swelling values have been observed when the hydrogels swollen in aqueous dye solutions. Here main characteristic effect is the hydrophilic character of dye molecules. So, the more hydrophilic groups in the aqueous dye solutions get the more the swelling of the PHA hydrogels. In the presence of dye, swelling of PHA hydrogels can easily follow the change of the hydrogen-bonded structure of water and polymer-solvent interaction.

Additionally, Fig. 1 represents the dynamic swelling behavior of the PHA hydrogels. Initially, the rate of the penetrant uptake sharply increases and then it begins to level off. The equilibrium swelling of PHA hydrogels was achieved after 80 min. A power law behavior is obvious from Fig. 1. The data may be well fitted with a Voigt-based equation [21, 22]:

S = p (1 - [e.sup.-t/[tau]]) (2)

where S (g [g.sup.-1]) is swelling at time t, p is equilibrium swelling (power parameter, g [g.sup.-1]), t is time (min) for swelling, and [tau] (min) stand for the "rate parameter." The value of rate parameter is a measure of resistance to water penetration. If the value of power parameter is high, the swelling is too high, if the value of rate parameter is small, it indicates that the swelling is fast.

The ratio of power parameter to the rate parameter gives the swelling rate (SR, g [g.sup.-1] [min.sup.-1]) at time [tau], while the inverse of the rate parameter gives the rate constant ([k.sub.v], [min.sup.-1]). The rate parameter value is a measure of the SR (i.e., the lower the [tau] value, the higher the rate of swelling) [23]. To calculate the parameters, nonlinear regression was applied to Eq. 2.

Power parameter, rate parameter, swelling rate, and swelling rate constant of PHA hydrogels are given in Table 2.

The rate parameters are found to be 34, 43, and 44 min for the PHA hydrogels with crosslinker NNMBA, and 27, 29, and 41 min for the PHA hydrogels with crosslinker EGDMA, respectively (Table 2). It means that according to the model (Eq. 2), these samples absorb approximately 63% of their maximum absorption capacity during 21-44 min. These results show that the PHA hydrogel rapidly swollen, and the PHA hydrogels containing EGDMA swollen about 10-14 times more than those PHA hydrogels containing NNMBA. The rate parameters were used to find the type of transport mechanism and diffusion constant. Conversely, the SR and rate constant ([k.sub.v]) values of the PHA gels varied randomly. The results may be due to the anomalous swelling behavior affected by crosslinker type and dye nature.

Diffusion

Analysis of the mechanisms of fluid diffusion into swellable polymeric systems has received considerable attention in recent years, because of important applications of swellable polymers in environmental, biomedical, pharmaceutical, and agricultural engineering.

The following equation is used to determine the nature of diffusion of penetrate into hydrogels.

F = k [t.sup.n] (3)

where F is the fractional uptake at time t, k is a constant incorporating characteristic of the macromolecular network system and the penetrate, and n is the diffusion exponent, which is indicative of the transport mechanism. Equation 3 is valid for the first 60% of the fractional uptake. Fickian diffusion and Case 11 transport are defined by n values of 0.5 and 1.0, respectively. Anomalous transport behavior (non-Fickian diffusion) is intermediate between Fickian and Case II. That are reflected by n between 1/2 and 1 [24].

For PHA hydrogels, F versus t graphs are plotted and representative results are shown in Fig. 2 for Safranin T solution. Diffusion exponents (n) and diffusion constants (k) are calculated from nonlinear regression of the curves from the experimental data, and are listed in Table 3.

Table 3 shows that the number determining the type of diffusion (n) is over 0.50. They are between 0.59 and 0.77. Hence the diffusion of dye solutions into the PHA hydrogels is found to have a non-Fickian character [24, 25]. When the diffusion type is anomalous behavior, the relaxation and diffusion time are of the same order of magnitude. As solvent diffuses into the hydrogel, rearrangement of chains does not occur immediately. Diffusion constants of PHA hydrogels are between 4.05 x [10.sup.-2] [min.sup.-n] and 7.73 x [10.sup.-2] [min.sup.-n]. There is no good relation between the values of diffusion constants of PHA hydrogels.

Swelling Kinetics

To examine the controlling mechanism of the swelling processes, several kinetic models are used to test experimental data. The large number and array of different chemical groups on the polymeric chains (e.g., amine, amide, carbonyl, carboxyl, or hydroxyl) imply that there are many types of polymer-solvent interactions. It is probable that any kinetics is likely to be global. From a system design viewpoint, a lumped analysis of SRs is thus sufficient to the practical operation.

A simple kinetic analysis is a second order equation in the form of

d S/d t = [k.sub.2,S] [([S.sub.eq] - S).sup.2] (4)

where [k.sub.2,S] is the rate constant of swelling [S.sub.eq] denotes the swelling percent at equilibrium. After definite integration by applying the initial conditions S = 0 at t = 0 and S = S at t = t, Eq. 4 becomes

t/S = A + B t (5)

where A is reciprocal of initial swelling rate [r.sub.o] or 1/[k.sub.2,S][S.sup.2.sub.eq] and B is inverse of the degree of swelling at equilibrium [26].

To test the kinetics model, t/S versus t graphs are plotted and representative graphs are illustrated in Fig. 3 for Janus Green. The calculated kinetic parameters are tabulated in Table 4.

As can be seen from Table 4, kinetics model is agreement with swelling experiments, since, as depicted in Table 2, [S.sub.eq] (theoretical) is changed only with crosslinker type.

Plotting the swelling data as functions of the square roots of time provides valuable information for distinguishing between Fickian and Case II Transport mechanisms because the Fickian diffusion curve exhibits a monotonie inflection-free approach to equilibrium, whereas the Case II curves are clearly sigmoidal [27]. The equation can be expressed as follows:

S = [k.sub.i][square root of t]+C (6)

where [k.sub.i] is the rate constant.

S versus [t.sup.1/2] graphs are plotted and representative graphs are illustrated in Fig. 4 for Neutral Red.

At first look, the curves in Fig. 4 appear to be sigmoidal type, thus indicating that sorption mechanism is more inclined toward Case-II type rather than Fickian one. As shown in Fig. 4, there are two stages to change the hydrogel state during gel swelling after an initial lag phase. The times for initial lag phase and first stage of swelling of PHA hydrogels are approximately between 0 and 3 min, 3 and 72 min, respectively. The time for second stages is equilibrium time of swelling (t > 72 min). It can be appreciated that Fickian behavior is obeyed up to approximately 75 min of swelling, although for longer times the behavior deviates from the linearity. The same pattern was obtained for all experiments. The [k.sub.i] values are calculated from the slopes of plots and tabulated in Table 5.

The [k.sub.i] values of the PHA hydrogels with crosslinker NNMBA were very low than for these values of PHA hydrogels with cross-linker EGDMA. As mentioned earlier, these results show that the PHA hydrogel containing EGDMA rapidly swollen more than the PHA hydrogel containing NNMBA. [k.sub.i] values of the PHA hydrogels were affected by crosslinker type more than dye type.

The diffusion coefficients of the cylindrical PHA hydrogels were calculated from the following relations [27]:

D = [([k.sub.D]/4).sup.2] [pi] [r.sup.2] (7)

where D in [cm.sup.2] [s.sup.-1], t in second, and r is the radius of cylindrical polymer sample. Diffusional rate constant ([k.sub.D]) was calculated by dividing [k.sub.i] by [S.sub.eq] to work on the reduces swelling values.

Table 5 shows that the values of the diffusion coefficient of the hydrogels vary from 1.32 x [10.sup.-6] to 44.71 x [10.sup.-6] [cm.sup.2] [s.sup.-1]. In the case of cylindrical hydrogels, the diffusion coefficients are normally obtained in the order of [10.sup.-7] but in the present case, the relatively higher diffusion coefficients are indication of faster penetration of dye molecules into the PHA hydrogels. The D values of the PHA hydrogels with crosslinker EGDMA were very high than for these values of PHA hydrogels with cross-linker NNMBA.

Binding

Adsorption isotherm indicates the distribution of adsorbate molecules between liquid phase and solid phase when adsorption process reaches an equilibrium state [28, 29].

To observe uptake of some dyes PAAm or PHA hydrogels were placed in aqueous solutions of cationic phenazine dyes such as Neutral Red, Safranin T and Janus Green, and the aqueous solutions of anionic dyes such as alizarin yellow R, eosin yellowish, naphtol green, bromocresol purple, congo red, indigo blue, and evans blue, and allowed to equilibrate for 2 days. At the end of this time PAAm do not show any coloration in all dye solutions. PHA hydrogels in the phenazine dye solutions showed the dark coloration of the original solutions, but transparent colors of these hydrogels do not change in the other dye solutions.

Since PAAm is a non-ionic polymer, ionisable groups on the polymer were increased by the modification of PAAm with N[H.sub.2]OH.HCl. Therefore, these hydrogels have many hydroxamate groups which can cause an increase of interaction between cationic phenazine dyes and the hydroxamate groups in the hydrogels. The possible interactions between phenazine dyes molecules and PAAm or PHA hydrogels are shown in Figs. 5-7.

Adsorption

The amount of adsorption per unit mass of the PHA hydrogels was evaluated using the following expression [28, 29].

Q = [([C.sub.o] - C)/m] x V (8)

where, Q is the amount of dyes adsorbed onto unit dry mass of the PHA hydrogels (mg [g.sup.-1]), [C.sub.o] and C are the concentrations of the dyes in the initial solution and in the aqueous phase after treatment for a certain period of time, respectively (mg [L.sup.-1]), V is the volume of the aqueous phase (L), and m is the amount of dry PHA hydrogels used (g).

To determine the sorption of the phenazine dyes into PHA hydrogels a plot of the amount of adsorption (Q) against the free concentration of dye solutions are shown in Fig. 8.

Figure 8 shows that sorption of phenazine dyes with in PHA hydrogels corresponds to S type sorption isotherms in the Giles classification system for sorption of the dyes from its solution [30].

In the S curves in the Giles classifications system, initial direction of curvature shows that adsorption easier as concentration rises. In practice, the S curve usually appear when three conditions are filled: the solute molecule (a) is monofunctional, (b) has moderate intermolecular attraction, causing it to pack vertically in regular array in the adsorbed layer, and (c) meets strong competition, for substrate sites, from molecules of the solvent or of another absorbed species [30].

The S-type isotherm depends on the Freundlich assumption about the heterogeneity of the surface. The presence of various planes leads to heterogeneous adsorption behavior. Heterogeneity is a usual and a general feature of surface properties due to different unsaturated adsorption sites of different energetic behavior. The Freundlich equation was developed mainly to allow for an empirical account of the variation in adsorption heat with concentration of an adsorbate on an energetically heterogeneous surface [31]. It has the general form:

Q=[k.sub.F] [C.sup.1/n] (9)

where Q is the amount adsorbed per unit mass of the solid (adsorbent), C is the solute concentration at equilibrium, [k.sub.F] is the Freundlich constant, equal to adsorption capacity at C = 1; and [n.sub.F] is a measure of deviation of isotherm from the linear form, that is, heterogeneity factor.

For adsorbents, it is desirable that the [k.sub.F] coefficient is as large as possible. For this reason, the [k.sub.F] values obtained in the study are very useful if compared with the k values obtained in other studies or studies. Higher values of [k.sub.F] represent an easy uptake of adsorbate from the solution. Conversely, the [n.sub.F] values are related to the Giles classification, S, L, and C type isotherm. [n.sub.F] < 1 correspond to S shape, [n.sub.F] = 1 to C type, and [n.sub.F] > 1 to L type [31, 32].

Freundlich parameters are calculated from the nonlinear regression of the plots in Fig. 8, and have been summarized in Table 6.

It is clear that the Freundlich exponent [n.sub.F], for the hydrogels with varying crosslinker and dye type, lies between 0.58 and 0.75, thus suggesting S type isotherm.

The values of the [k.sub.F] of the PHA hydrogels are between 0.04 and 1.86 [(mg [g.sup.-1]) [(L [mg.sup.-1]).sup.1/n]] for PHA-NNMBA and, 0.45 - 21.18 [(mg [g.sup.-1])[(L [mg.sup.-1]).sup.1/n]] for PHA-EGDMA. The values shows the adsorption capacities of the two adsorbents follow an order of PHA-EGDMA hydrogels > PHA-NNMBA. The reason that PHA-EGDMA hydrogels adsorbs much more phenazine dyes than the other material is most possibly related with the fact that it has the highest hydrophilicity. The largest [k.sub.F] values in both hydrogels were found for Janus green. According to the size of the [k.sub.F] value, the PHA hydrogel containing EGDMA and the Janus green system appear to be the best binding system.

Removal Efficiency and Partition Coefficient

The removal efficiency, ([phi]) and partition coefficient, ([K.sub.PC]) of the hydrogels were calculated as;

[phi] = [[[C.sub.o] - C]/[C.sub.o]] x 100 (10)

[K.sub.PC] = [[C.sub.o] - C]/C (11)

where [C.sub.o] and C are the concentrations of the dye solutions in the initial solution and the aqueous phase after treatment for a certain period time, respectively (mg [L.sup.-1]).

For the influence of crosslinker type and dye type to the removal efficiency and the partition coefficient, the hydrogels were put into 50 mL of concentration of 80 mg [L.sup.-1] dye solutions and left for 48 h at 25[degrees]C.

The values of [phi], [K.sub.PC], and [DELTA]G (Gibbs free energy) [[DELTA]G = -RT ln [K.sub.PC]] for the hydrogel-dye systems were calculated and tabulated in Table 7.

The [phi], [K.sub.PC], and [DELTA]G values of the PHA hydrogels with cross-linker NNMBA were lower than for these values of PHA hydrogels with crosslinker EGDMA. All partition coefficient values were found to be greater than the unity. It shows that the binding is spontaneous (If [K.sub.PC] > 1, [DELTA]G is negative). [DELTA]G demonstrates a spontaneous and favorable adsorption process. The higher negative value reflects a more energetically favorable adsorption. The Gibbs free energy value of PHA hydrogels with crosslinker EGDMA is the highest negative value than the values of other systems. For that reason, more energetically favorable adsorption is occurred in the PHA hydrogels with crosslinker EGDMA-dye systems. Conversely, as it can be seen from Table 7, the best binding by PHA-EGDMA and PHA-NNMBA hydrogels is binding of Janus Green.

Effect of the Quantity of Adsorbent on Adsorption

The quantity or mass of adsorbent is an important factor in large scale industrial application of adsorbent in the removal of a desired solute. To investigate the effect of mass of PHA hydrogels on the adsorption, a series of adsorption experiments were carried out with different masses of adsorbent at fixed initial solute concentration of 80 mg [L.sup.-1]. The effect of PHA mass was determined on the adsorption of the phenazine dyes by PHA hydrogels.

Figure 9 shows the amount of each dye adsorbed with varying amounts of PHA hydrogels. The results in each case indicate that dye removal efficiency increased with increase in mass of PHA hydrogels. The increase in mass of PHA hydrogels increases the contact surface of adsorbent which means that it will be more probable for dye molecules to be adsorbed on adsorption sites, and thus adsorption efficiency is increased. However, the removal increases up to certain limit and then remains constant, the limit in each case is still related to effect of adsorbate on adsorption. Further, Fig. 9 exhibits that the PHA-EGDMA hydrogels adsorb more dyes than the PHA-NNMBA hydrogels due to the reasons already mentioned.

There can be some reasons for non-covalent interactions in the binding of dye molecules by PHA hydrogels. The main interactions between the hydrogel and dyes may be electrostatic interactions. Electrostatic interactions will be expected to occur between positive charge of nitrogen atoms on the phenazine dye molecules and negativities charge of hydroxamic groups on the PHA. The other interactions may be hydrogen bonding. Specially, hydrogen bonding will be expected to occur: (a) between nitrogen atoms on the phenazine dyes and carbonyl groups on the repeating monomelic unit of crosslinked polymers, (b) between nitrogen atoms on the phenazine dyes and oxygen atoms and carbonyl groups on the EGDMA crosslinker, or (c) between nitrogen atoms on the phenazine dyes and carbonyl groups on the NNMBA crosslinker. Proposed mechanisms for the adsorption of the phenazine dyes onto PHA hydrogels are illustrated in Fig. 10.

The results of sorption studies are parallel behavior to the results of swelling studies. The ionic charge content in the polymeric structure is important. PHA contains acidic group such as hydroxamic acid, -HN -OH. The swelling degree of the hydrogels increases due to increase of the hydrophilic units on hydrogel structure. Therefore, PHA hydrogels have many ionic groups that can increase interaction between the dye molecules and anionic groups of hydrogels. So, it can be seen that swelling or sorption capability of PHA hydrogels are increased. The most important effect is hydrophilicity of copolymeric gels.

CONCLUSIONS

This work has given the quantitative information on the swelling and binding characteristic of the phenazine dyes such as Janus Green, Neutral Red and Safranin T with PHA hydrogels. Some swelling parameters have been calculated. The values of equilibrium swelling, [S.sub.eq] are 2.16 - 2.45 g [g.sup.-1] for PHA hydrogels crosslinked by NNMBA (PHA-NNMBA), and 23.21-33.25 g [g.sup.-1] for PHA hydrogels crosslinked by EGDMA (PHA-EGDMA). The diffusion exponents (n) are over 0.50. They are between 0.59 and 0.77. So, diffusion type of dye solutions into PHA hydrogels has been found as non-Fickian character.

PHA hydrogels have sorbed the phenazine dyes. S type sorption isotherm in Giles classification system was found using the Freundlich equation. The removal efficiency of phenazine dyes by PHA hydrogels was between 51.59 and 96.93%.

All parameters for PHA-EGDMA were found to be higher than those for PHA-NNMBA in this study. The type of crosslinker in the hydrogels influenced the swelling, binding, and sorption more than the type of dye.

At the end of this study, it can be said that PHA hydrogels may be used a sorbent for removal of some agents (such as organic molecules) and dye molecules. The utilization of these types of hydrogels, in biomedicine, controlled drug delivery, pharmaceuticals, agriculture, biotechnology, environment, sorption, separation, purification, immobilization, and enrichment of some species makes hydrogel more popular.

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Dursun Saraydin, (1) Yasemin Icikver, (1) Erdener Karadag (2)

(1) Science Faculty, Chemistry Department, Cumhuriyet University, Sivas 58140, Turkey

(2) Arts & Sciences Faculty, Chemistry Department, Ad nan Menderes University, Aydin 09010, Turkey

Correspondence to: D. Saraydin; e-mail: saraydin@cumhuriyet.edu.tr

Contract grant sponsor: Scientific Projects Commission of Cumhuriyet University; contract grant number: F-037.

DOI 10.1002/pen.24574

Caption: FIG. 1. Swelling isotherms of PHA hydrogels in phenazin dye solutions, *; PHA crosslinked with EGDMA, (); PHA crosslinked with NNMBA, and --; model fit.

Caption: FIG. 2. Plots of F versus t for PHA hydrogels in Safranin T solutions, *; PHA crosslinked with EGDMA, (); PHA crosslinked with NNMBA, and --; model fit.

Caption: FIG. 3. Second-order swelling kinetics curves of PHA hydrogels in Janus Green solutions, *; PHA crosslinked with EGDMA, (); PHA cross-linked with NNMBA.

Caption: FIG. 4. Swelling kinetics curves of PHA hydrogels in Neutral Red. *; PHA crosslinked with EGDMA, (); PHA crosslinked with NNMBA and--; model fit.

Caption: FIG. 5. An interaction between PAAm hydrogels and ionizable all dyes in solutions.

Caption: FIG. 6. Dissociation of polyhydroxamic acid hydrogel in aqueous media.

Caption: FIG. 7. A possible interaction between polyhydroxamic acid hydrogels and the phenazine dyes.

Caption: FIG. 8. Binding isotherms of PHA hydrogels in phenazine dye solutions; *; PHA crosslinked with EGDMA, (); PHA crosslinked with NNMBA and --; model fit.

Caption: FIG. 9. Effect of the mass of the hydrogel on phenazine dye adsorption; *; PHA crosslinked with EGDMA, (); PHA crosslinked with NNMBA.

Caption: FIG. 10. Proposed mechanisms for the adsorption of the phenazine dyes onto the hydrogels, (a) PHA crosslinked with EGDMA and (b) PHA crosslinked with NNMBA.
TABLE 1. Some properties of the phenazine dyes.

Dye                    Chemical formula             Molar mass

Safranin T (ST)        [formula not reproducible]     350.85
Basic Red 2
Tolusafranine
Cotton Red,
Gossypimine

Janus Green (JG)       [formula not reproducible]     511.07
Janus Green B
Janus Green V
Diazin Green S
Union Green B

Neutral Red (NR)       [formula not reproducible]     288.78
Basic Red 5
Neutral Red chloride
Neutral Red W
Toluylene Red

Dye                      Color     [[lambda].sub.max]
                       index nr.          (nm)

Safranin T (ST)          50240            530
Basic Red 2
Tolusafranine
Cotton Red,
Gossypimine

Janus Green (JG)         11050            660
Janus Green B
Janus Green V
Diazin Green S
Union Green B

Neutral Red (NR)         50040            540
Basic Red 5
Neutral Red chloride
Neutral Red W
Toluylene Red

TABLE 2. Swelling parameters of PHA hydrogels
in the phenazine dye solutions.

                                  EGDMA

Crosslinker    [S.sub.eq]           P         [tau]
Dye           (g [g.sup.-1)   (g [g.sup.-1)   (min)

Safranin T        33.25           35.97       40.82
Janus Green       24.98           26.40       28.99
Neutral Red       23.21           24.28       27.03

                  EGDMA                    NNMBA

                   SR          [k.sub.v] x
Crosslinker   (g [g.sup.-1      [10.sup.2]      [S.sub.eq]
Dye           [min.sup.-1])   ([min.sup.-1])   (g [g.sup.-1)

Safranin T        0.881            2.45            2.45
Janus Green       0.952            3.70            2.40
Neutral Red       0.838            3.45            2.16

                                   NNMBA

                                           SR          [k.sub.v] x
Crosslinker         P         [tau]   (g [g.sup.-1      [10.sup.2]
Dye           (g [g.sup.-1)   (min)   [min.sup.-1])   ([min.sup.-1])

Safranin T        2.57        42.55       0.053            2.45
Janus Green       2.52        34.36       0.073            3.70
Neutral Red       2.33        44.25       0.060            3.45

TABLE 3. Diffusion parameters of PHA hydrogels
in the phenazine dye solutions.

                         EGDMA                   NNMBA

Crosslinker    k x [10.sup.2]    n     k x [10.sup.2]    n
Dye            ([min.sup.-n])          ([min.sup.-n])

Safranin T          4.05        0.77        5.99        0.63
Janus Green         6.83        0.68        7.73        0.59
Neutral Red         4.09        0.77        6.00        0.63

TABLE 4. Second order swelling kinetics parameters of
PHA hydrogels in the phenazine dye solutions.

                                     EGDMA

Crosslinker      [r.sub.o],       [k.sub.2] x     [S.sub.max]
Dye            [(dS/dt).sub.o]    [10.sup.3]     (g [g.sup.-1])
                (g [g.sup.-1]    ([g.sup.-1] g
                [min.sup.-1])    [min.sup.-1])

Safranin T          1.008            9.17            48.08
Janus Green         1.245            6.93            31.06
Neutral Red         1.116            7.36            29.24

                                     NNMBA

Crosslinker      [r.sub.o],       [k.sub.2] x     [S.sub.max]
Dye            [(dS/dt).sub.o]    [10.sup.3]     (g [g.sup.-1])
                (g [g.sup.-1]    ([g.sup.-1] g
                [min.sup.-1])    [min.sup.-1])

Safranin T          0.094            9.17             3.19
Janus Green         0.075            6.93             3.29
Neutral Red         0.066            7.36             2.99

TABLE 5. Swelling kinetics parameters of PHA hydrogels
in the phenazine dye solution.

                                    EGDMA

                                                    D x
                 [k.sub.i]        [k.sub.D]     [10.sup.6]
Crosslinker    (g [g.sup.-1]     [10.sup.3]     ([cm.sup.2]
Dye           [min.sup.-1/2])   [s.sup.-1/2])   [s.sup.-1]

Safranin T         4.29             80.62          38.27
Janus Green        3.61             80.12          38.30
Neutral Red        3.52             64.84          44.70

                                    NNMBA

                                                    D x
                 [k.sub.i]        [k.sub.D]     [10.sup.6]
Crosslinker    (g [g.sup.-1]     [10.sup.3]     ([cm.sup.2]
Dye           [min.sup.-1/2])   [s.sup.-1/2])   [s.sup.-1]

Safranin T         1.53             6.48           1.32
Janus Green        1.51             9.66           3.53
Neutral Red        1.46             10.90          4.47

TABLE 6. Freundlich parameters of PHA hydrogels
in the phenazine dye solutions.

                                EGDMA

Crosslinker   [k.sub.F][(mg [g.sup.-1])
Dye           [(L [mg.sup.-1]).sup.1/n]]   [n.sub.F]

Safranin T               0.45                0.75
Janus Green             21.18                0.73
Neutral Red              0.99                0.69

                              NNMBA

Crosslinker   [k.sub.F][(mg [g.sup.-1])
Dye           [(L [mg.sup.-1]).sup.1/n]]   [n.sub.F]

Safranin T               0.04                0.58
Janus Green              1.86                0.64
Neutral Red              0.42                0.73

TABLE 7. Removal efficiency, partition coefficient and
free energy parameters of PHA hydrogels in the concentration
of 80 mg [L.sup.-1] phenazine dye solutions.

                        EGDMA

Crosslinker                            [DELTA]G
Dye           [phi]   [K.sub.PC]   (kJ [mol.sup.-1])

Safranin T    73.13      2.72            -2.48
Janus Green   96.93     31.52            -8.55
Neutral Red   85.26      5.79            -4.35

                        NNMBA

Crosslinker                            [DELTA]G
Dye           [phi]   [K.sub.PC]   (kJ [mol.sup.-1])

Safranin T    52.59      1.11            -0.26
Janus Green   88.85      7.97            -5.14
Neutral Red   72.10      2.58            -2.35
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Author:Saraydin, Dursun; Isikver, Yasemin; Karadag, Erdener
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2018
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