**7.3. Effect of solution pH**

**•** Uptake of C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 by some anion exchangers of different matrices and functionalities was not affected by the presence of

**•** Insignificant decrease of the sorption capacities was observed with the increasing amount of above mentioned salts as a consequence of competition in sorption between the dye anions and the chloride, sulfate and carbonate anions. Decrease of the sorption capacities in some cases is reflected in the affinity series of the anions towards the weakly basic anion

< H2PO4

2– < MoO4

– <CH3COO–

2– < CrO4

**•** Increasing salts concentration in the system, enhancement of the reactive and direct dyes occurred. The salting out effect reduces solubility of the dyes in the aqueous phase and promotes their sorption onto the hydrophobic part of sorbent. Kind of salts has a great effect on salting out. This phenomenon depends on the size of ions, their effective charge and

**•** The affinity of large organic anions such as dyes anions for the resins is influenced not only

**•** The type of functional groups of the anion exchangers, matrix structure (gel or macroporous) and composition play an important role in the dyes solution treatment in the presence of salts. The sieve effect is of significance when the sorption of the dyes anions is considered

Wastewater and after-process water from textile plants can contain surface-active substances. Surfactants are often applied as wetting, penetrating, dispersing and leveling agents in dyeing processes. They increase the solubility of dyes in water, in order to improve the dye uptake and dye fastness. Many articles published recently about the treatment of textile wastewaters focused on dyes removal and often neglected the influence of surfactant. Effect of the addition of such surfactants as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) on the dye removal by the used anion exchangers was studied from the systems

Investigating the uptake of C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 by the applied weak, intermediate and strong base anion exchangers from the systems containing 100 mg/L of the dye in the presence of the cationic surfactant CTAB, major decrease in the sorption capacities with the increasing concentration of CTAB from 0.1 to 1 g/L was observed.

– < HCO3 – < Cl–

2– < WO4

< H2PO4

2– < MoO4

2– < S2O3

– < HCO3

2– < WO4

< CN–

2– < I–

– < Cl–

2– < S2O3

 < NO2 – < Br– <

> < ClO4 – <

 < NO2 – <

< SCN– <

< SCN–

< CN–

2– < I–

inorganic electrolytes such as NaCl, Na2SO4 and Na2CO3

< IO3

< HCOO–

2– < SO4

2– < C2O4

– <CH3COO–

2– < CrO4

< IO3

2– < C2O4

by the anion charge but also by the structure of the anion and its size.

in the presence of chloride, sulfate, and carbonate anions.

containing 100 mg/L of dye in the presence of 0.1–1 g/L SDS or CTAB.

< HCOO–

2– < SO4

and towards the strongly basic anion exchangers:

2– < SO3

2– < SO3

salicylate < citrate < OH–

60 Ion Exchange - Studies and Applications

 < ClO3 – < BrO3 –

< salicylate < citrate

ability of hydrates formation.

exchangers:

F– < ClO3 – < BrO3 –

NO3 – < HPO4

OH– < F–

ClO4 –

 < NO3 – < HPO4

Br–

The solution pH is a very important parameter during the adsorption process and is mainly influenced by two factors:


Weakly basic anion exchangers in the free base form (RN(CH3)2) function at low pH when the hydrogen ion concentration is sufficiently high to protonate the amine nitrogen atom. Proto‐ nation of the amine group through the donor lone pair of electrons on nitrogen atom occurred during equilibration with acid as described below [41]:

**Figure 9.** Influence of anionic surfactant sodium dodecyl sulfate (SDS) on uptake of C.I. Acid Orange 7 by the polystyr‐ ene anion exchangers (a) as well as the polyacrylic and phenol–formaldehyde anion exchangers (b), of C.I. Reactive Black 5 by the polystyrene anion exchangers (c) as well as the polyacrylic and phenol–formaldehyde anion exchangers (d) and of C.I. Direct Blue 71 by the polystyrene anion exchangers (e) as well as the polyacrylic and phenol–formalde‐ hyde anion exchangers (f)

The resins were washed with 1 M HCl before the use taking the above into account. The relations between the initial pH of dyes solutions in the range of 1–12 and the sorption capacities of the anion exchangers were studied in the system containing 100 mg of dye per 1 L at 20<sup>ο</sup> C. The decrease in the sorption capacities with the increasing initial solution pH was observed for the weakly basic anion exchangers because the capacity of the weak base anion exchange resin is a function of pH (it decreased with the increase of pH).

As the strongly basic anion exchangers function at any pH, there was observed no influence of solution pH of the dyes on their sorption on the strongly basic anion exchangers. As it was mentioned, formation of ion pairs due to the electrostatic attraction between the dye functional groups and the quaternary ammonium groups of strongly basic anion exchangers or the tertiary amine groups of weakly basic anion exchangers is probably a leading, though not the only one, mechanism of the acid, reactive and direct dyes retention. These dyes contain different groups such as –OH, –SO3Na, –N=N–, –NH2 that can participate in covalent, cou‐ lombic, hydrogen bonding or weak van der Waals forces. The occurrence of the double bond serves to enhance the interaction between the dyes and the anion exchangers macromolecule. The physical adsorption and π–π dispersion forces can arise from the aromatic nature of the resins and the dyes. A similar phenomenon was observed for the sorption of Acid Green 9 on the weak and strong base anion exchange resins [42], for the sorption of Sunset Yellow on the weakly basic anion exchanger Amberlite FPA 51 [43] and strongly basic Amberlite IRA 900 and IRA 910 [44] as well as for the sorption of Brilliant Yellow on Amberlite IRA 67, Amberlite IRA 458 and Amberlite IRA 958 [45].

### **7.4. Kinetic studies**

2 32 2 2 32

*hydration anhydrous weak dissociation*

62 Ion Exchange - Studies and Applications

( ) ( )

*RCH N CH H O RCH NH CH OH*

2 32 2 32 2

*acid addition*

+ - + -

+ + ¾¾¾¾¾® +

+ -

*weakdissociation*

( ) ( )

*RCH NH CH OH HCI RCH NH CH CI H O*

**Figure 9.** Influence of anionic surfactant sodium dodecyl sulfate (SDS) on uptake of C.I. Acid Orange 7 by the polystyr‐ ene anion exchangers (a) as well as the polyacrylic and phenol–formaldehyde anion exchangers (b), of C.I. Reactive Black 5 by the polystyrene anion exchangers (c) as well as the polyacrylic and phenol–formaldehyde anion exchangers (d) and of C.I. Direct Blue 71 by the polystyrene anion exchangers (e) as well as the polyacrylic and phenol–formalde‐

hyde anion exchangers (f)

+ ¾¾¾¾® +

To understand better the sorption process of dyes of various types of anion exchange resins, it is essential to determine the course of this process in time and the effect of different factors affecting their retention. The rate at which the dissolved dye is removed from the aqueous solution by solid sorbents is a significant factor for application in wastewater quality control, too. It is essential to evaluate the adsorption kinetics using theoretical models in order to design and control the sorption process units. Two common kinetic models, namely, the Lagergren pseudo first-order model (Equation 4) and the Ho and McKay pseudo second-order model (Equation 5) were fitted to the experimental data of dyes sorption on the anion exchangers:

$$\log(q\_1 - q\_t) = \log(q\_1) - \frac{k\_1}{2.303}t \tag{4}$$

$$\frac{t}{q\_t} = \frac{1}{k\_2 q\_2^2} + \frac{1}{q\_2}t \tag{5}$$

where *q1* and *q2* are the amounts of dye sorbed at equilibrium according to Equations (4) and (5), respectively (mg/g), *qt* is the amount of dye sorbed at time *t* (mg/g), t is the time (min), *k1* is the constant rate of pseudo first-order adsorption (1/min), *k2* is the constant rate of the pseudo second-order adsorption (g/mg min) [46–50].

The values of *k2* and *q2* can be determined from the slope and intercept of the plot *t/qt* vs. *t*, respectively. This dependence is defined as type 1 of the pseudo second-order expression or simply the pseudo second-order expression. Similarly, *k2* and *q2* can be calculated from the plots of *1/qt* versus *1/t*, *1/t* versus *1/qt* , *qt /t* versus *qt* and *1/q2-qt* versus *t* for type 2, type 3, type 4 and type 5 of the pseudo second-order expressions, respectively [51, 52].

The above mentioned equations were used due to the fact that the first one describes well the initial stage of dyes sorption, the second equation fits well experimental data in the whole range of process time for most adsorbate/adsorbent systems. Moreover, using these equations the dyes sorption process is considered as a chemical reaction (chemisorption) [46-52].

Kinetic behaviour of the anion exchangers towards C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 was examined in the systems of different initial dyes concentrations ranging from 100 to 500 mg/L (in most cases) or even 1000 mg/L [2, 15, 20, 23, 25–31]. It was noticed that the amounts of dyes uptake increased with the contact time, and at some point in time reached an almost constant value where the amounts of dyes retained by the anion exchangers were in a state of dynamic equilibrium with the amounts of dyes desorbed from the anion exchangers. For the initial dyes concentration of 100 mg/L, the time required to reach equilibrium differed from a few to dozens minutes or even 12 h depending on the type of dye, composition of resins matrices as well as their structure, hydrophilic character of the skeletons and type of functional groups as can be seen from the data presented in papers [2, 15, 20, 23, 25–31]. C.I. Acid Orange 7 of the smaller molar weight saturated faster the available anionexchanging sites compared with C.I. Reactive Black 5 and C.I. Direct Blue 71. The studies on the sorption of dyes and organic compounds on the ion exchangers reported so far showed that the size of the sorbate molecules has a considerable effect on the degree of fixation [53– 55]. According to Dragan and Dinu [55] investigating the interactions of azo dyes such as Ponceau SS, Crocein Scarlet MOO, Congo Red and Direct Blue 1, differing in either the position of sulfonic groups or the number of anionic groups, with quaternized poly(dimethylami‐ noethyl methacrylate) it was stated that the number of sulfonic groups, position of the anionic charges and the whole structure of the dyes determined the dyes removal by the sorbent [55]. From the papers of Wojaczyńska and Kolarz [53, 54] about sorption of Methyl Orange, Acid Orange 10, Acid Red 44 and Direct Blue 1 on the divnylbenzene weak base anion exchangers of mono- and diethanolamine functional groups, it can be found that the copolymer gel heterogeneity has a marked effect on the degree of sorption and its course [54]. With the constant anion exchange capacity, the sorption properties decreased with an increase in the gel heterogeneity [53]. The dyes with a higher content of sulfonic groups were sorbed mainly by the formation of aggregates in the anion exchanger phase whereas Direct Blue 1 dye because of large size has the tendency to form aggregates in the solution rather than in the resin [54].

As follows from the data included in papers [2, 15, 20, 23, 25–31], generally the Lagergren equation (PFO) is not applied for description of sorption kinetics of C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 on the chosen anion exchangers of different types. This results, among others, from the non-linear dependence *log(qe – qt )* vs *t*, confirmed by low values of the determination *R2* and significant differences between the values of sorption capacity obtained experimentally and that calculated from the Lagergren equation. Moreover, the condition *log qe = intercept* is not satisfied which also indicates that the above equation is not applied [56–59]. Only in one case it was observed that the pseudo first-order expression better predicts the sorption kinetics than the pseudo second-order one. The closeness of the pseudo first-order equilibrium capacity to the experimentally determined equilibrium capacity indicates the usage of the pseudo first-order model to describe the kinetics of C.I. Direct Blue 71 uptake by Amberlite IRA 478RF. Several authors have also shown the applicability of the PFO kinetics in describing the sorption of dyes onto anion exchangers [53, 54] and different adsorbents [60–67]. Numerous applications of the Lagergren equation in sorption of dyes and inorganic ions have also been reported in the paper by Ho and McKay [49]. anion exchangers [53,54] and different adsorbents [60–67]. Numerous applications of the

The values of *k2* and *q2* can be determined from the slope and intercept of the plot *t/qt*

*/t* versus *qt*

, *qt*

and type 5 of the pseudo second-order expressions, respectively [51, 52].

plots of *1/qt*

64 Ion Exchange - Studies and Applications

versus *1/t*, *1/t* versus *1/qt*

respectively. This dependence is defined as type 1 of the pseudo second-order expression or simply the pseudo second-order expression. Similarly, *k2* and *q2* can be calculated from the

The above mentioned equations were used due to the fact that the first one describes well the initial stage of dyes sorption, the second equation fits well experimental data in the whole range of process time for most adsorbate/adsorbent systems. Moreover, using these equations the dyes sorption process is considered as a chemical reaction (chemisorption) [46-52].

Kinetic behaviour of the anion exchangers towards C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 was examined in the systems of different initial dyes concentrations ranging from 100 to 500 mg/L (in most cases) or even 1000 mg/L [2, 15, 20, 23, 25–31]. It was noticed that the amounts of dyes uptake increased with the contact time, and at some point in time reached an almost constant value where the amounts of dyes retained by the anion exchangers were in a state of dynamic equilibrium with the amounts of dyes desorbed from the anion exchangers. For the initial dyes concentration of 100 mg/L, the time required to reach equilibrium differed from a few to dozens minutes or even 12 h depending on the type of dye, composition of resins matrices as well as their structure, hydrophilic character of the skeletons and type of functional groups as can be seen from the data presented in papers [2, 15, 20, 23, 25–31]. C.I. Acid Orange 7 of the smaller molar weight saturated faster the available anionexchanging sites compared with C.I. Reactive Black 5 and C.I. Direct Blue 71. The studies on the sorption of dyes and organic compounds on the ion exchangers reported so far showed that the size of the sorbate molecules has a considerable effect on the degree of fixation [53– 55]. According to Dragan and Dinu [55] investigating the interactions of azo dyes such as Ponceau SS, Crocein Scarlet MOO, Congo Red and Direct Blue 1, differing in either the position of sulfonic groups or the number of anionic groups, with quaternized poly(dimethylami‐ noethyl methacrylate) it was stated that the number of sulfonic groups, position of the anionic charges and the whole structure of the dyes determined the dyes removal by the sorbent [55]. From the papers of Wojaczyńska and Kolarz [53, 54] about sorption of Methyl Orange, Acid Orange 10, Acid Red 44 and Direct Blue 1 on the divnylbenzene weak base anion exchangers of mono- and diethanolamine functional groups, it can be found that the copolymer gel heterogeneity has a marked effect on the degree of sorption and its course [54]. With the constant anion exchange capacity, the sorption properties decreased with an increase in the gel heterogeneity [53]. The dyes with a higher content of sulfonic groups were sorbed mainly by the formation of aggregates in the anion exchanger phase whereas Direct Blue 1 dye because of large size has the tendency to form aggregates in the solution rather than in the resin [54].

As follows from the data included in papers [2, 15, 20, 23, 25–31], generally the Lagergren equation (PFO) is not applied for description of sorption kinetics of C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 on the chosen anion exchangers of different types. This

obtained experimentally and that calculated from the Lagergren equation. Moreover, the condition *log qe = intercept* is not satisfied which also indicates that the above equation is not

and significant differences between the values of sorption capacity

results, among others, from the non-linear dependence *log(qe – qt*

of the determination *R2*

and *1/q2-qt*

vs. *t*,

versus *t* for type 2, type 3, type 4

*)* vs *t*, confirmed by low values

The linear form of the second-order equation proposed by Ho and McKay (PSO) can be applied for description of sorption of C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 on ion exchangers of various types. In this case there are satisfied the following conditions: the dependence *t/qt* vs *t* is linear, the determination coefficient reaches high values close to 1 and the calculated value of sorption capacity is largely consistent with the sorption capacity obtained experimentally. Comparison of quality of PFO and PSO equations fitting with the experimental data is presented for the system C.I. Reactive Black 5 – intermediate base anion exchanger Lewatit MonoPlus MP 64 in Figure 10. Lagergren equation in sorption of dyes and inorganic ions have also been reported in the paper by Ho and McKay [49]. The linear form of the second-order equation proposed by Ho and McKay (PSO) can be applied for description of sorption of C.I. Acid Orange 7, C.I. Reactive Black 5 and C.I. Direct Blue 71 on ion exchangers of various types. In this case there are satisfied the following conditions: the dependence *t/qt* vs *t* is linear, the determination coefficient reaches high values close to 1 and the calculated value of sorption capacity is largely consistent with the sorption capacity obtained experimentally. Comparison of quality of PFO and PSO equations fitting with the experimental data is presented for the system C.I. Reactive Black 5

– intermediate base anion exchanger Lewatit MonoPlus MP 64 in Figure 10.

Figure 10. The pseudo second-order plot (a) and the fitting of the pseudo first-order and pseudo second-order equations (b) to the experimental data of C.I. Reactive Black 5 sorption on Lewatit MonoPlus MP 64 from the systems of different initial dye concentrations **Figure 10.** The pseudo second-order plot (a) and the fitting of the pseudo first-order and pseudo second-order equa‐ tions (b) to the experimental data of C.I. Reactive Black 5 sorption on Lewatit MonoPlus MP 64 from the systems of different initial dye concentrations

In order to emphasize the influence of phase contact time on industrial effluents purification, suitable experiments were performed using the polyacrylic anion exchanger of the quaternary ammonium functionalities Amberlite IRA 958. Its effectiveness was confirmed in the batch experiments. The effluents from the textile industry containing different dyes and auxiliaries were shaken with 0.5 g of Amberlite IRA 958 from 1 to 144 h. The changes in absorbance values at the maximum wavelength in UV–vis spectra of the wastewater before and after sorption on Amberlite IRA 958 are presented in Figure 11. Analyzing the absorption values of untreated and purified wastewaters (Figure 11 a)), a significant colour reduction was observed after 12 h of phase contact time. The efficiency of decolourization exceeding 87% after only 1 h of contact time for the wastewater after the ozonation step is shown in Figure 11 b). The absorbance value at the maximum wavelength was reduced from 2.439 (before purification) to 0.1901 (after 12 h) in the case of wastewater containing Synazol Yellow KHL, Synazo lBlue KBR and Synazol Red K3BS (Figure 11 c)). As shown in Figure 11 d), after 3 h of phase contact time the yield of decolourization was 88.3%, the In order to emphasize the influence of phase contact time on industrial effluents purification, suitable experiments were performed using the polyacrylic anion exchanger of the quaternary ammonium functionalities Amberlite IRA 958. Its effectiveness was confirmed in the batch experiments. The effluents from the textile industry containing different dyes and auxiliaries were shaken with 0.5 g of Amberlite IRA 958 from 1 to 144 h. The changes in absorbance values at the maximum wavelength in UV–vis spectra of the wastewater before and after sorption on Amberlite IRA 958 are presented in Figure 11. Analyzing the absorption values of untreated and purified wastewaters (Figure 11 a)), a significant colour reduction was observed after 12 h of phase contact time. The efficiency of decolourization exceeding 87% after only 1 h of contact time for the wastewater after the ozonation step is shown in Figure 11 b). The absorb‐ ance value at the maximum wavelength was reduced from 2.439 (before purification) to 0.1901

increasing phase contact time to 144 h did not increase the adsorption efficiency

The regeneration step is the key to the implementation of the anion exchange system on the commercial scale. Desorption studies help to evaluate the nature of adsorption process. Desorption experiments were performed using different regenerating agents such as 1 M

significantly.

**6.5. Desorption studies**

(after 12 h) in the case of wastewater containing Synazol Yellow KHL, SynazolBlue KBR and Synazol Red K3BS (Figure 11 c)). As shown in Figure 11 d), after 3 h of phase contact time the yield of decolourization was 88.3%, the increasing phase contact time to 144 h did not increase the adsorption efficiency significantly.

#### **7.5. Desorption studies**

The regeneration step is the key to the implementation of the anion exchange system on the commercial scale. Desorption studies help to evaluate the nature of adsorption process. Desorption experiments were performed using different regenerating agents such as 1 M NaCl, 1 M Na2SO4, 1 M Na2CO3, 1 M NaOH, 1 M HCl and even 1 M KSCN. As previously stated [2, 15, 20, 23, 25–31], the aqueous solutions mentioned above were ineffective for the dyes removal from the resin phase.

**Figure 11.** Influence of phase contact time on the purification of raw textile effluents of different compositions using Amberlite IRA 958: a) the absorbance values at max wavelength at 0 h and 1 h were recorded after ten times repeated dilution, b) and c) samples were not diluted before measurements, d) the absorbance values at max wavelength at 0 h was recorded after twice repeated dilution

Considering that the dye retention by the anion exchanger may not be only by ion exchanging but also by the hydrophobic interaction or hydrogen bonding, methanol was chosen for breaking these non-specific interactions. Regeneration of the anion exchangers using 10–90% methanol solutions was ineffective confirming that strong electrostatic attraction between the dyes and the anion exchange matrix is a predominant mechanism of adsorption. The mixtures of 90% methanol with 1 M KSCN, 1 M HCl or 1 M NaOH improved the dyes desorption from the anion exchangers in most cases. Greluk and Hubicki [28, 29, 31], Karcher et al. [68, 69], Liu et al. [70] and Wawrzkiewicz [20, 23, 25] confirmed that regeneration of the anion exchangers loaded with the acid, reactive and direct dyes was problematic and required usage of aggres‐ sive regenerants which could have negative impact on the cost of the process.
