A review of the use of pullulan derivatives in wastewater purification

Reactive and Functional Polymers 149 (2020) 104510

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Review

A review of the use of pullulan derivatives in wastewater purification Luminita Ghimici , Marieta Constantin ⁎

T

“Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda, 41A, 700487 Iasi, Romania

ARTICLE INFO

ABSTRACT

Keywords: Pullulan derivatives Contaminants Flocculation Adsorption Wastewater treatment

Polymer derivatives based on polysaccharides have received increasing attention in wastewater treatment due to their easy availability and environmental friendly nature. Thus, a lot of modified polysaccharides (chitosan, dextran, cellulose, starch, etc) have been used as flocculants/adsorbants for removal of different contaminants. However, from the literature study it was observed that in spite of pullulan advantages (high solubility and flexibility of the backbone compared with other polysaccharides) there was a gap in the current research regarding its application, as such or in derivatized form, in wastewater purification. Therefore, in this article we review some important up-to-date results collated from investigations regarding the flocculating and adsorption properties of some pullulan derivatives which have been already evaluated in synthetic wastewater containing inorganic (clay) or organic (dyes, complex formulations of pesticides) contaminants. The data analyzed highlight the driving forces which govern the polycation/contaminant interactions and hence the flocculation/adsorption mechanisms, and demonstrate there are some parameters related to the characteristics of contaminants, polymers and environment which greatly influence the separation efficiency; the best results in terms of optimum polycation dose/removal efficiency and adsorption capacity were established. The accumulated knowledge provides solid arguments for future research regarding the application of pullulan derivatives in wastewater purification.

1. Introduction Polysaccharides and their derivatives possess some properties such as low priced, non-toxicity, biocompatibility and biodegradability which enable their use in many fields such as industry, agriculture, medicine, environmental protection. The need of polysaccharide derivatives has become important not only to overcome some drawbacks of polysaccharides (very high molar mass, limited solubility, etc.) but also to optimize their final properties by a suitable control of the chemical structure and/or physical properties. The change of molecular mass, mainly, by physical and biological methods and the introduction of acidic, basic, hydrophobic groups by chemical modifications have led to polysaccharides with required properties (solubility, biodegradability, chemical stability, etc.) for a particular application [1,2]. For instance, in the wastewater area, compounds which combine the benefits of both polysaccharide derivatives (mentioned above) and synthetic polymers (low optimum polymer dose) are desired. Thus a high variety of

derivatives of dextran, chitosan, guar gum, cellulose, konjac glucomannan, starch, pectin have been reported as efficient compounds used in the adsorption/separation processes [3–17]. In the last decade, some cationic pullulan derivatives have been also tested as flocculants in inorganic and organic synthetic wastewater [18–22]. The starting compound, pullulan, is non-toxic and holds outstanding chemical and physical properties, which confer it (as well as its derivatives) an important role in numerous uses in food, pharmaceuticals, cosmetic, electronics industries, biomedical applications, etc. as additive, thinking, glazing, carrier agents, etc. There are some article reviews which provide valuable information regarding the synthesis pathway, physico-chemical properties and applications of pullulan [14,23–29]. These aspects will be shortly discussed a little bit later. Till then, we have to mention that the pullulan modification has been performed mostly to insert charged groups for functionality. Thus, pullulan derivatives bearing quaternary ammonium or amino groups are mentioned as important modified polysaccharides [18,30–38]. Both the presence

Abbreviations: P, pullulan; P-g-pAPTAC, pullulan derivatives with grafted cationic chains, poly[(3-acrylamidopropyl)-trimethylammonium chloride]; DMAPAX-P, pullulan derivatives with tertiary amine groups, dimethylaminopropyl carbamate; P-g-pNIPAAm, pullulan grafted with poly(N-isopropylacrylamide); F, Fastac 10EC; NP, Novadim Progress; BM, Bordeau Mixture; KZ, Karate Zeon; AzB, Azocarmine B; AO, Acid Orange 7; MO, Methyl Orange; MB, Methylene Blue; RB, Reactive Blue 2; DS, substitution degree; [η], intrinsic viscosity; c⁎, critical overlap concentration; PD, polymer dose; OPD, optimum polymer dose; FW, flocculation window; RE, removal efficiency ⁎ Corresponding author. E-mail address: [email protected] (L. Ghimici). https://doi.org/10.1016/j.reactfunctpolym.2020.104510 Received 22 November 2019; Received in revised form 16 January 2020; Accepted 24 January 2020 Available online 25 January 2020 1381-5148/ © 2020 Elsevier B.V. All rights reserved.

Reactive and Functional Polymers 149 (2020) 104510

L. Ghimici and M. Constantin

of cationic groups and the high flexibility of the pullulan backbone represent two important advantages for the use of pullulan derivatives in the efficient wastewater purification. Nevertheless, the application of pullulan derivatives in this field was less investigated [39,40]. For instance, a composite flocculant (Pullulan and poly‑aluminum-chloride (PAC)) has been used to increase the performance of enhanced primary treatment processes for low-concentration municipal wastewater from South China [39]. The optimum composite flocculant (0.6 mg L−1 Pullulan+15 mg L−1 PAC) had high removal efficiency: over 95% of turbidity, over 91% of TP (total phosphate), over 58% of CODCr (chemical oxygen demand determined with potassium dichromate), and over 15% of NH3-N. The purification of wastewater has become a compulsory process as the quality of the water, whose reserves are limited, deteriorates, mainly, because of some industrial and agricultural activities. The presence of toxic contaminants, both of inorganic and organic type, in wastewater could have carcinogenic effect on the human being, impede photosynthesis and the growing of aquatic biota [41]. These undesirable and threatening effects have conducted the efforts of scientific community toward investigations for reduction or even elimination of pollutants from wastewater using various methods: adsorption, oxidation, flocculation, nano filtration, etc. [32,42–51]. Quite often, in order to obtain water of requested quality at small costs, combined methods are used. The flocculation technique demonstrated to be the most important purification method as it is easy of operate and cheaper as against to other ones [44]. The efficiency of this separation method is influenced by many parameters which are related to polymer (chemical structure, ionic groups content, molecular mass, dose), medium (temperature, dielectric constant ionic strength, pH), particles (size and concentration), etc. The influence of some of the above mentioned parameters on the adsorption/flocculation properties of some pullulan derivatives are considered in this review article. The separation efficiency has been evaluated either by the optimum polymer dose (OPD) which represents the polymer dose (PD) where the lowest value of the residual contaminants is achieved or the residual contaminants turbidity/absorbance (%)/the removal efficiency (%) (RE). PD is the polymer concentration in its mixture with the pollutant dispersion. The adsorption capacity at equilibrium (qe) has been used to assess the removal efficiency of the adsorbents based on pullulan [18,40].

derivatized in various forms due to the presence of nine hydroxyl groups on its pyranose rings per maltotriose unit. The modifications of pullulan, performed via chemical reactions such as esterification, etherification, sulfation, oxidation, copolymerization, etc. [2], were intended to enhance its use in as many areas as possible, including wastewater treatment. 2.2. Pullulan derivatives The structures and characteristics of the pullulan derivatives used in these studies are presented in the Figs. 1b,c and 2 and Tables 1a, 1b–3. Their use as flocculants is recommended, on the one hand, by the easiness of the pullulan flexible backbone to better arrange on the particle surface and on the other hand, by the presence of (i) positive charges (quaternary ammonium salt/pendent tertiary amino groups able to attach strongly to negatively charges on the contaminant particles via electrostatic attractions) or (ii) thermoresponsive chains (pNIPAAm) which enhance the flocculation efficiency. Concerning the adsorbents based on pullulan, the possibility to regenerate and reuse them with high adsorption efficiency over multiple adsorption/desorption cycles represents an economic advantage for water treatment applications. 2.2.1. Soluble ionic pullulan derivatives Pullulan derivatives with (i) different content and length of grafted chains, poly[(3-acrylamidopropyl)-trimethylammonium chloride] onto the pullulan (P-g-pAPTAC) (chemical structure in Fig. 1b) and (ii) different content of tertiary amine groups (DMAPAX-P) (chemical structure in Fig. 1c) were obtained by chemical modification of the polysaccharide sample (molar mass Mw = 200 kg mol−1), as reported by Constantin et al. [31,32]. In the polymers abbreviations P means pullulan, pAPTAC is poly[(3-acrylamidopropyl)-trimethylammonium chloride], DMAPA is dimethylaminopropyl carbamate group; X = DS, the substitution degree (number of amino groups per anhydroglucose unit in pullulan) was determined by N%, conductometric titration and 1 H NMR. The synthesis parameters of these pullulan derivatives are summarized in Tables 1a, 1b. 2.2.2. Pullulan microspheres The microspheres of Pullulan-graft-poly(3-acrylamidopropyl trimethylammonium chloride) (P-g-pAPTAC) have been synthesized by suspension cross-linking of the pullulan formerly grafted with cationic moieties, as reported by Constantin et al. [18]. 1H NMR spectroscopy and nitrogen elemental analysis were used to establish the copolymer composition while the microspheres morphology was investigated with a scanning electron microscope. The reaction conditions and physicochemical characteristics of the P-g-pAPTAC microspheres are presented in Table 2. The size of microspheres was between 10 and 300 μm [18]. In this article we present only the results obtained when P-g-pAPTAC microspheres with the diameter between 120 and 250 μm were used for adsorption studies.

2. Pullulan derivatives and contaminants used in the flocculation and adsorption studies The chemical structures and the main characteristics of the pullulan derivatives used in the separation processes will be presented after the subsection below where some data regarding the synthesis and physicochemical properties of the starting polysaccharide, pullulan, are briefly discussed. 2.1. Pullulan Pullulan (chemical formula: ((C6H10O5)n) is a linear, non-ionic polysaccharide consisting of maltotriose units: α-(1 → 6)-linked (1 → 4)-α-d-triglucosides [14,23] (Fig. 1a). It is mainly produced by fermentation of a yeast-like fungus, Aureobasidium pullulans. In their exhaustive review, Singh et al. [23] have mentioned (i) some other microorganisms (Tremella mesenterica, Teloschistes flavicans, Cryphonectria parasitica, Cytariaharioti, etc.) and (ii) different carbon sources (Asian palmkernel, cassava baggase, jackfruit seed, ricehull, etc) which produce pullulan. The values of molecular weight from 4.5 × 104 to 6 × 105 Da were reported for the produced pullulan and influenced in a great measure by cultivation conditions [24]. It is white powder, odorless and tasteless and has unique physicochemical properties owing to a glycosidic linkage pattern, such as high solubility in water and in diluted alkali, low viscosity, high flexibility of the chain, ability to form biodegradable film, adhesivity, etc. [14,23,24]. Pullulan can be

2.2.3. Nonionic thermosensitive pullulan copolymer The thermosensitive polysaccharide (P-g-pNIPAAm) was prepared by graft-polymerization of p(N-isopropylacrylamide) (pNIPAAm) onto pullulan (P) according to Ghimici and Constantin [20]. Ce(IV) ions were used as initiator. The grafted pullulan was characterized by elemental analysis and 1H NMR and FT-IR spectroscopy (see Table 3). The chemical structure is shown in Fig. 2. 2.2.4. Nonionic pullulan-graft-polyacrylamide hydrogel The pullulan-graft-polyacrylamide hydrogel was synthesized by free radical polymerization in the presence of a crosslinking agent (N,N′methylenebisacrylamide) and calcium carbonate as a porogen, according to Saber-Samandari et al. [40]. 2

Reactive and Functional Polymers 149 (2020) 104510

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Fig. 1. General chemical structure of pullulan (a) and of cationic pullulan derivatives: P-g-pAPTAC (b) and DMAPAX-P (c).

Fig. 2. The chemical structure of pullulan derivative P-g-pNIPAAm.

2,2-dimethylcyclopropanecarboxylate (chemical structure in Fig. 3b) (Syngenta Limited, England), Novadim Progress (named in the article NP) with active ingredient Dimethoate (chemical structure in Fig. 3c) (Cheminova A/S, Denmark), Bordeaux mixture MIF type (a mixture of copper (II) sulfate (CuSO4) and slaked lime (Ca(OH)2)) (named in the article BM) (IQV, Spain). Some other characteristics of contaminants and model dispersions are presented in Table 4.

Table 1a Synthesis parameters of pullulan derivatives P-g-pAPTAC. Polymer

P-g-pAPTAC1 P-g-pAPTAC2 P-g-pAPTAC3

P (g)

1.0 1.0 1.0

APTAC (×10−2 mol)

KPS (×10−2 mol)

0.484 0.967 0.484

0.0369 0.0369 0.0924

Product pAPTAC (wt%)

Graft ratioa (%)

22.53 29.05 34.51

29.09 40.94 52.69

2.3.2. Contaminants used in the adsorption studies The dyes Methyl Orange (MO), Acid Orange 7 (AO), Azocarmine B (AzB) used in the adsorption experiment onto P-g-pAPTAC microspheres, and Methylene Blue (MB) as well as Reactive Blue 2 (RB) used in the adsorption experiment onto pullulan-graft-polyacrylamide hydrogel were acquired from Sigma–Aldrich (St. Louis, MO, USA). Their chemical structure and main characteristics are shown in Table 5. The flocculation and adsorption experiments were carried out as it was mentioned in the already published articles [18–22,40]. Turbidity/ UV–Vis spectroscopy measurements as well as zeta potential (ζ) ones were used for separation efficiency evaluation and to obtain information about the flocculation mechanism. Moreover, data regarding the particle aggregates size (using laser diffraction technology) and surface morphology (using a Scanning Electron Microscope type Quanta 200) were collected. In the flocculation experiments the measurements mentioned above were performed as a function of PD. The supernatant samples were collected and analyzed after the optimum settling times

a

Graft ratio is calculated with the equation: (weight of grafted polymer – weight of substrate)/weight of substrate.

2.3. Contaminants 2.3.1. Contaminants used in the flocculation studies Clay powder-SSM Blend 14888 and kreutzonit were supplied by Romanceram Co., Romania. Fastac 10 EC (named in the article F) with active ingredient αCypermethrin which is a racemic mixture of (S)-alpha-cyano-3-phenoxybenzyl-(1R,3R)-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate (chemical structure in Fig. 3a) (BASF), Karate Zeon (named in the article KZ) with active ingredient lambda-Cyhalothrin which is a mixture of (R)-lambda-cyano-3-phenoxybenzyl (1S)-cis-3-[(Z)-2-chloro-3,3,3-trifluoropropenyl]-2,2-dimethylcyclopropanecarboxylate and (S)-lambdacyano-3-phenoxybenzyl (1R)-cis-3-[(Z)-2-chloro-3,3,3-trifluoropropenyl]Table 1b Synthesis conditions and main characteristics of pullulan derivatives DMAPAx-P. Sample code

Molar ratio UG/CDI/DMAPA

DS determined from 1

DMAPA0.16-P DMAPA0.4-P DMAPA0.9-P

1/0.25/1.84 1/0.56/4.12 1/1.12/4.12

H NMR

0.14 ± 0.01 0.39 ± 0.03 0.92 ± 0.06

UG = glucosidic unit of pullulan. 3

Conductometric titration

%N

0.16 ± 0.02 0.41 ± 0.03 0.89 ± 0.09

0.18 ± 0.03 0.49 ± 0.05 0.96 ± 0.1

Reactive and Functional Polymers 149 (2020) 104510

L. Ghimici and M. Constantin

Table 2 Reaction parameters and the physico-chemical characteristics of P-g-pAPTAC microspheres. Sample

P# 1 a b

Reaction parameters

Microspheres characteristics

P-g-pAPTAC concentration (%, w/v)

Crosslinking degree (mol ECH/ UG)a

pAPTAC content in polymer (wt %)

E.C.b of ms. (meq g−1)

pAPTAC content of ms. (wt%)

20

1.14/1

14

0.72 ± 0.04

13.66

The crosslinking degree is expressed as the molar ratio between ECH and glucose unit of pullulan. E.C., exchange capacity.

different conformations (tails, loops, trains) of the adsorbed chains and consequently, various flocculation mechanisms (bridging, charge neutralization and charge patch, depletion and shear flocculation, etc) could contribute, alone or simultaneously, to the particles separation [44,53,54]. Bolto and Gregory reported that in case of the neutral polymers and polyelectrolytes with high molecular masses (M) and low ionic content the bridging mechanism is the dominant one, while in case of ionic polymers with medium or low (M), high or medium ionic content the neutralization and/or patch mechanisms are the predominant ones [44]. Also, in the particular case of the grafted copolymers it turned out that the former flocculation mechanism (bridging) is the prevailing one [55–57]. With this information in mind we suggested that the bridging mechanism played an important role in the separation of contaminant particles. At small dosage, there was not enough polymer amount to create bridging links between particles. This is improved as the polymer dose increased; a rapid aggregation and hence, separation of the clay particles took place. At higher doses than OPD, the suspension was stabilized again. This happened because (i) there was insufficient free surface on the clay particles for attachment of polymer chains; (ii) the steric hindrance between the extended polymer chains may impede the bridge development. Another aspect which has to be emphasized is the much lower value of OPD when the ionic sample was used (4 mg L−1) against the nonionic one (60 mg L−1) (Table 6). The residual turbidity values corresponding to OPD were about 1% and around 6% in case P-g-pAPTAC2 and P-g-pNIPAM, respectively. These findings were explained by the different grafted chains on the pullulan backbone: ionic in case of P-g-pAPTAC2 and nonionic in case of P-g-pNIPAM. Consequently, one may infer the electrostatic interactions between the clay particles (negative charges) and the polymer chains (positive charges) had a significant role in the particle flocculation by the former pullulan derivative, while the H bonds between the OH groups on the particle surface and the amide groups of polymer by the latter one. The important role of the electrostatic attraction interactions, when the cationic sample was used for clay particle separation was emphasized by measurements of ζ as a function of P-g-pAPTAC2 dose (Fig. 4b). The ζ values enhancement from (−24.6 mV) (the initial ζ of the clay particle suspension) to (+7.58 mV) at the end of the investigated dose interval along with the negative value of ζ at OPD dose plead for the contribution of both bridging and neutralization mechanisms for the particles removal by the ionic pullulan derivative. According to Kleimann et al. [58], in neutralization mechanism the maximum separation happens at PD around that needed to give ζ close to zero. This finding was also sustained by particle size measurements performed on the P-g-pAPTAC2 flocs obtained at OPD as well as on the

Table 3 Characteristic data of pNIPAAm homopolymer and P-g-pNIPAAm copolymer. Sample

pNIPAAm P-g-pNIPAAm a b

NIPAAm/AGU (mol %) determined from %N

1

100/0 54.34/ 45.66

100/0 56.61/ 43.38

Length of pNIPAAm grafts, Mv × 10–4a

Mw × 10–4b

7.92 6.13

n.d. 26.0

H NMR

Calculated from viscosity measurements. Obtained by SLS in neutral water.

established for each particles (see Table 4); the optimum settling time refers to the period of time after which the pollutants residual turbidity/absorbance (%) remained almost constant. Adsorption experiments onto P-g-pAPTAC microspheres and pullulan-graft-polyacrylamide hydrogel were performed at pH, initial concentrations and temperatures shown in Table 5. The concentration of dye solutions (collected after 24 h) was found out at the wavelength corresponding to the maximum of absorbance (λmax) (see Table 5). 3. Parameters followed in the separation processes 3.1. Chemical structure of the pullulan derivatives and contaminant In order to illustrate the impact of the pullulan derivatives and contaminant chemical structure on the separation efficiency, the results obtained when (i) two types of soluble pullulan derivatives have been used for the flocculation of a clay suspension and (ii) the same type of pullulan derivatives, either soluble for pesticides removal or as microspheres and neutral hydrogel for adsorption of different dyes will be presented. 3.1.1. Pullulan derivatives type Fig. 4a shows the impact of two grafted pullulan copolymers, P-gpAPTAC2 (ionic) [19] and P-g-pNIPAAm (nonionic) [20] on the clay particles removal. Looking at the experimental data one finds that the residual turbidity varied with PD in a similar fashion for both grafted copolymers investigated; the residual turbidity raised at higher polymer doses than those where the maximum efficiency of clay particles removal was obtained, indicating a decrease in efficacy of the pullulan derivatives. This behavior is clarified in the following. Several interactions, such as hydrophobic, electrostatic, ion binding and hydrogen bonding could have a significant role in the flocculation process [52]. As a result,

Fig. 3. Chemical structure of pesticides: alpha-Cypermethrin (a), lambda-Cyhalothrin (b), Dimethoate (c). 4

Reactive and Functional Polymers 149 (2020) 104510

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Table 4 Contaminants and suspensions/emulsions characteristics. Particle Clay F

b

KZb NPb BM a b

a

Chemical composition (wt%)

Particle size (μm)/distribution (%)

Zeta potential (ζ), mV

λ (nm)

Settling time (min)

SiO2, 66–72; Al2O3, 23–26; TiO2, max.1.8; Fe2O3, max 1.5; Na2O, 0.1; K2O, 1.9–2.4 α-Cypermethrin: 100 g L−1; solvent naphtha (petroleum), light arom.) lambda-Cyhalothrin: 50 g L−1; solvent naphtha (petroleum), heavy aromatics, propylene glycol Dimethoate: 400 g L−1; solvent cyclohexanone, xylened) (20% copper as copper sulfate).

< 20, 90–95; < 10, 82–89; < 6, 70–80; < 2, 55–65; < 1, 45–57. 0.151

−25.6

500

60

−24.6

276

1200

0.165

−26.5

270

1200

0.132

−35.3

601

60

–

−20

652

1200

Characteristics of these materials according to the technical sheets supplied by producer. The hydrodynamic size of particles, Z-Average (d, nm), determined by DLS measurements are corresponding for F, KZ and NP in emulsion.

clay particles from the initial suspension (Fig. 5). Thus, the charge neutralization is indicated by the unimodal distribution of the flocs size along with their narrower distribution and smaller size, compared with the untreated clay [19]. The bridging mechanism was emphasized by the two larger - size peaks. This was probably possible, as the grafted chains may be involved in some interparticle associations, which led to bigger flocs. As regard P-g-pNIPAAm, the flocs size exhibited a unimodal distribution, too [20]. These investigations led to the conclusion that, in terms of both OPD and RE (more than 99%) the ionic pullulan derivative is more efficient, so preferably than the nonionic one for the separation of this type of clay particle. The chemical structure influence on the separation of this type of clay was also monitored in case of some amphiphilic cationic dextran derivatives, D40-R30 (R: dodecyl, octyl butyl and ethyl) [59]. The OPD values decreased for the sample containing longer alkyl chains, D40Oct30 and D40-Dod30. This result can be determined by the intramolecular hydrophobic associations which prevail in solution of low concentrations leading to a more compact conformation of the polymer coil.

Notable changes in the residual absorbance with polymer dose have been observed that means interactions between the partners in system took place. The pesticide/pullulan derivatives interactions could be possible as an outcome of their chemical structures (see Section 2.3.1). For initial emulsions of F and KZ, ζ are −24.6 mV and − 26.5 mV, respectively. For the pullulan samples containing tertiary amine groups (pKa from 3 to 11), we consider that at the working pH (4.4), the greatest part of the amine groups are protonated. The presented herein chemical structures imply that the electrostatic attraction forces between the F or KZ particles and ionic samples lead to the formation of aggregates which settle down, hence determining the pesticide particle separation. One could also observe a lower RE for KZ (85%) than for F (around 90%) (Table 6). This finding may be ascribed to the more voluminous trifluoropropenyl groups in the active ingredient of KZ, lambda-Cyhalothrin which could impede, to some extent, the interactions between the polycations and contaminant particles. The large interval of maximum flocculation efficiency recorded in case of F can be explained by its higher amount of active ingredient (20 mg L−1) than that in KZ (10 mg L−1) at the concentration used for pesticide formulation, c% (v/v) =0.02 (Table 6). The dye chemical structure (shown in Table 5) effect on the adsorption capacity of P-g-p(APTAC) microspheres (P#1) (Table 2) has been investigated by Constantin et al. [18]. It turned out that the electrostatic attractions between the cationic sites of the pullulan microspheres and the anionic groups of the dyes had a significant role in the adsorption process. Nevertheless, the order of adsorption capacity of the dyes investigated does not follow the order of the number of their charges. This means there are other forces beside the electrostatic ones which contribute to the dyes adsorption. This finding is clearly shown for the dyes AO and MO (Fig. 7) which contain one sulphonic group

3.1.2. Contaminant type We have undertaken investigations using pullulan derivatives with pendent tertiary amino groups (DMAPAx-P) for the removal of different pesticide formulations (F, KZ, NP, BM) [21,22], P-g-pAPTAC microspheres and pullulan-graft-polyacrylamide hydrogel for removal of various dyes (AzB, AO, MO) [18] and MB, RB, respectively [40]). As the results for the soluble samples followed the same trend, only the data obtained when DMAPA0.16-P was used in the separation of the pesticides of pyrethroid type (F and KZ) are presented (Fig. 6).

Fig. 4. The residual turbidity of clay suspension (%) in dependence on the polymer dose: P-g-pAPTAC2 (solid star); P-g-pNIPAAm (empty star), pH = 4.4 (a); Zeta potential dependence on the polycation dose (P-g-pAPTAC2) (b). 5

6

N

CH3

N

HN

S

N

N

NH2

N

Cl

N

SO 3-

N

N

N

+

NH

+

O O-Na +

O

CH3

S

-N CH3

O

Cl

SO 3

NH

-

S O Na

O

SO 3- Na +

+ SO 3 Na

NH

SO 3

k2 = the pseudo-second order rate constant (g mg−1 min−1).

O

O

Reactive Blue-2 (RB) λmax (nm) = 607

H3C

Methylene Blue (MB) λmax (nm) = 665

H3C

H3C

Methyl Orange (MO) λmax (nm) = 510

N

Acid Orange 7b (AO) λmax (nm) = 485

N+

N

Pullulangraftpolyacrylamide hydrogel

Pullulangraftpolyacrylamide hydrogel

P-g-APTAC microspheres P #1

Azocarmine B (AzB) λmax (nm) = 516

SO 3

Matrix

Acid dye

Table 5 Pullulan based adsorbents for dyes removal.

1.8 5.3 10.5 7.1

1.8 5.3 10.5 7.1

7

pH

20 35 55 70 20

20

20 35 55 70 20

20

25

25

35 45 55

25

T (°C)

50 100 300 500 2000

100

50 100 300 500 2000 100

100

100

100

100

100 300 500 500

C (gmL−1)

24

24

24

24

24

Equil. time (h)

Langmuir

Langmuir

Langmuir

Equil. model

97.2 248.4 360.8 386.8 389.1 393.8 398.5 196.4 386.8 1050 1502 2728 244.9 340.2 175.6 273.3 291.6 314.1 356.5 143.8 273.3 688.8 986.1 1822

55.19

65.42

57.14 84.03 113.83 89.75 76.504 72.08

qm (mg g−1)

Pseudosecond order k2 = 5.89 × 10−5

Pseudosecond order k2 = 3.76 × 10−5

k2 = 10.1 × 10−4

k2 = 7.82 × 10−4

Pseudosecond order k2 = 1.43 × 10−4

Kinetic model

Chemisorption

Chemisorption

Adsorption mechanism

Endothermic and spontaneous sorption process, thermodynamically favorable

Endothermic and spontaneous sorption process, thermodynamically favorable

Exothermic and spontaneous sorption process, impaired at high temperatures

Thermodynamic

L. Ghimici and M. Constantin

Reactive and Functional Polymers 149 (2020) 104510

Reactive and Functional Polymers 149 (2020) 104510

L. Ghimici and M. Constantin

Table 6 Pullulan based flocculants for clay and pesticide formulations removal. Polymer

Contaminant

Ionic groups content (%)

c (g L−1/%, v/v)

Temperature (°C)

Polymer concentration (ci, g L−1)

Natural pH

OPD (mg L−1)

RE (%)

Flocculation mechanism

P-g-pNIPAAm P-g-pNIPAAm P-g-pAPTAC1 P-g-pAPTAC2

Claya

– – – –

1

20 60 20

1

4.5 4.5 4.5

16

0.01 0.02

1 0.5 1 2 3 1

4.4 3 4.4 7.5

60 60 9 4 4 4 4 1.6 2 2.8 4

94.01 98.43 97.7 98.42 98.98 97.69 96 80 90.6 90.2 86.35

4.4 4.4 5 5.5 7

6 9 4 20 6

92.6 95.2 85 89.5 98.48

4.4 5.5 7 4.4 5.5 7

2 10 4 1.6 6 3

89.8 90.26 98.41 90.5 90.42 98.37

H bondings Hydrophobic associations Neutralization and bridging Neutralization and bridging Neutralization – – – Neutralization and H bondings BM (Cu2+ and SO42 - ions)/ amide/amino groups on the polymer chains interactions Neutralization Neutralization and H bondings Same as DMAPA0.16-P Neutralization Neutralization and H bondings Same as DMAPA0.16-P

DMAPA0.16-P

Claya

Fb

KZ NPb BMa

DMAPA0.9-P

a b

Fb NPb BMa Fb NPb BMa

20

0.03 0.04 0.02 0.7 0.5

b

DMAPA0.4-P

1

40 90

0.02 0.7 0.5 0.02 0.7 0.5

20

1

20

1

c (g L−1). c (%, v/v).

Fig. 6. The residual pesticides absorbance (%) dependence on the polymer (DMAPA0.16-P) dose: F (solid star); KZ (empty star); c%, (v/v) =0.02; settling time 1200 min.

Fig. 5. Particle size distribution for P-g-pAPTAC2 (star) and P-g-pNIPAM (square) flocs obtained at OPD and clay particles from the initial suspension (circle).

diffusion. The third one corresponds to the final equilibrium process caused by a decrease in the dye concentration in solution.

(Table 5). The adsorption kinetics followed the pseudo second-order model (Fig. 7 inset, Table 5). The different adsorption capacity of these dyes onto P-g-p(APTAC) microspheres could be assigned to the various aromatic substituents, namely a naphthalene ring in case of AO and a benzene one in case of MO. The greater adsorption ability of AO than that of MO (Fig. 7, Table 5) indicated that the bulkier aromating substituent determined a stronger binding force of the dye; hence one concludes the hydrophobic attraction forces could also have a contribution to the binding of the dye by the ionic microspheres. The adsorption of RB and MB on pullulan-graft-polyacrylamide hydrogel followed the same kinetic model, namely the pseudo secondorder one [40]. The authors found that the adsorption process of both dyes takes place in three steps, as follows: 1. a faster step due to the availability of the adsorption centers, 2. a gradual interparticle

3.2. Ionic groups content It is well recognized that the flocculation ability of a polyelectrolyte is influenced by the chains conformations in solution, which in turn, are influenced by the ionic groups content. The flocculation efficacy of three cationic pullulan samples with different charged groups content (see Table 1b) in pesticide (F, NP, BM) model wastewater has been examined [21,22]. Table 6 shows lower OPD values for the sample with higher ionic groups content. A higher ionic groups content increases the expansion of chain as a result of the stronger intramolecular repulsive interactions between the ionic groups as demonstrated by viscosity measurements [32]; [η] values (dl g−1) obtained by the Wolf method [60] are 1.41 for DMAPA0.16-P, 10.69 for DMAPA0.4-P and 15.14 for DMAPA0.9-P. Consequently, the attraction of 7

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measurements. The maximum removal efficacy for F particles [21] and NP ones [22] was noticed at PD corresponding to ζ values near zero; this means the simple charge neutralization, mainly, operates in the separation of these pesticides. We must mention here that the negative charge of NP emulsion (ζ = −35.3 mV) could come from some Dimethoate molecules (of organophosphoric type) ([O,O-Dimethyl S-(Nmethylcarbamoylmethyl) phosphorodithioate]) which can hydrolyze at the amide groups [65]; the solubility in water of Dimethoate compound is 39.8 g L−1 [66]. Also, the H bondings formed between the amine and/or amide groups of polymer and the amide units of Dimethoate can also have a contribution to the contaminant separation. In regard to BM, it can interact with DMAPAx-P by means of (i) Cu2+ ions which can interact with amide and amine groups of the pullulan derivative chains (ii) the electrostatic interactions between the positive sites of the polyelectrolyte and the negative species of BM (ζ = −20 mV). SO42− ions are able to interact with tertiary amine groups leading to intra−/intermacromolecular associations of the polyelectrolyte chains via binding of two nitrogen atoms, and consequently to aggregation of pesticide particles. The binding of SO42− anions to quaternary ammonium salt groups or tertiary amine N-atom belonging to some cationic polyelectrolytes has been already reported [67]. The findings above were supported by the negative values of ζ over FW (between −11.5 and −9 mV for DMAPA0.16-P, between −9 and −2.9 mV for DMAPA0.4-P and between −9 and −5.7 mV) DMAPA0.9-P [22].

Fig. 7. Effect of the dye structure on the adsorption onto P-g-p(APTAC) microspheres (P# 1 in Table 2) and the pseudo-second-order model for the analysis of the adsorption data (inset), at pH 7.0 and 25 °C. Adsorbent mass = 50 mg; [dye] = 100 mg L−1; shaking rate = 200 rpm.

pesticide particles by the more exposed cationic groups of the chain is favoured, and hence the neutralization process. On the other hand, a wider flocculation window (FW) for the sample with lower ionic groups content was noticed; FW refers to the range of PD where the lowest value of the residual contaminants is achieved. Thus, in case of F, FW was between 2.8 and 3.6 mg L−1 for DMAPA0.16-P, the lowest charged sample, unlike the other two polycations where restabilization took place faster [21]. The same trend was observed for NP and BM (Fig. 8a,b) [22]. In case of NP, DMAPA0.16-P revealed no important restabilization in a broader PD range (20 mg L−1–26 mg L−1) while DMAPA0.9-P exhibits restabilization at higher doses than 6 mg L−1. For BM, the FW width decreased with ionic content enhancement: 6–20 mg L−1 for DMAPA0.16-P, 4–12 mg L−1 for DMAPA0.4-P, 3–8 mg L−1 for DMAPA0.9-P. The larger FW noticed for DMAPA0.16-P is due to weaker repulsion between the lowest charged adsorbed polymer chains, which causes a delayed redispersion. Hence, one could conclude that the application of polycations with lower charge density in real situations could be an advantage as the particles redispersion risk at higher PD than OPD is eliminated. Similar behavior was reported for chitosan on bacteria suapension [61], cationic starch on kaolin suspension [62] and cationic dextran derivatives on clay suspension [63] and F emulsion [64]. The flocculation mechanism was established by zeta potential

3.3. Grafted charged chains length The application of grafted polysaccarides (chitosan, starch, gum guar, konjac glucomannan, etc) in the flocculation process is often encountered [55,56,68,69]. In our investigations, the pullulan derivatives P-g-pAPTAC1 and Pg-pAPTAC2 (see Table 1a) were chosen to evaluate their flocculation properties in dependence on PD as well as grafted chains length in a clay suspension (Fig. 9a) [19]. The results show a higher flocculation efficiency for P-g-pAPTAC2 than P-g-pAPTAC1, OPD for the former sample being lower (4 mg L−1) than that for the latter one (9 mg L−1) (see Table 6); this finding may be assigned to the longest pAPTAC chains grafted on the pullulan in P-gpAPTAC2 which may interact with the highest number of clay particles. This may happen due to the more extended hydrodynamic coil of this polycation (higher [η]), as demonstrated by the viscometric measurements (see Fig. 9b) [19]; [η] = 67mL g−1 and 500mL g−1 for P-gpAPTAC1 and P-g-pAPTAC2, respectively. The [η] values were determined by the Rao method [70]. These results are in line with some data found in the literature [56,68,69,71]. Furthermore, P-g-pAPTAC2 showed a wider FW compared to the other one. This, probably, happens as the grafted chains of highest length can be attached to a higher

Fig. 8. The residual pesticides absorbance (%) dependence on the polycation dose: DMAPA0.16-P (star), DMAPA0.4-P (circle) and DMAPA0.9-P (square); NP, settling time 120 min (a) BM, settling time 1200 min (b). 8

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Fig. 9. Residual turbidity of clay suspension (T %) dependence on the grafted cationic polysaccharide dose for: P-g-pAPTAC1 (circle); P-g-pAPTAC2 (star) (a), pH = 4.4, settling time 15 min; Reduced viscosity (ηsp/c) dependence on the grafted cationic polysaccharide concentration (c) for: P-g-pAPTAC1 (circle); P-gpAPTAC2 (star); inset, fitting of the Rao equation for: P-g-pAPTAC1 (circle); P-g-pAPTAC 2 (star) (b).

calculated as stated in Frish and Simha, c* = 1/[η] [72]. Close values were found for OPD (about 4 mg L−1) regardless of ci (Table 6). However, the residual turbidity value has modified with changing ci. Fig. 10 shows the residual turbidity dependence on ci at OPD. The residual turbidity values decreased if ci raised from 0.5 g L−1 to 1 g L−1 and increased for higher ci. At ci = 2 and 3 g L−1, located in the semidilute concentration regime, the bridge formation may be hindered as the polycation chains are overlapped or even entangled. Moreover, this led to a lower availability of ionic groups of P-g-pAPTAC2 and, hence to lower flocculation performance. So, an appropriate ci is required to obtain the best flocculation performance. Taking into account the best separation efficacy (of about 100%) corresponding to OPD, one may appreciate ci = 1 g L−1 as the optimum one for P-g-pAPTAC2, in the concentration interval considered. The optimum initial concentration of flocculant was also found in the case of other polymers, namely, star, linear and hybrid polyacrylamides [73] and cationic dextran derivatives, hydrophilic [63] and amphiphilic ones [74]. Thus, OPD was almost the same irrespective of ci for the hydrophilic polymers (as for the pullulan derivative investigated), while the FW width increased with ci enhancement for the amphiphilic one; this was assigned to different conformations/association of the polyelectrolyte chains in solutions with different concentrations.

Fig. 10. Residual turbidity of clay suspension (T %), at the optimum polycation dose, dependence on the initial solution concentration, (ci); settling time 15 min, pH = 4.4.

number of clay particles that means a less number of free (neadsorbed) cationic chains; the weaker repulsion between them led to a delayed redispersion. Evidence for the flocculation mechanisms were obtained by particle size and zeta potential measurements [19]. Both methods have revealed contribution from both bridging and neutralization mechanisms.

3.4.2. Concentration of contaminant dispersion The effect of various initial concentrations (c (%, v/v) between 0.01 and 0.04) of the F emulsions on the flocculation efficacy was monitored using of DMAPA0.16-P as flocculant [21]. As shown in Table 6, the increase of c determined an increase of OPD; this finding is in line with those obtained in case of other pesticide formulations (Decis, Dithane) using some cationic dextran derivatives [6,64] and chitosan [46] as flocculants. Also, Yang and Cheng [75] found higher OPD for higher initial turbidity when investigated the flocculation efficiency of an amphoteric chitosan derivative in a kaolin suspension. Removal degrees about 80% and more than 90% were found for pesticide emulsion with c = 0.01% and for higher concentrations, respectively. Moreover, for higher c than 0.01%, FW was larger, a residual F absorbance less than 10% being observed in the polymer dose range from 5 to 7 mg L−1 for c = 0.03% and 8 to 12 mg L−1 for 0.04% [21]. Also, the aggregates appeared faster; presumably, with rising number of pesticide particles, the collisions frequency raised and the aggregation of particles was facilitated. The effect of initial concentration of AzB on its removal by P-gpAPTAC microspheres (Table 2) was also determined [18]; the dye concentration varied from 100 to 500 mg L−1 (Table 5). AzB has three

3.4. Initial concentration of polymer solution (ci) and contaminant dispersion (c) The properties of ionic polymers in solution, including the flocculating one, are influenced by concentration (ci). Nevertheless, there are few investigations focused on the relationship between the flocculating efficacy and ci, i.e. the concentration of polyelectrolyte solution added to the contaminant dispersion. As regard the contaminant concentration, c, it is well known that the wastewater could contain different amounts of pollutant, hence it is necessary to find out the optimum polymer dose values for dispersions of differentc. 3.4.1. Initial concentration of polymer solution In order to determine the right ci for the polymer solution used in the flocculation process, experiments were performed on a clay suspension using the cationic pullulan derivative which showed the best flocculating efficiency, P-g-pAPTAC2 [19]. ci was in the range 0.5–3 g L−1 which includes the overlap concentration (c* = 2 g L−1), 9

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Fig. 11. Residual turbidity of clay suspension (%) dependence on the polymer dose: pNIPAAm (star); P-g-pNIPAAm (circle); settling time 30 min, T = 60 °C (a); Floc size distribution for P-g-pNIPAAm; T = 60 °C (b).

SO3 − groups which can interact with the quaternary ammonium groups of P-g-pAPTAC microspheres. As expected, the amount of the adsorbed dye at equilibrium, qe,exp increased with the dye concentration increase. The dye uptake enhancing could be assigned to the higher number of sorbate ions which can interact with the accessible binding sites of the adsorbent. Similar results were obtained in case of RB and MB [40]; enhancing the dye concentration between 50 and 2000 mg L−1 led to an increase of the amount of adsorbed dye from 143.8 to 1822 mg g−1 and 196.4 to 2728 mg g−1, respectively (Table 5).

explained that at acidic pH, the surface of the hydrogel is positively charged and hence the electrostatic interactions with the negatively charged groups of RB are predominant improving its adsorption efficiency against MB which at this pH is neutral. At basic pH, the higher amount of MB retained on the hydrogel is the outcome of the strong electrostatic attractions between the cationic sites of MB with the negative ones of the adsorbent. 3.6. Temperature The impact of temperature on the flocculation performance of a polymer is not obvious. For instance, higher temperatures led to better efficiency in the separation of kaolin suspension with cationic starches [79] or cationic konjac glucomannan [80] and of barium chromate with poly(diallyldimethylammonium chloride) [81], while contrary effect was observed when wastewater contaminated with dyes were purified by two commercial cationic polyelectrolytes [82]. The temperature impact becomes noteworthy when thermosensitive polymers are used as flocculants. It is well known that solubility of these polymers changes with temperature; they are soluble at small temperatures and become insoluble as temperatures rise. The temperature at which the phase separation takes place is called the lower critical solution temperature (LCST) [83]. In our investigations the flocculation properties of a thermoresponsive sample, pullulan grafted pNIPAAm copolymer (P-gpNIPAAm) (LCST = 34 °C) and poly(N-isopropylacrylamide) (pNIPAAm), a well known termosensitive synthetic homopolymer (LCST = 33 °C), were evaluated in a clay suspension (Fig. 11a) [20]. We have to mention here that in the flocculation experiments at higher temperature than LCST, the adsorption of copolymer was carried out at room temperature and afterwards the clay suspension was heated at 60 °C. The pNIPAAm homopolymer had no influence on the clay particles flocculation while the addition of P-g-pNIPAAm led to a quick reduction of the residual turbidity until a plateau was attained; a further rise of P-g-pNIPAAm dose led to restabilisation of suspension. Above LCST the dehydration of the polymer chains takes place (the hydrogen bonds between the water molecules and amide groups of pNIPAAm are disrupted) and the pNIPAM chains, and consequently the particle surface become hydrophobic; in this case, the hydrophobic associations have the main role in the separation of the clay particles. One observes that OPD of 60 mg L−1 was the same as that recorded when the experiments were conducted at room temperature (see Fig. 4a); the corresponding RE % was greater at temperature above LCST (over 98%) than at room temperature (94%) (Table 6), suggesting a better separation performance in the former case. Also, the broader FW above LCST (between 60 and 200 mg L−1) than that at room temperature (between 60 and

3.5. Dispersion medium pH pH is a significant parameter which affects the separation process as it can alters the charges of polymer chains (and hence, their conformations), of the suspended particles or both. The flocculation efficacy in response to changes of environmental pH (3, 4.4 and 7.5; c (v/ v) = 0.02%) was followed for DMAPA0.16-P in F emulsion [21]. For the pH corrections, solutions of 0.1 N HCl and 0.1 N NaOH were used. Similar removal degrees of pesticide (around 90%), were noticed at pH 4.4 and pH 3, but at a different OPD (Table 6). The decrease of OPD from 2.8 mg L−1 (pH 4.4) to 2 mg L−1 (pH 3) may be explained by the diminished charge of the pesticide particle emulsion (ζpH=3 = −16.2 mV) as well as by the higher number of protonated amine groups which can interact with the contaminant particles. At pH 7.5, the removal of F particles was slightly slower in the whole interval of polymer dose investigated despite the more negative ζ value of the F emulsion (ζ = −30 mV); the removal efficiency corresponding to OPD was around 86%. This finding could be determined by the lower number of charged groups (protonated amine groups) and hence, the abatement of the attractive forces between the polymer and contaminant particles. The effect of pH on the adsorption of AzB by sample P# 1 (Table 2), ([AzB] = 100 mg L−1; contact time = 24 h; shaking rate = 200 rpm) was investigated in the pH interval between 3.0 and 9.0, at 25 °C [18]. The dye adsorption was not affected by pH over the entire range investigated. This could be due to the fact that in this pH interval the dye as well as the adsorbent are entirely dissociated; the dye sulfonate is derived from a strong acid and the adsorbent is a strong basic exchanger. The average value of qe was 57.48 mg g−1. These results are in line with those found for the adsorption of anionic dyes by cross-linked cationic starch [76,77] and cross-linked quaternary chitosan [78]. Contrary, the impact of pH was important in case of the hydrogel based on pullulan with pHzpc = 6.12 (the adsorbent surface has net electrical neutrality) [40]. Thus at a lower pH (less than 6) the hydrogel adsorbed RB more than MB, and at higher pH values (pH > 6), the amount of MB adsorption was higher than that of RB (Table 5). The authors 10

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80 mg L−1) could be an outcome of the supplementary adsorbed chains of polymer which bring about further intermolecular hydrophobic associations and clay particles aggregation. The flocs size distribution above LCST (P-g-pNIPAAm dose = 140 mg L−1) confirmed this assumption (Fig. 11b). The two supplementary peaks, compared to the size distribution below the LCST (see Fig. 5) points out that a part of clay particles covered with hydrophobic chains may be implicated in inter-particle associations, which led to more voluminous flocs. This finding was sustained by SEM observations of the morphology of the clay particles in the presence and absence of P-g-pNIPAAm; the clay particles are almost separate in the absence of P-g-pNIPAAm, and assembled together in high size aggregates below LCST (polymer dose = 60 mg L−1) and even in large network above LCST (polymer dose = 140 mg L−1) [20]. In the adsorption process, temperature can affect the swelling capacity of the adsorbent, and the equilibrium point reflecting the exothermicity of the adsorption phenomenon [84]. This behavior has been confirmed when the influence of temperatures on adsorption of AzB onto P-g-pAPTAC microspheres has been investigated [18]. qe,exp decreased from 113.83 to 72.46 mg g−1 with increasing temperature from 25 to 55 °C (Table 5). This could be determined by the aromatic chains of AzB which are ramified and bulky and make difficult its diffusion into the microsphere pores. One concludes the AzB adsorption occurred only onto the surface of the microspheres, and the dye diffusion into the microspheres, which is influenced by the temperature, is neglected. Increasing the temperature from 20 to 70 °C determined an increase of pullulan-graft-polyacrylamide hydrogel adsorption efficiency from 96.7 to 99.6% for MB and from 63.8 to 83.2% for RB [40]. The thermodynamic study of dyes adsorption onto P-g-pAPTAC microspheres and the porous hydrogel showed negative values of the Gibbs free energy, ∆G, for both adsorbents, this indicating a spontaneous adsorption process [18,40]. For the P-g-pAPTAC microspheres, the negative values of the enthalpy, ∆H (−21.163 KJ mol−1) and the entropy, ∆S (−60.67 J K−1 mol−1) reveal an exothermic process and a scarce disorder at the solid–solution interface during dye adsorption [18]. In case of the hydrogel adsorbent, both ∆H and ∆S have positive values (∆H = 34.46 and 20.71 KJ mol−1 for MB and RB, respectively; ∆S = 143.4 and 76.10 J K−1 mol−1 for MB and RB, respectively) that point out an endothermic nature and the freedom of the dye ions during adsorption and the high degree of randomness at the hydrogel–dye solution interface [40].

5. Conclusions and perspectives Pullulan derivatives showed high removal efficiency of both inorganic (clay) and organic (dyes, pesticide formulations) contaminants through adsorption and coagulation/flocculation methods. The characteristics of polyelectrolytes, contaminants and medium affected the separation efficiency as follows: (i) The ionic pullulan derivative is more efficient (OPD: 4 mg L−1) than the nonionic one (OPD: 60 mg L−1) for the flocculation of clay particles; the bulkier groups in lambda-Cyhalothrin than in alpha-Cypermethrin led to a slightly lower removal efficiency for KZ than for F, while the bulkier aromating substituent in AO than MO determined a stronger binding force of the dye; (ii) The higher ionic content, the lower quantity of pullulan derivatives was needed to obtain the best separation efficacy of pesticide particles; larger FW was also found for samples with lower ionic content; (iii) The pullulan derivative with longer grafted chains had better separation performance than those with shorter ones; (iv) The ci values had no influence on OPD, but the smallest percentage of residual turbidity has been noticed at ci = 1 g L−1; (v) the percentage of contaminants (pesticide, dye) removal increased with their initial concentration in dispersion; (vi) the emulsion pH decrease led to OPD reduction; (vii) The flocculation investigations revealed that the pullulan derivative had good flocculation efficiency for clay particles at temperature below as well as above LCST; a higher RE% and broader FW were found a temperature above LCST than below it. The adsorption investigations showed that qe,exp decreased from 113.83 to 72.46 mg g−1 with temperature increase from 25 to 55 °C in case of the P-g-pAPTAC microspheres; the opposite effect was recorded in case of pullulan-graftpolyacrylamide hydrogels where the adsorption capacity increased from 273.3 to 398.5 mg g−1 with the temperature augmentation from 20 to 70 °C; (viii) the hydrogel adsorbent exhibited high adsorption efficiency over four consecutive sorption/desorption cycles. The higher separation efficiency proved by the pullulan derivatives investigated recommends their use as well as of other new derivatives of this polysaccharide, both in the soluble form and as hydrogel for the removal of different contaminants (clays, dyes, pesticides (insecticides, fungicides, herbicides) drugs, cosmetics, etc). Moreover, in order to obtain an improvement of the removal efficiency, and hence low (as much as possible) residual amounts of contaminants, the influence of some parameters related either to the particle dispersion properties (lower initial concentrations of particle dispersions, salt content and nature, solvent nature, mixture of contaminants) or materials ones (molar mass, hydrophilic-lipophilic balance, crosslinking degree, etc) has to be taken into account in the next investigations. The experimental results acquired from using the pullulan derivatives in flocculation/adsorption processes could contribute to application of the results to real/industrial wastewater purification.

4. Regeneration dye-adsorbed hydrogels Quite often, it has been demonstrated that the adsorption process is a cost competitive one for wastewater treatment due to the possibility to regenerate and reuse the adsorbents with high adsorption efficiency over multiple adsorption/desorption cycles [10,16]. In this context, the ability of the hydrogel adsorbent based on pullulan to be regenerated has been evaluated [40]. The analysis of the desorption process of RB and MB indicated that the percentage of desorption was high for the four cycles with a very small decrease in the adsorption (RB: from 273.3 to 235.6 mg g−1; MB: from 386.8 to 359.6 mg g−1) and desorption capacities: (RB: from 263.6 to 171.5 mg g−1; MB: from 376.7 to 297.4 mg g−1). The deprotonation and protonation of charged groups led to weaker electrostatic interactions between the dyes and hydrogel and hence the desorption of dyes from the dye-loaded hydrogel. It is also worth noting that the abatement in the desorption amounts of RB compared to MB from the dye adsorbed hydrogels was higher in each cycle. The authors explain this by the relatively large size of the RB dye molecules compared to that of the MB molecules, which accumulate on the small pores of the hydrogel. The results above confirm that the hydrogel can be successfully regenerated without losing its original activity and stability that makes it cost effective for using, in the water treatment, especially, in textile industry [40].

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