Flocculative ability of uncharged and hydrolyzed graft and linear polyacrylamides

    Flocculative Ability of Uncharged And Hydrolyzed Graft And Linear Polyacrylamides N. Kutsevol, A. Naumenko, V. Chumachenko, A. Balega, L. Bulavin PII: DOI: Reference:

S0167-7322(16)33113-0 doi:10.1016/j.molliq.2016.11.132 MOLLIQ 6679

To appear in:

Journal of Molecular Liquids

Received date: Accepted date:

31 October 2016 29 November 2016

Please cite this article as: N. Kutsevol, A. Naumenko, V. Chumachenko, A. Balega, L. Bulavin, Flocculative Ability of Uncharged And Hydrolyzed Graft And Linear Polyacrylamides, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.11.132

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ACCEPTED MANUSCRIPT FLOCCULATIVE ABILITY OF UNCHARGED AND HYDROLYZED GRAFT AND LINEAR POLYACRYLAMIDES N. Kutsevol, A. Naumenko, V. Chumachenko, A. Balega and L. Bulavin

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Taras Shevchenko National University of Kyiv 64/13, Volodymyrska 01601 Kyiv, Ukraine

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Abstract: Uncharged and anionic copolymers Dextran-graft-Polyacrylamide (D-g-PAA) and linear PAA of similar molecular weight were tested as flocculants in model kaolin polydisperse suspensions (C = 3 g/dl) having high content of particles less than 2 μm in size (more than 60%) which are difficult to settle down. The flocculation process was characterized by suspension sedimentation rate and degree of supernatant clarification. Dependence of flocculation on the degree of branched copolymers compactness as well as the degree of hydrolysis of the amide groups in the polymers was investigated. The main regularities of selection of suitable branched flocculants based on D-g-PAA and optimal their concentration for effective flocculation process was obtained. It was shown that D-g-PAA in ionic form with the lower grafting efficiency and long PAA grafts ensured the highest degree of supernatant clarification and the highest rate of the suspension sedimentation.

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Keywords: water purification; flocculation; branched copolymers

Introduction

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Mineral industry waste water contains problem minerals such as clay, the majority being kaolinite. Heavy metal ions characterized high toxicity and migration ability. Kaolinite due to the small particle size and particular structural and charge properties are difficult to settle and dewater. These compounds should be removed in an environmentally and economically acceptable manner. Flocculation is one of the generally used techniques for industrial processes of mineral processing, wastewater clarification, sludge dewatering etc. Polyacrylamide is the most common type of polymeric flocculants used due to low cost and possibility to synthesize polyacrylamide with variable functionality (positive, neutral, negative). In the last years many investigations are focused on replacing linear flocculants with branched polymers. Branched polymers become of special interest because of their controlled internal molecular structure. There is now a good evidence that such polymers possess unique properties since the number of variable parameters are overwhelmingly large, namely, initial polymer architecture, average degree of polymerization, solubility properties, distance between grafts, nature and flexibility of backbone and grafts, etc. [4-6]. The additional factors affecting the internal structure appear for branched polyelectrolytes: charge density or pH, nature of the charge distribution, valence and nature of counter ions, ionic strength, solvent quality [7-10]. Due to the structure peculiarities the local concentration of functional groups in branched polymers is notably higher than in linear ones [11], therefore they should be perspective materials for technological applications namely for pollution problems solving: sorption of toxic metal ions [12] from water medium or regulation of stability of disperse systems. Star-like copolymers Dextran-graft-Polyacrylamide and linear PAA of similar molecular weight in neutral and anionic forms were tested as flocculants using model kaolin polydisperse suspensions.

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Experimental

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Dextran-g-Polyacrylamide (D-g-PAA) copolymers consist of Dextran core (Mw=20 000 or Mw=70 000) and Polyacrylamide grafts were synthesized by radical polymerization [13]. The copolymers were designed as D20-g-PAA and D70-g-PAA, respectively. A theoretical number of grafting sites per Dextran backbone for both series of copolymers according to synthesis condition were equal to n = 20. The samples were designed as D20(70)-gPAA20. Linear Polyacrylamide (PAA) with Mw=1.4×106 was synthesized for comparative investigations. The samples were characterized self-exclusion chromatography coupled with light scattering (LS) and refractometry (Rf) after synthesis and their internal structure very analyzed in detail in [13]. Alkaline hydrolysis of synthesized copolymers and linear samples was used to obtain branched polyelectrolytes dextran-graft-polyacrylamide/polyacrylic acid (D20(70)-g-PAA20 PE). Synthesis and characterization of polyelectrolytes was described in details in [13, 14]. Branched copolymers and linear PAA (in uncharged and anionic form) were tested as flocculants in model kaolin suspension.

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Methods

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Self-exclusion chromatography SEC analysis was carried out by using a multidetection device consisting of a LC-10AD Shimadzu pump (throughput 0.5 ml∙mn-1), an automatic injector WISP 717+ from WATERS, 3 coupled 30 cm-Shodex OH-pak columns (803HQ, 804HQ, 806HQ), a multi-angle light scattering detector DAWN F from WYATT Technology, a differential refractometer R410 from WATERS. Distilled water containing 0.1M NaNO3 was used as eluent. The polymer concentrations in solutions prepared for the SEC analysis (C = 1 g/l) were below overlap concentration C‫=٭‬1/[η], therefore the intermolecular interactions can be neglected. Potentiometry Potentiometric titration was performed using a pH meter pH-340 (Russia). Solutions of HСl (0.2N) and NaOH (0.2N) as titrants were used. Polymer concentration was 2 g/l. The measurements were performed at 25.0C under argon, with constant stirring. Flocculation ability test Flocculation tests were performed in 50 ml graduated cylinders. Kaolin KOM (Poland) with high content of 2 μm particles (more than 60%) was applied in 30 g/l suspensions. Polymer concentrations were varied in the range C = 110-4 ÷ 110-1 g/l. The cylinders were inverted 12 times in order to mix the kaolin suspension with a dose of flocculant solution. The kaolin sedimentation was measured by observing the height of clarified liquid vs time. The sedimentation rate (V, mm/sec) and the supernatant clarity were analyzed by measuring the absorbance at 540 nm (D) in 20 min of flocculation.

Results and discussion Molecular parameters of graft copolymers obtained by SEC are represented in Table 1.

ACCEPTED MANUSCRIPT Таble 1 Molecular parameters of polymers

Rg, nm 64 46 68

(Rg2/Mw)×103, m2∙mol/g 2.85 2.75 -

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D70-g-PAA20 D20-g-PAA20 PAA

Mw×10-6, mol/g 1.43 0.77 1.40

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Sample

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Synthesized samples are branched star-like copolymers with dextran core and PAA-arms [13]. They characterized by higher compactness in comparison with linear analogues. Compactness can be estimated as Rg2/Mw [13] (see Table 1). When the value of Rg2/Mw is lower, the compactness is higher. The compactness becomes higher with decreasing the distance between grafts (as for D70-g-PAA20 sample). The branched and linear samples were saponified in the aim to obtain branched polyelectrolytes. The degree of conversion (, %) of amide groups into carboxylate ones is shown in Table 2. The degree of conversion is higher for branched polymers than for linear PAA for each hydrolysis time. Star-like structure of molecules provides higher local concentration of functional groups resulted in higher degree of hydrolysis. Table 2

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Conversion degree of polymers samples at various hydrolysis time

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Sample Hydrolysis time, min D70-g-PАА20 D20-g-PАА20 PАА

α, % 7.5 33 20 -

15 34 21 21

30 37 36 28

60 52 50 34

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D20(70)-g-PAA20 and PAA samples were analyzed as flocculants at wide concentration range. Increasing in concentration of polymer-flocculant leads to enhancement of flocculation ability both uncharged and anionic branched and linear polymers (Tables 3 and 4). The D20(70)g-PAA20 in uncharged form having close Rg2/Mw value demonstrated similar sedimentation rate of kaolin suspension, but supernatant clarification was better for sample D20-g-PAA20 with shorter distance between PAA-grafts. Linear PAA was just above of branched flocculant in sedimentation rate of clay suspension, but the supernatant clarification was significantly lower. Table 3 Sedimentation rate (V) of kaolin dispersion after treatment with different concentration (Cfloc) of flocculant

Sample Cfloc, g/l D20-g-PAA20 D70-g-PAA20 PAA D20-g-PAA20(PE) D70-g-PAA20(PE)

V, mm/s 1∙10-4 1.04 0.98 0.93 0.95 0.76

1∙10-3 1.46 1.55 1.75 1.72 2.18

1∙10-2 3.20 3.18 3.60 4.79 5.10

1∙10-1 6.15 6.18 6.91 10.89 11.34

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1∙10 0.385 0.517 0.990 0.826 0.954 0.481

1∙10-2 0.229 0.261 0.282 0.228 0.221 0.265

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1∙10 0.815 1.000 1.287 0.645 0.879 0.734

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Sample Cfloc, g/l D20-g-PAA20 D70-g-PAA20 PAA D20-g-PAA20(PE) D70-g-PAA20 (PE) PAA(PE)

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Optical density (D) of supernatant after treatment of kaolin dispersion with different concentration (Cfloc) of flocculant

1∙10-1 0.059 0.081 0.240 0.199 0.158 0.191

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The ability to remove the smallest kaolinite particles is the most important characteristic for flocculants used in mineral industry waste water recycling. Supernatant optical density after kaolin suspension treatment by flocculants of various molecular structure are shown in Fig. 1 and Fig. 2 (the smallest and highest flocculant dose within studied concentration range). The branched polymer-flocculant at Cfloc=1.10-4g/l has stabilized the smallest particles of clay dispesion during flocculation process. This effect was not observed for linear PAA. The similar results were reported in [15] for kaolin dispersion at much lower concentration (Ckaol = 4 g/l). The stabilizing effect increases for star-like copolymer with closer distance between PAAgrafts decreasing (or higher compactness of macromolecule).

Fig. 1. Supernatant clarity in 20 min after kaoline dispersion treatment by polymerflocculants. Cfloc=110-4 g/l

Fig. 2. Supernatant clarity in 20 min after kaolin dispersion treatment by polymerflocculants. Cfloc=1. 10-1 g/l

The star-like PAA-flocculant at Cfloc=110-1g/l has revealed the higher ability for suppernatant clarification then linear PAA. The branched flocculant with closed distance between PAA-glafts was most efficient for binding and removal small clay particles in flocculation process. Obviously such polymer structure works as a „dense brush“. The degree of polymer-frocculant hydrolysis considerably affects on the flocculation rate for samples with 7,5 and 15 min of saponification (Fig. 3). Then, the charged macromolecules have extremely extended conformation, which don’t change more with increase in charge density in polymer chains during father saponification. Analysis of the sedimentation rate of kaolin

ACCEPTED MANUSCRIPT suspension reveals the close flocculation efficiency of polymer samples with 15, 30 and 60 min ofhydrolysis. “Falling snow” effect was observed at Cfloc = 110-1 g/l. However it was not possible to resume the influence of polymer internal structure and charge appearance on flocculation process at low concentration of anionic flocculant.

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Fig. 3. Dependence of suspension setting rate on polymer-flocculant hydrolysis time: 1- D70-gPАА20(PE); 2- D20-g-PАА20(PE); 3- PАА(PE). Сfloc=110-2 g/l

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Effect of molecular structure on supernatant clarity has complicated character at low concentration of polymer-flocculant, Cfloc =110-4÷110-3 g/l (Table 4). With polymer concentration increasing to Cfloc =110-2÷110-1 g/l some regularity in supernatant clarification verse internal polymer-flocculant structure and degree of samples hydrolysis can be observed (Table 4, Fig. 4, Fig. 5).

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Fig. 4. Dependence of optical density on hydrolysis time of flocculant in 20 min after setting of kaolin suspension: 1- D70-g-PАА20; 2- D20-g-PАА20; 3- PАА. Сfloc = 110-2 g/l

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Fig. 5. Dependence of optical density on hydrolysis time of flocculant in 20 min after setting of kaolin suspension: 1- D70-g-PАА20; 2- D20-g-PАА20; 3- PАА. Сfloc = 110-1 g/l.

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The dependences have extremal character for star-like flocculants at Сfloc = 110-2 g/l. Increasing in hydrolysis time leads to decreasing the suppernatant optical density. For linear PAA the effect is reverse. Branched flocculants clarify supernatant better at Сfloc = 1.10-1 g/l than linear PAA irrespective of hydrolysis time of polymer-flocculant (Fig. 5). The copolymer D70-g-PAA20 hydrolysed 60 min are the most efficient for supernatant clarification (Fig. 6) as well as for rate of flocculation process (Fig. 3) at optimal flocculant dose.

Fig. 6. Supernatant clarity in 20 min after kaoline dispersion treatment by various doses of anionic polymer-flocculants (60 min of hydrolysis).

Finally, comparative analysis of uncharged and anionic polymer-floccunlant testifies that D20(70)-g-PAA20(PE) are more efficient in flocculation rate at optimal concentrations in (Fig. 7). It can be explained by drastic increasing of macrocoil size due to straightening of PAA-arms in star-like structures with charge appearance [14]. Meanwhile supernatant clarity for anionic flocculants was worse in comparison with uncharged samples (Fig. 8). Obviously the smallest particles of kaolin dispersion cannot be caught due to Coulomb repulsion of the negatively charged functional groups of the polymer chain and the negatively charged surface of kaolin particles.

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Fig. 7. Suspension setting rate in flocculation by uncharged and anionic polymer-flocculants.

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Cfloc = 110-2 g/l.

Fig. 8. Supernatant clarity in 20 min after kaolin dispersion treatment by noncharged and anionic polymer-flocculants. Cfloc = 110-1 g/l

Conclusions Branched noncharged copolymers Dextran-g-Polyacrylamide and their anionic derivatives were synthesized. The degree of conversion of amide groups is higher for branched polymers due to the higher local concentration of functional groups. Branched noncharged copolymers Dextran-g-Polyacrylamide and their anionic derivatives were tested and compared with linear polymer-flocculants in flocculation aim for clay dispersion setting. All investigated branched copolymers exhibit high flocculation activity as in noncharged and ionic forms. Moreover, in contrast to linear polymers not only the size of branched macromolecule polymer-flocculants, but its molecular structure influences on flocculation parameters in model kaolin polydisperse suspensions (sedimentation rate and degree of supernatant clarification).

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The analysis of the kinetics of flocculation process revealed that the flocculation activity is more significant for the branched sample having higher compactness. It was obtained, that at low concentrations of branched flocculants (Сfloc = 110-4 g/l) stabilization of residues of the kaolin suspension takes place. Degree of supernatant clarification for branched polymers at optimal flocculants concentrations is comparatively higher than for linear PAA. The branched polyelectrolytes Dextran-graft-(Polyacrylamide-co-Polyacrylic acid) are less efficient in clarification but rather more effective in sedimentation rate in flocculation process of polydispersed kaolin suspension in comparison with noncharged Dextran-g-Polyacrylamide copolymers. In the case of 20-34 % conversion degree of the branched samples the sedimentation rate increases, but further conversion of amide groups into carboxylate ones do not influence on flocculation substantially. Nevertheless, the best supernatant clarification was achieved for less compactness branched polyelectrolytes at flocculant consentration 10-210-1 g/l. Thus, to target specific technical tasks it can choose copolymer flocculant-D20(70)-PAAn (in nonionic or ionic form), which is due to peculiarities of the molecular structure of macromolecules in solution will provide high rate of flocculation and the best supernatant clarification.

Acknowledgements

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The authors are grateful to M. Rawiso and A. Rameau for SEC experiment at Institute Charles Sadron, Strasbourg, France.

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ACCEPTED MANUSCRIPT Highlights 

branched noncharged copolymers Dextran-g-Polyacrylamide and their anionic derivativeswere

synthesized copolymers were testedas flocculants in model kaolin polydisperse suspensions

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suspension sedimentation rate and degree of supernatant clarification were studied at different

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analysis of the kinetics of flocculation process allows to choose the most effective flocculant

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flocculants concentration