Flocculation of dilute titanium dioxide suspensions by graft cationic polyelectrolytes

The flocculation of a dilute titanium dioxide (TiO₂) suspension using homopolymers and graft copolymers of acrylamide (AM) and diallyldimethylammonium chloride (DADMAC) was investigated. The graft copolymers produced by γ-irradiating the mixtures of polyacrylamide (PAM) and polyDADMAC gave better flocculating performance than homopolymers, reflecting the higher fractions of large particles and bigger floc size. A kinetic delay in the onset of flocculation was observed after adding the copolymers in the dose range 5–30 [mg polymer]/ [g TiO₂]. Increasing dosage resulted in a longer delay period. No significant flocculation was observed when the dose was above 50 [mg polymer]/[g TiO₂]. This delay was interpreted in terms of the re-conformation of polymer chains driven by charge neutralization, between the positively charged polymer branches and the negative particle surface. Depending on the dosage used, the flocculation behavior of the graft copolymer has been suggested to be equilibrium and non-equilibrium flocculation. It was also observed that re-conformation is not affected by the ion strength of the media, but a strong shear force significantly reduces the chain reconformation time. Received: 9 April 1998 Accepted in revised form: 28 August 1998     

Formation and flocculation of graft copolymer micelles

Criteria for formation and flocculation of micelles from pure graft copolymers were investigated in single selective solvents by turbidimetry with the use of two series of graft copolymers from poly(vinyl acetate) (PVAC), i.e., PVAC–styrene graft copolymers with one branch and PVAC–methyl methacrylate graft copolymers with one and several branches. These graft copolymers could be completely coagulated through two processes in the selective solvents which had widely different ? temperatures. The first process is the formation of micelles. One sequence, i.e., either backbone or branch of the graft copolymers, becomes desolvated under conditions similar to those for the corresponding homopolymer. This results in formation of the core of the micelle, the other soluble sequence extending from the surface of the core into the solvent phase. As the soluble chains cover the micelle core, no macroscopic phase separation occurs, but a stable dispersion is formed. The second process is that the micelle becomes too unstable to exist as dispersed when the solvency of the medium for the soluble sequence decreases to a certain degree. As a result, flocculation of the micelle finally takes place.     

Ammonium persulfate-initiated polymerization of cationic starch-grafted-cationic polyacrylamide flocculant for the enhanced flocculation of oil sludge suspension

In this study, a novel, highly efficient and environmentally friendly flocculant, namely, cationic starch-grafted-cationic polyacrylamide (CS-g-CPAM), was synthesized by initiation polymerization of ammonium persulfate. First, CS-g-CPAM was polymerized with cationic starch(CS), acrylamide(AM) and diallyl dimethyl ammonium chloride (DMDAAC), and then the influence factors of graft polymerization were investigated, including total monomer concentration, initiator dosage, the monomer mass ratio of m_AM: m_CS: m_DMDAAC, post-polymerization temperature and post-polymerization time. And the intrinsic viscosity of the CS-g-CPAM was measured by the one point method accurately. The chemical structures and morphology of the samples were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermo-gravimetric and differential scanning calorimetry (TG-DSC), and scanning electron microscope (SEM). The CS-g-CPAM was utilized to flocculate the oil sludge suspension, the effects of CS-g-CPAM dosage, temperature and pH value on the flocculation performance were investigated. The results show that CS-g-CPAM has outstanding flocculation effect.     

Estimation of flocculation structure in filled polymer composites by dynamic rheological measurements

 The frequency and concentration dependences of the storage modulus (G ^′) for carbon black and short-carbon-fiber-filled polymer composites were investigated by means of dynamic rheological measurements. It was found that G ^′ at low frequencies and amplitudes could be used as a sensitive experimental parameter for detecting the flocculation structure of the ultra-fine-particle-filled polymer composites. Correlation of electrical resistivity of the composites to the relative storage modulus, G ^′ _r(=G ^′ _c/ G ^′ _p), revealed that the three-dimensional interparticle networks start to construct through the matrix when G ^′ _r increases to 7 regardless of the composite systems. Quantitative calculations in order to determine the flocculation structure were carried out by means of the modified Kerner equation. A plot of the calculated value, defined as the floc index A, dependence of electrical resistivity for various systems was found to be a universal curve. Accordingly, we suggest that A might universally correspond to the flocculation structure of the filler, which is independent of the nature of the filler, the molecular weight, the chemical composition of the polymer and the temperature at which the measurement is made. This method is particularly effective for estimating the flocculation structure of ultra-fine-particle-filled polymer composites no matter whether the filler is conductive or not. Received: 26 May 1999/Accepted in revised form: 28 September 1999     

Adsorption and flocculation by polymers and polymer mixtures

Polymers of various types are in widespread use as flocculants in several industries. In most cases, polymer adsorption is an essential prerequisite for flocculation and kinetic aspects are very important. The rates of polymer adsorption and of re-conformation (relaxation) of adsorbed chains are key factors that influence the performance of flocculants and their mode of action. Polyelectrolytes often tend to adopt a rather flat adsorbed configuration and in this state their action is mainly through charge effects, including ‘electrostatic patch’ attraction. When the relaxation rate is quite low, particle collisions may occur while the adsorbed chains are still in an extended state and flocculation by polymer bridging may occur. These effects are now well understood and supported by much experimental evidence. In recent years there has been considerable interest in the use of multi-component flocculants, especially dual-polymer systems. In the latter case, there can be significant advantages over the use of single polymers. Despite some complications, there is a broad understanding of the action of dual polymer systems. In many cases the sequence of addition of the polymers is important and the pre-adsorbed polymer can have two important effects: providing adsorption sites for the second polymer or causing a more extended adsorbed conformation as a result of ‘site blocking’.     

Synthesis, characterization and flocculation performance of zwitterionic copolymer of acrylamide and 4-vinylpyridine propylsulfobetaine

A novel zwitterionic polyacrylamide AMVPPS copolymer containing sulfobetaine groups was synthesized by copolymerizing acrylamide (AM) and 4-vinylpyridine propylsulfobetaine (4-VPPS) in 0.5 mol/L NaCl solution with potassium persulfate (K₂S₂O₈) and sodium bisulfite (NaHSO₃) as initiator. The structure and composition of AMVPPS copolymer were characterized by FT-IR spectroscopy, ¹H NMR and elemental analyses. Thermal stability and solution properties of AMVPPS copolymer were studied by thermogravimetry analysis (TGA) and viscometry. Anti-polyelectrolyte behavior was observed and was found to be enhanced with increasing 4-VPPS content in copolymer. The flocculation performance for 2.5 g/L kaolin suspension and 2.5 g/L hematite suspension was evaluated by transmittance measurement and phase contrast microscopy. The effects of 4-VPPS content in the copolymer, intrinsic viscosity and the added salt on the flocculation performance were investigated. It was found that AMVPPS copolymer was a good flocculant for both anionic kaolin and cationic hematite suspensions and the flocculation performance of copolymer was much better than that of pure polyacrylamide (PAM). A very wide range of the optimum flocculation concentration, named as “flocculation window”, was found for both suspensions. These flocculation characteristics were mainly dependent on the charge neutralization, the intragroup conformation transition from water to NaCl solution and then the interchain bridging of the zwitterionic AMVPPS copolymer.     

Investigation of adsorbed polymer layers by flocculation of oppositely charged sols

Measuring the flocculation of oppositely charged sols is a suitable method for investigating the stabilizing effect of thick adsorbed polymer layers since the values of the electrical attractive potential are much higher than those of the van der Waals-London attraction. In this case flocculation occurs at low electrolyte concentrations and thus the precipitation of the polymer that normally occurs at high electrolyte concentration can be prevented. These observations were proved by the mutual flocculation of positively charged Fe₂O₃-sol and negatively AgI-sol, in the presence of large amounts of adsorbed polyvinylalcohol. The total free energy of interaction has been calculated for 1.5 mg/m² adsorbed polymer amount and for two electrolyte concentrations, and the results were in saticfactory agreement with the experimental values.     

Synthesis and flocculation performance of graft and random copolymer microgels of acrylamide and diallyldimethylammonium chloride

Graft (from linear homopolymers) and random (from a linear random copolymer) copolymer microgels of diallyldimethylammonium chloride (DADMAC) and acrylamide were synthesized via a free-radical mechanism using a γ-radiation technique. These copolymer microgels were evaluated as flocculants on a model dilute TiO₂ colloid suspension using a turbidimeter and a disc centrifuge photosedimentometer, and their performances were compared with the linear homopolymers and their blends. It was found that microgels produced after an appropriate irradiation time showed improved flocculation behavior over their nonirradiated linear counterparts. The graft microgels performed better than the corresponding random microgels. For a γ-radiation dosage of 100 krad/h, the graft microgels obtained by irradiating a 30% DADMAC (by weight) homopolymer blend for 3 h showed the maximum reduction in the relative turbidity of the TiO₂ suspension as well as the largest fraction of larger particles flocculated. Received: 18 May 1999 Accepted in revised form: 1 June 1999     

Particle adhesion in coagulation and bridging flocculation

The adhesive forces between solid particles mutually attached during coagulation or bridging flocculation are important for modelling floc stability. Results are presented in this study which are obtained from experiments on the adhesion of glass or quartz particles to a flat glass substrate (centrifugal method) or to the wall of a glass capillary through which an aqueous electrolyte solution was passed. Coagulation experiments carried out in 10^–2 mole/1 MgCl₂ showed the action of hydration layers on hydrophilic surfaces, whereas surface methylation is associated with adhesion in the inner potential minimum. In addition essential ageing effects interpreted as interparticle gelation were observed, especially on the interaction of alkali glass surfaces. Adhesive strength in the case of flocculation with hydrolysed polyacrylamide and a cationic Praestol mainly depends on the polymer concentration and on the preadsorption conditions before the particle-substrate attachment. A significant strengthening of adhesion due to reconformation of the bridging agents was not observed for a contact time greater than 3 min. The effect of steric stabilization with polymer overdosing could be proved by a special preparation technique.Publication no. 922 from the Research Institute of Mineral Processing of the Academy of Sciences of German Democratic Republic, Freiberg.     

Reversible flocculation of silica suspensions by water-soluble polymers

The possibility to concentrate, then redisperse colloidal suspensions is not only of great theoretical interest, but is also relevant to the industrial process of solid-liquid separation, which must often be followed by a redispersion stage. Up to now, these consecutive operations were unrealizable in the presence of polymers, since flocculation and adsorption were generally considered irreversible in this case. Previous studies have pointed out the occurrence of two main flocculation mechanisms: charge neutralization and interparticle bridging. The use of copolymers makes it possible to take into account these two different mechanisms together. Using this fact we prepared new copolymers of acrylamide with N-vinylimidazole via radical polymerization and characterized them by light scattering, viscometry, potentiometric titrations, and UV studies. One peculiarity of the chosen system is its dependence on pH: actually the degree of neutralization of such cationic polyelectrolytes does vary with pH, especially near the pK value. This paper shows that these copolymers may induce reversible flocculation of negatively charged suspensions, e.g., silica suspensions, by simple pH adjustment. Performances of the system were followed by various physico-chemical methods. The observed results are explained in terms of flocculation mechanisms as a function of pH.     

Effect of hydrolysis degree of hydrolyzed polyacrylamide grafted carboxymethyl cellulose on dye removal efficiency

In this present work, a series of hydrolyzed polyacrylamide grafted carboxymethyl cellulose (CMC-g-HPAM) was prepared. The structure and solution properties of CMC-g-HPAM were characterized by FTIR, ¹H-NMR, elemental analysis and zeta potential measurements. The graft copolymers were applied as flocculants to remove methylene blue (MB), a cationic dye, from aqueous solutions. In comparison with its precursors, carboxymethyl cellulose (CMC) and polyacrylamide CMC-g-PAM, CMC-g-HPAM exhibited higher removal efficiencies. Furthermore, the flocculation performance of the copolymers was significantly improved with the increase of the hydrolysis degree, and the MB removal efficiency was more than 90 % when the hydrolysis degree of CMC-g-HPAM was higher than 80 %. More importantly, image analysis in combination with fractal theory demonstrated that the graft copolymers could produce notably denser and larger flocs, which was of great significance in practical water treatment. The improved flocculation performance was ascribed to both charge neutralization and bridging effects.     

Synthesis and flocculation performance of graft copolymer of <Emphasis Type="Italic">N</Emphasis> -vinylformamide and poly(dimethylaminoethyl methacrylate) methyl chloride macromonomer

 A comb-structured polymeric flocculant was synthesized by the aqueous copolymerization of N-vinylformamide (NVF) with poly(dimethylaminoethyl methacrylate) quats (methyl chloride) macromonomer. The effects of temperature and macromonomer concentration on the copolymerization kinetics were determined experimentally. The copolymerization reactivity ratio was measured to be 3.82 and 6.39 for NVF and macromonomers with 50 and 100 repeating units when copolymerized with NVF. The copolymer samples were also subjected to a flocculation performance test and were found to be more effective than linear random cationic copolymers in terms of cationic content, flocculation rate, final turbidity levels, and floc strength. Received: 11 June 2001 Accepted: 9 August 2001     

A kinetic investigation of the flocculation of alumina with polyacrylic acid

Using a model colloidal system of alumina and polyacrylic acid (PAA), the kinetics of flocculation was investigated at low polymer concentrations and short durations (on the order of seconds). The polymer-induced flocculation processes obeyed Von Smoluchowski's bimolecular rate equation. Increases in the concentration of the polymer resulted in higher rate constants for the flocculation process. At a fixed concentration (say 50 ppb, parts per billion), the rate constant values showed a maximum value for 250,000 g mol(-1) polyacrylic acid. At this polymer concentration, calculations of the surface coverage of alumina by PAA molecules of different molecular weights show that for all the cases the coverage is nearly the same, approximately 1x10(-3), but the flocculation response and the rates are significantly different. This trend in flocculation characteristics is attributed to the critical polymer number density requirement for effective flocculation (at least partial charge neutralization and initiation of flocculation). The mechanism governing the flocculation at ultralow concentrations (50 ppb) is the synergistic effect of partial patch neutralization and bridging.     

Structural Effect and Composition of Anionic Copolymer on Protein Flocculation

Flocculation of lysozyme with anionic copolymers of acrylamide, acrylic acid, and sodium styrene sulfonate used as flocculants was performed in pursuit of high flocculation efficiency. Two major factors, pH and ionic strength, are used to investigate the relationship of the flocculation behavior of protein by copolymers and the functional group (–COOH, –NH₂, –SO₃H) compositions of these copolymers. The protein flocculation can be controlled by adjusting pH. In addition, the various copolymers exhibit differing effects on ionic strength induced protein flocculation. FT-Raman spectroscopy was applied to investigate the mechanism of interaction between protein and copolymer. An attempt was made to understand how the pH and ionic strength change the surface chemical characteristics of protein and copolymers, as well as the relationship between the structure of copolymer and the protein flocculation process.     

Temperature influence of nonionic polyethylene oxide and anionic polyacrylamide on flocculation and dewatering behavior of kaolinite dispersions

Nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM) flocculation of kaolinite dispersions has been investigated at pH 7.5 in the temperature range 20-60 degrees C. The surface chemistry (zeta potential), particle interactions (shear yield stress), and dewatering behavior were also examined. An increase in the magnitude of zeta potential of kaolinite particles, in the absence of flocculant and at a fixed PEO and PAM concentration, with increasing temperature was observed. The zeta potential behavior of the flocculated particles indicated a decrease in the adsorbed polymer layer thickness, while at the same time, however, the adsorbed polymer density showed a significant increase with increasing temperature. These results suggest that polymer adsorption was accompanied by temperature-influenced conformation changes. The hydrodynamic diameter and supernatant solution viscosity of both polymers decreased with increasing temperature, consistent with a change in polymer-solvent interactions and conformation, prior to adsorption. The analysis of the free energy (DeltaG(ads)) of adsorption showed a strong temperature dependence and the adsorption process to be more entropically than enthalpically driven. The polymer conformation change and increased negative charge at the kaolinite particle surface with increasing temperature resulted in decreased polymer bridging and flocculation performance. Consequently, the shear yield stress and the rate and the extent of dewatering (consolidation) of the pulp decreased significantly at higher temperatures (>40 degrees C). The temperature effect was more pronounced in the presence of PEO than PAM, with 40 and 20 degrees C indicated as the optima for enhanced performance of the latter and former flocculants, respectively. The results demonstrate that a temperature-induced conformation change, together with polymer structure type, plays an important role in flocculation and dewatering behavior of kaolinite dispersions.     

Synthesis and biodegradation of poly(2-pyrrolidone-co-ε-caprolactone)s

Copolyesteramides of 2-pyrrolidone with ε-caprolactone were synthesized by ring-opening copolymerization. The copolymers were random-like and their melting temperature and heat of fusion were dependent on the polymer composition. Biodegradation by a polyamide 4 (PA4) degrading microorganism showed rapid degradation in the region of amide-rich polymer composition. On the contrary, enzymatic hydrolysis using a lipase resulted in a different tendency, that is, ester-rich copolymers hydrolyzed rapidly. Activated sludge makes copolymers degrade to CO₂ in wide polymer composition ratio. Copolyesteramides are expected to be applied as an environmentally-friendly plastics or bioabsorbable polymers in medical fields.     

Imaging c-PAM-induced flocculation of paper fibers

The flocculation of paper fibers by cationic polyacrylamides (c-PAM) was studied by imaging the fibers that remain free during flocculation. Studies with fibers of different lengths showed that the degree of flocculation increases with fiber length, with the best flocs being formed with mixtures of short and long fibers. Short fibers did not flocculate by themselves but were captured by flocs formed with longer fibers. The short fibers strengthen the floc and give it shear resistance. Shear had the expected effect of promoting flocculation at low Reynolds number but disrupting it at higher values. For a given polymer the maximum floc size for a mixture of fibers is dictated by the length distribution of the fibers. The polymer dose governs the rate of flocculation. The technique is especially useful in following the tail end of the flocculation process. At this stage a floc is almost fully grown and a small increase in its size would be very difficult to measure by conventional techniques. In contrast, the number of free fibers measured by single fiber imaging decreases rapidly at this point.     

Kinetic aspects of flocculation of bentonite clay in the presence of anionic and cationic acrylamide copolymers

The flocculation kinetics in aqueous-salt medium in the presence of anionic and cationic high-molecular-weight random acrylamide copolymers was studied in the hindered sedimentation mode, with a suspension of bentonite clay as example. The influence exerted on the flocculation parameters by the concentration and order of addition of ionic acrylamide copolymers taken in various combinations was analyzed.     

Characterization and degradation of the poly(methylphenylsiloxane)–poly(methyl methacrylate) graft copolymer

Poly(methylphenylsiloxane)–poly(methyl methacrylate) graft copolymers (PSXE-g-PMMA) were prepared by condensation reaction of poly(methylphenylsiloxane)-containing epoxy resin (PSXE) with carboxyl-terminated poly(methyl methacrylate) (PMMA), and they were characterized by gel permeation chromatography (GPC), infrared (IR), and ²⁹Si and ¹³C nuclear magnetic resonance (NMR). The microstructure of the PSXE-g-PMMA graft copolymer was investigated by proton spin–spin relaxation T₂ measurements. The thermal stability and apparent activation energy for thermal degradation of these copolymers were studied by thermogravimetry and compared with unmodified PMMA. The incorporation of poly(methylphenylsiloxane) segments in graft copolymers improved thermal stability of PMMA and enhanced the activation energy for thermal degradation of PMMA. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2521–2530, 1998     

Dynamic aspects of coagulation and flocculation

The destabilization of colloidal suspensions by salt (i.e., coagulation) or by polymer flocculants (i.e., flocculation) is a phenomenon of scientific interest as well as industrial importance. The most characteristic feature is that the system undergoes an irreversible process controlled by hydrodynamic and physicochemical conditions. In this article, we pay attention mainly to the dispersion of a model colloid composed of uniform spheres undergoing rapid aggregation. The term “rapid” means that every collision leads to the formation of a floc. With this system, theoretical predictions on the kinetics of coagulation can be compared with experimental data, by which a standardized system can be established. This was ultimately useful for analyzing the kinetics of polymer flocculation.