Coagulation of colloidal particles in water by chitosan

The feasibility of applying chitosan, as prepared from the crab chitin, was assessed in this study for the coagulation of colloidal particles. A series of batch flocculation tests with chitosan under different conditions was also conducted. The results indicate that chitosan is a potent coagulant for bentonite suspension. The relationship between the optimum chitosan dosage and the turbidity of the bentonite suspension is presented as a linear correlation. The evidence infers that an adequate range of the coagulant dosage is the primary consideration in determining the removal efficiency for the turbidity of the source water. It also indicates that the coagulation behavior for kaolinite by chitosan is different from that of bentonite, i.e., chitosan fails to form a good aggregate with kaolinite. Turbid water containing particles which show behavior similar to kaolinite apparently need to have some bentonite particles added as coagulant aid, thereby improving the aggregation of the colloid particles with chitosan. Moreover, the effect of pH on the coagulation efficiency of chitosan is insignificant. The evidence infers that charge neutralization is not a major mechanism controlling the formation of floc for chitosan coagulation.     

A Review of the Use of Chitosan for the Removal of Particulate and Dissolved Contaminants

Abstract

Chitosan has unique properties among biopolymers, especially due to the presence of primary amino groups. Chitosan has been used for the chelation of metal ions in near‐neutral solution, the complexation of anions in acidic solution (cationic properties due to amine protonation), the coagulation of negatively charged contaminants under acidic conditions, and for precipitative flocculation at pH above the pKa of chitosan. The coagulation and flocculation properties can be used to treat particulate suspensions (organic or inorganic) and also to treat dissolved organic materials (including dyes and humic acid). This paper will give an overview of the principal results obtained in the treatment of various suspensions and solutions: (a) bentonite suspensions; (b) organic suspensions; (c) anionic dye solutions; and (d) humic acid solutions. Stoichiometry and charge restabilization were determined for the coagulation of humic acid, kaolin, and organic dyes with chitosan, indicating charge neutralization as the dominant mechanism for removal. Charge patch destabilization and bridging mechanisms were inferred in other cases, based on the effects of the apparent molecular weight of the chitosan preparations and effectiveness of sub‐stoichiometric doses of chitosan. For dye solutions, results showed that color can be removed either by sorption onto solid‐state chitosan or by coagulation‐flocculation using dissolved‐state chitosan; the reactivity of amine groups was significantly increased when dissolved chitosan was used. For humic materials, chitosan can be used as a primary coagulant or as a flocculant after coagulation with alum or other inexpensive coagulants. The influence of the degree of deacetylation and the molecular weight of chitosan on its performance as coagulant/flocculant is illustrated by several examples.     

Parameters affecting the performance of polyelectrolytes as aids to water clarification

An investigation of the flocculation of silica and alumina suspensions by a series of synthetic flocculants of a wide range of charge density both anionic and cationic in nature has been carried out. The extent of flocculation was determined by measurements of settling rate, sediment volume and supernatant clarity. The effect of the pH of the suspension on the performance of a number of selected polymers has been investigated. In the systems examined, flocculation was largely controlled by the charge carried by the suspended particles and polymer molecules. The dosage/settling rate characteristics of kaolin suspensions flocculated with non-ionic polymers of varying molecular weight are reported. The dosages of non-ionic polymers required for optimum flocculation of kaolin suspensions of varying solids content have been measured. At low particle concentrations bridging is less effective, and is probably reduced due to the adsorption of polymer molecules on to single particles.     

Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite

An analogous study to 2:1 type montmorillonite [Tombácz, E., Szekeres, M., 2004. Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75–94.] was performed on 1:1 type kaolinite obtained from Zettlitz kaolin. Clay minerals are built up from silica tetrahedral (T) and alumina octahedral (O) layers. These lamellar particles have patch-wise surface heterogeneity, since different sites are localized on definite parts of particle surface. pH-dependent charges develop on the surface hydroxyls mainly at edges besides the permanent negative charges on silica basal plane due to isomorphic substitutions. Electric double layers (edl) with either constant charge density on T faces (silica basal planes) or constant potential at constant pH on edges and O faces (hydroxyl-terminated planes) form on patches. The local electrostatic field is determined by the crystal structure of clay particles, and influenced by the pH and dissolved electrolytes. The acid–base titration of Na-kaolinite suspensions showed analogous feature to montmorillonite. The initial pH of suspensions and the net proton surface excess vs. pH functions shifted to the lower pH with increasing ionic strength indicating the presence of permanent charges in both cases, but these shifts were smaller for kaolinite in accordance with its much lower layer charge density. The pH-dependent charge formation was similar, positive charges in the protonation reaction of (Si–O)Al–OH sites formed only at pHs below ∼ 6–6.5, considered as point of zero net proton charge (PZNPC) of kaolinite particles. So, oppositely charged surface parts on both clay particles are only below this pH, therefore patch-wise charge heterogeneity exists under acidic conditions. Electrophoretic mobility measurements, however, showed negative values for both clays over the whole range of pH showing the dominance of permanent charges, and only certain decrease in absolute values, much larger for kaolinite was observed with decreasing pH below pH ∼ 6. The charge heterogeneity was supported by the pH-dependent properties of dilute and dense clay suspensions with different NaCl concentrations. Huge aggregates were able to form only below pH ∼ 7 in kaolinite suspensions. Coagulation kinetics measurements at different pHs provided undisputable proofs for heterocoagulation of kaolinite particles. Similarly to montmorillonite, heterocoagulation at pH ∼ 4 occurs only above a threshold electrolyte concentration, which was much smaller, only ∼ 1 mmol l^− 1 NaCl for kaolinite, than that for montmorillonite due to the substantial difference in particle geometry. The electrolyte tolerance of both clay suspensions increased with increasing pH, pH ∼ 6–6.5 range was sensitive, and even a sudden change occurred above pH ∼ 6 in kaolinite. There was practically no difference in the critical coagulation concentration of kaolinite and montmorillonite (c.c.c.∼ 100 mmol l^− 1 NaCl) measured in alkaline region, where homocoagulation of negatively charged lamellae takes place. Rheological measurements showed shear thinning flow character and small thixotropy of suspensions at and above pH ∼ 6.7 proving the existence of repulsive interaction between uniformly charged particles in 0.01 M NaCl for both clays. The appearance of antithixotropy, the sudden increase in yield values, and also the formation of viscoelastic systems only at and below pH ∼ 6 verify the network formation due to attraction between oppositely charged parts of kaolinite particles. Under similar conditions the montmorillonite gels were thixotropic with significant elastic response.     

Influence of chitosan characteristics on coagulation and flocculation of organic suspensions

Several samples of chitosan with different degrees of deacetylation and of different molecular weights were tested for the coagulation–flocculation of organic suspensions. Organic suspensions were prepared by mixing mushroom powder with tap water. Experiments were carried out at pH 5, pH 7, and pH 9. Because decreasing the pH reduced the amount of chitosan required to reach the required turbidity, at pH 9, a high concentration of chitosan was required to achieve the required treatment levels, whereas the difference was less significant between pH 7 and pH 5 (the required concentration of chitosan was halved). Though viscosity, correlated to the molecular weight of chitosan, affected treatment performance, its influence on the efficiency of coagulation–flocculation could be substantially reduced by slightly increasing the concentration of the polymer. This is of importance in the processing of industrial effluents: the aging of a chitosan solution, which may cause partial depolymerization, and loss of viscosity, will have a limited impact on process efficiency. The degree of deacetylation also has a limited effect on treatment performance, especially when the degree of deacetylation exceeds 90%. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2070–2079, 2005     

Nucleation of Flocs in Dilute Colloidal Suspensions

In contrast to earlier theories which assume that coagulation of particles in dilute colloidal suspensions occurs by random collisions, we present evidence that coagulation is limited by a nucleation and growth phenomenon, typical of a phase transformation. We show that suspensions of well-dispersed particles, which are almost indefinitely stable, flocculate rapidly when seeded with pregrown flocs. The nucleation argument is supported further by the observation that flocs which are grown at low ζ potential, where the particle strongly attract, do not peptize when the pH is changed to high ζ potential, where the attraction is weak. The experiments were carried out with aqueous dispersions of alumina particles. The time-dependent change in cluster sizes was measured by in situ photon correlation spectroscopy. The observations raise the question of agglomeration as a phase transformation, involving a change in entropy, when the system moves from a dispersed to a flocculated state.     

Coagulation and flocculation of dye-containing solutions using a biopolymer (Chitosan)

Chitosan, dissolved in acetic acid, was used for the coagulation–flocculation of an anionic dye (Reactive Black 5). In acidic solutions protonated amine groups of chitosan attract dye sulfonic groups. Increasing chitosan dosage increases dye removal up to a concentration resulting in complete neutralization of anionic charges; above, the excess of cationic charges leads to suspension re-stabilization. Process efficiency increases with decreasing the initial pH of dye solution: the molar ratio between dye molecules and amine groups ([n]) respects the stoichiometry between sulfonic functions and protonated amine groups at initial pH 5; at initial pH 3 a possible dye aggregation phenomenon results in higher molar ratio [n]. The coefficient [n] depends on both the pH and the molecular weight of chitosan. The main mechanism for dye coagulation with chitosan sounds to be charge neutralization at acidic pH.     

Investigation of the role of interfacial chemistry on particle interactions, sedimentation and electroosmotic dewatering of model kaolinite dispersions

The influence of pulp chemistry on particle interactions and dewatering behaviour of colloidal kaolinite dispersions has been investigated under coagulation conditions. The dispersion shear yield stress, settling rate and consolidation showed strong dependence upon pH and ionic strength, indicating a maximum at ∼pH 3.2 which was established as the isoelectric point (iep) by particle zeta potential analysis. A “gel point” solid concentration at which the dispersion began to be significantly networked and gravity-driven consolidation of the pulp was completely suppressed, occurred at 13 vol.% (∼28 wt.%). The dewatering rates due to coagulation were significantly lower than those commonly achieved by polymeric flocculation, however the sediment consolidation was ∼25% higher when compared with flocculated pulps. Electroosmosis was found to be effective in consolidating pre-sedimented pulps to spadeable pastes (∼30 vol.%) at pH values away from the iep where zeta potential was higher and ionic strength low (10^− 3 M). This pulp consistency or markedly improved consolidation behavior is not achievable under coagulation and/or flocculation conditions.     

煤与高岭石表面性质对凝聚过程的影响机制

煤泥水中含大量高岭石等黏土矿物,为其絮凝沉降带来较大的困难。为明确矿物性质对凝聚过程的影响机制,在应用扩展的DLVO理论计算煤和高岭石颗粒间作用力的基础上,采用聚焦光束反射测量仪监测了CaCl2用量为4.50 mmol/L时20 g/L的煤和高岭石的悬浮液在60, 100和150 r/min的搅拌转速下的凝聚过程。结果表明,颗粒间的静电作用力在颗粒表面间距2?200 nm范围内起主导作用,高岭石的电负性较大,在凝聚过程中更难发生靠近和碰撞;较高的转速可为颗粒提供较大的动量,有利于提高碰撞频率,缩短完成凝聚所需时间,实验条件下,煤和高岭石的凝聚时间分别由74和123 s缩短至47和89 s。疏水性作用力在颗粒表面间距小于2 nm的范围内起主导作用,决定了颗粒的黏附效率;煤因强疏水性,在碰撞后更易黏附,且能抵抗更高的流体剪切作用,可由19.32 μm凝聚形成100 μm的大凝聚体,而高岭石则因其亲水性难以得到较大粒度的凝聚体,均小于30 μm。     

Stability of kaolinite dispersions in the presence of sodium and aluminum ions

The stability of colloidal kaolinite dispersions in the presence of NaCl and AlCl₃ was studied by measuring turbidity, electrophoretic mobility and adsorption. The kaolinite particles coagulated at pH 2.5–3.5 and were dispersed at pH >4.5. These results well obeyed the classic DLVO theory if the mean zeta potential of the kaolinite particles in aqueous solutions was taken into account in the computation of potential energy of electrical double layer repulsion, which suggests that the kaolinite particles might coagulate in the same way as normal colloidal particles. The kaolinite particles in aqueous aluminum salt solution only coagulated at a medium AlCl₃ concentration, and formed a stable dispersion at a high salt concentration. This is caused by Stern-layer adsorption of hydrolyzed aluminum species, probably adsorbed on the kaolinite surfaces through hydrogen bonds between the hydroxyl groups of the aluminum species and the oxygen atoms on the kaolinite surfaces.     

Comparison of the performance of chitosan and a cationic polyacrylamide as flocculants of emulsion systems

Food industries contribute significantly to the pollution of water streams. Oil in wasterwaters must be removed to prevent fouling of process equipment, to reduce interference with subsequent water-treatment units, and to comply with water-discharge requirements. Polyelectrolytes are frequently used to coagulate and flocculate colloidal systems. A model system of a sunflower oil/water emulsion was used to analyze the effect of:(i) emulsion droplet sizes, (ii) surfactant chain length (sodium dodecyl sulfate and sodium tetradecyl sulfate), (iii) ionic strength, and (iv) pH, on the doses of chitosan (natural cationic polyelectrolyte) and a cationic polyacrylamide, necessary to neutralize electrical charge and to get flocculation. Methods used were: turbidimetry, jar test, colloidal titration, and light microscopy observation. Results showed that the increase of NaCl concentration reduces the doses of chitosan and polyacrylamide to reach zero colloidal charge; these doses increase with the surfactant chain length. pH variation did not show any influence on the chitosan dose necessary to flocculate the system. Redispersion of the emulsion was observed in chitosan and polyacrylamide treatments.     

Influence of pressure on filtration of aqueous alumina suspensions

The dispersibility of colloidal alumina particles (median size 310 nm) was related to the surface potential, the solid concentration in a suspension and the pressure applied to the particles. The consolidation behavior of colloidal alumina particles with an isoelectric point pH 8.7 was examined using a developed pressure filtration apparatus at 1–10 MPa of applied pressure. The height of 7 or 20 vol% alumina suspensions at pH 3.0, 7.8 and 9.0 as a function of filtration time was fitted by a filtration model developed for a flocculated suspension rather than a traditional filtration model for a dispersed suspension. An increased pressure, a decrease of particle concentration and a porous microstructure of colloidal cake reduced the consolidation time of alumina suspension. The wet alumina compacts were significantly compressed during filtration but relaxed after the release of the applied pressure. However, the packing density of alumina compact after calcination at 700 °C was almost independent of the filtration pressure and controlled by the structure of network of alumina particles in a solution.     

Effect on 2,2′-bipyridine adsorption of poly(vinyl alcohol) adsorbed on Na-kaolinite

The adsorption (pH 6) of 2,2′-bipyridine (BP) on Na⁺-kaolinite and on samples of the same clay containing 4.35 and 7.45 mg/g, respectively, of previously adsorbed poly(vinyl alcohol) (PVA) was studied. Specific adsorption of BP decreases with increasing clay particle size in the suspension as a result of particle association. Aqueous suspensions with a 1–4% concentration of kaolinite exhibit flocculation in the absence or presence of PVA, forming clusters of 43–64 units of single clay particles. The PVA adsorbed on the surface of sodium kaolinite particles causes partial dispersion of the kaolinite in the aqueous suspension; however, 7.45 mg/g of PVA adsorption or even lower (4.35 mg/g) is not able to fully impede the association of kaolinite particles.     

Flocculation performance of modified chitosan in an aqueous suspension

A cationic moiety, N‐(3‐chloro‐2‐hydroxypropyl)trimethyl ammonium chloride (CHPTAC), was incorporated onto chitosan in an aqueous alkaline solution. Thus, modified chitosan was prepared. A series of modified chitosans with different molecular weights and charge densities were synthesized through the alteration of the molar ratio of CHPTAC to chitosan in the reaction mixture. The synthesized modified chitosans were thereafter characterized by a variety of physicochemical characterization techniques to confirm that modification did take place. Furthermore, the feasibility of applying modified chitosans as flocculants was assessed, and they were compared with the native chitosan in model colloidal suspensions of kaolin and iron‐ore powder. The results indicated that the unmodified chitosan itself was a good flocculating agent. The flocculation performance of the chitosan could be altered by the incorporation of the CHPTAC moiety. Studies showed that not all the modified chitosans had superior flocculation performance versus the native chitosan. Among the different grades, the modified chitosan with a moderate molecular weight and a moderate charge density showed the best flocculation performance in both model suspensions. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Dispersion—flocculation studies on a Goethite-clay system

Dispersion–flocculation studies on a Goethite–clay system using flocculants were carried out as a function of flocculant concentrations, pH of slurry, time of agitation and dispersant dosage. Also, the effects of pH and polymer concentrations on the adsorption behaviour of the system were investigated. Results show that well flocculated goethite was preferentially obtained from 4% goethite/kaolinite clay suspensions, with 50 ppm causticised starch in the slurry at pH values of 3–11·5 and with 50 ppm polyacrylamide at pH values of 3–8. Good flocs were also obtained on flocculation of the goethite suspension with 50 ppm polyacrylamide at pH values of 5–7, while the kaolinite suspension did not respond to the same dosage of causticised starch in the same pH range. Results further reveal that for the goethite/kaolinite suspension, the best results was obtained with 50 ppm polyacrylamide at pH values of 7–10. Based on the data generated in the study, it was concluded that causticised starch is a better flocculant than polyacrylamide for goethite suspensions although polyacrylamide is an excellent flocculant for kaolinite suspensions.     

An investigation of the flocculation characteristics of polyacrylamide‐grafted chitosan

To investigate the flocculation characteristics of polyacrylamide (PAM)‐grafted chitosan, a series of PAM‐grafted chitosan copolymer (Chito‐g‐PAM1 to Chito‐g‐PAM4) have been synthesized by ceric ammonium nitrate‐induced solution polymerization technique in nitrogen atmosphere. The flocculation characteristics of the polymer samples (PAM, grafted and ungrafted chitosan) were studied by settling test and jar test methods in the colloidal suspensions of kaolin, iron ore, silica, and bentonite powder. It was found that the settling performance of Chito‐g‐PAM3 is best among the polymer samples. The jar test results indicate that the ungrafted chitosan has better water clarifying performance than both the PAM and grafted chitosan. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Flux and rejection in the ultrafiltration of colloids

Charge on colloidal particles is shown to affect the filtration process in two ways. The effect of changing the charge of the particles, such as by varying pH, is to alter the observable flux which goes through a minimum as the charge is increased. At fixed charge the flux first decreases with particle size and then increases. Charge also influences the mechanism of particle rejection. If primary particles are stable colloids, smaller than the surface pores, they may penetrate the membrane but not necessarily pass all the way through. When particle charge is low enough for flocculation to be significant, rejection will most likely occur at the membrane surface as flocculation results in particles growing to become large enough to be rejected.     

Effects of different parameters on the characteristics of chitosan–poly(acrylic acid) nanoparticles obtained by the method of coacervation

Chitosan–poly(acrylic acid) polyelectrolyte complex nanoparticles were prepared by coacervation under mild experimental conditions without the use of any organic solvents or surfactants. The influence of some experimental parameters such as the pH of the polyelectrolyte solutions, their concentrations, and the purification procedure on the particle dimensions and their size distribution was studied in detail. The physicochemical properties of the obtained complex were characterized with Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. It was found that for solution concentrations below 0.1 wt %, it was possible to obtain suspensions of nanometer‐sized particles. Furthermore, it was established that the pH values of the reactant solutions had a great influence on both the particle size and the yield of the complex that was formed. The most convenient pH values for obtaining chitosan–poly(acrylic acid) particles with a nanometric size and optimum yield (near 90%) were found to be 4.5–5.5 for chitosan and 3.2 for poly(acrylic acid). Additionally, the effects of dialysis and ultrasonic treatment on the stability of complex suspensions, prepared under different experimental conditions, were clarified so that recommendations could be made to bring this system into practical use. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Surface force arising from adsorbed graphene oxide in alumina suspensions with different shape and size

The effects of graphene oxide (GO) on the yield stress‐pH of α‐Al₂O₃ (alumina) suspensions were investigated. For micron‐sized platelet alumina suspensions, micron‐sized GO additive increased the maximum yield stress by as much as six‐folds. This was attributed to GO‐mediated bridging interactions between the platelet particles. This type of bridging interactions was much less effective with submicron‐sized, spherical, and irregular shape alumina. Adsorption of the anionic GO reflected by the shift of pH of zero zeta potential to a lower pH is particularly high for platelet alumina. The 1.0 dwb % GO concentration added is sufficient to reinforce each platelet particle–particle bond, assisted by a directed GO–platelet interaction configuration. This is, however, not true with submicron‐sized particles as the particle concentration increases sharply with the inverse of the particle diameter to power of 3. Moreover, a GO sheet can adsorb several submicron‐sized particles and this does not produce the right interaction configuration. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3633–3641, 2013     

Effect of pH on the suspension stability of alumina, titania and their mixtures

The electrophoretic mobility, the rheology and the sedimentation analysis of alumina, titania and their mixtures (ratio 1:2 and 2:1) were investigated in aqueous dispersions in the pH range 3–10. These results suggest that the rheology of the slips and the sedimentation rate of alumina and titania particles from suspensions are charge related. Although the rheology of suspensions and the sedimentation behavior of the particles containing alumina and titania mixtures are charge related, the overall stability of the slips is controlled by the degree of heteroflocculation.