Search

The breakage or fragmentation kinetic function (FKF) of a given father size floc breaking to form two specific daughter size flocs cannot be measured experimentally. Indirectly it can be determined from the FKF model used to solve the dimensionless log-form population balance (PB) equation for floc fragmentation subjected to meeting three conditions: i) the computed and experimental particle size distribution function (PSDF) matched at all fragmentation time, ii) the PSDF data at zero fragmentation time was the input in the computation and iii) the duration of the experimental fragmentation be used in making the PB equation dimensionless. The fragmentation of compact polyelectrolyte-bridged silica flocs in a narrow gap concentric cylinder flow cell was investigated in the laminar Taylor vortex flow regime. The FKF data was correlated with the Taylor number. The FKF values were much smaller for large flocs as they tend to migrate to the low shear region. Cryo-SEM images of the flocs at different fragmentation times showed compact fractal structure with dimension larger than 2.

The zeta potential and yield-stress behaviour of gels prepared from as-received SHCa-1 hectorite (Ht) containing 50 wt.% impurities, mainly of calcite, dolomite and quartz, were characterized. The zeta potential is negative in the pH range 4–12. At pH 3 producing CO2 and Ca2+ increased the solution conductivity significantly and reduced the zeta potential. The buffering effect due to this chemical reaction was also observed in the yield stress–pH characterization below pH 8. Low yield stress was observed below pH 6 and above pH 12. The yield stress peaked at pH ~8, where Ht displayed a zeta potential of –20 mV. The relationship between maximum yield stress and Ht content (wt.%) was linear and was not affected by impurities. The ageing behaviour was characterized by an initial phase of rapid yield-stress increase and then by a phase of gradual increase. The Leong model performed better than the two-parameter glassy polymer ageing model in describing this ageing behaviour. However, the empirical power law model was more accurate. The timescale of the ageing process is long, lasting several days. An open cellular microstructure was observed to form in the gels. Impurities appeared not to interact with or bind to the Ht platelets.

The zeta potential-pH and yield stress-pH behaviour of washed spherical α-Al2O3 suspensions with and without adsorbed multilayer polyelectrolytes up to 4 layers of poly(styrene sulphonic acid) sodium salt or PSSNa (100 kDa) and polyethyleneimine or PEI (70 kDa) adsorbed sequentially were characterised. The maximum yield stress (τymax) of washed spherical α-Al2O3 is 81.8 Pa located at pHξ=0 at ~9.2. Particles with the first layer (layer 1) of PSSNa displayed negative zeta potential over the whole pH range of 3 to 8. Particles containing 3 layers with layer 3 being PSSNa also displayed negative zeta potential over the same pH range. The magnitude of the Layer 3 PSSNa is smaller. Similarly, the zeta potential remained positive for layer 2 and layer 4 PEI over the whole pH range of 3 to 8. At the dosage of 0.5 dwb% PSSNa and 0.1 dwb% PEI, monolayer coverage of PSSNa for layer 1 and 3 and of PEI for layer 2 and 4 should be achieved if the adsorption is uniform. The maximum yield stress of layer 1 PSSNa and layer 2 PEI was much larger than that without adsorbed additives despite the zeta potential being not zero. Additional attractive forces such as bridging, hydrophobic and hydrogen bonding must be present. TEM images showed that the adsorbed PSSNa-PEI layer surface is undulating and not uniform in thickness. This suggests that adsorption of these polyelectrolyes is not uniform. Extensive particle bridging showing the merging of layers at the interacting surface of the particles was observed from these TEM images. In contrast, α-Al2O3 suspensions with bilayer DIBMA-PEI, trilayer DIBMA-PEI-DIBMA and 4 layers DIBMA-PEI-DIBMA-PEI formed by 14 kDa DIBMA and 1.8 kDa PEI performed effectively as steric layer reducing the maximum yield stress significantly. TEM image of α-Al2O3 particles with bilayer DIBMA-PEI did not show the presence of polyelectrolyte layer suggesting that the polyelectrolyte layer is too thin to be observable.

A method of obtaining model-independent breakage rate or fragmentation kinetic function (FKF) data in the population balance (PB) equation (PBE) was proposed for the high shear breakage of compact polyelectrolyte-bridged alumina flocs. The continuous particle size distribution function (PSDF) data were obtained as a function of fragmentation time. The PSDF data at zero fragmentation time was and must be used as the input in the PB modelling. Computed temporal PSDF curves were obtained by solving the dimensionless log-form PBE using a FKF based only on the fragmenting particle and a daughter size. Another important requirement is that the experimental time must be used in the scaling. Using appropriate values for the parameters in the FKF, an excellent match between the temporal experimental and computed PSDF data was achieved. Upon meeting all these conditions, the FKF values used in the computation should truly represent the experimental values for all father-daughter size combinations encountered in the fragmentation process. The fragmentation occurred in the rotor-stator device under the conditions where the Kolmogorov microscale and characteristics velocity gradient of 37 μm and 1600 s−1. The ratio of the largest to smallest length scale turbulent eddies was 2047.

Purified sodium montmorillonite (SWy-2) gels of a few percent solids displayed pronounced time-dependent rheological or aging behavior with a long time scale. The aging behavior was characterized by an increasing yield stress with rest time. This increase continued even after a week of rest. An open sponge-like cellular microstructure of the aged gels was captured by cryo-SEM with samples prepared at high pressure. The size of the openings of the cellular structure is small, generally less than 1 μm formed by thin flexible platelet with curling edges. This structure was formed by strong attractive and repulsive forces. The rapid yield stress increase in the early stage of aging is due to rapid bond formation occurring between network platelets and free individual platelet, isolated aggregates, and platelet particles in network with free edges. Over time, all platelets are bonded in the network. During aging, the platelets in the structure would have to adjust continually in response to a net force acting on it by its neighbors. The high concentration of platelets responding to this force imbalance is the cause of the long aging time scale. The operation of the attractive and repulsive forces, and the shape and charge properties of the platelets are responsible for the cellular structure being built. At complete structural recovery, the structure should attain the state of lowest free energy. The repulsive force regulates the development of the microstructure. The aging data of the 3.3 wt % gel were fitted by different aging models.

Two superior adsorbents, namely bentonite and graphene oxide (GO), were hybridised to study the removal of copper and nickel ions from synthetic and industrial wastewater. The as-synthesised GO, bentonite/GO and bentonite were characterised by Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and N2 adsorption-desorption analysis. The factors influencing the adsorption behaviours including contact time, initial solution pH, ionic strength, initial concentration of metal ions, temperature and adsorbent dosage were systematically investigated by batch equilibrium method. The adsorption equilibrium for copper and nickel onto bentonite was attained in 90 min while equilibrium was reached in 60 min on bentonite/GO. The adsorption of copper and nickel was pH-dependent in the range from pH 2 to pH 7 and from pH 2 to pH 8. Pseudo-first-order kinetic model excellently described the adsorption of copper and nickel onto bentonite and bentonite/GO. The equilibrium adsorption data was well described by the Langmuir isotherm model and the maximum adsorption capacity was 248.9 mg/g, 558.4 mg/g, 215.8 mg/g and 402.5 mg/g for bentonite-copper, bentonite/GO-copper, bentonite-nickel and bentonite/GO-nickel adsorption systems, respectively. The bentonite/GO composite exhibited a higher adsorption capacity of both cations from synthetic wastewater than pure bentonite owning to the synergistic effect between bentonite and GO. In all adsorption studies, copper was more efficiently removed than nickel due to its higher tendency to form bond with adsorbent surfaces. The adsorption of copper and nickel on bentonite/GO was mainly due to cation exchange, intermolecular and electrostatic interactions and physisorption dominated the adsorption processes. The practical application of bentonite/GO on adsorption of copper was investigated using real wastewater and its removal efficiency was beyond 98%. The excellent adsorption performances of composites for the copper and nickel removal from wastewater demonstrated its significant potential for pollution mitigations.

Adsorption capacity and percentage removal efficiency of Cu(II) and Ni(II) ions were studied and compared between raw kaolinite and acid-activated kaolinite. Acid-activated kaolin was prepared by refluxing raw kaolinite with concentrated sulphuric acid followed by calcination to enhance its surface properties and adsorption ability. Both raw and acid-activated kaolinite samples were characterized by Fourier transform infrared spectroscopy, energy dispersive X-ray, scanning electron micrograph and zeta potential analysis. Upon acid treatment, acid-activated kaolinite was discovered to have altered chemical composition and larger BET surface area as compared with raw kaolinite. The batch adsorption studies on aqueous solution were performed under different factors such as contact time, pH condition, adsorbent dosage, initial metal ion concentration and temperature. The optimum condition was selected for each factor including a contact time of 60 min, pH of 7.0, adsorbent dosage of 0.1 g, initial metal ion concentration of 100 mg/L and temperature of 25 °C. Then, the adsorption studies on wastewater samples were carried out at the selected optimum conditions. Acid-activated kaolinite always had better adsorption capacity and percentage removal efficiency than raw kaolinite due to the increasing amount of negative charges on the adsorbent surface and the number of metal ion binding sites upon acid treatment. The adsorption kinetic obtained was well described by the pseudo-second-order model, whereas the adsorption isotherms obtained were well described by either the Freundlich or the Langmuir adsorption model. The results showed that acid-activated kaolinite adsorbent is a better option as a favourable and feasible commercial low-cost adsorbent for wastewater treatment.

The microstructures of the Clay Mineral Society’s kaolinite KGa-1b and KGa-2 suspensions obtained by cryo-SEM imaging published in “Au et al. Colloids and Surfaces A: Physicochem. Eng. Aspects, 484 (2015) 354−364” [1] are now believed to be affected by ice crystals formation. This short communication will provide the artefact-free microstructures. To prevent hexagonal ice-crystal formation, the suspension samples for microstructure cryo-SEM imaging were subjected to high pressure of up to 2000 bars and then freezing at a rapid rate of 25,000 °C s−1.

API bentonite, a key ingredient of drilling muds, displayed a negative zeta potential across the whole pH range of 2.5–12.3. It contained two very rheological active ingredients, sodium montmorillonite or Na+Mt (40.4%) and needle-shape sepiolite (16%). Due to the interplay of these two ingredients, the rheological behaviour displayed by the API bentonite suspensions is quite different to other bentonite suspensions. The gel displayed a very low or no yield stress at low acidic pH and a maximum yield stress at pH 8–12. Magnesium leaching from sepiolite at low pH and its hydrolysis plays a role in affecting the rheological properties at high pH. Yield stress and ageing behaviour was observed in suspensions with at least 3 wt% solids. The Herschel-Bulkley (HB) model described the flow behaviour of these gels. Both the HB yield stress and viscosity at a fixed shear rate of the 7 wt% gel peaked at pH 8.5 and were smallest at pH 5.2. The ageing or structural recovery behaviour is most pronounced at high pH for the most concentrated gels. Both the Leong and Nguyen-Boger ageing models described the structural recovery behaviour well. The recovery of the recently discovered very open honeycomb structure formed by Na+MT gel, after breakdown by shear was invoked to explain the ageing behaviour.

This work investigates the effect of inter-particle forces arising from adsorbed typical cationic and anionic surfactants on the morphology of ball milled gadolinium oxide (Gd 2 O 3 ). The experimental outcomes are interpreted in terms of stabilization and interaction mechanisms of fine washed Gd 2 O 3 particles (size diameter 2 O 3 particles suspension is at pH 11 where its maximum yield stress is observed. Because of hydrophobic interaction, the maximum yield stress increases by 30 times by adsorbed sodium dodecyl sulfate (SDS) and its IEP shifts slightly to a lower pH. By cetyl trimethyl ammonium bromide (CTAB), the yield stress also increases by a much smaller extent (3 times) and shifts to a higher pH of ⿼12.5. Without surfactants, the microstructure of dried Gd 2 O 3 displays the coarse particles of various shapes, i.e. rod, spherical and cubic shapes. This indicates that the milled particles remain agglomerated in dispersion. In the presence of adsorbed anionic SDS, the particles are refined together with numerous 2D nanowire or nano-rod particles at pH ⿼ 8. In contrast, coarser particles with absence of nano-rods are found when cationic CTAB is used to modify the Gd 2 O 3 surface at a pH of about 12.5. The SDS-modified suspension exhibits a much higher yield stress, which results from finer particles in suspension. This is invoked from an organic shell formed by the high adsorbability of negatively charged heads of SDS into the bare positive charge density of the particle. The organic SDS shell prevents the fine particles from re-welding during the dispersing and annealing route. This work develops an inexpensive ball-milling approach with assisted SDS surfactant for mass production of nanosized Gd 2 O 3 from bulky gadolinium material.

The yield stress and microstructure of washed, relatively monodisperse spherical zirconia (ZrO2) and titania (TiO2) suspensions at the isoelectric point (pI) were characterised. The yield stress was found to be dependent upon the particle size. At a given solid concentration, the finer suspensions produced a larger yield stress due to the higher particle concentration and hence, a greater density of attractive interaction. At pI, only the van der Waals force is in play. Vitrified fractal microstructures of these suspensions at pI were captured by cryo-SEM. A power law relationship described the (maximum) yield stress–volume fraction data for both oxides which is consistent with the prediction of scaling theory. An exponent value of ∼3 was obtained for both oxides. The fractal dimension (D f) extracted from this exponent value of the scaling law for large aggregate cluster interaction in the slow flow regime was ∼2.3. The theoretical fractal structure with the same D f constructed from monodisperse spherical particles based on the off-lattice variable-D f model showed strong resemblance to the cryo-SEM imagery of the vitrified structure for both oxides.

The mechanism of surface force, rheological and surface charge behaviours of kaolinite and graphene oxide (GO)-kaolinite suspension under various surface chemistry condition using kaolin mined in Malaysia are not clearly defined. Therefore, these properties were determined using zeta potential-pH measurements, Mular-Roberts (MR) pH-salt addition and yield stress-pH measurements at different solid concentrations. The KM40 kaolinite suspension displayed similar pH of zero zeta potential and point of zero charge (p.z.c.) at ∼5 where the maximum yield stress was located. With the presence of GO additive at 1.0dwb%, the pH of zero zeta potential was shifted to a higher pH of ∼8 similar with the maximum yield stress, with a significant increment of 63 % on the maximum yield stress. This was attributed to the arising of particle-bridging interactions between the kaolinite particles and relatively strong EDL interactions by overlapping negatively charged kaolinite particles and GO at higher pH region. However, the pH of zero zeta potential exhibited discrepancy that did not correlate well with the maximum yield stress at 0.5dwb%. The use of MR pH-salt addition method was also revealed not suitable for this case as GO was negatively charged throughout the entire pH region that did not show pH of zero zeta potential. Despite so, yield stress-DLVO relationships are still obeyed for all cases.

This study investigated the surface chemistry-microstructure-rheology relationship of two very well characterised, low impurity CMS source kaolinites; low defect KGa-1b and high defect KGa-2. The knowledge gained will improve the overall understanding by acting as a reference material for comparison with other studies. The trend of the yield stress-pH result was adopted to investigate kaolinite properties or factors that affect this trend. Both the KGa-1b and KGa-2 kaolinite slurries displayed similar zeta potential-pH and yield stress-pH behaviour. Crystallinity and particle size therefore did not affect the trend of the yield stress-pH and zeta potential-pH behaviour. The larger yield stress of the finer KGa-2 slurries is due to a higher particle concentration. Both kaolinite slurries exhibited a maximum yield stress at zero zeta potential. Their yield stress-solid volume fraction data were found to obey a power law model with the same exponent value of 3 accounting for a fractal dimension of 2.3 unaffected by pH. Cryo-SEM images of the vitrified microstructure of 35 wt% KGa-1b and 15 wt% KGa-2 suspensions at the maximum yield stress showed a 3-D microstructure formed by compacted clay aggregates which themselves are formed by a mixture of particle interaction configurations - face-face with stair-step feature and edge-face. Particle interaction modelling was discussed. Refereed/Peer-reviewed

The ageing and stability behaviour of Bentonite slurries were evaluated under the influence of Mg2 +, Ca2 + and Ba2 + ions at concentration ranging from 0.005 to 1 M. Stable gels were formed at low metal ion concentration of

The zeta potential result showed that the nano-discotic LAPONITE ® particles in dispersion were negatively charged across the whole pH range of 3–12. For the 3 wt% gel, no dispersed region was observed within this pH range. The yield stress decreased with increasing pH from 8 to 12. Particle agglomeration affected the yield stress result at low pH. Addition of 6 dwb% pyrophosphate peptizer increased the particle negative zeta potential and caused the 3 wt% gel to be completely dispersed at pH greater than 7. Again the yield stress increased with decreasing pH from 7 to 5. The critical volume fraction equation for the repulsive gel state or EDL caging effect to operate was found to be very useful for determining the phase state of the LAPONITE ® gel by the method of exclusion. A liquid sol state was deduced for the 0.45 and 0.89 wt% LAPONITE ® dispersions having conductivities equivalent to 0.0015 and 0.003 M KCl respectively. An attractive gel state was deduced for the 1.8 dwb% P 2 O 7 4− shear-thinning LAPONITE ® gels with concentration ranging from 1.5 to 5.5 wt% solids. The sol state was attributed to the Newtonian dispersions containing 1.1 to 1.5 wt% solids with 1.8 dwb% P 2 O 7 4− . The structural recovery behaviour of 3 wt% LAPONITE ® gels with 0.001 and 0.01 M KNO 3 , was well described by the two-parameter model of Rich et al. (2011b) and the three-parameter Leong model. The yield stress of the aged gel decreased with increasing temperature which contradicted a theoretical prediction.

Zeta potential (ζ)-pH and yield stress (τy)-pH behaviour of a number of kaolinite and montmorillonite slurries, including CMS-sourced materials, were compared. The CMS KGa-2 and Crown kaolin with very similar elemental composition displayed almost identical ζ-pH and τy-pH behaviour. Both displayed a pHζ=0 at 3–4 where the maximum τy was located. This pHζ=0 was higher at higher ionic strength as the pH-dependent charge increased with ionic strength and the permanent structural charge being invariant. The other kaolinite slurries (Reidel and Unimin) with different composition showed different behaviour. The surface chemistry and rheological properties of CMS-sourced SWy-2 (Na-) and STx-1b (Ca-) montmorillonite and bentonite slurries were also compared. They all displayed a negative zeta potential that was insensitive to pH. STx-1b slurries required a much higher solid concentration to form a gel. Maximum τy occurred over a broad range of pH. This pH is ∼5 for SWy-2, 8 for STx-1b

Based on flow curve of graphene oxide coated polymeric microsphere based electrorheological (ER) fluid, model independent yield stress and shear rate, γ ˙ ( τ ) , were determined using a Tikhonov regularization technique developed to solve the inverse problem of the shear rate in Couette flow viscometry. The model independent yield stress data were compared with that extracted from yield stress fluid models of Bingham plastic and Cho-Choi-Jhon (CCJ) used to fit the γ ˙ ( τ ) data. The agreement between model independent and Bingham yield stress was excellent, while the CCJ model yield stresses are generally slightly higher.

Mica slurries were found to be flocculated over the whole pH range of between 2 and 12. These slurries of different solid loading appeared to obey the yield stress–DLVO force model producing a single value for the deflocculation point zeta potential or the critical zeta potential (at point of flocculated-disperse state transition) of ~ 48 mV. Such a high value indicates the presence of an additional attractive force in addition to the van der Waals force. This force is attributed to edge-face unlike charge attraction. The use of pyrophosphate additive to minimize edge–face interactions via positive edge charge neutralization caused a large reduction in the critical zeta potential, to 22 mV. With this new value, the Hamaker constant of mica in water was determined to be ~ 13 zJ. This falls within the range of values reported for mica. Low Ca(II) kaolin suspension displayed a lower critical zeta potential of 41 mV and the use of pyrophosphate additive at 0.2 and 0.4 dwb% appeared not to decrease the magnitude of the critical zeta potential. This pyrophosphate-influence critical zeta potential is also independent of Ca(II) content in the kaolin. The established model of edge–face attraction for kaolin suspension at low pH may not be completely correct. A face–face interaction model may be more appropriate. A power law model relating the maximum yield stress with volume fraction showed a very high exponent value of 8 for mica slurries, only 3.1 for the low Ca(II) kaolin and 3.6–3.9 for the high Ca(II) kaolin suspensions. This exponent value may reflect the predominant particle–particle interaction configuration in the clay suspensions.

This investigation aims to answer the vexing question that has been with the colloidal science community for a long time, on whether we can use zeta potential data of dilute suspension to correlate with rheological parameters obtained at a much higher solid loading where the mismatch of surface properties is one of the main issue. In this study, sequential yield stress and zeta potential measurements were conducted on moderately concentrated alumina suspensions with different particle shape; spherical and platelet. The surface properties of the suspensions were varied by pH. The zeta potential–pH behaviour of the moderately concentrated suspension with a yield stress and of the dilute suspension was found to be identical for platelet alumina. The agreement was considered good for the spherical alumina in the pH range of 6 to 12. The correlation between the yield stress and the square of the zeta potential data was found to obey the yield stress–DLVO model for both platelet and spherical particles. This was found to be identical to the correlation evaluated from the yield stress and zeta potential data obtained from separate dilute and concentrated suspensions. The critical zeta potential characterising the flocculated–disperse state transition was found to be ∼40 mV for both spherical and platelet particles.

The results of the yield stress–pH and zeta potential–pH behaviour of pure kaolin and bentonite slurries, and mixed bentonite–kaolin slurries were evaluated. A high and low Ca(II) content kaolin clays were used. The location of the pH of maximum yield stress of the two pure kaolin slurries is very different, pH 9 for the high Ca(II) kaolin and 2.5 for the low. The trend of the variation of the yield stress with pH for the bentonite–kaolin composite slurries was similar to that of pure high Ca(II) kaolin irrespective of bentonite content. For the low Ca(II) kaolin, the maximum yield stress of the composite suspensions is located at the same low pH as that of pure low Ca(II) kaolin and bentonite suspensions, at pH ∼ 2.5. Bentonite content was found to determine the magnitude of the yield stress. The sequence of KOH addition, whether during or after the preparation stage, did have a significant effect on the yield stress but only for the composite slurries with a high bentonite content. A higher degree of swelling and delamination of the bentonite particles in one of the preparation methods was attributed as the cause. Zeta potential results showed that both the pure and mixed clays are negatively charged at all pH. However the magnitude is reduced by Ca (II). Low Ca(II) kaolin suspension was found to obey the yield stress–DLVO model. This study has provided an excellent approach of controlling and tailoring rheological properties of kaolin suspension with bentonite or vice versa for specific applications.

Fragmentation of polymer-bridged flocs is commonly encountered in many areas of suspension processing such as solid–liquid separation, washing and clarification. The effect of high shear impact on the fragmentation of polyelectrolyte-bridged silica flocs was investigated. The monodisperse silica suspension was first flocculated and then subjected to prolonged breakage in a shear field until a constant size distribution was attained. These flocs were then subjected to high shear impact breakage. The evolution of particle size distribution vs shearing time was monitored. This temporal evolution of size density function was compared with that obtained via population balance modelling converted to logarithmic scale to counter the 6–12 decades in particle size in volume. The Method of Lines (MoL) was employed to solve the equation. At lower breakage speed of 1500 rpm, excellent agreement between experimental and model size density functions was obtained where excellent matching was observed with the first and second moments. However, at the higher speed of 2500 rpm, the agreement is less satisfactory particularly with the variance. Scanning electron microscope images showed physical breakage of particles at this high shear rate. © 2015 Curtin University of Technology and John Wiley & Sons, Ltd.