Controlling attractive interparticle forces via small anionic and cationic additives in kaolin clay slurries

Interparticle forces govern slurry behavior in flow, mixing, sedimentation and thickening. This study evaluates the use of small anionic and cationic additives with pH to control the interparticle forces in kaolin slurry via the yield stress parameter. Both phosphate and citrate additives were found to reduce the interparticle attractive force or yield stress in the moderate pH region of 4–12. These relatively low charged additives were unable to impart a sufficiently strong repulsive interparticle force to completely disperse the slurry. Three linear relationships between yield stress and the square of zeta potential were observed in slurry with and without these additives, indicating that the yield stress–DLVO force model is obeyed in each linear region. The mid-range zeta potential region yielded a positive slope which was attributed to heterogeneous charge attraction between clay particles. It is this heterogeneous charge attraction that was weakened by the adsorbed additives. In contrast, cationic Polyethylenimine (PEI) of Mw 70,000 increases the yield stress at all pH level via bridging. Charge reversal was also observed at high PEI concentrations. In two cases, the pH of maximum yield stress and zero zeta potential coincided. A single linear yield stress–zeta potential squared relationship was observed despite particle bridging interaction being the dominant interparticle force.     

Incorporation of kaolin fillers into an epoxy/polyamidoamine matrix for coatings

The toughening of epoxy resins with the addition of organic or inorganic compounds is of great interest nowadays, considering their large scale of applications. The present work reports the results of the incorporation of two kaolin samples of different particle sizes into a polymeric base containing diglycidyl ether of bisphenol A (DGEBA) epoxy resin with polyamidoamine as hardener. A general review of the influence of fillers, especially clay ones, on epoxy matrices has been made. A prior surface treatment of the kaolin particles with anionic and cationic agents, sodium polyacrylate (Na-PAA) and dimethyldioctadecylammonium chloride (DOCA), respectively, helped to improve their dispersion into the matrix. Fourier transform infrared (FTIR), TGA, Zeta potential and surface energy measurements were conducted in order to assess the effectiveness of surface treatments. Microstructural observations and image analysis were also well suited to analyse particle dispersion and size distribution into the polymeric matrix. Microhardness tests realised on epoxy/polyamidoamine/kaolin systems allowed the study of the influence of particles pigment volume concentrations (PVC) on their behaviour.     

Sericite-chalcocite mineral particle interactions and hetero-aggregation (sliming) mechanism in aqueous media

Attractive particle interactions which lead to hetero-aggregation or “sliming” of gangue and valuable mineral particles are encountered in a number of hydrometallurgical and flotation processes. Sliming leads to poor recovery of the valuable, hydrophobic minerals and high recovery of hydrophilic gangue particles in flotation concentrates. In the present work, the influence of interfacial chemistry and dispersion conditions on particle interactions which underpin the mechanism of hetero-aggregation between sericite and chalcocite particles was investigated in the pH range 5-9 at 23 °C. Hetero-aggregation is shown to occur under aqueous dispersion conditions where the fluid shear rate was high and the individual chalcocite and sericite particles were negatively charged, as shown by the electrokinetic potential data. Continuous flow particulate adsorption and rheological studies revealed that the hetero-aggregation behaviour was strongly pH and oxidative environment dependant. Sliming was greater at lower than higher pH and under air saturation than N₂ gas purge. The unexpected hetero-aggregation is ascribed to chalcocite (Cu(I)₂S) surface oxidation and dissolution which accentuated with decreasing pH and Cu (II) ion hydrolysis effect. Specific adsorption of Cu (II) complexes onto the mix minerals’ surfaces had a striking impact on the interfacial chemistry, reflecting significantly enhanced dispersion shear yield stress. The attractive particles’ interactions are believed to be due to a combination of Cu(II)-mediated mechanisms including: electrostatic-charge patch attraction, van der Waals attraction, adsorbed ion-particle bridging, surface nucleation and cementation. Sliming mitigation was demonstrated by the use of N₂ gas and higher pH, as evidenced by markedly lower sericite-chalcocite dispersion shear yield stress and greatly reduced particulate adsorption behaviour.     

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.     

Preparation of anionic ion exchange latex particles via heteroaggregation

To prepare relatively large negatively charged polymer particles in a size range from 0.3 μm to 0.5 μm, having high surface charge densities, the heteroaggregation of small (50–100 nm), highly charged (185 and 421 μeq/g) anionic polystyrene particles onto the surface of larger (317–466 nm) poly(vinylbenzyl choride)‐based cationic (10, 614, and 830 μeq/g) particles was carried out. As a result, particles with different surface charges, having a core‐shell structure, were successfully prepared. First, aggregated particles were formed via heteroaggregation of the lowest surface charge density anionic particles (185 μeq/g) with the lowest surface charge density cationic particles (10 μeq/g). However, the anionic particles in the shell layer desorbed with time owing to the relatively weak interaction between the two particles. Second, aggregated particles comprised of the highest surface charge density cationic (830 μeq/g) and anionic latex particles (421 μeq/g) were formed. However, to prepare a stable system, an excess of the small anionic particles was required, leaving a large number of small particles present in the aqueous phase, which proved difficult to remove. Finally, aggregated particles were formed by heteroaggregation of cationic particles with an intermediate surface charge density (614 μeq/g) with the highest surface charge anionic particles (421 μeq/g). As a result, not only were core‐shell particles formed, but few free small anionic particles remained in the aqueous phase. In this article, the preparation and characterization of each of these aggregates are discussed in terms of particle size, morphology, and extent of incorporation of the functional groups. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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.     

Interaction mechanisms between Na montmorillonite clay and MPEG-based polycarboxylate superplasticizers

Sodium montmorillonite clay is shown to negatively impact the dispersion force of two methacrylate based polycarboxylates (PCEs) in cement paste. The PCEs tested consist of methacrylic acid/MPEG methacrylate-ester with molar ratios of 6:1 and 1.5:1. It was found that the PCEs sorb both chemically and physically onto clay. The sorbed amounts are ~ 100 times more than on cement. Chemisorption occurs via intercalation of the poly(ethylene oxide) side chains into the interlayer region between the alumosilicate layers, while physisorption occurs on clay surfaces which are positively charged through uptake of Ca²⁺. PCEs possessing high grafting density predominantly intercalate and show less surface adsorption, and vice versa. Also, the type of sorption is dosage dependent, whereby side chain intercalation dominates at higher PCE dosages, while electrostatic attraction via the anionic backbone prevails at lower dosages. Polyglycols can be utilised as sacrificial agents when highly grafted PCEs are employed at high dosages.     

The Effect of Ionic Surfactant Adsorption on the Rheology of Ceramic Glaze Suspensions

The rheological behavior of ceramic glaze suspensions containing limestone, quartz, feldspar, and kaolin, has been improved by the addition of either a cationic (cetylpyridinium chloride, CPC) or an anionic (sodium dodecylbenzenesulfonate, SDBS) surfactant. Additional stability and lower viscosity can result from either electrical double layer repulsion or steric repulsion depending on the surfactant type, concentration, ionic strength, and pH. Underdosing may result in high viscosity due to charge neutrality and hydrophobic attraction between particles while overdosing may result in high viscosity possibly due to micelles in the solution. The situation is particularly complicated for glazes containing limestone and anionic surfactant at moderate to low pH where calcium carbonate is soluble. The anionic surfactant and calcium ions can form complexes that are poorly soluble and strongly adsorb to the surface of the particles. The resulting thick steric layer of complexes produces a significant repulsion and stable, low viscosity suspensions. ζ potential measurements and adsorption isotherms are used to interpret the rheological behavior.     

Effect of sepiolite on the flocculation of suspensions of fibre-reinforced cement

Sepiolite is used to increase thixotropy of cement slurries for easier processing, to prevent sagging and to provide a better final quality in the manufacture of fibre-reinforced cement products. However, the effect of sepiolite on flocculation and its interactions with the components of fibre cement are yet unknown. The aim of this research is to study the effects of sepiolite on the flocculation of different fibre-reinforced cement slurries induced by anionic polyacrylamides (A-PAMs). Flocculation and floc properties were studied by monitoring the chord size distribution in real time employing a focused beam reflectance measurement (FBRM) probe. The results show that sepiolite increases floc size and floc stability in fibre-cement suspensions. Sepiolite competes with fibres and clay for A-PAMs adsorption and its interaction with A-PAM improves flocculation of mineral particles.     

Sintering and characterisation of ceramics containing paper sludge, glass cullet and different types of clayey materials

The present paper reports on the sintering behaviour of several ceramics prepared using a previously selected mixture of incinerated paper mill sludge and glass cullet in the ratio 60/40 which was blended with 10, 20, 30 and 40 wt.% of three different natural materials. The three natural products were: a red quartzitic clay, a yellow quartzitic clay and a kaolin. All mixtures were blended by attrition milling and dried; powders were sieved, pressed into specimens and fired for 1 h at temperatures ranging from 1040 to 1140 °C. The resulting materials were characterized by water absorption, shrinkage, crystallographic composition, microstructure and physico-mechanical properties. It was observed that materials containing kaolin display the best overall behaviour independently of the quantity of kaolin introduced. Conversely the optimal sintering temperature, and consequently the best properties of the materials prepared using red or yellow clay were measured on products fired at temperatures above 1080 °C; materials and temperatures are affected by the amount of clay added.     

Yield stress of oxide dispersions—intermolecular forces of adsorbed small ionic additives and particle surface roughness

The yield stress‐pH and zeta potential‐pH behaviour of α‐alumina and zirconia dispersions with adsorbed small ionic molecular additives such as phosphate and pyrophosphate were determined. The result for adsorbed citrate was included for comparison. Adsorbed phosphate at high surface coverage increased the maximum yield stress of low surface area α‐Al₂O₃ (AKP30 and AA07) dispersions slightly. This increase is attributed to the intermolecular hydrogen bonding between phosphates adsorbed on interacting particles. With high surface area ZrO₂ (Tosoh) dispersions, however, the adsorbed phosphate decreased the maximum yield stress. This is due to its very rough surface morphology limiting the extent of intermolecular hydrogen bonding between adsorbed phosphate layers. Unlike phosphate, pyrophosphate reduces the maximum yield stress of AKP30 α‐Al₂O₃. This is due to the presence of intramolecular hydrogen bonding, thereby impeding effective bridging. A similar result is observed with citrate. The adsorbed pyrophosphate acts as an effective steric barrier keeping interacting particles further apart, thereby weakening the van de Waals attraction. These dispersions with the presence of non‐DLVO forces, that is bridging and steric, did not affect the linear relationship between yield stress and the square of the zeta potential as predicted by the yield stress–DLVO force model. However the relative importance of these non‐DLVO forces affect the value of the critical zeta potential at the point of transition from flocculated to dispersed state. © 2011 Canadian Society for Chemical Engineering     

Effect of Alumina Content and Surface Area of Acid‐Activated Kaolin on Bleaching of Rice Bran Oil

This study investigated the effect of kaolin acid activation on alumina losses, surface area changes and oil bleaching performance. Ground kaolin was treated with hydrochloric or citric acid, and bleaching tests were performed on rice bran oil. The adsorption studies showed that the optimal bleaching of ~83 or ~81 % were achieved by activation with 0.5 M hydrochloric or citric acid, respectively, whereas bleaching with a commercial clay was ~82 %. The highest bleaching value was not associated with the maximum clay surface area or porosity. X‐ray fluorescence showed that alumina contents of 31–34 % were suggestive for optimum bleaching depending on the different acid used. Treating ground kaolin with the same hydrochloric acid strength by varying the acid concentration and clay/acid ratio also confirmed that the best Al³⁺ content was ~32 %, and a value lower than one indicated the extensively destruction of kaolinite proportions leading to a decrease in its bleaching capacity, even though it had the maximum surface area. The decrease in capacity was due to the reduction of alumina content, and the parallel formation of high amorphous silica was favorable for the adsorption of anionic pigments, such as chlorophyll‐a.     

Consolidation and aggregate densification during gravity thickening

A continuous laboratory column system has been used to investigate the consolidation behaviour of kaolin slurries flocculated by a nonionic flocculant under different conditions. Measurements of the steady-state bed density profile showed that higher agitation intensities during flocculation resulted in lower bed densities. The relative importance of compression versus shear for kaolin dewatering was determined by operating the steady-state column system at different bed heights above a mechanical rake. It was found that rake action, not compression, was the dominant dewatering mechanism. Samples taken from the consolidating bed before and after the rake were analysed by microscopy to determine the density–size relationship of the individual aggregates. This showed that the dewatering induced by the rake action occurred not only by removal of inter-aggregate liquor but also by densification of the aggregates (i.e. removal of intra-aggregate liquor). Measurements made on full-scale thickeners are also presented which demonstrate similar behaviour.     

The effect of polycations on early cement paste

This paper studies the possibility for improving the ductility of cement based materials by means of oligocationic additives. Actually, the setting of cement is due to ionic correlation forces between highly negatively charged C-S-H nanoparticles throughout a calcium rich solution. The main drawback of this strong attraction is its very short range that results in low elastic deformation of hydrated cementitious materials. A way to enlarge the attraction range between C-S-H particles would be to add cationic oligomers that would compete with calcium ions modifying the ionic correlation forces via a bridging mechanism of longer range, which could lead to a more ductile material. The studied parameters were the polymerization degree, the separation distance between the charged monomers and the balance between oligocations and monovalent and divalent cations in the solution. The results, both experimental and numerical by Monte Carlo (MC) simulations, demonstrate that cationic oligomers can compete with calcium cations as counterions to the C-S-H surface. The cohesive forces between C-S-H surfaces, calculated by MC simulations, show an interesting behaviour where range and magnitude can be tuned with oligomer concentration, polymerization degree and line charge density. Thus, it seems possible to modulate the ductility and critical strain of cement by addition of cationic oligomers.     

The influence of substrate absorbency on coating surface chemistry

The composition of the top surface of a coating layer can influence its functional properties or subsequent processing steps. The effect of the substrate absorbency on the coating surface chemistry is reported. Different coating systems containing a kaolin clay pigment, fine or coarse precipitated calcium carbonates, and a common latex binder were examined. The influence of a soluble polymer added into the coating was characterized. The surface chemistry was measured with attenuated total internal reflectance (ATR) and X-ray photoelectron spectroscopy (XPS).

Absorbent substrates generate bulky coatings with high voids and low gloss. Rapid dewatering by the absorbent substrate pulls the small particles, like latex binder, away from the top layers causing a low latex concentration at the surface. On non-absorbent substrates, the addition of the soluble polymer generates coating layers with higher void volume, lower gloss, and lower latex concentrations at the coating surface. However, on absorbent substrates, polymer addition causes coatings with lower void volumes and higher gloss. In this case, the rapid dewatering and mobility of particles is reduced by the polymer, which helps to retain the small particles at the surface. As a result, latex concentration at the surface increases with polymer addition on absorbent substrates.     

Relationship between surface chemistry and surface energy of different shape pigment blend coatings

The influence of pigment shapes and pigment blends on the surface energy was investigated and compared with the surface chemistry of pigmented latex coatings. The coatings were made of different volume ratios of two pigments: plate-like kaolin clay pigment and prismatic precipitated calcium carbonate (PCC) pigment. These were mixed together with carboxylated styrene–butadiene–acrylonitrile latex (SBA), and applied over nonabsorbent substrates as well as absorbent substrates. The composition of the surface of the coatings was investigated by X-ray photoelectron spectroscopy (XPS). Two approaches were used to estimate the total surface energy and the components of the coatings: a conventional approach—“the Kaelble approach”—and a more modern approach—“the van Oss approach.” Pigment blends with different shapes and increments caused a change in the surface chemistry and the surface energy of the latex coatings. As the prismatic PCC pigment particles increased in the kaolin/SBA coating system, the SBA latex content at the coating surface increased and the total surface energy of the coating decreased. This is valid for both nonabsorbent as well as absorbent substrates. It was found that there was a strong correlation between the surface energy and the surface composition. The surface energy of the coatings estimated by the Van Oss approach was always lower than that estimated by the Kaelble approach. Colloidal interactions between pigment–pigment and/or pigment–binder were thought to play an essential role in determining the final coating surface energy and its components. Changes in the surface latex content and the surface energy due to the different pigment blends investigated were found to fit straight-line equations.     

Adsorption of cationic-anionic surfactant mixtures on activated carbon

This paper reported the adsorption of cationic-anionic surfactant mixtures, such as octyltriethylammonium bromide/sodium dodecylbenzenesulfonate (OTEAB/SDBS) and dodecylpyridinium chloride/sodium octanesulfonate (DPC/SOS), on activated carbon (AC) in deionized water and in mineralized water systems. The AC surface chemistry was characterized by X-ray photoelectron spectroscopy and ζ-potential determinations. It was observed that in deionized water solution, the addition of SOS obviously promoted the adsorption of DPC, while the existence of OTEAB increased the adsorption of SDBS first and then decreased that slightly with increasing SDBS concentration. In mineralized water solution, the addition of cationic (anionic) surfactants reduced the adsorption of anionic (cationic) surfactants. It was shown that the adsorption of the surfactants on the AC was predominated mainly by the hydrophobic interaction between AC surface and surfactants because of the low oxygen content and very low ζ-potential on the AC surface. There might exist synergism between cationic and anionic surfactants when adsorbing on AC in deionized water due to the electrostatic interaction between oppositely charged surface active ions. Such synergism might be greatly weakened when a large number of inorganic salts exist owing to the “screen” effect of the counter ions of the salt on the electrostatic attraction of oppositely charged surface active ions.     

高岭土悬浮液的流变性质

<正>1引言高岭土在水中分散形成絮凝悬浮液,此体系中固相的体积分率、粒子形状和粒径分布、表面电荷和流动类型等,都在不同程度上影响其流变特性。有关此问题的研究工作尚不多。文献中已有的悬浮液流变模型大都是针对刚性球形粒子形成的悬浮液,粒于之间无相互作用,此时影响悬浮液表观粘度的主要因素是固相体积分率。     

Zeta Potential Measurement in Catalyst Preparations

Oxide surfaces are covered with hydroxyl groups. In contact with water, positive or negative surface charges can be developed. The surface charge of oxide particles can be fine‐tuned by changing the calcination temperature of the oxides before dispersion in water or by variation of the suspension pH. Strong negative or positive surface charges stabilize a suspension and avoid particle aggregation. Nano‐structured catalysts suspended in water show surface charges different from those of compact TiO₂. For spray drying, the cationic or anionic additives used have to be strongly attached via electrostatic forces to the surface of the suspended oxide particles. When noble metal complexes have to be brought to the support surface, the positively or negatively charged complexes must have an opposite charge relative to the surface charge. Zeta potential measurements can solve these problems.     

The role played by local pH and pore size distribution in the electrochemical regeneration of carbon fabrics loaded with bentazon

Adsorption of the herbicide bentazon on activated carbon fabrics was found to be enhanced in acidic medium, showing that bentazon is mainly adsorbed through dispersive interactions. Bentazon was reversibly desorbed by applying a cathodic polarization of the cloth, leading to its fast regeneration. The mechanisms involved were carefully examined in light of the nanoporous texture of carbon and surface functionality, using various electrolytes at different pH values. Results showed that bentazon was strongly adsorbed and that the negative charges on the carbon surface together with the local electrical field were not sufficient to induce desorption of anionic bentazon through electrostatic interactions. The pH increase inside the porosity, especially due to water decomposition, was crucial in favoring the dissociation of surface groups, thereby strengthening electrostatic repulsions between the negatively charged carbon surface and the anionic bentazon molecule. This mechanism occurred in the micropores and mesopores, the latter contributing to the desorption kinetics through transportation of the adsorbate from the carbon surface to the electrolyte. Additionally, the presence of acidic surface groups on the adsorbent or the use of a basic electrolyte improved the desorption process thanks to the increase in negatively charged functionalities and the promotion of electrostatic repulsions.