Particle aggregation in complex aquatic systems (original) (raw)
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Organic Geochemistry, 2004
Aggregation and dispersion of mineral particles spontaneously take place under changing environmental conditions in natural systems. The structure of particle network in soils, the retardation or release of colloidal particles, and their mobility and transport are inherently influenced by natural organic matter bound to the mineral matrix. Since the surface properties of clay mineral and metal oxide particles, and the electrified mineral-water interfaces play a major role in formation, structure and strength of aggregates, any surface modification, especially by polyanionic organic complexants such as humic substances, has a significant affect on particle interaction in a mineral assemblage. The permanently and/or conditionally charged clay minerals (montmorillonite and kaolinite) and iron oxides (hematite and magnetite), as known major mineral components in natural waters, were selected for studying their surface charge characteristics and pH dependent interactions. We discuss how the surface charge correlates with particle aggregation through some characteristic examples for homo and heterocoagulation of similar and dissimilar mineral particles under acidic condition (at pH $4) in the dilute and concentrated systems studied by means of light scattering and rheology, respectively. The adsorption of a brown coal derived humic acid, and its influence on the surface charge character and particle aggregation of clay and iron oxide particles were also studied in dilute and concentrated suspensions. Humic acids can be bound to the most reactive surface sites of clay and oxide particles, i.e. to Al-OH mainly at the edges of clay lamellae, and to Fe-OH on iron oxides, in surface complexation reaction, therefore their role in particle aggregation is specific. Relations between surface complexation, surface charge modification, and particle aggregation in pure and mixed montmorillonite-iron oxide systems are explained. #
Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite
An analogous study to 2:1 type montmorillonite . Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27,[75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][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.
The organic matter content and ionic composition of soil solutions have a dominant effect on soil structure, forming gradually an aggregation of mineral particles, in which clay fractions play a determining role. The colloidal interaction of particles, including repulsion and attraction, changes with their inherent surface properties and the interfacial layer composition around them, resulting in chemical and physical equilibria between solid and aqueous phases. These interactions and their relationship were studied in model systems containing soil constituents having a definite role in the formation of soil structure and are organized into two parts, here the interfacial processes as part I, with the particle network formation in the second part. The adsorption of humic acid (HA) on montmorillonite was studied under conditions relevant to the environment at pH $ 6.5 and 0.0014 M NaCl. The effect of Ca loading from 0.0013 to 0.0055 M was the focus of the work. Both HA and Ca 2+ adsorption were measured. To reveal molecular insight into the interactions between HA, Ca 2+ and montmorillonite, the HA concentration was related to the molar amount of its acidic functional groups. Adsorptive fractionation of HA was characterized using spectrophotometric parameters (SUVA 254 , E 3 /E 4 and E 4 /E 6 ). The fourfold increase in Ca loading resulted in more than ten times greater HA adsorption, but only three times greater Ca 2+ adsorption, during which the distribution between the solid and liquid phases changed considerably. The dominant processes were preferential Na + /Ca 2+ ion exchange and complexation by HA both in the aqueous phase and on montmorillonite (through Ca bridges on the faces and directly on Al-OH sites at the edges), causing adsorptive fractionation of organic matter and leaving the small, less humified HA fraction in the aqueous phase.
Applied Clay Science, 2004
Montmorillonite is the most often studied swelling clay mineral. The layers have permanent negative charges due to isomorphic substitutions, and pH-dependent charges develop on the surface hydroxyls at the edges. Wyoming montmorillonite samples with different extents of isomorphic substitutions (Swy-1 and Swy-2) were studied. The acid–base titration of Na-montmorillonite suspensions between pH 4 and 9 at 0.01, 0.1 and 1 M NaCl was used to characterize pH-dependent charge development on amphoteric edge sites and to determine the point of zero charge (PZC) of edges. The evaluation of reversible net proton surface excess vs. pH functions revealed that the OH groups at edges having PZC at pH ∼6.5 are less basic than the –OH and less acidic than the –OH groups. Positive charges can develop in a protonation reaction of –OH sites at edges only at pHs below ∼6.5, and deprotonation of –OH then that of the –OH sites takes place with increasing pH of solution resulting in negative charges at edges. Therefore, patch-wise charge heterogeneity of montmorillonite, i.e. oppositely charged surface parts of layers, exists only under acidic conditions. Coagulation kinetics measurements resulted in reliable stability ratio data for fine montmorillonite sols at different pHs, and provided undisputable characterization of hetero- and homocoagulation. Edge-to-face heterocoagulation occurs above NaCl concentration 25–26 mmol l−1 at pH ∼4, where the hidden electric double layer (edl) of positively charged edge region has emerged. Edge-to-face attraction between the poorly charged edges and negatively charged faces of platelets around the pH of PZC of edges (pHPZC, edge∼6.5) in relatively low concentration of the indifferent electrolytes (typically around 50 mmol l−1 NaCl) is probable. The homocoagulation of uniformly charged lamellae at pH 8–8.5, formation of face-to-face aggregates requires much higher salt concentration (typically around 100 mmol l−1 NaCl) to compress the dominant edl on the highly charged faces of particles. XRD patterns of montmorillonite films prepared from slightly acidic suspensions proved that formation of well-ordered layer packages is hindered by the attraction between edges and faces. Characteristic changes in gel formation and in rheological properties induced by decreasing pH in dense suspensions containing 0.01 M NaCl provided experimental evidence for the structure of particle network. A significant increase in thixotropy and yield values, and also the formation of viscoelastic gels only below pH ∼6.5 verify that attractive interaction exists between oppositely charged parts of lamellar particles.
Faraday Discuss., 2016
Na-montmorillonite nanoclay is a colloid of layered mineral silicate. When dispersed in water, this mineral swells on absorption of water and exfoliates into platelets with electric double layers on their surfaces. Even at low particle concentration, the aqueous dispersion can exhibit a spontaneous ergodicity breaking phase transition from a free flowing liquid to nonequilibrium, kinetically arrested and disordered states such as gels and glasses. In an earlier publication [Applied Clay Science, 2015, 114, 8592], we showed that the stability of clay gels can be enhanced by adding a salt later to the clay dispersion prepared in deionized water, rather than by adding the clay mineral to a previously mixed salt solution. Here, we directly track the collapsing interface of sedimenting clay gels using an optical method and show that adding salt after dispersing the clay mineral does indeed result in more stable gels even in very dilute dispersions. These weak gels are seen to exhibit a t...
2009
The electrical double-layer model (EDLM), generally used in order to describe the behaviour of the solids bearing only amphoteric groups at the surface (i.e. oxides), can be extended to solid based dispersion presenting simultaneously amphoteric sites (pH-dependent charge) and permanent structural charges (pH-independent charge) at the surface (i.e. clays). In this study, the acid-base behaviours of a fraction of colloidal clays extracted from a natural quartz sand have been modelled within the EDLM frameworks. These colloidal clays also exhibit heterogeneous macroscopic surface charge distribution. We have considered the case where the colloidal fraction presents only amphoteric sites at the surface (case 1) and the case where both pH-depending and pH-independent charges are present simultaneously at the surface (case 2). The modelling results show a great difference between the theory and the experimental data for case 1 but a good corroboration for case 2. Therefore, in order to describe correctly the physicalchemistry properties of the colloidal fraction it is necessary to take into account the permanent charges (microscopic surface charge heterogeneity) to explain the increase in the zero protonic charge (ZPC) and in the protonation amount with decreasing ionic strength. In case 1, the double-layer model leads to a constant ZPC and to an increase in the protonation amount as a function of ionic strength, which is in disagreement with the experimental results.
European Journal of Soil Science, 1995
The effect of changing pH and electrolyte concentration on the dispersion and zeta potential of Naand Ca-forms of kaolinite, illite and smectite was investigated in relation to changes in their net negative charge. The percentage of dispersible Na-clay and the percentage increase in net negative charge was positively correlated with pH, but the slopes varied from clay to clay. In general, the net negative charge was the primary factor in clay dispersion, and the pH affected clay dispersion by changing the net charge on clay particles. Na-smectite had larger net charge at all pHs than Na-illite and Nakaolinite, and it always had larger flocculation values. The role of electrolyte concentration could be due to its effect both on flocculation and variable charge component of the clay minerals. The zeta potential at different pHs also reflected the same trend of clay dispersion with net particle charge. In Ca-clays the trends were similar to Na-clays up to pH 7.0. In more alkaline solution CaCO? formation led to charge reduction on clay particles, resulting in flocculation and reduction of zeta potential. At similar pHs the electrophoretic mobilities of all the clays showed constant potential behaviour. However, the zeta potentials of Ca-clays were always smaller than those of sodic clays because the clays were more aggregated. Net particle charge was the most important factor in controlling clay dispersion for the whole range of pH and ionic strength and for all types of cations.
Sains Malaysiana, 2019
The scenario of released nanoparticles from consumer products into the environment especially natural waters has become a great concern nowadays. Assessing their aggregation and stability under environmental conditions is important in determining their fate and behavior in natural waters. The aggregation behavior of selected nanoparticles (iron oxide and alumina) was investigated at variable concentrations of humic acid (5, 10, 50 mg/L), and pH variation in solution. Dynamic light scattering was used to measure their z-average hydrodynamic diameter and zeta potential. Derjaguin-Landau-Verwey-Overbeak (DLVO) theory was used to explain the thermodynamic interactions between two particles. Then, the stability was evaluated by assessing their aggregation. The increasing of humic acid concentrations enhanced aggregation of iron oxide and alumina nanoparticles, particularly at low pH levels. The maximum aggregation was found in pH below the point of zero charge (PZC) due to electrostatic destabilization and electrostatic stabilization that took place at pH above the point of zero charge. Meanwhile, at pH point of zero charge, nanoparticles were coated with negative humic acid charged. From this study, properties of nanoparticles (size, surface charge, Hamaker constant) and environmental condition (humic acid concentration, pH) have their specific roles to control the fate and behavior of nanoparticles in environmental media.
Organic Geochemistry, 2007
Charge state and particle network formation were studied at pH 6-6.5 in model systems containing montmorillonite, humic acid (HA) and calcium ions (Ca 2+), which are responsible for microaggregate formation in soils. Since dispersing and aggregation of particles are governed by the composition of the coating layer on particle surfaces, the interfacial equilibria for the same systems were studied in Part I. Here, the composition of the systems was based on the HA and Ca 2+ adsorption results using a meaningful HA concentration unit related to the molar amount of its acidic functional groups. Approaching real conditions, the effect of Ca loading was also studied at low and high HA content, representing the colloid fractions of soil poor ($0.5%) and rich ($5%) in organic matter. The zeta potential was measured and the structure of dense suspensions was characterized using rheology. HA addition breaks down the shear-tolerant structure of the particle network forming in montmorillonite suspensions at pH $6.5. Suspensions become liquefied due to the dispersing effect of the HA. However, a much stronger structure can build up, if both HA and Ca 2+ are present in optimal ratio, which can be readily estimated from the charge balance between negative charges (cation exchange sites on montmorillonite and acidic groups of HA) and positive charges of Ca 2+. On the other hand, zeta potential determination did not support these changes, except the dispersing effect of HA. The strengthening of the particle network was quantified using the Bingham yield values. With increasing Ca loading, an almost fivefold increase in shear tolerance could be attained in HA-rich suspensions vs. those without HA or with low HA content. The aggregation of mineral particles is enhanced by the joint effect of HA and Ca 2+. The higher the OM content, the more the Ca loading is expected to reach optimal aggregation.