Clay aggregate

Fig. 8-12 Hypothetical particle arrangement in a sediment and a clay aggregate. (Adapted from Casa-grande, 1940.)...

As noted in Sect. 10.1, heterogeneities play a dominant role in the migration of contaminants in the subsurface. Nonuniform, preferential patterns of flow and transport are ubiquitous. It is important to recognize that, at the field scale, contaminant movement generally is very difficult to anticipate. In natural soils and aquifer materials, macropores, soil cracks and aggregates, fissures, solution channels, root paths, and wormholes, as well as variable mineral composition (e.g., clay aggregates... 

Haque, N., Morrison, G., Cano-Aguilera, I. and Gardea-Torresdey, J.L. (2008) Iron-modified light expanded clay aggregates for the removal of arsenic(V) from groundwater. Microchemical Journal, 88(1), 7-13. 

Experimental Details. A fair comparison between the apparent densities of clays immersed in water and of clays with a certain number of preadsorbed monolayers immersed in n-decane requires that each preadsorbed monolayer of water between the unit layers is completed, so that no vacant space within a mono-layer exists. The clay should be in the same state of hydration in the entire system. The selectively accessible void space should be completely filled, as well as capillaries in the clay aggregates. The homogeneous distribution of the adsorption water was achieved by slowly equilibrating thin flakes of clay with almost saturated water vapor. After about one month of equilibration, the uniform state of hydration of the clay was shown by the sharpness and order of the x-ray diffraction pattern. The completion of the monolayers was judged from the amount of water taken up by the clay, with the knowledge that about 100 mg. of water is needed per gram of clay for the formation of a monolayer. 

Inequality 11 was substituted into Equation 8, together with reasonable values of other parameters and 0.1 cm < a < 0.2 cm as a was found to be in this study. This leads to the conclusion that, for systems in which r is less than 0.1 cm, the local equilibrium assumption is applicable (i.e., q is sufficiently small) when D is nearly equal to as observed in the experiments. In soils in which the exchanging particles are not spherical, r would represent approximately the mean diffusion path within clay aggregates or within clay coatings on coarse particles. 

For soils without appreciable clay aggregation, the experimental results and theoretical analysis described here indicate that when diffusion is rate-limiting, the ratio of the hydrodynamic dispersion coefficient to the estimated soil self-diffusion coefficient may be a useful indicator of the applicability of the local equilibrium assumption. For reacting solutes in laboratory columns such as those used in this study, systems with ratios near unity can be modeled using equilibrium chemistry. 

Fig. 14. Different particles occurring in radiolarian ooze Black micronodules, patchy clay aggregates, glassy sharp dged fragment volcanic ash. (Clay aggregates can partly be formed as weathering products of volcanic ash or partly be new built).

Cation loss causes larger clay aggregates to fragment (deflocculate). Resultant smaller clay particles are more readily washed down profile (Eluvation)... 

Physicochemical reactions within the sea-sediment sphere tend to reach equilibrium. Those reactions that are so rapid that they occur prior to burial in the bottom sediments are referred to as "halmyrolysis" (e.g. formation of clay aggregates), while those that take place in the upper part of the sediment are termed "early diagenesis". The diagenetic processes include cementation, compaction, diffusion, redox reactions, transformation of organic and inorganic material, and ion exchange phenomena. A short... 

Natural soils are polydisperse systems of particles, which are rarely present in the form of loose beds. Nevertheless, if loose particles are available, they will be preferentially mobilized. Other factors that are important are the coverage of the soil surface with roughness elements like pebbles, stubbles, bushes, etc., which partially absorb momentum coherence forces between soil particles due to clay aggregation, organic material, or moisture content and soil texture, that is, the composition of the soil in terms of particle size classes (see Table 7-7). 

The chemical and physical properties of clay suspensions produced during oil production from oil sands are described. With a composition of approximately 70 wt% water (with some unrecovered bitumen) and 30 wt%solids (>90% less than 44 gm in size)9 these clay suspensions consolidate very slowly. Clay aggregate or floe morphology has been shown to be a function of the water chemistry and can be manipulated to produce a tailings suspension that is easier to consolidate and dewater. Commercial oil sands processing has been going on in northeastern Alberta since 1967, and in that time approximately 250 million m3 of this difficult to dewater clay suspension has been produced. The reclamation options for this material (mature fine tailings) on a commercial scale are also outlined. 

The application of Eq. 6.1 to the estimation of the number of unit layers per clay aggregate involves the additional assumptions that (1) the parameters a, 6, and Pc do not depend on the type of exchangeable cation on the clay and (2) each aggregate of either Na- or Li-montmorillonite comprises a single unit layer. With these two assumptions, the slopes of linear plots of rj p against c can be compared for several homoionic montmorillonites and used to calculate the number of unit layers, as given in Table 6.1. These estimates are relative values based on a number of simplifying assumptions. 

Figure 16. Wetting/diying of montmorillonite dry at 2 °C and 4.3 Ton (a), fully wetted after increasing pressure to 5.2 Ton (b), dry clay aggregate formed after droplet evaporated (c), aggregate partially (d) and fully wetted (e).

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