Using the Quirk-Schofield Diagram to Explain Environmental Colloid Dispersion Phenomena

Abstract

Colloid dispersion, through its role in soil science, hydrology, and contaminant transport, is a basic component of many natural resources and environmental education programs. However, comprehension of colloid dispersion phenomena is limited by the numerous variables involved. This article demonstrates how the Quirk‐Schofield diagram can be used as a pedagogical tool to explain the relationship between colloid dispersion and three key variables: ionic strength, counterion valence, and pH. The theory of colloid dispersion is briefly reviewed, in order to provide the background needed to define the Quirk‐Schofield diagram. These diagrams are then employed to qualitatively explain numerous published observations of environmental colloid dispersion, first in the case of constant pH and then in the case of variable pH. In addition, new Quirk‐Schofield diagrams are presented for three common clay minerals: kaolinite, illite, and montmorillonite. These diagrams are used for quantitative analysis of two key studies from the literature on colloid‐associated contaminant transport. The generality and simplicity of Quirk‐Schofield diagrams, which provide a single framework to conceptualize many phenomena, should greatly assist student learning.

Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

“…This diagram therefore provides a conceptual and quantitative model to distinguish chemical conditions favoring colloid dispersion, below the line for the relevant pH, versus colloid aggregation, above the line. Adapted from Mays [2007].…”

“…The mechanical threshold is complicated by chemical conditions. Colloid mobilization generally occurs when the ionic strength ( I ) drops below the critical coagulation concentration, (CCC) [ Mays , 2007]. The CCC, in turn, depends on the type of colloid, the pH, and the ratio of bivalent to monovalent cations, usually quantified using the sodium adsorption ratio (SAR).…”

Classification and Mitigation of Soil Salinization

“…As Figure 1 shows, for a given soil there is a clear relationship between the soil permeability and salinity versus sodicity. (Mays, 2007). This means that the dominance of divalent cations, such as calcium and magnesium (responsible for the formation of scale in hard water), will cause a shift in the direction of lower SAR values in Figure 1, and the soil will be soft, permeable, and well structured.…”

Modification of the Kozeny-Carman Equation To Quantify Formation Damage by Fines in Clean, Unconsolidated Porous Media

“…The dispersion of fines occur when the individual fines are more stable individually than they would be alongside other fines due to either collisions or chemical interactions. Fines that remain separate will remain suspended in solution for a longer period and this dispersion characteristic will enable fines to remain mobile and it may hinder adsorption to the grain surfaces (Mays, 2007). Dispersion is very similar to diffusion in that with a change in time, a single fine, which remains suspended, and will move down the concentration gradient, on average.…”