Soil Colloids and Water-Suspended Solids

Suspended solids have the potential to silt out stream channels, rivers, lakes, and reservoirs they inhibit aquatic life and are expensive to remove from water. In some industries (e.g., mining) suspended solids, along with various other pollutants, are regulated by law, which requires that sediment ponds at the base of disturbed watersheds be built with sufficient detention time so that the water released meets certain sediment and water chemistry criteria (Tables 9.1 and 9.2). 

Effluent Characteristics Average of Daily Values for 30 Maximum Allowable Consecutive Discharge Days  

Sample Location Cafl Mg° Naa K Al° CIa SO/ Alkalinity HC03 EC pHfc Ionic Strength SARC 

Source From Evangelou, 1989. Values given are in mmolc L 1. 6Values given are in dS m 1. cValues given are in mmol L 1. 

The purpose of this chapter is to introduce the fundamental soil-water chemistry processes controlling behavior of colloids in soil-water environments. 

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Fig. 7.1 gives a size spectrum of water-borne particles. Particles with diameters less than 10 pm have been called colloids. In soils, the clay-sized and fine silt-sized particles are classified as colloids. Colloids do not dissolve, but instead remain as a solid phase in suspension. Colloids usually remain suspended because their gravitational settling is less than 10 2 cm s 1. Under simplifying conditions (spherical particles, low Reynolds numbers), Stokes law gives for the settling velocity, vs... 

Vinten et al. (1983) demonstrated that the vertical retention of contaminated suspended particles in soils is controlled by the soil porosity and the pore size distribution. Figure 5.8 illustrates the fate of a colloidal suspension in contaminated water during transport through soil. Three distinct steps in which contaminant mass transfer may occur can be defined (1) contaminant adsorption on the porous matrix as the contaminant suspension passes through subsurface zones, (2) contaminant desorption from suspended solid phases, and (3) deposition of contaminated particles as the suspension passes through the soil. 

Colloid behavior in natural soil-water systems is controlled by dispersion-flocculation processes, which are multifaceted phenomena. They include surface electrical potential (El-Swaify, 1976 Stumm and Morgan, 1981), solution composition (Quirk and Schofield, 1955 Arora and Coleman, 1979 Oster et al., 1980), shape of particles, initial particle concentration in suspension (Oster et al., 1980), and type and relative proportion of clay minerals (Arora and Coleman, 1979). When suspended in water, soil colloids are classified according to their settling characteristics into settleable and nonsettleable solids. 

Geoiogy. An example of electrochemistry in geology concerns certain types of soil movements. The movement of earth under stress depends on its viscosity as a siurry that is, a viscous mixture of suspended solids in water with a consistency of very thick cream. Such mixtures of material exhibit thixotropy, which depends on the interactions of the double layers between colloidal particles. These in turn depend on the concentration of ions, which affects the field across the double layer and causes the colloidal structures upon which the soil s consistency depends to repel each other and remain stable. Thus, in certain conditions the addition of ionic solutions to soils may cause a radical increase in their tendency to flow. 

Water and wastewater treatment processes inevitably involve the removal of suspended solids (often referred to as turbidity), usually silt, clay, hydrous oxides and organic matter. Of these, the most difficult suspended solids to remove are the colloidal-sized fraction which, because of their small size, can easily escape both sedimentation and filtration. Examples of these would include spent protein and emulsions from domestic waters, bacterial cells, algae, viruses, amoeba, industrial waste colloids, silts, clays and organic matter from soil wash. Beyond drinking water treatment and industrial wastewater treatment, other application areas include mineral and petroleum processing, and pulp and papermaking, to name just a few. 

The distribution of chemical between environmental phases is at the core of understanding enviromnental fate. Examples of environmental phase distribntions are those between air and water, between atmospheric particles and the gas phase, between plants and air, between soil and air, between suspended matter and the dissolved phase in water, and between groundwater and snbsnrface solids. On a fundamental level, the distribntion behavior of a chemical determines where a chemical is residing in the environment. It further influences the natnre and extent of the transport and transformation processes it will experience. The distribution of a chemical between gas phase and particle phase in the atmosphere not only determines by which mechanism and how fast the chemical is being deposited to the Earth s sitrface, bnt also further determines the type and rate of reactions that it will experience in the atmosphere. A chemical in a water body experiences very different behavior depending on whether it is dissolved in water or whether it sorbs to colloidal or sohd matter suspended in the water colimm. Similarly, the mobility and reactivity of a contaminant in the snbsnrface enviromnent depend strongly on its distribution between water and solids. 

Fig. 3.1 shows that many suspended and colloidal solids encountered in waters sediments and soils have a surface charge and that this charge may be strongly affected by pH. 

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