ABSTRACT
The solid phase constituents in soil and water are comprised of a variety of components. Based on their chemical composition, they can be subdivided into inorganic minerals (specific density of about 2.6 g cm-3) and organic compounds (specific density of somewhat more than 1.0 g cm-3). Both types of constituents can also be classified according to the size of the particles. Figure 4.1 gives an overview of the particle size of all major components present in soil and water. Note that the size of the particles forms a continuum, from molecules to large inorganic particles. For convenience, when distinguishing between dissolved and particulate matter , a particle size of 0.45 μm is usually adopted; this size is based on the pore size of a certain type of membrane filter. Colloids constitute a special intermediate case between dissolved and particulate matter. They consist of particles dispersed in water and ranging in size from 1 nm (the size at which the particles approach molecular dimensions) to about 10 μm. A size of 10 μm is still small enough for Brownian motion (i.e. the apparently random motion experienced by any molecule or small particle immersed in a fluid) to prevent them from settling out. Examples of environmental colloidal particles are clay minerals and humic substances . Nanomaterials are natural and manmade particles ranging from 1 nm to 100 nm in size. Like colloidal particles, they form a special group of extremely small solids, and when dispersed in water, nanomaterials include colloidal particles. The types and properties of man-made nanoparticles and their health and environmental risks are discussed in section 4.2.5 In this chapter, both particulate matter and colloidal particles are considered to be solid phase constituents. Note, however, that some
of the colloidal particles are smaller than 0.45 μm and so may also be classed as dissolved matter.
