The smallest particles often have the highest contaminant concentrations

Figure 2-3. Electron microscope photograph of the structure of smectite clay particles. Source: USGS 2006.

Due to their small size, clays have a large surface area per unit mass to which all types of molecules, organic and inorganic, can adhere. The chemical makeup of clay minerals gives them a strong negative charge as well as a weaker positive charge, enabling adsorption of both positive and negative ions. Clay minerals take the form of thin sheets or plates, as depicted in Figure 2-3. This plate structure greatly adds to the surface area of these tiny particles.

The layered plates of clay particles also provide spaces for contaminants to absorb into clay particles, as illustrated in Figure 2-4,where the partial negative charges carried by the oxygen atoms on a dibenzo-p-dioxin molecule are attracted to a cation (such as Ca+2) nestled between two clay plates.

The propensity for smaller particles to attract contaminants was dramatically shown in a study of lead-contaminated soil, as presented in Table 2-1. The pattern is clear: the smaller the particle size (i.e., the larger the mesh size number), the larger the concentration of lead associated with that particle size. Although the smallest particle size, that less than 200-mesh (0.074 mm), made up only one-third of the whole sample mass, it carried nearly three-fourths of the lead mass in the sample. Many experimental studies have documented the finding that for most contaminants, soil fractions composed of smaller particle sizes have higher loadings than fractions composed of larger particles. Important exceptions include metal fragments at firing ranges, explosive/propellant fragments, and ore particles at some mining sites (Walsh et al. 2007; Pavlowsky, Owens, and Martin 2009). Compounds bound to soil particles seldom migrate independently of the particles; they migrate with the particles. The very small particles carrying most of the contaminant load in soil are able to migrate with the wind or be carried by water flow in streams and storm water runoff.

Proposed molecular model of DD-smectite complex showing the arrangement of DD in the interlayer space of the smectite clay.

Figure 2-4. Illustration of smectite clay plates and interstitial cation binding with dibenzo p dioxin.

Source: Superfund Research Program 2010.

Table 2-1. Relationship between particle size and lead concentration for a firing range site
(Adapted from ITRC 2003)
Soil grain size (standard sieve mesh size) Particle size (mm) Soil fractionization (%) Lead concentration in fraction by AA (mg/kg) Lead distribution (% of total lead)
Greater than 3/8 (0.375) inch >9.53 18.85 10 0.20
Between 3/8 inch and 4-mesh 9.53–4.76 4.53 50 0.24
Between 4- and 10-mesh 4.76–2.00 3.65 108 0.43
Between 10- and 50-mesh 2.00–0.297 11.25 165 2.00
Between 50- and 200-mesh 0.297–0.074 27.8 836 25.06
Less than 200-mesh <0.074 33.92 1970 72.07
Totals   100 927 (wt. averaged) 100