Soil structure heterogeneity

Contaminants in soil can partition between the soil and air, soil and water, and soil and solids however, the physical structure and chemical composition of surface and subsurface soil are highly variable. Compared to aqueous systems, soil is a complex heterogeneous media composed of solid, liquid, and gaseous phases. The four major components of soil are the inorganic (mineral) fraction, organic matter, water, and air. Soil consists of 50% pore space, which is occupied by air and water 45% minerals and 5% organic matter. Figure 2.5 illustrates a typical soil structure that is important to consider for remediation. 

Structural hierarchy and heterogeneity are intrinsic soil properties that have profound effects on soil functioning in natural and man-made ecosystems. Quantifying soil structural hierarchy and its effects on soil processes is both an imperative and a challenge for soil physicists. Continual advances in this field benchmark soil science history. One of the advances was the introduction of the scaling concept, which showed simple and consistent ways of quantifying inherent soil variability and structural hierarchy (Nielsen et al., 1998). 

Dumer, W. 1994. Hydraulic conductivity estimation for soils with heterogeneous porous structure. Water Resour. Res. 30 211-223. 

Shein, E. V., Milanovsky, E. Yu. Spatial heterogeneity of properties on different hierarchic levels as a basis of soils structure and functions. Scale effects in the study of soils. Moscow Publishing house of Moscow state University, 2001, P. 47-61. (in Russian). 

There are 48 naturally occurring zeolites (even found on Mars ), and more than 150 synthetic varieties (Figure 2.86). - The natural varieties are mostly used in applications such as concrete, soil treatment ( zeoponics - e.g., controlled release of fertilizer or nutrients such as or N2), and even kitty litter that are not affected by their high levels of compositional and structural heterogeneity. However, synthetic zeolites possess a uniform and precise composition and structure, which... 

The surfaces of sorbent materials, e.g., oxide particles in soil, are often less complex than the exterior of protein molecules. However, if such particles are (partly) covered with organic materials, e.g., humic acids and/or fulvic acids, their surface chemistry may be very complex as well. Also, surfaces of biological structures, such as those of plant roots, may be heterogeneous. 

Adsorption influences the reactivity of surfaces. It has been shown that the rates of processes such as precipitation (heterogeneous nucleation and surface precipitation), dissolution of minerals (of importance in the weathering of rocks, in the formation of soils and sediments, and in the corrosion of structures and metals), and in the catalysis and photocatalysis of redox processes, are critically dependent on the properties of the surfaces (surface species and their strucutral identity). 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

An accurate evaluation of kxa is complicated by the heterogeneous nature and poor definition of contaminant/soil systems. Some success has been achieved in modeling mass transfer from a separate contaminant phase. During degradation these nonaqueous phase liquids (NAPLs) often dissolve under conditions where phase equilibrium is not achieved and dissolution is proportional to k a. Experimental determinations and correlations for k-p depend on interfacial area of the NAPL and liquid velocity at the interface (Geller Hunt, 1993). For adsorbed contaminants, kxa varies with soil composition and structure, concentration and age of contamination, and therefore with time. For example, slurry reactor tests indicate that the rate of naphthalene mass transfer decreases with time, with media size, and with aging of the tar prior to testing (Luthy et al., 1994). 

---