32 Interaction of Pollutants with Soil components II

Meenakshi Nandal

Objectives

Mechanisms of soil-pollutant interaction Physical Mechanisms

• Adsorption
• Non-adsorptive Retention
• Macroscopic Dispersion

33.1 Physical Mechanisms

The physical and chemical properties of soil such as salinity, temperature or moisture content play an important role in soil- pollutant interaction mechanism. The pollutants in the Soil grains could either be retained by surface adsorption or could undergo intergranular accumulation concentrations maintaining their original chemical composition, or could undergo chemical reactions resulting in an altered/new form being organic, inorganic, or an amalgam of both. They could imbibe in the soil in various physical forms; as suspended particles, solutes or water immiscible liquids. The process of Adsorption and its associated phenomena is the most important physical chemical soil-pollutant interaction mechanism.

33.1.1 Adsorption

Pollutants can be adsorbed on soil grains surfaces by two ways: Physi-sorption– It is physical retention of pollutant molecules to the soil grains surfaces supported by weak and long Van der Waal forces. They consist of low magnitude energy and insufficient for breaking bonds. Therefore, the chemical confirmation of the pollutants that stick to surface of soil independent of the fact that they undergo stretching or bending due to proximity to the surface is retained.

Chemi-sorption– It is chemical retention of the pollutant on soil surface due to formation of covalent bonding. The attachment energy of is much higher when compared with physical adsorption process Thus, in chemisorption the molecule is ripped for satisfying valence considerations due to bond formation with the atoms on surface. The differentiation between physical and chemical adsorption is very difficult and generally with increasing temperature there is a decrease in amount of physically adsorbed material while it is reversed in case of relation for chemically adsorbed material. (Hassett and Banwart 1989).

Diffuse Double Layer Theory (DDL): This theory was adopted for deducing the relation between charged particles and their adsorption on the surface of solids. Between the colloidal particle and the dispersion medium an electrical double layer of positive and negative charges was assumed at the surface of separation According to this model, a layer more diffuse in character balances the charges on the surface of the solid instead of a single mobile layer in the surrounding phase. According to this theory colloid surfaces are considered planar with uniform distribution of electrical charges. When this layer is approached by a liquid or gaseous phase are opposed by a layer with equal but opposite ionic charged layer on colloid surface causing attachment of approaching ions on the surface of colloid. Simultaneously due to diffusion there is drifting of ions with similar charge leading to repulsion from the colloid surface. So, two layers lone with counter ions of opposite charges and the other repelling ions will surround the colloid surface – one of these moving towards the ambient liquid or gaseous medium.

Adsorption of Ionic Pollutants

As explained in the Diffuse Double Layer model (DDL), as the soil grain molecule is approached by a gaseous or liquid environment it will be surrounded by one or more layers of counter ions (ions of opposite charge) or co-ions (ions of similar charge) . Many soil components (e.g. clay minerals, silicates) have a palpable propensity of substituting few of their ions with species similar in the medium (solution or gaseous phase). The phenomenon is defined as cation exchange if lost or gained species is cations, otherwise it’s considered as anion exchange. Soil environment is majorly affected by cations with very rare anion exchange processes. This is because in association with hydrogen, anions could dissociate adsorbent materials such as the clay minerals into. Some examples include silicate minerals, silicate glasses, molybdates, arsenates, vanadates, and related species.

Factors Affecting Adsorption

Generally soil matrix, composition, and surface properties including physical and chemical properties of the pollutants are considered.

The factors affecting the intensity of adsorption are summarized below:

• Soil Mineralogical composition Soil grain size variation
• Soil humic substances
• Soil solution properties- Chemical and physical Chemical constitution of pollutants

Other external factors- climatic conditions and agricultural practices.

Soil Mineralogical Composition

When absorbance is considered the most efficient are clay minerals followed by silicates and organic components. Accordingly, clay portion of the soil defines the absorbance intensity of soils along with bulk of silicates present in the matrix composition. The principal role for determination of mechanism of adsorption, selection and intensity is played by the structure of individual clay minerals. Following pattern of adsorption could be identified regarding clay minerals:

Adsorption on planar external surfaces (e.g. kaolinite) –hydrogen bonds strongly hold the tetrahedral layers so that available sites for ion exchange are left with only external surfaces.

Exchange in the interlayer space – here hydration process leads to swelling of layers and conduce to the attainment of ion exchange in the interlayer space (e.g. Montmorillonite). In addition, only cations can bond the adjacent layers (e.g. Vermiculite) leading to cation exchange fulfilled by the size conditions. Hydroxides of aluminum and iron could be considerably increase the CEC’s of clay.

Soil Grain Size Variation

Finer sediments generally have a higher rate of adsorption compared to coarser ones. In sandy sediment the amount of total calcium and sodium varied from 90% with grain size range of 0.12–0.20

• mm to 10%in coarser fraction of size 0.2–0.50 mm being attributed to the high diffusion rates in finer fractions compared to the coarser ones. In the case of sandy sediments, the distribution of cations (calcium and sodium) among the various grain size fractions depends on the corresponding hardness and lower resistance to erosion of the silicate minerals (e.g. feldspars). The mineralogical framework of the contrasting grain size fractions should be deduced by X-ray diffraction methods. However, high surface energy released due to large surface area of sediments dispenses high rates of ion adsorption.

Soil Humic Substances

Humic substances consist of phenolic hydroxyl and carboxyl functional groups escalate the CEC of the soil depicting a positive influence on the CEC of a soil.

Soil solution Properties

The vadose or saturated groundwater zone is majorly affected by the pollutants. In the presence of clays, hydration shells are formed by water molecules that provide adsorption sites for pollutant molecules; increase in the exchange capacity of the soil surfaces is caused due to acidity and higher dissociation rates by adsorbed water on clay molecules by process of infiltration. In the vadose zone, the plunging contaminated fluids called leachates spread horizontally parallel to direction of the groundwater flow.

Chemical constitution of pollutants

The nature and composition of contaminants controls adsorption on the soil grains control to a considerable as well as the diffusion processes. This may be attributed to the sensitivity of (pH, Eh) and hydrolysis towards ion exchange and chemical parameters of environment created by contaminants in their surroundings. For example the adsorption of organophosphorus pesticides on the clay surfaces.

33.1.2 Non-adsorptive Retention

33.1.2.1 Trapping

One of the major processes for the retention of pollutants in the soil is enmeshing of solid particles and large dissolved molecules in the pore space of the soil. The figure 33.1 illustrates the three mechanisms of trapping which have been concisely described below:

1. Caking: It is a physical process that takes place when the size of pollutants is larger than the soil pores. When the pore sizes becomes too small a layer (cake) is formed on the entrapped particle surface or may cause clogging of soil pores due to biological activities by clustering of particles in bigger lumps. (Fortescue, 1980)

2. Straining: This process occurs if pollutant particles have the same size as the soil pores. As they percolate down the pore size becomes too small and entraps them at the entrance as shown in figure 33.1.

3. Physical-chemical trapping: when the molecular sizes exceed the soil pores physical or chemical transformation leads to production of new products which limit the flow. For example the precipitation of iron and manganese oxides causes flocculation of the colloidal material (Kozlovskiy,1972).

33.1.2.2 Precipitation

When the molecules pass from a dissolved form to an insoluble form the contaminants are retained in the soil controlled by the acid-base equilibria and redox conditions during the geochemical precipitation reactions. They cause dissolution of compounds that were precipitated and may produce reversible compounds if there are changes in conditions.

33.1.2.3 Infiltration

It is the most commonly used mechanism contamination for soil vadose zone and other deeper saturated zones of the groundwater. Leachates containing inorganic and organic constituents are formed with downward movement of fluids under the impact of gravity, the spreading of contaminates is caused in horizontal as well as vertical direction in the geochemical pathways as they reach the saturated zone of the groundwater. The three categories of patterns in which material flow in the landscapes have been described below:

1. Main migrational cycle (MMC). This type of flow is similar to geochemical cycles, i.e. vertical and upward movement of material from soil to plants and animals and then downward from plants and animals to the soil, finally achieving a steady state (Fig. 33.2 a).
2. Landscape geochemical flow (LGF). This involves a parallel progressive material transport to soil surface taking place within a prism alongwith atmosphere and pedosphere. (see Fig. 33.2b).
3. Extra landscape flow (ELF). Flow including accumulation (+ve flow) or outflow (–ve flow) of chemical substances (Fig. 33.2 c).

Contaminants Transport

Two major transport processes spread and transport the contaminants in course of any of these geochemical flows cycles

• (1) advection, movement caused by the groundwater flow
• (2) dispersion, movement caused during advection by the irregular mixing of fluids.

Advection

This is the mechanism that controls the fluid flow underlying earth layers in the soil. As assessed by Darcy’s law that relates the flow of a fluid through a porous medium.

Q = K A (hL/L)

Where Q represents total discharge of fluid per unit time (cm3 s–1), L is the length of the flow path, A is the cross-sectional area of flow path (cm2), h the hydraulic head or pressure drop across the flow path (g cm–2), K is the permeability constant.

Dispersion

Hydrodynamic dispersion is the net effect of the several microscopic, macroscopic and surrounding conditions affecting an aquifer by spreading of solute concentration which ultimately leads to contamination of soil.

33.1.3 Microscopic Dispersion:

In this the dispersion is caused due to:

• Molecular diffusion along concentration gradients
• Transport along viscosity or density gradients
• Transport by forces arising from change of pore geometry

Molecular diffusion: Diffusion is the main process that occurs in solids, liquids and gases along concentration gradients in the available pore space that again contributes to the geochemical gradient in vicinity regions to attain uniform concentration in a contaminant in soil water. Gases present in soil generated by volatile part of contaminants diffuse all through the pore system and the rate of diffusion (material transport) is proportional to the concentration gradient. Further, the transport of contaminants through pore space depends on the density and viscosity changes in soil (Kaufman and Mckenzie, 1975; Oberlander ,1989)

Transport by Forces Arising from Changes of Pore Geometry

As described above in Darcy’s law, permeability is property of transport of fluids without any structural changes in the structure of medium. Soil texture and geometry depends on the pore size and porosity. However, due to inherent properties of the geometrical pattern of the pores the permeability may change without a change in the porosity. The pore wall small-scale roughness is an example. Enhancement of transport can occur by porosity with higher transport rates due to dissolution cavities in soil.

33.1.4 Macroscopic Dispersion

This classical advection/dispersion contaminant transport model for is effective only when dispersion occurs on micro scale. The physical situation varies with change in hydraulic conductivity in soils due to aggregation of particle or crack development. An example of this is clay rich soils in which the consecutive pattern of wetting and drying leads to development of shrinking cracks in the soil matrix and roughness of the pore walls reducing the retention of fluid on soil surface.

Behavior of Non-Aqueous Phase Liquids (NAPLs) in Soils

The non-aqueous liquids/Synthetic organic solvent pure chemicals are either insoluble or slightly soluble immiscible in water. The level of contamination caused by NAPL in the soil depends on many factors like density, viscosity of the NAPL, the volume or capacity of spill and the saturation degree of the soil relative to water. The contaminants added in large volumes can spread on surface after being retained on the soil or infiltrate the soil to the vadose zone. Absolute permeability depends on hydrologic principles and coefficient of permeability defined medium and is independent of fluid nature.

In water-saturated soils the water and NAPL compete for flow within the pore system. The capillary forces hold and drive the NAPL into finer and finer spaces in pore-water along with the soil air and any other existing gaseous phases. At the end the central portion of the pore is filled by the non-aqueous phase liquids while a thin lining is formed by non reducible pore-water held by capillary forces forming a thin layer lining the pore (see Fig.33.5). At this point, the

fluid saturation which is the fluid volume expressed in ratio to total pore space, of NAPL reaches maximum and that of water is decreased to almost zero. The fluid saturation of the transport of a given fluid is directly proportional to its velocity in the medium. If the case of small spills is considered soil being semi saturated with water the whole water could not be expelled out of the pores by NAPL volume. Thus the fluid saturation is distributed between the NAPL and the pore water, succeeded by a decrease in transport velocities the value being lesser than expected. The adherence of soil particles in the vadose zone to particular fraction of the NAPL is called residual saturation.

NAPLs Lighter than Water (LNAPLs)

LNAPLs may be retained on grain surfaces in small spills inside the vadose (unsaturated) zone. For further penetration, LNAPL is retained as residual saturation and has to be carried deeper parts of solution at contaminated site and dissolved by penetration of water. For example in xylene, toluene or benzene a further path of evaporation and diffusion may be used. In this the water is displaced in pore spaces by the NAPL’s in horizontal and vertical directions, the wetting properties are changed like capillary pressure, viscosity etc. are changed causing collapse of capillary fringe leaving a surged water table. With stoppage of the contaminant flow preferentially in the vadose zone in horizontal direction until residual saturation is achieved the water table rebounds (Palmer and Johnson 1989a).

NAPLs Denser than Water (DNAPLs)

In case of Denser NAPL’s, because of their denser nature the displacement is caused in the deeper layer from the vadose zone and as the water is viscous than the contaminant an unstable liquid-water boundary forming viscous fingers penetrating in the vadose zone til the residual saturation is achieved.A contaminate plume is formed with help of Penetrating waters and dense vapours that affects the capillary fringe. At local aggregation ganglia is formed at places when the NAPL’s are held in place between the soil grains

When there are large spills either dispersion is caused, residual saturation is achieved or deeper penetration occurs into the saturated zone. The spill could disperse near the water table or deeper penetration could cause formation of pools at the surface of impermeable layers majorly depending on the spill dimension. The mixing of both phases becomes difficult and miscibility is reduced due to the surface tension between the water wetting and NAPLs on soil grain surfaces. So the NAPL pollutants dissolve at a very low rate in the groundwater. Some processes to increase the dissolution rates have been given below: \

• 1) Hot water Flushing- In this process the viscosity of water is decreased and chemical solubility of the NAPLs is increased. But some studies suggest that this is not a very effective method (Imhoff et al. , 1995a)
• 2) Steam injection- It is a costly but effective method. when steam is injected in porous media like sand effective removal of immiscible pure phase liquids such as gasoline, trichloroethylene etc. was observed (Hunt et al., 1988)
• 3) Solvent Flushing- The interfacial tension between NAPLs and water that leads to mixing is decreased by some solvents and so chemical solubility of pure phases is increased e.g. methanol
• 4) Surfactant Flushing- As the interfacial tension between pure contaminant and water and decreases due to change in wetting property of associated water. A decrease in interfacial pressure occurs due to addition of surfactants through partitioning of the pure phase into surfactants micelles succeeded by an increase in NAPL chemical solubility

Summary:

By understanding the soil- pollutant interactions, what each fertilizer or amendment product offers with respect to nutrient availability, complexes formed and processes taking place ease of handling and application, reaction effects and understanding of the subject develops.

References

• Kaufman, M. I. and McKenzie, D. J. Upward migration of deep well – -waste injection fluids in Floridan aquifer, South Florida, U. S. Geol. Survey, Jour, of Research, 3(3), 261-265, 1975.
• Kozlovskiy, F. Structural Functional and Mathematical Model of Migrational Landscape Geochemical Processes. Pochvovedenive, 4, 122-8, 1972.
• Oberlander TM. Characterization of arid climates according to combined water balance parameters, J Arid Environ, 2: 219-241, 1979.
• Palmer, C.D and R.L. Johnson. Determination of Physical Transport Parameters. In: Transport and Fate of Contaminants in the Subsurface. EPA/625/4-89/019, Robert S. Kerr Environmental Research Laboratory, U.S. Environmental Protection Agency, Ada, OK, 1989a.
• Stumm, W., Morgan, J.J. Aquatic chemistry, John Wiley, New York, 624, 1980