31 Interaction of Pollutants with Soil components I

Meenakshi Nandal

Objectives:

• Understand the soil-pollutant interaction in general
• Understand the mechanism involved in soil-pollutant interaction

Introduction

The Soil systems consists of a dynamic and complicated array of inorganic and organic constituents and at any instance the concentration of pollutants in the soil liquid phase is defined by a series of reactions such as acid-base equilibria, oxidation-reduction equilibria, complexation with organic and inorganic ligands, precipitation and dissolution of solids and lastly ion exchange and adsorption. The reactions rates of occurring depend on both rates of degradation and/or biological uptake and also regulate the concentrations of the pollutants in the soil liquid phase. Solubility equilibrium is the climax stage that a toxic chemical and the soil water shall attain under optimum environmental conditions. The interaction between soil and pollutant can influence almost all the properties of soils. A better understanding can be developed, if the various factors affecting the soil pollutant interaction are isolated and considered independently. Soil response to environments is defined by soil structure, geochemical parameters, chemical characteristics, soil-water interaction. This is also influenced by mineral structure, like size of particle, bonds between particles, ion exchange capacity, etc. The smaller the size of soil particle greater ability it has to interact with the environment. Other factors included are ion exchange; cation and anion exchange, nature of pore fluid; electrolyte concentration, dielectric constant, acidity and alkalinity. The various process involved are listed below in fig.1.

Process Involved in soil-pollutant interaction

Acid-Base   Equilibrium:    The    array   of   proton   transfer   reactions   governs   the    acid-base equilibrium processes. If this case is considered proton donor is an acid and base is the acceptor e.g.  If  soil  solution  is  being  considered  as  an  aqueous  system,  H2O  is  always  the  present acceptor. For example, during the dissociation of an acid in the soil solution, one of the dissociation products will be H3O+ and the acid strength of the H+ acid relative to H3O+ will be a quantitative parameter. In a dilute solution, the activity of the hydrated protons equals that of H3O+ and the pH value represents the H+ ion activity. Under these conditions, substituting for pH into the following equation gives:

[base] pH – pKA = log [acid];   Acid ~ base + proton     (Henderson–Hasselbalch equation)

Figure 32.1: showing various processes of soil-pollutant interaction

Precipitation-Dissolution- In soil medium this is a chemically occurring heterogeneous reaction. Under various physicochemical conditions, a process of dissolution and precipitation of minerals in water occurs. This reaction plays a major role in the natural systems while a lesser one in the fate of pollutants. However, the dissolution of a particular chemical in the soil -water phase could alter the speciation of water phase pollutants. (Stumm and Morgan, 1981).

Equilibrium constant can be considered to estimate the vicinity of the dissolution or precipitation reaction of the systems. As the dissolution ends only when water saturation occurs, supersaturation is the stage where precipitation of chemicals occurs. In case of soil medium, a heterogeneous chemical reaction i.e. dissolution reaction occurs and the solubility in terms of a solubility product with equilibrium constant is characterized to predict the solubility and the ways in which the solubility is affected by solution variables. A chemical substance can be present in one or more crystalline form but, excluding a transition temperature, considering chemical composition of the system just one form is thermodynamically stable. However, the stable forms are less soluble than the metastable stable ones. Similarly, the allotropic forms have different activities, the highest being with the most unstable one. Coprecipitation of inorganic trace elements, for example, as contaminants in their oxide forms, may blend the solubility product access by producing ill-defined stoichiometric solid solutions. So, for deducing the changes in solubility in terms of solubility product the estimation of solubility product in affiliation to equilibrium constant is necessary. Figure 32.2 shows schematic solubility diagram showing relationship between solubility and saturation.

Figure 32.2: A schematic solubility diagram showing relationship between Solubility and

saturation                                                               Source: http://www.xray.bioc.cam.ac.uk

The Ligand Effect – Ligand is defined as a coordination compound formed by a central atom of cations and molecules of anions. In this a complex formation takes place by combination of cations possessing molecules or anions possessing free pairs of electrons. The complex can either be electrostatic, covalent, or a mixture of both. The central atom is cation and anions being ligand in which the molecules form coordination compound. The molecule forms a multidentate complex called chelates consisting of more than one ligand. So, conceptually the complex formation is in co-ordination with the acid-base and precipitation-dissolution reactions. After reaching the soil solution, the base ion initiates a continuing search for its partner. The cations undergo exchange reactions by following coordination reactions in which exchange occurs between the soil solution with the coordinated water molecules and preferred ligands. A complex called an inner-sphere complex is thus formed between a Lewis acid and base. While an outer-sphere complex is formed, when the acid and the base molecules are interposed by the water molecule. The hard Lewis acid has high electronegativity, small size, low polarizability, high oxidation state and whereas a molecular unit with low oxidation large size, low electronegativity and high polarizability is the case of a soft Lewis acid. A hard Lewis base has a high electronegativity and a low polarizability while a soft Lewis base displays low electronegativity and high polarizability. (Hartley and Graham-Bryce, 1980)

Oxidation-Reduction Equilibrium– These reactions involve transfer of electrons from donors to acceptors. A redox reaction consists of two half reactions, a reduction and an oxidation. Oxidation number is the concept used to designate a reduced and oxidized chemical species a hypothetical valency to every atom is assigned. Every redox reaction includes two half reactions:

• a reduction half-reaction and an oxidation half-reaction. A reduction half-reaction, for example, includes acceptance of electrons by a chemical species while an oxidation reaction has donor chemical species.

A here denotes a chemical species in any phase and the subscripts ox and red denote the reduced and oxidized states, respectively. The parameters a, b, s and g are stoichiometric coefficients, H+ and e denote the proton and the electron in the aqueous solution. Though the potential in soil solution always exists for redox reactions to occur, built is not necessary that the proficiency of the reaction also occurs. If the redox reaction is favored thermodynamically it is not necessarily that it is favored kinetically also. Under these conditions and in the presence of a catalysis process, as the reactions are slow the equilibrium could be achieved. In case of redox reactions in soil medium, the microbial organisms may act as catalysts play an effective role under favorable conditions. The redox reactions can only be controlled by the thermodynamic processes while reaction kinetics controls the soil microbial-induced catalysis.

Effects of Mixed Solvents and Surfactants – Dispersing agents or surfactants are responsible for penetration of the potentially toxic organic chemicals in soil either in mixture of solvents or in other forms. The major effect is an increase in solubility of the solvent. Most of the organic chemicals that are potentially toxic reach the soil either as a mixture of solvents or in formulations with dispersing agents (surfactants). In a solvent such as water these formulations increase the solubility of the active compound. In a solvent which forms nearly optimum solutions with another, limiting the solubility of a third substance and a lower crystal stability value, the logarithm of its solubility and the mole fraction composition of the solvent are nearly in a linear function. In case of a solvent pair, where the solubilities of the third substance vary greatly, these solvents – due to their varying solvent powers -are typically much less soluble in one another and form miscible solutions which are non-linear. Thus the amount expected to be dissolved by the more powerful solvent is much larger as compared to quantity of solute dissolved in a mixture of two equal amounts of the solvents. When a case of powerful organic solvent miscible with water is considered, a more linear slope for the log solubility vs. solvent composition relationship can be derived when the compositions plotted as volume fraction instead of mole fraction.

Effect of Temperature on Solubility: The solubility of most inorganic and organic pollutants in water is in direct proportional relation with temperature. The temperature along with seasonal variation directly as well as indirectly affects the solubility of contaminant in the soil aqueous phase. Also, the seasonal variation in temperature could affect the toxic chemicals solubility in the soil solution. So, at a given time and ambient temperature the recognized solubility equilibria defines only the solubility of the compound.

An exceptionally complex and heterogeneous structure defines a soil environment. This soil environment supports plant and animal life by developing a particle-pore matrix consisting of solid, liquid and gas phases. This is necessary for an effective cycling and storing of nutrients. Also, the migration of pollutants is influenced by many other factors as well as the complexity of situation. As the heavy metals penetrate in the soil, ionic composition and concentration changes in the medium that disrupt the equilibrium state of the soil and cations in the adsorption layer. After this, in the free medium the cations displace the one present in adsorption layer making the soil attain a stable state. Simultaneously, due to the developed electrostatic force, the cations invade the heavy metal that are adsorbed by the surface of soil particles and gets neutralized by the anions in the adsorption layer. With a gradual increase in concentration of invading cations, more anions become neutralized leading to reduction in electrokinetic potential, resulting in diffusion of the double electric layer. In addition, other reactions by heavy metals like complexation reaction, replacement reaction, redox reaction etc. with cement particles generates soft organic plastid, causing rupture of the inter-particle aggregation

The physical, chemical, and biological processes influence the behaviour and interaction of pollutants with soil in all three components (solid, gas, and liquid). The physical processes include transport and retention and mainly defined by physical parameters of the medium (temperature, electric charges and grain size, etc.), while in case of chemical processes they rely largely on the type and chemical nature of pollutants. However, biologically controlled processes include the biotransformation and biodegradation process, depending on their microbial ecology,  depth and oxygen availability at the pollution site. As the pollutants encounter the soil grains, may retain on the surface of grains by adsorption, or accumulate in the intergranular space, forming concentrations that retain their original chemical composition or form elements that have been altered due to chemical reactions. The contaminants enter a natural environment resulting in instability, disorder or harm to the ecosystem i.e. physical or biological system. These elements of pollution can be alien or foreign substances, energies, or naturally occurring (contaminants that exceed the natural levels). Pollutant migration has noticeable impact on engineering aspects of the soil, so is being taken up to study by environmental geotechnical scholars. The evaluation of fate of contaminants occurring in soil is important in terms their possible effects on exposure to humans. Another significant component is to understand the complexity of pathways controlled by emission sources, relation with soil surfaces, and changes in the chemical and biological conditions with time, in the environment. Soil degradation has a direct influence on water, air quality and climate change and can also affect human health and food safety.

The comportment and interaction mechanism of soil pollutants consists of various biological, physical and chemical processes functioning in all three phases (solid, gas and liquid) of the soil avenues. They generally include three main groups of processes:

1. Confinement on and in soil body
2. Soil solution movement and diffusion
3. Chemical transformations within the soil

The transport and confinement of soil and pollutant components are the major physical processes, shown in fig 2. They mainly rely on the physical variables of the soil medium i.e temperature, size, charge etc. On the other hand chemical procedures are defined mainly on the type and chemical nature of pollutants while biologically controlled process majorly depend on microbiological biotransformation and biodegradation process depending on microbial species, depth of their occurrence and the amount of oxygen available at the site of pollution.

Fate of contaminants

The fate of contaminants in soil follows either through point source or diffuse pollution; the main difference existing in between the two lies the transferring mechanism of contaminants to the soil. Point sources, such as landfills and incinerators etc. use soil as a support and are linked to the events that necessarily transport pollutants into the soil. The diffusion sources are linked with natural phenomena including long range transport, sedimentation, recycling, inadequate waste treatments and agricultural practices. Generally, the most hazardous contaminants in soil are persistent organic pollutants, inorganic pollutants and heavy metals. Persistent organic pollutants are derived anthropic and characterized by high lipid affinity, semi-volatility and resistance to decomposition. If the heavy metals, cannot be degraded their presence in the soil could be due to certain natural processes, like the soil formation or due to anthropogenic activities. Some of the important essential elements (Cu, Fe, Mn, Zn, Co), when present in ideal concentration are suitable, while others (Hg, Pb, Cd) are potentially toxic elements.

The bioavailability of a pollutant is decided by soil and on-site characteristics. The bioavailable portion is known to affects the plants, animals or humans directly, as it is taken up by their bodies. The site conditions affect then contaminant binding to the soil particles and its solubility in water. Higher solubility generally means that more bioavailable of the contaminant, but it also relates the contaminant likeliness to leach out of the soil. As the contaminants penetrate in soils the contaminants undergo chemical changes and degrade into products that mayhave more or less toxicity in comparison to original compound. The different contaminants differ in their tendency to:

• ♦ through leaching they end up in soil water or in groundwater
• ♦ Volatilize into the air
• ♦ Bind tightly to the soil.

The soil characteristics that may also affect the fate of contaminants. characteristics that may affect the behaviour of contaminants include:

• ♦ Soil mineralogy and soil texture
• ♦ pH of the soil
• ♦ organic matter of soil
• ♦ Moisture content
• ♦ Temperature existing in soil
• ♦ Presence of other chemicals in soil

Nature and behaviour of inorganic contaminants

Heavy metals are one such group of the numerous classes of substances, if they reach the critical levels considering the human health, soil fertility and ecological risks food safety. These metals being the common contaminants in the soil can bioaccumulate, in the organism and thus their absorption by the organism increases with time in comparison to the level evaluated in the environment. This is due to the higher absorption rate than the excretion rate of the organism. Also, the allocation of heavy metals amongst the solid phase and soil solution is deemed to be the main factor to assess the environmental outcomes of the heavy metal accumulation in the soil. An analysis of soil profile along with the physical and chemical analysis is essential for assessing if soil is a barrier against the inorganic contaminants, especially heavy metals. In the solid phase, retention of heavy metals in soil is dependent mainly on the pH, linked to clay minerals, hydroxides, humic substances, iron oxides and manganese present in the soil, that control the attenuation effect on anionic forms as well. The processes involved in retention and release of heavy metals involve precipitation, decomposition, ionic exchange, adsorption and desorption. The precipitation reactions may involve release of discrete solid phases that are absorbed by the soil surface. While, the ion-exchange reactions are related to exchange between an ionic species present in the soil medium and an ionic species retained at sites possessing a permanent charge on the soil surface. The process of absorption and desorption can influence all ionic or molecular species and generally affect the absorbent sites with a pH-dependent charge like iron, hydroxides, aluminium, manganese oxides, humic substances and clay minerals.

Summary:

We studied the soil systems and processes involved in the soil-pollutant interaction. We understand the factors affecting the soil-pollutant interaction. Understanding the mechanism involved in soil-contaminant interaction  The process depicting the fate of contaminants in soil should be well understood, to evaluate their possible harmful effects on exposure to humans. We study the significant element that is complexity of pathways, interactions with soil surfaces, the chemical and biological changes over time in the environment.

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