Chapter 2: Geochemistry Background

2.1: Introduction In this chapter we study the behavior and reactions of chemicals in soils and groundwater. The chemical aspects of soil are generally studied in geochemistry courses, which apply the principles of chemistry to the study the chemical reactions and processes that are associated with earth materials. We are particularly deals with the distribution and behavior of chemicals in soils and groundwater.

Introduction In this chapter, we review briefly the geochemistry of inorganic and organic chemicals. This background is essential in addressing geo-environmental problems, particularly issues dealing with the soil and groundwater remediation process and the chemical-resistant barriers. The following sections cover basic concepts of inorganic and organic geochemistry.

2.2 Chemistry of Soils

Overview Soils are the interface between the biosphere, lithosphere, hydrosphere and atmosphere: •Nutrients in soils determine the biological productivity of an ecosystem. •Soils buffer the chemical inputs to groundwater. •Soils act as a source/sink of CO2. •Biogeochemical processes in soils are the primary chemical weathering reactions.

Components of Soils

Soil (and Sediment) Texture and Mineralogy

2.3: Soil pH and Soil Acidification

Significance of Soil pH/Acidity Soil pH determines the solubility (and bioavailability) of essential nutrients and toxic species (e.g., Al(OH)n3-n).

Soil Acidity Active acidity: Acidity associated with the solution phase. Reserve Acidity: all titratable acidity associated with the solid phase. Two components: Non-exchangeable acidity: bound H+ and Al 3+ that is not displaced by concentrated neutral salts. Associated with organics and bound Al-OH cations. Exchangeable Acidity: exchangeable Al 3+ in mineral soils and exchangeable H+ in organic soils.

Acid-base Buffers in Soils

Acid-base Buffers in Soils

Consequences of Acidification (In Soils)

Consequences of Acidification Acid input also releases cations such as K, Ca, Mg from smectite clays and decreases soil fertility. Release of Al also leads to the formation of AlPO4 (at low pH) and limits phosphate bioavailability. At higher pH, Al(OH)3 and related phases sorbs HPO4-2 by surface complication.

Nutrients

Soil Nitrogen Cycle

2.4: Soil Pollution

2.4.1: Inorganic Geochemistry One aspect of inorganic chemistry is the behavior of inorganic chemicals, such as heavy metals, in soils and groundwater. In this section we present various types of toxic metals found at contaminated sites. We then explain briefly the distribution of metals in soils, geochemical processes affecting the distribution of metals in soils, and mathematical models that can predict the metal distribution in soils and groundwater.

2.4.2: Metal Contamination

2.4.2: Metal Contamination Of the many different inorganic chemicals, we will concern ourselves only with those that are identified as toxic by the USEPA. The toxic inorganic chemicals most commonly found at contaminated sites and in waste leachate include: lead (Pb), chromium (Cr), arsenic (As), cadmium (Cd), nickel (Ni), zinc (Zn), copper (Cu), mercury (Hg), silver (Ag), and selenium (Se).

2.4.2: Metal Contamination These chemicals are also commonly referred to as metals, toxic metals, heavy metals, trace metals, transition metals, or micronutrients. Strictly speaking, a metal is any element that has a metallic luster and is a good conductor of heat and electricity. Although As and Se have both metallic and nonmetallic properties (metalloids), they are also commonly called metals. Unlike organic compounds, the unique characteristic of all metals is that they are not biodegradable.

2.4.3: Distribution of Metals in Soils When metals are introduced into soils, they may be distributed in one or more of the following forms: 1. Dissolved in soil solution (pore water) 2. Occupying exchange sites on inorganic soil constituents. 3. Specifically adsorbed on inorganic soil constituents. 4. Associated with insoluble soil organic matter. 5. Precipitated as pure or mixed solids. 6. Present in the structure of minerals.

2.4.3: Distribution of Metals in Soils In situations where metals have been introduced into the environment through human activities, they only exist in the first five forms. The last fraction generally exists if metals occur in natural soils that have been subjected to various geologic processes. For simplicity, the metal forms in soils can be grouped under three phases: (1) aqueous phase, representing the first form; (2) adsorbed phase, representing the second, third, and fourth forms; (3) solid phase, representing the fifth form.

2.4.3: Distribution of Metals in Soils The aqueous phase represents the metals existing in soil solution as free (uncomplexed) metal ions (e.g. Cd2+ ,Ni2+, Zn2+, Cr3+) and as various soluble complexes (e.g. CdSO4 ,ZnCl+, CdCl3 -). Metals can form soluble complexes with inorganic and organic ligands. Common inorganic ligands are and organic ligands include soluble constituents of fulvic acids, and low molecular weight aliphatic, aromatic, and amino acids.

2.4.3: Distribution of Metals in Soils The potential for migration (or mobility) of metals in soils will be higher if higher amounts of metals are present in the aqueous phase; this is because metals in the aqueous phase are readily transported by processes such as advection, dispersion, and diffusion. The adsorbed phase represents the accumulation of metal ions at the interface between soil solids and the aqueous phase. This phase is associated with the surfaces of soil solids such as organic matter, soil minerals, iron and manganese oxides, and hydroxides, carbonates, and amorphous alumino silicates. .

2.4.3: Distribution of Metals in Soils The solid phase represents metals present in precipitate form. These precipitates exist in three dimensional solid phase and may consist of pure solids [e.g., CdCO3, Pb(OH)2, ZnS] or mixed solids [e.g.,(FexCr1-x)(OH)3.] Mixed solids are formed when various elements coprecipitate.

2.4.3: Distribution of Metals in Soils The solid phase may be present in soils when metal concentrations are significantly higher than their solubility limits. The metals that exist in solid phase are immobile in soils, meaning that the solid-phase metals remain in place and cannot migrate. Certain remediation technologies such as stabilization aim to convert all of the metals in aqueous and adsorbed phases into the solid phase.

2.4.3: Distribution of Metals in Soils The conversion of metals into the solid phase eliminates the potential for migration or spreading of the contaminants in soils, thereby reducing the risk to public health and the environment.

2.5: Geochemical Processes Controlling in the Distribution of Metals in Soils

2.5: Geochemical Processes Control The amounts of metals present in different soil phases (i.e., the aqueous, adsorbed, and solid phases) are controlled by the following interdependent geochemical processes: (1) adsorption and desorption, (2) redox reactions, (3) complex formations, (4) precipitation and dissolution of solids, and (5) acid-base reactions.

(1) Adsorption and desorption Adsorption is defined as the accumulation of ions at the interface between a solid phase and an aqueous phase. Desorption is the opposite of adsorption, defined as the decrease of ions at the interface of solid and aqueous phases.

(1) Adsorption Edges to Ferric oxide

(2) Redox reactions Oxidation-reduction reactions involve the transfer of electrons and are important for metals that possess more than one oxidation state. Important toxic inorganic chemicals that possess multiple oxidation states are U, Cr, As, Mo, V, Se, Sb, W, Cu, Au, Ag, and Hg. Redox reactions also involve other major elements that possess more than one oxidation state, such as H, O, C, S, N, Fe, and Mn.

(2) Redox reactions Eh is used instead of p' (or pE) to express redox potential. pE is the negative common logarithm of electron activity or electron concentration, pE = -log10[e-]. Eh and pE are related by Eh : 0.059 pE Eh is measured in aqueous samples using the standard hydrogen electrode (SHE), which is formed by bubbling hydrogen gas at 1 bar pressure over a platinum electrode in a I N HCI solution.

(2) Redox reactions – pE (or Eh) - pH diagram for chromium

(2) Redox reactions – pE (or Eh) - pH diagram for Pb

(3) Complex formations Depending on the inorganic and organic present in soil, metals may form various complexes. These complexes may be cationic, anionic or neutral This will then affect the surface reactions and hence adsorption and desorption. For instance, Cr 3+ may complex with hydroxyl to form Cr(OH) 2+ Cr(OH)2 + or Cr(OH)3. The adsorption of complexes is expected to be reduced and becomes negligible when Cr(OH)3 solids are formed.

(4) Precipitation and dissolution of solids Precipitation and dissolution of solids can significantly influence the phase distribution of metals. The dissolution of solid minerals and metal oxides and hydroxides increases the aqueous phase, while precipitation of the solids decreases the aqueous phase. The solubility property of the solids controls the extent of dissolution. The solubility of solids is influenced by the soil solution composition, including ionic strength.

(5) Acid-base reactions Acid-base reactions affect the pH and the ion chemistry of soil solution. CO2, and water in soil solution generate H+ and carbonates species( H2CO3,HCO3 -. and CO3 2- ) through a series of acid-base reactions that increase the pH. This increase in pH affects the adsorption and desorption of metals.