5.3 Sorption and Ion Exchange Processes in Groundwater Systems

Sorption processes play a crucial role in groundwater systems, affecting contaminant mobility and water quality. These processes involve adsorption, absorption, and ion exchange, which determine how substances interact with solid surfaces in aquifers.

Various factors influence sorption and ion exchange in groundwater, including surface area, pH, temperature, and mineral composition. Understanding these mechanisms is essential for predicting contaminant transport and developing effective remediation strategies in groundwater management.

Sorption Processes in Groundwater Systems

Sorption and ion exchange processes

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• Sorption encompasses dissolved substances adhering to solid surfaces through adsorption, absorption, and ion exchange
• Adsorption accumulates substances on solid surfaces via physical or chemical bonding mechanisms
• Absorption incorporates substances into the bulk of a solid material
• Ion exchange involves reversible swapping of ions between solid phase and aqueous solution maintaining charge balance

Mechanisms of sorption

• Physical sorption (physisorption) uses weak van der Waals forces creating reversible process with limited long-term impact on contaminant mobility
• Chemical sorption (chemisorption) forms strong chemical bonds between sorbate and sorbent often irreversibly reducing contaminant mobility significantly
• Electrostatic attraction influences sorption of ionic species through interaction between charged particles and surfaces
• Hydrophobic interactions attract non-polar compounds to solid surfaces impacting organic contaminants (pesticides, PCBs)

Factors Affecting Sorption and Ion Exchange

Factors controlling sorption reactions

• Surface area of sorbent increases sorption capacity as larger area provides more binding sites
• pH of aqueous solution affects surface charge of sorbents and influences speciation of contaminants (metals, organic acids)
• Temperature generally decreases sorption at higher levels due to increased molecular motion
• Ionic strength impacts electrostatic interactions between ions and surfaces
• Presence of competing ions reduces sorption of target contaminants through site competition
• Organic matter content enhances sorption of hydrophobic compounds (PAHs, chlorinated solvents)
• Mineral composition of aquifer material determines varying sorption capacities (clay minerals, iron oxides)
• Contact time allows for more complete sorption as longer periods enable diffusion into pore spaces

Ion exchange in groundwater chemistry

• Cation exchange capacity (CEC) measures soil's ability to hold positively charged ions affecting retention of metal cations (Ca²⁺, Mg²⁺, Na⁺)
• Anion exchange capacity (AEC) represents ability to hold negatively charged ions important for oxyanions (NO₃⁻, PO₄³⁻)
• Selectivity coefficients describe preference of exchange sites for different ions influencing competition (K⁺ vs Na⁺)
• Impact on water hardness through exchange of calcium and magnesium with sodium in water softening processes
• pH buffering helps maintain stable pH in groundwater through ion exchange reactions
• Contaminant retardation slows movement of ionic contaminants in aquifers (heavy metals, radionuclides)
• Breakthrough curves describe contaminant transport affected by ion exchange characterized by $C/C_0$ vs pore volumes
• Regeneration of exchange sites occurs through natural flushing or artificial remediation processes (pump and treat)
• Impact on redox conditions through exchange of redox-sensitive ions affects groundwater chemistry (Fe²⁺/Fe³⁺, Mn²⁺/Mn⁴⁺)