Sorption and Ion Exchange

Sorption and ion exchange at mineral and organic surfaces control the retention and release of nutrients, metals, and contaminants in soils and sediments.

Definition

The study of the accumulation of ions and molecules at solid-solution interfaces in soils and sediments, including reversible ion exchange and specific adsorption.

Scope

This topic covers cation and anion exchange on charged surfaces, specific adsorption and surface complexation of metals and oxyanions, sorption isotherms and partitioning of organic compounds, and the dependence of these processes on pH, ionic strength, and surface properties.

Core questions

• What distinguishes nonspecific ion exchange from specific adsorption?
• How do pH and ionic strength control sorption?
• How are sorption data described by isotherms and surface-complexation models?
• Why do organic contaminants partition to soil organic matter?

Key theories

Surface complexation and ion exchange

Ion retention spans weak, reversible electrostatic exchange on permanent-charge sites and strong, pH-dependent specific adsorption through surface complexation on variable-charge oxides, providing a unified description of soil sorption.

Mechanisms

Permanent negative charge on clays drives reversible cation exchange, while variable-charge oxide and edge sites form inner- and outer-sphere surface complexes whose strength depends on pH and the ion. Nonpolar organic compounds partition mainly into soil organic matter, described by partition coefficients linked to organic-carbon content.

Clinical relevance

Sorption and ion exchange determine the bioavailability, leaching, and mobility of nutrients and contaminants, and are central to predicting transport and to remediation design.

History

From early studies of base exchange in soils, the field advanced through surface-complexation modeling and spectroscopic confirmation of bonding mechanisms in the late 20th century.

Key figures

• Garrison Sposito
• Donald L. Sparks

Seminal works

• sparks2003
• sposito2008

Frequently asked questions

What is cation-exchange capacity?

It is the quantity of exchangeable cations a soil can hold on its negatively charged surfaces, a key measure of its ability to retain nutrients.