DOM-montmorillonite system. Snapshot from a simulation of protonated DOM-montmorillonite system. Water molecules are represented with cylinders, whereas DOM and clay are represented as balls and sticks. Color codes for atom types are as follows: carbon (gray), hydrogen (white), oxygen (red), nitrogen (blue), sulfur (yellow), and calcium ions (brown). Reprinted with permission from Sutton and Sposito . Copyright 2006 Elsevier.