Amino acid adsorption by clay minerals in distilled water

The adsorption of 15 protein amino acids from dilute (~ 10 μM) distilled water solutions onto organic-free kaolinite and montmorillonite clay minerals (1 wt% suspensions) was determined at room temperature over a 48 hour period. The systems came to steady state within 2 hours. Basic (positively charged) amino acids were strongly adsorbed (40-80% removal) by both clay minerals. Neutral (uncharged) amino acids were taken up appreciably (10-15%) by montmorillonite, but little if any (<5%) by kaolinite. Acidic (negatively charged) amino acids were adsorbed (20-35%) only by kaolinite. These adsorption patterns appear to be related in part to electrostatic interactions between the clay mineral surfaces and the different amino acid types. The measured extents and selectivities of adsorption onto these clay minerals are sufficiently great to potentially affect the distributions and reactions of free amino acids in natural environments.