ADSORPTION OF AQUEOUS NUCLEOBASES, NUCLEOSIDES, AND NUCLEOTIDES ON COMPOST-DERIVED HUMIC-ACID .1. NATURALLY-OCCURRING PYRIMIDINES

Adsorption of the naturally occurring pyrimidine nucleobases and their corresponding nucleosides and nucleotides on compost-derived humic ac id (HA) has been studied in aqueous solution. All these nine solutes i nteract with HA. Adsorption isotherms were measured at 19 solute conce ntrations from 12-600 mu M and 6 temperatures in the range 10.0-40.0 d egrees C. Analysis with the Langmuir model indicates that adsorption o ccurs in three detectable steps A, B, and C, depending on the solute a nd the experimental conditions. Most of the data fit the Langmuir mode l, which generates a site capacity vi and an equilibrium constant K-i for each detectable adsorption step. Site capacity and solute selectiv ity data indicate that the HA sites responsible for adsorption steps A , B, and C are chemically different and that pyrimidine solutes are ad sorbed through their nucleobase units. Adsorption of pyrimidines on HA is exothermic, thermoneutral, or endothermic, depending on the solute and the adsorption step. Linear correlation of adsorption enthalpies and entropies points to similar surface interactions.