ADSORPTION OF AQUEOUS NUCLEOBASES, NUCLEOSIDES, AND NUCLEOTIDES ON COMPOST-DERIVED HUMIC-ACID .2. NATURALLY-OCCURRING PURINES

This paper describes adsorption of the aqueous purines adenine (10), a denosine (11), adenosine 5'-monophosphate (12), guanosine (13), and gu anosine 5'-monophosphate (14) on compost-derived humic acid (HA) over the solute concentration (12-1200 mu M) and temperature (10.0-40.0 deg rees C) ranges used with pyrimidine solutes in Part 1. Nucleobase 10 a nd nucleosides 11 and 13 are neutral under the experimental conditions , while nucleotides 12 and 14 are monoanions. All the purines interact with HA, but only adenine causes HA dissolution at pH 5-6 and tempera tures above 25.0 degrees C, as found with cytosine and (-)nicotine. Th e TR spectrum of the soluble adenine-HA product indicates strong inter action with carboxylic and phenolic HA groups that is not due to simpl e proton transfer. Pyrimidines and purines are adsorbed in sequence on three HA sites A, B, and C. Most of the data fit the Langmuir model t o generate site capacities v(A), v(B) and v(C) and equilibrium constan ts K-A, K-B, and K-C for each solute and site. Average site capacities increase (v(A)) < (v(B)) < (v(C)), while equilibrium constants at 25. 0 degrees C decrease K-A > K-B > K-C. Site capacities v(i) are indepen dent of temperature and solute molar mass or charge, indicating adsorp tion through solute nucleobase units. Adenine has unusually high avera ge (v(A)) and (v(B)) related to its ability to dissolve HA at pH 6, bu t (v(C)) is unexceptional. The equilibrium constant data show that com post-derived HA sites have quite different solute selectivities. For e xample, site A discriminates poorly between the solutes thymine (4), t hymidine (5), and 11 while site C has a strong preference for 4. These selectivities could influence animal, plant, soil, and sediment bioch emistry. Linear correlation of the enthalpies and entropies for adsorp tion of every solute at each HA site points to a common underlying ads orption mechanism.