A ''site creation'' model for specific adsorption of aqueous nucleobases, nucleosides, and nucleotides on compost-derived humic acid

Aqueous nucleic acid constituent solutes cytosine (1), cytidine (2), cytidine 5'-monophosphate (3), thymine (4), thymidine (5), thymidine 5'-monophosphate (6), uracil (7), uridine (8), uridine S-monophosphate (9), adenine (10), adenosine (11), adenosine 5'-monophosphate (12), guanosine (13), and guanosine 5'-monophosphate (14) adsorb on compost-derived humic acid in sequential steps A, B, and C, Langmuir analysis of the isotherms gives a site capacity v(i) and equilibrium constant K-i for a given solute and adsorption step i that increases v(A) < v(B) < v(c) and decreases K-A > K-B > K-B > K-C at 25.0 degrees C. Data at six temperatures in the range 10.0-40.0 degrees C indicate that structurally related solutes like 7, 8, and 9 adsorb through their nucleobase units. Adsorption of a solute in step A, B, or C can be endothermic, thermoneutral, of exothermic. This paper uses the sire capacity v(i) at 25.0 C, the average site capacity over the experimental temperature range (v(i), the adsorption equilibrium constant K-i at 25.0 degrees C, and the adsorption enthropy Delta H-i and entropy Delta S-i for each solute to propose a model consisting of four components or each step i = A or B or C: (a) solute adsorption on a specific HA site A, B, or C involved in respective step A, B, or C; (b) as a result of primary adsorption on site A or B or C for many solutes, freeing up or ''creation'' of more sites A or B or C, respectively, from interactions A...X, Y, etc. with other humic acid (HA) structural components X, Y, etc.; (c) solute adsorption at the newly available sites A or B or C; and (d) site hydration by water. The model accounts for the wide range of measured adsorption enthalpies and entropies. Among several conclusions is that adenine has an outstanding ability to interact with A and B sites of compost-derived HA and that B sites are harder for other solutes to access than A and C sites, Humic acids change their dimensions and stale of aggregation to suit their environment. It now appears that selectivity for a solute is related to HA's ability to make more adsorption sites available near the point of primary solute interaction.