Adsorption of Aqueous Nucleobases, Nucleosides, and Nucleotides on Humic Acids. 3. Adsorption of Uracil, Uridine, and Uridine-5‘-Monophosphate on a German Peat-Derived Humic Acid and Its Tightly Bound Mercury(II) Form

Abstract

Aqueous nucleic acid constituents uracil, uridine, and uridine-5‘-monophosphate are selectively adsorbed by a solid German peat-derived humic acid (GHA) and its tightly bound mercury(II) form (MGHA). Adsorption isotherms with GHA and MGHA adsorbents were obtained at 19 solute concentrations from 0 to 1.20 mM and seven fixed temperatures from 5.0 to 35.0 °C. Langmuir data analysis indicates that adsorption occurs in sequential steps A, B, and C, depending on the solute and experimental conditions. MGHA very strongly adsorbs low concentrations of uracil and site capacity vA for uracil adsorption on MGHA in step A increases with increasing temperature. The last steps of adsorption of relatively high concentrations of uridine, and uridine-5‘-monophosphate on MGHA are S-curves ascribed to increasing site capacity with increasing solute concentration. Comparison of the new data for adsorbents GHA and MGHA with data for adsorption of uracil, uridine, and uridine-5‘-monophosphate on compost-derived humic acid (CHA) from part 1 reveals similarities and differences. Nevertheless, linear plots of adsorption enthalpies vs entropies in all detected steps with CHA, GHA, and MGHA as adsorbents indicate that selective adsorption of nucleic acid constituents on different HAs is free energy buffered by similar mechanisms.

Adsorption of nucleic acid constituent uracil on copper(II)-loaded, solid peat and soil-derived humic acids

“…The slope of Fig. 6 is 313K (40°C) and the intercept is -4.5 kcal mol -1 , similar to parameters for nucleic acid constituent binding to HAs from different sources (Davies et al 1997a;Ghabbour et al 1997).…”

“…HA samples GHA and NHA were isolated with the different protocols in Davies et al (1997b) and Ghabbour et al (1997). We verified that the HA products isolated with these methods from a given source are indistinguishable.…”

“…Linear segments in plots of Eq. 2 indicate successive adsorption steps with increasing c (Khairy et al 1996a,b;Davies et al 1997a,b;Ghabbour et al 1997). Also, an adsorbent with a fixed number of sites per gram theoretically has v independent of solute molar mass and temperature (Adamson 1982;Ghabbour et al 1997).…”

“…We are studying the sorptive (Khairy et al 1996a,b;Davies et al 1997a;Ghabbour et al 1997) and metal binding (Davies et al 1997b;Ghabbour et al 2001) properties of solid HAs isolated from composts, peats and soils. Our objective is to establish how solid HAs perform these functions and whether HAs from different countries behave similarly or not.…”

“…That is, they are free energy buffers. Free energy buffering is made possible by compensating enthalpy and entropy changes for adsorption (Davies et al 1997a;Ghabbour et al 1997) and metal binding (Ghabbour et al 2001). The sorption of solutes by a solid HA reflects its macroscopic properties.…”