title: Investigation of the ionic strength dependence of Ulva lactuca acid functional group pK(a)s by manual alkalimetric titrations.
abstract: We performed a series of manual alkalimetric titrations in NaCl solutions (0.01-5.0 M) at T = 25 degrees C on both fresh and dehydrated samples of the marine chlorophyte Ulva lactuca (sea lettuce), a strong metal accumulator holding considerable promise in biosorbent and biomonitor applications. Functional groups were characterized in terms of their number, site densities, and acid dissociation constants (pK(a)s). FITEQL4.0 modeling shows that, at any ionic strength, titration curves for dehydrated biomass in the pH range 2-10 are adequately described by three functional groups with remarkably uniform site densities of about 5 x 10(-4) mol/g. Lower site densities for fresh U. lactuca are consistent with approximately 87% water content. The pK(a)s display pronounced ionic strength dependent behavior obeying an extended Debye-Huckel relation. Extrapolation to I = 0 yields values of 4.26 +/- 0.04, 6.44 +/- 0.02, and 9.56 +/- 0.04. This information by itself is insufficient to unambiguously identify the groups. Similar site densities suggest that all three are linked to major molecular building blocks of the cell material, pointing to carboxylic acids, phosphate esters, and amines as likely candidates. Highly acidic sulfate esters, not detected in our titrations, may also play a role in trace metal adsorption on U. lactuca.
authors.affiliation: Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, P.O. Box 38, Solomons, Maryland 20688, USA. schijf@cbl.umces.edu
authors.forename: Johan
authors.initials: J
authors.lastname: Schijf
authors.forename: Alina M
authors.initials: AM
authors.lastname: Ebling
mesh_terms: D000327:Adsorption; D000704:Analysis of Variance; D001673:Biodegradation, Environmental; D018533:Biomass; D003102:Colloids / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009994:Osmolar Concentration; D012044:Regression Analysis; D014131:Trace Elements / Q000032:analysis; D044445:Ulva / Q000254:growth & development*; D014874:Water Pollutants, Chemical / Q000032:analysis
chemical_list: D003102:Colloids; D014131:Trace Elements; D014874:Water Pollutants, Chemical