M.E. Sastre de Vicente

Physicochemical studies of cadmium(II) biosorption by the invasive alga in Europe, Sargassum muticum.

In recent years, there has been a significant increase in the studies concerning brown seaweed as biosorbents for metal removal owing to their high binding ability and low cost. This work reports the results of a study regarding the cadmium binding equilibria of dead biomass from the seaweed Sargassum muticum; this alga is a pest fouling organism that competes with the local fucalean species and may also interfere with the “sea industry”; therefore, it would constitute an ideal material to be used as biosorbent. Seven different treatments were tested in order to obtain a stable biomass that could be suitable for industrial use under a broad range of operational conditions. The treatments employed were protonation, chemical cross‐linking with formaldehyde, KOH, Ca(OH)2 and CaCl2 or physical treatments with acetone and methanol. The equilibrium adsorption isotherms of Langmuir, Freundlich, and Langmuir–Freundlich were obtained for the quantitative description of the cadmium uptake. The effect of pH on biosorption equilibrium was studied at values ranging from 1 to 6, demonstrating the importance of this parameter for an accurate evaluation of the biosorption process. Maximum biosorption was found pH higher than 4.5. The maximum biosorption uptake for the raw biomass was 65 mg g−1, while for formaldehyde cross‐linking biomass the uptake increases to 99 mg g−1 and for protonated biomass to 95 mg g−1. Potentiometric titrations were carried out to estimate the total number of weak acid groups and to obtain their apparent pK value, 3.85, using the Katchalsky model. Kinetic studies varying cadmium concentration, algal dose, and ionic strength were carried out. Over 95% of the maximum cadmium uptake was achieved within 45 min, so the process can be considered relatively fast. A pseudo‐second‐order model, for the kinetics of cadmium biosorption, was shown to be able to reproduce experimental data points with accuracy.

The protonation constant of triethanolamine in KBr and KNO3 solutions at 25°C

We determined the stoichiometric acid-base constants of triethanolamine at 25°C in KBr and KNO3 at different ionic strengths by the potentiometric technique using glass electrodes sensitive to the H+ ion. By applying the Pitzer equations for the activity coefficients of the species present in the medium we obtained equations accounting for the dependency of pK on the ionic strength. The coefficients thus obtained are critically analyzed.

Effect of ionic strength on the protonation of various aminoacids analysed by the mean spherical approximation

The dependence of the ionic strength of stoichiometric protonation constants, obtained potentiometrically, of several amino acids has been analysed by the mean spherical approximation (MSA), a mathematically simple statistical mechanical treatment based on integral equations. Unlike the Debye–Huckel equation, which overestimates the effects of ion–ion interactions as the sizes of all ions are neglected, the MSA represents the electrolytes as a mixture of hard spheres immersed in a dielectric continuum which represents the solvent. This approach fits experimental data using concentration, electrical charge and diameter concepts and is an alternative to semiempirical models based on different ionic strengths with coefficients that defy theoretical interpretation. Analysis of data using both semiempirical models and a quasi-lattice model, not containing Debye–Huckel terms, is also included.

Trend and energetics of pK* vs. ionic strength for o-chlorobenzoic, m-nitrobenzoic and benzoic acids in aqueous KNO3 solutions at 298 K

Ionization constants for benzoic, m-nitrobenzoic and o-chlorobenzoic acids are experimentally determined in aqueous KNO3 solutions and ionic strength dependence of data are analysed by using Guggenheims model for activity coefficients. Moreover, for benzoic acid, energetic contributions ΔGex in the minimum of the pK–ionic strength curve are calculated. It is found that ca. 20% of the total excess energy is due to the non-Debye–Huckel term. A discussion of pK*(I) in terms of solvation energy is also given.

Protonation constants of α-alanine in electrolyte mixtures at constant ionic strength

The stoichiometric acid-base equilibrium constants for α-alanine in tetraethylamonium iodide-potassium nitrate solutions were determined at a constant ionic strength of 1.4 m at 25°C. The results obtained are discussed on the basis of the Friedman and the Pitzer model for electrolyte mixtures.