Lorenzo Spadini

Cation adsorption on oxides and clays: The aluminum case

The sorption mechanisms for trace metal ions on montmorillonite have been investigated. Complexation with surface hydroxyl groups located on the brocken edges of platelet particles is found to occur over a pH range similar to that observed on silica and other oxides, at comparable metal/site ratios. A second mechanism involving cation exchange on the negatively charge basal plane, which does not involve proton exchange in our experimental conditions, has been invoked to explain the low pH behavior. Consistent with this cation exchange mechanism, adsorption at low pH is strongly ionic strength dependant. A quantitative model which involves both mechanisms is presented and tested against both cation and proton adsorption data.

Speciation study in the sulfamethoxazole–copper–pH–soil system: Implications for retention prediction

Sulfamethoxazole (SMX) is a persistent sulfonamide antibiotic drug used in the veterinary and human medical sectors and is widely detected in natural waters. To better understand the reactive transport of this antibiotic in soil, the speciation of the SMX-Cu(II)-H(+) system in solution and the combined sorption of these components in a natural vineyard soil were investigated by acid-base titrimetry and infrared spectroscopy. Cu(II) is considered to represent a strongly complexing trace element cation (such as Cd(2+), Zn(2+), Pb(2+), Ni(2+), etc.) in comparison to more prevalent but more weakly binding cations (such as Ca(2+) and Mg(2+)). Titrimetric studies showed that, relative to other antibiotics, such as tetracycline, SMX is a weak copper chelating agent and a weak soil sorbent at the soil pH (pH6). However, the sorption of SMX in soil increases strongly (by a factor of 6) in the presence of copper. This finding strongly supports the hypothetical formation of ternary SMX-Cu-soil complexes, especially considering that copper is dominantly sorbed in a state at pH6. The data were successfully modelled with PhreeqC assuming the existence of binary and ternary surface complexes in equilibrium with aqueous Cu, SMX and Cu-SMX complexes. It is thought that other strongly complexing cations present on the surface of reactive organic and mineral soil phases, such as Cd(II), Ni(II), Zn(II), Pb(II), Fe(II/III), Mn(II/IV) and Al(III), affect the solid/solution partitioning of SMX. This study thus suggests that surface-adsorbed cations significantly increase the sorption of SMX.

Xanthan Exopolysaccharide: Cu(2+) Complexes Affected from the pH-Dependent Conformational State; Implications for Environmentally Relevant Biopolymers.

The conformational impact of environmental biopolymers on metal sorption was studied through Cu sorption on xanthan. The apparent Cu(2+) complexation constant (logK; Cu(2+) + L(-) ↔ CuL(+)) decreased from 2.9 ± 0.1 at pH 3.5 to 2.5 ± 0.1 at pH 5.5 (ionic strength I = 0.1). This behavior is in apparent contradiction with basic thermodynamics, as usually the higher the pH the more cations bind. Our combined titration, circular dichroism and dynamic light scattering study indicated that the change observed in Cu bond strength relates to a conformational change of the structure of xanthan, which generates more chelating sites at pH 3.5 than at pH 5.5. This hypothesis was validated by the fact that the Cu sorption constants on xanthan were always higher than those measured on a mixture of pyruvic and glucuronic acids (logK = 2.2), which are the two constitutive ligands present in the xanthan monomer. This study shows the role of the structural conformation of natural biopolymers in metal bond strength. This finding may help to better predict the fate of Cu and other metals in acidic environmental settings such as aquatic media affected by acid mine drainage, as well as peats and acidic soils, and to better define optimal conditions for bioremediation processes.