Characterization of Ca- and Al-pectate gels by thermal analysis and FT-IR spectroscopy

Thermal analysis (TG-DTA) and FT-IR spectroscopy have been performed on calcium-pectate membranes to investigate their structure and the consequent variation caused by aluminium sorption. Calcium-polygalacturonate (Ca-PG) membranes, system models of the soil-root interface, were exposed to aluminium solutions at different concentrations (25-800 mM). Three different pHs (3.50, 4.00 and 4.50) were chosen to study also the influence of different aluminium species such as Al3+, Al(OH)2+ and Al(OH)2+ on the structure of the Ca-PG membrane. The DTA profiles and the FT-IR spectra showed how the aluminium sorption induces structural modifications leading to a reorganisation of the chain aggregates and a weakening of the structure. Higher pHs, i.e. 4.00 and 4.50, and thus the presence hydrolytic aluminium species and the related higher calcium content would maintain a more regular structure than at pH 3.50. At pH 3.50, both the effect of Al3+ and a major calcium release induces a greater weakening of the structure causing an earlier collapse of the membrane. This suggests that Al3+ could be more toxic to plants than the hydroxyl-Al species.