Characterisation 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, model systems of the soil–root interface, were exposed to aluminium solutions at different concentrations (25–800 μM). Three different pHs (3.50, 4.00 and 4.50) were chosen to study the influence of different aluminium species, such as [Al(H2O)6]3+, [Al(OH)(H2O)5]2+ and [Al(OH)2(H2O)4]+, on the structure of the Ca–PG membrane. The DTA profiles and FT-IR spectra showed how aluminium sorption induces structural modifications leading to a reorganisation of the chain aggregates and a weakening of the structure. Higher pH, that is, 4.00 and 4.50, and thus hydrolytic aluminium species and related higher calcium content maintain a more regular structure than at pH 3.50. At pH 3.50, both the effect of [Al(H2O)6]3+ and a major calcium release had a greater impact and thus induced a greater weakening of the structure.

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