Quasi-quantitative determination of elemental relationships and surface properties in aqueous aluminium-silicon systems

•Novel analytical combination has been used for the study of Al and Si interaction.•Effect of dissolved Si on the formation of charged Al species was quantified.•Mechanism for the reaction of silicic acid and Al(OH)3 was proposed.

Interactions of aluminium (1 mM) and silicon (0.1–2 mM) in aqueous solutions were studied at a low ionic strength medium representing fresh water and water treatment process environments, attempting to quantify elemental relationships of the formed precipitates as well as their surface charge properties with a novel combination of analytical techniques. FESEM analyses showed that the Al:Si ratio in the obtained precipitates was clearly affected by the Al:Si ratio in the initial aqueous solution. According to XRD, the precipitates originated from the 1 mM Al and the 1 mM Al + 0.1 mM Si solutions were gibbsite, while for the initial Al:Si ratios equalling to 1:0.5 or higher, only amorphous material was precipitated. Zeta potential decreased as a function of the initial silicon content, and reached negative values at pH 6.2 with the initial concentrations of 1 mM Al + 2 mM Si. In contrast, for the total surface charge, a maximum value was found between pH 4.5 and 5, and it clearly decreased with the amount of silicon present. To conclude, the dissolved silicon had a significant effect on the formation of charged aluminium species. This may have considerable environmental and health effects. A reaction mechanism for the formation of hydroxyaluminosilicates from silicic acid and aluminium hydroxide was proposed.

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