Effects of silicic acid on the chemistry of monomeric and polymeric (Al13) aluminium species in solutions

Abstract

Polymeric aluminium (Al13) has been shown to be more toxic to plant root growth than monomeric aluminium (Almono) in simple solution culture studies. However, doubt has been expressed on the existence of Al13 in soil solutions due to the synthetic laboratory conditions needed for its production. To examine the formation of Al13 in solutions, comparable in cation composition to soil solutions, solutions containing Al and silicic acid (Si), but not SO4 and PO4, were hydrolysed to a base:Al ratio of 2. The amount of Al13 detected in these solutions, using both 27Al NMR and ferron, decreased exponentially with increasing Si concentration until none was detected in a solution containing 441 µM Si. On the basis of ferron analysis, other unidentified Al species (Alb2) were assumed to have formed in these solutions. The Alb2 species did not have the tetrahedral Al atom detected by 27Al NMR suggesting that Al13 is not a precursor for their formation. They were also observed to react more slowly than both Almono and Al13 with ferron and pyrocatechol violet (PCV). The slow reaction of Si to form the molybdosilicate complex suggests that the unknown soluble Alb2 species have Si incorporated into their structure.