Reactivity with phosphate and phytotoxicity of hydroxyaluminosilicate ions synthesized by instantaneous mixing of aluminum chloride and sodium orthosilicate solutions

Hydroxyaluminosilicate ions are important as precursors of imogolite in synthetic systems and as less toxic aluminum-bearing species in natural aquatic systems. Despite their importance, their structures and properties have not been fully elucidated. We prepared a solution of hydroxyaluminosilicate ions by instantaneous mixing of solutions of sodium orthosilicate and aluminum chloride and carried out phosphate-binding experiments and phytotoxicity testing. To avoid possible alteration by drying, phosphate binding was measured in hydroxyaluminosilicate solutions contained in dialysis bags. The amount of phosphate bound to hydroxyaluminosilicate ions linearly increased as the equilibrium phosphate concentration increased up to 0.7 mmol L^-1. The amounts of bound phosphate were lower than those of imogolite and allophanes at relatively low phosphate concentrations; however, phosphate binding by hydroxyaluminosilicate ions surpassed binding by allophane and imogolite at equilibrium phosphate concentrations > 0.4 mmol L^-1. Phytotoxicity was evaluated by measuring hindrance to root elongation of maize seedlings. The hydroxyaluminosilicate ions hindered root elongation by approximately 10% at a total aluminum concentration of 0.5 mmol L^-1, at which concentration the hindrance by hydroxyaluminum and aluminum ions was 80-85%. These results suggest that hydroxyaluminosilicate ions have very low biological activity in natural waters.