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Hydroxyl reactivity of synthetic goethite, gibbsite, and two series of silica-alumina gels having a wide range of Si/Al composition (one series having characteristics of allophane) were studied by phosphate adsorption and “ammonium oxalate reactivity” (Ro), where Ro = amount of hydroxyls released in the reaction of amonium oxalate with hydroxylated surfaces at pH 6.3 over 25 min. Ro ranged from 14 mmole/100 g for goethite to 61 and 106 mmole/100 g for gibbsite prepared at pH 4.5 and 6.5, respectively. In the silica-alumina gels, Ro ranged from 90 to 442 mmole/100 g. In both series of gels, Ro reached a maximum at a SiO2/Al2O3 ratio near 1. At this composition the Al and Si that dissolved during the reaction had the highest Al/Si ratio.

Ro was also determined on samples heated to 105°C. In all but one sample of the allophanic series, Ro decreased on heating, the largest decreases being observed for the more reactive samples. In the most Si-rich member of the series (SiO2/Al2O3 = 1.5), however, Ro increased on heating by 24 mmole/100 g. This result may explain the Ro values observed for Andisols containing Si-rich allophanes. Here, Ro increased on heating to 105°C. Active OH groups on goethite, as determined by Ro, were found to be in agreement with those determined by phosphate adsorption. The maximum phosphate adsorption for the allophanic gels was displayed by samples having the highest Ro values.

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