Activation energy of the slow reaction between phosphate and goethites of different morphology

Abstract

Relatively little is known about the nature of the slow reactions between phosphate and iron oxides. For this reason, the sorptive behaviour of 15 goethites, ranging widely in specific surface area, aluminium substitution, morphology and other properties, was examined in detail. The slow sorption could be described by a modified Freundlich equation including time and activation energy (E) terms. The value of E ranged from 38 to 80 kJ mol-1, suggesting that a mechanism common to all samples, such as a simple penetration into the crystalline matrix, was unlikely. Multidomainic goethites showed higher E values than the monodomainic goethites. It is hypothesized that the wedge-shaped interdomainic grooves of the multidomainic crystals have sorption sites posing a more energetic barrier to phosphate than sites on the smooth surface of the mondomainic crystals. Sites in interparticle pores and bridging of particles by phosphate might explain why E values are relatively high, even for monodomainic crystals.