The transformation of α-(Al, Fe)OOH in natural fire: Effect of Al substitution amount on fixation of phosphate

Abstract The transformation of Al-substituted goethite in oxidative and reductive atmosphere, which models natural fire, was investigated in detail. Various characterization results indicated that Al-substituted goethite transformed into hematite in an oxidative atmosphere and magnetite followed by zero-valent iron in a reductive atmosphere. Interestingly, the substitution of Al for Fe not only changed the crystal morphology affecting crystal surface reactivity, but also restrained the transformation of goethite into hematite, magnetite and zero-valent iron. In addition, the Al in the goethite was embedded into the crystal structure of thermally formed hematite and magnetite, considerably influencing their surface reactivity. The macroscopic adsorption results indicated that the substitution of Al for Fe increased the adsorption capacity of goethite and the corresponding derivatives except zero-valent iron. For the same Al amount, the adsorption capacity followed an order of goethite > hematite > magnetite, implying a loss of phosphate in goethite-rich soil after experiencing natural fire. However, with increasing in temperature in the reductive atmosphere, Al-magnetite transformed into a mixture of Al-magnetite and zero-valent iron which displayed excellent phosphate adsorption capacities increasing to 1.21–5.96 mg/g. The phosphate adsorption behaviors to thermally formed products were fitted well by surface complexation modeling with five complexation sites, which were more obvious for goethite than for hematite and magnetite. These findings presented in this study represent significant progress toward an understanding of the migration, enrichment and transformation of phosphate in Al-substituted goethite-rich soil in cases experiencing natural fire.