Reduction of artificial iron(III) phosphate complexes by hydroxylammonium chloride

The amenability of iron(III) phosphate complexes towards reduction by hydroxylammonium chloride at different pH was tested in laboratory experiments. Higher pH and higher phosphate contents of the precipitates reduced both the extent and the rate of iron reduction. The dissolution of Fe(III) phosphate complexes was generally incongruous. This can be due to redistribution of phosphate or preferential reduction. The course of iron reduction and phosphate adsorption were simulated with a model relating the reduction rate to the level of phosphate sorption to the surface as described by a Langmuir isotherm. Hampering of the reduction was more than proportional to the fraction of sorption sites occupied by phosphate. The ecological significance of the results is that in phosphate-rich freshwater sediments, the seasonal variation in depth of the oxic layer may result in incomplete reduction of the iron complexes ultimately buried by sediment accumulation. This would enhance the sediment fixation capacity and reduce the internal loading of phosphate.