Phosphate inhibition of schwertmannite nucleation

The iron(III) phosphate mineral schwertmannite has shown potential for the removal of oxyanions such as phosphate and arsenate through their adsorption [1,2] and incorporation into the schwertmannite structure [3]. Here we have used dynamic light scattering (DLS) to explore the inhibitory effect of mM concentrations of phosphate on the nucleation dynamics of schwertmannite at different temperatures. Our experiments show that potential prenucleation clusters are present in the solutions prior to nucleation of schwertmannite. In the presence of phosphate these become increasingly stabilized and in long-term experiments are observed to grow slowly. Nucleation of schwertmannite was determined as a change in the system behavior, where a sudden increase in the size of the scattering particles was observed. After nucleation, particle growth was characterized by a much faster rate until the particles began to settle from the solution. Both the growth rate after nucleation and the induction time increased with increasing phosphate concentration, but, were even more retarded by decreasing temperature. Phosphate likely alters the system dynamics through direct interactions with the iron(III) as evident even at the ion pair level in UV-Vis spectroscopy measurements and the incorporation of phosphate into the schwertmannite structure based on X-ray diffraction analysis.

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