While FFF recovered essentially all of the iron in sodium silicate suspensions, complexation by polyphosphate species may have resulted in losses of iron to waste in separations of hexametaphosphate suspensions. And since recovery of 31 P from a sodium hexametaphosphate standard was 111%, the partial 31 P recovery from the iron oxide suspension suggests that hexametaphosphate hydrolyzed to orthophosphate or depolymerized to trimetaphosphate, 41 species that readily pass through the FFF membrane to waste (Figure ). The partial recovery of iron may be due to chelation by trimetaphosphate in particular: the resulting complex would also escape retention by the FFF membrane. In drinking water systems, hexametaphosphate reversion is expected to be significant, especially in older pipe networks. 43