Mechanistic Study of Iron Sequestration by Phosphates

Abstract

Iron sequestration by phosphate was examined from the perspective of mechanisms, water chemistry impacts, and inherent limitations. Phosphates slowed Fe²⁺ oxidation above about pH 7–8, but a combination of ferric complexation and colloid stabilization caused iron to remain invisible. Orthophosphate was a weak sequestrant, but at relatively low pH and hardness, it could be effective and is not thought to worsen corrosion control. Increased phosphate chain length, phosphate concentration, and silica concentration caused more effective sequestration, whereas calcium, magnesium, and increased pH could make it ineffective. When polyphosphate was dosed, the percentage of iron less than 10K apparent size decreased linearly by about 10% for every 100 mg/L increase in CaCO₃. Furthermore, up to 4× more tripolyphosphate was needed to effectively sequester iron at pH 9 versus pH 7. Contrary to some published guidelines, iron at concentrations above 1 mg/L could sometimes be sequestered effectively with exponentially increasing doses of polyphosphate, but at some point, higher chemical costs or precipitation (e.g., calcium phosphate or iron phosphate) became limiting.

The mechanisms, water chemistry effects, and limitations of iron sequestration are systematically investigated.