Iron oxyhydroxide nanoparticles (Fe-NPs) are natural vectors of phosphate (PO4) in the environment. Their mobility is determined by colloidal stability, which is affected by surface composition. This might be manipulated in engineered NPs for environmental or agricultural applications. Here, the stability of PO4-Fe-NPs (HFO/goethite) was determined across contrasting environmental conditions (pH, Ca concentration) and by using fulvates (FA) and polyphosphates (poly-P's) as coatings. The PO4-Fe-NPs are unstable at Ca concentrations above 0.1 mM. Addition of FA and some poly-P's significantly improved stability. Zeta potential explained colloidal stability across treatments; surface charge was calculated with surface complexation models and explained for phytic acid (PA) and hexametaphosphate (HMP) by a partial (1-4 of the 6 PO4 units) adsorption to the surface, while the remaining PO4 units stayed in solution. This study suggests that Ca concentration mainly affects the mobility of natural or engineered PO4-Fe-NPs and that HMP is a promising agent for increasing colloidal stability.