We constrain the iron abundance in a sample of 33 low-ionization Galactic planetary nebulae ( PNe ) using [ Fe iii ] lines and correcting for the contribution of higher ionization states with ionization correction factors that take into account uncertainties in the atomic data . We find very low iron abundances in all the objects , suggesting that more than 90 % of their iron atoms are condensed onto dust grains . This number is based on the solar iron abundance and implies a lower limit on the dust-to-gas mass ratio , solely due to iron , of M _ { dust } / M _ { gas } \geq 1.3 \times 10 ^ { -3 } for our sample . The depletion factors of different PNe cover about two orders of magnitude , probably reflecting differences in the formation , growth , or destruction of their dust grains . However , we do not find any systematic difference between the gaseous iron abundances calculated for C-rich and O-rich PNe , suggesting similar iron depletion efficiencies in both environments . The iron abundances of our sample PNe are similar to those derived following the same procedure for a group of 10 Galactic H ii regions . These high depletion factors argue for high depletion efficiencies of refractory elements onto dust grains both in molecular clouds and asymptotic giant brach stars , and low dust destruction efficiencies both in interstellar and circumstellar ionized gas .