Just as a follow up, the iron in GFO (Fe+++) becomes more and more soluble as the pH rises from pH 8 to higher values. 1 M NaOH is about pH 14. At that pH, the amount of iron that dissolves (as Fe(OH)4- ) is just under 0.1 mM (about 5 ppm). So you will be dissolving away some of the surface. That may be beneficial for this process.