The Hanford Site near Richland, WA, has 177 high-level nuclear waste tanks containing 56 million gallons of waste derived from nuclear fuel reprocessing. The site is removing waste from old single-shell tanks, but some gibbsite [gamma-Al(OH)(3)] has been left behind by the current retrieval technology and would remain in the tanks when grouted on site, if not removed first. This study develops a new process to remove this gibbsite by converting it into water-soluble NaAl(OH)(4)center dot 1.5H(2)O in highly concentrated NaOH(aq). Gibbsite was incubated in NaOH(aq) solutions of 8, 11, or 19.4 M concentration at 25 or 50 degrees C for up to 312 h. Inductively coupled plasma atomic emission spectroscopy and polarized light microscopy were used to monitor the reaction. Gibbsite rapidly converted into NaAl(OH)(4)center dot 1.5H(2)O at all NaOH concentrations and temperatures for nonradioactive and real waste gibbsite samples. NaAl(OH)(4)center dot 1.5H(2)O initially crystallized with a needle-shaped morphology, but subsequently transformed into a rectangular plate. These results demonstrate that gibbsite can be readily converted into NaAl(OH)(4)center dot 1.5H(2)O in concentrated NaOH at ambient temperatures, suggesting that this is a promising technology for removing gibbsite from high-level waste tank heels.