FISHING FOR LEAD: METAL REMEDIATION THROUGH USE OF FISH BONE

The high adsorption efficiency and reaction rates of nanocrystalline metal oxide phases, which make them so effective for remediation purposes, derive from an abundance of surface sites, small crystallite size that ensures high surface area:volume ratio, and negative surface charge over an important pH range. Nanocrystalline biological apatite, such as that in bone, also is effective and efficient in removing heavy metals from groundwater and soil. Apatite II (patent #6217775), a thoroughly cleaned form of various types of fish bones from non-oily fish, such as pollock, has been used successfully to remediate lead at firing ranges and acid mine sites. Our goal is to understand better the products and processes of phosphate remediation. We conducted experiments in which 1 g L^-1 aqueous suspensions of Apatite II fish bone were reacted with 10^-4 M Pb(NO₃)₂; the solutions were buffered at pH 6 and agitated on a rotary shaker. The solutions were analyzed for dissolved lead, calcium, and phosphate after 4, 24, and 72 hrs. Within 4 hours, the dissolved Pb concentration had decreased below 5x10^-8 M and a fine white precipitate had formed in suspension. The residual bone and some of the precipitate in the reaction flask were recovered after 72 hrs. Raman microprobe spectroscopy was done on individual Apatite II bone samples before and after reaction. The Raman spectra showed the original fish bones consisted of collagen and carbonated apatite, with lower carbonate concentration than is typical for mammalian bones. Reduced spectral ratios of apatite:collagen bands showed that apatite had dissolved in some portions of the bones after reaction. Samples with the greatest apatite depletion showed weak bands for the Pb phosphate phase (hydroxy)pyromorphite, in addition to those for apatite. Preliminary data suggest at least two possible mechanisms of Pb sequestration: 1) Pb substitution in the apatite nanocrystals of the bone and 2) dissolution of low-solubility bioapatite, release of dissolved phosphate, followed by nucleation on the remaining bone of highly insoluble lead phosphate. The proven higher effectiveness of fish bone than cow bone or synthetic hydroxylapatite in heavy-metal remediation demands better characterization of the mineralogical differences among these apatitic materials.