Zinc Sorption Onto Meat and Bone Meal (MBM) Char: Taking a Closer Look At a Soil Amendment for in-Situ Metal Remediation.

Forest dieback as a result of mining/smelting activities for metal ores is not an uncommon problem. At Flin Flon, Manitoba this condition has been exacerbated by forest fires as well as the harvesting of trees for fuel and lumber to support the mine and smelter. Over the years, contaminants (sulfur dioxide & heavy metals) from the mine/smelter entered the environment, changing the chemistry of the soils and impeding recovery of the forest. Zinc (Zn) has been identified as the metal primarily responsible for plant toxicity and, as such, is the primary target for soil amendments aimed at reducing toxicity, encouraging plant establishment and enhancing regrowth of the forest ecosystem. Meat and bone meal (MBM) charÑmixed with willow charÑfrom a commercial source has been identified as a possible soil amendment to immobilize Zn in-situ. The immobilization potential for Zn by charred biological apatite is well recognized, but there is disagreement between whether adsorption, incorporation, or precipitation of a new metal phosphate phase are the dominant mechanisms. The goal of this project is to establish the mechanism(s) of Zn sorption for the MBM-char and identify whether application of the MBM-char to a contaminated soil will yield the same Zn species. Single component sorption experiments were conducted to determine the extent of Zn sorption as a function of: reaction time (5-min to 48-h), pH (4-9), and Zn concentration (10-5 to 10-2 M). Zn speciation was assessed on select sorption samples using synchrotron-based Zn K-edge XANES and EXAFS at the Canadian Light Source HXMA beamline. The Ca-Phosphate mineral in the MBM char dissolved at pHs below 6.0 and resulted in Zn immobilization via precipitation of a Zn-phosphate. A pH of 6.2 was used to study sorption kinetics and isotherms. The kinetics were best described with an Elovich equation, which some have suggested could indicate a changeover from one binding site to another. Sorption samples from both portions of the kinetic curve resulted in the same monodentate, inner-sphere bonding to phosphate according to the EXAFS fitting of the Zn K-edge spectra. The combination of sorption experiments and synchrotron-based EXAFS spectroscopy will provide a better understanding of how this novel remediation material interacts with Zn and how its use may be optimized in the field.