EVALUATING A MEAT AND BONE MEAL BIOCHAR AMENDMENT FOR IMMOBILIZATION OF ZINC IN A SMELTER IMPACTED SOIL

Abstract

Non-ferrous smelter emissions have prompted revegetation efforts near the border towns of Flin Flon, Manitoba and Creighton, Saskatchewan to facilitate regrowth of the surrounding boreal forest. Previously, several soil amendments were tested for plant response in soils from the smelter-impacted area and one amendment, a pyrolyzed meat and bone meal (MBM) biochar, was of particular interest because of its potential to immobilize Zn. Hydroxyapatite (HAP), with a small degree of carbonate substitution, was identified as the major component of the MBM biochar. The solubility of this Ca-mineral was pH dependent, with dissolution occurring at pH 6.3; consequently, adsorption experiments were performed at a slightly lower pH (6.1 ± 0.1). Zinc adsorption kinetics were bi-phasic and could be modeled using the Elovich equation, suggesting a diffusion limited reaction likely related to material aggregation. Adsorption also was modeled using the Langmuir equation, which indicated a moderate affinity of the biochar for Zn and an adsorption maximum of 0.650 mmol Zn g-1. Synchrotron-based Zn K-edge x-ray absorption spectroscopy (XAS) indicated an inner-sphere monodentate, tetrahedral bonding of Zn to phosphate groups in the MBM biochar. This was consistent with Zn adsorbed to a HAP standard, indicating that the same sorption mechanism was involved. The ability of MBM biochar to affect Zn speciation in soils was investigated using four soils from four locations in the smelter-impacted region around Flin Flon. A 1:10 (w/w) mixture of the MBM and soil was suspended in 200-mL deionized water (pH 6.1 ± 0.1) and equilibrated for 30 d. Whereas all the soils showed a decrease in extractable Zn following equilibration, only one exhibited a change in Zn speciation—with ca. 25% of the Zn adsorbed onto the MBM biochar. Ore-derived minerals were present in all soils and strong backscattering made identification of minor Zn species difficult. However, using microprobe-based x-ray absorption near edge structure (XANES) spectroscopy, several minor Zn species were identified; including hopeite, a ZnPO4 mineral. The presence of both hopeite and adsorbed Zn are indicative of a direct Zn-phosphate reaction. These results indicate that, under certain condition, MBM biochar can be an effective soil amendment.