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Pyrolysis as a technique for separating heavy metals from hyperaccumulators
Abstract
Plants capable of accumulating very high concentrations of metals are termed hyperaccumulators. Phytoremediation involves removing metals from soil or water for environmental reasons and phytomining implies extraction of metals from soils rich in metal content, using hyperaccumulators for economic gain. A technique for separating heavy metals present in hyperaccumulators will have application in phytoremediation and phytomining. The overall objective of this study was to determine the feasibility of using pyrolysis to produce metal (Ni, Zn, Cu, Co or Cr) ‘ore’ from hyperaccumulator biomass. While pyrolysis would be the concentration step, processing of the ‘ore’ in a conventional ore-processing unit to extract the metal would be the final separation step. “Synthetic hyperaccumulator biomass” (SHB) was prepared by impregnating the metal of interest (Ni, Zn, Cu, Co or Cr) into the biomass (corn stover) by using metal acetate and metal citrate salt solutions. The goal was to mimic the metal concentration and/or chemistry found in actual hyperaccumulators. The purpose for developing a technique to produce SHB was to prepare sufficient quantities of samples for use in pyrolysis experiments. Nine out of the thirteen samples of SHB that were prepared mimicked the metal concentration ranges found in the literature. The metal ‘ore’ was prepared using lab-scale and pilot-scale reactor systems. The overall material balance, distribution of the metal among the product streams, and carbon, ash and metal component balances were documented. On an N2-free basis, the mass balance closure was between 79% and 98%. More than 98.5% of the metal recovered in the product stream was present in the char or ‘ore’. The concentration of metal in the char increased 3.2–6 times compared to the feed (SHB). The chemical form of the metal in the ‘ore’ was characterized using X-ray diffraction. Ni was predominantly identified in the metallic form in the pyrolyzed Ni-SHB. Zinc was found in the oxidized form and Cu was found in metallic and oxidized forms. The form of metal in pyrolyzed Co-SHB and Cr-SHB could not be conclusively identified as metallic or oxidized in nature. Finally, the economics of SHB pyrolysis as an avenue to produce metal ‘ore’ was evaluated.
Subject Area
Chemical engineering|Environmental engineering|Agricultural engineering
Recommended Citation
Koppolu, Lakshmi, "Pyrolysis as a technique for separating heavy metals from hyperaccumulators" (2002). ETD collection for University of Nebraska-Lincoln. AAI3045523.
https://digitalcommons.unl.edu/dissertations/AAI3045523