Case study: taking zinc speciation into account in terrestrial ecotoxicity considerably impacts life cycle assessment results

Abstract

Metals are a major concern in life cycle assessment (LCA): they dominate the ecotoxicological impacts due to the use of models that do not consider metal speciation. The objective of this study was to measure the influence of zinc (Zn) speciation on LCA results and evaluate the importance of including it in LCA. A case study in which ecosystem quality impacts are dominated by zinc (Zn) when using the current characterization factor (CF) for Zn was performed on the market for electricity, low voltage CA-QC (Canada-Quebec) ecoinvent data using the IMPACT 2002 + methodology. Different CFs for Zn were tested: IMPACT 2002 + CF and USEtox-derived terrestrial CF without speciation, generic default CF values for the world, and minimum and maximum regionalized CF values including speciation based on either soluble or true solution Zn. For each scenario, the following outcomes were determined and compared with those obtained using the other CFs for Zn: the contribution of Zn emitted to soil to the terrestrial ecotoxicity impact category and ecosystem quality damage category scores, the rank of Zn emitted to soil as a contributor to the terrestrial ecotoxicity impact category, and the total terrestrial ecotoxicity impact score. To our knowledge, this is the first study that includes zinc speciation in soil in a real LCA case study. In this case study, when including speciation, the contribution of Zn emitted to soil to the terrestrial ecotoxicity impact score falls from 26% to a maximum of 1.27%; the impact scores of Zn emitted to soil drop by 1.44–14.37 orders of magnitude; the total terrestrial ecotoxicity and ecosystem quality impact scores decrease by approximately 25% and 21%, respectively; and among the major contributors to terrestrial ecotoxicity, Zn falls from the 2nd to between the 9th and 255th position. Considering that the contribution of all metals emitted to soil represents between 56.2% and 75.7% of the total terrestrial ecotoxicity impact score for this case study, the results highlight the need to include metal speciation in LCA and the considerable influence it could have on ecotoxicological impacts.