Driving sustainability at early-stage innovation in production of zinc oxide nanoparticles

Abstract

Despite its industrial relevance and the methods that have been described for its synthesis, little is known about the performance of the production processes of ZnO nanoparticles (ZnO NPs), either pure or doped, from the sustainability perspective. The Safe-and-Sustainable-by-Design (SSbD) framework brings to this context an excellent opportunity to 1) evaluate the impacts of chemical processes from the safety and sustainability perspectives, and 2) design and test safety and sustainability strategies to study and optimise these key aspects in early innovation stages. This work aims at assessing the production of ZnO NPs using this approach, testing the sustainability of the materials, designed and produced by Phornano, an Austrian SME, under this scheme. Three scenarios were analysed: the original process (BS) and two alternatives resulting from the application of SSbD strategies to the former (S1 and S2). BS is a linear process in which Zn(NO₃)₂·6H₂O, whey, water and a dopant (a Mn salt) are used as starting materials. However, obtention of the desired product entails the release of toxic fumes (SO_x and NO_x) to the atmosphere. S1 and its scale-up version, S2, are circular processes in which SO_x emissions are avoided, due to the replacement of whey by a non-aminated starch, and NO_x are transformed into HNO₃, which reacts with Zn powder to produce Zn(NO₃)₂·6H₂O; in this way, no harmful substances are freed and the zinc salt employed as a raw material in BS is generated during the manufacture of ZnO NPs. Four well-known evaluation tools were employed to achieve a holistic sustainability perspective: Environmental Life Cycle Assessment (LCA), Material Flow Cost Accounting (MFCA), Social Life Cycle Assessment (S-LCA) and Multi-Criteria Decision Analysis (MCDA), according to the standardised methodologies or the most broadly spread ones; the study was complemented with an uncertainty analysis. The results for the production of 1 kg of ZnO NPs show that the after-SSbD scenarios are remarkably more sustainable than BS: the environmental evaluation reveals that S2 outperforms BS for 10 environmental indicators, allowing a reduction of 67 % in terms of total aggregated impact (from 13.7 to 4.4 mPt); from the economic viewpoint, synthesis of ZnO NPs through S2 is around four times cheaper than that achieved via BS (512 vs 2206 €); finally, the social footprint is reduced from 159 mPt in the original process to 21 mPt in S2. MCDA of BS, S1 and S2 considering the three assessments performed confirms that S2 is, with almost 100 % probability, the best-performing alternative from the sustainability perspective, followed by S1. Overall, this work, the most complete in this field to date, contributes to the sustainable synthesis of ZnO NPs and to the methodological advance of the SSbD framework through the revision of its limitations and opportunities.