Zero-valent iron nanoparticles (nZVI) are highly effective in removing numerous contaminants from water, as they combine the reducing action of metallic iron with adsorption and coprecipitation mechanisms. The traditional synthesis method for nZVI involves the reduction of Fe(II) and Fe(III) salts with sodium borohydride. Given the need to optimize the synthesis protocol for scalability, Life Cycle Assessment (LCA) emerges as a useful tool with the potential to identify critical stages, enabling the minimization of environmental impacts from both production and remediation processes. In this study, two synthesis protocols based on the borohydride reduction method, previously tested in the laboratory, were compared using LCA.
For the LCA application, a functional unit of 1 g of synthesized nZVI was defined, adopting a cradle-to-gate approach. The Life Cycle Inventory was constructed for two synthesis protocols: S2015 and S2023, considering three stages: reaction, filtration, and drying.
In the S2015 protocol, the reaction was conducted in a glass reactor with stirring under anoxic conditions maintained by N2 bubbling for 60 minutes. FeCl3·6H2O and NaBH4 were mixed in 40% ethanol with a BH4⁻:Fe molar ratio of 3.5. During the filtration stage, the resulting suspension was vacuum filtered and washed with diluted ethanol. The liquid effluents were disposed of as hazardous waste for incineration, while the filters were treated as solid waste for landfill disposal. In the drying stage, the nZVI were resuspended in absolute ethanol, which was subsequently removed using a rotary vacuum evaporator, recovering the ethanol through condensation.
For the S2023 protocol, the reaction time was reduced to 30 minutes, and 40% ethanol was replaced with absolute ethanol. After filtration, the nZVI were dried in a vacuum desiccator using silica gel to remove residual moisture. The dried nZVI were stored in a nitrogen-purged container and subsequently characterized.
Both protocols were successful in producing nZVI. The S2015 protocol yielded nZVI with 58% Fe(0) and particle sizes ranging from 10 to 60 nm, whereas the S2023 protocol produced nZVI with 87% Fe(0) and sizes between 50 and 103 nm. Additionally, S2023 demonstrated better reaction efficiency, yielding a greater mass of nZVI per synthesis: 0.380 g compared to 0.220 g for S2015.
The next step was to conduct the impact assessment using SimaPro® v9.5 software, selecting the IMPACT World+ (Midpoint V1.03) method and the Ecoinvent 3.9.1 database. Eighteen impact categories were evaluated. For S2015, the drying stage was the main contributor to 10 categories, while liquid effluents were the major contributors in 6 categories. Combined, drying and liquid effluents accounted for over 50% of the impact value in each category (except for freshwater eutrophication). The reaction stage contributed significantly (24% to 38%) to six categories.
For S2023, the reaction, liquid effluents, and filtration stages were the primary contributors to 8, 6, and 6 categories, respectively. In absolute terms, the S2023 protocol reduced impacts by more than 50% in 8 evaluated categories compared to S2015. Key hotspots included the use of ethanol, liquid effluents generated, drying energy in S2015, and filtration energy in S2023.
In conclusion, the S2023 protocol demonstrated improved reaction conditions, yielding a higher mass of nZVI with a greater Fe(0) content. Additionally, it achieved significant reductions in most impact categories, making it a more environmentally sustainable alternative compared to S2015.