Definition of agronomic circular economy metrics and use for assessment for a nanofertilizer case study

Circular economy has become global priority, and fertigation make large contribution. Modern circular methodologies base their definitions, besides on waste minimisation and recovery, on the product usage U and lifetime L. We have modified a commonly used equation for the mass circularity indicator (MCI) to permit MCI determination for agricultural cultivation. We defined U as intensity for diverse investigated parameters of plant growth and L as the bioavailability period. In this way, we compute circularity metrics for the plantgrowth performance when exposed to three nanofertilizers and one biostimulant, as compared to no-use of micronutrients (control 1), and micronutrients supplied via conventional fertilizers (control 2). We determined an MCI of 0.839 for best nanofertilizer performance (1.000 denotes full circularity), while the MCI of conventional fertilizer was 0.364. Normalised to control 1, U was determined as 1.196, 1.121 and 1.149 for manganese, copper and iron-based nanofertilizers, respectively, while U was 1.709, 1.432, 1.424 and 1.259 for manganese, copper, iron nanofertilizers and gold biostimulant when normalised to control 2, respectively. Based on the learning of the plant growth experiments, a tailored process design is proposed for the use of nanoparticles with pre-conditioning, post-processing and recycling steps. A life cycle assessment shows that the additional use of pumps for this process design does not increase energy costs, while preserving environmental advantages related to the lower water usage of the nanofertilizers. Moreover, the impact of the losses of conventional fertilisers by missing absorption of plant roots, which is presumed to be lower for the nanofertilizers.