Orange peel waste valorization for nanomaterials: from colorimetric sensing applications to environmental remediation

The orange fruit is one of the most common and widespread fruits cultivated worldwide: the global annual production was 76.4 million tons in 2022, with the highest producers being Brazil, India and China [1]. In EU, the main producers are Spain, Greece and Italy; considering that the 70% of the harvested production ends up in the juice industry, and that around 50% of the orange mass results in waste during the juice extraction, approximately 2.5 Mtons of orange waste are annually produced in EU [1]. The orange peel contains many organic water-soluble compounds with reducing ability (organic acids, flavonoids, phenolic compounds), which can be easily extracted in water. In this study, a water extract of citrus sinensis fruit peel (OPE) was used as green reducing agent for the facile and cost-effective synthesis and preparation of silver nanoparticles (AgNPs@OPE), used as colorimetric sensor, and silver nanoparticles based-reduced graphene oxide (OPE-rGO@AgNPs) used as photocatalyst material. The optimized protocols permitted the valorisation orange peel waste, and lowers the environmental impact of the process, with respect to the conventional synthetic procedure. The nanoparticles and the photocatalyst were deeply characterized by UV–Vis, FT-IR, Raman and SEM analysis. AgNPs were used as colorimetric sensors for different metal ions in aqueous solution. The colorimetric assay showed that AgNPs@OPE were able to detect Pb2+ and Cd2+, as demonstrated by the splits of surface plasmon resonance (SPR) and change in colour, due to the aggregation process. For the quantification of each of the two target cations, a calibration was performed by using the univariate linear regression, plotting the absorbance ratio between the main SPR band and the second band relative to the aggregate formation [2]. Then a multivariate approach was followed to perform both Cd2+ and Pb2+ quantification and try to discriminate the two cations in water. The photocatalyst material (OPE-rGO@AgNPs) showed prominent results for the photodegradation of Methylene Bue (MB), as target pollutants, under Vis-light, demonstrating an increasing of photodegradation efficiency as function of the Ag loading on the materials, with an optimum at 20% wt. of Ag; this photocatalyst showed a 95% of MB degradation at 10 ppm. The photodegradation results correlated with the calculated band gap values of the different tested materials, showing a progressive decrease in band gap for the AgNPs- modified material, with 2.90 eV for the OPE- rGO@AgNPs that exhibited the best efficiency as photocatalyst.