The effects of size and surface-coating of CuO-nanoparticles on extractable Cu and enzyme activities in soil
Kaushik Adhikari, Anil Timilsina, Hao Chen
Abstract
The application of nano-enabled agricultural chemicals will introduce metal oxide nanoparticles (NPs) into agricultural soils, with copper oxide NPs as one of the most common nano-agrichemicals. Because CuO NPs dissolute to release Cu ions and they often display different environmental impacts, it is essential to conduct a comprehensive examination of the bioavailability of Cu in CuO NPs-contaminated soils and its impacts on soil enzyme activities, which are critical for soil health. This research comprehensively examined how variations in CuO NP size (10, 40, 80 nm), surface coatings (Polyvinylpyrrolidone (PVP), Polyacrylic acid (PAC)), and concentrations (100, 250, 500 mg/kg) influenced the extractable Cu in soil and their impact on selected soil enzymes (acid phosphatase and dehydrogenase). Main indicators were quantified within a 30-day incubation period. The bioactive Cu related to CuO NPs as determined via CaCl2 and diethylenetriaminepentaacetic acid extractions indicated that 10 nm CuO NPs had the highest extractable Cu across all incubation times. Besides, PVP and PAC coating had little impact on extractable Cu compared to uncoated ones. Unlike CuSO₄, extractable Cu concentrations in CuO NPs spiked soil increased over time. CuO NPs dissolution is negatively correlated with soil pH. CuO NPs after 24 h short-term exposure significantly inhibited both enzyme activities across all tested concentrations, with smaller NPs showing greater effect. However, reduced toxicity to enzyme activities was observed at 30 days. A strong negative correlation was observed between CuO dissolution and enzyme activities over 24 hours and 30 days, indicating Cu ions are the main toxicity source. This study elucidates that CuO NP size, concentration, surface coating, and exposure duration collectively impact the interactions of CuO NPs with soil enzymes, providing critical insights into the sustainable use of nanotechnology in agriculture.