Size, concentration, coating, and exposure time effects of silver nanoparticles on the activities of selected soil enzymes

Abstract

Silver nanoparticles (AgNPs) have a wide variety of applications in consumer products and can directly or indirectly leak into environmental compartments such as soil. The environmental risks associated with AgNPs must be assessed to assist in the development of regulatory guidelines. Because they are linked to numerous soil ecosystem functions, soil enzyme activities are very sensitive to any changes in soil management caused by environmental or human factors. Therefore, changes in soil enzymatic activities upon exposure to AgNPs can serve as early indicators of any adverse effect that these particles might have on soil quality. The objective of this study was to investigate the effect of the concentration of AgNPs (1, 10, and 100 mg kg−1 soil), size of particles (30, 80, and 200 nm), nanoparticle coatings (citrate, lipoic acid, and polyvinylpyrrolidone), and contact time between nanoparticles and soil (3 h, 3 d, and 30 d) on two soil enzymes, phenol oxidase and arylamidase, in a silt loam soil. Soil samples were treated with silver ions with similar treatments, and enzyme assays were performed to understand how the enzyme activities differed under the influence of ionic silver and nano-silver. The results revealed that the effect of AgNPs and silver ions on the soil enzymatic activities depended on the Ag dose and length of exposure more than on the particle size and coating. Soil arylamidase and phenol oxidase activities were generally inhibited in the presence of a high concentration of AgNPs and AgNO3 (100 mg kg−1 soil), though in low Ag concentrations, there was no significant inhibition. The levels of inhibition varied notably in relation to the enzyme type, wherein the arylamidase activity was affected severely compared to phenol oxidase. The effect of silver ions dissolved from AgNPs was not significant, indicating that the adverse effects on enzyme functioning was caused by the AgNPs themselves.