Binary mixture of nanoparticles in sewage sludge: Impact on spinach growth

Present study carried out pot experiments and evaluated effects of single and binary mixture of nanoparticles (exposed via sludge as soil conditioner) on spinach plant. Exposure of Ag₂O nanoparticles (NPs) (1 and 10 mg/kg soil-sludge) did not show significant reduction in plant as compared to control. On the other hand, TiO₂ NPs (exposed as single and in binary mixture) resulted in significant increase in root length (29% and 37%) and fresh weight (60% and 48%) at highest exposure concentration. Total chlorophyll content decreased for Ag₂O and binary mixture (7% and 4%, respectively) and increased for TiO₂ (5%) at 10 mg/kg soil-sludge. The toxic interaction between Ag₂O and TiO₂ NPs was additive at both exposure concentrations. Ag₂O NPs had higher tendency of root surface adsorption than TiO₂ NPs. Metal content in spinach leaves at highest exposure concentration was Ag: 2.6 ± 0.55 mg/g plant biomass(for Ag₂O NPs) and 1.02 ± 0.32 mg/g plant biomass (for Ag₂O + TiO₂ NPs) and for Ti: 1.12 ± 0.78 (for TiO₂ NPs) mg/g plant biomass and 0.58 ± 0.41 mg/g (for Ag₂O + TiO₂ NPs). The inadvertent ingestion of NPs- contaminated spinach resulted in projected daily intake (DI) of Ag and Ti for different age-mass classes (child to adult) exceeding the oral reference dose for toxicity during oral ingestion. In conclusion, we report no acute toxicity of single and binary mixture of NPs to spinach but significant accumulation of Ag and Ti metals in spinach leaves. There are high chances that ingestion of spinach grown in such environment might lead to human health risks.