Influence of humic acid on the stability and bacterial toxicity of zinc oxide nanoparticles in water

• Humic acid stabilizes ZnO NPs.
• Adsorption varies with pH and salt concentration.
• Presence of HA increased particle toxicity.
• HA decreased the photocatalytic activity of ZnO.
• HA increased the photostability of ZnO.

The present study investigated the stability of zinc oxide nanoparticles (ZnO NPs) by the adsorption of humic acid (HA) and the mechanism of adsorption. The effect of humic acid on NP toxicity was determined by Escherichia coli (ATCC 13534), E. coli (ATCC 25922), and Pseudomonas putida (MTCC 4910). The nanoparticles showed low zeta potential and were least stable in the absence of HA. However, the negative surface charge of the particles increased in the presence of HA (0–50 mg/L) that reduced the propensity of nanoparticles to aggregate in water. A decrease in absorbance of ZnO NPs at 375 nm (plasmon peak) was noted in the presence of HA by UV–visible spectrophotometric analysis. A blue shift towards 370 nm was noted when the concentration of HA was above 20 mg/L. The HA adsorbed ZnO NPs showed higher zeta potential (>− 30 mV) and were highly stable. HA reduced the photocatalytic activity of ZnO and at the same time increased the photostability of ZnO.