Humic acid-induced silver nanoparticle formation under environmentally relevant conditions

Area of Science:

• Environmental Science
• Nanotechnology
• Chemistry

Background:

• Silver nanoparticles (AgNPs) are increasingly prevalent in aquatic environments.
• The natural formation pathways of AgNPs are not fully understood.
• Humic acids (HAs) are abundant natural organic matter that can interact with metal ions.

Purpose of the Study:

• To investigate the direct formation of AgNPs from silver ions (Ag+) using various humic acids (HAs).
• To determine the influence of HA type, concentration, and temperature on AgNP formation kinetics and stability.
• To assess the potential for natural AgNP formation in aquatic systems.

Main Methods:

• Utilized ultraviolet-visible spectroscopy to monitor the surface plasmon resonance (SPR) of AgNPs.
• Employed transmission electron microscopy (TEM) and atomic force microscopy (AFM) for AgNP characterization.
• Applied dynamic light scattering (DLS) to analyze particle size distribution.
• Tested HAs from Suwannee River (SUW), sedimentary HAs (SHAs), and soil HAs at different temperatures (22 °C and 90 °C).

Main Results:

• AgNP formation was observed with SUW and SHAs at 22 °C within 2-4 days.
• Soil HAs did not form detectable AgNPs at 22 °C within 25 days.
• Elevated temperature (90 °C) significantly accelerated AgNP formation to within 90 minutes for SUW and SHAs.
• TEM and AFM revealed spherical AgNPs with broad size distributions.
• DLS indicated polydisperse particle size distributions.
• HAs demonstrated colloidal stabilization of AgNPs over two months.

Conclusions:

• Humic acids can directly mediate the formation of silver nanoparticles from silver ions under environmentally relevant conditions.
• Temperature plays a critical role in accelerating AgNP formation kinetics.
• The findings suggest that naturally formed AgNPs may contribute to the AgNP pool in natural waters, not solely anthropogenic sources.