Soil–pore water distribution of silver and gold engineered nanoparticles in undisturbed soils under unsaturated conditions

• Both Ag and Au ENPs added to soils were retained almost completely within 24 h.
• The short term solubility of Au was partly controlled by natural soil colloids.
• Ag from Ag ENPs added to soils was present in the pore water mostly as ionic Ag.
• Under aerated soil conditions the actual availability and mobility of Ag and Au ENPs in soils are low.

Release of engineered nanoparticles (ENPs) to soil is well documented but little is known on the subsequent soil–pore water distribution of ENPs once present in soil. In this study, the availability and mobility of silver (Ag) and gold (Au) ENPs added to agricultural soils were assessed in two separate pot experiments. Pore water samples collected from pots from day 1 to 45 using porous (<0.17 μm) membrane samplers suggest that both Ag and Au are retained almost completely within 24 h with less than 13% of the total added amount present in pore water on day 1. UV–Vis and TEM results showed that AuENPs in pore water were present as both homoaggregates and heteroaggregates until day 3 after which the concentration in pore water was too low to detect the presence of aggregates. A close relation between the concentration of Au and Fe in pore water suggests that the short term solubility of Au is partly controlled by natural soil colloids. Results suggest that under normal aerated soil conditions the actual availability of Ag and AuENPs is low which is relevant in view of risk assessment even though the impact of environmental conditions and soil properties on the reactivity of ENPs (and/or large ENPs aggregates) retained in the solid matrix need to be addressed further.