Effect of NOM on copper sulfide nanoparticle growth, stability, and oxidative dissolution

Abstract

Metal sulfide nanoparticles have recently been discovered in natural environments subject to fluctuating redox conditions such as wetlands and river floodplains, which are often very rich in natural organic matter (NOM). Strong binding of chalcophile metals like copper by NOM is expected to interfere with sulfide precipitation and is likely to affect the growth of these nanoparticles. We therefore conducted experiments on the formation of copper sulfide nanoparticles (Cu_xS NPs) in anoxic solutions with varying Cu (50, 500 μmol L⁻¹) and sulfide (100, 1000 μmol L⁻¹) concentrations in the absence and presence of Suwannee River fulvic acid (SRFA, 0, 5, 50 mg C L⁻¹). The size development of the Cu_xS nanoparticles and their stability were tracked over 4 weeks using transmission electron microscopy (TEM) and size-exclusion chromatography coupled to ICP-MS (SEC-ICP-MS). Additional dissolution experiments were performed in closed containers in the absence and presence of O₂ and fulvic acid over several months providing insights into the oxidative dissolution behavior of Cu_xS. Our results highlight the high colloidal stability of Cu_xS nanoparticles in anoxic environments irrespective of the NOM concentration. Median particle diameters ranged between a few and a few tens of nanometers with larger particles forming in more concentrated suspensions. At low Cu and S concentrations, fulvic acid restricted particle growth by up to 25% compared to SRFA-free suspensions and metal sulfide clusters even smaller than 1 nm were detected. The Cu_xS nanoparticles exhibited a remarkable chemical stability against oxidative dissolution and were only dissolved when both fulvic acid and O₂ were present.