Humic Acids

The sulfidation process was monitored by measuring time-resolved depletion of soluble sulfide using a sulfide ion selective electrode (sulfide-ISE) following nanoparticle removal by centrifugal ultrafiltration. In a typical experiment, 4.9 mL DI water was added into a 15 mL plastic tube containing desired amount of AgNP-30 nm powder (0.324 – 5.393 mg), followed with sonication in a bath sonicator for 10 min to disperse aggregates, then 0.1 mL of 50 mM Na₂S solution was added to initiate the sulfidation reaction at a starting Na₂S concentration of 1 mM. The 1 mM Na₂S solution in DI water has an initial pH of 11.1 (Orion 8165BNWP pH electrode, Thermo Scientific), and the predominant sulfide species is HS⁻ (>99.9%) as calculated by visual MINTEQ (version 3.0). The reaction mixture was rotated at 20 rpm for up to 48 hrs, after which the sulfide-containing solution was separated from the solids using centrifugal ultrafiltration (Amicon Ultra-4 3K, cellulose membrane with 1–2 nm pore size, Millipore), at an relative centrifugal force of 4000 g for 25 min (Allegra X-15R, Beckman Coulter, Inc.). Then, a sulfide antioxidant buffer (SAOB, Orion 941609 Thermo Scientific) was added in equal volume to the collected sulfide-containing filtrate to prevent sulfide oxidation and volatilization during analysis. Soluble sulfide concentrations were measured with a sulfide-ISE (9616BNWP silver/sulfide combination electrode, Thermo Scientific) at room temperature, based on linear calibration curves constructed daily from fresh Na₂S standards in SAOB with detection limit of 6.25×10⁻³ mM. Any interference of silver ion with the sulfide-ISE was prevented by solution pretreatment with EDTA as the manufacturer recommends. This was confirmed by a special control experiment in which sulfide was measured in the presence of excess AgNO₃ (Figure S2).
The sulfidation stoichiometry was determined by measuring soluble sulfide depletion over 5-hr for a range of AgNP-30 nm concentrations (0.2 – 10 mM on Ag-atom basis, equivalent to 21.6 – 1079mg/L) and selected sulfidation experiments were carried out under Ar purge to investigate the role of oxygen. Two silver samples with different particle size of 5 nm and 1~3 μm were also used to study surface area dependence. Selected experiments were conducted in media including humic acids (Suwannee River humic acid II standards, International Humic Substances Society) of 20 mg/L, at lower sulfide (0.1 and 0.25 mM), and at lower pH (50 mM pH7 phosphate buffer). All sulfidation experiments were conducted at room temperature and protected from room light.
The role of oxygen was investigated by carrying out sulfidation experiments under Ar purge (99.9% purity) using macroscopic silver foils (4 mm×4 mm×0.127 mm) followed by solid product characterization. In addition, dissolved oxygen (DO) levels were monitored in-situ during batch silver sulfidation experiments in a closed amber glass bottle under magnetic stirring using a DO probe (Orion 083010MD, Thermo Scientific) at 60 sec sampling frequency.