Effects of natural organic matter (NOM), metal-to-sulfide ratio and Mn2+ on cadmium sulfide nanoparticle growth and colloidal stability

Abstract

Redox-dynamic environments such as river floodplains and wetlands have been identified as sources of natural metal sulfide nanoparticles (MS NPs). However, little information is available on how their growth and colloidal stability are affected by the concentrations of metals and sulfide in solution, the presence of natural organic matter (NOM), and the possible incorporation of other metal cations such as Fe²⁺ or Mn²⁺. Here, we performed experiments on the formation of CdS nanoparticles (CdS NPs) in anoxic solutions with varying Cd (50, 100, 500 μmol L⁻¹) and sulfide (50, 100, 1000 μmol L⁻¹) concentrations in the absence and presence of Suwannee River fulvic acid (SRFA, 0, 5, 50 mg C per L). Additionally, we studied the influence of different metal-to-sulfide ratios and varying Mn²⁺ (0, 0.1, 0.5, 1 mmol L⁻¹) concentrations on CdS aggregation using dynamic light scattering (DLS), transmission electron microscopy (TEM), and electrophoretic mobility measurements. The incorporation of Mn into the crystal lattice of CdS over 8 weeks was investigated with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Our results show that after 24 hours, small CdS primary particles with median diameters of a few nanometers (_TEM = 2–14 nm) formed large aggregates (_TEM = 167 nm) and that increasing SRFA concentrations progressively constrained the size of these aggregates (down to 19 nm) irrespective of the initial reactant stoichiometry. When NOM was absent or at low concentration, higher metal-to-sulfide ratios (≥1) and Mn²⁺ concentrations (≥0.5 mmol L⁻¹) led to reduced colloidal stability of the suspensions. We found that in suspensions containing Mn²⁺, 10–30% of the Cd atoms in the crystal lattice were substituted by Mn during the formation of CdS, which was prevented by NOM.