Reciprocal effects of NOM and solution electrolyte ions on aggregation of ferrihydrite nanoparticles

The effects of natural organic matter (NOM) types and electrolyte ions are crucial to the aggregation of ferrihydrite nanoparticles (Fh NPs) in the environment. Dynamic light scattering (DLS) was employed for the aggregation kinetics of Fh NPs (10 mg/L as Fe) in the present study. The critical coagulation concentration (CCC) values of Fh NPs aggregation in NaCl were obtained in the presence of 15 mg C/L NOM as SRHA (857.4 mM) > PPHA (752.3 mM) > SRFA > (420.1 mM) > ESHA (141.0 mM) > NOM-free (125.3 mM), indicating Fh NPs aggregation was inhibited as the above order. Comparatively in CaCl₂, the CCC values were measured in ESHA (0.9 mM), PPHA (2.7 mM), SRFA (3.6 mM), SRHA (5.9 mM), NOM-free (76.6 mM), implying NPs aggregation was enhanced following the order of ESHA > PPHA > SRFA > SRHA. To investigate the dominant mechanisms, the aggregation of Fh NPs was comprehensively studied under the effects of NOM types, concentrations (0-15 mg C/L) and electrolyte ions (NaCl/CaCl₂ beyond CCC). In NaCl/CaCl₂, the low concentration of NOM (<7.5 mg C/L) could accelerate NPs aggregation mainly due to patch-charge attraction. When NOM concentration was high (> 7.5 mg C/L), the inhibition effect on NPs aggregation occurred in NaCl due to steric repulsion, whereas the enhancement effect in CaCl₂ of aggregation was dominated by the bridging effect. The results indicated that the effects of NOM types, concentration and electrolyte ions should be carefully considered for the environmental behavior of NPs.