Natural organic matter inhibits aggregation of few-layered black phosphorus in mono- and divalent electrolyte solutions

Extensive synthesis and applications of few-layered black phosphorus (BPs) are accompanied by increasing concern over its stability and potential risk. However, the colloidal stability of BPs under environmentally relevant conditions remains unclear. Hence, we investigated the influences of two representative electrolytes (i.e., NaCl and CaCl2) and natural organic matter (NOM) on the aggregation behavior of BPs. Ca2+ ions exhibited a stronger destabilization effect on BPs than Na+ due to their stronger surface charge screening as well as intersheet bridging by the complexes of Ca2+ ions and oxidized phosphorus (POx) species on the BPs surface. Apart from the Ca2+-induced enhanced aggregation in the presence of high concentration of Ca2+, the aggregation behavior of BPs in the two electrolytes at different concentrations and their ratios of critical coagulation concentrations (CCCs) generally followed classical colloidal theory such as the Schulze-Hardy rule. Moreover, in the presence of 10 mg C/L NOM the CCC values of BPs in NaCl and CaCl2 solutions were both three times higher than that obtained without NOM, and the aggregation kinetics of BPs in these electrolytes containing NOM were qualitatively consistent with extended DLVO theory. Specifically, NOM significantly improved the stabilization of BPs in CaCl2 solutions via steric repulsion and isolation of Ca2+ ions from interaction with POx species on the surface of BPs. This stabilization mechanism derived from "NOM corona" structures was elucidated by a wide spectrum of characterization and quantification techniques. These findings provide new insights into evaluating the stability and fate of this nanomaterial in natural aquatic environments.