Effects of natural organic matter on aggregation kinetics of boron nanoparticles in monovalent and divalent electrolytes

Abstract

Nano boron is a promising new propellant being considered for military and civilian applications; however, the impact of its release on the environment is largely not known. The early stage aggregation kinetics of boron nanoparticles was investigated in the presence of two kinds of natural organic matter—Suwannee River humic acid (SRHA) and sodium alginate—by dynamic light scattering and transmission electron microscopy (TEM). The addition of SRHA caused the boron nanoparticles to stabilize and resulted in (1) decreased attachment efficiency for the reaction-controlled regime and (2) an increase in the critical coagulation concentration, in CaCl2 and MgCl2 solutions. The increase in the electrostatic repulsion is suggested as a main cause of the induced stabilization as indicated by the electrophoretic mobility measurements. Similar behavior was observed in the presence of alginate and MgCl2. However, the attachment efficiency kept increasing in the presence of CaCl2 and alginate with the increase in the electrolyte concentration and was greater than unity at >4 mM CaCl2. The destabilization was attributed to bridging of the nanoparticles by the alginate–Ca2+ system. Results from this study suggest that various NOM and electrolytes play significant and differing roles in the aggregation of boron nanoparticles in natural aquatic environments.