Aggregation and deposition behaviors of dissolved black carbon with coexisting heavy metals in aquatic solution

Abstract

The aggregation and deposition behaviors of colloidal or dissolved black carbon (DBC) with coexisting heavy metals are important to understand the fate of DBC and the associated contaminants in environmental and engineering systems. Time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation (QCM-D) techniques were employed to study the effect of cation concentration and properties on the aggregation and deposition behaviors of DBC with divalent heavy metals (Pb²⁺, Cu²⁺, Cd²⁺ and Zn²⁺). The aggregation experiments suggested that divalent heavy metals significantly destabilized DBC due to strong complexation and cation–π interaction, and the destabilization ability of these metal cations was related to their electronegativity (critical coagulation concentration (CCC): Pb²⁺ < Cu²⁺ < Cd²⁺ < Zn²⁺). The deposition experiments indicated that the presence of heavy metals favored the deposition of DBC on a silica surface, which was characterized by a relatively rigid layer in the initial rapid phase and a relatively swollen layer in the subsequent slow phase. Additionally, the initial deposition rate first increased as the cation concentration increased, while a further increase in cation concentration was unfavorable to the deposition process because of the diffusion limitation. Similarly, the deposited film was more rigid at first and then became more swollen with the increase in cation concentration. Notably, the electronegativity of the metal cation dominated its retention ability (critical deposition concentration (CDC): Pb²⁺ < Cu²⁺ < Cd²⁺ < Zn²⁺) and surface roughness, while the hydration shell thickness had a greater influence on the viscoelastic properties of the deposited film (rigidity of the adlayer: Pb²⁺ > Cd²⁺ > Cu²⁺ > Zn²⁺). Overall, these results provide new insights into the aggregation and deposition behaviors of DBC with coexisting heavy metals in aquatic solution, which are helpful to understand the fate of DBC and the associated contaminants.