Deposition and release of clay colloids on alumina and natural organic matter surfaces via heterogeneous pathways

Abstract

Clay colloids are ubiquitous on land and in aquatic environments. Because of their overall negative surface charge, the impact of surface charge heterogeneity has long been overlooked. In this work, we demonstrate that both the deposition and the release of montmorillonite colloids (MONTs) are influenced by surface charge heterogeneity, both on positively charged metal oxide (alumina) surfaces and negatively charged natural organic matter (NOM: humic acid and alginate) surfaces. Deposition trends show that MONTs has the highest attachment on alginate, due to the positive charge and rich hydroxyl functional groups at the edges of MONTs, as well as the coarser surface produced by the extended conformation of alginate macromolecules. At very low ionic strength (IS), MONTs can deposit onto alumina in a face-face adsorption manner due to electrostatic attraction, while no deposition occurs on NOM because of electrostatic repulsion. At high IS, the compression of the basal surface electrical double layer (EDL) exposes the edge EDL of the MONTs, leading to significant deposition on the NOM via the edge. In addition, on the MONT layer already deposited on the alumina surface, small particles of MONT also further deposit via the edge adsorption. When divalent ions are present, in addition to the EDL compression a cation bridge effect between the edges of the MONTs and the NOM surface occurs, thereby reducing release. These findings provide new insights into the study of the deposition behavior of charge-heterogeneous colloids, such as MONTs, in the environment.