Probing molecular interactions between humic acid and surface-grafted polyacrylamide using quartz crystal microbalance with dissipation and atomic force microscopy: implications for environmental remediation

Abstract

Environmental contextPolyacrylamide and its derivatives may enter the natural environment as a consequence of their wide use in various industrial applications. This study demonstrates the application of a quartz crystal microbalance and atomic force microscopy to study the molecular interactions between polyacrylamides and humic acids under various solution chemistries. The knowledge obtained can be used to understand and predict the environmental behaviour of polyacrylamides. AbstractA fundamental understanding of the environmental behaviour of polyacrylamide (PAM) is of importance for guiding environmental remediation. We create a framework for understanding the molecular interactions between PAM and a major constituent present in all natural waters and soil, humic acid (HA), using a quartz crystal microbalance with dissipation (QCM-D) and an atomic force microscope (AFM). A thin film of PAM was grafted on a silica surface silanised with 3-(trimethoxysilyl)propyl methacrylate and the resulting surface was characterised by X-ray photoelectron spectroscopy for the chemical bonds and composition, secondary ion mass spectrometry for the composition and molecular weight, water contact angle measurements for the hydrophilicity, AFM for the morphology, and ellipsometry for the thickness. Surface-grafted PAM was used to study its interactions with HA in aqueous solutions at different pH (2, 7, and 10) and NaCl salt concentrations (1, 10, and 100 mM, within the range of salt concentrations of fresh water) using QCM-D. QCM-D measurements showed that compared with bare silica, the adsorption of HA by PAM-coated silica was greatly reduced at all pHs and salt concentrations, and the adsorption of HA on PAM-coated silica depended on the solution chemistry including solution pH and salt concentration. Hydrogen bonding between PAM and HA is the major driving force for HA to adsorb on PAM. AFM force measurements showed that adhesion between PAM and HA was observed only at acidic conditions. The knowledge obtained from this study will benefit the prediction of the environmental behaviour of PAMs under different conditions in natural/engineered environments and provide guidance for the design of remediation technologies for water and soil.