Real-time evaluation of natural organic matter deposition processes onto model environmental surfaces

Natural organic matter (NOM) is ubiquitous in aqueous systems and dynamically partitions onto/from environmental surfaces. However, such interfacial processes have not been uniformly quantified in situ and in real time. In this work, adsorption and deposition processes of Suwannee River humic acid (SRHA) and Suwannee River fulvic acid (SRFA), as model NOM, were evaluated for a series of environmentally relevant interfaces. Real-time, interfacial phenomenon, including deposition, release, and adlayer viscoelastic properties, were quantified over a variety of water chemistries via quartz crystal microbalance with dissipation monitoring (QCM-D). Specifically, adlayer mass and deposition rates of SRHA and SRFA were evaluated as a function of NOM concentration/molecular weight (fraction), pH, electrolyte composition (type and concentration), and collector surface type. For these, the adsorption of SRHA onto aluminum oxide (Al₂O₃) and polystyrene (PS) surfaces follows the Langmuir isotherm model. Rapid, near-monolayer formation of SRHA/SRFA adlayers were observed on Al₂O₃, hydroxyapatite (HAP), and poly (l-lysine) (PLL) surfaces, but not on PS or iron oxide (Fe₃O₄) surfaces. The presence of divalent cations (Ca²⁺/Mg²⁺) at relatively low concentrations (0.5-5.0 mM) significantly enhances the mass of SRHA/SRFA deposited onto the surfaces of silica (SiO₂), Al₂O₃, and PS. Viscoelastic properties of the adsorbed layer based on the ratio of dissipation to frequency revealed a relatively unique adlayer structure for SRHA in the presence of 5.0 mM Ca²⁺.