Natural organic matter at solid/liquid interfaces: Complexation, conformation and colloidal stabilization

Natural organic matter (NOM) adsorbs on most particle surfaces and can dominate their surface properties. In most cases, adsorbed NOM increases the colloidal stability of particles. The purpose of this research is to provide a better understanding of the adsorption process and the consequent stabilization effects. A hybrid SC (surface complexation)/SF (Scheutjens and Fleer) model is developed to describe the adsorption of weak polyelectrolytes including NOM on metal oxide surfaces. The hybrid model has the advantage that both the macromolecular nature of NOM and NOM/oxide complexation interactions are addressed.; In the model, the average magnitude of the specific interactions between NOM (polyelectrolyte) functional groups and surface sites, expressed as {dollar}rmchisb{lcub}s{rcub}{dollar}(hyb), varies systematically with surface and NOM (polyelectrolyte) speciation. Over a broad range of solution conditions, good agreement of model simulations and experimental adsorption density is achieved. Of the reactions tested, a 1:1 stoichiometric reaction represented by {dollar}>{dollar}MeOH + L{dollar}sp-{dollar} = {dollar}>{dollar} MeOHL{dollar}sp-{dollar} provides the best representation of the experimental adsorption density data.; Depending on solution chemistry, adsorbed NOM exhibits different conformations at solid/liquid interfaces. At low pH and high ionic strength, adsorbed NOM has an extended conformation. At high pH and low ionic strength, adsorbed NOM exhibits a flat conformation. A detailed description of the conformation of adsorbed NOM at solid/liquid interfaces is described by the model. The effects of pH and ionic strength on adsorbed hydrodynamic layer thickness are predicted well by the model for the oppositely charged case (pH {dollar}{dollar} pH{dollar}rmsb{lcub}pzc{rcub}),{dollar} the contribution of tails to adsorbed hydrodynamic layer thickness is underpredicted by the model.; Depending on solution conditions, adsorbed NOM stabilizes colloidal particles to different extents and by different mechanisms. At high pH and low ion strength, colloidal stability is determined by the electrostatic interactions between ionic diffuse layers on colliding particles. At low pH and high ionic strength, the interactions between adsorbed NOM (polyelectrolyte) segments on colliding particles probably also play a role in determining colloidal stability.