The role of natural organic matter in suspension stability: 2. Modelling of particle–particle interaction

The role of natural organic matter (NOM) in changing the interaction between particles and subsequently the flow properties of concentrated particulate slurries has been modelled using electrokinetic and shear yield stress measurement data. Natural organic matter from soil, coal and peat sources as well as full and fractionated samples from aquatic sources has been examined. The data shows that the majority of the humic and fulvic rich samples adsorb in a flat configuration on a model alumina surface where the interaction is driven by a combination of electrostatics and chemical contributions to the adsorption free energy. Using a DLVO based model that assumes that the rheology response of a concentrated suspension of particles at low strain is the result of the summation of pair-wise particle interactions, the assumption of either a flat or molecularly dense adsorption configuration for most of the species at high additive addition rates was found to be valid. The model showed a consistent relationship between the measured rheology and electrokinetic data in terms of colloidal stability. The same modelling exercise for selected NOM samples at low dose showed a discrepancy between the electrokinetic and rheology data. This was for the NOM samples dominated by the neutral and hydrophilic fractions. The model indicated that the measured electrokinetics were poorly representative of the colloid stability possibly due to the presence of a poorly contiguous adsorbed layer that affects the hydrodynamic flow during electrokinetic measurements.