Aggregation of colloidal silica particles in the presence of fulvic acid, humic acid, or alginate: Effects of ionic composition

We have studied the aggregation of model silica particles in the presence of NOMs (fulvic acid, humic acid, or alginate) by using dynamic light scattering. Experiments were performed systematically by changing CaCl2 concentration, the types and concentration of NOMs: Suwannee river fulvic acid (SRFA), Suwannee river humic acid (SRHA), and alginate. Experimental results demonstrate that the aggregation rate in the presence of the SRFA and Ca2+ ions was on par with the rate of the diffusion limited aggregation. On the other hand, the addition of SRHA or alginate extremely enhances the rate of aggregation in the presence of CaCl2 compared to the rate of diffusion limited aggregation. The enhancement of the rate is probably due to the bridging of silica particles by the microgels formed by the Ca-mediated bridging of SRHA/alginate molecules. Also, the enhanced aggregation with alginates/SRHA and Ca2+ ions is reduced by the coexistence of K+ or Mg2+ ions. We suppose that the breaking of Ca-mediated bridging of alginates/SRHA by ion-exchange reduces the rate of the enhanced aggregation.