Ca ²⁺ and Cu ²⁺ Induced Aggregation of Variably Charged Soil Particles: A Comparative Study

The different surface reactions of metal ions with variably charged soil particles can considerably influence the particle interactions, which will further influence soil particle aggregation, sedimentation, dispersion, and transportation. This paper presents a study of the aggregation kinetics of variably charged soil colloidal particles at different Ca 2+ and Cu 2+ concentrations by using dynamic light scattering (DLS) and then investigating the formed aggregates structures by static light scattering (SLS). From the comparison of the experimental results, we found Cu 2+ adsorption‐induced aggregation was much faster than Ca 2+ adsorption‐induced aggregation. An aggregation process referred to as “attraction‐diffusion‐limited cluster aggregation” (ADLCA) was observed in the Cu 2+ adsorption‐induced aggregation of the soil particles, resulting from electrostatic attraction forces between oppositely charged particles. The fractal dimensions of the Cu 2+ specific adsorption‐induced aggregation were much higher than those of the Ca 2+ electrostatic adsorption‐induced aggregation, irrespective of whether aggregation for the former was by the ADLCA, diffusion‐limited cluster aggregation (DLCA), or an ADLCA + DLCA process. Also, the ADLCA process led to the lowest fractal dimension, the DLCA process led to the highest fractal dimension, and the ADLCA + DLCA process led to an intermediate moderate fractal dimension, which shows the complexity of interactions between variably charged particles interactions when specific adsorption occurs at a particle surface. The above observations were well explained by analysis of the net interaction forces between particles in suspension.