Time-resolved observation of ZrO₂-colloid agglomeration

The agglomeration of ZrO₂-nanoparticles is investigated in situ by a combination of light scattering photon correlation spectroscopy (PCS) and laser-induced breakdown detection (LIBD). Surface charge measurements suggest a point of zero charge (PZC) for ZrO₂-colloids at pH 8.2 as confirmed with PCS by agglomeration rate measurements at higher concentrations (10¹¹ particles cm^-3). The agglomeration rate is investigated further at the PZC as a function of particle concentration. Utilizing the high sensitivity of LIBD the linear dependence of the agglomeration rate was confirmed for the first time at concentrations typical for natural systems, i.e. down to 2×10⁶ particles cm^-3. The results agree with the simple diffusion limited aggregation model of Smoluchowski (sticking factor 1) and no indication of an activation barrier are found. Zr(IV) is known for its similarity to Pu(IV) regarding colloid formation and hence the present investigations can serve as a model for the aggregation of Pu(IV)-colloids, the stability of which represents an important aspect in the safety assessment of nuclear waste repositories.