Time-resolved observation of ZrO 2 -colloid agglomeration  Claudia Bitea a , Clemens Walther a, *, Jae Il Kim a , Horst Geckeis a , Thomas Rabung a , Franz J. Scherbaum a , Dan G. Cacuci b a Institut fu ¨r Nukleare Entsorgung, Forschungszentrum Karlsruhe, Postfach 3640, D 76021 Karlsruhe, Germany b Institut fu ¨r Reaktorsicherheit, Forschungszentrum Karlsruhe, Postfach 3640, D 76021 Karlsruhe, Germany Received 13 May 2001; accepted 21 August 2002 Abstract The agglomeration of ZrO 2 -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 ZrO 2 -colloids at pH 8.2 as confirmed with PCS by agglomeration rate measurements at higher concentrations (10 11 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 /10 6 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. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Colloid agglomeration; Zirconia; Plutonium; LIBD 1. Introduction Aggregation plays an important role in the manufacturing of granular matter, often called ‘nanomatter’. Production of grams or kilograms on technical scales requires highly concentrated suspensions (typically several weight percent), where theory is based on solid grounds and agrees well with experimental observation [1,2]. Particle aggregation kinetics was investigated by many groups [3  /6] for either high concentrations or large particles. However, to our knowledge, ag- gregation was not yet investigated for very dilute suspensions (below 10 7 part cm 3 ) of small  This article comprises part of the dissertation of C. Bitea. * Corresponding author. Tel.:  /49-7247-82-6064; fax:  /49- 7247-82-3927 E-mail address: walther@ine.fzk.de (C. Walther). Colloids and Surfaces A: Physicochem. Eng. Aspects 215 (2003) 55  /66 www.elsevier.com/locate/colsurfa 0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7757(02)00415-6