Radio;. Phys. Chem. Vol. 26, No. 6, pp. 641-645, 1985 printedin Great Britain. 0146-5724185 $3.00 + .OO 0 1985 PergamonPress Ltd. ENHANCEMENT OF NEUTRALIZATION REACTION IN COLLOIDAL FERRIC HYDROUS OXIDE? Auos VERTES~ Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, U.S.A. (Received 11 February 1985; in revised form 3 April 1985) Abstract-Neutralization reaction was investigated in negative ferric hydrous oxide ~01s. The ad- sorption isotherm of OH- ions on the surface of colloidal particles was measured to provide the variation of their charge with pH. Impulse radiolysis generated the reacting ions in the sol. Using a fast DC conductivity setup for detection, a significant increase in the neutralization rate coefficient was found at low OH- concentrations. The phenomenon was discussed in terms of the theory of diffusion controlled reactions for particles interacting with Coulomb and Yukawa potential. 1. INTRODUCTION TRANSPORT OF corrosion products in the primary cooling system of water cooled nuclear reactors has drawn attention to the behavior of iron oxide col- loids undergoing irradiation.“’ Suspensions of other fissile oxides had been examined earlier.“’ The basic question these investigations have posed is how activated corrosion products travel around in the system and how sedimentation terminates their trip causing contamination. Both of these investi- gations were performed under stationary condi- tions; dissolution”’ and fragmentation”’ effects were found to be caused by substantial doses. The search for candidates for water splitting electrodes led to experiments with colloidal metal “microelectrodes”-see, e.g. Refs. (3,4)-and re- cently with colloidal oxide semiconductors”s6’ such as TiOz and a-FezOJ dispersions. The suspected ad- vantage inherent in the small particle size, besides their enormous surface, is the possibility of avoid- ing recombination of light generated electrons and holes before these reach the surface. This of course could improve the yield of water cleavage systems. Our aim was to explore fast processes immedi- ately after irradiation in ferric hydrous oxide sols in order to understand the fate of water radiolysis products in such systems. The contribution of these processes to radiation induced coagulation of iron oxide colloids and to the consequent contamination of the primary cooling circuit of nuclear power plants may more clearly be understood in this way. t The research described herein was supported by the Oflice of Basic Energy Sciences of the Department of Energy. $Present address: Central Research Institute for Phys- ics P. 0. Box 49, H-1525 Budapest, Hungary. The present paper deals with the effect of neg- atively charged colloidal particles on the rate of the neutralization reaction (I) H+ + OH-LH20, where k1 is the rate coefficient. This effect is orig- inated in the possibility of a new reaction, namely the reaction of H+ ions with OH- ions adsorbed on the surface of colloidal particles. Since the charge of the colloidal particle depends on the bulk concentration of OH- ions, cOH- , the effective neutralization rate coefficient, keE, will be a func- tion of this concentration. In order to characterize the colloid as a reactant we needed the relation between the bulk concen- tration of OH- and the charge of individual parti- cles. The measurement of adsorption isotherms of hydroxide ions on the surface provided information on the charge of single particles. The second step was to measure the effective rate coefficient of the decay of radiation generated hydrogen ions in a wide CoH- range. This was made possible by the nanosecond conductivity setup of Janata.“’ In the Discussion we invoke the theory of dif- fusion controlled reactions to describe our experi- mental findings. 2. EXPERIMENTAL 2.1. Preparation and characterization of the sols Negative ferric hydrous oxide ~01s’~’ were pre- pared by the method of Powis”): 200 cm3 of a so- lution 0.0033 M in FeC13 and 0.0015 M in HCl was added gradually to 300 cm3 of well stirred 0.016 M KOH. The solutions were kept under Nz atmos- phere and at 0°C before and during the mixing. 641