PARENT SESSION

1B+C Abiotic Transformation
Hall 9
8:30 AM - 12:30 PM, Monday, 28 April 2003
Chair: Peijnenburg, W.¹, ¹
Co-chair: Jafvert, C.T.², Gombar, V.K.³, ^{2 3}

(MO9/10) Kinetics and mechanism of the abiotic hydrolysis of phenylureas.

Capasso, Sante ¹, Salvestrini, Stefano ¹, ¹ 2° University of Naples, Caserta, Italy

ABSTRACT- The hydrolysis of phenylureas has been studied in soil/water system and in homogeneous water phase by analysing the dependence of the temperature, pH and buffer concentration on the reaction rate. The study was carried out on 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 3-(4-isopropylphenyl)-1,1-dimethylurea (isoproturon), 1-butyl-3-(3,4-dichlorophenyl)-1-methylurea (neburon) and 3-(3-chloro-p-tolyl)-1,1-dimethylurea (chlorotoluron). Under all the experimental conditions, phenylureas underwent spontaneous irreversible hydrolysis at an appreciable rate giving detectable amounts of a single aromatic product, which was identified as the corresponding aniline derivative by HPLC co-injection of pure compounds and mass spectrometry techniques. Kinetic evidence suggests that the formation of phenylisocyanate, the initial product of the reaction, occurs via an intermediate zwitterion. In water homogeneous phase, depending on pH and buffer concentrations, the zwitterion can be produced through three parallel routes. Bifunctional acid-base buffers such as HCO3-/CO3- -, H2PO4-/ HPO4- - and CH3COOH/CH3COO- are highly efficient catalysts. At high buffer concentration, as well as at pH < 3 or > 12, the breakdown of the zwitterion is rate-determining. In sterile soil/water mixtures, experimental data show that two processes occur simultaneously: a) rapid ripartition of the herbicides between soil and water, and b) hydrolysis of the molecules present in the water phase. These molecules react with a rate constant that is independent of the amount of soil and equal to the value recorded in homogeneous aqueous phase in the presence of high catalyst concentrations. Analysis of the reacting mixtures indicates that in the presence of soil the catalytic effect is due principally to reactive groups presents

Key words: abiotic degradation, phenylurea herbicides, hydrolysis