Catalytic Hydrodehalogenation of Haloacetic Acids: A Kinetic Study

Abstract

Haloacetic acids (HAAs) are undesired halogenated byproducts commonly generated upon oxidation treatments carried out in drinking water treatment plants. In this work, the removal of a representative group of these hazardous species (monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, dibromoacetic acid, and bromochloroacetic acid) by catalytic hydrodehalogenation (HDH) was investigated and kinetic models were accordingly developed. Complete dehalogenation of all these pollutants (0.05 mmol L–1) was achieved using a Pd/Al2O3 catalyst (0.5 g L–1), although it was found that their reactivity depended on the nature and number of halogen substituents present in the HAA molecule. In general, bromoacetic acids showed higher reactivity than the chlorinated ones, which was attributed to the lower C-X bond dissociation energy. On the other hand, while the chloroacetic acid reactivity increased with the chlorination degree of the molecule, particularly from one to two Cl substituents, the bromoacetic acids did not show higher reaction rates with the increase in Br substituents. Based on the results obtained, different reaction pathways, via stepwise and/or concerted reactions, were proposed for the HDH of the HAAs. Consistent with those pathways, kinetic models were also developed, which allowed describing successfully the experimental data.

Trace Br– Inhibits Halogenated Byproduct Formation in Saline Wastewater Electrochemical Treatment

“…The formation of halogenated byproducts during the electrochemical treatment of saline wastewater is a serious concern. ,, Herein, the concentrations of typical indicator compounds and halogenated oxyanions (e.g., THMs, HAAs, chlorate, perchlorate, and bromate) were measured and compared between different treatments. ,, Figure c clearly shows that in the absence of Br – , the electrochemical treatment of phenol solution containing Cl – resulted in the production of a considerable amount of byproducts, including THMs (22.5 nM) and HAAs (mainly in the form of dichloroacetic acid) (33.4 nM). However, when Br – was present, although a greater variety of byproducts were observed, such as bromomethane and bromoacetic acid, the total amount of halogenated byproducts was significantly reduced.…”

Palladium-based Catalytic Membrane Reactor for the continuous flow hydrodechlorination of chlorinated micropollutants

“…Up to now, the application of HDC for chlorinated micropollutants removal has been successfully performed at lab scale by conventional catalytic processing (i.e. slurry batch reactors) using powdered Pd/Al2O3 catalysts [5,22]. Nevertheless, this configuration does not allow optimizing hydrogen consumption, and the microenvironment created by the solid-liquid-gas interface cannot be controlled.…”