Abiotic degradation of 2,4-D (dichlorophenoxyacetic acid) on synthetic birnessite: A calorespirometric method

Abiotic transformation of organic pollutants in soil environments can contribute significantly to remediation. As a model for the examination of abiotic remediation, the birnessite-catalyzed degradation of 2,4-D(dichlorophenoxyacetic acid) and ethyl ether was monitored by calorespirometric methods, and the products were examined by calorespirometry, chromatography and electron spin resonance (ESR) spectral analysis. Birnessite is a common component of many soils. Birnessite (modeled as the oxide mineral delta-MnO2) is an efficient solid-state catalyst of the breakdown of organic pollutants. Our results show that 2,4-D and ethyl ether adsorbed on the surface of birnessite are rapidly oxidized. Both compounds produce CO2 as a major product, but by somewhat different mechanisms. Analysis of the reaction products by high-performance liquid chromatography shows no accumulation of intermediates. Following reaction with 2,4-D, but not with ethyl ether, ESR analysis detected Mn2+ extractable into methanol. These experiments show (a) that birnessite catalyzes the breakdown of organochlorine herbicides, such as 2,4-D and organic solvents, such as ethyl ether; (b) that CO2 is a major product of degradation for both compounds; (c) that the reaction proceeds via the birnessite-catalyzed oxidation of hydrocarbon structures, but the 54-D-assisted dissolution of birnessite to produce Mn2+ is also involved in the degradation of organic solvents; and (d) that solid-state analysis by calorespirometry allows simple, rapid determinations of the reaction kinetics of pollutant breakdown.