Activated and nonactivated forms of zinc powder: Reactivity toward chlorocarbons in water and AFM studies of surface morphologies

This work was carried out with the purpose of developing effective reagents for decontamination of groundwater contaminated with chlorocarbons. Zinc metal as a reducing agent for carbon tetrachloride (CT), chloroform (Chl), and methylene chloride (MC) in aqueous solution has been studied in some detail, especially regarding activated forms of the metal. Chlorocarbon concentrations were monitored at certain time intervals by gas chromatography/mass spectrometry (GC/MS) analysis of the headspace and water phase. Reaction mixture headspace was additionally studied by a GC/headspace analysis system to detect the formation of hydrocarbons. Chloroform, methylene chloride, methyl chloride, methane, and acetylene were found to be products from CT reduction. For methylene chloride reduction, traces of cis and trans-1,2-dichloroethene (DCE) were also found. Activated by cryo or mechanical treatment, metallic zinc caused an increase in CT dechlorination rate and conversion into methane. After the first 2.5 h, more than 20% of CT was converted into methane by cryochemically activated zinc in comparison to 1.2% by conventional zinc dust. Furthermore, CT reduction by activated zinc caused the formation of DCEs and TCE. Pathways are proposed to account for the observed methane/methylene chloride ratio and DCEs and TCE formation that include sequential reductive dechlorination through organometallic and carbonoid species on the Zn surface. Furthermore, it seems likely that some methane can be formed in "one metal contact", since significant amounts are formed early in the reaction. In attempts to learn more about morphological changes in the zinc during its consumption, pore volume/pore radii were determined, and atomic force microscope images were obtained. Zinc corrosion takes place rapidly at edges/corners leading to the formation of cavities with wide openings, large volumes, and increased specific surface areas. Pyramidal zinc "pillars" are formed during the process.