Dechlorination of Recalcitrant Polychlorinated Contaminants Using Ball Milling

Abstract

The persistence of hazardous pollutants in the environment or waste streams and materials is frequently due to the thermodynamic and/or kinetic stability related to their molecular structure. Further these pollutants tend to be sorbed strongly to other components of the contaminated matter such as polychlorinated biphenyls (PCBs) or pentachlorophenol (PCP) to clayish soils or polychlorinated dibenzodioxins (PCDDs) or dibenzofuranes (PCDFs) to filter dusts. Adsorption renders such pollutants virtually inaccessible to physical, chemical or biological remediation and/or natural attenuation processes. These difficulties of stability and inaccessibility can be effectively overcome by mechanical activation of appropriate reductive dehalogenation reactions inside a ball mill. Such recalcitrant compounds, like PCBs, PCDDs, PCDFs or PCP, can be completely dechlorinated under strikingly benign, reductive conditions, i.e., at room temperature, ambient pressure and in a short time, by ball milling the contaminated material with a base metal and a hydrogen donor. A ball mill is utilized as a mechanochemical dehalogenation reactor in one single, almost universally applicable, operational key step. This novel, versatile ex situ dehalogenation approach can be successfully applied both to contaminated materials and highly concentrated or pure contaminants and of their mixtures, virtually regardless of their state, and is designated as “Dehalogenation by Mechanochemical Reaction” (DMCR). For instance, PCBs in contaminated soil, filter dust, transformer oil, or as pure PCBs oils from transformers or capacitors can be rapidly dechlorinated to harmless chloride and their parent hydrocarbon biphenyl by applying magnesium, aluminum or sodium metal and a low acidic hydrogen source like an ether, alcohol and/or amine. DMCR offers several economic and ecological benefits. Ball milling requires a low energy input only, toxic compounds can be converted to defined and usable products, the method facilitates the re-use of scrap metals such as magnesium or magnesium-aluminum alloys, and detoxified materials like transformer oils can be readily recycled after a DMCR treatment. No harmful emissions to the environment have to be expected. This paper presents selected results of various treatability and feasibility studies demonstrating the versatility and efficiency of DMCR regarding the defined destruction of numerous hazardous persistent organic pollutants (POPs) like PCBs, PCDD/Fs and related compounds (PCP and dichlorobenzene (DCB)) in different solid and liquid contaminated materials.