Reduction of Polychlorinated Dibenzodioxins and Dibenzofurans in Contaminated Muds by Mechanically Induced Combustion Reactions MARZIO MONAGHEDDU, GABRIELE MULAS, STEFANIA DOPPIU, GIORGIO COCCO,* AND STEFANO RACCANELLI † Dipartimento di Chimica, Universita ` di Sassari, Via Vienna 2, I-07100 Sassari, Italy Some selected results are reported from an ongoing study concerning mechanochemical dehalogenation reactions carried out under autopropagating high-temperature regimes. Our work shows that instantaneous burning reactions can be ignited in highly exothermic systems involving chloroorganics when subjected to intensive mechanical treatments. A reference trial using calcium hydride-hexachlorobenzene is first presented. No residual traces of the organohalide compound were found in the reacted powders or in the gaseous phase sampled from the headspace of the milling reactor. The solid products of the reaction were restricted to CaHCl and CaCl 2 salts and graphite. Similar combustion-like phenomena were observed when contaminated mud samples were added to initial reactants. The contaminated mud was obtained from a waste site and contained traces of polychloro- dibenzodioxins and polychlorodibenzofurans. It was observed that an effective chemical oven was created which caused the thermochemical breakdown of the hazardous compounds. A transformation yield greater than 99.6% was obtained with a decrease of the toxic equivalent concentration from 2492 to 0.788 μg/kg. Unlike conventional incineration methods, the mechanically induced burst processes run under a strictly confined environment and controlled conditions. Introduction Disposaloftoxicchemicalssuch aspolychlorinated biphenyls (PCBs), chlorinated pesticides, and polychlorinated diben- zodioxins (PCDD) and -furans (PCDF) has become a major problem for our technology (1-3). Traditional oxidative degradation, such as incineration in waste combustors, represents a significant environmental hazard since it can often lead to even worse congeners (4, 5).Alternative remedy technologies have not been as effective as incineration and have met only limited success (6). Basic research therefore continuesin thedevelopmentofnewdisposalmethodologies. A mechanochemical degradation route was developed by Rowlands et al. (7). Following this line of research, we described the mechanochemical reduction of organochlo- rines carried out over ionic hydrides in high energy impact ball mills (8, 9). The reaction rates were found to be determined bythe intensityofthe mechanicaltreatment.In the specific case of solid hexachlorobenzene, a self-ac- celerating or explosive-like behavior was observed beyond a millingintensitythreshold (9),which led to an instantaneous transformation of the reactants to hydrogen, graphite, calcium halide, and/or the calcium hydride-halide mixed salt.Such combustive behavior wasprimarilyconnected with the inherent highly exothermic character of these reactions and with the ultimate limit of the reactant stability under progressively induced structural deformations. Two points are ofconcern here;a veryhigh localtemperature is achieved during reaction, and the adiabatic temperature (10), which determines the maximum local temperature of the powder, was calculated to be approximately 2890 K (9). In addition to this, the combustive-like region extends to compositions with a large excess of calcium hydride. These qualities prompted us to exploit the heat released at the combustive event to create a chemical oven. Our intention was to treat difficult systems,for instance,contaminated soilor sediment and mud . Real samples, containing PCDD and PCDF traces, were obtained from an actual waste site for testing. The aim ofthis paper is to show that such materials can be efficiently handled under combustive regimes when mixed with a primer mixture such as hexachlorobenzene and calcium hydride and that the contaminants are burnt by the heat produced by the principal reactants. Experimental Section Mechanical Treatment. The milling reactor consisted of a hollowhardened-steelcylinder,3.7cm internaldiameterand 5.7 cm high, equipped with leak-proof valves. The reactor was clamped in a millingdevice (SpexMixer-Millmodel8000) swinging at 875 rpm along a three-dimensional course. A single hardened-steel milling ball was used. As the mill operated, the ball was released inside the cylinder and, at each impact,its kinetic energywas transferred to the trapped layer of powder. A detailed account of the procedure and protocols we have developed to determine the impact energy, E, and the milling intensity, I, has been reported elsewhere (11). E (joule/hit) is defined as 1 / 2mv 2 , where v is the relative velocityofthecollidingballand m its mass. I (watt)isobtained by multiplying E by the number of collisions per second. High-temperature combustion phenomena occurring under milling treatments are characterized by two distin- guishing parameters: the sharp increase of the reactor vial temperature, ΔT, and the milling time at which the rapid temperature rise is registered,referred to as the ignition time, t ig. As shown later, the first parameter, ΔT, can be related to the reaction heat released at the combustion-like event, whereas the product between t ig and the milling intensity, I, gives the total amount of mechanical energy delivered to the reactants (9). Most of this energy is dissipated as heat while another part is utilized in the powder fragmentation, in defect generation, and to increase the particle contact area.Thislatter effect favorsintermixingon a molecular level and ultimately rises the excess free energy to a threshold beyond which the process spreads spontaneously. In the present workthe temperature ofthe reactor was continuously monitored during milling by a thin lamella-shaped Pt resistance thermometer fixed, under an insulating cover, on the outside surface of the reactor. Reagents. CaH2 (95%, -4 + 40 mesh) and C6Cl6 (99%) were purchased from Aldrich.The main hexachlorobenzene impurities were benzene and the isomers of bi-, tri-, tetra-, and pentachlorobenzenes. *Corresponding author phone: (079)229553/ 6; fax: +39 79 229559; e-mail: cocco@ssmain.uniss.it. † Consorzio Interuniversitario la Chimica per l’Ambiente,Via della Liberta ` 5/12, I-30175 Marghera (VE), Italy. Environ. Sci. Technol. 1999, 33, 2485-2488 10.1021/es9809206 CCC: $18.00  1999 American Chemical Society VOL. 33, NO. 14, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2485 Published on Web 06/05/1999