SURFACE INTERACTION BETWEEN METALLIC COBALT AND TUNGSTEN CARBIDE PARTICLES AS A PRIMARY CAUSE OF HARD METAL LUNG-DISEASE

Hard metal dusts, typically WC/Co, but not pure WC or Co particles, ca use the so-called 'hard metal lung disease' when inhaled over long per iods of time at the workplace. In order to investigate the chemical na ture of the dust which originates the disease, the surface behaviour o f pure cobalt, pure tungsten carbide, an industrial hard metal dust an d a mechanical mixture of cobalt and tungsten carbide have been compar ed. Electron microscopy reveals an intimate contact between metal and carbide in the mixed dusts. The mixed dust is more active than the sin gle components in the adsorption of water vapour in both adsorbed amou nt and interaction energy(lll kJ mol(-1) for the mixture, 95 kJ mol(-1 ) for pure cobalt and 84 kJ mol(-1) for pure WC). Bath industrial and mechanical mixtures are more active than pure components in the cataly tic decomposition of hydrogen peroxide. Incubation of the mixed dusts in phosphate buffered solutions causes a progressive release of cobalt (II) ions in solution and the appearance of round smooth aggregates (d iameter ca. 300-400 mu M) at the expense of smaller particles. The mix ed dusts, but not the pure components, promote the homolytic rupture o f a carbon-hydrogen bond in aqueous suspension, as revealed by the for mation of carboxylate radicals from formate ions. This is evidenced by the use of DMPO as a spin trap, which yields the DMPO - CO2.- adduct whose EPR spectrum intensity measures the amount of radicals generated . Radicals are only formed in aerated solutions indicating a crucial r ole of atmospheric oxygen in their generation. The hydroxyl radical, h owever, does not appear to be implied, for two main reasons: (ij no fr ee oxygen radicals are detected in the absence of formate as target mo lecule; and (ii) free-radical release is insensitive to the addition o f mannitol (an (OH)-O-. scavenger). The formation of the carboxylate r adical CO2.- is an activated process: an induction time of ca. 30 min is required to produce detectable amounts of radicals, while radical g eneration continues for several hours. Samples withdrawn from the solu tion, washed, dried and re-employed are still active, as long as some metallic cobalt is present. A model is proposed whereby in the mixture electrons from oxidized cobalt are translocated at the carbide surfac e where they reduce atmospheric oxygen in a surface-active form which is responsible for the generation of carboxylate radicals from formate ions. The implication of this reaction in health related effects as w ell as possible hazards from particulates in enviromental pollution is discussed.