Elemental carbon can reduce transition metal ions in aqueous solutions. This redox reaction may have significant effect on particle toxicity due to the valence state dependence of the metal's ability to induce oxidative stress. Results from some studies have suggested a synergistic effect between carbon and transition metals on the toxicity of particulate matter. The observed synergistic effect may be due to the carbon-metal redox reaction. Because of the ubiquitous abundance of both elemental carbon and transition metals in ambient particulate matter, understanding the implication of carbon-metal redox reaction in particle toxicity is of great importance to developing air pollution control strategies.

In this study we investigated the redox reaction between various elemental carbon nanoparticles and ferric ions in an acidic aqueous solution at 40 and 80 ˚C. The objective was to determine the amount of Fe³⁺ reduced to Fe²⁺ (reduction yield) in batch mode reactions, as a function of reaction time, temperature, and the type of elemental carbon particles. We found that the reduction yield reached a steady-state value after about 1 hour of reaction, regardless of particle type and temperature. The carbon mass based reduction yield was dependent on the type of elemental carbon, as well as on the temperature. The carbon particle surface area based reduction yield showed weak dependence on the particle type at 40 ˚C, but at 80 ˚C it was significantly particle type dependent. X-ray photoelectron analysis of the carbon particles before and after the reaction suggested that the surface functional groups of the carbon particles were involved in the redox reaction.

In conclusion, the preliminary data have shown that elemental carbon of various origins can reduce ferric ions in aqueous solutions. This reaction appears to be dependent on the surface functional groups of the elemental carbon particles. Further investigation is needed to confirm the hypothesized biological effect of the redox reaction between elemental carbon and transition metals.