Figure 4. Direct electron transfer from pyrogenic carbon matrices to minerals.

(a) Linear sweep voltammograms of minerals on pyrogenic carbon. All linear sweep voltammetry were performed by immobilizing minerals on the surface of a pyrogenic carbon (pyrolysed at 800 °C) working electrode. Scan rates varied from 50 to 250 mV s⁻¹ with an interval of 50 mV s⁻¹, except for iron(III) chloride with scan rates from 10 to 40 mV s⁻¹ at an interval of 10 mV s⁻¹. Fe(II) catalysed H⁺ reduction followed Fe(III) reduction in iron chloride at more negative potential, and was not separately prepared. The cyclic voltammetric chart shows the direct electron transfer by pyrogenic carbon matrices to surface quinone groups (SQGs, measured using 0.4 mg pyrogenic carbon produced at 500 °C). The actual range of peak potentials from low to high scan rates are shown at the main x axis for all tested species. (b) The linear relationships between scan rate and peak current of all tested species. Error bars are s.d. of triplicate measurements. (c) The reduction rate comparison among minerals. Numbers on the top of columns indicate the Brunauer–Emmett–Teller surface area for each mineral.