Figure 1: A schematic diagram of the experimental system.

The system consists of two chambers of 350 ml. each in size. The P-Chamber contains rutile or other semiconducting minerals (anode). On irradiation by a Xe lamp, the electrons are excited from the valence band (VB) with a reduction potential of 2.70 V to the conduction band (CB) with a reduction band of −0.50 V, whereas the positively-charged holes remain in the valence band. The holes were neutralized by the chemical reductant ascorbic acid. The redox couple of the reduced and oxidized ascorbic acid has a reduction potential of −0.28 V (ref. 13); thus, the transfer of electrons from ascorbic acid to the valence band of rutile is a spontaneous process. The photoelectrons are transferred to the C-chamber through an externally connected wire, and the resulting current is determined by measuring the voltage through a 1,000 Ω external resistor using a data logger (ADC-16, Pico Technologies, UK). The electrons in the C-Chamber are captured by oxidants such as Fe³⁺ that is produced by oxidation of Fe²⁺ by A. ferrooxidans. This electron capture process is spontaneous because the reduction potential of graphite (slightly more positive than −0.50 V owing to the potential drop across the resistor) is more negative than the reduction potential of Fe³⁺/Fe²⁺ at pH 2 (0.77 V). The proton exchange membrane (PEM) is used to allow exchange of protons across it to achieve charge balance between the two chambers.