Figure 2. Change of HCl-extractable Fe(II) with time in the non- reduced control (NC), bioreduced smectite (BS), and aerobically inoculated smectite (AIS). The measured HCl-extractable Fe(II) concentration (mM) was normalized to milligrams of smectite in each tube, and reported as mmol/g. The extent of reduction by the end of incubation reached 18%, based on the measured Fe(III) concentration and the amount of smectite used in the tubes. The vertical errors are typically <10%.