Ionpac as11 analytical column

Ferrihydrite reduction was monitored by measuring the concentration of aqueous Fe(II) and total Fe(II) (total Fe(II) is comprised of aqueous Fe(II) and Fe(II) extracted from the solids by 0.75 M HCl) with the ferrozine assay [24 ]. Briefly, 1 mL of HEPES (50 mM)-buffered ferrozine reagent [25 (link)] was added to 0.05 mL of sample, and the Fe(II) concentration was measured at 562 nm with a spectrophotometer. The concentrations of sulfate and phosphate in samples were determined by using an ion chromatograph (Dionex 3000) with an IonPac AS11 analytical column (250 x 2 mm, Dionex) and 1–20 mM KOH at a flow rate of 0.5 mL min^-1 (Dionex Corporation, Sunnyvale, CA, USA). The concentrations of glucose and acetate, lactate, and other organic acids were measured by high-performance liquid chromatography (HPLC) with an Agilent 1100 series HPLC equipped with refractive index and ultraviolet absorbance detectors (Aglient Technologies, Santa Clara, CA, USA). The samples were diluted with an equal volume of 10 mM H₂SO₄, and 50 μL of the diluted sample was injected on a Bio-Rad Aminex HPX-87H ion-exchange column (7.8 × 300 mm; Bio-Rad Laboratories, Hercules, CA, USA). The column was eluted isocratically with 5 mM H₂SO₄ at a flow rate of 0.6 mL min^-1 at 50°C, with analyte detection at 210 nm. The aqueous-phase pH was measured by Semi Micro pH electrode (Thermo Scientific, Waltham, MA, USA).

The reduction of Fe^III in the bioreactors was monitored by measuring the production of Fe^II over time. Samples for total Fe^II (i.e., dissolved Fe^II and acid-extractable Fe^II) analysis were prepared by adding 0.75 mL of anoxic 1 M HCl to a 0.25 mL subsample of the well-mixed bioreactor suspension. The samples were mixed on an end-over-end shaker for two weeks, and then centrifuged at 25,000 × g for 10 min. The concentration of Fe^II in the supernatant was determined spectrophotometrically using the ferrozine assay [58 (link)]. Briefly, 1 mL of HEPES-buffered ferrozine reagent [59 (link)] was added to 50 μL of supernatant and the absorbance was measured at 562 nm using a Cary 100 UV-Vis spectrophotometer. The detection limit for ferrous iron using this method is approximately 10 μM.
Samples for sulfate and acetate analysis were prepared by centrifuging a subsample of the well-mixed suspension at 25,000 × g for 10 min, then removing 50 μL of supernatant and combining it with 950 μL of an isopropanol solution (15% v:v) to preserve the sample until analysis. The concentrations of sulfate and acetate were measured using a Dionex ICS 3000 ion chromatograph equipped with an IonPac AS11 analytical column (250 × 2 mm, Dionex) and a 1-20 mM KOH eluent gradient at a flow rate of 0.5 mL min⁻¹. The detection limit for these anions was approximately 1 μM.