Table 2.

Fe-normalized electrons transferred over 28 days relative to the redox state measured (a) directly after addition of HS or RHS, i.e. t = 0, in the HS or RHS magnetite sample and (b) t = 28 days bicarbonate control sample

Sample	meq e− Fe−1 a	meq e− Fe−1 b
Biogenic magnetite + HS	0.5	− 12.4
Biogenic magnetite + RHS	− 47.7	− 67.5
13 nm magnetite + HS	17.9	− 6.3
13 nm magnetite + RHS	10.7	− 16.8
7 nm magnetite + HS	0.2	− 7.8
7 nm magnetite + RHS	− 0.7	− 11.7
500 nm magnetite + HS	2.9	3.1
500 nm magnetite + RHS	2.0	2.6

Positive numbers are net-oxidation reactions and negative numbers are net-reduction reactions with respect to Fe

Using the values in Additional file 1: Table S5, the electrons transferred (z) were calculated as

$z=\frac{\left(\frac{{(F{e}^{2+}(\%)+Fe(II)(\%))}_{{}_{ref.}}-{(F{e}^{2+}(\%)+Fe(II)(\%))}_{{}_{sample}}}{100}\right)\times \left[Fe(II)\right]{}_{tot.avg}}{\left[Fe\right]{}_{tot.avg}}$ where ${\left[Fe(II)\right]}_{tot.avg}=\frac{{[Fe(II)]}_{tot.ref.}+{[Fe(II)]}_{tot.sample}}{2}$ and $\left[Fe\right]{}_{tot.avg}=\frac{\left[F{e}_{tot}\right]{}_{ref.}+{\left[F{e}_{tot}\right]}_{{}_{sample}}}{2}$, Fe²⁺ represents dissolved Fe²⁺, Fe(II) represents solid phase Fe(II), [Fe(II)]_tot is the sum of dissolved Fe²⁺ and solid phase Fe(II), [Fe_tot] is the total concentration of Fe (i.e. the sum of Fe²⁺, Fe(II), Fe³⁺ and Fe(III)) and ref. is (a) the t = 0 sample of the HS or RHS sample as indicated on the lines in the table or (b) the bicarbonate control at t = 28 days