. 2020 Sep 8;117(37):22873–22879. doi: 10.1073/pnas.2002659117

Table 2.

Exploration of the role of the microfluidic pH gradient across the mineral precipitate in CO₂ reduction

Experiment	Vent-side pH	Ocean-side pH	Vent-side postprecipitation solute^*	Ocean-side postprecipitation solute/gas^†	Formate concentration, μM^‡
1^§	12.3	3.9	Na₂S/K₂HPO₄/Na₂Si₃O₇	None/CO₂	1.5 (1.66; 1.40)
6	7.0	3.9	None	None/CO₂	n.d.
7^¶	7.0	3.9	Na₂S/K₂HPO₄/Na₂Si₃O₇	None/CO₂	n.d.
8^¶	3.9	3.9	Na₂S/K₂HPO₄/Na₂Si₃O₇	None/CO₂	n.d.
9	12.3	9.8	Na₂S/K₂HPO₄/Na₂Si₃O₇	Na₂CO₃/CO₂	n.d.
10	12.3	3.9	Na₂S/K₂HPO₄	None/CO₂	1.5 (1.33; 1.69)
11	12.6	3.9	Na₂S	None/CO₂	1.8 (2.12; 1.44)
12	9.1	3.9	K₂HPO₄	None/CO₂	(<0.37)^#
13	12.1	3.9	K₃PO₄	None/CO₂	0.9 (1.04; 0.76)

n.d., none detected.

Vent fluid concentrations for precipitation in all reactions were as follows: Na₂S, 100 mM; K₂HPO₄, 10 mM; Na₂Si₃O₇, 10 mM. The same concentrations were used postprecipitation, as relevant. K₃PO₄ (100 mM) was used postprecipitation in experiment 14. In all reactions, both during and after precipitation, vent fluids were driven by H₂ at 1.5 bar and a flow rate of 5 µL/min.

^{^†}

Ocean fluid for precipitation was composed of FeCl₂ (50 mM) and NiCl₂ (5 mM) in H₂O, pushed by CO₂ (1.5 bar) at a flow rate of 5 µL/min. Following precipitation, the ocean fluid for reaction was changed to contain no solutes besides dissolved CO₂ from the driving gas, except for experiment 9 with Na₂CO₃ (100 mM).

^{^‡}

Average calculated concentrations listed, with results from duplicate samples in parentheses.

^{^§}

Same as in Table 1.

^¶

Vent fluid titrated with 1 M HCl to pH 7.0 (experiment 7) or pH 3.9 (experiment 8).

^{^#}

Peak was observable, but concentration was below the limit of quantification (0.37 µM; SI Appendix, Fig. S16).