Concrete waste as a source of calcium and alkalinity for coccolithophore cultivation: an experimental test of carbon dioxide removal and sequestration

Cultivation of calcifying microalgae has been proposed as a route to carbon dioxide removal (CDR), but biogenic calcification releases CO ₂ unless the calcium supplied also adds alkalinity. We used cultures of the coccolithophore Chrysotila sp to test whether hydrated lime (Ca(OH) ₂ ), supplied as a reagent or within ground waste concrete, can provide both calcium and alkalinity. In a 26-day factorial batch experiment, we monitored pH, nitrate and dissolved calcium. Calcite was quantified by thermogravimetric analysis. Nitrate concentration decreased and calcite accumulated in all algal treatments, indicating that lime or concrete did not inhibit growth or calcification. Lime and concrete raised culture pH significantly whereas the algal growth rate inferred from nitrate drawdown did not differ among treatments. Calcite production was 3.6-fold higher with lime than in seawater (4.5 vs. 1.2 mg L ⁻¹ d ⁻¹ ) and highly variable with concrete; lime and concrete cultures remained supersaturated with respect to calcite (Ω ≈ 7-10 vs. 0.2). Carbonate-system calculations indicate that the added alkalinity raises the equilibrium CO ₂ uptake capacity of the medium by ca. 3.4 mmol kg ⁻¹ , offsetting CO ₂ released by calcification. These results provide proof of concept for coupling coccolithophore cultivation with concrete waste reuse for CDR.