Combining biochar and basanite rock powder enhances carbon dioxide removal by carbonate alkalinity production

Introduction The combination of enhanced rock weathering (ERW) with pyrogenic carbon capture and storage (PyCCS) has been proposed to harness synergistic effects on carbon dioxide removal (CDR). Synergies may arise from co-application of silicate rock powder and biochar, or from co-pyrolysis of rock powder and biomass to produce rock-enhanced (RE-)biochar. While co-pyrolysis with silicate rock powder is well documented not to affect the carbon yield nor the aromaticity of RE-biochar, the effect of co-pyrolysis and co-application on alkalinity production by ERW remains poorly constrained. Methods We quantified the daily and cumulative production of carbonate alkalinity (TA carb g −1 basanite) in a controlled weathering experiment conducted in columns, comparing 14 treatments consisting of basanite rock powder, biochar, co-applications or RE-biochars produced at contrasting highest treatment temperatures (HTT) of 450 °C and 750 °C. The experiment was run under two conditions: sandy, agricultural topsoil under ambient p CO 2 , and washed, quasi non-reactive quartz sand under elevated p CO 2 , the latter designed to better isolate leachate signals originating from the amendments alone. Column flushing with demineralized water prior to the experiment and subtraction of TA carb signals from the matrix material and biochar amendments enabled quantification of the net TA carb signal from ERW, here referred to as Net_TA carb . Complementary pseudo-lysimeter experiments (i.e., the same treatments set up in larger vegetated soil columns) were used to assess effects on plant growth. Results Cumulative Net_TA carb production was significantly higher from co-deployments than from pure basanite ( p < 0.05), with RE-biochars producing more alkalinity than co-applications under both p CO 2 regimes. Thermal treatment of pure basanite increased its mean specific surface area ( p > 0.05) but decreased its Net_TA carb production. The release of dissolved silica correlated with Net_TA carb production. In the pseudo-lysimeter experiments, no significant effect of any amendment on plant growth was observed. Discussion Carbon sinks from PyCCS and ERW have complementary sequestration curves. Silicate rock powder may therefore be co-deployed with biochar to hedge carbon losses from biochar mineralization while increasing alkalinity production and unlocking additional agronomic co-benefits.