A sustainable approach for the production of formate from CO₂ using microalgae as a clean biomass and improvement using potassium-doped g-C₃N₄

An effective strategy to combat the adverse effects of rising CO₂ levels in the atmosphere is to convert CO₂ to value-added chemicals. Recently, microalgal technology has received attention for CO₂ utilization considering that it provides a green pathway for producing the chemicals. The current study demonstrates the production of formate from CO₂ using the Chlamydomonas reinhardtii and Spirulina microalgae. The production is enhanced using potassium (K)-doped graphitic carbon nitride (g-C₃N₄), a non-toxic and environmentally benign catalyst. The experiments performed in a photobioreactor show the maximum formate production over the K-g-C₃N₄-supported Chlamydomonas reinhardtii and Spirulina to be 1192 and 751 mg/L, respectively, which are 59 and 13 % higher than those in the corresponding uncatalyzed algal processes. Starch synthesis in the microalgae is aided by g-C₃N₄ in light, while potassium doping and a nitrogen blanket in the reactor increased the microalgae's ability to convert starch to formate in dark. The photocatalytic role of gCN in CO₂ conversion by the microalgae was confirmed by scavenger analysis. A positive interaction between the microalgae and K-gCN was observed using the morphological and physicochemical characterization tests. Accordingly, the study confirmed that K-gCN played a dual role. It not only provided a suitable surface for the microalgae but also enhanced photocatalytic activity, which in turn improved formate production. A mechanistic insight of the step-wise conversion of CO₂ to formate is proposed. In a first, this study has proposed a clean route to producing a low-carbon alternative.