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.