Moisture-Driven CO2 Direct Air Capture and Delivery for Cultivating Cyanobacteria

A system was developed and demonstrated that uses solid sorbents to capture CO2 passively from ambient air and to subsequently release CO2 when immersed in an alkaline medium. The system was used to cultivate the cyanobacterium Synechocystis sp. PCC 6803 at the flask (50 mL) and bench (12 L) scales. A small pilot-scale system installed in a 4.2-m2 outdoor raceway pond (840 L) was evaluated for over 300 days of outdoor wet/dry cycling and in cultivation trials over four seasons. Sorbent CO2-release capacity and kinetics were reduced due to competitive binding of nitrate and biofouling, and sorbent performance could be partially restored using a wash protocol. Over time, the sorbent beads showed significant reduction in the force needed to induce mechanical failure (i.e., fracture) and loss of quaternary ammonium functional groups necessary for CO2 capture, which may have been due to cumulative UV-induced damage to the polymer. Under the assumption that sorbent performance can be retained during cultivation, preliminary techno-economic analyses showed the potential for the viability of a small biorefinery producing 7.6 million gasoline gallon equivalents of biofuel per year ($3.62/GGE) by offsetting the cultivation costs by first extracting bulk protein as a supplement ($6 kg-1) and phycocyanin as a natural food and beverage dye ($50 kg-1).