The most promising geoengineering proposal I've seen is spreading "green sand" on beaches which is being researched by project vesta[1]. One of the outputs is

The most promising geoengineering proposal I've seen is spreading "green sand" on beaches which is being researched by project vesta[1]. One of the outputs is calcium carbonate which also has the added benefit acting as a pH buffer. 1. https://www.projectvesta.org/ https://www.projectvesta.org/