Great question! The clue lies in the names. “Synthetic” means something that is made to imitate a natural product. And “bio-based” refers to something wholly or partly extracted from materials that naturally or biologically exist.
With that in mind, synthetic (and petrochemical) surfactants are modern chemicals. They are manufactured into new ingredients that are typically not found in nature. Unfortunately, because the energy-intensive manufacturing process uses a large amount of fossil fuels, it leads to more air and water pollution (yuck!). And even at very low concentrations, synthetic surfactants in surface water can be highly toxic.
On the other hand, bio-based surfactants are derived from natural ingredients. Bio-based surfactants represent a good answer to address environmental issues because they are extracted from biomass such as cereals, vegetables, and oilseeds.