Novel adsorbents can be synthesized through tannin gelation, and they are effective agents for the removal of specific contaminants, for example the pharmaceutical species Trimethoprim. The current paper presents an optimization process for obtaining the best adsorbent from four tannin feedstock: Acacia mearnsii de Wild, Schinopsis balansae, Cupressus sempervivens, and Pinus pinaster bark extract. The cross-linking was undergone with formaldehyde and acetaldehyde, hence the type of aldehyde and its concentration in the gelation mixture were considered operative variables as well as the tannin source. The best categories resulted to be Cupressus sempervivens and Pinus pinaster with formaldehyde (3.68 mmol pure formaldehyde per g of of tannin extract) and Schinopsis balansae with diluted formaldehyde (1 mmol of pure formaldehyde per g of tannin extract). Tannin-derived rigid gels were very effective adsorbents for the removal of this dangerous pharmaceutical: trimethoprim, with maximum adsorption capacities even higher than 300 mg of trimethoprim per g of adsorbent.