Adsorption modification for nitrate removal

I am trying to remove high levels of nitrate from water by adsorption using pumice and perlite aggregates.

The mean diameter of aggregates is ~5 mm.

I tried using iron-modified pumice and adding the hydrochloric and sulfuric acid but these methods are not giving the satisfactory results.

I would appreciate your advice on how to remove nitrate more efficiently. Which parameters should I change and how? Which modifications can I apply to get better results?