Synthesis and surfactant modification of clinoptilolite and montmorillonite for the removal of nitrate and preparation of slow release nitrogen fertilizer

► Clinoptilolite and montmorillonite have been synthesized hydrothermally. ► SMCHDTMAB appears to be better adsorbent. ► Surfactant modification increases the adsorption capacity of silicates to retain nitrate. ► The maximum monolayer capacity for nitrate at 303 K was 125.0 mg g−1. ► The adsorption data fitted well with the Langmuir and Freundlich isotherms.