NO3−-N removal from water using raw and modified fibrous clay minerals and their potential application as nitrogen fertilizers in hydroponic lettuce cultivations

Abstract Raw, thermal- and acid-treated palygorskite (Pal/ T-Pal/ H-Pal ) and sepiolite (Sep/ T-Sep/ H-Sep) were applied to adsorb NO 3 − -N from aqueous solutions. A series of batch kinetic experiments was conducted on NO 3 − -N removal while investigating the effects of initial pollutant concentrations, adsorbent dosages and solution pH. Adsorbent dosages of 4.0 g resulted in NO 3 − -N removal within 20 min. Removal capacities of raw Pal and Sep were examined in solutions with 15–100 mg NO 3 − -N/L. Acid treatment enhanced the capacity of Pal and Sep to remove initial concentrations of 15 mg NO 3 − -N/L from 0.15 and 0.10 mg/g to 0.28 and 0.32 mg/g, respectively, whereas thermal treatment did not improve NO 3 − -N removal capacities of either mineral. The NO 3 − -N adsorption was better expressed by Langmuir isotherm and pseudo-first order kinetic model, except from Pal sample where Freundlich and pseudo-second order kinetic presented better fit. Saturation tests showed that all adsorbents were saturated within 24 h, while desorption tests showed that most nitrate was released within 12 h. Despite their rapid nitrate release, raw and modified Pal and Sep were applied as effective nitrogen fertilizers in 40-day hydroponic cultivations of lettuce seedlings. Both minerals proved to be effective N fertilizers for hydroponic lettuce cultivations. Raw Pal and Sep potentially boosted N uptake more than the modified clay samples. Pal fertilizers significantly improved root growth and Sep fertilizers enhanced shoot height.