Scalable fabrication of LDH-based adsorbent for the removal of nitrate with enhanced performance

Abstract

The exploration of LDHs for the selective adsorption of NO₃⁻ from water has been proposed for a long time; however, their complex fabrication methods and unclear adsorption mechanism limit their further utilization. Considering this, herein, a systematic investigation was conducted by synthesizing Mg₃Al-LDHs with different morphologies via the co-precipitation method with different alkali sources including individual NaOH and Na₂CO₃, and mixed alkali sources of NaOH and Na₂CO₃ to remove NO₃⁻ from water. Their detailed microstructure and morphologies were characterized and the influence of the alkali source on the NO₃⁻ removal rate for the Mg₃Al-LDHs was fully investigated. Based on the results, LDHs prepared with NaOH as the alkali source could weaken the agglomeration of the Mg₃Al–LDH layers, thus exposing the active adsorption sites on their surface and reducing their mesoporous pore size to 3.38 nm, which enhanced their adsorption capacity for NO₃⁻. Consequently, the adsorption performance of LDHs for NO₃⁻ was investigated considering the influencing factors including time, temperature, and initial concentration. It was found that the sorption process could be well described by pseudo-second-order kinetics and the Langmuir isotherm model. The maximum monolayer adsorption capacity on NO₃⁻ was calculated to be 6.1 mg g⁻¹ at 298 K. Meanwhile, the thermodynamic results indicated that the adsorption removal of NO₃⁻ is spontaneous (ΔG⁰ < 0) and exothermic (ΔH⁰ < 0). Additionally, the enhanced adsorption performance is attributed to the combination of physisorption driven by intermolecular forces and chemisorption caused by ion exchange. Therefore, due to the scalable fabrication of Mg₃Al-LDHs with the addition of NaOH as an alkali source, they can be a potential adsorbent for the removal of NO₃⁻–N from water.