Mechanism of boron removal and stabilization by in-situ formation of layered double hydroxides: Insight from spectroscopy and DFT studies

Abstract

A method for the effective in-situ formation of boron-containing Mg-Al layered double hydroxides (LDHs) was developed for boron removal and stabilization. The influence of the B/Al molar ratio and pH on the formation of Mg-Al-B–LDHs was investigated. Compared with the adsorption method, under a high B/Al ratio, the coprecipitation method increased the boron sorption density from 0.256 to 0.472 of Al. The Toxicity Characteristic Leaching Procedure showed that the boron-coprecipitated LDHs exhibited higher stability than the boron-adsorption LDHs. The synthesized LDH samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and solid-state ¹¹B-NMR. The results showed that boron was effectively incorporated into the LDH structure for the coprecipitation method. Combined with the experimental results, a potential in-situ formation pathway for Mg-Al-B–LDHs was elucidated through density functional theory calculations. The boron tended to directly incorporate into the LDH structure in the coprecipitation method, whereas it was predominantly adsorbed on the LDH surface in the adsorption method. The adsorption energy demonstrated that boron preferentially bonded to Mg²⁺ sites on the surface. The mechanism of boron incorporation in the LDHs for the coprecipitation method involved precipitation of amorphous aluminum hydroxide, layered boehmite transformation, nucleation, and layer stacking. During these processes, boron formed complexes to enhance its stability. Residual boron underwent further reactions with the LDHs, including surface adsorption and ion exchange. These findings provide theoretical insight into the effective removal and long-term immobilization of boron in landfill leachate self-remediation processes.