Mechanistic insights into selenite and selenate immobilization using brucite-rich magnesium precipitate derived from seawater electrochlorination facility

Abstract

This study explored the application of magnesium precipitate (MP) obtained from seawater electrochlorination storage tanks as an innovative and sustainable adsorbent for Se(IV) and Se(VI) in water. Initially, the collected pristine MP mainly consisted of the low crystalline Mg(OH)2 (brucite) phase. Subsequently, the MP underwent calcination (CMP), and analyses via scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed an improved crystallinity, displaying a typical hexagonal plate structure in the aqueous phase. Notably, the CMP exhibited a significant adsorption capacity (qm: 85.0 mg/g) for Se(IV), while the adsorption capacity for Se(VI) was lower, determined as 3.0 mg/g using the Langmuir adsorption model. Se(IV) adsorption remained relatively consistent across a wide pH range and ionic strength in the presence of competing anions. In contrast, Se(VI) adsorption varied under the same conditions. This differing adsorption behavior of Se(IV) and Se(VI) on CMP could be ascribed to different mechanisms: inner-sphere predominant surface complexation for Se(IV) and outer-sphere complexation for Se(VI). These mechanisms were confirmed via X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), XRD, and SEM analyses, and surface complexation modeling (PHREEQC–PEST) of the batch experimental results. In conclusion, these findings underscored the potential utility of MP as an effective adsorbent for selenium, positioning it as an environmentally sustainable and significant novel material. Furthermore, the study offered mechanistic insights into the interaction between CMP, Se (IV), and Se(VI) in the aqueous phase.