Valorization of magnesium precipitate from electrochlorination facility for sustainable selenite and selenate sequestration

Abstract

Selenium (Se) is known both as an essential and toxic element for humans, depending on the exposure level. It is therefore often described as a ‘double-edged sword’ element. In aquatic environments, Se(IV) and Se(VI) are the predominant species, and magnesium oxide (MgO) has recently been introduced as a potential candidate for removing Se species. However, their effectiveness and underlying adsorption mechanisms remain insufficiently understood. Meanwhile, current research increasingly focuses on developing waste-derived adsorbents as more sustainable and low-cost alternatives. Such adsorbents often require complex chemical modification, which compromises their economic and environmental advantages. In this context, a MgO-based adsorbent was prepared for Se(IV) and Se(VI) removal via a facile thermal transformation of magnesium precipitate (MP), which is a brucite-rich byproduct from an electrochlorination facility. The calcined MP (CMP) showed a remarkable adsorption capacity for Se(IV) (85.0 mg/g), but a much lower value for Se(VI) (3.0 mg/g). In kinetic experiments, Se(IV) and Se(VI) adsorption reached equilibrium within 360 min, but partial Se(VI) desorption was observed. Moreover, Se(IV) adsorption remained stable over a wide range of pH, ionic strength, and the presence of other anions, whereas Se(VI) uptake varied under the same experimental conditions. The contrasting behaviors were attributed to inner-sphere predominant complexation for Se(IV) and outer-sphere complexation for Se(VI), which were supported by spectroscopic analyses and geochemical modeling. To enhance Se(VI) removal, a sustainable in situ layered double hydroxide (LDH) formation strategy using CMP was proposed. Optimal removal conditions were achieved by controlling solution pH and Al input, which allowed much higher Se(VI) removal efficiency compared to using CMP alone. The removal mechanism involved Mg release from CMP through Al ion hydrolysis, followed by Se(VI) sequestration in the LDH interlayer via outer-sphere complexation. In addition, the resulting sludges (LDHS and calcined LDHS (LDOS)), potential secondary wastes, were further investigated as Se(VI) adsorbents. LDOS exhibited a maximum adsorption capacity of 42.1 mg/g, comparable to that of other conventional adsorbents. These results not only demonstrated a sustainable and practical route using magnesium precipitate for Se(IV) and Se(VI) removal but also provided mechanistic insights into their sequestration mechanisms. Furthermore, a techno- economic analysis revealed that the adsorbent based on CMP and the in situ LDH formation strategy for Se removal achieved a significantly lower treatment cost than conventional methods, highlighting their cost-effectiveness and applicability for Se sequestration.