A novel layered double hydroxide coupled with zero valent iron system for selenate removal under anaerobic condition: Batch and continuous studies

Abstract

Batch and continuous stirred tank reactor (CSTR) tests were conducted to evaluate Se (VI) removal in a hybridized Fe(II)-Al(III)-Cl− layered double hydroxide/zero valent iron system (Fe-Al-Cl-LDH/ZVI) under anaerobic condition. This novel system was created by employing a nitrate-AlCl3 pretreatment method to coat the ZVI grains with an LDH surface and produce discrete LDH suspended solids (SS) with diameter of 475.3 nm through a rapid ZVI-AlCl3-nitrate reaction. In batch test, Se (VI) was quickly removed by Fe-Al-Cl-LDH/ZVI grains through adsorption via electrostatic attraction, anion exchange by releasing 1.7 mM Cl−/mM Se (VI) and Se (VI) reduction to Se (IV), Se (0) and Se (-II) by ZVI. In CSTR test, the anaerobic Fe-Al-Cl-LDH/ZVI system created a self-sustaining highly-reactive mixture that steadily reduced 2 ppm Se (VI) to below EPA discharge limit (50 ppb) in a single-stage reactor, where 200–300 ppm Cl− and 6.5–20 ppm SO42− coexisted. Externally-supplied FeCl2 and AlCl3 were essential to maintain LDH as the iron corrosion product and thus the high reactivity of ZVI. This study suggests that the Fe-Al-Cl-LDH/ZVI system could overcome ZVI passivation and thus provide an effective platform for Se (VI) and potentially other water contaminants removal.