Enhanced Cr(VI) removal by using the mixture of pillared bentonite and zero-valent iron

Abstract

In this work, a novel combination of pillared bentonite (OH-Al-bent) with zero valent iron (ZVI) was studied in column experiments so as to enhance Cr(VI) removal efficiency and improve the longevity of Fe0-based permeable reactive barriers. The influence of mixing manner, bentonite type, initial pH and OH-Al-bent amount on the removal efficiency were evaluated. The enhanced removal was attributed to the synergetic effect between adsorption by OH-Al-bent and reduction by ZVI. The adsorption facilitated the mass transfer of Cr(VI) from aqueous to iron surface, and thus accelerated the reduction of Cr(VI) by ZVI. Additionally, the higher removal efficiency was observed under lower pH, and the complete Cr(VI) removal could last longer due to the buffering effect of OH-Al-bent. XPS analysis indicated that Cr(VI) was reduced to Cr(III), and then precipitated in the forms of Cr(III) and Fe(III) hydroxides or Cr(III)–Fe(III) hydroxides. But fewer precipitations on iron surface were detected when OH-Al-bent was combined with ZVI, which might also contribute to the extended longevity of reaction medium.