Improved longevity of nanoscale zero-valent iron with a magnesium hydroxide coating shell for the removal of Cr(VI) in sand columns

Abstract

Nanoscale zero-valent iron (NZVI) has been engineered as an attractive tool for in-situ groundwater remediation. However, the poor mobility and aqueous corrosion of NZVI in the porous subsurface have hindered its practical applications. In this research, the NZVI surface was coated with a novel Mg(OH)₂ shell (NZVI@Mg(OH)₂) to improve the feasibility of NZVI for remediation. In the column tests for continuous removal of Cr(VI) from the flowing water, the Mg(OH)₂ shell greatly improved the delivery of NZVI into the sand columns. Coating NZVI with Mg(OH)₂ shell also showed considerably greater chemical stability than bare NZVI and thus greater resistance to aqueous corrosion. In addition, the dissolution of Mg(OH)₂ allowed the reactivity to be gradually recovered along the sand column for Cr(VI) reduction. As a result, compared to bare NZVI in the columns, NZVI@Mg(OH)₂ significantly prolonged the breakthrough period of Cr(VI) and hence increased the columns' Cr(VI) removal capacity. Moreover, the Cr(III) produced was effectively immobilized by NZVI@Mg(OH)₂, even under an acidic condition (pH 4.0). The results show that Mg(OH)₂ coating is a promising technique to improve the longevity and capacity of NZVI for full-scale in-situ soil and groundwater remediation.