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

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.