Encapsulating nanoscale zero-valent iron with a soluble Mg(OH)2 shell for improved mobility and controlled reactivity release

Abstract

Nanoscale zero-valent iron (NZVI) is a highly reactive material for decontamination but it often suffers from problems such as agglomeration, aqueous corrosion, and poor mobility for field applications. In this study, a new modification technique was developed by coating the NZVI surface with a soluble Mg(OH)₂ shell to form Mg(OH)₂-coated NZVI (NZVI@Mg(OH)₂) nanoparticles. The Mg(OH)₂ shell significantly reduced the magnetic attraction between NZVI particles, preventing particle aggregation. Consequently, NZVI@Mg(OH)₂ with an appropriate coating ratio (Mg/Fe, wt%) had an increased stability in suspension and considerably improved mobility in saturated porous media. According to the filtration tests, the Mg(OH)₂ shell effectively decreased the attachment, blocking, and straining effects during the transport of NZVI particles through a sand column. With the Mg(OH)₂ shell, the reactivity of NZVI was well preserved during the NZVI storage and transport. Encapsulation with 100 wt% Mg(OH)₂ could protect the NZVI core from aqueous corrosion such as oxidation by H⁺ ions. When NZVI@Mg(OH)₂ is used in field applications, the NZVI reactivity would be progressively released and fully recovered with the dissolution of the Mg(OH)₂ shell in the background water. The test results showed that after the dissolution of the Mg(OH)₂ shell, the NZVI exhibited a similar reactivity and capacity to bare NZVI for the reduction of Cr(VI). Overall, the novel Mg(OH)₂ coating technique not only provides NZVI with a high mobility for transport and delivery but also protects the NZVI core from rapid corrosion and enables a controlled release of NZVI reactivity for environmental remediation.