Phosphate and Humic Acid Inhibit Corrosion of Green-Synthesized Nano-Iron Particles to Remove Cr(Vi) and Facilitate Their Cotransport

Abstract

Major components such as phosphate and humic acid (HA) in soil and groundwater can impact the remediation of hexavalent chromium (Cr(VI)) by green-tea-synthesized nanozerovalent iron (GT-nZVI), which is composed of an Fe 0 core and tea polyphenols (TPs). The interplay of naturally occurring phosphate and HA on the reactions and cotransport of GT-nZVI with Cr(VI) in water-saturated porous media was investigated, and the underlying mechanisms were revealed using multiple technologies including TEM, SEM-EDS, FTIR, XPS and a transport model. The results showed that the formation of Cr(OH) 3 and Cr 2 O 3 happened on GT-nZVI after reacting with Cr(VI). When phosphate and HA present, both adsorption inhibited the Fe 0 core corrosion and the release of TPs from GT-nZVI at pH ≤ 6.0, especially, leading to Cr(VI) reduction by 39.9% (6.7 mg/g) and 70.6% (11.9 mg/g) at pH 3.0, respectively. In particular, the HA presence favored the formation of Cr 2 O 3 , and then facilitated the cotransport with GT-nZVI with the complexation of Fe(III)–HA–Cr 2 O 3 due to the steric repulsive forces of HA. In contrast, phosphate ion reduced the Cr(VI) adsorption and subsequent generation of Cr 2 O 3 on GT-nZVI and accordingly increased the Cr(VI) mobility through water-saturated porous media. To our knowledge, this is the first study unraveling the mechanisms regarding the impact of phosphate and HA on the change of physicochemical properties of GT-nZVI coupling with Cr(VI)/Cr(III) transformation. Our discovery informs a better strategy for in situ remediation of Cr(VI) by GT-nZVI where phosphate and HA commonly cooccur.

Keywords: Humic acid, Green-tea-synthesized nanozerovalent iron (GT-nZVI), phosphate, Chromium, corrosion, Transport