Phosphate and humic acid inhibit corrosion of green-synthesized nano-iron particles to remove Cr(VI) and facilitate their cotransport

Abstract

Green tea synthesized nano-zero-valent iron (GT-nZVI), composed of a Fe0 core and tea polyphenols (TPs), has been attracting attention for effective removal of hexavalent chromium (Cr(VI)) from water and soils. However, the relevant environmental factors which may affect GT-nZVI removal of Cr(VI) are still unclear. Thus, it is necessary to investigate the interplay of naturally occurring phosphate and HA on the reactions and stability of GT-nZVI with Cr(VI) in water-saturated porous media to mimic soils and groundwater. The inherent mechanisms were revealed using multiple technologies, including TEM, SEM-EDS, FTIR, XPS, and a transport model. We found that the Cr(OH)3 and Cr2O3 were formed on GT-nZVI upon reacting with Cr(VI). When phosphate and HA were present, their adsorption inhibited the Fe0 core corrosion and the release of TPs from GT-nZVI at pH ≤ 6.0. Thus, the generation of soluble Cr(III) was reduced by 39.9% (6.7 mg∙g−1) and 70.6% (11.9 mg∙g−1) at pH 3.0, respectively. In particular, HA favors the formation of Cr2O3, and then facilitates the cotransport with GT-nZVI, forming Fe(III)–HA–Cr2O3 complex due to the steric repulsive forces of HA. In contrast, phosphate ion dereases Cr(VI) adsorption and subsequent generation of Cr2O3 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.