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

Green tea synthesized nano-zero-valent iron (GT-nZVI), composed of a Fe-0 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 Fe-0 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) and 70.6% (11.9 mg/g) 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.

Automated summary

Green tea synthesized nano-zero-valent iron (GT-nZVI) is effective for removing hexavalent chromium (Cr(VI)) from water and soils. The presence of phosphate and humic acid (HA) inhibits Fe-0 core corrosion and reduces the generation of soluble Cr(III), while HA facilitates the formation of Cr2O3. Phosphate ion, on the other hand, decreases Cr(VI) adsorption and increases Cr(VI) mobility in water-saturated porous media when coupled with GT-nZVI.