Fate and mechanistic insights into nanoscale zerovalent iron (nZVI) activation of sludge derived biochar reacted with Cr(VI)

While nanoscale zero-valent iron modified biochar (nZVI-BC) have been widely investigated for the removal of heavy metals, the corrosion products of nZVI and their interaction with heavy metals have not been revealed yet. In this paper, nZVI-BC was synthesized and applied for the removal of Cr(VI). Batch experiments indicated that the adsorption of Cr(VI) fit Langmuir isotherm, with the maximum removal capacity at 172.4 mg/g at pH 2.0. SEM-EDS, BET, XRD, FT-IR, Raman and XPS investigation suggested that reduction of Cr(VI) to Cr(III) was the major removal mechanism. pH played an important role on the corrosion of nZVI-BC, at pH 4.5 and 2.0, FeOOH and Fe3O4 were detected as the major iron oxide, respectively. Therefore, FeOOH-BC and Fe3O4-BC were further prepared and their interaction with Cr were studied. Combining with DFT calculations, it revealed that Fe(3)O(4 )has higher adsorption capacity and was responsible for the effective removal of Cr(VI) through electrostatic attraction and reduction under acidic conditions. However, Fe3O4 will continue to convert to the more stable FeOOH, which is the key to for the subsequent stabilization of the reduced Cr(III). The results showed that the oxide corrosion products of nZVI-BC were subjected to the environment, which will eventually affect the fate and transport of the adsorbed heavy metal.

Automated summary

This study investigates the removal of heavy metals using nanoscale zero-valent iron modified biochar (nZVI-BC) and the interaction between nZVI corrosion products and heavy metals. The results show that nZVI-BC follows the Langmuir isotherm for Cr(VI) adsorption, and the main removal mechanism is the reduction of Cr(VI) to Cr(III). The corrosion of nZVI-BC is influenced by pH, with different iron oxide products formed at different pH levels. DFT calculations reveal that Fe3O4 has higher adsorption capacity for Cr(VI) under acidic conditions, and it effectively removes Cr(VI) through electrostatic attraction and reduction. However, Fe3O4 continues to convert to more stable FeOOH, which stabilizes the reduced Cr(III).