Amidoxime-functionalized corn straw for efficient Cr(VI) adsorption from water: Waste utilization, affinity character and DFT simulation

To endow the added value of agricultural waste straw resources, we introduced amidoxime groups onto corn straw to fabricate amidoxime-functionalized corn straw sorbents (AO-CS) capable of efficiently adsorbing Cr(VI). The presence of surface-anchored functional moieties significantly accelerated the adsorption rate, achieving equilibrium in just 10 min. The Langmuir isotherm model and experimental data agreed well, with the maximum adsorption capacity reaching 199.7 mg/g at pH = 2.0, 303 K. Experiments with various temperatures were used to obtain the thermodynamic parameters (ΔG, ΔH, and ΔS). The Cr(VI) adsorption mechanism on AO-CS was found to involve electrostatic attraction, hydrogen bonding, redox reactions, and complexation, as confirmed by DFT calculations, FT-IR, and XPS analysis results. Under conditions of coexisting anions (Cl-, NO3-, SO42-, and PO43-), AO-CS exhibited a high selectivity for Cr(VI) due to the establishment of a more stable complex between amidoxime groups and Cr2O72-. Moreover, AO-CS could be easily regenerated using EDTA, with only a slight decrease in adsorption capacity even after four cycles. In conclusion, these cost-effective sorbents with rapid adsorption toward Cr(VI) held great promise for practical applications in wastewater treatment.