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 10min. The Langmuir isotherm model and experimental data agreed well, with the maximum adsorption capacity reaching 199.7mg/g at pH = 2.0, 303K. 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.