Comprehensive Experiment of Oxidative Modification of Corn Straw and Its Adsorption of Cu (II)

Corn straw (CS) is a resourceful adsorption material that can be prepared easily at low cost and exhibits high specific surface area. However, in the adsorption of heavy metals it has limitations, such as unstable properties and small adsorption capacity. The adsorption capacity can be enhanced by modifying surface microstructure, such as increasing functional group content or enriching specific surface and pore structure. However, the toxic reagent, complicated process, long cycle, intense reaction conditions and high preparation cost have become the limitations of modification. New economic and sustainable technologies are urgently required to convert postharvest residues into value-added products with minimal impact on the environment. A low-temperature oxidation method has been suggested as an economic and effective strategy for turning biomass into environmental remediation materials. The CS were modified in O₂ and N₂ atmospheres from 140 to 220 ℃, respectively, and Cu(II) was used as the heavy metal target. With the increase of modification temperature, the contents of total acid value, phenolic hydroxyl group, lactone group and total base value of CS (CS-O₂-T) modified at O₂ atmosphere were significantly higher than those modified by N₂ (CS-N₂-T). At the modified temperature of 220 ℃, the adsorption capacity of Cu(II) by CS-O₂-220 is 34.5% higher than that by CS-N₂-220, which is 2.2 times of original CS. The results demonstrate that acid-base functional groups on the surface of CS were increased significantly, and the adsorption capacity of Cu(II) was effectively increased after modified by oxidation. The results of SEM, BET, FTIR and XPS prove that the adsorption of Cu(II) is mainly due to the complexation and cation-π coordination of endoester group, phenolic hydroxyl group and aromatic structure by modified CS. This experiment helps students construct their theoretical knowledge such as the adsorption treatment of heavy metals in wastewater and master the experimental skills of straw modification, and it also cultivates their scientific research and innovation ability.