Performances of residues from hydrolyzed corn-cobs for the adsorption of Congo red

The use of agricultural wastes as raw materials to produce adsorbents for treating dye wastewater is a remarkable research topic. Given that maize (Zea mays L.) is one of the most productive crops, a large amount of discarded corn-cobs is not utilized every year. In this work, solid residues from corn-cobs hydrolyzed by oxalic acid were modified with polyethyleneimine (PEI) and catechol to form adsorbents. The adsorption performances for Congo red (CR) dye were measured, including the effects of initial solution concentration, temperature, and adsorption time. The maximum equilibrium adsorption capacity for CR was ∼650 mg/g at 30 °C, which exceeded that of many biomass-based adsorbents reported in recent literature and even reached the level of some carbon materials. The adsorption of CR molecules followed the pseudo-second-order kinetics. The thermodynamic interaction between CR molecules and the adsorbents was an endothermic process. The adsorption state of the CR molecules on the adsorbents was Langmuir monolayer adsorption, and the adsorption type was physical adsorption. The thermal stability of the adsorbents was similar to that of corn-cobs.