Phosphate-functionalized ramie stalk adsorbent for efficient removal of Zn from water: adsorption performance, mechanism, and fixed-bed column treatment of real wastewater.

A highly efficient adsorbent functionalized with phosphate groups made from a local agricultural waste, ramie stalk, was designed for Zn removal from water. SEM, EDS, FTIR, zeta potential, and XPS tests were used to study the morphology and properties of modified ramie stalk (RS-P). The results showed that the phosphate groups were successfully grafted to the surface of the ramie stalk, which has a multilayered and porous structure and can provide large adsorption sites. Adsorption performance and mechanism were investigated in the static and dynamic adsorption experiments. The adsorption kinetics of Zn by RS-P were better fitted by the pseudo-second-order model, indicating chemical adsorption. Adsorption isotherm was better described by Redlich-Peterson isotherm, which suggested heterogeneous and multi-site adsorption, with a maximum adsorption capacity of 0.558 mmol g. The characterization of adsorbents before and after adsorption indicated that a combined action of electrostatic interaction and ion exchange was the primary mechanism of adsorption. Dynamic adsorption experiments with fixed-bed column displayed excellent water treatment capabilities. RS-P exhibited good reusability in 5 cycles without much deterioration in its adsorption performances. Complex co-existing ions impaired Zn adsorption during real wastewater treatment. This research benefits agricultural waste recycling and provides safe water to ensure economic, social, and environmental sustainability.