Eco-friendly synthesis of rod-like hydroxyapatite on spherical carbon: A dual-function composite for selective cobalt removal and enhanced oxygen evolution reaction.

The presence of cobalt ions (Co) and radionuclides (Co) in industrial and radioactive effluents pose serious threats to environmental ecosystems and human health. This paper presents the synthesis of dual-functional hydroxyapatite (HAp)-incorporated spherical carbon (SC) composite (HAp/SC) towards the selective adsorption of cobalt from wastewater and the utilization of the Co-adsorbed HAp/SC composite (Co- HAp/SC) as an electrocatalyst for the oxygen evolution reaction (OER). Herein, we prepared a series of HAp/SC composites by varying HAp weight percentages of 10 %, 20 %, 30 %, 40 %, and 50 %. Among the prepared composites, 20 wt% HAp/SC exhibited the highest Co adsorption capacity of 111.03 mg g⁻ which was higher than those of individual HAp and SC. The excellent Co adsorption performance of 20 wt% HAp/SC composite might be due to the synergistic effects of phosphate groups in HAp, which selectively capture Co, along with large number of surface -OH and -COOH functional groups of SC through electrostatic, ion-exchange, and surface complexation mechanisms. Batch adsorption experimental data fit well with the Langmuir model (R = 0.97) suggesting monolayer adsorption of Co onto the adsorption sites of HAp/SC. Also, the 20 wt% HAp/SC composite exhibited rapid Co adsorption kinetics and effectively describing the pseudo-first-order model (R = 0.97) with a rate constant (k) of 0.14893 min. Additionally, the Co-HAp/SC composite demonstrates potential as an electrocatalyst for the oxygen evolution reaction (OER), exhibiting an overpotential of 380 mV and a Tafel slope of 39.3 mV dec. This dual functionality suggests the HAp/SC composite for the cobalt removal, with the resulting product serving as an electrocatalyst for OER.