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^{2 +}) and radionuclides (⁶⁰Co) 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 Co²⁺-adsorbed HAp/SC composite (Co²⁺- 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 Co²⁺ adsorption capacity of 111.03 mg g⁻¹ which was higher than those of individual HAp and SC. The excellent Co²⁺ adsorption performance of 20 wt% HAp/SC composite might be due to the synergistic effects of phosphate groups in HAp, which selectively capture Co²⁺, 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 (R² = 0.97) suggesting monolayer adsorption of Co²⁺ onto the adsorption sites of HAp/SC. Also, the 20 wt% HAp/SC composite exhibited rapid Co²⁺ adsorption kinetics and effectively describing the pseudo-first-order model (R² = 0.97) with a rate constant (k₁) of 0.14893 min⁻¹. Additionally, the Co²⁺-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⁻¹. This dual functionality suggests the HAp/SC composite for the cobalt removal, with the resulting product serving as an electrocatalyst for OER.