Cobalt removal capacity on hydroxyapatite-zeolite composite upcycled from blast furnace slag

Abstract

Upcycling industrial by-products into water treatment materials is a promising strategy for simultaneously achieving resource circularity and water purification. In this study, a hydroxyapatite-zeolite (HAp-ZE) composite upcycled from blast furnace slag, an abundant industrial by-product, was investigated as a sorbent for toxic cobalt ions (Co²⁺). HAp-ZE removed Co²⁺ from the solution within a short contact time of 1 h and achieved a removal efficiency of approximately 100 % at Co²⁺ concentrations of up to 0.4 mmol/L. The sorption isotherm of HAp-ZE fitted well with the Langmuir model. Furthermore, the maximum Co²⁺ sorption capacity of HAp-ZE, estimated from the sorption isotherm, was comparable to that of a physical mixture of high-purity zeolite and HAp. Sodium ions (Na⁺) and calcium ions (Ca²⁺) were released from HAp-ZE during the Co²⁺ sorption tests. Elemental analysis revealed that Co was distributed in both the zeolite and HAp phases of HAp-ZE. These results suggest that Co²⁺ removal occurs through cation-exchange reactions in both the zeolite and HAp phases of HAp-ZE. This study demonstrates that the low-cost HAp-ZE produced from blast furnace slag exhibits a Co²⁺ removal performance comparable to that of its high-purity counterpart, highlighting its potential as a sustainable and economically attractive water purification material.