In Situ Synthesized Hydroxyapatite—Cellulose Nanofibrils as Biosorbents for Heavy Metal Ions Removal

The synthesis of hydroxyapatite (HAp) in the presence of cellulose nanofibrils (CNF) or TEMPO-oxidized CNF (TCNF) was performed in situ by wet chemical precipitation process, using chemical precursors, to hybrid a low-cost biosorbents for removal of Co2+ as a model heavy metal ion. The removal is investigated by batch adsorption method depending on the pH value, the dosage of adsorbent, initial Co2+ concentration and the contact time of adsorption. The removal of Co2+ reached the maximum (87%) at pH 6 by using a dosage of 0.5 g L−1. The TCNFs is shown to increase the nucleation and growth of the HAp synthesized, providing higher surface area (138 m2 g−1) with lower pore diameter (11.89 nm), compared to the CNFs based hybrid (135 m2 g−1, 14.42 nm) or pure HAp (118 m2 g−1, 13.32 nm), however, both resulting to a higher adsorption capacity (25 mg g−1) of Co2+ compared to HAp or TCNF (20–22 mg g−1). The adsorption is follows primarily by pseudo-first and Elovic order kinetic models which is due to the physisorption of Co2+ and surface ionic interactions with available negative phosphate (from HAp) and/or carboxylic (from TCNF) groups, fitting well with the Freundlich adsorption isotherm, and secondly due to the ion-exchange mechanism of Co2+ with Ca2+ from HAp. Both hybrid adsorbents show good adsorption (≥40%) capacity even after third reusing cycle, and high temperature stability (weight loss of 14%) up to 1000 °C, however, the CNF-HAp hybrid represent a high-value alternative to relatively costly TCNF.