Green synthesis of sulfonated hydroxyapatite incorporated carbonaceous composite (C-aHAp-S) for efficient adsorptive separation of lead in water: an experimental and theoretical approach

Abstract

A facile in-situ co-precipitation strategy was devised to produce novel sulfonated aluminium substituted hydroxyapatite nanoparticle (designated as aHAp-S) immobilized macro-scale granular charcoal composite (referred as C-aHAp-S). The green synthesis strategy (water used as reaction medium) involving minimal energy and non-toxic nature of the products enhance the scalability and sustainability of the material. The developed aHAp-S showed higher specific surface area (S_BET = 120.36 m²/g) and pore volume (v_p = 0.20 cm³/g) relative to the unmodified counterpart, aHAp (S_BET = 81.91 m²/g, v_p = 0.14 cm³/g). High S_BET of C-aHAp-S (310.56 m²/g) facilitated enhanced pore diffusion of lead ions to be adsorbed on the active sites inside the carbon matrix. aHAp-S showed enhanced maximum Pb uptake capacity (Q_L) in aqueous phase (Q_L = 1282.0 mg/g at 30 °C using Langmuir model) than aHAp (Q_L = 823.0 mg/g). C-aHAp-S demonstrated Q_L of 200.0 mg/g. High activation energy (E_a > 20 kJ/mol) indicated chemisorption of Pb. A fundamental pore diffusion-adsorption model was utilized for analyzing the kinetics of the batch process along with the conventional pseudo-first order and pseudo-second order models. The strong selectivity and credible reusability (5 cycles) was also highlighted. Fixed bed columns with C-aHAp-S packing were used to elucidate an effective continuous mode Pb adsorption performance with breakthrough volume as high as 860 times the fixed bed packing volume. An appropriate diffusion-convection-adsorption based transport model was employed to generate simulated concentration profiles and analyze the breakthrough trends for column studies which were subsequently used to predict the long-term performance.