Adsorptive properties of rod-shaped silver nanoparticles-functionalized biogenic hydroxyapatite for remediating methylene blue and congo red

Abstract

Periwinkle shelled-hydroxyapatite (HAP) was functionalized with Terminalia catappa-mediated silver nanoparticles (AgNPs) for effective methylene blue (MB) and congo red (CR) adsorption. The adsorbent, AgNPs-functionalized HAP, was characterized using scanning electron microscopy (SEM), energy dispersive X-ray (XRD) and Fourier Transform Infra-red (FTIR) spectroscopy. The adsorption process was significantly influenced by all batch adsorption parameters. The rod-shaped AgNPs-functionalized HAP with a 1.85 ± 0.84 µm thickness had a pH point of zero charge (pHpzc) of 7.09 and adsorbed congo red (CR) and methylene blue (MB) maximally at pH 2 and 11, respectively. Freundlich isotherm model accurately predicted the adsorption of both dyes. The monolayer adsorption capacities (qmax) ranged from 47.75 to 159.11 mgg−1 for CR and 1039.07 to 205.49 mgg−1 for MB, from 303 to 313 K. The kinetics of adsorption was best fitted to pseudo-second order while the rate-determining step was both controlled by adsorption and intramolecular diffusion. Adsorption process was spontaneous (-ΔG) with decreasing feasibility with increasing temperature, exothermic (-ΔH) with a decreased degree of randomness (-ΔS) and physical in nature. The considerably higher qmax of AgNPs-functionalized HAP implies its superiority over previously reported adsorbents for dyes. Hence, AgNPs-functionalized HAP is a more promising adsorbent for dye wastewater remediation.