Macromolecular humic acid modified nano-hydroxyapatite for simultaneous removal of Cu(II) and methylene blue from aqueous solution: Experimental design and adsorption study

Abstract

Humic acid (HA) is well-known for its chelating activity towards various contaminants, but it dissolves in water thus cannot be used alone as an adsorbent. By immobilizing HA to the surface of nano-hydroxyapatite (nHAP), a novel HA-nHAP adsorbent was prepared. The composition and surface morphology of the obtained HA-nHAP composite were studied in detail. Adsorptive removal of Cu(II) and Methylene blue (MB) using HA-nHAP in single and binary systems were investigated. Results indicated that HA-nHAP exhibited efficient removal of Cu(II) and MB with a favorable adsorption at higher pH. The mechanisms involved in adsorption of Cu(II) and MB were electrostatic interaction and surface complexation. Optimization study using central composite design (CCD) based response surface methodology (RSM) was performed and 3-D response surfaces were plotted from the mathematical model. The optimum conditions were found to be 2.2 g/L (adsorbent dosage), 8 h (contact time), 25 mg/L (initial Cu(II) concentration), and 85 mg/L (initial MB concentration). At optimum conditions, removal rates of Cu(II) and MB were 97.68% and 100%, respectively. The adsorption kinetics and equilibrium fitted well with Elovich and Langmuir models, respectively. Desorption studies confirmed that HA-nHAP adsorbent could be effectively regenerated and reused.