Removal of Congo red dye from aqueous solution with hydroxyapatite/chitosan composite

Chitosan (CS) has marked potential in environmental adsorption applications, but typically suffers from low specific gravity and sensitivity to pH. In this work, a hydroxyapatite-CS (HAp-CS) composite was developed via embedding of HAp into CS and used for removal of Congo red (CR) dye from aqueous solution. Effects of different weight contents of CS in HAp-CS composite, adsorbent dosage, initial pH, contact time and the initial dye concentration on CR removal were studied in detail using batch adsorption at room temperature. The kinetic data were best described by the pseudo-second-order model (R2 > 0.9999), while thermodynamic investigation of CR adsorption by HAp-CS composite confirmed a spontaneous adsorption. Further investigations demonstrated that the adsorption pattern fitted well with Langmuir model (R2 > 0.97) but less-satisfied with Freundlich model (R2 < 0.94). FT-IR and X-ray photoelectron spectroscopy studies showed that possible pathways for CR adsorption may include surface complexation, ion exchange and hydrogen bonding. HAp-CS composite containing 50 wt% of CS exhibited adsorption capacity of 769 mg g−1 for CR (Langmuir model), which is much higher than that of pure CS, HA and other previously reported adsorbents.

► Hydroxyapatite–chitosan composite for adsorption of dyes is reported. ► The adsorption capacity of HAp-CS composite for Cango red removal is 769 mg g−1. ► The adsorption capacity of composite is much higher than previously reported adsorbents. ► The adsorption mechanism was studied with FT-IR and XPS. ► It can be useful in water purification for its easy shape and high adsorption capacity.