Influence of alginate fixation on the adsorption capacity of hydroxyapatite nanocrystals to Cu2+ ions

Nano-hydroxyapatite (HAP) can be used as an environmentally friendly adsorbent of heavy metal ions. However, the agglomeration and precipitation of nano-HAP will result in the loss of adsorption capacity to heavy metal ions. Herein, the stable nano-HAP suspension was synthesized with the stabilization effect of polyacrylic acid and further fixed by alginate to form nano-HAP-alginate composite. The effect of alginate fixation on the adsorption capacity of nano-HAP to Cu2+ ions was studied. Results showed that the adsorption capacity to Cu2+ ions of alginate fixed nano-HAP was depended on the mass ratio of nano-HAP to alginate. With the increase of mass ratio of nano-HAP to alginate from 5: 95 to 95: 5, the displayed adsorption capacity of nano-HAP to Cu2+ ions was gradually decreased due to the increasing agglomeration of nano-HAP. Especially, while the mass ratio of nano-HAP to alginate was at 5: 95-10: 90, the nano-HAP was fixed in alginate with single-particle distribution, showing adsorption capacity comparable to that of nano-HAP suspension (60.99 mg g(-1)). The adsorption of Cu2+ on nano-HAP was attributed to Langmuir isotherm model (monolayer sorption). After fixation by alginate, the resulting nano-HAP-alginate (50:50) composite showed the adsorption to Cu2+ followed both the Langmuir and Freundlich isotherm models, indicating the multilayer sorption of Cu2+ on heterogeneous surface. Accordingly, the alginate fixation is an alternative way for nano-HAP as adsorbent of heavy metal ions.