Composite of magnetite and Zn/Al layered double hydroxide as a magnetically separable adsorbent for effective removal of humic acid

Abstract

In the present study, we synthesized a magnetically separable adsorbent, i.e., composite of magnetite and Zn/Al layered double hydroxide (Fe3O4−Zn/Al LDH), for effective removal of humic acid (HA). The composite was synthesized through conventional co-precipitation method using NaOH as precipitating agent. The presence of well-dispersed Fe3O4 outside the interlayer space of Zn/Al LDH made the Fe3O4−Zn/Al LDH magnetically active and thus the adsorbent was easily separated from aqueous solution by application of an external magnetic field. The adsorption test on HA demonstrated that Fe3O4−Zn/Al LDH was optimum in removing HA at pH 4.0 with removal capacity estimated according to the Langmuir isotherm model was 1.21 × 10−4 mmol g−1 or equivalent to 35.28 mg g−1, while the removal rate constant according to the pseudo-second order kinetic model was 443.6 (mol/g)−1 min−1. The results strongly suggest that the removal of HA on Fe3O4–Zn/Al LDH occurs solely through simple adsorption on the outer surface through an electrostatic interaction. In addition, when the material was tested in a real raw water sample from a drinking water treatment agency in Pontianak City, West Kalimantan, Indonesia, the performance was far surpassing commercially available alum.