Synergistic effect of humic and fulvic acids on Ni removal by the calcined Mg/Al layered double hydroxide

Abstract

Discharge of Ni(II) containing wastewater to terrestrial environment can lead to serious pollution issues. This study presents a facile, cost-effective and highly efficient method to rapidly remove Ni(II) from wastewater using a calcined Mg/Al layered double hydroxide (LDH), which is a pioneering investigation of using the calcined LDH to remove the positively charged Ni(II) ions. The kinetic studies showed that the removal of Ni(II) was very rapid with a pseudo first order k of up to 3.6 ± 0.5 h⁻¹, and maximum removal of Ni(II) was up to 204.2 ± 10.6 mg g⁻¹. In addition, our results reveal that both humic (HA) and fulvic acids (FA) can strongly bind to the surface of the LDH, which demonstrates that the calcined LDH can effectively remove both cationic and anionic pollutants in a single process. With the increase of pH up to 7.5, the removal of Ni, HA and FA by the calcined LDH are all drastically enhanced. Our study also demonstrated a strong synergetic effect of FA and HA on the Ni(II) removal by the calcined LDH, with a maximum Ni(II) removal of 290.6 ± 16.4 mg g⁻¹ and 480.4 ± 21.3 mg g⁻¹ at pH 7, respectively. The possible mechanism of the Ni(II) removal can be due to the incorporation of Ni into the layered structure by replacing the leached Mg in the layers of the LDH during the process of rehydration of the calcined LDH. The HA or FA bound on the surface of the rehydrated LDH can induce an additional Ni(II)-HA or -FA complexation consequently leading to the enhancement of the Ni removal. The trial of treating a high Ni-containing wastewater in practice indicates that the calcined LDH is a promising material for metal containing wastewater treatment.