Preparation of a MgAl-layered double oxide@polypyrrole composite for efficient and selective Hg(Ⅱ) removal from aqueous solutions

In this study, a MgAl-layered double oxide@polypyrrole composite (MgAl-LDO@PPy) was facilely prepared by in-situ calcination and polymerization, and used for Hg(Ⅱ) adsorption from aqueous solutions. The synthesized adsorbent was characterized by FTIR, XRD, SEM, TEM, BET, and XPS, verifying that MgAl-LDO@PPy with abundant functional groups and large specific surface area was successfully prepared. The crucial parameters affecting the adsorption performance were investigated and optimized by batch adsorption experiments and response surface methodology, including solution pH, adsorption temperature, initial Hg(Ⅱ) concentration, and adsorbent dosage. The results showed that the adsorption of Hg(Ⅱ) by MgAl-LDO@PPy was highly pH-dependent, and the predicted maximum adsorption capacity by central composite design was 352 mg/g at pH = 7.5, temperature = 45 ℃, initial concentration = 45 mg/L, and dosage = 0.05 g/L. The adsorption process was spontaneous and endothermic, and well described by the pseudo-second-order kinetic model and the Redlich-Peterson isotherm model. In binary heavy metal ion water system and actual water/wastewater system, MgAl-LDO@PPy presented a highly selective adsorption capability for Hg(Ⅱ), due to the great affinity provided by its abundant nitrogen/oxygen-containing groups. As revealed by FTIR and XPS analyses, adsorption mechanism was mainly attributed to the electrostatic attraction and surface complexation. After saturated adsorption, the exhausted MgAl-LDO@PPy was easily regenerated by HCl solution, and successfully reused in five adsorption-desorption cycles. In summary, with superior adsorption capacity, high selectivity, and satisfactory reusability, MgAl-LDO@PPy is expected to be an ideal material for Hg(Ⅱ) removal in the future. [Display omitted] •A MgAl-layered double oxide@polypyrrole composite adsorbent was facilely prepared.•MgAl-LDO@PPy presented a highly selective adsorption capability for Hg(Ⅱ).•The crucial adsorption conditions were optimized by response surface methodology.•Adsorption mechanism included surface complexation and electrostatic attraction.•The exhausted MgAl-LDO@PPy was effectively and easily regenerated by HCl solution.