Soft Ion Divalent Metals toward Adsorption on Zn/Al-POM Layered Double Hydroxide

Development of Zn/Al layered double hydroxide by intercalation using polyoxometalate (POM) $K_4 [α-SiW_12O_40]$ to Zn/Al-POM was investigated. The success of the modification is evidenced by the XRD, FT-IR, and BET characterization data. XRD characterization showed an increase in the interlayer distance from 8.59 Å in Zn/Al LDHs to 10.26 Å in Zn/Al-POM. This success is also supported by the FT-IR data with the appearance of vibrations around $779–979 cm^{-1}$ which indicates the vibration of the polyoxometalate compound in Zn/Al-POM. Other supporting data in the form of BET also prove an increase in surface area from 1.968 $m^2$/g in Zn/Al LDHs to 14.042 $m^2$/g Zn/Al-POM. The ability of Zn/Al-POM as an adsorbent is proven through several parameters such as kinetics, isotherm, thermodynamics, and regeneration for $Cd^{2+}$, $Pb^{2+}$, $Ni^{2+}$, and $Co^{2+}$. Adsorption kinetics showed that Zn/Al-POM was more likely to follow the pseudo-second-order adsorption kinetics model for $Cd^{2+}$, $Pb^{2+}$, $Ni^{2+}$, and $Co^{2+}$. The results of determining the adsorption isotherm parameters of Zn/Al-POM tend to follow the Freundlich isotherm model with a maximum adsorption capacity of 74.13 mg/g on $Pb^{2+}$. The regeneration process showed that Zn/Al-POM was more resistant than Zn/Al LDHs up to 3 cycles. It was proven that Zn/Al-POM was able to survive in the last cycle up to 69.19% on $Ni^{2+}$.