Immobilization and coordination chemistry of Friedel’s salt (Ca/Al-LDHs) on heavy metals removal

Friedel’s salt (F’s salt, FS, Ca/Al-LDHs) was prepared via coprecipitation synthesis to remove Pb(II) and Zn(II). The adsorption of F’s salt on Pb(II) and Zn(II) was a physical adsorption process and could be well described by pseudo-second order kinetics. The adsorption process is mainly completed by surface adsorption of pore structure and interlayer structure. In addition, the crystal microstructure parameter, molecular structure and elemental spatial distribution analysis results demonstrated that Pb(II) and Zn(II) could form new solid solutions such as Pb/CaAl-Cl-LDHs, CaZn/Al-Cl-LDHs and Zn/CaAl-Cl-LDHs via lattice replacement, and then effectively immobilized in F’s salt crystals. Unlike Pb(II), there are two fixed lattice substitution pathways for Zn(II), that is, Ca(II) and Al(III) can be used as replacement objects. Notably, the chemical bonding mechanism is more prominent for the solidification of Pb(II) than that of Zn(II) in F’s salt. Moreover, the superficial coordination chemistry analysis results revealed that the abundant OH groups distributed in the main layer laminate structure can also serve as the binding sites for Pb(II) and Zn(II). This study provides a theoretical basis and novel insights into the immobilization mechanism of Pb(II) and Zn(II) in F’s salt.