Remediation of chromium-contaminated soil using calcined layered double hydroxides containing different divalent metals: Temperatures and mechanism

MgAl and CaAl were calcined at different temperatures and applied to the immobilization of Cr(VI) in the soil. The maximum adsorption capacity of Mg-500 degrees C and Ca-900 degrees C was 13.89 mg/g and 33.78 mg/g, respectively. TCLP and phytotoxicity experiments showed that Mg-500 degrees C and Ca-900 degrees C could immobilize Cr(VI) stably in the soil, prevent the biological transfer of Cr(VI) from soil to plants. XRD, SEM, FTIR, and other characterizations were utilized to analyze the sorption mechanism. The adsorption mechansim of MgAl and Mg-300 degrees C are mainly interlayer anion exchange and surface adsorption. But for the Mg-500 degrees C, memory effect is predominant. Since the main structure of Mg-900 degrees C is mainly spinel. After long-term adsorption, only a small part returns to the original layered structure of LDHs. So its adsorption performance is poor. Immobilization of Cr(VI) by CaAl is mainly through interlayer anion exchange and surface adsorption. After calcination, the main structure of Ca-300 degrees C is Ca4Al2O3(OH)(6)center dot(NO2)(2), which can regeneration into LDHs during adsorption. But part of it dissolved and combined with CO32- to form CaCO3 leading to a poor performance. Main form of Ca-500 degrees C is CaO and Ca (OH)(2), which dissolves to produce Ca2+ during adsorption and mainly produces CaCO3 after reaction, so the adsorption of Cr (VI) was also poor. Differently from others, two stages were included in the immobilization of by Ca-900 degrees C. In the first, Cr(VI) were adsorbed on the alumina gel resulted from the hydration of calcined products. In this stage, Cr(VI) can be attracted as an interlayer anion into the new forming LDHs in the following stage and then immobilized into the interlayer of LDHs.