Removal of Cr(VI) from aqueous media on calcined (Mg-Zn)/(Al-Fe)-(CO3)/Cl layered double hydroxides

The three newly reported calcined layered double hydroxides (CLDHs) coded CLDH10, CLDH21 and CLDH22 were synthesized by co-precipitation method. The characterization of CLDHs before and after loading with CrO42– with Fourier transform infrared (FTIR), scanning electron microscope (SEM), electron dispersive X-ray analysis (EDX) and flame atomic absorption spectrometry (FAAS) showed a typical hydrotalcite structure and confirmed the loading of Cr(VI). The sorption process of Cr(VI) as chromate was investigated onto these CLDHs using batch method. The sorption parameters; initial pH, shaking time, sorbent dose, initial concentration of chromate and temperature were studied. The optimum conditions for Cr(VI) sorption were obtained at initial pH 7.1, sorbent dose 25 mg and shaking time 3 h. The complete removal of 50 mg L–1 of chromate (>99%) was achieved at sorbent dosage ≥ 50 mg (0.2% w/v) of CLDH sorbents. The sorption process was suggested to follow the pseudo-second-order kinetics and Langmuir-type isotherm with monolayer capacities 40.0, 38.5 and 52.6 mg g–1, onto CLDH10, CLDH21 and CLDH22, respectively. The measured thermodynamic parameters ∆Gads, ∆Hads and ∆Sads indicated that the sorption process was spontaneous and endothermic in nature. The loaded chromate was best recovered from CLDH sorbents using 1.0 mol L–1 Na2CO3. CLDH22 showed best sorption capacity (45.7 mmol/100 g) with possible magnetic separation. It was applied successfully for the removal of Cr(VI and III)from standard reference material, tanning effluents,leather scraps and cement extract with removal efficiencies of 88.5–100%.