Ultrasound assisted synthesis of Ca–Al hydrotalcite for U (VI) and Cr (VI) adsorption

Adsorption of chromium (VI) and uranium (VI) nitrate on Ca–Al hydrotalcite (Ca–Al LDHs) compounds is investigated. The powdered materials are synthesized in a modified co-precipitation method under the ultrasonic treatment with different conditions for Cr (VI) and U (VI) adsorption. Factors that affect the compounds synthesis such as temperature, reaction time, calcination and aging conditions are reported. The effects of various parameters on adsorption process including adsorption/desorption kinetics, isotherms and influencing factors such as contact time, solution pH and effect of competing ions are studied. It is found that Ca–Al LDHs has the largest capacity for both Cr (VI) and U (VI), adsorbing 104.82 ± 0.02 mg/g and 54.79 ± 0.02 mg/g, and highest rate of adsorption up to 98.78 ± 0.02% and 90.28 ± 0.02% respectively. Cr (VI) and U (VI) adsorption decreased obviously with pH levels outside the range of 7–11. Adsorption/desorption kinetics of Cr (VI) and U (VI) shows that desorption K values of U (VI) are larger than Cr (VI) while adsorption K values U (VI) are smaller than Cr (VI), suggesting that Ca–Al LDHs efficiency for the removal of U (VI) is fast adsorption and slow release compare to Cr (VI). The effect of competing ions such as nitrate radical and chloridion was negligible.