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. (C) 2012 Elsevier B.V. All rights reserved.