[Comparison of the adsorption of arsenite and arsenate anions from aqueous solution by calcined Mg-Al layered double hydroxides].

The comparison of calcined Mg-Al layered double hydroxides (Mg-Al CLDH) for adsorption of arsenite and arsenate anions from aqueous solution was investigated by batch method. The results show that Mg-Al CLDH is an effective adsorbent for the removal of arsenite and arsenate anions. The adsorption processes are followed 'memory effect'. When the initial concentration of arsenic is lower than 10 mg x L(-1) for arsenite and 40 mg x L(-1) for arsenate, the equilibrium concentration of arsenic is lower than 10 microg x L(-1). In the range of 4-500 mg x L(-1) for initial arsenic concentration, the maximum adsorption capacity of arsenite and arsenate on Mg-Al CLDH from 298 to 323 K is 150.46-224.03 mg x g(-1) and 149.62-224.76 mg x g(-1), respectively, which are much larger than other adsorbents. But the adsorption capacity is not reached saturation, it is continued to increase significantly. L and H model can be used to describe the adsorption isotherm of arsenite and arsenate, respectively. The adsorption data are corresponded to the Freundlich model for arsenite and Langmuir model for arsenate at lower equilibrium concentration, but corresponded to Freundlich model for arsenite and arsenate at higher equilibrium concentration. Under the same temperature and initial concentration, the adsorption rate and removal rate of arsenate are higher than those of arsenite. And they are increased with temperate but decreased with increase in initial concentration. Adsorption processes follow the pseudo-second-order kinetic model for both arsenite and arsenate. The removal rate is not influenced by the initial pH (3.0-10.0). The adsorption capacity is influenced insignificantly by ionic strength. Arsenate anions are adsorbed firstly from arsenite and arsenate solution on Mg-Al CLDH.