Abstract: Mg-Al-HT (Hydrotalcite) and Mg-Fe-HTLcs (Hydrotalcie-like compounds) were used as precursor. Calcined at 550℃ and 380℃ for 2h, they were transformed into Mg-Al-LDO (Layered Double Oxide) and Mg-Fe-LDO respectively. Both of them have strong tendency of obtaining anion and water from ambient to recover their primary crystal structure of HT and HTLcs. At room temperature, arsenous absorption capacity is 83.2mg/g for Mg-Al-LDO, and 87.45mg/g for Mg-Fe-LDO. It is found that pH values increase while arsenous anion is absorbed by Mg-Al-LDO, which is predicted by the equation that describes the chemical reaction between Mg-Al-LDO and arsenous solution. During the reaction of Mg-Fe-LDO with the same solution, pH ascends first and then descends. It is believed that there is another reaction besides arsenous absorbed by Mg-Fe-LDO, i.e., arsenous anion will be deposited by Fe(III) in the LDO. Theoretic absorption capability of the LDOs on arsenous anion calculated from the chemical equation is 219.3mg/g for Mg-Al-LDO and 187.5mg/g for Mg-Fe-LDO, much higher than their real performance at room temperature. In believing that it is due to the competition from dissolved CO₂ with arsenous anion, the system is heated to 90℃ to expel
CO2 from the solution. Absorption capability on arsenous anion at 90℃ is 114.9mg/g for Mg-Al-LDO, and 199mg/g for Mg-Fe-LDO. Heating has double effects for the later in the system: the competition from dissolved CO₂ is eliminated, and the deposition of arsenous by Fe(III) in LDO is enhanced.

Key words: Mg-Al-LDO, Mg-Fe-LDO, arsenous anion, adsorption capability