Removal of anions from solution by calcined hydrotalcite and regeneration of used sorbent in repeated calcination-rehydration-anion exchange processes

Synthetic hydrotalcite calcined at 350-550 degrees C was used for the removal of arsenate, chromate, and vanadate ions from water solutions. The initial anion concentrations were 0.002 mol. l(-1). The sorption isotherms were measured at 20 degrees C and at neutral pH. The Langmuir adsorption isotherm was used for the sorption capacity evaluation. The ability of the calcined hydrotalcite to remove the anions from solution decreased in the order of vanadate - arsenate - chromate. The hydrotalcite calcined at 450 degrees C exhibited the best sorption ability for all the anions. The sorbed anions were released by anion exchange in a carbonate-containing solution and the hydrotalcite after subsequent calcination was used again for the removal of anions. The repeating cycles calcination-rehydration-anion exchange gradually reduced the adsorption capacity of the hydrotalcite. The sorption capacity decreased by 50% after the first two cycles but it did not change significantly in the subsequent cycles. When chromate anions were adsorbed, the decrease in sorption capacity was not observed during repeated calcination-rehydration-anion exchange cycles. The change in the sorption capacity was influenced by the ability of calcined hydrotalcite to regenerate the layered crystal structure during the rehydration process.