Sorption/desorption of arsenate on/from Mg-Al layered double hydroxides: influence of phosphate.

We have studied: (i) the sorption of arsenate on Mg-Al layered double hydroxides (LDHs) containing chloride (LDH-Cl) or carbonate (LDH-CO(3)) in the absence or presence of phosphate; (ii) the competitive sorption of arsenate and phosphate as affected by reaction time and pH and; (iii) the desorption of arsenate previously sorbed on the LDH by phosphate. The LDH samples were uncalcined (LDH-Cl-20 and LDH-CO(3)-20) or calcined at 450 degrees C (LDH-Cl-450 and LDH-CO(3)-450). More phosphate than arsenate was sorbed onto all the minerals but LDH-Cl-450 sorbed much lower amounts of both the ligands than LDH-Cl-20; vice versa LDH-CO(3)-450 showed a capacity to sorb arsenate and phosphate much greater than LDH-CO(3)-20. XRD analysis showed that arsenate was included into the layer spaces of LDH-Cl-20, but not in those of LDH-CO(3)-20. Competition in sorption between arsenate and phosphate was affected by pH, reaction time, surface coverage and sequence of addition of the ligands. Phosphate showed a greater affinity for LDHs than arsenate. The final arsenate sorbed/phosphate sorbed molar ratio increased with reaction time or by adding arsenate before phosphate, but decreased by increasing pH and by adding phosphate before arsenate. The effect of reaction time on the desorption of arsenate by phosphate was also studied.