Adsorption removal of organic phosphonate HEDP by magnetic composite doped with different rare earth elements

Abstract

Rare-earth-based magnetic adsorbents were prepared by co-precipitation of europium (Eu), lanthanum (La) and cerium (Ce) with Fe2+/Fe3+ and powdered activated carbon (PAC), respectively. The obtained magnetic adsorbents Fe/Eu@PAC, Fe/La@PAC and Fe/Ce@PAC were used to remove organic phosphonate 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) from reverse osmosis (RO) concentrates. The maximum adsorption capacities of Fe/Eu@PAC, Fe/La@PAC and Fe/Ce@PAC composites were 27.45, 16.58 and 15.99 mg-P/g at 298 K and pH=8.0, respectively. Fe/Eu@PAC composite with the highest adsorption capacity could completely remove HEDP from the solution at 1 g/L dosage. In addition, its adsorption capacity reached 80% of the equilibrium adsorption capacity at 30 min and 298 K, indicating a fast adsorption rate. The difference in adsorption performance of the three adsorbents was mainly due to the difference in zero charge point (pHpzc) and hydroxyl content. Both the pHpzc value and the hydroxyl content increased in the order of Fe/Eu@PAC>Fe/La@PAC>Fe/Ce@PAC. A higher pHpzc value was beneficial to improve the electrostatic interaction. Furthermore, the higher hydroxyl content was also conducive to improving the role of ligand exchange and hydrogen bonding. XPS analysis showed that the ligand exchange capacity of Eu 3d3/2 and La 3d3/2 was stronger than that of Eu 3d5/2 and La 3d5/2. In addition, the size of the ionic radius was not the main factor that affects the ligand exchange capacity of the rare-earth-based adsorbent. The results of this study suggest that magnetic Fe/Eu@PAC composites can be used as a promising high-performance adsorbent for the removal of phosphonates from water.

Keywords

• Rare earth composite materials
• Adsorption
• Organic phosphonate
• HEDP