Effect on uptake of heavy metal ions by phosphate grafting of allophane

The effect of phosphate grafting on the uptake of various heavy metal ions was investigated using allophane, a clay mineral having a chemical composition of 1-2SiO2⋅Al2O3⋅5-6H2O with hollow nanoparticles 3.5-5 nm in size. Phosphate grafting was performed on allophane with an atomic ratio Si/Al = 0.85 using the orthophosphates (NH4)2HPO4, Na2HPO4 and CaHPO4 and the triphosphate Na5P3O10. The cations in the phosphates were partially retained in the samples, along with the grafted phosphate. Uptake experiments were performed with various heavy metal ions (Cu2+, Zn2+, Ni2+, Co2+, and Mn2+) and alkaline earth ions (Mg2+ and Ca2+) using a batch method. With increasing amounts of grafted phosphate, the Cu2+ uptake capacity increases to about double of that of ungrafted allophane. Although part of the grafted phosphate is released during the uptake experiments, this can be largely suppressed by heat-treating the samples at 200-500 °C. The Cu2+ uptake ability of the (NH4)2HPO4-grafted sample showed a steep decrease with higher heating temperature according to the thermal decomposition of NH+4. The ratios of cations released from the samples to cations removed from solution (2Na/Cu and Ca/Cu) are close to unity. From these results, the replacement reaction is thought to be the main uptake mechanism of heavy metal ions in the present samples. The uptake abilities of the present samples for various cations are explained with respect to the solubilities of the corresponding metal phosphates.