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·Al₂O₃·5-6H ₂O 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, Na ₂HPO₄ and CaHPO₄ and the triphosphate Na ₅P₃O₁₀. 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 Cu ²⁺ uptake ability of the (NH₄)₂HPO ₄-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.