Development of melamine-impregnated alginate capsule for selective recovery of Pd(II) from a binary metal solution

An “ionic barrier” concept is proposed to introduce selectivity function to adsorbents towards precious metals (PMs) in the present study. As a model system, melamine-impregnated alginate capsule (MIAC) was prepared and used as a Pd(II) selective adsorbent from a binary metal solution containing Pt (IV) and Pd(II). The MIAC exhibited excellent Pd(II) selectivity for a pH around 4.3, where the selectivity coefficient reached a high value of 2190.66. In comparison to the very low Pt (IV) uptake, a high maximum uptake of Pd(II) was estimated of 316.92 ± 9.50 mg/g by the Langmuir isotherm model. Sorption-desorption studies showed that the MIAC had a good reutilization property. The selective process for Pd(II) recovery was proposed as follows: the electroneutral Pd(OH)₂ but not anionic PtCl₅⁻first penetrate the outer ionic barrier; then, the penetrated Pd(OH)₂ was bound by the inner melamine through chelation. The ionic barrier-based sorbent thus can be considered as an alternative one for separation and recovery of Pd(II).