Preparation of Thiocyanate-Retaining Tannin Gel for Adsorption Recovery of Palladium from Strong Hydrochloric Acid Solutions

Abstract

A tannin gel (TG) synthesized from condensed tannin molecules has a remarkable ability to adsorb various metal ions in aqueous solutions. In the present study, we have introduced thiocyanate ions (SCN À ) into the TG by adsorption process in acidic thiocyanate solutions to improve its adsorbability for palladium ion, Pd(II). To determine the optimum conditions for preparing the SCN À -retaining tannin gel (SCN-TG), we have investigated the introduction mechanism of SCN À into the TG at 298 K and HCl concentrations less than or equal to 1 M, where the decomposition of SCN À can be negligible. The adsorption amount of SCN À onto the TG increased with increasing HCl concentration, indicating that SCN À is adsorbed onto the TG through the electrostatic attraction between anionic SCN À and the protonated hydroxy groups in the TG at higher HCl concentrations. Using the resultant SCN-TG and the TG, we have conducted the Pd(II) adsorption experiment at molar-level concentrations of HCl and found that the SCN-TG can adsorb Pd(II) efficiently, whereas the TG adsorbs almost no Pd(II). Besides, the adsorbability of the SCN-TG for Pd(II) decreased only slightly with increasing HCl concentration up to at least 5 M.

Improved adsorption and separation of palladium(II) and platinum(IV) in strong hydrochloric acid solutions using thiocyanate‐retaining tannin gel

“…One possible problem of the SCN‐TG is that SCN − may be released from the gel to the solution during the metal ion adsorption, because it is retained in the gel only by the electrostatic attraction. In the previous study,41 however, we confirmed that the amount of SCN − released from the SCN‐TG is negligible small during the metal ion adsorption in strong HCl solutions.…”

“…The SCN‐TG was prepared by soaking the TG particles in an acidic thiocyanate solution. Previously, we investigated the introduction mechanism of SCN − into the TG in detail to determine the optimum procedure for preparing the SCN‐TG and found that SCN − can be adsorbed onto the TG through the electrostatic attraction with the protonated hydroxy groups of the TG under acidic conditions 41. From the results obtained in this previous study, we determined the optimum procedure for the SCN‐TG preparation as follows.…”

“…The TG particles were added to a vial containing an acidic thiocyanate solution at 2 g‐gel/L on a dry basis, where the concentrations of SCN − and HCl were [SCN − ] = 0.01 M and [HCl] = 1 M , respectively. The vial was sealed and then placed in a thermostatic shaker with a sufficient shaking speed at 298 K for 72 h: under this condition, the loading amount of SCN − was 0.6 mmol/g‐dry gel 41…”

“…All adsorption experiments in the present study were carried out in a batch system at a specific temperature, and the SCN‐TG was used immediately after SCN − loading without drying: once the SCN‐TG was dried, its adsorbability decreased because of the precipitation of a thiocyanate salt on the gel surface 41. For the adsorption experiments in the single metal ion systems, the TG or the SCN‐TG particles were added to HCl solutions containing Pd(II) or Pt(IV) at 1 g‐gel/L, where the initial metal ion concentration, [Pd] ini or [Pt] ini , was 0.0001 M .…”

“…This suggests that if SCN − can be introduced into the TG in some way, precious metal ions could be adsorbed even in such strong HCl solutions through the ligand‐substitution reaction of metal ions with SCN − in the gel. Based on this idea, we prepared the SCN − ‐retaining tannin gel (SCN‐TG) by adsorption of SCN − onto the TG in acidic thiocyanate solutions and then demonstrated that the resultant SCN‐TG can adsorb Pd(II) efficiently at molar‐level concentrations of HCl 41…”