Novel reusable hydrogel adsorbents for precious metal recycle

A novel polyethylene glycol diacrylate-allylthiourea (ATU-PEGDA) hydrogel was simply synthesized through photo-reaction. Modified thiourea simultaneously employed chelation and electrostatic force to selectively recycle Ag(I) and Pd(II) from electrolytic wastewater. Sorption efficiency was nearly 100% for Ag(I) and Pd(II), which occurred at initial pH of 1 within 300 min. The adsorption characteristics of ATU-PEGDA followed Langmuir isotherm model and the maximum adsorption capacity of Ag(I) and Pd(II) achieved 83.33 and 152.81 mg g^-1 sorbent, respectively where Pseudo-first order model demonstrate the adsorption kinetics. In the presence of other heavy metals, ATU-PEGDA performed high selectivity, 0.89 and 1.31 towards Ag(I) and Pd(II). ATU-PEGDA can be completely regenerated within 120 min using 0.5 M thiourea-0.001 M HNO₃ and 1 M thiourea-4 M HCl after the adsorption of Ag(I) and Pd(II), respectively. Thiourea-branched structure was created after regeneration, improving the adsorption capacity. Compared to initial hydrogel, the adsorption capacity of Ag(I) and Pd(II) increased 31.83 ± 3.08% and 75.12 ± 11.02%, respectively. Over 10 consecutive adsorption-desorption cycles, ATU-PEGDA performed 111.34 and 263.79 mg g^-1 sorbent in adsorption capacity of Ag(I) and Pd(II). Chromism of ATU-PEGDA hydrogel was a benefit to determine adsorption saturation and completely desorption of Ag(I) and Pd(II). Potentially, ATU-PEGDA can be extended to industrial applications.