Selective Adsorption and Recovery of Silver from Acidic Solution Using Biomass-Derived Sulfur-Doped Porous Carbon

It is vital to recycle precious metals effectively such as silver from waste sources because of limited natural reserves. Herein, passion fruit (Passiflora edulis Sims) shell-derived S-doped porous carbons (SPCs) were newly synthesized by hydrothermal carbonization and following with activation by KOH/(NH₄)₂SO₄, and the adsorption of Ag⁺ on SPC under acidic solutions was investigated. It was found that the activator of (NH₄)₂SO₄ can not only introduce the doping of S elements but also increase the proportion of mesopores in the as-prepared SPC. As the active site, the increasing S doping can improve the adsorption of Ag⁺ on SPC. The kinetic data of Ag⁺ adsorption by SPC was consistent with the pseudo-second-order kinetic model. The Langmuir isothermal model was used to well fit the Ag⁺ adsorption isotherms of SPC, and the maximum adsorption capacity of the optimized SPC-3 for Ag⁺ is up to 115 mg/g in 0.5 mol/L HNO₃ solution. SPC-3 showed good selectivity toward Ag⁺ over diverse competing cations, which is mainly attributed to the strong bonding between Ag⁺ ions and the sulfur-containing functional groups on the surface of SPC-3 resulting in the formation of Ag₂S nanoparticles. The adsorbed Ag could be recovered as an elemental form by a simple calcination. This study provides a new insight into the design of an environmentally friendly and efficient adsorbent for the selective recovery of silver from acidic aqueous media.