Synthesis of a magnetic nanomaterial with core–shell structure for effective Au(III) recovery from solution with superior separation and one-step reduction

Abstract

To synthesize a novel magnetic nanomaterial for effective Au(III) recovery from the aqueous phase with excellent capability, superior selectivity, one-step reduction and ease of separation after loading Au(III) still remains a huge challenge. In this study, a magnetic nanomaterial (γ-Fe2O3@PDA-NAM) with a core–shell structure was synthesized for effective Au(III) recovery by sorption. The maximum Au(III) sorption capacity by γ-Fe2O3@PDA-NAM reached 2910 mg/g at pH = 6.10 ± 0.15, 44.29 times that by γ-Fe2O3 (65.7 mg/g). Its Au(III) sorption efficiency was over 97.75% at very low concentrations (i.e., 5.5 × 10-4 and 5.5 × 10-5 mg/mL). TEM, XPS and FTIR analysis verified that excellent Au(III) recovery by γ-Fe2O3@PDA-NAM resulted from N/S-containing soft base moieties introduced to different layers. N and/or S elements at these sites provided lone pair electrons to Au(III) to form stable coordination bond. Meanwhile, the hydrogen bond between O-containing (–OH, -C=O) and N-containing (-C-N, -N(-C)-N-, -C=N) moieties on γ-Fe2O3@PDA-NAM and –OH on the main speciations of Au(III) (AuCl3(OH)-, AuCl2(OH)2-, and AuCl(OH)3-) also greatly enhanced Au(III) recovery. About 78.03% of Au(III) was directly reduced to elemental gold by -C-S- and -N(-C)-N- group on the tertiary amine structure during sorption. More than 98.84% of Au(III) was selectively sorbed on γ-Fe2O3@PDA-NAM at pH 4.0 even though Pt(IV), Cu(II), Zn(II), Pb(II), and Ni(II) were co-present at the same concentration as that for Au(III) (0.01 mg/mL). In contrast, sorption efficiency of other interfering ions was within 1.75–26.05%. A relatively poor sorption selectivity for Pt(IV) could be acceptable because it is also a noble metal.