Heterogeneous activation of persulfate by reduced graphene oxide–elemental silver/magnetite nanohybrids for the oxidative degradation of pharmaceuticals and endocrine disrupting compounds in water

Reduced graphene oxide hybridized with zero-valent silver and magnetite nanoparticles (NPs) (rGO-Ag⁰/Fe₃O₄ nanohybrids) prepared via in situ nucleation and crystallization was used to activate peroxydisulfate (PDS) for degradation of pharmaceuticals and endocrine disrupting compounds (phenol, acetaminophen, ibuprofen, naproxen, bisphenol A, 17β-estradiol, and 17α-ethinyl estradiol). The deposition of Ag⁰ and Fe₃O₄ in rGO nanosheet enhanced the catalytic removal of phenol in the heterogeneous activation of PDS. The adsorption capacities of rGO-Ag⁰/Fe₃O₄ for 10 μM phenol were 1.76, 1.33, and 2.04 μmol g⁻¹-adsorbent at pH 4, 7, and 10, respectively, which are much higher than those of single NPs studied (Ag⁰, nanoscale zero-valent iron, and rGO). The rGO-Ag⁰/Fe₃O₄ effectively activated PDS to produce strong oxidizing SO₄[rad]⁻ and facilitate an electron transfer on the surface of the nanohybrid. The initial pseudo-first-order rate (k_ini) constant for phenol degradation in PDS/rGO-Ag⁰/Fe₃O₄ system was 0.46 h⁻¹ at pH 7, which is approximately eight times higher than that in the presence of single NPs (k_ini = 0.04-0.06 h⁻¹) due to the synergistic effects between adsorption and catalytic oxidation. Among various organic contaminants tested, the simultaneous use of rGO-Ag⁰/Fe₃O₄ (0.1 g/L) and PDS (1 mM) achieved more than 99% degradation of acetaminophen and 17β-estradiol at pH 7. The radical scavenging studies with methanol and natural organic matter indicated that phenol was more likely to be degraded via free SO₄[rad]⁻ and [rad]OH formation or a non-radical oxidative pathway. Our findings indicate that the rGO-Ag⁰/Fe₃O₄ nanohybrids can be used as an efficient magnetically-separable nanocatalyst for removal of organic compounds in water and wastewater treatment.