Synergistic adsorption removal of heavy metal oxoanions and cations from aqueous solution by a polyacrylate-coated ferrous sulfide and layered double hydroxide nanocomposite

Heavy metal oxoanions and cations often co-exist in the environment and it is a critical challenge to achieve their simultaneous and efficient removal due to their presence of opposite charges. Here, a polyacrylate-coated ferrous sulfide and layered double hydroxide (PAA/FeS@LDH) nanocomposite was developed. Results illustrated that PAA/FeS@LDH exhibited excellent reductivity and stability due to the dispersion of PAA and the support by LDH. The removal efficiency of single Cr(VI) (9.73 mg/L) using PAA/FeS@LDH reached 100 % within 120 min, which was significantly higher than that of LDH (35.39 %), FeS (73.13 %), and FeS/LDH (96.47 %). Attractively, an enhanced removal of Cr(VI) (19.46 mg/L) by PAA/FeS@LDH in the presence of Cu(II), Ni(II), or Cd(II) was observed, with Cr(VI) removal efficiency increasing from 49.63 % (single Cr(VI)) to 99.51 %, 82.46 %, and 72.18 % within 30 min, respectively. The adsorption capacities of Cr(VI) significantly enlarged from 140 mg Cr/g (single Cr(VI)) to 221, 175, and 171 mg Cr/g when co-existing with Cu(II), Ni(II), or Cd(II), respectively. Another interesting outcome was the significant enhancement of reusability of PAA/FeS@LDH by the co-existence of Cr(VI) and Cu(II). After 5 cycles, PAA/FeS@LDH could still remove 96.97 % of Cr(VI) together with >70 % of Cu(II) in the mixed solution, by contrast, the removal efficiency of Cr(VI) was only 38.04 % in a single solution. The characterization analysis and density functional theory calculations demonstrated that besides surface reduction, complexation and precipitation, the cation-anion bridge interaction was an essential mechanism of achieving the synergistic removal of heavy metal oxoanions and cations.