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The presence and removal of heavy metals such as Cu(II) as well as Cr(VI) in petroleum refinery wastewater calls for concerted efforts due to their mobility, toxicity, bioaccumulation, and non-biodegradability in the environment. In this present work, zinc oxide (ZnO), iron oxide (Fe 3 O 4 ) nanoparticles and ZnO/Fe 3 O 4  nanocomposites were synthesized via simple sol-gel and chemical reduction methods; characterized using different analytical tools and then applied as nanoadsorbent to sequester Cu(II) and Cr(VI) ions from Petroleum Refinery wastewater via batch adsorption process. Cu(II) and Cr(VI) adsorption processes were examined with respect to contact time (kinetic effect), nanoadsorbent dosage, isotherm equilibrium, and thermodynamic parameters. ZnO/Fe 3 O 4  nanocomposites with higher surface area (39.450 m 2 /g) have a mixture of rod-like and spherical shapes as compared to ZnO and Fe 3 O 4  nanoparticles with spherical shape only and surface areas of 8.62 m 2 /g and 7.86 m 2 /g) according to the high-resolution scanning electron microscopy (HRSEM) and Brunauer–Emmett–Teller (BET) analysis. The X-ray diffractometer (XRD) results revealed the formation of hexagonal wurtzite structure of ZnO and the face-centered cubic structure phase of Fe 3 O 4  nanoparticles, after the formation of the ZnO/Fe 3 O 4  nanocomposites the phases of the nanoparticles were not affected but the diffraction peaks shifted to higher 2θ degree. The average crystallite size of ZnO and Fe 3 O 4  nanoparticles and ZnO/Fe 3 O 4  nanocomposites were 20.12, 26.36 and 14.50 nm respectively. The maximum removal efficiency of Cu (II) (92.99%) and Cr (VI) (77.60%) by ZnO/Fe 3 O 4  nanocomposites was higher than 85.83%; 65.19% for Cu (II) and 80.57%; 62.53 for Cr (VI) using ZnO and Fe 3 O 4  nanoadsorbents individually under the following conditions: contact time (15), dosage (0.08 g) and temperature (30 °C). The experimental data for Cu (II) and Cr (VI) ion removal fitted well to the pseudo-second-order kinetic and Langmuir isotherm models. The thermodynamic study suggested that the removal of the two metal ions from petroleum wastewater was endothermic. The reusability study after the fourth adsorption-desorption cycle indicated the stability of ZnO/Fe 3 O 4  nanocomposites with 85.51% and 69.42% removal efficiency of Cu (II) and Cr (VI). The results showed that ZnO/Fe 3 O 4  nanocomposite achieves higher performance than ZnO and Fe 3 O 4  alone in the removal of Cu (II) and Cr (VI) ions from the petroleum refinery wastewater.

Simultaneous removal of Cu (II) and Cr (VI) ions from petroleum refinery wastewater using ZnO/Fe 3 O 4 nanocomposite