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• The PEG/Fe 3 O 4 /GO-NH 2 nanoadsorbent removed Ca2+ and Mg2+ from oilfield wastewater by 61–70 %. • The nanoadsorbent had relatively high reuse rates of 69–80 % at the fifth recycling run. • The oil recovery significantly increased by 11.8 % when the treated oilfield wastewater was reinjected. The high salinity of oilfield-produced water caused by the presence of metal ions, such as Ca2+ and Mg2+, has a significantly adverse effect on oil recovery. It is crucial to remove these metal ions efficiently before the oilfield wastewater is reinjected down into the oil wells. To address this issue, we prepared a PEG/Fe 3 O 4 /GO-NH 2 nanoadsorbent by aminating GO, incorporating magnetic Fe 3 O 4 nanoparticles and coating with PEG (PEG = poly(ethylene glycol); GO-NH 2 = aminated graphene oxide). We investigated the removal ratios of the nanoadsorbent for Ca2+ and Mg2+. The respective removal ratios for Ca2+ and Mg2+ reached 69.8 % and 61.1 % at 10 min, indicating that the nanoadsorbent was capable of efficiently removing Ca2+ and Mg2+. We also evaluated the reusability of the nanoadsorbent by recycling and reusing it 5 times. At the fifth recycling run, the nanoadsorbent retained relatively high reuse rates (79.1 % for Ca2+, 69.5 % for Mg2+) and removal ratios (69.5 % for Ca2+, 64.3 % for Mg2+), suggesting that the nanoadsorbent possessed relatively high reusability. Oilfield wastewater treated with the nanoadsorbent was employed for oil recovery in core displacement experiments. The oil recovery efficiency significantly increased by 11.8 % compared with untreated oilfield wastewater. Therefore, the PEG/Fe 3 O 4 /GO-NH 2 nanoadsorbent exhibits considerable potential for the treatment of oilfield wastewater to enhance oil recovery. [ABSTRACT FROM AUTHOR]