Raw walnut shell modified by non-thermal plasma in ultrafine water mist for adsorptive removal of Cu(ii) from aqueous solution

Non-thermal plasma in ultrafine water mist (UWM) is proposed to increase the content of COOH groups on the surface of raw walnut shell in order to improve its performance in the removal of Cu(ii) from wastewater. The modified walnut shell surface was characterized by various techniques (BET, SEM-EDX and XPS), and it was observed that more COOH groups were generated. Oxygen disassociated from water mist by plasma bonded with the walnut shell to form activated sites of COOH groups. After Cu(ii) adsorption, the COOH group content in the walnut shell decreased because some groups were changed into C–O groups by Cu(ii) chemisorption with COOH groups. The Cu(ii) removal efficiency was 33.5% for raw walnut shell; however, the efficiency increased to 98% after plasma modification for 15 min under 3 g min(−1) water mist. The maximum Cu(ii) adsorption capacity of the UWM-plasma-modified WNS was 39.4 mg g(−1) at pH 5.3 and 25 °C, around 8 times that of the raw WNS. This implies that UWM-plasma modification is a potential method for improving the Cu(ii) adsorption performance of raw biomass.