Eggshell-enhanced biochar with in-situ formed CaO/Ca(OH)2 for efficient removal of Pb2+ and Cd2+ from wastewater: Performance and mechanistic insights

Abstract

This study proposes a green and simple strategy for efficiently removing heavy metals from wastewater by activating biochar with eggshell and in-situ generating CaO/Ca(OH)2. Characterization results revealed that the composite carbon material (BCEg15) successfully incorporated CaO/Ca(OH)2 active sites compared to the raw biochar (BC). Additionally, eggshell activation increased the specific surface area and pore volume of BCEg15 by 4.36 and 6.80 times, respectively, compared to BC. These structural enhancements increased the maximum adsorption capacity of BCEg15 for Pb2+ and Cd2+ by 20.68 and 42 times, respectively, compared to BC. Importantly, the loading of CaO and Ca(OH)2 significantly enhanced the ion exchange and mineral precipitation capacities of the biochar. For instance, compared to BC, the adsorption capacities of BCEg15 attributed to ion exchange (Qe) for Pb2+ and Cd2+ rose by 403.12 mg/g and 197.65 mg/g, respectively, while the adsorption capacities attributed to mineral precipitation (Qp) increased by 19.60 times and 45.20 times, respectively. BCEg15 demonstrated excellent reusability, maintaining adsorption capacities of 461.18 mg/g for Pb2+ and 265.26 mg/g for Cd2+ after five cycles. Additionally, BCEg15 outperformed commercial activated carbon in treating heavy metals from desulfurization wastewater from coal-fired power plants, achieving a removal efficiency of 99.98 % compared to 48.22 %. The primary mechanisms by which BCEg15 removes Pb2+ and Cd2+ were identified as ion exchange, mineral precipitation, complexation with oxygen-containing functional groups, and interactions with π electrons. Density functional theory (DFT) was utilized to delve deeper into the microscopic mechanisms of adsorption. The findings showed that the O-top site of CaO exhibited the greatest stability for heavy metal adsorption. Electron density difference maps and partial density of states (PDOS) results revealed a denser electron cloud and greater orbital overlap between CaO and Pb2+ compared to Cd2+, suggesting a higher affinity and adsorption capacity for Pb2+, consistent with the experimental findings. This study presents an effective method for preparing adsorbents capable of removing Pb2+ and Cd2+ from wastewater and offers theoretical insights into the adsorption mechanisms of metal oxide-incorporated biochar.