CO2 capture performance of biochar prepared from sewage sludge after conditioning with different dewatering agents

Abstract

The appropriate conditioning of sewage sludge can facilitate its reuse as an adsorbent, such as that for CO2 capture to mitigate greenhouse effects. Herein, we treated raw sludge with two common dewatering conditioners (cationic polyacrylamide and polymeric aluminum chloride) and prepared the corresponding biochar (denoted as B-CPAM and B-PAC, respectively). These biochars were compared to that prepared from untreated sludge (B-SS) in terms of surface elements, surface functional groups, specific surface area, pore volume, and the capacity for CO2 capture. The mechanism of CO2 capture by sludge-derived biochars was analyzed, and the environmental risk due to the heavy metals in different forms was evaluated using risk assessment criteria. CO2 capacities of B-PAC (31.86 mg/g) and B-CPAM (48.54 mg/g) were higher than that of B-SS (28.36 mg/g). The optimal performance of B-CPAM for capturing CO2 is attributed to its largest microporous volume and microporous surface area. Oxygen-containing functional groups on the surface of biochar can react with CO2 by forming hydrogen bonds, and the amino groups can also condense with CO2, thereby improving the CO2 capture performance of biochar. And it was found that in B-CPAM, apart from Fe and Al, other heavy metals are more easily bound to CO2, such as Cd. The environmental risks of heavy metal pollution from B-CPAM and B-PAC were less than that from B-SS. Overall, all three sludge-derived biochars, especially B-CPAM, could potentially be used as adsorbents for CO2. Our results also highlight the importance of using a suitable dewatering conditioner at wastewater treatment plants.