The performance and mechanism of biochar-enhanced constructed wetland for wastewater treatment

Biochar is considered as black gold in environmental remediation field due to its high surface area, light density, great adsorption capacity. Biochar has high potential to improve the wastewater purification in constructed wetlands (CWs), which has been increasingly investigated in this decade. This paper reviews the characteristics of biochar, methods of preparation, pollutant removal effects, and the greenhouse gas (GHG) emission reduction in CWs systematically, which could guide further studies of biochar application in CWs. It was found that the biochar applied in CWs were mainly produced by pyrolysis of terrestrial plants, while the biochar produced from marine biology or by other procedures may have different performance due to the various properties, which needed further investigation. Biochar affected CW performance through multiple pathways, including influencing the physicochemical & hydraulic conditions and biological behavior in CWs. The quantified enhancement of pollutant removal by biochar addition in CWs was firstly summarized. Research data proved that medians of COD, ammonia and TN removal in biochar-added CWs were 109%, 145% and 178% as much as those in non-biochar CWs, respectively. Moreover, the most significant improvement showed under oxygen-shortage condition in CWs, rather than aeration condition. However, it remains unclear which pathway dominates the enhanced pollutant removal. Besides, biochar commonly decreased N2O emission from CWs, while the opposite results were also reported and a comprehensive explanation has not been reached. Therefore, future mechanism studies are needed to advance our understanding on biochar's behavior in CWs for wastewater treatment.