Biochar application in constructed wetlands for wastewater treatment: A critical review

Constructed wetland (CW) is a cost-effective and ecologically sustainable technique for the restoration of polluted aqueous ecosystems through physical, chemical, and biological purification processes. The substrate of CWs is considered as the integral component that significantly influences the wastewater treatment efficacy and ecological associations. Biochar's high surface area and pore volume, suitable pore size distribution, high cation exchange capacity, surface functionality, fixed carbon, and stability along with low-cost availability enhance its potential application as an adsorbent besides ensuring microbiological diversity and stability in CWs. This review provides an overview of significant properties of biochar associated with bioremediation, biochar-plantbiological association and mechanisms involved in the efficient removal of organic and inorganic contaminants, emerging contaminants, and pathogens from wastewater under CWs. Biochar obtained from various feedstocks and pyrolysis conditions significantly influences the composition, functionality, porosity, surface properties, and stability of biochar, and hence its applicability and efficacy in CWs. Factors such as substrate properties, hydraulic retention, oxygenation, and redox condition also influence the pollutant removal efficiency in CWs. This review presents the feasibility and practicalities of biochar application in CWs and identifies the existing research gaps, uncertainties, and future research needs for large-scale wastewater treatment using biochar in CWs. Future research should evaluate biochar's long-term stability and performance in real-world CW settings. This includes modifying biochar properties for targeted pollutant removal and exploring synergies with technologies such as electrochemistry and advanced oxidation.