Assessing the life cycle and economic impact of cement-modified biochar compared to conventional adsorbents for heavy metal removal in stormwater

Abstract

Modified biochar is used for heavy metal removal from stormwater, but current modifications rely on high-cost chemicals and result in highly toxic effluents. Additionally, there is limited research on these modifications' environmental impact and cost. This study aims to investigate the life cycle analysis (LCA) and economics of the process of absorption of Cu, Pb and Zn from synthetic stormwater utilizing cement-modified biochar adsorbent and compared it with commonly available adsorbents, activated carbon and zeolite. The cradle-to-gate LCA used experimental data and the Ecoinvent 3.0 database. Adding 1.5 % cement to biochar reduced its negative environmental impact by two to three times compared to plain biochar. Cement addition enhances heavy metal removal by raising pH, promoting precipitation, and reducing metal mobility. Combined with biochar, it increases the available surface area for adsorption, further boosting the system's efficiency in contaminant removal. The cement-modified biochar had the lowest global warming potential (kg CO2 eq), about half of Paddy Husk Biochar (PHBC). Approximately 50–90 % of the environmental impact of cement-modified biochar, zeolite, and PHBC was attributed to raw material collection. The Saw Dust Biochar (SDBC) had the highest sensitivity towards transportation distances among the adsorbents considered. Due to its heightened adsorption capacity, the 98.5 % PHBC composite demonstrated the lowest environmental impacts for Cu and Zn removal in multi-metal systems. Transportation of raw materials and labor costs were the primary cost drivers for biochar-based adsorbents. Among the adsorbents, 98.5 % PHBC was the best, being cost-effective and efficient for heavy metal removal. It is suitable for at-source pollutant removal in low-rainfall and high-pollution areas. Overall, this study helped identify performance weaknesses and modification opportunities for cement modification of biochar in heavy metal pollution control.