Innovative solutions for coastal wetlands pollution: Application of Spartina alterniflora Loisel. - biochar for cadmium removal and carbon sequestration

Abstract

Coastal wetland ecosystems are extremely fragile, with heavy metal contamination and uncontrolled growth of invasive plants (e.g. Spartina alterniflora Loisel.) could result in threaten the ecological security and resource wastage. In this study, Spartina alterniflora-biochars at two different temperatures (400 and 600 °C), as well as phosphorus-modified biochars (400BC, 600BC, 400PBC, and 600PBC) were produced, and evaluated for their removal capacity in cadmium (Cd)-contaminated sediments. The maximum sorption capacity, as determined by the Langmuir equation, was in the order of 600BC (838.81 mg/g) > 400BC (707.25 mg/g) > 600PBC (581.69 mg/g) > 400PBC (455.82 mg/g) for Cd, with adsorption occurred through single-layer chemical processes. In the Cd removal tests, increasing incubation times led to a significant reduction in bioavailable Cd (from 84.74 mg/kg at 1 h to 34.79 mg/kg at 7 day). There were significantly negative correlation (p < 0.01) and highly linear regression correlation (R2 > 0.81) between bioavailable Cd and three enzymes’ activities (urease, sucrase, and acid phosphatase). Moreover, 63.52%–98.75% of the desorbed Cd can be reabsorbed and fixed in the presence of biochar after 56 d, which effectively reducing the potential for secondary release of heavy metals into the environment. Notably, a comprehensive evaluation through cost-benefit analysis (CBA) and life cycle assessment (LCA) revealed that the biochar system can generate an overall profit of $302.8/tonne and result in a net reduction of CO2 emissions by 1455.479 kg/tonne. The application of biochar could reduce carbon footprints and promote carbon neutrality, and be an effective in-situ remediation strategy for Cd-contaminated coastal wetland.