(583d) FeCl3 Activated Bermudagrass-Derived Biochar for Removal of Cyanotoxins in Water: Characterization, Adsorption and Regeneration

Microcystin-LR (MC-LR), one of the most harmful cyanotoxins, can create a great risk to the ecosystem and human health. Due to the high toxicity of MC-LR, the World Health Organization (WHO) has regulated the MC-LR concentration limit of 1 µg/L in the drinking water guideline. Given its environmental impacts, it is significant to eliminate the MC-LR from water and wastewater. Biochars (BCs), made by the pyrolysis of biomass and solid wastes with oxygen-limited environment, have been recognized as low-cost and sustainable adsorbents for various wastewater and water treatment. However, the low surface area and poorly developed pore structure of raw BCs limit their adsorption capacities for various contaminants. The activation has been considered as the most effective way to improve the pore structure and surface area of BCs. However, to our knowledge, there are a few reports to investigate the adsorption of MC-LR by activated BCs. Compared to other activating agents, FeCl₃ as an activating agent is inexpensive and environmental-friendly. Moreover, iron oxides can be attached onto BCs after FeCl₃ activation, which enable the BCs to be easily separated by the magnet after adsorption. Additionally, the regeneration of contaminants saturated BCs was significant to reduce the cost of practical application.

In this study, the FeCl₃-activated biochar (Fe-BC) was prepared by one-step FeCl₃-mediated pyrolysis and activation of bermudagrass. Bermudagrass (BG), one of the most productive forage grasses in U.S, can be used as a viable feedstock for BC. For the first time, the present study investigated the detailed characteristic and mechanisms for MC-LR adsorption on Fe-BC. After the FeCl₃ activation, Fe-BC showed significantly increased BET surface area of 835 m²/g and high adsorption capacity of 9.31 mg/g BC at pH 6 for MC-LR, which is higher than various BCs reported from the previous studies. The adsorption experiments for various initial pH, kinetic and isotherm studies indicated various interactions between the MC-LR and Fe-BC via Ï€-Ï€ EDA, hydrophobic and hydrogen bond interactions. The thermodynamic studies suggested that the adsorption process was spontaneous and endothermic. Moreover, compared to chemical oxidation (Fenton oxidation and persulfate oxidation) and alkaline desorption (NaOH), thermal oxidation was proved to be the most effective method to regenerate the MC-LR saturated Fe-BC. After 4 cycles of regeneration under the treatment of 300 °C, over 99% of regeneration efficiency were still achieved. Therefore, the Fe-BC in this study can be a cost-effective and practical adsorbent for elimination of MC-LR in water.