Characteristics and mechanisms of microcystin-LR adsorption by giant reed-derived biochars: Role of minerals, pores, and functional groups

Abstract

The powerful toxin microcystin-LR (MCLR), which is released during cyanobacterial blooms in eutrophic waters, has become a serious threat to the ecological environment and to human health. The relationship between adsorption of MCLR and the variable temperature-dependent properties of biochars is poorly understood. Thus, MCLR sorption on the biochars derived from giant reed at 300–600 °C (BC300−BC600) and the corresponding deashed biochars (BCF300−BCF600) was investigated. MCLR sorption was nonlinear (n = 0.24–0.70) and adsorption-dominant for all the biochars. The high-temperature biochars (≥500 °C, HBCs) exhibited higher sorption capacity (41.2–42.4 mg g−1) for MCLR. This is larger than, or at least comparable to, the results from commercial adsorbents like carbon nanotubes (5.9 mg g−1), mesoporous silica (5.99 mg g−1), and activated carbon (16.1–83.3 mg g−1). Deashing barely altered the MCLR sorption on the low-temperature biochars (≤400 °C, LBCs), whereas the mineral components (e.g., Mg2P2O7) in HBCs greatly contributed to the overall sorption. Higher mesoporosity of HBCs facilitated their sorption capacity and affinity toward MCLR relative to LBCs. MCLR was prone to interact with O-containing functional groups via H-bonds and electrostatic attraction to LBCs, while MCLR sorption onto the carbon fraction in HBCs was mainly ascribed to hydrophobic interaction and π+–π or π–π interaction. Electrostatic repulsion reduced MCLR sorption on BCF400 with increasing pH from 2.0 to 10.0. In contrast, MCLR molecules more easily diffused into the mesopores of BCF600, which had a nearly electroneutral surface (pH 3.5) relative to other pH conditions. This resulted in the highest MCLR sorption. These findings are useful for understanding the roles of minerals, pores, and functional groups in MCLR sorption by plant-derived biochars; and for understanding the benefits from producing engineered biochar sorbents to remove MCLR from water.