Evaluation of the mechanisms of adsorption of microcystins and nodularin-R onto rice husk-based biochar

Adsorption mechanisms of individual and collective sorption of microcystins (MCs) and nodularin-R (NOD-R) onto heat-treated rice husk (RH) at neutral pH were evaluated. The functional groups on cyanotoxin standards and RH surface as well as their charge indicate that non-electrostatic and electrostatic interactions are present. Surface area and porosity of RH were determined indicating that MCs and NOD-R can be adsorbed in secondary micropores and mesopores of RH. Positively charged guanidinium ion in arginine-containing MCs and NOD-R created attractive electrostatic forces with the negatively charged RH surface (pHPZC ∼3.3). MCs and NOD-R experience electrostatic repulsion with the surface due to the presence of negatively charged carboxylate groups. MC-RR, even at significantly high toxin concentrations, could be removed by RH as it had a strong attraction to the surface due to two arginines and its neutral charge. Other congeners adsorbed on RH experienced lateral intermolecular repulsion and repulsion with molecules in solution. At low sorbate concentrations, MC-LR and NOD-R had electrostatic attraction to the RH surface and MC-LA, MC-LW, and MC-LF experienced electrostatic repulsion with the surface at all concentrations. Non-electrostatic forces govern the adsorption process of MC-LA, MC-LW, and MC-LF, and electrostatic forces are significant for the adsorption of MC-LR, MC-RR and NOD-R at low concentrations (1-100 ng/mL) as well as the adsorption of MC-RR at high concentrations (1000 ng/mL). The presence of 0.3 M NaCl increases the adsorption of MC-LA and MC-LF proving that those molecules undergo repulsive interactions with RH. The adsorption of MCs and NOD-R onto RH is pH dependent, and it appears to be improved at pH values < pHPZC. Overall, the adsorption of MCs and NOD-R onto RH biochar depends on congener concentration, structure, and charge, and can be modified through optimization of structural properties of RH sorbent and sorption conditions, such as ionic strength and pH.