Bifunctional porous polyethyleneimine-grafted lignin microspheres for efficient adsorption of 2,4-dichlorophenoxyacetic acid over a wide pH range and controlled release

Herbicides pollution is currently a big threat to the environment due to their widespread use in the agricultural field. Herein, we proposed the creation of bifunctional porous polyethyleneimine-grafted lignin microspheres (PLMs) by grafting polyethyleneimine onto lignin via an inverse suspension polymerization method by using polyethylene glycol as a porogen. The porous structure and abundant functional groups of PLMs allowed it to efficiently capture the representative herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), steadily over a wide pH range of 4–10. The maximum adsorption capacity was 909.09 mg/g at 45 °C that was much higher than the reported activated carbon (515.46 mg/g) and metal-organic framework (MIL-53, 556 mg/g). The adsorption process followed the Langmuir isotherm model and pseudo-second-order kinetic model, respectively. More importantly, the 2,4-D-loaded PLMs showed a distinct sustained release behavior as the environmental pH values varied. Under acidic (pH = 2) or alkaline (pH = 11) conditions, 57% or 88% of 2,4-D was released after 72 h, respectively, while only 10% of 2,4-D was released even after 96 h under pH = 7. FTIR and XPS analysis indicated that the adsorption mechanism was mainly involved with the electrostatic attraction, hydrogen bonding, π-π stacking and van del Waals forces between the PLMs and 2,4-D molecules. This work proposed an environmentally-friendly biomass-based bifunctional microspheres that could be potentially applied in the management of herbicide pollution through adsorption and controlled release formulations.