Facile construction of amphoteric lignin-based hydrogel to simultaneously capture ammonium and phosphate ions and reutilization as bio-based slow-release fertilizer

Herein, a type of amphoteric lignin-based hydrogel adsorbent (ALH) was synthesized via free radical polymerization of lignosulfonate with methylacrylloxyethyl trimethyl ammonium chloride and acrylic acid through a simple one-pot approach. The ALH exhibited excellent adsorption capacity in removing ammonium (NH4+) and phosphate (H2PO4- ) ions, with a maximum theoretical adsorption capacity (qe) of 200.49 and 257.22 mg center dot g-1, respectively. Meanwhile, the effects of ALH dosage, pH values of solution, and interfering ionic ions on the qe of ALH were also investigated. The adsorption process of ALH for ammonium and phosphate ions obeyed the pseudo-first-order kinetic and Freundlich isotherm model. The possible mechanism of the adsorption processes were the physical and chemical adsorption. The reusability results indicated that ALH exhibited outstanding recyclability and stability, and the adsorption capacity was maintained more than 91.04 % and 91.00 % after five cycles of regeneration. The ALH prepared in this manuscript exhibited relatively higher adsorption capacity to ammonium and phosphate ions than the majority of non-metallic adsorbents. The adsorbed ALH could be reused as a slow-release fertilizer for the culture of rice hydroponics. In addition, the recycled ALH displayed excellent slow-release capacities and 71.5 % of ammonium and phosphate ions were released from ALH-N&P within 30 days. The products in this manuscript would not only open ways on the high-value application of lignin and recirculation of the adsorbent, but also give highlights on developing cost-effective and eco-friendly slow-release fertilizers in agriculture or substrates for oilless culture.