Cascade utilization of crop straw through a FeCl3-mediated deep eutectic solvent biorefinery: Lignin-containing cellulose nanofibers flocculant fabrication followed by fertilizer production

The current agriculture relies heavily on the input of fossil resource-based chemicals while continuously generating crop straw residue and phosphorus-containing farmland drainage, which has been a serious threat to its sustainable development. This work proposed a closed-loop technique route to cascade utilization of crop straw residue through lignin-containing cellulose nanofibers (LCNFs) flocculant fabrication followed by fertilizer production, with the goal of farmland drainage phosphorus recovery and reuse. The technical feasibility of FeCl3-mediated deep eutectic solvents (DES) processing of wheat and rice straw into iron oxide immobilized LCNFs (Fe-LCNFs) flocculant, the flocculation performance of total phosphorus for representative farmland drainage, and the potential for co-production of glucose and phosphorus-containing fertilizer were comprehensively assessed. Results showed that the FeCl3 incorporation significantly enhanced the deconstruction and fractionation ability of DESs, resulting in nearly 100% hemicellulose removal and 86.4% delignification. The prepared Fe-LCNFs flocculant exhibited efficient total phosphorus removal of 99.9%. The spent flocs exhibited considerable glucose conversion of ∼52.4% even with iron oxide and phosphorus complex. It was calculated that 8.6 g reactive lignin, 27.9 g phosphorus-containing fertilizer and 30.7 g glucose additive for foliar fertilizer could be mostly obtained using this integrated technique. It was proposed that the dual function of biomass deconstruction and lignin fragmentation/relocation of the employed FeCl3-mediated DESs showed a determined role in enhancing the flocculation performance of Fe-LCNFs and enzymatic hydrolysis efficiency of the spent flocs. This work showed a chemical engineering approach for cascade and closed-loop utilization of crop straw toward more sustainable and green agriculture application.