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Conventional fertilizers often face difficulties in controlling nutrient release rates and exhibit relatively low utilization efficiency. Thus, environment-responsive fertilizers that modulate nutrient release in response to environmental changes are progressively emerging as pivotal technological approaches for advancing sustainable agriculture.
Lignin (L) was hydrolyzed using urea/NaOH (UR/SH) at low temperatures to obtain hydrolyzed lignin (HL). The hydrolyzed lignin was then cross-linked with polyethylene glycol diglycidyl ether (PEGDGE) to prepare a hydrolyzed lignin hydrogel (HLHG). The HLHG was subsequently employed as a carrier for diammonium phosphate (DAP) fertilizer to create a pH-responsive hydrogel fertilizer.
The optimal hydrolysis conditions were determined to be 18 h and −10℃. Under these conditions, the contents of hydroxyl (—OH) and carboxyl (—COOH) groups reached their maximum levels, increasing by approximately 68% and 99%, respectively, compared to untreated L. The obtained pH-responsive lignin hydrogel [HLHG-(−10℃)-18 h] showed an equilibrium swelling ratio of 1216%, with a water retention swelling ratio of 163% after 11 hours. These values were approximately 3 and 12 times of the unhydrolyzed lignin hydrogel (LHG), respectively. pH responsiveness analysis indicated that the hydrogel demonstrated significant pH sensitivity, with equilibrium swelling ratio of 364%, 518%, and 634% at pH 6, 7, and 8, respectively. Between the adjacent gradients of pH 6−7 and pH 7−8, the equilibrium swelling ratios increased significantly by 41% and 70%, respectively, compared to LHG. SEM analysis demonstrated pH-dependent microstructural evolution of the lignin hydrogel. As the pH increased from 5 to 8, the honeycomb network exhibited a progressive pore channel expansion with concomitant wall thinning. When the pH increased to 9, the network structure contracted, resulting in a reduction in pore channels and thickening of the pore wall. This pH-responsive behavior was mechanistically attributed to the ionization/protonation equilibria of abundant —OH and —COOH groups in lignin, as validated through integrated functional group quantification and pH responsiveness assays. Furthermore, the pH-responsive hydrogel was employed as a carrier for DAP to control the release of phosphate fertilizer nutrients. The nutrient release results showed that the cumulative release rate of P2O5 from the lignin hydrogel fertilizer in the period of 60 h was 61%, 76%, and 85% at pH 6, 7, and 8, respectively, exhibiting a prominent pH-responsive tendency.
UR/SH hydrolysis of lignin effectively modifies the content of active functional groups and the internal spatial network structure of lignin, it is possible to create pH-responsive lignin hydrogels with enhanced water absorption and retention capacities. Utilizing this pH-responsive lignin-based hydrogel as a carrier for fertilizers offers a valuable reference for the effective utilization of lignin resources and the design of bio-based green fertilizer for nutrient control release.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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