Kinetics of manganese ions release from cross-linked hydroxymethyls alkali lignin manganese fertilizers

Abstract: With people's increasing concern about food safety, slow/controlled release fertilizers have become a hot-point in the research and development of fertilizers because of their increasing efficiency of fertilizers, reducing environmental pollution, and increasing crop yields. Lignin can absorb metal ions via carbonyl oxygen and phenolic oxygen of the ligand, while biodegradation of lignin were a slower course, thus progressing to humus formation and the quality of organic matter were enhanced. In a word, lignin showed excellent properties for low cost, safety, and biocompatibility. It has the potential of a kind of agricultural slow/controlled release fertilizer carrier. Although grafting derivatives of alkali lignin get a modified S nutrient release curve, its cost is high. The problem hinders the application of alkali lignin in the agricultural fields as a slow/controlled release carrier. 　　According to Mn deficiency occurs on alkaline soils in northern of china, hydroxymethyls alkali lignin (HMAL) was synthesized with wheat straw alkali lignin and formaldehyde via reacting for 2 hours at 90℃. Manganese fertilizers of cross-linked hydroxymethyls alkali lignin (CHMAL-Mn) were prepared from HMAL and an appropriate amount of manganese sulfate by physical mixing. Its yield of CHMAL-Mn was 93.7%, and the release data of manganese ion in the static water indicated that the maximum capacity of CHMAL-Mn to loading manganese was 48 mg/g. Cross-linking of hydroxymethyls alkali lignin was proved by IR. CHMAL can significantly increase the amount of Mn load on alkali lignin. The study, which can promote comprehensive utilizations and applications in the field of biodegradable slow/controlled release carrier, is of great significance. In addition, kinetic data of the Mn2+ cumulative release rate were fitted with seven fit models of dynamics and probability distribution. The best-fit model was determined by a comparison of the effectiveness of fit models using the values of R2, e, Af and Bf. The results show that the best-fit model was the Ritger-Peppas model from kinetic data of the Mn2+ cumulative release rate. Those arithmetic means of R2, e, Af and Bf were 0.9443, 0.0083, 0.0223 and 0.0003 separately. When CHMAL-Mn to loading manganese was 12 mg/g, the release index (n) of its Ritger-Peppas model from kinetic data of the Mn2+ cumulative release rate is 0.1774. Because of n<0.45, slow/controlled release mechanism of CHMAL-Mn shows as fickian diffusion. When CHMAL-Mn to loading manganese was higher than 24 mg/g, the release index (n) of its Ritger-Peppas model from kinetic data of the Mn2+ cumulative release rate is among 0.45≤n≤0.89, slow/controlled release mechanism of CHMAL-Mn shows as following synergy of non fickian diffusion and drodible matrix. Especially, when CHMAL-Mn to loading manganese was 48 mg/g, release index (n) of its Ritger-Peppas model from kinetic data of the Mn2+ cumulative release rate is 0.811. At that time the Ritger-Peppas model predicts that the maximum of Mn2+ cumulative release rate was 79.28%, effective Mn2+ release period of CHMAL-Mn was 95.92 h and the model fitting accuracy rate was 96.57%.