| Urea formaldehyde slow-release fertilizer(UF-SRF)is a kind of oligomers that can be slowly degraded by microorganisms in soil to release nitrogen.UF-SRF is a mixture of methylene urea with various polymerization degree(DP)(H2NCONH(CH2CONH)nH).It is composed of cold-water soluble(CWS,25℃)and cold-water insoluble methylene urea(CWI),the latter of which contains methylene ureas only soluble in hot water(only HWS,100℃)and insoluble in hot water(HWI).The only HWS is responsible for nitrogen slow release.Compared with water soluble urea,UF-SRF is merited with higher nitrogen efficiency and lower pollution.Activity index(AI,characterized by the nitrogen content or weight content of only HWS in CWI)is commonly employed to indicate the slow-release duration of UF-SRF.Previous studies have suggested that the slow-release duration is closely related to the DP of methylene ureas.Here we further propose that,slow-release rate,as another important indicator for slow-release performance,should be determined by the content of methylene urea with specific DP values.Unfortunately,little attention has been paid to the DP and content of methylene ureas,neither to their correlation with AI and slow-release of UF-SRF.This work aims to establish methods for quantifying the DP and content of various methylene ureas in UF-SRF and thereby find their correlation with AI and slow-release performance of UF-SRF.To realize this aim,we first synthesized UF-SRFs with various AI values;then the feasibility to quantify the DP and content of various methylene ureas was examined by using proton nuclear magnetic resonance(~1H NMR),elemental analysis(EA),and high performance liquid chromatography-mass spectrometry(HPLC-MS);finally,the dependence of AI and slow-release on DP and methylene urea content was explored.The main research contents and conclusions are as follows:①Synthesis of UF-SRFBy using constant molar ratio of U to F(U/F=1.8)and constant hydroxymethylation conditions(T=50℃,t=50 min,pH=8),the effect of methylene reaction condition(T,t,pH)on the yield and AI’(weight-based activity index)of UF-SRF was investigated.The results revealed that,T=50℃ and pH=3 may promise fast methylenation and high yield,and AI’ can be regulated by adjusting reaction time.②Construction of methods for quantifying DP and content of methylene ureas1)The feasibility of ~1H NMR and EA to detect DP was investigated.It was found that both ~1H NMR and EA can quantify the average DP((?))values.The ((?)) values of UF-SRFs changed with similar trend to AI’ yet those for only HWS had no correlation with AI’.Specifically,for UF-SRFs with AI’ values ranging from 52.2%to 80.2%,their only HWS had similar ((?)) values of～3.5,suggesting that only HWS may have similar ((?)) values regardless of the AI’ values.2)The HPLC-MS method was established:Agilent 6460 triple series quadrupole liquid chromatography-mass spectrometry,Shimadzu C18 column(4.6×150 mm,5 μm),5%methanol aqueous solution as mobile phase(isocratic elution),column temperature as 35℃,flow rate as 0.7 mL/min.By using this method,six methylene ureas including methylenediurea(MDU),dimethylenetriurea(DMTU),trimethylenetetraurea(TMTU),tetramethylenepentaurea(TMPU),pentamethylenehexaurea(PMHU)and hexamethyleneheptaurea(HMHM)were effectively separated within 20 min,and the relative content of each methylene urea was quantified.③Prediction of the slow-release performance by using HPLC-MS results1)The results from HPLC-MS detection indicated that,regardless of the AI’ values,all UF-SRFs and their CWI and only HWS had identical methylene ureas yet the content of each methylene ureas was divergent.The main methylene ureas in UF-SRFs were MDU,DMTU and TMTU,whereas in CWI and only HWS was TMTU.These results imply that UF-SRFs may have similar slow-release duration yet different slow-release rate.2)The contents of only HWS and its respective methylene ureas in UF-SRF were calculated,which were then employed to predict the total slow-release amount of methylene ureas and the corresponding average slow-release rates during short-term(40～80 days),mid-term(80～120 days)and long-term(>120 days).The results indicated that both the total release amount and average release rate had no definite correlation with AI’.TMTU(long-term releasing)in UF-SRF demonstrated obviously higher release amount(g/100 g UF-SRF)and release rate(g/(100 g UF-SRF×10 days))than those shortterm(DMU)and mid-term methylene ureas(DMTU).To sum up,~1H NMR and EA can quantify the average polymerization degree while the established HPLC-MS method can quantify both the specific polymerization degree and the content of methylene ureas in UF-SRF,CWI and only HWS.The HPLC-MS results can be further employed to predict the slow-release performance of UF-SRF,though further experiments are required to verify it.This work helps to take a more comprehensive understanding on the compositions and slow-release of UF-SRF and provides a new promising method to evaluating the slow-release performance. |
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