表层水体中典型小分子有机氮的光铵化反应

地表水体中溶解性有机氮(DON)向无机氮的光化学转化是氮循环的重要过程之一。本文以自然水体中典型小分子DON(如氨基酸、核苷酸、尿素等)为对象,通过检测不同条件下的光铵化速率,探究小分子DON的结构、光源及水质条件对其光铵化过程的影响。结果表明:模拟自然光下,含有芳香环结构的氨基酸可以发生不同程度光铵化反应,酪氨酸和色氨酸的光铵化速率最大,6 h转化率可达约50%,而尿素和核苷酸则无明显光铵化反应发生;自然光下,酪氨酸和色氨酸在缓冲液中发生明显光铵化反应,而自然水样中组氨酸和苯丙氨酸也可生成NH4+;可见光波段对DON的光铵化几乎没有贡献,说明DON的光铵化速率受光源与水质组分影响。分子结构也影响DON的光铵化反应,如组氨酸分子中的氮可以完全转化为NH4+,而一个色氨酸分子中则仅有一个氮原子可以发生转化。基于密度泛函理论计算发现,氨基酸光铵化速率与其分子轨道能隙值有较好的负相关性。系列实验表明了水体中DON光化学转化的复杂性,应当在氮循环、水质变化和生态环境的评估中得到重视。

Photoammonification of typical low-molecular-weight dissolved organic nitrogen in surface water system

Phototransformation,a process from dissolved organic nitrogen(DON)to dissolved inorganic nitrogen(DIN),is an important component of nitrogen cycle in surface aquatic system.In this study,we measured photoammonification rates of typical low-molecular-weight DON(e.g.,amino acids,nucleotides,and urea)under different conditions to elucidate the potential role of DON structures,light sources and water matrices during photoammonification.Results showed that under simulated sunlight,amino acids with aromatic moieties produced different amounts of NH4+.L-tyrosine and L-tryptophan exhibited the highest ammonification rates,up to50%transformation within 6 hours,whereas there was negligible photoammonification rate of urea,adenine and cytosine.Under natural sunlight,L-tyrosine and L-tryptophan produced NH4+by phototransformation in both buffer solution and natural water sample,while all aromatic amino acids and L-histidine produced NH4+only in natural water sample by phototransformation.Visible light did not contribute to photoammonification of the low-molecular-weight DON.These results indicated that photoammonification was affected by light sources and matrix constituents.Meanwhile,the structures of DON affected photoammonification.For example,at optimal conditions of irradiation,all nitrogen atoms of L-histidine could be converted into NH4+by photoammonifica-tion,whereas only one of N atoms in L-tryptophan was transformed into NH4+.Based on density functional theory calculation,there was significantly negative correlation between photoammonification rates and ELUMO-EHOMOof amino acids.Our results demonstrated the complex of DON photoammonification in surface water system.The significant role of photoammonification of lowmolecular-weight DON in surface water system should be considered in nitrogen cycle,water quality evolution and the assessment of eco-environment.