ZHANG Hua, WANG Kuan, SONG Jian*, ZHANG Yong, HUANG Ming,HUANG Jian, ZHU Jing, HUANG Shan, WANG Meng

1. School of Environment and Energy Engineering, Anhui Jianzhu University, Hefei 230601, China

2. Key Laboratory of Anhui Province of Water Pollution Control and Wastewater Reuse, Anhui Jianzhu University, Hefei 230601, China

The Fluorescent Properties of Dissolved Organic Matter and Assessment of Total Nitrogen in Overlying Water with Different Dissolved Oxygen Conditions

ZHANG Hua1,2, WANG Kuan1,2, SONG Jian1,2*, ZHANG Yong1,2, HUANG Ming1,2,HUANG Jian1,2, ZHU Jing1,2, HUANG Shan1,2, WANG Meng1,2

1. School of Environment and Energy Engineering, Anhui Jianzhu University, Hefei 230601, China

2. Key Laboratory of Anhui Province of Water Pollution Control and Wastewater Reuse, Anhui Jianzhu University, Hefei 230601, China

This paper used excitation-emission matrix spectroscopy (EEMs) to probe the fluorescence properties of dissolved organic matter (DOM) in the overlying water with different dissolved oxygen (DO) conditions, investigating the relationship between protein-like fluorescence intensity and total nitrogen concentration. The resulting fluorescence spectra revealed three protein-like components (high-excitation wavelength tyrosine, low-excitation wavelength tyrosine, low-excitation wavelength tryptophan) and two fulvic-like components (ultraviolet fulvic-like components, visible fulvic-like components) in the overlying water. Moreover, the protein-like components were dominant in the overlying water’s DOM. The fluorescence intensity of the protein-like components decreased significantly after aeration. Two of the protein-like components—the low-excitation wavelength tyrosine and the low-excitation wavelength tryptophan—were more susceptible to degradation by microorganisms within the degradable organic matter with respect to the high-excitation wavelength tyrosine. In contrast, the ultraviolet and visible fulvic-like fluorescence intensity increased along with increasing DO concentration, indicating that the fulvic-like components were part of the refractory organics. The fluorescence indices of the DOM in the overlying water were between 1.65～1.80, suggesting that the sources of the DOM were related to terrigenous sediments and microbial metabolic processes, with the primary source being the contribution from microbial metabolism. The fluorescence indices increased along with DO growth, which showed that microbial biomass and microbial activity gradually increased with increasing DO while microbial metabolism also improved, which also increased the biogenic components in the overlying water. The fluorescence intensity of the high-excitation wavelength tyrosine peak A showed a good linear relationship with the total nitrogen concentration at higher DO concentrations of 2.5, 3.5, and 5.5 mg·L-1, withr2being 0.956, 0.946, and 0.953, respectively. This study demonstrated that excitation-emission matrix spectroscopy can distinguish the transformation characteristics of the DOM and identify the linear relationship between the fluorescence intensity of the high-excitation wavelength tyrosine peak A and total nitrogen concentration, thus providing a quick and effective technique and theoretical support for river water monitoring and water restoration.

Excitation-emission matrix spectroscopy; Dissolved oxygen; Overlying water; Dissolved organic matter; Fluorescence intensity; Total nitrogen

Introduction

Dissolved organic matter (DOM) is a complex mixture, composed of different chemical substances with various chemical properties, with high activity and migration since it is rich in activity groups with carboxyl, phenolic hydroxyl, aminos, aromatic nuclei, and so on[1-2]. DOM is able to react with hydrophobic organic components, affecting the migration and transformation of such components, and can also form complexes with metals that promotes their toxicity and causes more serious water pollution. Therefore, to effectively combat water pollution, it is very important to research the transformation and migration of the DOM in water systems.

Three-dimensional fluorescence technology is widely used in the research of rivers, lakes, and sewage because the technique provides both high selectivity and sensitivity and quick detection[3-5]. Results from this type of analysis has shown that DOM has different structures and fluorophores in different environments, resulting in significantly different fluorescence peak positions and fluorescence intensities. Further studies have found that the fluorescence peak and fluorescence index can characterize the sources and conversions of organic matter in water. At present, the relationship between fluorescence intensity and dissolved organic carbon (DOC) or dissolved oxygen (DO) have been established, and the levels of organic pollution have been effectively tracked through quantitative analysis of the fluorescence intensity of DOM characteristic peaks[8]. However, there has been little study on the fluorescent properties of the DOM in the overlying water, or on the relationship between protein-like fluorescence intensity and total nitrogen concentration with different DO conditions. While nitrogen is one of the major nutrients contributing to water eutrophication, chemically measuring nitrogen concentration is time-consuming, energy-intensive, and easily causes secondary pollution. In this study, the fluorescent properties and their transformation laws of the DOM in the overlying water are analyzed using fluorescence spectrometry. In addition, the relationship between protein-like fluorescence intensity and total nitrogen concentration is studied with different DO conditions. This work provides a more convenient way to intensively research DOM and indirectly detect total nitrogen concentration in overlying water, and provides a theory reference for the application of aeration technologies in the treatment of black and odorous river water.

1 Experimental materials and methods

1.1 Sample collection and experimental control

Experiment samples were collected from the Ershibu River, China, located at 31.84 degrees north latitude and 117.39 degrees east longitude, in June 2014. The Ershibu River is an important river that flows alongside both residential and industrial zones, and which has become extremely polluted; in fact, the river is one of the main pollution sources for Chaohu, Anhui province. Surface sediment samples to a depth of 20 cm were collected by gravity type column, then stored in a sealed plastic bag away from light. Overlying water samples were collected at the same time.

Four two-liter reactors were used in the experiment, numbered 1#, 2#, 3#, and 4#for this work. Fresh sediment was added to the bottom of each reactor up to a depth of 11 cm, followed by raw river water. The ratio of sediment to raw water in each reactor was one to three. A black thin film in the bottom of the four reactors was used to keep each sample in darkness. Each reactor was allowed to stand for two days until stabilized, then aerated. Following this step, 100 mL of the overlying water was removed from each cylinder to be detected, and raw water was supplied to the original volume to replace the water removed. Each water sample was centrifuged and isolated, then filtered using a 0.45 μm filter membrane. The DO concentration in each reactor were controlled to 0.3, 2.5, 3.5, and 5.5 mg·L-1, respectively.

1.2 Experimental instruments

Three dimensional fluorescence spectra were determined using an F-7000 fluorescence spectrophotometer (HITACHI, Japan). Each spectra was taken using a voltage of 700 V at excitation wavelengths of 200～450 nm (every 5 nm), and with an emission wavelength of 250～550 nm (every 5 nm), respectively. Scanning speed was 2400 nm·min-1with an automatic response time. The blank water used as a control was ultrapure water supplied by Milli-Q. The DO concentration was detected using an HQ30D portable DO meter (HACH, USA). Total nitrogen in each sample was measured using a TU-1901 UV visible spectrophotometer (Beijing Puxi General Company, China).

2 Results and Discussion

2.1 The fluorescence characteristics of raw overlying water

The fluorescence characteristics of the raw overlying water are illustrated in Fig.1, and show that three kinds of protein-like fluorescence peaks (a high-excitation wavelength tyrosine peak A, a low-excitation wavelength tyrosine peak B, and a low excitation-excitation wavelength tryptophan peak C) and two kinds of fulvic-like fluorescence peaks (an ultraviolet fulvic-like peak D and a visible fulvic-like peak E) appear in the raw overlying water. The center position of peak A and peak B is Ex/Em=275/305 nm and Ex/Em=225/310 nm, respectively, while peak C occurs at Ex/Em=225/330 nm. Peak D and peak E appear at Ex/Em=240/400 nm and Ex/Em=325/420 nm, respectively[4].

Fig.1 Fluorescence spectra of the raw overlying water

As shown in Fig.1, the fluorescence intensity of protein-like peaks A, B, and C in the raw overlying water are relatively stronger than the fluorescence intensity of the fulvic-like fluorescence peaks D and E, which are weaker. This result suggests that the fluorescence intensity of the fluorescence peak can distinguish the relative content of the given organic matter, while the fluorescence intensity of a given DOM and the total fluorescence intensity can indicate the proportion of that given DOM in a sample. Fig.1 shows that the main fluorescent components in the overlying water from the Ershibu River come from protein-like material. The fluorescence intensity of these protein-like components can properly characterize pollution sources in water such as sewage or strong microbial activity, which will show an obvious protein-like fluorescence. The results also suggest that the DOM in the Ershibu River is the result of both terrigenous sediments and microbial metabolism.

2.2 Fluorescent properties of the DOM in the overlying water with different DO conditions

Fig.2 shows the fluorescence spectra of the five peaks after stabilization with DO concentrations of 0.3, 2.5, 3.5 and 5.5 mg·L-1, respectively, which change to some degree. The protein-like fluorescence intensities significantly weaken with increasing DO concentration, peaks A and peak B are more obvious than peak C. The reduction of peaks A, B, and C shows that microorganisms significantly degraded the protein-like components during the aeration process, in addition to some differences in the biodegradability of the different protein-like components. Thus, the low-excitation wavelength tyrosine and the low-excitation wavelength tryptophan belong to easily degradable organic matter by microorganisms within the degradable organic matter with respect to the high-excitation wavelength tyrosine.

Fig.2 Fluorescence spectra of the DOM in the overlying water with different DO conditions

As the changes in fluorescence intensity show in Fig.3, the decomposition rate of the protein-like components increases with increasing DO, which shows that microbial activity gradually increased with growth in the sediment aerobic layer depth, accelerating the decomposition rate of protein-like components.

However, the fluorescence intensity of the fulvic-like components also increased with increasing DO, with the rise of visible fulvic-like fluorescence intensity especially prominent (Fig.2 and Fig.3). Fulvic-like components belong to humic, which are chemically stable and are resistant to degradation by microorganisms. Fulvic-like components are considered refractory organics[9].

Fig.3 Changes in fluorescence intensity for characteristic fluorescence peaks

The fluorescence index is often used to characterize and study the source of DOM, and is the ratio of the fluorescence intensities for Ex/Em=370/450 nm and Ex/Em=370/500 nm[10]. The changes in fluorescence index as a function of time with the different DO conditions are shown in Fig.4.

The fluorescence indices of the DOM in the overlying water were 1.65～1.80 with the different DO conditions, with average values of 1.67, 1.72, 1.74, and 1.76, respectively. The typical fluorescence index for DOM from terrigenous sediment is 1.40, and 1.90 for DOM derived from microbial metabolic processes[11]. The fluorescence indices of the DOM in the samples collected from the Ershibu River in this work were between 1.65～1.80, values relatively closer to 1.90 than 1.4. This indicates that the DOM source of the samples

Fig.4 Fluorescence index with different DO conditions

was terrigenous sediments and microbial metabolic processes, but mainly derived from the microbial metabolic processes. Moreover, the fluorescence indices increase with increasing DO, which indicates that the microbial biomass and microbial activity also gradually increase with DO growth, as well as the corresponding microbial metabolic rate. These changes lead to an increase in the biogenic components of the DOM in the overlying water.

2.3 Correlation of the high-excitation wavelength tyrosine peak A and total nitrogen concentration

Our analysis of every protein-like fluorescence intensity and total nitrogen concentration in the overlying water with different DO conditions shows a strong correlation existing between the fluorescence intensity of the high-excitation wavelength tyrosine peak A and total nitrogen concentration, as shown in Fig.5. DO concentrations of 2.5, 3.5, and 5.5 mg·L-1produced correlation coefficients of 0.956, 0.946, and 0.953, respectively. However, the correlation coefficient was only 0.412 for a DO concentration of 0.3 mg·L-1, indicating that the fluorescence intensity of the high-excitation wavelength tyrosine peak A exhibits a good linear relationship with the total nitrogen concentration at higher DO concentrations. Based on water environment function and protection objectives in China, the surface water were divided into five categories, where the lowest sufficient DO concentration was 2 mg·L-1. Below this concentration, steps must be taken to increase total DO concentration in the restored water treatment, so that total nitrogen concentration can be accurately evaluated through monitoring the fluorescence intensity of the high-excitation wavelength tyrosine peak A in the overlying water.

Fig.5 Correlation between fluorescence intensity of the high-excitation wavelength tyrosine peak A and total nitrogen

3 Conclusions

DOM analyzed in the over overlying water collected from the Ershibu River for this work primarily contained five different fluorescent components: high-excitation wavelength tyrosine, low-excitation wavelength tyrosine, low-excitation wavelength tryptophan, ultraviolet fulvic-like components, and visible fulvic-like components. The protein-like components were the main fluorescent components.

The fluorescence intensity of the protein-like components significantly decreased with increased DO, while the fluorescence intensity of the fulvic-like components increased. This suggests that protein-like components are more easily degraded by microorganisms, while fulvic-like components are chemically stable and harder for microorganisms to degrade. The fluorescence indices of the DOM also increased along with DO growth, having values of between 1.65 and 1.80. These values indicate that the DOM sources for water in that region of the Ershibu River are terrigenous sediments and microbial metabolic processes, with the microbial processes the dominant source.

The fluorescence intensity of the high-excitation wavelength tyrosine peak A exhibited a good linear relationship with total nitrogen concentration at high DO concentrations. This suggests that total nitrogen concentration could be rapidly analyzed through survey of the fluorescence intensity of the high-excitation wavelength tyrosine peak A, and that these results provide a good method for the rapid detection of total nitrogen concentration.

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O657.3

A

不同溶解氧水平上覆水中DOM熒光特性及總氮含量評估

張 華1,2， 王 寬1,2， 宋 箭1,2*， 張 勇1,2， 黃 明1,2，黃 健1,2， 朱 菁1,2， 黃 珊1,2， 王 萌1,2

1. 安徽建筑大學環(huán)境與能源工程學院， 安徽 合肥 230601

2. 安徽建筑大學水污染控制與廢水資源化安徽省重點實驗室， 安徽 合肥 230601

采用熒光光譜技術研究不同溶解氧(DO)水平下二十埠河底泥上覆水中溶解性有機物(DOM)轉(zhuǎn)化特性及類蛋白熒光強度與總氮濃度的關系。 三維熒光光譜顯示： 上覆水中DOM主要由三種類蛋白物質(zhì)(高激發(fā)波長類酪氨酸、 低激發(fā)波長類酪氨酸、 低激發(fā)波長類色氨酸)和兩種類富里酸物質(zhì)(紫外區(qū)類富里酸、 可見區(qū)類富里酸物質(zhì))組成， 類蛋白物質(zhì)是上覆水中DOM的主要成分。 經(jīng)過曝氣后類蛋白熒光強度均存在明顯降低， 其中低激發(fā)波長酪氨酸和低激發(fā)波長色氨酸相對于高激發(fā)波長酪氨酸更易被微生物降解。 而類富里酸熒光強度則均呈現(xiàn)增強趨勢， 表明類富里酸物質(zhì)屬于難降解有機物。 上覆水中DOM熒光指數(shù)介于1.65～1.8之間， 表明上覆水體DOM既有陸源又有生物源但以生物源為主。 熒光指數(shù)隨DO增加而增大， 說明隨著DO增加微生物量及微生物活性逐漸增加， 微生物代謝功能增強， 使得上覆水中DOM的生物源成份加大。 在較高的溶解氧水平下， 即DO分別為2.5， 3.5和5.5 mg·L-1時， 高激發(fā)波長類酪氨酸峰A的熒光強度與上覆水中總氮濃度有良好的相關性， 相關系數(shù)(r2)分別為0.956， 0.946， 0.953， 說明可以通過三維熒光技術監(jiān)測高激發(fā)波長類酪氨酸峰A的熒光強度而快速分析上覆水中總氮濃度， 為河道水體診斷、 治理及修復提供快速有效的技術參考和理論支持。

熒光光譜； 溶解氧； 上覆水； 溶解性有機物； 熒光強度； 總氮

2014-12-13，

2015-04-16)

2014-12-13; accepted： 2015-04-16

Major Science and Technology Program for Water Pollution Control and Treatment of China(2014ZX 07303-003-09, 2014ZX07405-003-03), Science and Technology Project of Anhui Province(1301032137-04), The Natural Science Project for Colleges of Anhui Province(KJ2013B049), Doctor Foundation of Anhui Jianzhu University(2013-6)

10.3964/j.issn.1000-0593(2016)03-0890-06

Biography： ZHANG Hua, (1978—), female, Ph.D., Associate Professor of Anhui Jianzhu University e-mail: zhanghuateacher@163.com *Corresponding author e-mail: songj@ahjzu.edu.cn

*通訊聯(lián)系人