【作者】 欧晓霞；

【导师】 全燮；

【作者基本信息】 大连理工大学 ， 环境工程， 2008， 博士

【摘要】 腐殖酸是自然水体中广泛存在的一类天然有机大分子物质，对环境污染物的迁移转化具有重要影响。光解是持久性有毒污染物在环境水体中经历的重要过程之一，主要以间接光解为主。铁元素也是自然地表水的重要组分，当腐殖酸和Fe（Ⅲ）络合在一起后，必然会潜在的影响共存系统中环境污染物的光解过程。有报道显示，腐殖酸对污染物的光解作用具有多面性，即一些报道证实腐殖酸对光解具有催化作用；而另一些报道显示腐殖酸对光解有抑制作用。本论文首先研究了腐殖酸和铁络合物存在下阿特拉津光降解的机理，以及这个过程中涉及到的铁循环过程和H₂O₂的形成消失规律；然后采用分子排阻色谱（SEC）对腐殖酸进行分级，通过对阿特拉津的光解实验考察不同腐殖酸级分的光敏活性，重点研究腐殖酸不同级分和Fe（Ⅲ）的络合物对阿特拉津光降解的影响。本项研究对正确认识天然地表水中污染物的光解行为，尤其是深入了解和掌握腐殖酸这样一种重要的天然水组分和铁的络合物对污染物光解行为的作用具有重要的意义。主要实验结果如下：（1）鉴于腐殖酸这种大分子的结构复杂性和异质性，本研究选择先从腐殖酸光解产物中最小分子之一的柠檬酸分子入手，考察了柠檬酸和铁络合物对阿特拉津光降解的影响。实验发现：氙灯光照下，阿特拉津在Fe（Ⅲ）-柠檬酸溶液系统中的光降解主要是由^·OH引发的；而且Fe（Ⅲ）-柠檬酸体系生成^·OH的能力要高于单独的Fe（Ⅲ）体系（pH 3．5）。随着Fe（Ⅲ）-柠檬酸溶液的pH从3．5升高到8．6，阿特拉津的降解速率迅速降低，这主要归因于铁物种的形成受pH影响很大。光照强度的增强和柠檬酸浓度的增大都有利于Fe（Ⅲ）-柠檬酸体系中阿特拉津的光降解；柠檬酸同时起到配体和还原剂的作用。UV-Vis和FTIR光谱对柠檬酸和Fe（Ⅲ）的络合物表征的结果说明柠檬酸和Fe（Ⅲ）通过配体交换络合在一起。（2）氙灯光照下，腐殖酸的存在抑制了溶液中阿特拉津的光降解，腐殖酸浓度越大，对阿特拉津光分解的抑制作用就越明显。而当腐殖酸和Fe（Ⅲ）共存于溶液中时，表现出对阿特拉津光解的促进作用。为了研究腐殖酸和Fe（Ⅲ）的结合作用，采用了SEM、EDX、UV-Vis、FTIR和荧光光谱等表征方法。SEM和EDX给出了腐殖酸和Fe（Ⅲ）络合物的形貌特征：从UV-Vis光谱看出腐殖酸和Fe（Ⅲ）络合后在长波长方向吸收强度有轻微的提高；FTIR光谱表明二者通过配体交换络合在一起，1385 cm^-1处为-COO-Fe的峰；由荧光光谱的数据计算得到腐殖酸中有74％的荧光团参与络合，Fe（Ⅲ）-HA络合物条件稳定常数的对数值（logKc）为4．28。（3）为了更好的模拟自然环境，给氙灯加上了滤光片，使得透过的光是波长大于290nm的光。氙灯（λ>290 nm）光照60 h，在腐殖酸、Fe（Ⅲ）和Fe（Ⅲ）-HA络合物的溶液（pH 6．1）中，分别有8．5％、25％和56．3％的阿特拉津发生了光降解。分别测定了这些过程中生成的Fe（Ⅱ）和H₂O₂的浓度，结果发现Fe（Ⅲ）-HA络合物的存在明显提高了溶液中Fe（Ⅱ）和H₂O₂的浓度，从而生成更多的^·OH，促进阿特拉津的光降解。光照Fe（Ⅲ）-HA络合物溶液60 h的过程中，Fe（Ⅱ）和总铁的浓度比（Fe（Ⅱ）／Fe（t））在20-32％的范围内，当加入阿特拉津之后，Fe（Ⅱ）和H₂O₂的生成量都有所下降，Fe（Ⅱ）／Fe（t）降到10-22％，Fe（Ⅱ）的浓度几乎降到未加入阿特拉津前的一半，进一步证明腐殖酸和铁的络合物引发^·OH生成是阿特拉津光降解的主要原因。腐殖酸对铁循环和活性自由基的产生起催化作用；随着腐殖酸浓度的提高，Fe（Ⅱ）和H₂O₂的生成呈增加趋势，阿特拉津的降解也随之加速；但当腐殖酸浓度相对较高时，也会对^·OH起到淬灭作用。（4）为了研究腐殖酸的结构和其光化学性质之间的关系，采用体积排阻色谱（SEC）对腐殖酸进行分级，得到三个级分，分子量排序为：F_A>F_B>F_C。采用元素分析、¹H NMR、FTIR和荧光光谱各种方法表征，结果表明与级分F_A和F_B相比，小分子量的级分F_C含有较多的含氧官能团，芳香族含量高，荧光响应值大，对Fe（Ⅲ）络合能力强。氙灯（CHF-XM35-150W）照射下，阿特拉津在小分子量级分F_C的作用下降解较快，这可以归因于F_C含有较多的荧光团，具有更好的光敏性。阿特拉津在Fe（Ⅲ）与F_A、F_B、F_C组成的络合体系中的光降解是由^·OH引起的，Fe（Ⅱ）和H₂O₂的生成量在F_C和Fe（Ⅲ）共存的溶液中较多，阿特拉津的降解较快。用Fe（Ⅱ）的生成量来考察三种腐殖酸级分F_A、F_B、F_C的还原性，表明F_C由于具有高的芳香性和含有较多的含氧官能团而表现出最高的还原能力。总之，本研究考察了铁循环和H₂O₂的形成在腐殖酸和铁共存体系降解污染物过程中的作用，证明腐殖酸和铁络合物光照作用下产生的^·OH是引发阿特拉津光降解的主要原因；初步确认腐殖酸中的光敏级分为小分子量的级分，芳香结构和含氧官能团相对含量高，与铁络合前后都表现出高于其它级分的光催化降解阿特拉津的能力。本项研究对正确认识腐殖酸的环境行为及持久性有毒污染物的生态风险性具有重要意义和参考价值。更多还原

【Abstract】 Humic acids （HA）, the most widespread organic substances in natural waters, play important roles in the environmental behaviour of organic pollutants. In natural aquatic systems, photochemical processes are important pathways for the transformation of persistent toxic substances （PTS） that are poorly biodegradable. Iron, a ubiquitous element in natural water, is involved in complexation by HA, and the formed Fe（Ⅲ）-HA complex could affect the photodegradation of PTS significantly. Previous studies have shown that humic materials often control many photochemical reactions of PTS in two ways. That is, the presence of HA could either enhance or inhibit the photolysis of organic pollutants. In this work, the photochemical formation of Fe（Ⅱ） and hydrogen peroxide （H₂O₂） coupled with HA was studied to understand the significance of iron cycling in the photodegradation of atrazine under simulated sunlight. Then HA are separated based on the molecular weight, and relationship between the photoinductive activity and the structural properties of HA fractions were investigated with and without Fe（Ⅲ）. These works are helpful in understanding the potential and mechanism of PTS photodegradation in natural waters and getting more insights into photosensitizing properties of Fe（Ⅲ）-HA complex. The main results are as following:（1） Because of the chemical complexity, variable chemical composition, and polydispersity of HA, citrate is selected as the analogue of HA considering that citrate is one of the small molecules produced from HA photodecomposition. The photodegradation of atrazine in aqueous solutions containing citrate and Fe（Ⅲ） was studied under Xe lamp irradiation. It was found that the presence of Fe（Ⅲ）-citrate complex enhanced the photodegradation rate of atrazine as a result of ^·OH attack. The rate of atrazine degradation was considerably reduced with increasing pH from 3.5 to 8.6, which could attributed to that the pH also controls the iron speciation. Higher light intensity and citrate concentrations lead to increased photodegradation of atrazine. Citrate not only acted as a carboxylate ligand but also a reductant of Fe（Ⅲ）. The interaction of Fe（Ⅲ） with citrate was characterized using UV-visible absorption and fourier transform infrared spectroscopy （FTIR）, indicating that the hydrogen ions on the carboxyl groups were exchanged for Fe（Ⅲ） ions.（2） Under Xe lamp irradiation, atrazine photodegradation was inhibited by the presence of HA at pH 6.1, and the rate decreased with increasing HA concentration. However, the rate for atrazine photolysis was promoted in solutions containing both HA and Fe（Ⅲ）. Interactions of Fe（Ⅲ） with HA were characterized by SEM, EDX, UV-Vis and FTIR, revealing that Fe（Ⅲ）-HA complex was formed by ligand exchange between oxygen groups of HA and Fe（Ⅲ）. Using fluorecence spectrometry the stability constant （Ax） and the fraction of fluorophores available for complexation （f） were obtained as logKc = 4.28 and f= 74%.（3） The Xe lamp in combination with a special glass filter restricting the transmission of wavelengths below 290 nm was used for sunlight simulation. At the irradiation time of 60 h, the photodegradation of atrazine was observed with 8.3%, 25% and 56.3% removal, corresponding to the presence of HA, Fe（Ⅲ） and Fe（Ⅲ）-HA complex, respectively. The formation of Fe（Ⅱ） and H₂O₂ was significantly enhanced by the presence of Fe（Ⅲ）-HA complex, and the subsequent product of Fe（Ⅱ） oxidation by H₂O₂, hydroxyl radical （ ^·OH）, was the main oxidant responsible for the atrazine photodegradation. During 60 h of irradiation, the fraction of iron presented as Fe（Ⅱ） （Fe（Ⅱ）/Fe（t）） decreased from 20%-32% in the presence of Fe（Ⅲ）-HA complex to 10%-22% after adding atrazine. The rate of atrazine photodegradation in solutions containing Fe（Ⅲ） increased with increasing HA concentration, suggesting that the complexation of Fe（Ⅲ） with HA accelerated the Fe（Ⅲ）/Fe（Ⅱ） cycling. At a relatively high concentration, HA could act as a scavenger of ^·OH that are produced in the photo-Fenton reaction and hence compete with atrazine for ^·OH.（4） HA were separated based on the molecular weight （M_W） and three fractions were obtained following the order of M_W: F_a > F_b > F_C. The characteristic results of elemental analyses, ¹H NMR, FTIR and fluorescence spectra showed that the small size fraction （F_C） was characterized by greater aromaticity, more oxygen groups and higher fluorophore. In addition, F_C are more efficient at binding Fe（Ⅲ） than F_a and F_b. Photodegradation of atrazine under simulated sunlight （CHF-XM35-150W） was much faster in solution containing Fc since the structure of F_C was dominated by more fluorophores. In the presence of Fe（Ⅲ） complexes with F_a, F_b and F_C, ^·OH was responsible for atrazine photodegradation. Due to the higher aromaticity and oxygen groups involved in F_C, more Fe（Ⅱ） and H₂O₂ were generated in solution containing Fe（Ⅲ）-Fc complex, leading to the rapid degradation of atrazine under Xe lamp irradiation. The capacity of electron transfer, estimated from the amount of photoformed Fe（Ⅱ）, was also highest for F_C.In summary, a better understanding of indirect photodegradation of PTS by ^·OH generated by sunlight interacting with sensitizers （e.g., HA and Fe（Ⅲ）） will contribute to elucidating the potential photochemical process occurring in natural waters; the relationship between the structure and the photoinductive activity of HA in the presence of iron would provide valuable insights into the different role of humic materials on pollutants fate in natural surface waters.更多还原