| 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 (MW) and three fractions were obtained following the order of MW: Fa > Fb > FC. The characteristic results of elemental analyses, 1H NMR, FTIR and fluorescence spectra showed that the small size fraction (FC) was characterized by greater aromaticity, more oxygen groups and higher fluorophore. In addition, FC are more efficient at binding Fe(Ⅲ) than Fa and Fb. Photodegradation of atrazine under simulated sunlight (CHF-XM35-150W) was much faster in solution containing Fc since the structure of FC was dominated by more fluorophores. In the presence of Fe(Ⅲ) complexes with Fa, Fb and FC, ·OH was responsible for atrazine photodegradation. Due to the higher aromaticity and oxygen groups involved in FC, 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 FC.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.
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