| As active agents in cleaning and disinfecting products, antimicrobials are an important group of pharmaceuticals and personal care products (PPCPs). Antimicrobials have widely spread in the environment, and drawn extensive attention for their potential threats to ecological system and human health. Chlorophene (CP) and dichlorophen (DCP) are broad-spectrum emerging antimicrobials. CP functions as an active agent in disinfectant formulations in hospitals, households, industrial and farming environments. DCP functions as a bacteriocide and fungicide in soap and other personal care product formulations, and it has high activity against intestinal tapeworms. CP and DCP are relatively hydrophobic with estimated logarithmic n-octanol/water partition coefficient (LogKow) of 3.6-4.2 and 4.3, respectively, and are likely accumulated in organisms and sediments once released to the environment. CP and DCP are potential persistent organic pollutants and may remain in soil or water environment for prolonged period. It was documented that CP had carcinogenic and mutagenic activity in animals, and DCP has been recognized as very toxic to aquatic animals. These properties cause low removal efficiency of CP and DCP by general wastewater treatment systems, thus huge amount of produced and consumed antimicrobials would inevitably bring on the continual releasing of them to the environment. It has been reported that these two kinds of antimicrobials were detected in a number of natural waters. Thus, the degradation process of CP and DCP needs to be further studied.Enzyme-catalyzed degradation reaction, especially laccase, is a widely used method for the treatment of pollutants,Laccases are blue multi-copper oxidases, which are widely found in nature. Laccase-catalyzed reaction has been examined as a highly efficient strategy to remove certain trace contaminants from water/wastewater. The most common substrates are phenolic pollutants. Thus, the study of the enzyme-catalyzed degradation process of antimicrobials can help to comprehend the transformation mechanisms of organic contaminants, and have significance to the development of aquatic remedial methods.Natural organic matter (NOM) is ubiquitous in natural waters and is well known to influence various environmental processes. As a relatively smaller and more soluble fraction of NOM, fulvic acid (FA) is usually present in water at concentration orders of magnitude higher than the target contaminants. FA could react with organic contaminants through several pathways in enzyme-catalyzed process. Furthermore, FA would affect the environmental behavior and fate of these pollutants. Consequently, study on the effect of NOM, especially FA, on the enzymatic degradation process is important for the treatment of antimicrobials.In the present study, enzyme-catalyzed degradation process of CP and DCP were systematically investigated. The influence of ambient FA on the enzymatic degradation process was also studied. Meanwhile, we also discuss the effects of simulated solar light irradiation on the degradation process. The primary results are summarized as follows:(1) Laccase-catalyzed removal of CP. Batch experiments were conducted to determine CP removal at various reaction conditions. It came to a conclusion that the optimum reaction conditions were at pH 6 and at 25℃. Laccase-catalyzed CP removal was a second-order reaction, and the second-order rate constant was 0.21 U-1·mL·min-1.According to the degradation products, the possible pathways of laccase-catalyzed CP may occur via three different pathways. In pathway I, the nucleophilic substitution of chlorine by hydroxy group and further oxidation products were found. Pathway Ⅱ led to the direct polymerization. Pathway Ⅲ involved the etherification reaction between CP and its products. Furthermore, taking both charge density and spin density into account, the possibility for coupling between C1 on one radical and C1’on the other and between the hydroxyl group 08 and C1’ tended to be greater. The toxicity assay demonstrated that the laccase-catalyzed reaction in toxicity removal for CP was highly effective.(2) Laccase-catalyzed removal of DCP. Similar to CP, laccase-catalyzed DCP removal was also a second-order reaction, and the second-order rate constant was 0.29 U-1·mL·min-1. Laccase-catalyzed removal of DCP follows two main possible reaction pathways. Pathway I lead to the formation of a hydroxylated and mono-dechlorinated product. Pathway II included two dimer species. Similarly, the possibility for coupling between C1 on one radical and C1’on the other and between the hydroxyl group 08 and C1’tended to be greater. In addition, the toxicity of DCP was nearly completely removed.(3) The effects of FA on the behavior of laccase-catalyzed removal of CP and DCP. The kinetic experiments revealed that the presence of FA significantly inhibited the laccase-catalyzed removal of CP and DCP, and the inhibition was quantitatively decreased with the elevated CP and DCP concentration. Mechanism study showed that FA may act as a free radical scavenger to reduce the intermediate products to the substrate in laccase-catalyzed reaction. Furthermore, Michaelis-Menten reaction kinetics suggested that DCP had a higher affinity to the enzyme than CP did. Therefore, compared with CP, DCP was a more suitable substrate for laccase. It seemed that the presence of FA could weaken the affinity between the substrate and the enzyme.(4) The effects of simulated solar light irradiation on the laccase-catalyzed removal process of CP. It is found that the degradation efficiency of CP by simultaneous photocatalytic-enzymatic process was significantly increased as compared with the results obtained when laccase or simulated solar light irradiation were separately used. The removal percentage of CP could reach 72% within 45 min with simultaneous 0.05 U/mL laccase and simulated solar light irradiation, greater than that obtained when simulated solar light (55%) or laccase (52%) was separately applied. Laccase could be rapidly deactivated upon exposure to simulated solar light, which was the main factor limiting the degradation efficiency of combined action. |
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