| Organochlorine (OC) pollutants are typical contaminants with long half-life periods and easily bioaccumulation character, which cause carcinogenesis, teratogenesis and mutagenesis effects in the environment, and even human health. Constructed wetlands (CWs) have outstanding technical advantages in OC pollutants removal for its low investment and operating cost, convenient management and the ability to beautify the environment, etc. However, the mechanisms of OC pollutants removal in CWs are still lacking. In addition, the concentration of OC pollutants in the CW effluent is usually high. Therefore, there is an essential need to study the mechanisms and optimization of OC pollutants removal in CWs, provide scientific basis on the application of CWs in OC pollutants removal.This research focused on two typical OC pollutants, endosulfan and triclosan, which belong to organochlorine pesticides (OCPs) and organochlorine pharmaceutical and personal care products (PPCPs), respectively. The removal mechanisms of endosulfan and triclosan were revealed through analysis of results from both field investigation and lab-scale studies. Biological enhancement and plant optimization method for endosulfan and triclosan removal in CWs was proposed and their strengthening mechanisms were elucidated. Main conclusions are as follows:(1) Nitrogen (N) and phosphorus (P) nutrients were important influence factors of endosulfan migration and transformation in soil. And bacteria were important microorganism category for endosulfan biodegradation. Investigation of polluted soil in pesticide factory indicated that β endosulfan was the prevail form of endosulfan ranged from 43.30 to 100% because of its slow degradation and fast spread. In addition, endosulfan residue had significant correlation with the concentration of P and C in soil. According to the study of microbial degradation mechanism, bacteria contributed more efforts in endosulfan removal than fungi, and the difference between them enlarged with the increase of endosulfan concentration. Besides, bacteria exhibited higher tolerance ability of endosulfan than fungi. Therefore, bacteria played a more important role in the degradation of endosulfan.(2) Biological enhancement promoted endosulfan removal in CWs, and the single biological enhancement was more appropriate than the combined ones. Bioaugmentation and biostimulation with extra carbon source probably promoted microbial degradation process of endosulfan, and biostimulation with extra phosphorus source promoted absorption process by plants. Both the microbial degradation process and the absorption process by plants were enhanced by combined biological enhancement. However, comparing with single biological enhancement, the combined one only slightly improved endosulfan removal efficiency with much higher cost and more complicated operation. Therefore, single biological enhancement, especially bioaugmentation and biostimulation with extra carbon source, were recommended in optimizing endosulfan removal in CWs.(3) Triclosan can be efficiently removed by CWs, and the absorption by plants had great potential for triclosan removal. The removal efficiency of triclosan were 22.44～78.82% in typical large-scale CWs in winter. And since the average daily triclosan released to downstream water bodies by CW effluent was 4.40～92.60 g, the potential threat should not be ignored. Triclosan removal efficiency in lab-scale CWs with Lemna minor (L. minor) was higher than 80%, among which 44～56% of triclosan were removed by substrate adsorption and 30% by biodegradation. The absorption by plants increased with the operation period.(4) Plant species optimization increased the removal efficiency of triclosan, and the bio-sediment accumulation factors (BSAFs) could be used as indicators for the interaction of substrates, plants and microbes. Emergent plants showed better removal ability of triclosan than submerge plants and the worst removal efficiency was obtained in floating plants systems. Microbial structure and abundance in the substrates of CWs were all affected by triclosan, as well as plant species. Beta-Proteobacteria played the most important role in triclosan biodegradation of Typha angustifolia and Hydrilla verticillata systems, while delta-, and gamma-Proteobacteria, as well as Sphingobacteria, worked in Salvinia natans system. By analyze the correlation among BSAFs, bioaccumulation ability of wetland plants, and microbial degradation, it is indicated that BSAFs could be used in optimization strategy like plant species selection which expanded our knowledge on BSAFs.(5) According to the different characters of OC pollutants, different biological optimization methods were suggested to improve OC pollutants removal. For OC pollutants with long half-life and volatile properties, biological optimization measures with enhanced microbial degradation process were recommended. While for those none persistent OC pollutants with short half-life, their removal efficiency could be optimized by optimization in plant species and configuration of CWs.The application of biological optimization in CWs can not only improve the removal efficiency of OC pollutants, but also save the operation costs. In addition, it has great meaning in water quality safeguards, economic values incensement of CWs, and improvement of environmental and social economic benefits. This research provides theory basis for applying biological optimization to improve OC pollutants removal, and has engineering significance in the optimization of OC pollutants removal in CWs and practical significance to water purification and human health protection. |
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