| Poly Brominated Diphenyl Ethers (PBDEs) were widely detected in the air, water, sediment, animal and human body in the environment has been recognized to be a kind of global Emerging Persistent Organic Pollutants (POPs). PBDEs may affect human nerve, endocrine, thyroid, liver and kidney; they may also lead to embryonic malformation. At present, studies of environmental problems of PBDEs have become hot spots of international academia.Water environment is an important part of PBDEs global cycle. PBDEs can go into water environment through surface runoff, atmospheric dry and wet deposition, or other paths. They can also be released back into the soil and atmospheric, or be used by organisms and accumulated as food chain under certain conditions, which make them into the global cycle again. One important source of PBDEs into water environment is wastewater discharge or irrigation, because it was found that water treatment processes in the Sewage treatment plant cannot significantly degrade or eliminate PBDEs. Investigation results showed that wastewater discharge or irrigation has become a significant pollution source of PBDEs into water environment.Although PBDEs are hydrophobic and can be strongly adsorbed on soil and sediment, they have been detected in groundwater. It was worth noting that PBDEs in groundwater may accumulate in human body or crops by drinking water or plant growth, these will be a potential risk for human health and water environment. However, our knowledge of PBDEs pollution in groundwater and their behavior mechanism in the unsaturated zone is limited. It is still unknown that how these hydrophobic POPs breakthrough unsaturated zone and into groundwater system, and what has happened during their migration and transformation in this dark environment.This study focused on this global Emerging POPs-PBDEs and the vacant area of their global circulation-groundwater environment. The typical sewage irrigation area of Xiaodian Irrigation Area in Taiyuan city was selected as the study area to study PBDEs behavior in groundwater environment. We are aimed to figure out three problems, that is,"How does PBDEs distribute in groundwater","How does hydrophobic PBDEs go through unsaturated zone and into groundwater environment", and "What has happened for PBDEs in soil-groundwater system". For the above questions, in this study:(1) Wastewater, soil, sludge and groundwater samples were collected for PBDEs measurement to analyze the distribution characteristics of PBDEs in the Irrigation Area and identify their major species and pollution sources.(2) Soil, sand and colloid were used as adsorbent to study the adsorption characteristics of major PBDEs on them.(3) Column experiments were carried out to test the effect of HA colloid on the adsorption and migration of PBDEs in saturated porous media. The numberical models of PBDEs migration in saturated porous media were built to fit experiment data. In addition, a new method of fluorescence spectra for aqueous PBDEs analysis was developed and used in this experiment.(4) Interaction of HA colloid and PBDEs were modeled to identify the effect of colloid on PBDEs geochemical behavior on the molecular scale.It was found that:(1) The major species of PBDEs were variable as the environment change, the major species of PBDEs in soil samples in2011were BDE47, BDE100, BDE154and BDE153, while that of PBDEs in soil samples in2012were BDE71, BDE85, BDE95and BDE154. The major species of PBDEs in wastewater and shallow groundwater samples were BDE47and BDE28, and they mainly distributed in the colloid or particles with the size of<2μm in water samples. Wastewater irrigation was the main pollution source for PBDEs in this area, meanwhile, widespread use of agricultural film could be a potential source of PBDEs in soil and groundwater.(2) Adsorption of BDE28and BDE47on soil and sand was mainly controlled by chemical action, their adsorption processes were divided into three stages:(i) highly quick absorption, their contents decreased to be35%-40%of original concentration in0.5-1h, which is controlled by surface diffusion process;(ii) quick adsorption, their contents decreased to be25%of original concentration in1-10h, which means that PBDEs take place of water molecular and are adsorbed on the inside/outside of surfaces; and (iii) slow adsorption, their contents decreased to be about24%of original concentration in10-24h, PBDEs have go to the inside of particle.(3) Under the influence of organic matter content, adsorptive capacity of both BDE28and BDE47on soil was higher than that on quartz sand. In the equilibrium concentrations of4-30μg/L, the Kd of BDE28on soil and sand were773-1456L/kg and456-1103L/kg, respectively. Their Log Koc was in the ranges of4.59-5.87. In the equilibrium concentrations of1-15μg/L, the Kd of BDE47on soil and sand were901-1547L/kg and482-850L/kg, respectively. Their Log Koc was in the ranges of4.5-5.3and4.8-5.4, respectively.(4) After added HA, equilibrium adsorption of BDE47on quartz sand decreased obviously. The Kd value of BDE47on sand with0,0.1and1mg/L HA in solutions were calculated by linear adsorption model to be853.3L/kg (616.33-972.63L/kg),12.63L/kg (8.99-14.76L/kg) and2.47L/kg (2.12-3.48L/kg), respectively. These indicated that there is competitive adsorption of BDE47on HA colloid and sand. The higher of HA content in the liquid phase, the inhibition of hydrophobic BDE47adsorption on solid was stronger. (5) Compared with conventional GC-based analytical methods, the fluorescence spectroscopy method is more efficient for PBDEs migration study, because it only uses a small amount of samples (4ml), avoids lengthy complicated concentration and extraction steps, and has a low detection limit of a few ng/L. Based on this method, models of PBDEs transport in saturated sand column under the effect of HA colloid were conducted to fit the experimental data well. When HA concentrations were0,0.1and1mg/L, the breakthrough time of BDE47into column were8×10,(7-8)×102and120pore volumes, respectively. These further confirmed that HA colloid significantly promoted BDE47migration.(6) HA interacted with PBDEs mainly by hydrogen bonding or hydroxyl. After interaction with HA, the surface area, volume and polarity of PBDEs increased while their hydrophobicity decreased, which increased PBDEs migration in water environment.Finally, based on the reported study, field investigation and laboratory experiments, migration model of PBDEs in soil and groundwater system in irrigation area was constructed. Speciation change analysis result showed that the key processes of PBDEs go through unsaturated zone and into groundwater were (i) aqueous PBDEs reacted with HA colloid to form colloidal PBDEs, and (ii) adsorbed PBDEs on soil and sediment transferred to colloidal PBDEs under the effect of rainfall and irrigation.The innovations of this paper are:(1) We first proposed that "Colloid may be the key path of PBDEs into groundwater". The effect of colloid on PBDEs transportation between soil and groundwater was studied to explain that how PBDEs go through soil and into groundwater and3D structures of PBDEs and HA were modeled to further explain the geochemical behavior of PBDEs in the system of soil and groundwater.(2) A new method was developed for rapid and direct measurement of aqueous PBDEs using fluorescence spectroscopy, which can be well applied on migration mechanism study of PBDEs under the effect of HA colloid, and will supply a new tool for PBDEs in situ measurement in water environment. |
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