The Occurrence And Fate Of PAHs In The Guozhuang Karst Water System Of Northern China

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutants (POPs) that are carcinogenic, mutagenic and resistant to be degraded. They mainly derive from the incomplete combustion of fossil fuels and biomass. The destructiveness of PAHs to the central nerve and blood system of human beings is very strong. Especially for PAHs with alkyl chain, they cause extremely strong irritability and anesthesia to the mucous membrane. PAHs are widely distributed and persistent in the ecological environment due to their long-distance migration through environmental media (air, soil, suspended solids, water, biological), causing serious harm to the ecological environment, animals, plants and human health.Karst aquifers provide abundant high-quality groundwater resources for drinking water supply of the human society and more than25%of the world’s population lives in karst areas. The distribution area of carbonate rocks in China accounts for about14%of the whole land area. And about1/4groundwater resources across the country are located in karst area. However, the groundwater quality has been under threat of contamination.At Guozhuang karst system, the carboniferous are deposited directly on the middle Ordovician strata due to the missing of Ordovician, Silurian system, Devonian system and Lower Carboniferous strata. There are only layers of the Benxi formation between coal-bearing strata and the carbonate aquifers, showing a pattern of co-existence of "water with coal". The thickness of the Benxi formation is commonly more than30meters in Shanxi, but only20meters in the southern part of the province. It is bound to have an impact on the groundwater environment due to the coal mining activities that are so close to the limestone aquifers.Karst water is particularly vulnerable to the impact of anthropogenic activities. It is necessary to study the source, composition, migration features of POPs, and the influence of human activities in karst water system. In this paper, taking Guozhuang karst water system in Shanxi province as an example, the photodegradation, adsorption and biodegradation of PAHs in the process of surface water leakage were investigated. The concentration, distribution, formation mechanism and controlling factors of the transport of PAHs were also evaluated to provide scientific evidence and technical support for the protection of karst water resources. The research contents of this thesis are summarized as follows:1. The source and occurrence of PAHsThe concentration, spatial distribution and source of polycyclic aromatic hydrocarbons (PAHs) in topsoil, groundwater and groundwater suspended solids (SS) at Guozhuang karst water system of northern China were investigated. The total concentration of PAHs ranged from622to87882ng/g dry weight in topsoil, from4739to59315ng/g dry weight in SS, and from2137to9037ng/L in groundwater, with mean values of17174ng/g,11992ng/g and5020ng/L, respectively. The study area could be considered moderate to highly polluted with PAHs. The PAHs in groundwater samples from R1may originate from the desorption of raw coal. The PAHs in groundwater of R2may mainly originate from the recharge area and automobile exhaust emission. PAHs in R3could be related to the discharge of coking wastewater into the river water which may later leak into the subsurface. PAHs of discharge area R4mainly came from the migration from above three areas. The contents of PAHs in samples from R3and R4were higher than those from R1and R2.In order to distinguish the proportions of low molecular, medium molecular and high molecular (LM, MM and HM) PAHs in different environment media,16PAHs compounds were divided into three groups:2+3rings,4-rings and5+6rings. The composition of PAHs indicated that LM PAHs were predominant in groundwater samples, and HM PAHs such as6-ring PAHs were not detected. The contents of MM PAHs were elevated in SS samples, and carcinogenic HM PAHs were frequently detected in topsoil. The high contents of low-medium molecular weight PAHs in groundwater and SS suggested a relatively recent local source of PAHs that were transported into the aquifer via leakage of contaminated river and/or infiltration of PAHs-containing precipitation. ANT/ANT+PHE versus Fla/Fla+PYR was plotted, which illustrated that PAHs in groundwater, SS and topsoil mainly originated from wood and coal combustion, and PAHs in very few topsoil samples originated from petroleum combustion.Although the composition and ratio of selected PAHs compounds could be taken as indirect indicators of their sources of PAHs, the relationship among different factors and their percentage of contribution to total contamination was not clear. So five factors (oil, coal combustion, vehicle, biomass combustion, coal tar) were selected to analysis the sources of PAHs using PMF model. The relative contributions for the1-5factors were2%,32%,22%,27%,18%, respectively. The major sources of PAHs in groundwater were categorized in this study as pyrogenic origin, especially coal combustion, accounting for50%. And like many other areas in northern China, large amounts of consumption in both industrial and domestic sectors may have made coal the primary contributor to PAHs in Guozhuang karst water system. 2. Photodegradation of PAHsThe photodegradation of PAHs in surface water was studied using self-designed cylindrical photodegradation reactor. The photolysis rates of the selected three PAHs in water were quite fast and the photolysis half-lives of PAHs only need several minutes (fluorine (FLU),4.17min; phenanthrene (PHE),3.79min; pyrene (PYR)4.77min). The photodegradation data fitted first-order kinetics well. The absorption spectrum and peak values were different (PYR absorption spectrum was the least, and the peak value was the smallest, when PHE absorption spectrum included lots of different intensity of wavebands). So the photolysis rates of PAHs, followed the decreasing order: PHE> FLU> PYR.The influence of main anions (SO42-, HCO3-and NO3-) of karst water to the PAHs photodegradation was investigated. The addition of bicarbonate radical led to different degrees of increase in the removal rates and efficiency of PHE, which indicated that there was no free radicals reaction involving in the PHE degradation. However, in the presence of HCO3-, the photodegradation of FLU and PYR was significantly inhibited. And further increasing of the HCO3-concentration from200to400mg/L resulted in additional decrease in the degradation of FLU and PYR, indicating that oxidation by hydroxyl radicals was the main degradation pathway of FLU and PYR. NO3-is a main source of OH in natural water, which can improve the efficiency of the PAHs photodegradation. However, at higher concentration (20mg/L) of NO3-, a slight decrease in photodegradation of the PAHs was observed. There was a strong adsorption of NO3-as an inner inert filter in the ultraviolet region, preventing light through the solution. There were both "·OH" generation and capture reaction in sulfate reaction in water. The photolysis rates of FLU and PYR decreased with the addition of SO42-, which indicated that the "·OH" capture reaction was dominated in the experiment process.In the presence of HA, it was more likely to protect it from oxygen and photodegradation due to the adsorption of PAHs within the complex HA structure. And the photolysis rates decreased due to the light absorption by HA. In our experiments, When HA concentrations were0,20and40mg/L, the photolysis rates k were0.032,0.029and0.026for FLU;0.045,0.036and0.031for PHE;0.016,0.0081and0.0042for PYR, respectively. Photodegradation of PYR was restricted most significantly.3. Adsorption of PAHs The kinetics of PHE was well fitted with the pseudo-first order model, and isotherms could be described with the liner isotherms. The sorption of PHE could be divided into three stages: the first stage, PHE was adsorbed by the exterior surface of limestone; the second stage, the molecular PHE might enter intraparticle pores of the limestone and adsorbed by the interior surface of the particle; the third stage only occurred after12h. The adsorption of PHE on limestone was controlled by particle diffusion. The adsorption capacity of limestone to PHE was calculated to be12.31μg/g.In order to reveal the effect of carbonate and organic carbon (OC) on PHE sorption, limestone sample was treated sequentially to remove carbonate and OC. The Kd value decreased after carbonate remove, especially after OC remove, which indicated that carbonate played a minor role in the sorption, in spite of its dominant content in limestone. The R2of linear isotherm decreased from0.960to0.922after removal of carbonate and OC. Because the sorption of PAHs by inorganic fraction (i.e. carbonate, residue) represented by Freundlich isotherm, was primarily a physical process and the contaminants were adsorbed on many sorption sites.It can be seen that the sorption significantly decreased under both acidic and alkaline conditions. The FTIR spectra results showed that the existence of hydroxyl and carboxyl groups on the surface of limestone samples. Under acidic conditions, the hydroxyl groups were becoming protonated to OH2+groups, resulting in the decrease of the sorption of PHE. And acidic condition was thought to lead the dissolution of carbonate, which reduced the sorption sites for PHE; CH2groups indicated organic carbon desorbed from the surface at alkaline condition. Also the decrease of PHE sorption could be attributed to the change of the carboxy groups on the surface of limestone. In the presence of low ionic strength, calcium ions would occupy part of adsorption sites, which led to the decrease in PHE adsorption. Standard free energy (ΔG0) became larger negative when the concentration of CaCl2increased from0.005M to0.1M, which was consistent with the increase of adsorption amount.To investigate the effects of dissolved organic matter (DOM) on PHE adsorption by limestone, three conditions were designed to investigate by considering interaction of humic acid (HA) with PHE.(1) After pre-equilibrium of HA adsorbed on limestone, and subsequent sorption of PHE. It was observed that the HA concentration in solution did not obviously increase but the PHE concentration sharply decreased. This phenomenon indicated that there were still some pores for PHE after the HA adsorption equilibrium. And there was a competition adsorption between PHE and HA, the affinity of HA and limestone was stronger than that of PHE. So PHE can only replace a small amount of HA.(2) After pre-equilibrium of PHE adsorbed on limestone, and subsequent sorption of HA. The results showed that the added HA replaced PHE which was released into the solution, and HA was then adsorbed on limestone. This replacement phenomenon implied that competitiveness of HA on sorption sites of limestone was stronger than PHE.(3) In simultaneous sorption of PHE and HA on limestone, competition of HA on sorption sites of limestone was much stronger than PHE. This finding implied that even in contaminated groundwater containing DOM, limestone was effective for PAHs adsorption.4. Biodegradation of PAHsMicrobial degradation is the major degradation process in the transport and transformation of pollutants. A bacterial strain Acinetobacter sp. WSD with PAHs-degrading ability was identified based on biochemical tests and16S rDNA gene sequence analysis, which was isolated from PAHs-contaminated groundwater. Acinetobacter sp. WSD could utilize PAHs as its sole carbon source and degrade them with high efficiency. Low molecular weight PAHs such as naphthalene and anthracene were completely degraded after6d. And for high molecular weight PAHs, the degradation percentage could reach65%. Approximately12h lag phase of biodegradation and subsequent high biodegradation rate were observed for each compound. This result indicated that an acclimatization process, such as induction or de-repression of enzymes or adaptation to the toxic chemical, occurred and allowed the bacteria to cope with the toxicity of PAHs before further degradation. The results of adsorption experiments showed that a maximum of7.5%of FLO,8.5%of PHE,5%of PYR were adsorbed on the biomass.Acinetobacter sp. WSD could use PAHs, glucose and HCO3-as its carbon source at the same time. And the additional carbon source in the form of glucose or HCO3-may have increased the metabolism of PAHs in our experiments. And Acinetobacter sp. WSD more inclined to use organic carbon than inorganic carbon. The degradation rates of PAHs were stimulated in the presence of HA. And the amount of PYR degradation was increased from50%to70%with the addition of HA. HA contains hydroxyl groups, which can be converted to the corresponding polarity matrix and affect enzyme activity. And HA can provide nutrition elements to promote the growth of microorganisms.At Guozhuang karst water system, before their transport into karst aquifer, the PAHs-containing wastewater or gas emissions were first discharged or deposited in the river water containing free oxygen. Leaking river water then transported PAHs into the karst aquifer. Thus the initial step may include the oxidation of the benzene ring by mono or dioxygenase enzymes, converting the aromatic compounds to hydroxy aromatic intermediates which are further dehydrogenated to form carbonyl compounds. The final products were alkanes, carbon dioxide and water. It was important to note that after2d biodegradation phenol,2,5-bis(1,1-dimethylethyl) was formed. And it could not be degraded or utilized by Acinetobacter sp. WSD. There are two major novel points in this thesis:(1) to reveal the proportion of each pollution source using PMF software;(2) to isolate high efficient degradation bacteria of PAHs from karst groundwater in a coal-mining area and explore their biodegradation mechanism.