Occurrence And Influencing Factors Of PAHs And PAHs Degraders In Sediments Of Knysna,South Africa And Major Estuaries Of China

Polycyclic aromatic hydrocarbons(PAHs)are a kind of organic pollutants widely existing in the nature environment.Because of their carcinogenic,teratogenic,mutagenic effects and bioaccumulation characteristics,PAHs can pose a certain threat to human and ecological environment health.The estuarine area is a typical area with intensive human activities,and also a typical area of land sea interaction.It not only receives the pollutants from the upstream river,but also is an important"sink"of urban pollutants in the downstream estuary.Its complex hydrodynamic conditions and environmental factors form a natural physical and chemical property gradient in the estuary,which makes the pollutants have its unique occurrence characteristics in the estuary,and at the same time,it is also an important"sink"of urban pollutants in the downstream estuary It is also an ideal habitat for various microbial communities,so the importance of its ecological environment is self-evident.Although a lot of studies have been done on the occurrence characteristics,source identification and ecological risk of PAHs in estuaries,the research on temperate estuaries in the southern hemisphere which are less affected by human activities is not enough,and especially the study on bacterial communities and PAHs degraders is not complete.In addition,there are few studies on the source analysis of each monomer PAH.In this study,the sediment of the Knysna estuary in South Africa was taken as the research object,and the PAHs occurrence characteristics of the Knysna estuary in South Africa were compared with those of the main estuaries in China.The concentrations of 16 PAHs in the estuarine sediments that were preferentially controlled by the U.S.Environmental Protection Agency(EPA)were determined,and the differences of PAHs occurrence characteristics between the temperate estuaries in the southern hemisphere and the densely populated estuaries in the northern hemisphere were analyzed.Stable carbon isotope(?¹³C)method was used to identify the source of PAHs in the sediments of the Knysna estuary in South Africa.Based on the existing literature,the potential PAHs degrading bacteria community in the sediments of the Knysna estuary in South Africa was explored,and the influencing factors of PAHs and PAHs degraders were analyzed,so as to provide strong scientific data support for the protection of estuarine ecological environment The main conclusions are as follows:(1)The concentrations of 16 PAHs ranged from 94-356ng/g in the surface sediments and 28-1675ng/g in the sediment column of the Knysna estuary,South Africa.The concentrations of PAHs were in the dry season>wet season,and in the sample with greater impact of human activities>the sample with less impact.In the sediment column,PAHs showed a pattern of middle enrichment,which may be related to the forest fires in the Knysna area.In terms of composition,low-molecular-weight PAHs and middle-molecular-weight PAHs were the main components in the sediments of the Knysna estuary.The concentration of PAHs in the sediments of the main estuaries in China ranged from 50.4 to 3310.3 ng/g,which also showed the characteristics of dry season concentration>wet season.In the wet season,the concentration of PAHs in the southern estuary was higher than that in the northern estuary,but a higher concentration of PAHs was still detected near the estuary.This may be due to the higher level of urbanization in Beijing Tianjin Hebei region and the traditional heavy industry in this region.The dry season showed the opposite characteristics,which may be due to the use of coal heating in the north in winter,the additional emission of PAHs.In general,the concentration of PAHs in the Knysna estuary of South Africa,which was less affected by human activities,was lower than that in the main estuaries of China,and it was also at a lower pollution level in the world.(2)The results showed there was a weakly positive correlation between the concentration of PAHs and the concentration of TOC.In the main estuaries of China,there was a significant correlation between TOC and PAHs,indicating that TOC played an important role in the occurrence and distribution of PAHs.There was a significant positive correlation between the total PAHs concentration and clay content in the study area,and a significant negative correlation between the total PAHs concentration and the size of the sediment.This may because that the fine particles have a larger specific surface area and are able to adsorb more PAHs.(3)According to the diagnostic ratios,most PAHs in the sediments of the Knysna estuary in South Africa and the main estuaries in China were generated from coal and biomass combustion.Through quantitative analysis by PMF model,it was found that the main sources of PAHs in the Knysna estuary were coal and biomass combustion,loading 43%of the total PAHs,while the main sources of PAHs in the estuaries of China were traffic vehicle emissions and gasoline motor vehicles.The combined contribution of vehicles and diesel vehicles accounted for 43%of the total PAHs,which may be due to the population density of the two places and the forest fire in Knysna area.The results of stable carbon isotope(?¹³C)method showed that the value of?¹³C in PAH of Knysna estuary sediments ranged from-31.31‰to-25.79‰.The low-molecular-weight PAHs and middle-molecular-weight PAHs were mainly from biomass combustion such as agricultural straw combustion and forest fire,while the middle and high ring PAHs were mainly from various forms of coal combustion with a small amount of wood combustion.(4)Based on the risk quotient method,the ecological risk of estuarine sediments was evaluated.The results showed that most of the Knysna in South Africa and the estuaries in China were at low ecological risk level,while the Knysna sediment columns(30-50cm)in South Africa and some of the samples in Hangzhou Bay and Haihe estuary were at medium risk level due to more Pyr and Bb F emissions.(5)The main PAH degraders in the Knysna estuary of South Africa had obvious temporal variation.The dominant PAH degraders were Marinobacterium and Vibrio in April,Alcanivorax and unclassified?Sphingomonadaceae in September,Mycobacterium,Bacillus and Microbubifer in July,and Pseudoalteromonas in sediment column.PH,salinity,NO₃^-and TOC were the main environmental factors that affect the distribution and structure of bacterial community and PAH degraders.Salinity had a negative correlation with most PAH degraders,and NO₃^-and TOC content had a positive correlation with the distribution of PAH degraders.No significant effect of PAHs concentration on PAHs degraders was found,which may be due to the low PAHs concentration in Knysna estuary and other geographical conditions.In general,human activities played an important role in the occurrence of PAHs in temperate estuaries.We should reduce PAHs production by reducing and controlling the discharge of industrial waste and limiting traffic flow,to reduce the threat of PAHs and other pollutants to microbial communities,to enrich the diversity of local microbial communities,in order to protect the stability and health of estuarine ecosystem.