The Study On Removal Of PAHs In Sediments Of Tiaozi River,Siping By Zero-valent Iron

Persistent organic pollutants (POPs) are kinds of organic chemicals which aredifficult to degrade in the environment. They show a potential risk to the globalenvironment and great influences on the human health because of their persistence,high toxicity and liquidity. Polycyclic aromatic hydrocarbons (PAHs) are typicalpersistent organic pollutants, which are teratogenic, carcinogenic and mutagenic.PAHs have a lot of pollution source and are easy to migrate in soil and water, causinggreat harm to human health. PAHs in sediment increase the local concentrationthrough the food chain enrichment in ecological system, which results in the pollutionof aquatic ecosystems. The higher the molecular weights of PAHs are, the less harm tothe environment they bring. Because of low solubility in water and volatile of highmolecular weight of PAHs, generally compounds with2to7benzene rings have theenvironmental significance.At home and abroad, PAHs remediation has attracted great attention for thesevere contamination. The approaches to remove PAHs mainly include physicalremediation, chemical remediation and bioremediation. This research used the zero-valent iron to remove PAHs in sediment. The pollutants are converted into low-toxicor indissolvable forms by adding chemical reductant(zero-valent iron) into pollutedsediments. With a strong reducibility and active chemical property, zero-valent ironcan reduce strong oxidizing ions or compounds, and certain organic compounds. As aresult of the zero-valent iron’s extensive sources, low cost and no secondary pollution,it is a kind of good reductant. In addition, the zero-valent iron also has adsorption. It isgenerally believed that zero-valent iron removes pollutions by absorbing them to thesurface, then following by the redox reaction. At present, zero-valent iron is widelyused in remediation of soil and groundwater, but rarely in the removing of PAHs inthe sediments.This research focused on removal of PAHs in the sediments by zero-valent ironbased on national “Major projects of science and technology for water pollutioncontrol and management” subject (No.2012ZX07202-009) and “The project of Environmental Protection Department in Jilin Province”(No.2010-14). In this studythe sediment of Tiaozi River in Siping was selected as the research object. After freezedrying, autoclaved sterilization and sieving, the ultrasound extraction combining withsilica gel column chromatography purification, rotary evaporation and nitrogen-blowwere used to extract PAHs in the samples. The contents of PAHs were determined byGC-MS. Single factor experiment and orthogonal experiment were conducted to getthe best conditions and the highest removal rate, so that scientific reference basiscould be provided for engineering and reasonable methods to control water pollution.The results showed that in the range temperature of5℃to25℃of theexperiment, the higher temperature, the higher rate in the zero-valent iron removingPAHs in sediment. In the range of zero-valent iron particle size of0.15mm to1-2mmof the experiment, the smaller of zero-valent iron particle size, the higher the removalrate. Within the limit content of0.5g to2g of zero-valent iron in the experiment, themore content of zero-valent iron adding, the higher rate is in the zero-valent ironremoving PAHs in sediment. In the study, the highest removal rate appeared at theexperiment temperature of25℃, zero-valent iron particle size of0.15mm, zero-valent iron quantity of2g to deal with the sediment5g. The initial total contents of16kinds of PAHs in sediment were2111.3ng/g~2151.2ng/g, while the repaired totalcontents of PAHs were879.4ng/g~942.9ng/g. The removal rate of16kinds of PAHswas from50.25%to69.82%and the average removal rate is56.95%. Of all the PAHs,the removal rate of Phenanthrene was highest, whose content in the sediments wentfrom202.6ng/g down to61.1ng/g. the removal rate of Benzo (a) anthracene was thelowest, whose content in the sediments went from45.6ng/g down to22.7ng/g. Theaverage removal rate of PAHs in low ring is higher than the middle ring, the averageremoval rate of PAHs in middle ring is higher than the high ring. In addition, therange analysis of removal effects of16kinds of PAHs showed that temperature hadthe most influences on removal effects, followed by zero-valent iron particle size,influences of zero-valent iron content on the removal rate were the least.