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Efficiency and Mechanism of Chlorobenzene Removal by Air Stripping and Adsorption Coping with A Sudd

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Tutor: CuiFuYi
School: Harbin Institute of Technology
Course: Municipal Engineering
Keywords: sudden pollution,chlorobenzene,emergency treatment,air stripping,adsorption,deso
CLC: TU991.2
Type: PhD thesis
Year:  2012
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Abstract:
As the fast development of national economy, industrialization process and theescalation of people¡¯s living standard, the water quality of drinking water sourcedecreased caused by the high development and adequate utilization of waterresources, the serious water pollution caused by organic material, and the frequentlysudden water pollution accidents. However, the relatively undeveloped watertreatment process had weak ability to cope with sudden pollution in raw water formost waterworks in china. Consequently, the study of emergency water treatmenttechnology and process had an important current significance.Emergency water treatment technology based on air stripping and PACadsorption was established to resolve the problem of volatile organic compound(VOC) sudden pollution in raw water. Chlorobenzene (CB) was used as the targetvolatile organic compound (VOC) in this paper. Experiments were conducted tostudy efficiency and influencing factors of VOCs removal by PAC adsorption andair stripping. Based on their respective advantages and characteristics, the twotechnologies of PAC adsorption and air stripping was coupled and then combinedwith the following conventional water treatment process in order to improve VOCsremoval efficiency and decrease the operation cost. Additionally, experiments wasconducted to study the adsorption of VOCs onto ACF from the off-gas emitted byair stripping and the desorption of VOCs from PAC in water, in order to developsome technology to avoid secondary contamination.Air-water ratio and temperature were the main factors influencing the removalof CB by air stripping, and the removal of CB was positively correlated with air-water ratio and temperature. However, the removal of CB by air stripping had littlerelationship with CB initial concentration and turbidity. Synergistic removal effectswas found between different VOCs while air stripping.A static mass transfer kinetic model was fitted to the experimental data toobtain the overall mass transfer coefficient. The result showed that the overall masstransfer coefficient had linear increasing relationship with CB initial concentration,and had nonlinear increasing relationship with air flowrate and temperature. Adynamic air stripping model was used to study the removal efficiency at differentair-water ratio. It was found that air stripping was efficient and economical at theair-water ratio of500-100with the removal rate of83.73%-91.14%and theoperation cost of0.033-0.065Yuan/t. However, the operation cost would increasewhen the air-water ratio was more than100.PAC can adsorb CB from water rapidly with80%and98%of the equilibrium adsorption capacity at5min and30min, respectively. PAC surface area and mixingintensity had an important influence on the adsorption efficiency by PAC, whilefactors such as temperature, pH value and ionic strength had little effect on CBremoval by PAC adsorption. Adsorption kinetics and equilibrium could besuccessfully simulated by the pseudo-second kinetic model and Freundlich model,respectively. Based on the parameters obtained from the models above, a theoreticalformula of PAC dose with initial CB concentration and adsorption time wasestablished and verified by experimental results. And the recommended PAC dosewas given based on the result above. It is concluded from pilot test that the mixtureof PAC and water and the coagulation clarification were the key process to removeCB, and the removal could reach62.5%-98.9%; the conventional treatment processcould intercept the micro flocs and particles carrying CB in water, resulting in thefurther reduce of CB; as a safety guarantee, granular activated carbon filter columnwas the last process to remove CB.The combining technology of PAC adsorption and air stripping could integrateinto the conventional water treatment process, and take the advantage of the twotechnology. PAC adsorption and air stripping shared different CB removal load,which could improve the ability of CB removal, and obtain an efficient andeconomical outcome. Emergency treatment process, with the combining technologyof air stripping and PAC adsorption being the core, the conventional water treatmentprocess being the main part, and GAC filter being the last safety guarantee, could beused to cope with VOCs sudden pollution in source water for most waterworks inour country.Competitive adsorption and concentration gradient drive would causecontaminant desorption from activated carbon. CB desorption happened withvarying degrees under the influence of competitive adsorption by stronglycompeting background compounds (CB) and pore-blocking background compounds(PSS), and the driving force by concentration gradient. CB desorbed rapidly fromactivated carbon, and reached equilibrium within20-40min. The percent of CBdesorption was3%-63%at equilibrium. It was found that the rate and amount of CBdesorption was positively correlated with the concentration of competitioncompounds (ethylbenzene). PSS had slightly effect on CB adsorption or desorptiononto activated carbon, while it slowed the rate of CB desorption due to the pore-blocking effect of PSS in macropore of activated carbon. The driving force byconcentration gradient influenced CB desorption notably. CB desorption amountincreased with increasing concentration gradient. The percent of CB desorption was32%and40%when the solution of CB and activated carbon equilibrium system wasdiluted to5and10times, respectively. The risk of CB desorption from activatedcarbon was assumed low when competitive adsorption existed, but very high if concentration gradient drive effect became notable, which always existed in theGAC filter having used for a sudden pollution. Therefore, these used activatedcarbon in GAC filter should be replaced if the filter would run after the suddenwater pollution.CB in the off-gas emitted by air stripping could be removed efficiently byactivated carbon fiber (ACF) adsorption, whose removal efficiency was concernedwith BET surface area, micropore volumn and surface chemical character of ACF,relative humidity (RH) and temperature. The larger the BET surface area andmicropore volumn, the greater the adsorption rate and capacity. RH had aninhibitory effect on CB adsorption onto ACF. Moreover, with increasing RH, theinhibitory effect becomes more significant. When RH was higher than50%, CBadsorption amount reduce remarkably by more than13%. The main factorsrestraining CB adsorption onto ACF was the competitive adsorption and capillarycondensation of water molecular in micropore. Surface modification of ACF wasconducted by being impregnated with urea or copper sulphate solution and thenheated at800¡æin the environment of nitrogen. On the one hand, it was proved thatsurface modification could decrease oxygen content, and reduce acid functionalgroups, and improve surface hydrophobicity of ACF surface, and then lower wateradsorption amount. As a result, the inhibitory effect of RH on CB adsorption wasrelieved. On the other hand, the generation of basic functional groups could increasethe adsorption rate and amount of CB adsorption onto ACF due to the increase in ¦Ğ-¦Ğ dispersive interactions between the ¦Ğ-electron system of the carbon graphenelayers and the ¦Ğ-electrons of the ring of CB molecular. Additionally, the load ofcopper oxide microcrystal enhanced VOCs adsorption because of the highadsorption bond between copper oxide microcrystal and CB molecular. Temperatureis an important factor to affect CB adsorption onto ACF. Experimental resultsshowed that CB adsorption amount decreased with increasing temperature.
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