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Çeşitli adsorbanlar ile hümik bileşiklerinin adsorbsiyonu üzerine bir çalışma

Başlık çevirisi mevcut değil.

  1. Tez No: 46119
  2. Yazar: SELMA KAYA
  3. Danışmanlar: PROF.DR. NURSEN İPEKOĞLU
  4. Tez Türü: Yüksek Lisans
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 96

Özet

ÖZET Doğal suların çoğu bitki ve hayvan yaşamından gelen organik maddeler ile organiklerin ve minerallerin sentezinden kaynaklanan bileşikler içermektedir. Bu bileşiklerin sulardan giderilmesi ve suyun içme suyu olarak kullanılabilir hale gelmesi için yasalarca belirlenen konsantrasyonların altına indirilmeleri gerekmektedir. Suda çözünmüş halde bulunan doğal organik maddeler arasında hümik bileşikler önemli bir yer kaplamak tadır. HUmik maddeler birçok fonksiyonel grup (alkol, fenol, kinon, metoksi, karboksil) içeren büyük molekül ağırlıklı kompleks yapıda maddelerdir. Bu bileşikler sudaki rengin nedenidirler, iyon değiştirme ve kompleks oluşturma özelliklerine sahiptirler, toksik maddeler (ağır metaller, pestisidler) için taşıyıcı görevi görürler, su dağıtım sisteminde aşınmaya, reçineler ile membranların tıkanmasına neden olurlar. Klorlama sırasında oluşan ve kanser yapıcı özelliği olan triha- lometanlar ve kötü tat ile kokuya neden olan klorafe- nol bileşiklerinin oluşumunda rol oynarlar. Bu yüzden günümüzde hümik maddeleri sudan uzaklaştırmak zorunluluğu doğmuştur. Daha önce bu amaçla oksidasyon, flokülasyon, filtrasyon, aktif karbon, iyon değiştirici reçine, ultrafiltrasyon ve ters asmoz gibi çeşitli ayırma teknikleri kullanılmıştır. Bu çalışmada değişik adsarbanlar denenerek hümik maddelerin adsorpsiyonu araştırılmıştır. Bu amaçla organik kökenli bileşikler (mısır koçanı, pirinç kabuğu.) ve doğal mineraller (bentonit, zeolit, aktif alumina) kullanılmıştır. En yüksek adsorpsiyon kapasite sine sahip adsorban olarak bentonit bulunmuştur. Tanecik boyutu, pH, sorbent konsantrasyonu, zaman gibi çeşitli parametreler incelenmiş ve yüksek pH'larda, küçük tanecik boyutunda ve uzun süreli adsorpsiyonda % 100'e varan adsorpsiyon verimleri elde edilmiştir.

Özet (Çeviri)

A STUDY GN THE ADSORPTION DF HUMIC SUBTANCES BY DIFFERENT ADSORBENTS SUMMARY Humic acid and related substances are widely dist ributed in terrestrial sails, natural waters, marine and lake sediments, peat bags, shales, and brown caals. They are important in the transportation and enrichment of mineral surfaces in sediments and sedimentary rocks and they serve as precursors of coal and kerogen. It has been concluded that the amonut. of organic carbon in the earth as humic acids (6D x 10 tans^ exceeded that which occurs in living organisms (7 x 10 tons). Humic substances are best described as a series of acidic, yellow to black-colored, moderately high-mole cular-weight polymers having characteristics unlike any organic compounds occurring naturally in living organisms. The modern view is that they represent a heterogeneous mixture of molecules which in any given soil or sedi ment, may range in molecular weight from as low as around 3 x 102 and 2 x 10. In classical terminology, humic acid is defined as the material which is extracted from soils and sedi ments by alkaline solutions and which is precipatated upon acidification; ful vie acid is the alkali-soluble material which remains in solution upon acidification and humin is the fraction not soluble in water at any pH value. The origin of humic substances in soils and sedi ments has yet to be resolved. The classical view is that they represent modified lignins. According to contemporary views, humus synthesis is a two-stage process which involves decomposition of all plant components, including lignin and the subsequent polyme rization of products of microbial metabolism into high- molecular-weight polymers. The role of lignin is rele gated to that of serving as a source of building blacks (polyphenols and quinones) for humus synthesis. Accordingly, fulvic acids represent initial products of“humif ication”XIand futher condensation results in the formation of humic acids and ultimately coal. Much of the reactivity of humic substances in aqueous solution is attributed to their functional groups that contain oxygen, including carboxyl, car- bonyl, phenolic and methoxyl, carboxyl and phenolic groups. Humic substances are fairly stable (i.e. their BDD is low). However, these substances are chemically oxidisable and therefore, can readily affect the results of COD determinations and they are a very important carbon resource in the natural carbon cycle. Although humic and fülvic acid molecules exhibit structural complexity, several unique features are noted: (1 ) a humic acid molecule can be considered to be a polymer with an aromatic ring structure representing the monomer and (2) acidic functional groups such as carhoxylic acids, -CDDH, are attached to the rings and can ionize as a function of pH (to-CDO“), giving a negative charge. At pH values typical of natural waters, a fulvic acid molecule is of a large molecular size with a net negative charge. Humic materials are not usually toxic, but exert major controlling effects on the hydrochemical and bio chemical processes in a water body. They significantly affect the quality of water for certain uses, especially those which depend on organoleptic properties (tasta and smell). During chlorination for drinking water disin fection, humic and fulvic acids act as precursor subs tances in the formation of trihalomethanes such as chloroform, a suspected carcinogen. In addition, substances included in aquatic humus determine the speciation of heavy metals and some other pollutants because of their high complexing ability. As a result humic substances affect the toxicity and mobilitiy of metal complexes. Therefore, measurement of the con centrations of these substances can be important for determining anthropogenic impacts on water bodies. Fulvic and humic acid concentrations in river and lake waters are highly dependent on the physico- geographical conditions and are usually in the range of tens and hundreds of micrograms of carbon per litre However, levels can reach miligrams of carbon per litre Xllin waters of marshy and woodland areas. In natural conditions fulvic and humic acids can comprise up to 80 cent of the DOC, which can be used as an approximate of their concentrations. Divalent cations may bind to humic acids the four following configurations: (a) chelation by a carboxylate and an orthophenolate ; (b) complexation by two carboxy- lates on separate molecules; (c) chelation by two carboxylates on the same molecule; and (d) complexation between a carboxylate and a partially neutralized hyd rolysis species. Humic acids have a large cation exchange capacity (CEC)=ca. 200-400 mequiv./100 g. It is mainly caused by carboxyl groups. At higher pH values especially phenol groups became active. Because of their large capacity to retain cations by exchange reactions, humic acids are of great importance for soil fertility. The relationship between corrosion and incrustation and the humic content of water is both complex and important. Small amounts of humic substances (1-2 mg/ litre) assist in the deposition of a protective layer of a calcium carbonate in distribution systems. Where lime has been added as a post- treatment corrective step for corrosive waters, larger amounts of humic acid may be responsible for the deposition of f low-restristive ”humus mud“ in distribution systems. Water containing very little dissolved humic material can be more metal- corrosive than water containing larger amounts. Since humic acid and certain of its metal complexes are poorly soluble at the pH of potable water, they may be partly responsible for turbidity in a water sample. Furthermore, since ”dissolved“ humic substances in.water exist predominantly as colloidal dispersions, and since optical measurements of turbidity are influenced by particles in the colloidal size range, such colour in water will affect turbidity values. Dissolved organic matter (DOM) is ubiquitous in surface waters and its role in their chemistry and biology is increasingly under investigation. Fundamental research concerning the properties of DOM is greatly facilitated if significant quantities of DOM can be isolated from aqueous media and separated from inorganic solutes. The overall isolatin process can conceptually xixibe divided into (1) concentration, in which all solutes are concentrated by removal of water; (2) purification, in which the DOM is separated from inorganic salutes; and (3) fractionation in which some components of DOM are accidentally or deliberately separated from others. In practice, more than one of these conceptual steps could occur in a single experimental step. Because DOM appears to play such an important bio chemical and geochemical role in aquatic ecosystems, there is considerable interest in incorporating its chemical properties into predictive models of kinetic and equilibrium processes. Characterization of DOM is generally using samples that have been isolated from environment, so it is never certain that m perties are relavant in the unperturbed na It has been suggested that DOM samples tha isolated by adsorption an XAD resins, far highly fractionated and are remarkably uni devoid of site-specific information that m present in the original water samples. He increased efforts to isolate a higher perc DOM than is normally obtained, so that the DOM will be more representative of natural conducted the.aquatic easured pro- tural system, t have been example, are form and ight be recommends entage of isolated DOM. would quant fract metho water this have recog examp rever The unattainable ideal system for isolation of DOM be able to rapidly extract and purify large ities of DOM from large volumes of water without ionation or other loss of DOM. Further, the ideal dology would work equally well in fresh and saline s. All real isolation methods fall far short of ideal; however, some relatively unfamiliar methods greater potential than has been previously nized. Among these methods, we can say for le XAD resins, DEAE cellulose, ultrafiltration and se osmosis. Artificial humic acid-like substances may be for med from hydroquinane or other phenols by oxidative polymerization at a pH=8 in alkaline medium and by air oxidation of catechol in the presence of various amino acids. Humic acids has not been used in a wide range in the technique. They are used in the production of paper pulp, tanning materials, herb protection units, xivwood palish and dye materials. Humic preparates are used alsa far medical purposes due to their adsorption and buffering properties and bactericidal and astringent action. The removal of humic substances is today one of the most important targets in water purification technology. The conventional process is coagulation- flocculation seperation to remove humic substances. Although coagulation is traditionally a pretreatment process for removing particulates, many investigators have demonstrated that it can also reduce the concent ration of naturally occurning organics. Its ability to reduce organics, however, is limited, and if unac- ceptably high concenrations remain, activated carbon must be employed. Reverse osmosis (RO) has been widely used to sepa rate inorganic salutes from water (desalination); however its use to separate organic salutes and more specifically dissolved organic matter (DDM), has been quite limited. The mechanisms by which solutes and solvent are separated in the RD process are not well understood, but some insights can be gained by studying the use af RG to concentrate pure organic compounds in water. The sepa ration of salutes and solvent by RD appears to result from preferential diffusion across the membrane of smaller molecules and/or molecules that are preferentially adsorbed at the membrane surface. The development of macroporous resins since the late fifties has created an increasing interest in humic acids removal by ion exchange in surface waters; there are today several different types of resins available for this purpose. However, study is continued in order to modify this process especially Lewatit which exists at only one full-scale plant in Europe. For utilization as ion exchangers and particularly as purification agents for textile industrial effluents, celluloses substituted by quaternary ammonium functions have been prepared and manufactured by the Institut Textile de France. For this synthesis, the quaternary ammonium cellu loses have been obtained by cellulose condensation with epoxy propyltriethylammonium chloride. Synthesis of this salt in the presence of celluloses has been achieved xvso that the grafted ammonium cellulose could be obtained from celluloses and epichlorhydrin in one step. These modified celluloses are very strong anion exchangers. Activated alumina (AA) has been used to remove various inorganic substances from dringking water supplies. The use of AA has also been suggested”for the removal of organic acids, surfactants and dissolved humic substances. It was recently reported that more adsorbed on AA after they have been preoxidized with ozone. Adsorption capacity increased as pH descreased and ozone dosage increased. The increase in adsorption caused by.ozonation is consistent with several earlier studies that showed color, particles, organic compounds, and trihalamethane (THM) precursors were more effectively removed after ozonation by coagulation with aluminium salts or adsorption on AA. An explanation of the increased molecular polarity caused by ozonation results in stronger adsorption bonds with the polar alumina surfaces. GAC has been designated as a baseline technology far the removal of dissolved organic matter of natural origin, often in the form of humic substances. Humic substances may decrease the adsorption capacity of activated carbon for selective organic compounds. Thus a through understanding of humic substance adsorption is essential in the optimal use of activated carbon. Only a few studies have addressed the mass transfer aspects of humic substance removal by activated carbon. Adsorption of humic substances is slow to reach equilibrium when the grain size is in the range typical of commercial adsorbents and that previous kinetic studies have been limited to investigations of the initial portion of the adsorption-diffusion process. In water treatment, the carbon can be either powdered activated carbon or granular activated carbon, consisting of larger particles. The adsorptive pro perties of the PAC and GAC are not fundamentally different, since they depend on pore size and the internal surface area of the pores which are independent of overall particle size. Using the gel filtration technique, humic substan ces from water may be separated into several fractions. The method is also called the molecular sieve techique. At present there are only two different types of gels XVIavailable on the market,“Biogel-P”, which is spherical beads of polyacrylamide-gel and“Sephadex”; there are seven different grades of Sephadex available, each made of the same material but with different of crass linkage and consequently different pare sizes. It is alsa used ta estimate the molecular distribution of humic substances in different types of water. In this study, we investigated the adsorption af a humic substance by several different adsorbents. As adsorbents corncobs, rice hulls and also natural minerals like bentonites and zeolites were used. The synthetic solutions of humic acid (from Fluka) were made in 0.1 N NaOH. The adsorption experiments were carried out using a mechanical mixer and an orbital shaker. In all tests the supernatant was analysed by ÜV visible spectrophotometry at 656 nm to measure humic subsutances left in solution after centrifugatian. In all adsorption experiments either solutions of humic substance or adsorbents were not handled any pretreatment. It was concluded that organic originated matter can't be used in humic substance uptake because it influences the composition and concentration of humics by decomposition. Bentonite has the highest capacity among the others. The best pH, time, and particle size values for this adsorbent are found to be pH 12, 5 h and 53-75 fxm. xvn

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