[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Removal of Aquatic Humus by Ozonation and Activated-Carbon Adsorption

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<ul><li><p>39 Removal of Aquatic Humus by Ozonation and Activated-Carbon Adsorption </p><p>Ellen Kaastrup1 and Terje M . Halmo2 </p><p>Department of Civil Engineering, Norwegian Institute of Technology, N-7034 Trondheim-NTH, Norway </p><p>The separate and combined effects of treatment with ozone and ac-tivated carbon were studied for three different humus sources: Nor-wegian brook water, Norwegian bog water, and commercial humic acid. The effect of ozonation on solution properties was determined by ultrafiltration, color, UV, and dissolved organic carbon (DOC) analysis. Adsorption prior to and after ozonation was studied in laboratory isotherm studies and a pilot-scale column experiment. Ozonation caused significant reductions in the content of high-mo-lecular-weight material, UV extinction, and color; DOC reductions were insignificant for 1 mg of ozone per mg of DOC. Both isotherm and pilot-scale studies showed significant increases in adsorption ca-pacities resulting from preozonation. </p><p>SURFACE WATERS HIGH IN COLOR-IMPARTING HUMIC SUBSTANCES are commonly used for drinking water in Norway. Such water used to be considered harmless, and the brownish color was treated as an aesthetic nuisance. The discovery of possible health threats caused by formation of trihalomethanes (THM) and heavy metal complexes (1) has led to more restrictive treatment requirements and extensive research on treatment alternatives. </p><p>1Current address: Ebasco Services, Inc., 143 Union Boulevard, Lakewood, CO 80228 </p><p>2Current address: Elf-Aquitaine Norway A/S, P.O. Box 168, Dusavik, N-4001 Stavanger, Norway </p><p>0065-2393/89/0219-0697$08.50/0 1989 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Sept</p><p>embe</p><p>r 4,</p><p> 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Dec</p><p>embe</p><p>r 15</p><p>, 198</p><p>8 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>a-19</p><p>88-0</p><p>219.</p><p>ch03</p><p>9</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>698 AQUATIC HUMIC SUBSTANCES </p><p>It is difficult to find suitable treatment methods because of the wide variety of compounds in such water. The treatment combination investigated in this study was ozonation and activated-carbon adsorption. Neither of these methods has been successful in efficient removal of humic materials when used alone. The study objectives were to </p><p> study the adsorption of organic matter from different humic-water sources onto activated carbon, </p><p> determine how ozonation affects solution properties such as molecular-size distribution, color, U V extinction, and dissolved organic carbon (DOC), </p><p> determine if the adsorbability of organic matter is increased (or changed) as a result of preozonation, and </p><p> relate the changes in adsorptive properties to the changes in solution properties. </p><p>Background Humic Substances in Water. Humic substances in water are gen</p><p>erally divided into humic acids and fulvic acids. The humic acid fraction, containing the larger base-soluble molecules, precipitates on acidification to p H </p></li><li><p>39. KAASTRUP &amp; H A L M O Ozonation and Activated-Carbon Adsorption 699 </p><p>different from natural humus. The commercial materials had a lower content of aromatic molecules, longer chains of carbon-carbon double bonds, and lower carboxyl-group content. </p><p>Activated-Carbon Adsorption of Humic Substances. Most organic compounds are removed from water by activated-carbon adsorption. The adsorbability of individual compounds depends on a number of factors, such as polarity and hydrophilicity, solubility, molecular size and structure, and p H . </p><p>Generally, relatively insoluble and nonpolar compounds are most easily adsorbed. Size and structure may be limiting factors for molecules that are too large to have access to the smaller carbon pores that make up the major part of the surface area. The carbon surface can be divided by size into different groups of pores. According to the IUPAC (International Union of Pure and Applied Chemistry) definition, pores with diameter </p></li><li><p>700 AQUATIC HUMIC SUBSTANCES </p><p>of it) is most often used. Previous investigators have proposed that the collective organic matter in humic water can be considered as consisting of fractions of varying adsorbability, often including a nonadsorbable fraction (8-10). The Freundlich model can be modified to account for a nonadsorbable fraction of organic matter, and different parameters can be used for segments of different adsorbability. By use of a substitution term for nonadsorbable matter, the equation takes the form </p><p>qe = KF(Ce - Cn)l/n (1) </p><p>where qe is the amount adsorbed per unit weight of carbon at equilibrium (or most often "pseudoequilibrium"), Ce is the equilibrium concentration of organic substances, Cn is the concentration of nonadsorbable matter, KF is the Freundlich coefficient, and 1/n is the Freundlich exponent. </p><p>Ozonation. Ozone is one of the strongest chemical oxidants known. It is used in water and wastewater treatment for disinfection, for decolori-zation, and as pretreatment for filtration and adsorption processes. The ozonation products are generally smaller, more polar, and hydrophilic than their precursors. Therefore, they are considered less adsorbable and more easily biodegradable. </p><p>The decolorization of humic water occurs because ozone is a typical double-bond reagent. It reacts with double bonds in the conjugated chains of large, color-imparting molecules, and thereby reduces the color and size of the molecules. Efficient and satisfactory decolorization of humic water has been reported in several studies (11-14). </p><p>Ozonation breaks up molecules into smaller units, but does not remove organic matter to any significant extent. Complete oxidation to carbon dioxide and water is too slow to be of significance in water treatment, and only minor reductions in the content of organic matter are therefore observed. Ozone treatment is consequently not sufficient as a removal method for organic matter. </p><p>Ozonation and GAC Adsorption. Ozonation is commonly used as a pretreatment for granular activated carbon (GAC) adsorption. Because the formation of more polar molecules generally causes reduced adsorbability, the intention of such pretreatment is usually to enhance biological activity in the filter and thereby to increase the overall removal efficiency. </p><p>Several studies have reported increased organic-substance removal in adsorption filters as a result of preozonation. However, in most cases ozonation had an adverse effect on adsorption, and the improved removal resulted from increased biological growth that led to increased biodegradability. Such results have been reported in several studies (15-18). However, the character of the organic matter is important. Most of the reported studies have </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Sept</p><p>embe</p><p>r 4,</p><p> 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Dec</p><p>embe</p><p>r 15</p><p>, 198</p><p>8 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>a-19</p><p>88-0</p><p>219.</p><p>ch03</p><p>9</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>39. KAASTRUP &amp; H A L M O Ozonation and Activated-Carbon Adsorption 701 </p><p>dealt with organic matter that was relatively easily adsorbable prior to ozonation and in which intermediate-to-small molecules, rather than extremely large molecules, dominated. The situation is different for large humic molecules, for which adsorption is restricted by carbon pore size. Such molecules are adsorbed to a very low extent; they use large pores, which are only a small fraction of the carbon surface. These large molecules are easily oxidized by ozone. The reaction between ozone and the larger molecules wil l dominate as long as these are present because of ozone's high double-bond reactivity. On the basis of these observations, the following hypotheses were proposed for this study: </p><p> Ozone will react with large color-imparting molecules and thereby reduce the color and molecular size. </p><p> The adsorption capacity of organic matter from humic solutions will be increased as a result of reduced molecular size, as long as the reduced size dominates the increase in polarity. </p><p> The adsorption rate will increase as a result of reduced diffu-sional restrictions. </p><p> Increased biological activity is expected, in addition to increased adsorbability, in long-term studies. </p><p>Approach </p><p>Because two treatment processes were involved in this study, it was important to study them both separately and in combination. To study the influence of ozonation on solution properties, a thorough characterization was required. It was therefore decided to use three different concentration parameters: color, U V extinction, and D O C . Another goal was to determine the molecular-weight distributions for the different doses of ozone applied. </p><p>The color reduction resulting from ozonation demonstrates the bleaching efficiency of ozone for different ozone doses and different humic substances. U V extinction is commonly used as a measure for the content of natural organic matter in water. Most small, easily biodegradable molecules are not detected by this parameter. The D O C value represents a collective measure of all the organic substances in solution. </p><p>Because of the confusion and disagreements among studies that deal with humic substances and adsorption of organic matter in general, it was decided to use humic substances of different origins. These substances included a commercial product that has frequently been used in adsorption studies but is considered very different from aquatic humus (4), in addition to two different natural humus sources. </p><p>Two characteristic properties of the solution to be treated, adsorption rate and adsorption capacity, are important for the design of adsorber sys-</p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Sept</p><p>embe</p><p>r 4,</p><p> 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Dec</p><p>embe</p><p>r 15</p><p>, 198</p><p>8 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>a-19</p><p>88-0</p><p>219.</p><p>ch03</p><p>9</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>702 AQUATIC HUMIC SUBSTANCES </p><p>terns. The emphasis of this work was to study the adsorption capacities of the different humic substances and changes in them caused by preozonation. Most of the reported studies of ozonation-adsorption systems have been carried out in full-scale plants. Because biological degradation is likely to interfere when long contact times are used, the effect of preozonation on adsorptive properties was studied in laboratory experiments, with precautions against biodgradation. </p><p>Adsorption capacities before and after ozonation were determined from isotherm experiments. The Freundlich model is known for its wide applicability to natural water and mixtures of differing adsorbability. This model was therefore selected to describe the adsorption isotherms and changes in adsorbability. The adsorption was modeled by "pseudo-single-solute" isotherms, with D O C and U V extinction, respectively, as concentration parameters. </p><p>Absolute equilibrium for large humic molecules is not reached within the reaction times chosen in this study. Summers (19) found that, for commercial humic acid, equilibrium was not reached even after 50 days of reaction time. He did, however, choose 5 days of reaction time in his study because a pseudoequilibrium state was reached within this period. This reaction time also agreed with the theoretical equilibration time calculated by the method of Suzuki and Kawazoe (20). </p><p>Finally, it was important to see how well the laboratory results agree with larger-scale results. A pilot study for comparison of organic-substance removal from nonozonated and ozonated water was therefore designed. The experimental conditions were chosen to allow for biological growth, and thus to determine the importance of biological removal compared to adsorptive removal. </p><p>Experimental Procedures </p><p>Humus Sources. Different humus sources were used in the experiments in order to be able to make general conclusions. Three kinds of humic materials were studied: a commercial humic acid and two different Norwegian waters from Helle-rudmyra and Heimdalsmyra. </p><p>The commercial humic acid was prerinsed according to the procedure described by Altmann (21). Stock solutions of humic acid were made by dissolving 1 g of the solid in 1 L of distilled deionized water (Milli-Q) at pH 11, obtained by adding 1.0 NaOH. This solution was placed in an ultrasonic bath for 30 min to achieve complete dissolution and then stored at 4 C. Dilutions of the stock solution were made up prior to each experiment. The pH was adjusted by addition of 1.0 HC1 and a phosphate buffer. </p><p>Brook water was collected from Hellerudmyra, a marsh area outside Oslo, Norway. The water was immediately filtered through glass fiber filters (Whatman GF/C) and stored at 4 C until use. Prior to each adsorption experiment, the water was filtered through 0.45- microfilters (Millipore). </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Sept</p><p>embe</p><p>r 4,</p><p> 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Dec</p><p>embe</p><p>r 15</p><p>, 198</p><p>8 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>a-19</p><p>88-0</p><p>219.</p><p>ch03</p><p>9</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>39. KAASTRUP &amp; HALMO Ozonation and Activated-Carbon Adsorption 703 </p><p>Heimdalsmyra water was collected from a bog outside Trondheim, Norway. This water has a color of approximately 700 mg of Pt per liter, and was diluted with tap water for experimental use. The filtration procedure was the same as for Hellerud-myra water. </p><p>Carbon. The carbon selected for this study (Calgon F-400) was ground and sieved to the actual particle sizes, washed in distilled deionized water (Milli-Q), and dried at 110 C for 48 h. It was stored in an airtight bottle placed in a desiccator until use. </p><p>Ozonation. Ozonation was carried out with an ozone generator (BBC LN 103). The apparatus was calibrated, and unreacted ozone was quantified by the potassium iodide absorption method, as described by Standard Methods for the Examination of Waters and Wastewater (22). </p><p>The contact column used for batch ozonation was a 7-L glass column. Ozone was applied through a glass sinter of porosity 0 in the bottom of the column. The ozone dose was adjusted by keeping the gas flow constant and varying the contact time. </p><p>Continuous ozonation for the pilot-scale study was carried out in a 1.5-L glass column with countercurrent flow of ozone through a sinter in the bottom. The applied dose was 1 mg of ozone per milligram of DOC. </p><p>Analytical Methods. Various total organic carbon (TOC) analyzers were used for DOC analysis (Dohrmann DC-80, Astro Model 1850, Sybron-Barnstead Pho-tochem Analyzer). The instruments were calibrated with a potassium hydrogen phthalate (KHP) standard and tested with humus standards. Excellent agreement between the different instruments was obtained. </p><p>UV-visible absorption spectra were determined by transmission measurements on spectrophotometers (Bausch and Lomb Spec...</p></li></ul>

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