[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Effect of Coagulation, Ozonation, and Biodegradation on Activated-Carbon Adsorption

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<ul><li><p>40 Effect of Coagulation, Ozonation, and Biodegradation on Activated-Carbon Adsorption </p><p>Gregory W. Harrington1, Francis A. DiGiano, and Joachim Fettig2 </p><p>Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, NC 27514 </p><p>The ability to describe humic substance adsorption is important for the design of activated-carbon filters in water treatment. Humic so-lutions are composed of a multitude of unknown molecular species, and competitive adsorption among these species, as well as with trace anthropogenic organic chemicals, needs to be understood better. In this research, ideal adsorbed solution theory was used to describe an aquatic humic solution as a set of several pseudocomponents and to evaluate the effects of two treatment processes (alum coagulation and a combination of coagulation, ozonation, and biodegradation) on the solution's equilibrium and kinetic adsorption behavior. </p><p>OUR LIMITED UNDERSTANDING OF THE STRUCTURE of aquatic humic substances (HS) is manifested in our poor understanding of how these materials are adsorbed on activated carbon. When measured collectively by a surrogate parameter such as total organic carbon (TOC), HS solutions are not considered well adsorbed by activated carbon. Nevertheless, their adsorption is important because the removal of synthetic organic chemicals may be decreased by the presence of HS. Moreover, the removal of HS by adsorption </p><p>1Current address: Malcolm Pirnie, Inc., Newport News, VA 23606 2Current address: Division of Hydraulics and Sanitary Engineering, Norwegian Institute of Technology, N-7034 Trondheim-NTH, Norway </p><p>0065-2393/89/0219-0727$06.00/0 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>ch04</p><p>0</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>728 AQUATIC HUMIC SUBSTANCES </p><p>may become important if more stringent maximum contaminant levels (MCLs) for trihalomethanes and other chlorination byproducts are set. Water treatment facilities wil l be forced to rely more on adsorption if future M C L s cannot be met by moving the point of chlorination or increasing the efficiency of coagulation. Therefore, the ability to describe H S adsorption and its effect on the adsorption of pollutants is of utmost importance in the design of activated-carbon filters. </p><p>Because of the heterogeneous nature of HS solutions, this research was aimed at testing the applicability of a competitive adsorption model to describe equilibrium and kinetic adsorption behavior. The approach is similar to that used by Frick and Sontheimer (J) and Crittenden et al. (2), wherein an unknown mixture is described as a set of several pseudocomponents by using the ideal adsorbed solution theory (IAST) (3). This approach was used to evaluate the effects of coagulation, ozonation, and biodgradation on the adsorption behavior of HS solutions. </p><p>Preparation of Samples Raw water was obtained from Lake Drummond in southeastern Virginia. The water is highly colored, low in alkalinity (50 m g / L as C a C 0 3 ) , and p H 4. Upon return to the laboratory, the water was prefiltered with 1.0- honeycomb filters to remove leaves and sediment. The water was then stored in a cool, dark storage area prior to treatment. </p><p>The first stage of treatment used was alum coagulation. The p H of the raw water was adjusted to 6.5 by the addition of 2.4 X 10 2 M sodium carbonate. Alum was added at high mixing intensity and constant p H to a concentration of 205-210 m g / L as Al 2 (S0 4 ) 3 -18 H 2 0 to achieve destabili-zation. This step was followed by 40 min of flocculation, overnight settling, and filtration with 0.45- membrane filters to achieve 76% removal of U V absorbance at 254 nm and 50% removal of T O C . The coagulated water was then stored in a refrigerator for prevention of biodgradation. </p><p>The coagulation stage was followed by ozonation and biodgradation. Distilled, deionized water was ozonated to a concentration of 25 mg of 0 3 / L and then combined with an equal volume of the coagulated sample to yield an ozone dosage of 1.15 mg of 0 3 / m g of T O C . The ozonated sample was placed in the reservoir of a recycle batch reactor, the column of which was previously seeded with return activated sludge from the local waste-water-treatment plant. The reactor was allowed to run until no further reduction in T O C was observed (10 days of run time) and the sample was then stored in a refrigerator. T O C values of each stage are given in Table I. </p><p>Adsorption Isotherms Equilibrium studies were performed by using the bottle point technique for determining adsorption isotherms. Granular activated carbon (GAC, F-400) </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>ch04</p><p>0</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>40. HARRINGTON ET AL. Coagulation, Ozonation, and Biodgradation 729 </p><p>Table I. Total Organic Carbon Levels in the Humic Mixtures Tested </p><p>TOC Humic Mixture (mg/L) Prefiltered 43.8 Alum coagulated 21.7 Ozonated and biostabilized 7.0 </p><p>was washed, dried, stored, and ground to a 200-325 mesh size to ensure that a representative sample of carbon was used (4). In order to adequately describe the isotherms, bottles were filled with powdered carbon dosages ranging from 5 to 4000 mg/L. Samples were buffered with a 5 m M phosphate buffer to yield a p H of 6.5. Each bottle was then filled with 100 m L of sample and tumbled for 7-10 days at 23 C. After the period of tumbling, the powdered carbon was removed by using 0.45- membrane filters and the T O C of each sample was measured. Each component in a given mixture must satisfy the following mass balance equation for each bottle: </p><p>* = "MT- </p><p>where q{ is the surface loading of component i at equilibrium, C{ is the bulk liquid-phase concentration of component i at equilibrium, C 0 i is the initial liquid-phase concentration of component i , and M/V is the carbon dosage. </p><p>Prediction of Multicomponent Equilibrium IAST was assumed capable of describing the multicomponent nature of the resulting HS isotherms through the use of the following five equations: </p><p>qT = Qi (2) </p><p>% = for i = 1 to (3) QT </p><p> N </p><p> = 0 (4) C, = z,C, for i = 1 to (5) </p><p>T l A d , TMads (iP d[\ll (C, 0)] </p><p>~W = ~W = I M t f J * 1 f o r ' = l t o N ( 6 ) </p><p>Total surface loading, qT, is defined as the sum of individual solute surface loadings by equation 2; the surface mole fraction, z t , of an individual solute is defined by equation 3. Equation 4 defines the surface loading of </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>ch04</p><p>0</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>730 AQUATIC HUMIC SUBSTANCES </p><p>the mixture as a function of single-solute surface loadings, q, achieved when the single-solute systems adsorb at the same temperature and spreading pressure as the mixture. By equating the chemical potentials of a solute in the adsorbed and liquid phases, one arrives at equation 5, where would be in equilibrium with q? in a single-solute system. Equations 4 and 5 are key equations in IAST, because both assume that the adsorbed phase forms an ideal solution. Finally, equation 6 equates the spreading pressure of the mixture, , with the spreading pressures of the single-solute systems, ir t . A a d s is the adsorbent surface area per unit mass of adsorbent. The spreading pressure of a single-solute system is evaluated by the integral shown in equation 6. </p><p>Each H S solution was assumed to contain a set of several individual pseudocomponents, each having a single-solute adsorption behavior that could by described by the Freundlich isotherm equation. The linearized form of the Freundlich equation is given by </p><p>In (q?) = In (K.) + ^ j In (C ) (7) where K f and ni are the Freundlich isotherm constant and exponent, respectively, for component i. When equation 7 is substituted into equation 6, the following expression results: </p><p>niqo = njqj for j = 2 to (8) </p><p>Combining equations 1-5, 7 and 8 yields the following objective function, which was derived by Crittenden et al. (2): </p><p>F i = 0 = C 0 t &lt; ' y %~ </p><p>Thus, the equilibrium state of each isotherm bottle may be described by a set of equations when the Freundlich parameters, nt and K and the initial concentration, C 0 of each component are known and when the carbon dosage, M / V , is also known. The set of equations may be solved by a Newton-Raphson algorithm, as shown by Crittenden et al. (2). </p><p>In this case, the Freundlich constants and initial concentrations of each component were unknown, although the initial concentration of the total mixture was known. Therefore, a search routine was combined with the Newton-Raphson algorithm to find the Freundlich constants and initial con-</p><p> M for i = 1 to </p><p>(9) 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>ch04</p><p>0</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>40. HARRINGTON ET AL. Coagulation, Ozonation, and Biodgradation 731 </p><p>centrations of the pseudocomponents that yielded the best fit to the isotherm of the total mixture. The objective of the search routine was to minimize </p><p>SSR = </p><p>where SSR is the residual sum of squares, Nohs is the number of observations, ^ T f . o b s</p><p> a n d C T i c a l c are the observed and calculated concentrations of the mixture in bottle i , and { is the standard deviation of replicate measurements O f C Ti.obs-</p><p>Because of the unknown nature of HS mixtures, their concentrations must be measured through the use of surrogate parameters such as T O C . The IAST equations (equations 2-6), however, are based on a thermodynamic derivation in which concentrations are expressed in molar rather than mass units. If the Freundlich model (equation 7) is used to describe single-solute adsorption on a T O C basis, equation 8 is still valid on a molar basis. However, equation 9 is arrived at only by assuming that each component contains the same number of carbon atoms per molecule. The need for this assumption produces a dilemma in using T O C data in the IAST model to search for Freundlich adsorption constants of individual components if, in fact, these components do not all contain the same number of carbon atoms per molecule. This dilemma has not been adequately addressed by others who have used the IAST model (5, 6). On-going work by this research group is addressing further manipulation of the IAST equations to include the number of carbon atoms per molecule for each unknown fraction. However, for the purposes of this chapter, we assume that all fractions have the same number of carbon atoms per molecule. </p><p>Prior to determining pseudocomponent properties, the number of statistically valid pseudocomponents was determined. This procedure was begun by determining the root mean square error (RMSE) of the fit obtained using one adsorbing pseudocomponent. The value of R M S E was calculated from </p><p> S S R N , I 0 5 </p><p>R M S E- - [ r t J (11&gt; where IV a d s is the number of adsorbing pseudocomponents, p N a d $ is the number of adjustable parameters for N a d s pseudocomponents, SSR i V a d s is the residual sum of squares for N a d s pseudocomponents, and N o b s is the number of observations. For the IAST-Freundlich model used in this work, </p><p>( 1 2 ) </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>ch04</p><p>0</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>732 AQUATIC HUMIC SUBSTANCES </p><p>Equation 11 shows that the value of R M S E can increase with increasing values of N^s. The remainder of the procedure involved calculating R M S E N a d s for succeeding values of N a d s and comparing the values obtained. For the isotherms presented here, R M S E N a d j for IV a d s = 3 was always greater than or not sufficiently smaller than the value of R M S E N a d t for Nads = 2. Thus, the largest number of statistically valid adsorbing pseudocomponents was limited to two. </p><p>Another modeling technique, reported by Jayaraj and Tien (7), reduces the number of parameters to </p><p>= A U - 1 (13) </p><p>and, as a result, reduces computational limits to the maximum number of pseudocomponents. However, this reduction in parameters is obtained by arbitrarily assigning Freundlich parameters to pseudocomponents and searching only for pseudocomponent concentrations, whereas the technique employed in this work searches for all of these parameters. Statistical l imitations to the Jayaraj and Tien technique are unknown, but the feet that they exist is demonstrated by the inability of this technique to produce a unique description of one of the wastewaters tested when using five adsorbing pseudocomponents (7). The use of three or four adsorbing pseudocomponents may have produced a more valid result. In any event, further research is required to determine the statistical limitations to this approach. </p><p>Rate Tests and Kinetic Models </p><p>External diffusion rates were studied through the use of the minicolumn rate test (8). This test uses a high flow rate and a short G A G bed length to allow for the domination of external mass transfer. External mass transfer coefficients were calculated from </p><p>where Q is the volumetric flow rate through the column, M is the mass of G A G in the column, A e f f is the effective external specific surface area of the adsorbent (as determined by the use of a solute with a known diffusion coefficient), and CTt and CTJ=0 are the effluent plateau and influent concentrations of the mixture, respectively. Free liquid diffusivities (Dt) were calculated from Gnielinskis correlation; these measures of D z were independent of the equilibrium parameters. Gnielinski's correlation was used because experiments by Roberts et al. (8) determined that the particle shape factor and, therefore, A e f f were not affected by the Reynolds number. The Dt </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>ch04</p><p>0</p><p>In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. </p></li><li><p>40. HARRING...</p></li></ul>

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