1044
Langmuir 1994,10, 1044-1053
Surface Thermodynamics of Ozone-Induced Particle Destabilization Pradeep Chheda and Domenic Grasso' Departments of Civil Engineering and Chemical Engineering and The Environmental Research Institute, The University of Connecticut, Storrs, Connecticut 06269-3037 Received July 6,1993. In Final Form: October 25, 199P
The phenomenon of ozone-induced particle destabilizationwas studied employing a colloidal suspension of 150mg/L sodium montmorillonite(Na-M)suspended in river water containing 3.1 mg/L natural organic matter (NOM). The suspensionwas treated with high ( 106pM) and low ( 10pM) ozone doses. Extended DLVO theory was utilized to investigatethe surface thermodynamics of unozor?ated and ozonated Na-M coated with NOM (cNa-M). Ozonation decreased the surface charge and the Lewis base parameter, and increased the Lewis acid parameter and Lifshitz-van der Waals component of the surface energy. The overall result of these changes was a decrease in the change in the totalfree energy of interaction and hence a decrease in the stability of cNa-M with increasing ozonation. Modification in the surface thermodynamics responsible for destabilization could possiblybe attributed to partial dealuminizationof Na-M and wociated NOM transformations, increasing the autophilicity of the colloids.
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Introduction Ozone has been increasingly used in the field of environmental engineering for a variety of applications. A principal use of ozone is as a preoxidant in water treatment trains. Raw water, apart from other constituents, contains natural organicmatter (NOM) and colloidal particles which refract incident light (turbidity). Naturally occurring colloidal particles in raw water can be both organic and inorganic. Organic aquatic particles include bacteria, humic substances, algae, and plant debris. Inorganic aquatic colloids may include clays (e.g., montmorillonite, kaolinite), metal oxides (e.g., silica, alumina), and metal oxyhydrides (e.g., goethite). Aquatic colloidal particles are typically coated, to varying degrees, with NOM, resulting in thermodynamically stable suspensi0ns.l The predominant fraction of NOM in raw water is comprised of humic substances, which are anionic polyelectrolytes of low to moderate molecular weight having complex structures of both aromatic as well as aliphatic character and are typically surface active. Humic substances have been reported to increase the colloidal stability of a variety of inorganic particle^."^ An area of increased research is the interaction between ozone and particulates. These interactions are of specific concern in water quality operationss as well as the treatment of contaminated soils.g There have been a variety of studies exploring ozone-particle interactions6JJ0-18 in water treatment. In certain natural
* To whomcorrespondence shouldbe addressedat theDepartment of Civil Engineering. Abstract published in Advance ACS Abstracts, March 1,1994. (1) Gibbs, R. J. Enuiron. Sci. Technol. 1983, 17, 237. (2) Kuhn, W.; et al., J.-Am. Water Works Assoc. 1978, 70, 326. (3) Tipping, E. Geochim. Cosmochim. Acta 1981,45, 191. (4) Jekel, M. R. Water Res. 1986,20, 1543. (5) Felix-Filho, J. A. Doctoral Dissertation, University of North Carolina, Chapel Hill, NC, 1985. (6) Grasso,D.; Weber, W. J. J.-Am. Water Works Assoc. 1988,BO (B), 73. (7) Dowbiggin, W. B.; Singer, P. C. J.-Am. Water Works Assoc. 1989, 81 (6), 77. (8)Singer, P. C.; Chang, S. D. Impact of Ozone on the Removal of Particles, TOCand THMRecursors;American Water Works Association Research Foundation: Denver., CO. - - , 1989. --(SCMasten, S. J. Ozone: Qci. Eng. 1991, 13, 287. (10) Pak,H.; et al. Estuarine, Coastal Shelf Sci. 1981, 13, 327. (11) Richard, Y. Ozone: Sci. Eng. 1982, 4 , 59. (12) Richard, Y.; et al. Ozone: Sci. Eng. 1983,5, 3.
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waters, ozonation has been reported to partially destabilize colloidal suspensions (ozone-induced particle destabilization, OIPD), leading to a decrease in the required coagulant doses, longer filter runs,and improved particle r e m o ~ a l , 8 J J ~ Jwhereas ~ J ~ ~ ~in other waters, it ha^ been obeerved to increase colloidal stability and turbidity.12Ja1s21 The objective of the work presented herein is to investigate the impact of ozone on the stability of sodium montmorillonite (Na-M) coated with NOM (cNa-M) and to explain the observed phenomena of OIPD in terms of surface thermodynamics for this specific model system. Na-M is used as a particulate substrate because it is representative of naturally occurring abiotic colloidal matter. An experimental and analytical protocol was designed and executed to gain insight into salient transformations. Controlled ozonation of cNa-M suspensions was followed by traditional critical coagulation concentration (CCC) determinations. Analytical techniques were employed to ascertain, to the extent possible, surface transformations. Various surface parameters of cNa-M, such as the surface charge, electron-donor parameter, electron-acceptor parameter, and Lifshitz-van der Waals (LW) component of the surface tension, were determined. Upon quantifying these parameters, the components of the change in the total surface free energy of interaction were determined. Subsequently, the extended DLVO model was employed to identify and quantify energy fields impacted by ozonation. An integrated interpretation of both analytical and modeling results is presented.
0
(13) Graseo, D. Doctoral Dissertation, The University of Michigan, Ann Arbor, MI, 1987. (14) Edwards, M.;Benjamin, M. M. J.-Am. Water Works Aseoc. 1991, 83 (6). ,~,.96. -(15) Farvardin, M. R.; Collii, A. G. Water Res. 1989,23,307. (16)Mastronardi, R. A.; Fulton, G. P.; Farrar, M.; Collins, A. G. Ozone: Sci. Eng. 1993,16, 131. (17) Tobiason, J. E.; Edzwald, J. K.; Schneider, 0. D.; Fox, M. B.; Dum, H. J. J.-Am. Water Works Assoc. 1992,84 (12), 72. (18) Wilczak, A. W.; Howe, E. W.; Aieta, E. M Lee, R. G. J.-Am. Water Works Assoc. 1992,84 (12), 85. (19) Maier, D. Oxidation Techniques in Drinking Water Treatment; Report No. EPA-570/9-79-020; USEPA Waehington, DC, 1979. (20) Jekel, M. R. Ozone: Sci. Eng. 1983,5, 21. (21) Chheda, P.; Grasso, D.; van Ose, C. J. J. Colloid Interface Sci. 1992,153, 226.
0743-7463/94/2410-1044$04.50/00 1994 American Chemical Society
Langmuir, Vol. 10, No.4, 1994 1045
Surface Thermodynamics of Particle Destabilization
Background The theory of colloid stability was first developed independently by Derjaguin and Landau22 (1941) and Verwey and Overbeekm (1948) and is denoted by the abbreviation DLVO. The theory states that the change in the total surface free energy of interaction, AGT, is the sum of repulsive electrostatic (EL) interactions and attractive electrodynamicor van dw Waals interactions. The van der Waals interactions are typically represented through the use of the Lifshitz continuum theory which modified Maxwell’s equation to incorporate a random electricfield with Fourier componentshaving magnitudes governed by a fluctuationJdissipation theorem (LW interactions). A positive value of AGT indicates particle stability, whereas if AGT I0, the suspension is unstable and will coagulate spontaneously given sufficient probability of particle contact. However, a number of anomalies have been observed when the DLVO theory is used to interpret interfacial interactions in polar media such as water. Hence, it had long been surmised that physical interactions other than electrodynamicand electrostatic may play a role in colloidal stability. The origin of these forceslies in electron-acceptorJelectrondonor interactions. Lewis acid-base (AB) or polar interactions refer to all such interactions.24*26 The Lewis acid-base concept first formulated in 1923 has been modified in recent In the paradigm of molecular orbital (MO) theory, a Lewis acid is any species that employs an empty orbital in initiating a reaction, and a Lewis base is any species that employs a doubly-occupied orbital in initiating a reaction. Major classes of chemical phenomena which are subsumedunder the category of Lewis acid-base reactions include ‘the following:%n systems covered by the Arrhenius, solvent system, Lux-Flood, and proton acid-base definitions; traditional coordination chemistry and nonclassical complexes; solvation, solvolysis, and ionic dissociation phenomena in both aqueous and nonaqueous solutions; electrophilic and nucleophilic reactions in organic and organometallic chemistry; charge-transfer complexes, socalled molecular addition compounds, weak intermolecular forces, H-bonding, etq molten salt phenomena;andvarious miscellaneous areas such as chemisorptionof closed-shell species, intercalations in solids, so-called ionic metathesis reactions, salt formation, etc. All of the above phenomena may be explained in terms of a generalized MO version of Lewis acid-base chemistry for closed-shell/closed-shell interaction. Electron-acceptor/electron-donorinteractions are asymmetric in nature and can be either attractive or repulsive. In an aqueous medium, attractive and repulsive AB forces are known as hydrophobic interactions and hydration pressure, respectively. Inclusion of AB forces in the traditional DLVO theory, referred to as extended DLVO or ExDLVO theory, yields an approach capable of predicting many aspects of colloidal behavior which are otherwise inexplicable. Several mechanisms have been postulated regarding A commori ozone-induced particle destabili~ation.~*~J~@ (22) Derjaguin, B. V.; Landau, L. Acta Physicochim. USSR 1941,14, 633. (23) Verwey, E. J. W.; Overbeek, J. T. G. Theory of Stability of Lyophobic Colloids; Elsevier Publishing Co.: Amsterdam, 1948. (24) Van Oss, C. J.; Chaudhury, M. K.;Good, R. J. Chem.Rev. 1988, 88,927. (25) Good, R.J.; v ~ OM, f C. J. In Modern Approach to Wettability: Theory and Applicatronu; Schrader, M. E., Loeb, G.I., E&.; Plenum Press: New York, 1991; p 1. (26) Jensen, W. B. Chem. Rev. 1978, 78, 1. (27) Jeneen, W.B. CHEMTECH 1982, December, 755.
aspect among all proposed mechanisms is the interaction of ozone with NOM. Ozonation appears to affect colloidal stability by transforming NOM and alteringita inbractions with the colloidal particles and aquatic matrix. Otoneinduced transformation of adsorbed NOM may include one or more of the following: formation of oxygenated functional groups such as=-tO-, W O H , >M -COO-, , and -COOH; decreases in molecular weight; and formation of metastable organics,resulting in condensation products and increases in molecular weight. The above phenomena may alter particle surfaces by increading electrostatic repulsion as a result of increases in surfacenegative charge; increasing surface Lewis base character as a result of the formation of 4 0 - , sC-0, -COO- groups; increasing surface Lewis acid character as a result of the formation of *OH and -COOH groups; and decreasing surface Lewis acid character as a result of “internal”neutralization, e.g., for the structure
,
C*(j
the proton wodd be incapable of furtber participation in hydrogen-bonding, but unshared pairs of electrons on the oxygen atoms may remain available for Lewis base interactions.29
Theory Building on classical DLVO theory, for a system consisting of particles 1suspended in medium 3, the total surface free energy, AGTsl, can be obtained by summing (a) the change in the surfacefree energy due toelectrmtatic forces (repulsive),AGE, and (b) the change in the surface free energy due to electrodynamicforces or Lifshitz-van der Waals forces (attractive), AGkz. Thus,
AGT~, = AGE+ A G ? ~ where
AGE= 64n&TtY2exp(-Kd) K
l9=
exp(zeJ/d2kT)- 1 exp(zeqd2kT) + 1
(3)
and no = number of counterions per volume, m-9, k = Boltzmann’s constant, 1.38 X J-K-l, T = absolute temperature, K, K = reciprocal Debye length, m-1, d = distance of separation between the particles,m,z= valence of the counterions, e = charge of an electron, 1.6 X 10-10 C, and 30= potential on the surface of the particles, V. For platelike particles such as Na-M,m AGkg = -Als1/12~d2
(4) where A131 = Hamaker constant, J. The Hamaker constant, A131, is related to the apolar or LW component of interfacial tension, $ -.: When d is set equal to the minimum equilibrium distance (do) which derives from Born repulsion and is calculated to be 0.158 nm,= (28) Reckhow, D.;Singer, P.; T.uesell, R. R. OzOnation: Recant Advances and Reaearch Needs Seminar, Kaneae City, MO, 1986. (29) Chiu, H.M.S.Thesie, State University of New York at Buffalo, 1989. (30)Hiemenz, P.C. Principles of Colloid and Surface Chemistry; Marcel Dekker, Inc.: New York, 1986.
1046 Langmuir, Vol. 10, No.4, 1994
Chheda and Grasso
AG~: = -irk: (5) Following argumentssimilar to those of Israelachvilisland satisfying the conditions of eq 4, Lwd
2
AIS, = 2 4 ~ ~ 1o3 Thus, from eqs 4 and 6,
(6)
AGk: = -2#d;/d2 (7) As per the Good-Girifalco-Fowkes combining rule:2-s3
(8) = ( d y k W- dy:w)2 where ykw = LW component of the surface tension of particles 1, J-m-2, and ykW = LW component of the surface tension of medium 3, J*m-2. By measuring the contact angle of an apolar liquid on particles 1, ykw can be determined using the Young-Good-Girifalco-Fowkes equation:aO :ty
rkW = ( y L p i+ COS q2
(9)
where yb = surface tension of the liquid, J*m-2,and Bi = contact angle between the solid 1 and liquid, Lip deg. An expression for the CCC of an indifferent electrolyte can be derived by invoking the following conditions:
A G T ~=~ o
(10)
a(AGTsl)/ad = 0
(11)
and Solving eq 10 and 11 simultaneously for the aforementioned conditions and substituting appropriate values for the constants, the critical coagulation concentration for monovalent ions (I = M),based on classical DLVO theory, is
CCC = 10-9e*ost9'/A1s~ moUL (12) Expanding classical DLVO theory to include AB interactions, extended DLVO theory may be written asu AG;rl = A G E + AGk: + A G E (13) where the first two terms were defined earlier and A G E is the change in surface free energy due to Lewis acid-base interactions. A G E can be expressed asu
AGE = AOGE exp
do-d ( x)
where A = decay length of the liquid molecules (- 1nm for water)% and A o G E = change in the surface free energy due to Lewis acid-base interactions at the equiA o G E is related to the polar librium distance, do, or AB component of interfacial tension, yf, according to
AOGE = -27f
(15)
The total surface tension of any compound i, Ti, can be expressed in terms of two yi = y y
+y p
surface tension of compound i, J*m-2. y p is resolved into two monopolar surface parameters, i.e., an acidic parameter and a basic parameter. Since yp is asymmetric in nature, its components are not linearly additive. Hence?%
(16)
where 7)" = LW component of the surface tension of compound i, and yp = AB component of the (31) Lraelnchvili, J. B. Q. Rev. Biophys. 1974,6,341. (32)Good, R. J.; Girifalco, L. A. J. Phys. Chem. 1%0,64,681. (33) Fowkw, F. M. J. Phya. Chem. lSS3,67, 2538. (94)Van Om,C. J.; Giese, R F.; Cost", P.M.Clays Clay Miner. 1990,38,151.
yp
= 2(y+y;)'/2
(17)
where 7; = electron-acceptor (Lewis acid) parameter of the surface tension of compound i, and yT = electron-donor (Lewis base) parameter of the surface tension of compound i, J*m-2. The polar component of the change in the surface free energy of interaction between materials 1 and 3 can be expressed via the combining rule as24*26
+
A O G =~- 2 ( ( ~ : 7 2 l / ~ (y;$P2)
(18)
which can also be expressed by the Dupr6 equationU as A O G =~yf
-y
y
-@
(19)
Equations 17-19 can be combined to yield yf = 2(dy:
- d?;)(dy; - dT2
(20)
and eqs 15 and 20 yield A O G =~- 4 ( d / ~ :- d y ; ) ( d~472 ;
(21)
The parameters required to quantify AoG$ and AGkz, i.e., ,:y y;, and ykw may be determined by measuring contact angles between the solid, 1, and at least three different liquids, Lip of which two are polar and one is apolar. These three contact angles can then be used to solve for the three parameters using the following equation9 (1 + COS ei)y, = 2 ( ( ~ 4 ~ ~ : + 7 )(T:T$/~ ~/~ + (Y;Y~I) + 1/21 (22)
where i = 1-3.
Materials and Methods Materials. Sodium Montmorillonite. Na-M was prepared from laboratory-grade bentonite (Fisher Scientific) clay as per methods described earlier.21 Purified Na-M was freeze dried and stored under a vacuum desiccator for further use. The chemical composition of Na-M was determined by X-ray fluorescence (XRF). One part of Na-M was mixed with two parts of Li2B40, and fused at 1100 OC. The melted mass was cast in the form of a disk (3-cm diameter X 1 mm thick) and analyzed for various metals usinga Kevex Delta 770 class energy dispersive XRF analyzer. A direct rhodium X-ray tube with an excitation voltage of 3.5 kV was used for the analysis of Na, Mg, Al, and Si, whereas K, Ca, Ti, and Fe were analyzed using a germanium target and an excitation voltage of 15 kV. The nominal composition of Na-M, in weight percent, was as follows: SiO2, 66.23; A l 2 0 8 , 24.73; Fe208, 3.53; MgO, 3.56; Na20, 1.73; TiO2, 0.09; CaO, 0.07; K20,0.04.X-ray diffraction data on the Na-M indicated a basal spacing of 1.47 nm. The average particle size of Na-M was 860 nm as determined by laserdiffractiontechniques (Zetasizer IIc, Malvern Instruments, Inc., Southborough, MA). Aquatic Matrix. Approximately 20 L of water was collected from the Little River in Hampton, CT, and stored at 4 OC until used. Table 1 summarizes a partial analysis of the water. Ozone. Ozone was prepared from a laboratory ozone generator (Model T-408, Polymetrics, Inc., San Jose, CA) using zero grade oxygen. Deionized (DI) water was obtained from a Milli-Q water system (Millipore, Bedford, MA). All other chemicals used were of analytical grade. Methods. The overall scheme of the experimental protocol is presented in Figure 1.
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Surface Thermodynamics of Particle Destabilization
Langmuir, VoZ.10,No. 4,1994 1047
Table 1. Analyses of Little River Water and Filtrates before and after Ozonation sample filtrate0 river treated with treated with parameteP water control 15.8 uM OS 108 uM OS PH 6.65 alkalinity (as CaCOs) 7000 sodium concn 3555 3895 3926 4330 potassium concn 1371 634 657 782 calcium concn 4258 306 524 670 magnesium concn 1266 111 195 274 aluminum concn 20
