Chemical Modeling of Aqueous Systems II - ACS Publications

Chapter 23

Coagulation of Iron Oxide Particles in the Presence of Organic Materials Application of Surface Chemical Model 1

Downloaded by FUDAN UNIV on March 19, 2017 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch023

Liyuan Liang and James J. Morgan Environmental Engineering Science, W. M . Keck Laboratories, California Institute of Technology, Pasadena, CA 91125 Experiments using hematite particles (70 nm in diameter) in the presence of organics (phthalic acid, fatty acids, polyaspartic acid, fulvic and humic acid) reveal important features of particle coagulation dynamics. A light scattering technique was used to determine quantitatively the initial coagulation rates of hematite particles. Electrokinetic measurements were taken to obtain the sign and magnitude of electrical potential at the oxide/aqueous solution inter face. Adsorption experiments were carried out to evaluate affinities of aqueous molecules for the metal oxide surface. Intrinsic equilibrium constants for surface complexes are derived from a Surface Complex Formation/Diffuse Layer Model ( S C F / D L M ) accounting for interfacial electrostatic charge and potential. Small organic molecules, such as phthalate, show specific chemical reaction with the hematite surface and influence colloidal hematite coagulation kinetics by alter ing interfacial charge and potential. For fatty acids, hydrophobic interaction, in addition to covalent and electrostatic interaction, offers a plausible expla nation for observed systematic changes in hematite stability and electrokinetic data. In the presence of polyelectrolytes, such as polyaspartic acid, fulvic and humic acids, a combination of specific chemical, electrostatic, and hydrophobic energies of carboxyl segments favors adsorption, and these materials have a relatively great impact on particle coagulation and stability. Suspended solids play an important role in the geochemical cycles of metals, such as iron and manganese, in lake, river, estuary and ocean waters. Small particles in the range of 10 nm to 1 / i m possess large specific surface areas and many physical, chemical processes readily take place at the solid-aqueous interface. Observed non-conservative behavior of iron in estuarine waters has been attributed to "flocculation of dissolved iron" (1-4). Rates of coagulation (i.e., the aggregation of colliding particles to form larger particles) may determine the fate of naturally occurring particles. Hence, an understanding of the process is desirable in natural water environments and in improving water quality in engineering facilities. Colloids are said to be "stable" wdien they experience slow coagulation, to be "unstable" when they experience transfer-controlled coagulation. A "stability ratio" for coagulation is defined as the ratio of the transfer-controlled coagulation rate to the actual one. Earlier experimental and theoretical studies of systems in which particles interact through Van der Waal's attraction and electrostatic repulsion led to the development of DerjaguinLandau-Verwey-Overbeek theory ( D L V O ) . This theory has been successful in providing a quan1

Current address: Department of Environmental Health Sciences, School of Public Health, Room 311, University of South Carolina, Columbia, SC 29208 0097-6156/90/0416-0293$06.00/0 © 1990 American Chemical Society

Melchior and Bassett; Chemical Modeling of Aqueous Systems II ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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CHEMICAL MODELING OF AQUEOUS SYSTEMS II


titative understanding of the relationship between critical coagulation concentrations and the valences of simple electrolytes (5). Multivalent ions are more effective in causing coagulation than monovalent ions, and polymeric species often are more effective than monomeric species. In a complex aqueous system, ionic species may interact specifically with particle surfaces in addition to electrostatic interaction. A s a result of specific adsorption, the particle surface charge and potential may be altered, which in turn may influence the coagulation rate of particles. A l i et al. (6) showed that dissolved organic matter increases the stability of lake particles while magnesium and calcium are able to decrease the stability. Adsorption of dissolved organic matter on metal oxides changes the particle surface characteristics (7), and the aggregation process is affected by the chemical interaction of bivalent ions with surface-adsorbed humics (8). Further evidence of the importance of adsorption on coagulation kinetics comes from the experiments of Gibbs (9), who studied the coagulation of river-borne particles under natural conditions and also with their organic coatings removed. Gibbs observed that naturally coated particles coagulate more slowly than uncoated ones. Surface chemistry influences the surface charge on particles and hence has an important effect on particle coagulation. T o model coagulation of particles in the presence of a specifically adsorbed species (such as organic solutes), it is necessary to apply surface chemical models to adsorption results, and thereby relate the adsorption to coagulation. OBJECTIVES This study examines the influence of organic species on particle stability with respect to coag ulation. Hematite particles were chosen because hematite is one of the naturally occurring iron oxides. From previous work and this work, the zero point of charge ( p H ) of hematite particles is found to be around 8.5, which allows the study of chemical interactions on both positively and negatively charged surfaces. Aqueous iiOn(III) is capable of forming complexes with many organic and inorganic species. A knowledge of relevant aqueous Fe(III) equilibrium constants should aid in interpretating iron oxide surface speciation. z p c

Several organic compounds were chosen to study electrostatic, specific chemical, hydropho bic effects on particle stability. The organics include phthalic acid as an example of small organic materials, polyaspartic acid as an example of polymeric organics and fatty acids with different numbers of carbons in the hydrophobic chains. Naturally occurring organic materials, such as Suwannee River fulvic and humic acids were also used in the study. To identify the factors that influence the rate of coagulation in natural waters, adsorption, electrokinetic and coagulation rate data are combined and interpreted on the basis of surface chemical and colloid stability models. EXPERIMENTAL

General Remarks. Details of experimental procedures are given elsewhere by Liang (14). Deionized distilled water was used to prepare all solutions. A l l reagents were analytical grade and were used without further treatment. Fatty acids and polyaspartic acids were obtained from Sigma and were at least 99% pure. Fulvic and humic acids were supplied by the International Humic Substances Society. Pyrex glassware was used and was cleaned first, in a detergent solution (Linbro) soaked in concentrated hydrochloric acid, rinsed with deionized distilled water, and then oven dried. A suction pump was used to facilitate the cleaning of syringes and electrophoresis cells. The solution ρ H was monitored in all coagulation, titration, electrophoresis and adsorption experiments. These measurements were made using a Radiometer glass combination electrode ( G K 2 4 0 1 C ) and a p H meter (Radiometer Model P H M 8 4 research pH meter). The electrode was calibrated by N B S buffers and, if extended measurements were made, calibration was checked against buffers every two hours.


23.

LIANG & MORGAN

295

Coagulation of Iron Oxide Particles

Particle Preparation and Characterization. Hematite particles were synthesized according to the method of Matijevic and Scheiner (15). The characterization of the bulk and surface properties of the particles has been described by Liang (L4). Table I summarizes the properties of hematite particles used in this study.

Table I: Hematite Surface Physical and Chemical Properties Sample Preparation Hydrolysis of F e ( C 1 0 ) at 100°C


4

pH

z p c

8.50

Surface Equilibrium Constants

Electrolyte

Specific Surface A r e a

NaCl

BET: Batch 1: 4 0 . 0 m / g Estimated from averaged diameter: Batch 1: 3 8 . 3 m / g Batch 2: 16.3m /g

3

2

Diffuse Layer Model ρΚ^ =7.25 ρΚ|#=9.75 Site d e n s i t y = 4 . 8 n m ι

-2

2

2

Adsorption Measurement. Adsorption of organic solutes, such as phthalate, 1 auric (fatty acid, C12) and Suwannee River fulvic acid, on hematite particles was studied using a total organic carbon analyzer. Adsorption isotherms at constant p H and adsorption as a function of p H at fixed organic concentration were obtained. The desired p H was achieved by adding a small amount of N a O H or HC1 and ionic strength was adjusted by adding N a C l . A n organic compound was introduced into a particle suspension of known solid concentration. Stirring was maintained for 24 hours and at the end of the period, pH measurements were taken. Particles were filtered out of suspension following adsorption. Because the solid concentration was low in the sample, the uptake of organic acid by particles was low and the difference between the initial amount of organics and the amount in the filtrate could not yield an accurate measure of the adsorbed acid by particles. Therefore, the amount of adsorbed organics was determined as follows. After filtration the particles collected on the filter were then resuspended in a small volume (~10ml) of 0.02N H 2 S O 4 solution. The carbon present in the H 2 S O 4 suspension was determined using a Dorhmann DC-80 T o t a l Organic Carbon Analyzer.

Electrophoretic Mobility Measurement. A Mark II particle micro-electrophoresis apparatus (Rank Brothers, London) was used to mea sure hematite electrophoretic mobility. These studies were performed at 2 5 ° C in a KC1 solution. In each measurement at least 20 particles were timed in each direction of movement (16).

Coagulation Experiments. Light scattering measurements were applied to determine the initial rate of hematite coagulation. The details of the procedure and interpretation are described in Liang (14). To perform a coagulation kinetics experiment, 3 ml of colloidal suspension of desired p H and solid concentration was placed in a cell of 1 cm light path length. Ten to 100 μΐ of aqueous organic solution (measured by a Hamilton syringe) were introduced to an adder-mixer (17). and then mixed with a particle suspension by plunging the mixer into the cell. Spectrophotometric measurements were begun immediately after the initial mixing time of approximately 0.4 sec. Extinction as a function of time was measured at wavelength A = 546nm and recorded automatically.


CHEMICAL MODELING OF AQUEOUS SYSTEMS II

296 R E S U L T S A N D DISCUSSION

Effects of inorganic species, such as phosphate, sulfate, magnesium and calcium were investi gated, and have been discussed in Liang (1_4). Hematite surface chemical equilibria were studied by acid-base titration and surface acidity equilibrium constants were derived using a surface chemical model. The model used was surface complex formation in which electrostatics were described with a Gouy-Chapman relationship for interfacial charges and potentials. A l l surfacebound species were assumed to be located at a surface mean plane. Thus, the model is a two layer model (a surface layer and a diffuse layer) and the fitting parameters are the surface species formation constants, e.g., for ^ F e O H Ï " , ^ F e O M g , ^ F e L , etc. We refer to this simple model as S C F / D L M , surface complex formation plus diffuse layer model. Similar models with different electrostatic description for interfaces have been used to interpret titration and adsorption data (18-22). The estimation of equilibrium constants with a S C F / D L M model can be facilitated by using computer programs, such as S U R F E Q L and F I T E Q L (23-24). Hematite surface equilibrium acidity constants are listed in Table I, together with other surface physical parameters. Here we summarize the effects of organic solutes on kinetics of hematite coagulation and discuss the results on the basis of surface chemical concepts.


+

Effects of Small Organic Adsorbates. Phthalate adsorbs specifically on iron oxide surfaces (14, 25). The agreement of experimental results on adsorption, electrokinetics and rate of coagulation demonstrates the effect of chemi cal interaction on colloid stability. In Figure 1 adsorption density, electrokinetic mobility and stability ratio are compared as a function of total phthalate concentration. A t low concen tration (total phthalate concentration of 1 micromolar), the adsorption density is of the order 8 χ 10 m o l e s / c m . This corresponds to a tenth of the total surface site density. Most surface sites are occupied by surface hydroxyl groups in protonated or neutral species, so that surface charge and potential are similar to values in the absence of organics, and mobility data show that at Ι μ Μ phthalate concentration the mobility is unaffected by the presence of phthalate ions. Consequently, particles have a high stability ratio, on the order of 100. A t higher organic concentration, adsorption of phthalate ions is able to reduce the interfacial potential, and at a total phthalate concentration of 2 χ 1 0 ~ M the potential is near zero, as is reflected in the mobility measurements in Figure 1(b). At this phthalate concentration the stability ratio is a minimum, and the coagulation rate is close to diffusion-controlled. A s phthalate concentration is further increased beyond 2 χ 1 0 ~ M , mobility data show that surface charge reverses sign. If phthalate did not specifically adsorb on the hematite surface, the mobility would be expected to level off, approaching zero when the phthalate concentration is increased beyond 2 χ 1 0 ~ M . The increase in adsorption density indicates that the chemical bonding between phthalate and surface hydroxyl groups is sufficiently strong, for phthalate ions to overcome a negative surface potential and be adsorbed. Specific adsorption results in a negative interfacial potential and a rise in the stability ratio. Although the tendency of increased stability ratio is evident when phthalate concentration exceeds the critical coagulation concentration, the absolute values of stability ratio are small. The stability ratio is not expected to increase substantially when ph thalate concentration is greater than 1 0 ~ M , for two reasons. First, using Langmuir's treatment, the maximum adsorption density corresponds to ~ 4 0 Â per molecule adsorbed on the surface. This specific area is of the same order of magnitude as the cross-sectional area calculated for phthalate ions. Hence, further adsorption of phthalate is restricted by lack of available surface to accommodate additional molecules. Consequently, surface charge will be approximately con stant when the phthalate concentration exceeds 1 χ 1 0 " M . Second, an increase in phthalate ion concentration is unavoidably accompanied by an increase in the concentration of counterions, such as Na+. In the presence of a high concentration of positive ions, stability is reduced through electrostatic interactions. - 1 1

2

4

4

4

3

2

3


Coagulation of Iron Oxide Particles

297


23. LIANG & MORGAN

Figure 1: Comparison of hematite adsorption density, Γ, mobility, and stability ratio, W , as a function of total phthalate concentration at p H 6.25. The typical variations of the data are less than 10% of the average plotted. exp


298

CHEMICAL MODELING OF AQUEOUS SYSTEMS II A quantitative treatment of surface speciation can be achieved through the equilibria 2

=FeOII + 211+ + L ~ ^ EEFelIL + H 0 2

2

= FeOH + 1 1 + + L ~ — EEFeL" + H 0 2

K

H

L

(1)

K

(2)

L

2


Where L ~ represents phthalate ion, which are considered here as potential determining ions. KHL and K L , are the equilibrium constants, obtained by fit to adsorption data. The adsorption density as a function of phthalate aqueous concentration is illustrated in Figure 2, where the solid line shows S C F / D L M model calculation with \ogI

Chemical Modeling of Aqueous Systems II - ACS Publications

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