Introduction
The Influence of Surface Chemistry on Colloid Stability
Adsorption of aqueous species by suspended particles changes the surface properties of the particles. The presence of negatively charged iron oxide particles in lake and river environments has been attributed to adsorption of dissolved organic matter (Tipping, 1981).
Coagulation in Natural Water Environments
Substituting equation (2.6) for (2.3), the rate of change in the number density of particles of volume vj~ for an initially monodisperse system is given by. At the pH conditions of the adsorption (pH 5.2), the titration data in the absence of organic acid show that the surface charge σo is 0.11 coul∕m2, which corresponds to 8 × 10“11 mol∕cm2. The critical coagulation concentration of hematite at pH 6.5 for a monovalent ion, for example Cl-, is of the order of 0.1M. The most important phosphate species at this pH are H2PO^ and HPO^-.
At pH around 6.5, the mobility is approximately zero due to adsorption of phthalate, and the corresponding stability is also a minimum. By substituting Φi = —32 mV, and t = 4.0Â, the stability ratios are calculated and experimental and model results are shown in Figs. In the range of pH 6.5 to 8.2 (which are the typical values from river water to sea water), the mobilities of hematite particles are negative in the presence of 0.1 mg/1 fulvic acid (Fig. 4.23), even though the hematite particles themselves are positively charged in simple salt solutions over the same pH range.
Fuerstenau, T.W., (1981), The Adsorption of Oleate from Aqueous Solution onto Hematite, Adsorption from Aqueous Solutions, Tewari, P.H.
Coagulation in Water and Wastewater Treatment
Motivation to Study Hematite
Iron is used as a coagulant in water treatment processes where colloidal iron oxides are produced. Order of magnitude estimates derived from these studies have been useful in designing the adsorption and coagulation experiments in this thesis.
Scope and Objectives
The use of hematite particles as a model system serves well to understand some of the surface properties of iron oxides, adsorption behavior and coagulation consequences. The types of electrolytes to be studied include (i) chloride salts of sodium, magnesium and calcium ions at pH greater than pHzpc, - (ii) sodium salts of chloride, sulphate and phosphate at pH less than pHzpc. 3) To determine the influence of organic species on particle stability and to understand the effects of hydrophobicity, configuration and size of the molecules on particle stability.
Introduction
Smoluchowski’s Equations for Brownian Coagulation
If Noa = ∑Sι ni is the total number of particles per unit volume of liquid, Smoluchowski showed that equation (2.7) is equivalent to. Similarly, it can be shown that the solution is for the concentration of doublets during solidification.
Fuchs’ Treatment
Figure (2.1) shows the change of the total number concentration with normalized time, (t∕τb), and the number concentration of singles, doubles, triples, etc. In experimental terms, W represents the diffusion-controlled coagulation ratio. the rate of decrease in the actual rate of coagulation in the presence of the force field.
DLVO Theory of Colloid Stability
Using SURFEQL, the adsorption data in Figs. 4.28 is modeled by the diffuse layer model. is fitted by a polynomial as follows: . 5.12) is the result of specific interaction and he. The mobility data and stability ratios from coagulation experiments show both systematic change with variations in concentration and in the carbon number of the molecules. Thesis, California Institute of Technology, Pasadena, CA Kerker, M., (1969), The Scattering of Light and Other Electromagnetic Radiation,.
L The Electrical Double Layer
Electrostatic Repulsion Energy
Electrostatic repulsion occurs when two particles of the same charge approach each other and their diffuse layers of ions overlap. They expressed the potential in terms of a distribution of electric dipoles on the surface of the particles, and developed an integral equation, from which a better approximation was derived.
London-Van der Waals Attraction Energy
Using expressions for Vr and Vχ, the total interaction energy Vp is feasible (Vγ, — Vr -∣- Va), and the stability of a colloid in the presence of electrostatic and Van der Waals forces can be calculated.
Applications of DLVO Theory
With the Stern layer correction, the surface potential can be replaced by the Stern potential when calculating the total interaction energy. In the calculation of the stability ratio, the integration of exp(V⅞∕RT) in the Stern layer makes a negligible contribution.
Hematite Surface Speciation
Surface Chemical Model
- The Origin of the Surface Hydroxyl Group
- Development of Chemical Speciation Model
Then the water molecules dissociate and react with the surface iron, forming surface hydroxyl groups. Therefore, for the sake of simplicity in this study, the surface hydroxyl groups are assumed to be the same and have the same energy level.
Models of the Solid/Aqueous Interface
Other parameters, such as the capacitance of the diffuse layer, are determined by the Gouy-Chapman theory, hence the electrostatic description. The diffuse layer model was chosen for this work because (1) the simple physical presentation of the interface gives rise to the fewest number of fitting parameters.
Adsorption of Anions on Hematite
The model parameters are given in the text. of the equilibrium constants for solution complexes. The surface complexation constants for these fatty acids are estimated from those of the simple acids (namely acetic and propionic acids), accounting for the additional contribution of the hydrophobic tails.
Materials and Methods
- General Remarks
- Particle Preparation and Characterization
- Preparation of Hematite Particles
- Particle Characterization
- Surface Properties
- Specific Surface Area
- Determination of Specific Surface Sites
- Acid-Base Titration
- Adsorption Measurements
- Adsorption of Phosphate
- Adsorption of Organic Matter
- Electrophoretic Mobility Measurements
- Coagulation Experiments
- Light Scattering Theory
- Total Light Extinction Measurement
- Interpretation of Light Scattering Data
- Experimental Conditions
Titration experiments yielded the same pHzpc for the three batches, demonstrating that the acid-base properties of the particle surfaces are the same in the three batches. The organic acid adsorption was determined by the difference between the initial acid concentration and the amount remaining in the solution phase.
Experimental Results
Coagulation Rates
- Effect of NaCl on Hematite Stability
- Effect of pH on Hematite Stability
- Effect of Temperature on Hematite Stability
- Effect of Bivalent Ions on Hematite Stability
- Effect of Phosphate on Hematite Stability
- Effect of Oxalate and Phthalate on Hematite Stability
- Effect of Polymeric Organic Compounds on Hematite Stability . 60
- Variation of Electrolyte Concentration
- Effect of Fatty Acids on Hematite Stability
Hematite stability was studied as a function of pH at room temperature for various ionic strengths. The dependence of hematite stability on pH in the presence of 0.1 millimolar phthalate ions is shown in Fig. The stability of hematite particles as a function of pH was studied in the presence of fulvic acid (0.1 mg/1 FA in ~17 mg/1) hematite).
Electrokinetic Properties of Colloidal Hematite
- pH and Mobility
- Influence of Bivalent Ions: Sulfate and Magnesium
- Influence of Phosphate on Hematite Mobility
- Influence of Phthalate on Hematite Mobility
- Influence of Fulvic Acid on Hematite Mobility
- Influence of Fatty Acids of Varying Chain Length
The mobility of hematite particles was studied as a function of pH in the presence of phosphate. The pH dependence of mobility in the presence of 0.1 mg/1 fulvic acid is illustrated in Fig. The electrophoretic mobility of hematite in the presence of caprylic (C8), capric acid (CIO) and lauric acid (C12) was studied and the results are summarized in Fig.
Adsorption Results
- Adsorption of Protons
- Adsorption of Phosphate
- Adsorption of Phthalate
- Adsorption of Fatty Acids
In this study, a titration of hematite particles was performed (see Section 3.2.3 for the procedure), and a detailed account of the modeling of the surface equilibrium constants is given in Sect. A general form of equilibrium constants based on the formation of surface complexes can explain the pH dependence; this is presented in Sect. The ionic strength of the adsorption medium was adjusted with NaCl to 0.05 M and the pH was 5.2.
Discussion of Results
Introduction
The Effect of pH Variation
- Description of the Physical Chemical Model
- Particle Mobility, Surface Potential and the ^-Potential
- The Dependence of Hematite Stability on pH
The constant capacitance model can be viewed as a special case of the Stern model where the diffuse layer disappears. The relationship between Φo and σo for a flat surface is given by (Verwey and Overbeek the surface charge is proportional to the square root of the ionic strength of the solution, so a lower surface charge is expected for low ionic strength. For small potentials the potential decays exponentially in the diffuse layer, the surface potential and is connected to ((-potential z,.
Hematite Particles in Inorganic Media
- The Effect of Temperature on Particle Stability
- Hematite Particles in the Presence of Phosphate
- Hematite and Sulfate, Calcium or Magnesium Ions
When the phosphate concentration exceeds 10μM, the adsorption of phosphate on the initially positively charged particles reverses the surface potential, and the stability is the result of the interactions of negatively charged particles. This small value of the phosphate concentration required to coagulate hematite is mainly due to the specific chemical reaction between phosphate and the surface groups. Firstly, the adsorption is responsible for the speciation on the surface and the change in the surface potential and charge that occurs as a result of surface chemical reactions.
Hematite in the Presence of Organic Solutes
- Hematite in the Presence of Small Organic Molecules
- Effect of Polymeric Organic Matter
- The Effect of Fatty Acids
The dashed line represents the reduced potential at t=4.0 nm. b), the species distribution is modeled by calculating the equilibrium. If the surface is assumed to be fully covered at pH<8.0, the ^-potential should be similar to the surface potential. In the case of lauric acid adsorption on hematite surfaces, the formation of a surface complex is expressed by the equation.
Summary and Conclusions
- Introduction
- Surface Chemical Properties
- Adsorption of Protons
- Adsorption of Phosphate
- Adsorption of Magnesium and Sulfate
- Adsorption of Phthalate
- Adsorption of Fatty Acids
- Adsorption of Polyelectrolytes
- Surface vs. Aqueous Complex Formation Constants
- Hematite Coagulation Kinetics
- Hematite Particles in Inorganic Media
- Hematite the Presence of Organic Solutes
- Effect of Temperature on Hematite Stability
- Modeling of Hematite Stability Ratios
In the presence of these ions, the coagulation of a colloidal hematite suspension is due to the compaction of the diffuse layer. The reduction in the stability ratio illustrated in fig. 6.3 is due to the compression of the diffuse layer as the sodium concentration increases. The value of the stability ratio is dependent on the potential, Φi, which can be estimated from the mobility data in the presence of the corresponding amount of phosphate.
Implications and Future Work
Introduction
The following section first describes the determination of hematite stability using membrane filtration techniques. Field studies of iron removal in natural water systems using filtration techniques can therefore be interpreted using the results in this paper.
Filtration Methods vs. Light Scattering Techniques
- Experimental Procedure
- Results and Interpretation
The light scattering technique is found to yield the same stability ratio for hematite as the filtration method. Nevertheless, the results of the initial stage filtration experiment are sufficient for comparison with the results obtained by the light scattering method, since the coag. Using a simple monodispersed hematite suspension, the stability ratio obtained from the light scattering method is in good agreement with the result of the filtration method.
Implications for Natural Water Systems
- The Origin of the Surface Charge
- Stability of Particles in Lake Waters
- Coagulation of Particles in Estuaries
- Temperature and Coagulation
The behavior of river iron oxide in estuaries can be interpreted based on the findings of this research. Over this range of ionic strength, the stability of iron oxide particles can vary greatly and the coagulation rate of the particles is determined by the chemical composition of the river water and the river flow rate (through the mixing ratio and, consequently, the particle number concentration and ionic strength). Based on our work, it can be deduced that at the ionic strength of his study (0.16M or 0.83% salinity), the stability of the particles is of the order of 1, and the overall solidification rate is given by ka = ⅛kT∕3μ.
Recommendations for Future Work
- The Structure of Iron Oxide
- Effects of Other Particles
The effect of the same adsorbed species on the stability of different oxides can vary because the adsorption energies of the aqueous species are not expected to be the same on the surface of different oxides. An understanding of the effect of the oxide crystal structure on adsorption is essential to predict the stability of these oxide particles. Sigg, L., (1987), Surface Chemical Aspects of the Distribution and Fate of Metal Ions in Lakes, Chap.
Equilibrium constants for the formation of aqueous complexes
Comparison of hematite properties
Critical coagulation concentration of inorganic ions
Properties of Suwannee River fulvic acid
Summary of hematite properties
Comparison of surface and aqueous equilibrium constants
Critical coagulation concentration of organics in terms of carbon
Hamaker constants for hematite
Stability of hematite as a function of NaCl concentration
Stability of hematite as a function of temperature
Stability of hematite as a function of bivalent ions
Stability of hematite as a function of phosphate concentration
Stability of hematite as a function of
Stability of hematite in the presence of phthalate and oxalate
Stability of hematite as a function of polyelectrolyte concentration
