Surfactants interfere with the ability of the molecules of a substance to interact with each other and, in this way, lower the surface tension of the substance. That is, they have two distinctly different characteristics, polar and nonpolar, in different parts of the same molecule. The surfactant lines up at the interface as shown below in Fig1.2. The hydrophobic end of the molecule faces away from the water and the hydrophilic end stays close to the water.
These hydrophobic regions are surrounded by the hydrophilic regions where the heads of the surfactant molecules interact with water. The surface tension of the water undergoes a sharp drop and the cleaning power of the mixture increases dramatically at the CMC. These surfactants are generally more expensive than anionics because of the high-pressure hydrogenation reaction that must be performed during their synthesis.
They can be anionic (negatively charged), cationic (positively charged) or non-ionic (no charge) in solution, depending on the acidity or pH of the water. While the positive charge is almost always ammonium, the source of the negative charge can vary (carboxylates, sulfates, sulfonates).
What is Surfactant Adsorption?
General behaviour of surfactant and its effect on various physical properties
Adsorption of more than one surfactant significantly increases the efficiency of many interfacial properties compared to the adsorption of a single surfactant. Although the adsorption of single surfactants at the solid-liquid interface has been intensively investigated, there have been only a few studies of mixed systems, despite their great importance. Adsorption of surfactants from the mixed systems mainly depends on the solution properties of mixed surfactant system.
Many researchers have studied the solution properties of mixed surfactant systems and the resulting adsorption. The physical properties such as surface tension, interfacial tension and detergency change below the CMC with concentration, but there is no change in these properties above the CMC. Some other physical properties, such as density and equivalent conductivity, show a change in slope below and above the CMC.
Adsorption of Surfactant at the Solid/Liquid Interface
Adsorption by polarization of electrons: When the surfactant contains electron-rich aromatic nuclei and the solid adsorbent has strong positive sites, attraction between electron-rich aromatic nuclei of the adsorbate and positive sites on the adsorbent leads to adsorption. v) Adsorption by dispersion forces: Adsorption by London-van der waals force between adsorbate and adsorbent increases with increasing molecular weight of the adsorbate. The charge on the mineral colloids depends on the nature of the colloid, pH, ionic strength and other solution conditions3. At the solid-liquid interface, the graph of the amount of surfactant adsorbed per unit mass or unit area of the solid vs.
The study of surfactant adsorption equilibrium is important to determine the maximum amount adsorbed per unit area or mass of adsorbent and to determine the adsorption isotherm. This is a measure of the extent of the adsorbent surface covered by the adsorbent molecules in a given state, and therefore determines the interfacial properties in many applications. Solid surfaces become positively or negatively charged in aqueous media by ionization/dissociation of surface groups or by adsorption of ions from solution onto a previously uncharged surface.
Therefore, the electric double layer at the solid-liquid interface is usually an important phenomenon for the adsorption of ionic surfactants. Surfactants are mainly used in aqueous dispersions, where they reduce the surface tension and consequently increase the spread and wetting of the weed surface. Adsorption of a suitable surfactant on the filter surface can lower the energy barrier between the particles and the filter surface; and thus increase the deposition of small particles on the filter surface.
Particle settling, which destabilizes the suspension, is often caused by the shielding of surface charges on the particles that will lead to coagulation and subsequent settling. The effects of adding conventional stabilizing agents (eg, ionic surfactants, polymers) have been found to increase the stability of the particle. Surfactant molecules are adsorbed on both soil and dust surfaces in the process of cleaning.
The cleaning performance of a surfactant in the absence of electrolyte can be improved if the adsorption of the surfactant is increased.
CHAPTER 2
LITERATURE SURVEY
- Surfactant adsorption kinetics
- Adsorption kinetics of ionic surfactant
- Adsorption kinetics of non ionic surfactant
- Groundwater Remediation
- Soil washing
- SEAR (Surface Enhanced Acquifer Remediation) Model
- Materials
- Sand sieving and cleaning
- Surfactant Analysis
- Surfactant solubilization of PAH’s
- Sorption of surfactant onto sand
- Distribution of PAH in sand-water-surfactant system
There appear to be implications in the literature for the fact that time variations in the extent of adsorption can be divided into three different regimes: The nature of the solid surface, i.e. hydrophobic or hydrophilic, and the electrical interactions play an important role in the kinetics of surfactant adsorption at the solid-liquid interface. Because the cellulose surface is negatively charged in the aqueous medium, the cationic surfactant can be preferentially adsorbed onto the cellulose surface.
The initial rate of adsorption increases steadily with surfactant concentration, which is on the left side of the graph, or 0.01 mmol l_ 1. The Kd value can be easily measured in the laboratory, and the reciprocal of the Kd value can be used as an index to quantify the efficiency of the system for floor washing with the help of surfactants. Soil remediation using surfactants has been investigated for some typical HOCs. It is assumed that there are two main sources of HOCs in the soil system before leaching begins.
One is the solid HOC that is physically or chemically adsorbed or bound in the soil medium, and another is the RS that resides in the liquid phase due to partitioning. Another important study was conducted by Lizhong Zhu6 etal on the solubilization of non-ionic organic compounds (NOCs) such as phenol, p-nitrophenol and naphthalene o Betonite and soil using Myristylpyridium bromide (MPB) surfactant. The results obtained are shown in fig. 2.4. These persistent sources were determined to be NAPLs, which are undissolved organic contaminants that become trapped in the subsurface due to capillary forces.
This is because of their occurrence as groundwater contaminants, the risk associated with their presence in the subsurface, and the lack of other established remedial alternatives. A post-wetting injection water flood is conducted to recover injected chemicals and soluble or mobilized DNAPL remaining in the aquifer. The sand was sieved in the sieve shaker and the 212-500 µm size sand as taken for the experiments.
An appropriate portion of the supernatant was then carefully withdrawn with a volumetric pipette and the PAH concentration in the solution was analyzed using the UV spectrophotometer using water as a reference in the cubes. 20 ml of surfactants with different concentrations are taken in bottles and then 10 g of cleaned sand is poured into each such bottle containing surfactants of different concentrations. Equilibrium sorption experiments for naphthalene on sand in the presence of surfactant were performed in duplicate using 50 mL plastic bottles.
RESULTS AND DISCUSSIONS
- Quantization of surfactant
- Surfactant solubilization of Naphthalene
- Sorption of surfactant on sand
- Sorption of Naphthalene on sand
Different concentrations of surfactant solution, some below the CMC value and some above it, were taken. Thus obtained values of absorbance at different wavelengths corresponding to different concentrations of surfactants are plotted as shown in Fig 4.1. A similar approach was followed for the quantification of Naphthalene and the relationship between concentration of Naphthalene and concentration of surfactant was found.
Different concentrations of surfactants were taken considering the limitations of UV spectrophotometer and sufficient amount of Naphthalene solution was added to saturate the solution (0.2ml of 400mM Naphthalene solution). Thus obtained solubilization curve is shown in Fig 4.2. Similarly, the solubilization curve for anionic surfactant (Sodium Lauryl Sulfate) was obtained as shown in Fig 4.3. It can be deduced. Therefore, it can be said that anionic surfactant is more efficient in PAHs removal from sand by solubilization mechanism Below the CMC, surfactant exists as a monomer and has minimal effects on the water solubility of organic substances.
Micellar solubilization occurs when the surfactant concentration exceeds the CMC, where the aqueous solubility of organics is increased by the incorporation of hydrophobic molecules into micelles. Since the CMC value of nonionic surfactant is lower than that of anionic surfactant, the former has a better dissolution tendency than the latter. Surfactant adsorption is studied to measure surface/interface coverage with surfactant.
From both isotherms, it can be deduced that anionic as well as nonionic surfactants are equally capable of removing PAHs. But from the literature it is believed that anionic surfactants are better than nonionic surfactants. The sand surface should therefore be covered with anionic surfactant, which increases the hydrophobicity of the sand and helps extract PAHs. As the micellar concentration in the system gradually increases, the remaining RS in the liquid phase is reduced, giving a better chance for the fixed HOC to contact the unoccupied micelles.
Because the surfactant concentration is relatively high, RS is completely dissolved in the micellar phase and the solid HOC in the sand appears to be the only source extracted by surfactant micelles, resulting in a rapid reduction of HOC in the sand . the sand media.
CONCLUSIONS
The solid-sorbed surfactant was not used to simply increase the functional organic carbon content of solid for PAHs sorption. When the sorption of surfactants on the sand reached saturation, the solid surface was completely covered with the surface micelle or bilayer. Similar to the partitioning of PAHs to surfactant micelle in solution, the sorption of the sorbed surfactant is attributed to PAHs partitioning to the surface micelle.
Comparison between anionic and non-ionic surfactant shows that non-ionic has a higher solvation capacity. A key aspect in the use of surfactants for in-situ remediation of soils is the effect of the soil matrix on surfactant solution behavior.
