ABSTRACT
In the previous chapter, extraction of sorbed phenanthrene from aquifer materials using anionic amphiphilic cross-linked polymer (ACP) nanoparticles having the same microstructure of anionic surfactant micelles was presented. ACP nanoparticles were shown to adsorb weakly to a sandy aquifer in batch experiments, which is attributed to the chemically cross-linked nature of their microstructure. In this chapter, soil-washing performance of ACP nanoparticles having different surface property will be presented. Soil-washing performance of a surfactant strongly depends on the degree of sorption onto soil. In general, cationic surfactants having severe biotoxicity show relatively high degree of sorption because of negatively charged soil particles, so that the use of cationic surfactants in soil washing is highly restricted. Although the degree of sorption of anionic surfactants is much lower than that of cationic surfactants,
Advances in Nanotechnology and the Environment Edited by Juyoung Kim Copyright © 2012 by Pan Stanford Publishing Pte. Ltd. www.panstanford.com
their toxicity to soil bacteria could hinder the whole remediation effort. Therefore, nonionic surfactants can be the most favorable materials for soil remediation because of their intermediate sorption and low biotoxicity. Several kinds of nonionic surfactants have been used for removal of various hydrophobic pollutants from the soil [1-10]. Representative nonionic surfactants, which have been the most widely used on soil remediation, are Triton X-100, Tween, and Brij series. All these nonionic surfactants have polyethylene oxide (PEO) chains or ethylene oxide (EO) component as hydrophilic segments within their backbone. Hydrophilic-lipophilic balance (HLB) of these surfactants can be controlled by the varying chain length of PEO or the number of EO in the backbone [11-13]. In aqueous phase, these surfactants form nano-aggregates, micelles, at the concentration higher than their own critic micelle concentration. Exterior of their micelles have PEO or EO chains expanded and oriented toward water phase, which provide dispersion stability and solubilization capability for the micelles. To synthesis amphiphilic polymer nanoparticles having the same microstructure of nonionic surfactant micelles, nonionic amphiphilic polymers should have PEO segments as hydrophilic segment within their backbone. Although tremendous number of nonionic surfactants have been being developed and used, very few number of amphiphilic polymers having hydrophilic PEO have been commercialized. One of representative nonionic amphiphilic polymers is Pluronic (polypropylene oxide (PPO)–PEO triblock copolymer). This polymer has been used in various fields as drug delivery carrier, dispersant, template of nanoparticles synthesis, etc., because of their amphiphilic property [14-21]. Even though Pluronic has very versatile functions and applications, its application for environmental remediation has been rarely reported because of their expensive price. These amphiphilic polymer nanoparticles containing hydrophilic PEO segments could also be synthesized using nonionic amphiphilic precursors, urethane acrylate nonionomers (UAN), which can be synthesized through much simple process. UAN chains have hydrophilic PEO pendant segment and PPO-based hydrophobic segment at the same backbone. In addition, UAN chains have reactive vinyl groups at the both ends of PPO-based hydrophobic segment, so that UAN chains can be converted into highly cross-linked amphiphilic
polymer through radical-initiated polymerization process. By mixing with water, UAN chains can form very stable amphiphilic oligomer nanoparticles in aqueous phase whose structure is similar to that of nonionic surfactant micelle. As UAA nanoparticles can be converted to anionic ACP nanoparticles via cross-linking polymerization process, these amphiphilic oligomer nanoparticles can be converted into nonionic ACP nanoparticles. This chapter will first represent interfacial activity and solubilization performance of nonionic ACP nanoparticles, which will be compared with three kinds of nonionic surfactants based on the microstructural difference between nonionic ACP nanoparticles and nonionic surfactant micelles. Desorption capability of nonionic ACP nanoparticles for sorbed hydrophobic pollutants from soil was also presented and compared with nonionic surfactants. 3.1 NONIONIC AMPHIPHILIC REACTIVE
based hydrophobic segments. These two segments are connected with each other through urethane linkage such as UAA chains. That is, OH groups of hydrophobic PPO triol was first reacted with excess amount of NCO groups of diisocyanate compounds such as toluene diisocyanate (TDI) or methylene bis(phenyl isocyanate) (MDI) to synthesis NCO-terminated PPO precursors. Then, OH groups of hydrophilic poly(ethylene glycol) and hydroxyethyl methacrylate (HEMA) are reacted with NCO groups of the obtained PPO precursors in a separate step. Through two-or three-step reaction between OH groups and NCO groups, hydrophilic PEO segments are connected with hydrophobic PPO segments containing reactive vinyl groups [22-29]. Detailed recipe and chemicals used for the synthesis of UAN chains are summarized in Table 3.1. Molecular weights of PPO triol and polyethylene glycol were varied to control the HLB of UAN. Nonionic ACP nanoparticles synthesized using UAN 700-1, 700-2, and 1000 chain are named as NACP 700-1, 700-2, and 1000 nanoparticle, respectively.
