Superfund Research Program
Chloro-Organic Degradation by Polymer Membrane Immobilized Iron-Based Particle Systems
Project Leader: Dibakar Bhattacharyya
Grant Number: P42ES007380
Funding Period: 2000-2019
Chlorinated organic compounds (such as PCBs and TCE) constitute a large group of pollutants of concern due to their high toxicity, persistence, and various sources of distribution in the environment. In recent years, nanosized metal (especially iron) and iron oxide particles have attracted growing attention in groundwater remediation of chlorinated organics. The researchers have made significant technology advancements for the remediation of these chlorinated compounds via reductive and oxidative (requiring an additional chemical, H2O2) pathways, using supported iron-based systems, such as membranes and hydrogels. Based on their progress they have also submitted a treatability study work plan to DOE for potential implementation of their membrane-immobilized nanotechnologies at the Paducah Superfund site for TCE remediation.
In the case of oxidative pathway, Fe (II) has been immobilized in polyacrylic acid (PAA) coated-microfiltration (MF) membranes and reacted with H2O2 (modified Fenton type reaction), generating hydroxyl radicals (OH.) for destruction of chlorinated pollutants such as TCE and trichlorophenol. In the researchers’ newest approach, H2O2 for the modified Fenton reaction was generated in situ using a two-stack membrane system (Proc Natl Acad Sci, 2011). Alternately, Fe (II) was reduced to zero-valent iron and oxidized to form ferrihydrite nanoparticles (NPs) immobilized on membranes. These oxidized NPs are highly efficient catalysts for chloro-organic destruction. Their reactivity toward TCE dechlorination has been investigated in both batch and convective flow; depending on the NP loading, H2O2 concentration and residence time, conversions as high as 100% were achieved.
For the reductive pathway where no additional chemicals are needed, the same platform (PAA-MF membranes) has been employed for the immobilization of zero-valent iron NPs and used in the remediation of chlorinated organics. In addition, the researchers have done some studies with temperature responsive (use of N-isopropylacrylamide, NIPAAm, and PAA copolymer) hydrogels containing metal NPs. Preliminary results have shown that TCE dechlorination rates in water are enhanced three-fold by a small change in temperature (only 4°C, from below to above Lower Critical Solution Temperature) because of enhanced chloro-organic portioning in the collapsed hydrogel phase. They have also demonstrated (J Memb Sci, 2010) substantially increased NP longevity for PCB dechlorination by additional Fe(II) incorporation in the PAA domain. In their latest approach, the researchers show that NPs can also be synthesized using “green” reducing agents (polyphenols from tea extract) as a substitute for sodium borohydride. Tea extract contains a number of polyphenols that can directly complex and then reduce iron (II) to iron NPs, supported in the membrane. The NP’s reactivity has been tested toward TCE remediation, showing 70% dechlorination after 20 h contact time in batch mode. For faster TCE degradation and for the remediation of PCBs, Fe/Pd bimetallic NPs have been used, the secondary metal (Pd) being deposited by post-coating. The addition of Pd led to a two-fold increase in TCE dechlorination rate. Polyphenols act as both reducing and capping agents (J Memb Sci, 2011) that increase NP’s longevity and resistance to oxidation (but with a reduced reaction rate). More studies are required to achieve both an increase in the dechlorination reaction rates and longevity of the NPs. Nanoparticles were also synthesized using ascorbic acid (Vitamin C) as a reducing agent. In this case, the reduction potential is sufficient to reduce Pd2+ only, but not Fe (II). However, ascorbic acid can simultaneously reduce both Fe and Pd, resulting in a formation of 30 nm bimetallic Fe/Pd NPs; this was confirmed from SEM imaging, EDX and XRD spectra (ChemSusChem, 2011).