Hyeok Choi

Arsenic sorption on TiO2 nanoparticles: Size and crystallinity effects

Single solute As (III) and As (V) sorption on nano-sized amorphous and crystalline TiO(2) was investigated to determine: size and crystallinity effects on arsenic sorption capacities, possible As (III) oxidation, and the nature of surface complexes. Amorphous and crystalline nanoparticles were prepared using sol-gel synthesis techniques. For amorphous TiO(2), solute pH in the range of 4-9 had a profound impact on only As (V) sorption. As (III) and As (V) sorption isotherms indicated that sorption capacities of the different TiO(2) polymorphs were dependent on the sorption site density, surface area (particle size) and crystalline structure. When normalized to surface area, As (III) surface coverage on the TiO(2) surface remained almost constant for particles between 5 and 20 nm. However, As (V) surface coverage increased with the degree of crystallinity. X-ray absorption spectroscopic analysis provided evidence of partial As (III) oxidation on amorphous TiO(2) rather than crystalline TiO(2). The data also indicated that As (III) and As (V) form binuclear bidentate inner-sphere complexes with amorphous TiO(2) at neutral pH.

Voltammetric determination of catechol using a sonogel carbon electrode modified with nanostructured titanium dioxide

In this study, we investigate highly efficient sonogel carbon electrode (SGC/TiO(2)) modified with nanostructured titanium dioxide synthesized via sol-gel method employing surfactant template for tailor-designing the structural properties of TiO(2). The stable SGC/TiO(2) electrode detects catechol, a neurotransmitter, in the presence of ascorbic acid, a common interferent, using cyclic voltammetry. A possible rationale for the stable catechol detection of SGC/TiO(2) electrode is attributed to most likely the adsorption of catechol onto highly porous TiO(2) (surface area of 147m(2)g(-1) and porosity of 46.2%), and the formation of C(6)H(4)(OTi)(2) bond between catechol and TiO(2). The catechol absorbed onto TiO(2) rapidly reaches the SGC surface, then is oxidized, involving two electrons (e(-)) and two protons (H(+)). As a result, the surface of TiO(2) acts as an electron-transfer accelerator between the SGC electrode and catechol. In addition to the quantitative and qualitative detection of catechol, the SGC/TiO(2) electrode developed here meets the profitable features of electrode including mechanical stability, physical rigidity, and enhanced catalytic properties.

The influence of capsular extracellular polymeric substances on the interaction between TiO2 nanoparticles and planktonic bacteria

The role of capsular extracellular polymeric substances (EPS) at the surface of planktonic microorganisms was investigated for possible toxicity mitigation from titanium dioxide (TiO₂) nanoparticles, using variable EPS producing wild-type and isogenic mutant strains of Pseudomonas aeruginosa. Membrane integrity assays revealed that increased capsular EPS reduced cell membrane damage. Acting as a barrier to the cell membrane, capsular EPS permitted attachment of nanoparticles to the cell, while simultaneously delaying cellular damage caused by the production of reactive oxygen species (ROS). Modulations in ROS production were monitored in situ; while changes in the chemical composition of the microorganisms before and after exposure were examined with Fourier transform infrared spectroscopy (FTIR). The addition of methanol, a known radical scavenger, was shown to vastly reduce ROS production and membrane integrity losses, while not affecting physical interactions of nanoparticles with the microorganism. The results support that EPS provides an attachment site for nanoparticles, but more importantly act as a barrier to cell membrane oxidation from ROS. These observations provide better understanding of the overall importance of ROS in TiO₂ microbial toxicity.

Chapter 8 TiO2-Based Advanced Oxidation Nanotechnologies for Water Purification and Reuse

Abstract Advanced oxidation technologies (AOTs) produce highly reactive radical species, which readily attack and decompose organic pollutants in water eventually mineralizing them to water, carbon dioxide, and other simple inorganic species. AOTs involve nonselective oxidizing species and are among the most efficient chemical oxidation processes for the treatment of water contaminated with biologically toxic and nondegradable chemicals. Among such technologies, TiO 2 photocatalysis has attracted great attention for the development of efficient water purification and reuse systems due to the effectiveness of TiO 2 to generate the highly oxidizing hydroxyl radicals in the absence of any externally added chemicals in the system. In this chapter, we provide an overview of TiO 2 -based AOTs; from fundamentals to environmental applications. The mechanism of radical generation from TiO 2 /UV systems, principle of photocatalytic water purification, and applications of TiO 2 photocatalysis are described. The chapter also deals with some challenging issues in TiO 2 -based AOTs, including the need for enhancement of photocatalytic activity, challenges in reactor design with immobilized TiO 2 , the concept and multifunction of photocatalytic membranes, aspects on solar energy-based TiO 2 photocatalysis, and catalyst deactivation and fouling problems. Current advances in the technology such as tailor-design of TiO 2 materials at the nanoscale and simultaneous generation of hydroxyl radicals and sulfate radicals are of high interest and are also discussed. Finally, previous and current efforts, point to further development of TiO 2 -based AOTs, and more growth as a competitive process for full-scale applications in the near future are described in the end of this chapter.

Effect of reaction environments on the reactivity of PCB (2-chlorobiphenyl) over activated carbon impregnated with palladized iron

Reactive activated carbon (RAC) impregnated with palladized iron nanoparticles has been developed to treat polychlorinated biphenyls (PCBs). In this study, we evaluated the effects of various reaction environments on the adsorption-mediated dechlorination of 2-chlorobiphenyl (2-ClBP) in the RAC system. The results were discussed in close connection to the implementation issue of the RAC system for the remediation of contaminated sites with PCBs. Adsorption event of 2-ClBP onto RAC limited the overall performance under condition with a 2-ClBP/RAC mass ratio of less than 1.0x10(-4) above which dechlorination of 2-ClBP adsorbed to RAC was the reaction rate-determining step. Acidic and basic conditions were harmful to 2-ClBP adsorption and iron stability while neutral pH showed the highest adsorption-promoted dechlorination of 2-ClBP and negligible metal leaching. Coexisting natural organic matter (NOM) slightly inhibited 2-ClBP adsorption onto RAC due to the partial partitioning of 2-ClBP into NOM in the liquid phase while the 2-ClBP absorbed into NOM, which also tended to adsorb onto RAC, was less available for the dechlorination reaction. Common anions slowed down 2-ClBP adsorption but adsorbed 2-ClBP was almost simultaneously dechlorinated. Some exceptions included strong inhibitory effect of carbonate species on 2-ClBP adsorption and severe detrimental effect of sulfite on 2-ClBP dechlorination. Results on treatment of 2-ClBP spiked to actual sediment supernatants implied site-specific reactivity of RAC.

PCB congener sorption to carbonaceous sediment components: macroscopic comparison and characterization of sorption kinetics and mechanism.

Sorption of polychlorinated biphenyls (PCBs) to sediment is a key process in determining their mobility, bioavailability, and chemical decomposition in aquatic environments. In order to examine the validity of currently used interpretation approaches for PCBs sorption, comparative results on 2-chlorobiphenyl sorption to carbonaceous components in sediments (activated carbon, carbon black, coal, soot, graphite, flyash, wood) were macroscopically correlated with the structural, morphological, crystallographic, and compositional properties of the carbonaceous components. Since the Freundlich sorption constant, K(F) (Lkg(-1)) spanned several orders of magnitude, ranging from logK(F) of 6.13-5.27 for activated carbon, 5.04 for carbon black, 3.83 for coal to 3.08 for wood, organic carbon partitioning approach should be more specifically categorized, considering the various forms, nature and origins of organic carbon in sediment. Sorption rate constants and fraction parameters, which were numerically defined from empirical kinetic model with fast and slow sorption fractions, were closely related to the physicochemical properties of the carbonaceous components. Sorption interpretation approaches with a specific property and viewpoint, such as organic carbon partitioning, soot carbon distribution, or surface area correlation, did not properly explain the overall results on sorption capacity, fast and slow sorption kinetics, and partitioning coefficient. It is also important to emphasize the heterogeneous nature of sediment and the difficulties of encompassing the partitioning among its carbonaceous components.

Nanostructured Titanium Oxide Film- and Membrane-Based Photocatalysis for Water Treatment

Titanium oxide (TiO2) photocatalysis, one of the ultraviolet (UV)-based advanced oxidation technologies (AOTs) and nanotechnologies (AONs), has attracted great attention for the development of efficient water treatment and purification systems due to the effectiveness of TiO2 in generating highly oxidizing hydroxyl radicals, which readily attack and decompose organic contaminants in water. In this chapter, we provide an overview of how the physicochemical properties of TiO2 are precisely controlled and functionalized at the nanoscale for versatile, practical, and full-scale applications. Some challenges in TiO2 photocatalysis, including enhancement of the catalytic activity, controllability of the structural properties, immobilization to form films and membranes, narrowing of the bandgap energy, and selective decomposition of target contaminants, could be solved by introducing nanotechnological synthesis routes, noble material processing approaches, and new reactor design and concepts. The nanostructured TiO2 films and membranes inherently possess multiple functions under UV and even visible-light irradiation, including photocatalytic decomposition of organic pollutants, inactivation of microorganisms, anti-biofouling action, and physical separation of water contaminants.

Phenomenological and spectroscopic analysis on the effects of sediment ageing and organic carbon on the fate of a PCB congener spiked to sediment

This study assesses the full cycle transport and fate of a polychlorinated biphenyl (PCB) congener spiked to sediment to empirically and spectroscopically investigate the effects of sediment ageing and organic carbon on the adsorption, desorption, and reaction of the PCB. Caesar Creek sediment (CCS) was oxidized to remove amorphous organic carbon (AOC) followed by soot carbon (SC), spiked and aged with 2-chlorobiphenyl (2-ClBP), mixed with various aquatic solutions, and treated on reactive activated carbon (RAC) impregnated with palladized iron. Results showed that 2-ClBP sorption isotherms and kinetic parameters well reflected the critical influence of AOC and SC on the sorption behavior of 2-ClBP. Infrared analysis implied the presence of preferred 2-ClBP sorption sites within the sediment matrix. The shift in the CH vibrational frequencies of 2-ClBP bound to CCS was more apparent in cases of higher organic content (particularly SC) and longer ageing time, which made 2-ClBP more sorbed and strongly bound to CCS and thus made it more difficult to desorb 2-ClBP. The ageing effect on 2-ClBP binding was more prominent in the presence of organic carbon. Only desorbed 2-ClBP was transported to the target RAC for its physical adsorption and chemical dechlorination.

Desorption, partitioning, and dechlorination characteristics of PCBs in sediments in interaction with reactive activated carbon

Sediment (WHS) in Waukegan Harbor, Illinois, heavily contaminated and aged with polychlorinated biphenyls (PCBs), was treated with reactive activated carbon (RAC) impregnated with palladized iron nanoparticles. Lab test proceeded in a direct mixing configuration of RAC and WHS. A compartment configuration, where RAC was physically separated from WHS, was also designed to trace the sequential transport and fate of PCBs, including desorption, adsorption, dechlorination, and re-partitioning. PCBs, once desorbed from WHS, were immediately sequestrated to RAC and subject to dechlorination. Direct mixing of WHS with RAC was one-order of magnitude more effective for dechlorination than compartment configuration. Compared to their desorption-followed by-adsorption route, direct physical contact of RAC with PCBs bound to WHS exhibited negligible contribution to the availability of PCBs for dechlorination reaction. Addition of RAC even in compartment configuration facilitated PCBs desorption from WHS. However, slow desorption of PCBs limited overall performance, resulting in a five-order of magnitude lower dechlorination yield when compared with treatment of purely aqueous PCBs. The low dechlorination yield reflected real world complexities in treating 3.19% organic carbon-containing WHS aged with PCBs for 40 years. These observations were further supported when compared with results on clean Cesar Creek sediment spiked with 2-chlorinated biphenyls.

Chapter 8 – Nanostructured Titanium Oxide Film- and Membrane-Based Photocatalysis for Water Treatment

Titanium oxide (TiO2) photocatalysis, one of the ultraviolet (UV)-based advanced oxidation technologies (AOTs) and nanotechnologies (AONs), has attracted great attention for the development of efficient water treatment and purification systems due to the effectiveness of TiO2 in generating highly oxidizing hydroxyl radicals, which readily attack and decompose organic contaminants in water. In this chapter, we provide an overview of how the physicochemical properties of TiO2 are precisely controlled and functionalized at the nanoscale for versatile, practical, and full-scale applications. Some challenges in TiO2 photocatalysis, including enhancement of the catalytic activity, controllability of the structural properties, immobilization to form films and membranes, narrowing of the bandgap energy, and selective decomposition of target contaminants, could be solved by introducing nanotechnological synthesis routes, noble material processing approaches, and new reactor design and concepts. The nanostructured TiO2 films and membranes inherently possess multiple functions under UV and even visible-light irradiation, including photocatalytic decomposition of organic pollutants, inactivation of microorganisms, anti-biofouling action, and physical separation of water contaminants.