Advanced Oxidation Process Based on the Cr(III)/Cr(VI) Redox Cycle

Bokare, Alok D.,Choi, Wonyong American Chemical Society 2011 Environmental science & technology Vol.45 No.21

<P>Oxidative degradation of aqueous organic pollutants, using 4-chlorophenol (4-CP) as a main model substrate, was achieved with the concurrent H<SUB>2</SUB>O<SUB>2</SUB>-mediated transformation of Cr(III) to Cr(VI). The Fenton-like oxidation of 4-CP is initiated by the reaction between the aquo-complex of Cr(III) and H<SUB>2</SUB>O<SUB>2</SUB>, which generates HO<SUP>•</SUP> along with the stepwise oxidation of Cr(III) to Cr(VI). The Cr(III)/H<SUB>2</SUB>O<SUB>2</SUB> system is inactive in acidic condition, but exhibits maximum oxidative capacity at neutral and near-alkaline pH. Since we previously reported that Cr(VI) can also activate H<SUB>2</SUB>O<SUB>2</SUB> to efficiently generate HO<SUP>•</SUP>, the dual role of H<SUB>2</SUB>O<SUB>2</SUB> as an oxidant of Cr(III) and a reductant of Cr(VI) can be utilized to establish a redox cycle of Cr(III)–Cr(VI)–Cr(III). As a result, HO<SUP>•</SUP> can be generated using both Cr(III)/H<SUB>2</SUB>O<SUB>2</SUB> and Cr(VI)/H<SUB>2</SUB>O<SUB>2</SUB> reactions, either concurrently or sequentially. The formation of HO<SUP>•</SUP> was confirmed by monitoring the production of <I>p</I>-hydroxybenzoic acid from [benzoic acid + HO<SUP>•</SUP>] as a probe reaction and by quenching the degradation of 4-CP in the presence of methanol as a HO<SUP>•</SUP> scavenger. The oxidation rate of 4-CP in the Cr(III)/H<SUB>2</SUB>O<SUB>2</SUB> solution was highly influenced by pH, which is ascribed to the hydrolysis of Cr<SUP>III</SUP>(H<SUB>2</SUB>O)<SUB><I>n</I></SUB> into Cr<SUP>III</SUP>(H<SUB>2</SUB>O)<SUB><I>n</I>-<I>m</I></SUB>(OH)<SUB><I>m</I></SUB> and the subsequent condensation to oligomers. The present study proposes that the Cr(III)/H<SUB>2</SUB>O<SUB>2</SUB> combined with Cr(VI)/H<SUB>2</SUB>O<SUB>2</SUB> process is a viable advanced oxidation process that operates over a wide pH range using the reusable redox cycle of Cr(III) and Cr(VI).</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2011/esthag.2011.45.issue-21/es2021704/production/images/medium/es-2011-021704_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es2021704'>ACS Electronic Supporting Info</A></P>

Review of iron-free Fenton-like systems for activating H₂O₂ in advanced oxidation processes

Bokare, A.D.,Choi, W. Elsevier Scientific Pub. Co 2014 Journal of hazardous materials Vol.275 No.-

Iron-catalyzed hydrogen peroxide decomposition for in situ generation of hydroxyl radicals (HO<SUP>*</SUP>) has been extensively developed as advanced oxidation processes (AOPs) for environmental applications. A variety of catalytic iron species constituting metal salts (in Fe<SUP>2+</SUP> or Fe<SUP>3+</SUP> form), metal oxides (e.g., Fe<SUB>2</SUB>O<SUB>3</SUB>, Fe<SUB>3</SUB>O<SUB>4</SUB>), and zero-valent metal (Fe<SUP>0</SUP>) have been exploited for chemical (classical Fenton), photochemical (photo-Fenton) and electrochemical (electro-Fenton) degradation pathways. However, the requirement of strict acidic conditions to prevent iron precipitation still remains the bottleneck for iron-based AOPs. In this article, we present a thorough review of alternative non-iron Fenton catalysts and their reactivity towards hydrogen peroxide activation. Elements with multiple redox states (like chromium, cerium, copper, cobalt, manganese and ruthenium) all directly decompose H<SUB>2</SUB>O<SUB>2</SUB> into HO<SUP>*</SUP> through conventional Fenton-like pathways. The in situ formation of H<SUB>2</SUB>O<SUB>2</SUB> and decomposition into HO<SUP>*</SUP> can be also achieved using electron transfer mechanism in zero-valent aluminum/O<SUB>2</SUB> system. Although these Fenton systems (except aluminum) work efficiently even at neutral pH, the H<SUB>2</SUB>O<SUB>2</SUB> activation mechanism is very specific to the nature of the catalyst and critically depends on its composition. This review describes in detail the complex mechanisms and emphasizes on practical limitations influencing their environmental applications.

Reductive dechlorination of octachlorodibenzo-p-dioxin by nanosized zero-valent zinc: Modeling of rate kinetics and congener profile

Bokare, V.,Jung, J.l.,Chang, Y.Y.,Chang, Y.S. Elsevier Scientific Pub. Co 2013 Journal of hazardous materials Vol.250 No.-

Polychlorinated dibenzo-p-dioxins (PCDDs), a group of recalcitrant toxic compounds, are ubiquitous in nature. Amongst them, octachlorodibenzo-p-dioxin (OCDD) is not only prevalent in soil and sediment due to its high lipophilicity and hydrophobicity, but also detected in ground water and surface water. The present study examined the degradation of OCDD in aqueous solutions using four different zero-valent metal nanoparticles; zero-valent aluminum (nZVAL), zero-valent zinc (nZVZ), zero-valent iron (nZVI) and zero-valent nickel (nZVN). Only nZVZ was found to efficiently degrade OCDD into lower chlorinated congeners [OCDD→1,2,3,4,6,7,9-HxCDD (63%)→1,2,3,6,8,9-HpCDD (21%)→1,2,4,7,8-PeCDD (46%)→1,2,4,7-TeCDD (19%)] under ambient conditions. Simulations were also performed to predict the OCDD dechlorination pathway using a linear free energy relationship (LFER) model. Additionally, toxic equivalent quantity (TEQ) and homologue patterns were calculated by LFER modeling. The experimentally observed congener profiles were in excellent agreement with the model-predicted results, especially considering the complexity of the OCDD dechlorination pathway (256 theoretically possible reactions). This study proposes nZVZ as a suitable candidate for OCDD dechlorination and constitutes the first report on OCDD degradation using zero-valent metal nanoparticles under ambient conditions.

Dual modification of hematite photoanode by Sn-doping and Nb₂O₅ layer for water oxidation

Jeon, Tae Hwa,Bokare, Alok D.,Han, Dong Suk,Abdel-Wahab, Ahmed,Park, Hyunwoong,Choi, Wonyong Elsevier 2017 Applied catalysis. B, Environmental Vol.201 No.-

<P><B>Abstract</B></P> <P>Porous hematite (α-Fe<SUB>2</SUB>O<SUB>3</SUB>) films doped with Sn(IV) and coated with an ultrathin (∼2nm thick) Nb<SUB>2</SUB>O<SUB>5</SUB> passivation layer were synthesized, and the photoelectrochemical (PEC) water oxidation performance and durability of the hematite were examined in detail. As compared to hematite samples modified by either doping or passivation, dual-modified hematite exhibited a promising PEC water oxidation performance under AM 1.5 irradiation. A stable photocurrent was maintained under prolonged irradiation over 24h, while O<SUB>2</SUB> was produced from water with a Faradaic efficiency of over 80% without showing any sign of deactivation. This performance and durability could be decoupled into separate effects of Sn doping and Nb<SUB>2</SUB>O<SUB>5</SUB> layer via in-depth surface characterization and electrochemical analyses. Sn doping increased the donor density (<I>N</I> <SUB>d</SUB>) of bare hematite by a factor of 20 and significantly improved its conductivity, leading to enhanced charge transfer efficiency. The Nb<SUB>2</SUB>O<SUB>5</SUB> layer exerted an effect similar to Sn doping because of the diffusion of a fraction of Nb(V) into the hematite lattice during the annealing process at 700°C. The primary effect of the Nb<SUB>2</SUB>O<SUB>5</SUB> layer is to passivate the hematite surface and make the surface more reactive toward the oxygen evolution through water oxidation. These effects are synergistically combined in the dual-modified hematite electrode.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Hematite doped with Sn(IV) and coated with an ultrathin Nb<SUB>2</SUB>O<SUB>5</SUB> layer are synthesized. </LI> <LI> Dual-modified hematite exhibits remarkably high photoelectrochemical efficiencies. </LI> <LI> A stable photocurrent is maintained under prolonged irradiation over 24h. </LI> <LI> O<SUB>2</SUB> is linearly produced from water with an efficiency of >80% without deactivation. </LI> <LI> The effect is in-depth studied in terms of charge separation and injection. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Heterogeneous Catalytic Oxidation of As(III) on Nonferrous Metal Oxides in the Presence of H₂O₂

Kim, Dong-hyo,Bokare, Alok D.,Koo, Min suk,Choi, Wonyong American Chemical Society 2015 Environmental science & technology Vol.49 No.6

<P>The oxidation of As(III) (arsenite) to As(V) (arsenate), a critical pretreatment process for total arsenic removal, is easily achieved using chemical oxidation methods. Hydrogen peroxide (H<SUB>2</SUB>O<SUB>2</SUB>) is widely used as an environmentally benign oxidant but its practical use for the arsenite oxidation is limited by the strong pH dependence and slow oxidation kinetics. This study demonstrated that H<SUB>2</SUB>O<SUB>2</SUB>-induced oxidation of As(III) can be markedly enhanced in the presence of nonferrous metal oxides (e.g., WO<SUB>3</SUB>, TiO<SUB>2</SUB>, ZrO<SUB>2</SUB>) as a heterogeneous catalyst working over a wide pH range in ambient reaction conditions. In particular, TiO<SUB>2</SUB> is an ideal catalyst because it is not only active and stable but also easily available and inexpensive. Although the photocatalytic oxidation of As(III) on TiO<SUB>2</SUB> was intensively studied, the thermal catalytic activities of TiO<SUB>2</SUB> and other nonferrous metal oxides for the arsenic oxidation have been little investigated. The heterogeneous oxidation rate increased with increasing the TiO<SUB>2</SUB> surface area and [H<SUB>2</SUB>O<SUB>2</SUB>] and weakly depended on pH whereas the homogeneous oxidation by H<SUB>2</SUB>O<SUB>2</SUB> alone was favored only at alkaline condition. The oxidation rate in the TiO<SUB>2</SUB>/H<SUB>2</SUB>O<SUB>2</SUB> system was not reduced at all in the absence of dioxygen. It was not retarded at all by OH radical scavengers but markedly inhibited by hydroperoxyl radical scavengers. It is proposed that the surface complexation of H<SUB>2</SUB>O<SUB>2</SUB> on TiO<SUB>2</SUB> induces the generation of the surface hydroperoxyl radical through an inner-sphere electron transfer, which subsequently reacts with As(III). The catalytic activity of TiO<SUB>2</SUB> was maintained without showing any sign of deactivation. The heterogeneous catalytic oxidation is proposed as a viable method for the preoxidation treatment of As(III)-contaminated water under ambient conditions.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2015/esthag.2015.49.issue-6/es5056897/production/images/medium/es-2014-056897_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es5056897'>ACS Electronic Supporting Info</A></P>

LiOH-embedded zeolite for carbon dioxide capture under ambient conditions

조영민,이지윤,Alok D. Bokare,권순박,박덕신,정우성,최진식,양영민,이주열,최원용 한국공업화학회 2015 Journal of Industrial and Engineering Chemistry Vol.22 No.-

In the present work, commercial zeolites 13X and 5A are embedded with LiOH (LEZ-13X and LEZ-5A) toremove CO2 under simulated indoor conditions. Although the BET surface area of both LEZ sorbents wasmuch smaller than that of bare zeolite material, it showed highly enhanced CO2 adsorption capacity. Therequirement of optimummoisture content for maximum CO2 adsorption was established by correlatingthe relative humidity (RH) values and CO2 uptake. The modified sorbents performed effectively even inpresence of air or oxygen carrier gas, which demonstrated its viability to adsorb CO2 from indoorenvironments at ambient conditions.

Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO₂

Choi, Yeoseon,Koo, Min Seok,Bokare, Alok D.,Kim, Dong-hyo,Bahnemann, Detlef W.,Choi, Wonyong American Chemical Society 2017 Environmental science & technology Vol.51 No.7

<P>We investigated a sequential, photocatgysisdark reaction, wherein organic pollutants Were degraded on Ag/TiO2 under UV irradiation and the dark reduction of hexavalent chromium (Cr(VI)) was subsequently followed. The photocatalytic oxidation of 4-thlorophenol (4-CP), a test organic substrate, induced the generation of degradation intermediates and the storage of electrons in Ag/TiO2 which were then utilized for reducing Cr(VI) in the postirradiation period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO2 (87%), compared with bare TiO2 (27%) and Pt/TiO2 (22%). The Cr(VI) removal by Ag/TiO2 (87%) was contributed by adsorption (31%), chemical reduction by intermediates of 4-CP degradation (26%), and reduction by electrons stored in Ag (30%). When formic acid, humic acid or ethanol was used as an alternative organic substrate, the electron storage effect was also observed. The postirradiation removal of Cr(VI) on Ag/TiO2 continued for hours, which is consistent with the observation that a residual potential persisted on the Ag/TiO2 electrode in the dark whereas little residual potential was observed oh bare TiO2 and Pt/TiO2 electrodes. The stored electrons in Ag/TiO2 and their transfer to Cr(VI) were also indicated by the UV-visible absorption spectral change. Moreover, the electrons stored in the preirradiated Ag/TiO2 reacted with O-2 with showing a sign of low-level OH radical generation in the dark period.</P>

Enhanced removal of chromate from aqueous solution by sequential adsorption-reduction on mesoporous iron-iron oxide nanocomposites

Kim, Jae-Hwan,Kim, Ji-Hun,Bokare, Varima,Kim, Eun-Ju,Chang, Yoon-Young,Chang, Yoon-Seok Springer-Verlag 2012 JOURNAL OF NANOPARTICLE RESEARCH Vol.14 No.8

Solar production of H₂O₂ on reduced graphene oxide–TiO₂ hybrid photocatalysts consisting of earth-abundant elements only

Moon, Gun-hee,Kim, Wooyul,Bokare, Alok D.,Sung, Nark-eon,Choi, Wonyong The Royal Society of Chemistry 2014 Energy & environmental science Vol.7 No.12

<P>A superior cocatalytic behavior of reduced graphene oxide (rGO) was observed for the photocatalytic production of H<SUB>2</SUB>O<SUB>2</SUB> in the TiO<SUB>2</SUB>-based system. The adsorption of phosphate on TiO<SUB>2</SUB> enhanced the production of H<SUB>2</SUB>O<SUB>2</SUB> up to a millimolar level. The <I>in situ</I> formation of cobalt phosphate on rGO/TiO<SUB>2</SUB> enabled the photocatalytic production of H<SUB>2</SUB>O<SUB>2</SUB> even in the absence of organic electron donors.</P> <P>Graphic Abstract</P><P>A superior cocatalytic behavior of reduced graphene oxide (rGO) was observed for the photocatalytic production of H<SUB>2</SUB>O<SUB>2</SUB> in the TiO<SUB>2</SUB>-based system. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4ee02757d'> </P>