ARTICLE pubs.acs.org/est
Advanced Oxidation Process Based on the Cr(III)/Cr(VI) Redox Cycle Alok D. Bokare and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
bS Supporting Information ABSTRACT: Oxidative degradation of aqueous organic pollutants, using 4-chlorophenol (4-CP) as a main model substrate, was achieved with the concurrent H2O2-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 H2O2, which generates HO• along with the stepwise oxidation of Cr(III) to Cr(VI). The Cr(III)/H2O2 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 H2O2 to efficiently generate HO•, the dual role of H2O2 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• can be generated using both Cr(III)/H2O2 and Cr(VI)/H2O2 reactions, either concurrently or sequentially. The formation of HO• was confirmed by monitoring the production of p-hydroxybenzoic acid from [benzoic acid + HO•] as a probe reaction and by quenching the degradation of 4-CP in the presence of methanol as a HO• scavenger. The oxidation rate of 4-CP in the Cr(III)/H2O2 solution was highly influenced by pH, which is ascribed to the hydrolysis of CrIII(H2O)n into CrIII(H2O)n‑m(OH)m and the subsequent condensation to oligomers. The present study proposes that the Cr(III)/H2O2 combined with Cr(VI)/H2O2 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).
’ INTRODUCTION Advanced oxidation processes (AOPs) using hydrogen peroxide (H2O2) as a precursor of hydroxyl radical (HO•) have emerged as efficient technologies for the rapid destruction of recalcitrant organic pollutants.1 The nonselective reactivity of HO• toward most organic pollutants with near diffusion-limited bimolecular rate constants (108109 M1 s1), combined with the easy availability (million metric ton-scale), low price (ca. 1.0 $/kg of 100% H2O2), and environmentally benign nature of H2O2, facilitates large-scale applications.2 The success of H2O2based oxidation processes depends critically on the choice of reagent used to enhance the formation of HO• from H2O2 decomposition. Transition metal ions (Fe2+, Fe3+, Cu2+) have been extensively used in classical or modified Fenton (photoFenton and electro-Fenton) and Fenton-like reactions for the oxidation of various organic contaminants.38 However, the fact that the active metal species is consumed as a reagent and lost through precipitation severely limits the process efficiency. As a result, the continuous supply of metal reagent is needed to sustain the activation of H2O2, which causes the problem of metal sludge. Heterogeneous transition metal catalysts may provide an alternative solution for such problems but suffer from mass transfer limitation and metal leaching.9 Therefore, the ideal process of the metal-induced decomposition of H2O2 should require the regeneration of the active metal species through a redox cycle. To achieve this objective, the redox states of the involved metal species should be stable over a wide r 2011 American Chemical Society
pH range. Compared to iron and copper, chromium exists in a wider range of oxidation states (from 2 to +6), with the trivalent [Cr(III)] and hexavalent [Cr(VI) or chromate] species commonly found in water. Being an oxyanion, chromate is completely soluble over the entire pH range.10 However, due to its extreme toxicity and carcinogenecity, any deliberate addition of Cr(VI) as a reagent into wastewaters is not sensible, even if various physicochemical or biological post-treatments11 can easily remove it from aqueous solution. Trivalent chromium [Cr(III)], on the other hand, is the most thermodynamically stable oxidation state of chromium, kinetically inert, and significantly less toxic. Although the two chromium species (CrVI vs CrIII) are characterized by different chemical behavior, bioavailability, and toxicity,12 they are readily interconverted in aqueous solution. Cr(VI) is a strong oxidant [E0(HCrO4/Cr3+ = 1.35 VNHE)]13 and reacts rapidly with numerous reducing agents (like Fe0, Fe2+, S2‑, and natural organic matter) to form Cr(III).14 On the other hand, Cr(III) is thermodynamically stable under reducing conditions and is oxidized to Cr(VI) by Mn(III,IV) (hydr)oxides15 or photo-oxidized by FeOH2+.16 H2O2 alone can interconvert Cr(III) and Cr(VI) into each other because of its Received: June 25, 2011 Accepted: September 22, 2011 Revised: September 21, 2011 Published: October 11, 2011 9332
dx.doi.org/10.1021/es2021704 | Environ. Sci. Technol. 2011, 45, 9332–9338
Environmental Science & Technology
ARTICLE
Scheme 1. Schematic Illustration of HO• Generation from H2O2 using Cr(III)Cr(VI) Redox Cyclea
a
The numbered paths indicate the following: (1) hydrolysis of Cr(III)aquocomplex; (2) Fenton-like oxidation of Cr(III) to Cr(VI) by H2O2; (3) Cr(VI)-mediated decomposition of H2O2 via dissociation of Cr(V)peroxo complex (demonstrated in the previous work20); (4) regeneration of Cr(III) by H2O2-mediated reduction of Cr(VI).
ability to act as both an oxidizing agent [E0(H2O2/H2O) = 1.77 V] and a reducing agent [E0(O2/H2O2) = 0.68 V].17 The pe-pH relationship of Cr(VI)/Cr(III) and O2/H2O2 couples indicates that H2O2 can oxidize Cr(III) at pH > 8 and reduce Cr(VI) at lower pH.18 Since the reducing strength of H2O2 strongly increases with decreasing pH, the H2O2-induced reduction of Cr(VI) to Cr(III) at pH < 3 is used for removing chromate from wastewaters.19 In our previous work,20 we demonstrated that Cr(VI) can also activate H2O2 and generate HO• for the oxidative degradation of aqueous organic pollutants although the toxicity of Cr(VI) limits practical applications only to the degradation of organics in chromate-contaminated wastewaters. The oxidation mechanism involves the formation of tetraperoxochromate(V) complex and works over a wide range of pH 311. However, the previous method of H2O2 activation can utilize only Cr(VI), not Cr(III). The present study successfully demonstrates that Cr(III) can also activate H2O2 to generate HO• along with the stepwise oxidation of Cr(III) to Cr(VI). This enables a redox cycling of Cr(III)/Cr(VI) by H2O2 that serves as both an oxidant of Cr(III) and a reductant of Cr(VI). As a result, a new AOP that generates HO• repeatedly based on the Cr(III)/Cr(VI) redox cycle is developed (see Scheme 1). We also demonstrate that the Cr(III)/Cr(VI) redox transformation can be easily manipulated by H2O2 in pH-controlled reactions and H2O2 serves the dual roles of a precursor of HO• and an oxidant/reductant of Cr(III)/ Cr(VI). Through this reversible chromium catalytic cycle coupled with the decomposition of H2O2, the hydroxyl radicalmediated degradation of organic compounds can be achieved in repeated cycles in a single batch reactor.
’ EXPERIMENTAL SECTION Chemicals and Materials. Chemicals that were used as received in this study included chromium(III) nitrate (Sigma), sodium chromate (Sigma), hydrogen peroxide (30%, Kanto), 4-chlorophenol (4-CP, Sigma), phenol (Aldrich), aniline (Aldrich), nitrobenzene (Aldrich), benzoic acid (Aldrich), p-hydroxy
Figure 1. (a) Effect of initial pH on the degradation of 4-CP in the Cr(III)/H2O2 system. [4-CP]0 = 100 μM, [Cr(III)]0 = 2 mM, and [H2O2]0 = 20 mM. (b) Degradation of 4-CP and the concurrent generation of chloride and chromate (Cr(VI)) under the condition of (a) and pHi = 7. The degradation of 4-CP in the presence of Cr(III) only (no H2O2) or H2O2 only (no Cr(III)) is denoted by open triangle (r) and closed triangle (2), respectively.
benzoic acid (p-HBA, Aldrich), β-cyclodextrin hydrate (Aldrich), methanol (Daejung), and acetonitrile (Merck). All solutions were prepared in ultrapure water (18 MΩ cm) obtained from a Barnstead purification system. Procedure and Analytical Methods. The reactions were carried out in 50-mL glass beakers stirred on a magnetic stirrer. An aliquot of stock solution of 4-CP (or other substrate, 1 mM) was added to make a desired concentration (typically 0.1 mM), and then the reaction was initiated by the sequential addition of Cr(III) and H2O2. Unless otherwise mentioned, [Cr(III)] was fixed at 2 mM. The initial pH (pHi) of the solution was adjusted to a desired value with 1 N NaOH standard solution. Sample aliquots (1 mL) were withdrawn at regular time intervals from the reactor and injected into 4-mL glass vials containing 50 μL of sodium sulfite (Na2SO3, 2 M) to quench residual H2O2. All experiments were carried out in triplicate for a given condition. Quantitative analysis of substrates was done using a highperformance liquid chromatograph (HPLC Agilent 1100) equipped with a C-18 column (Agilent Zorbax 300SB) and a diode-array detector. The eluent compositions were as follows: (a) 0.1% phosphoric acid aqueous solution and acetonitrile 9333
dx.doi.org/10.1021/es2021704 |Environ. Sci. Technol. 2011, 45, 9332–9338
Environmental Science & Technology
ARTICLE
Figure 2. Degradation of phenol, nitrobenzene, and aniline (separate single-component experiments) in the presence of Cr(III) and H2O2 ([Cr(III)]0 = 2 mM, [H2O2]0 = 20 mM, [substrate]0 = 100 μM, and pHi = 7).
(80:20 v/v) for 4-CP, (b) water, acetonitrile, and acetic acid (78:20:2 v/v) for phenol, (c) water and methanol (50:50 v/v) for nitrobenzene and aniline, and (d) 0.1% phosphoric acid aqueous solution and acetonitrile (85:15 v/v) for benzoic acid. Quantification of ionic intermediates/products was performed using an ion chromatograph (IC, Dionex DX-120) equipped with Dionex IonPac AS-14 column and a conductivity detector. The eluent composition was 3.5 mM Na2CO3 + 1 mM NaHCO3. Cr(VI) concentration was determined using a modified diphenylcarbazide (DPC) method. H2O2 interferes in the Cr(VI) determination by the standard DPC method21 due to its ability to rapidly reduce Cr(VI) to Cr(III) in acidic solution. In the modified method,22 2-mL sample aliqouts were quenched by sequential addition of 0.5 mL of DPC in acetone (20 g/L) followed by the addition of 0.2 mL of 9 M H2SO4. The absorbance of the colored Cr-DPC complex was analyzed spectrophotometrically at 540 nm within 5 min of the color development. Total organic carbon (TOC) was measured using a TOC analyzer (TOCVCSH, Shimadzu). Various species of Cr(III) aquo-complexes were analyzed by forming their inclusion complexes with β-cyclodextrin (β-CD), which were then determined by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS, Bruker REFLEX III). The inclusion complexes were prepared by mixing Cr(III) and β-CD at 1:1 molar ratio and adjusting the pH to the desired value with 1 N NaOH. The matrix used for the MALDI-TOF experiments was α-cyano-4hydroxycinnamic acid (CHCA, Aldrich) dissolved in acetone at 80 mg/mL. The CHCA and Cr(III) + β-CD solutions were mixed at 4:1 ratio (matrix:analyte, v/v), and the mixed solution was dropped onto the MALDI plate and air-dried.
’ RESULTS AND DISCUSSION Oxidation in Cr(III)/H2O2 System. To evaluate the oxidative capacity of the Cr(III)/H2O2 system, 4-CP degradation in aqueous solution was investigated at different pHi under airequilibrated conditions. As shown in Figure 1a, 4-CP degradation was completely inhibited at pH 3, but increased with increasing
Figure 3. (a) Comparison of the time profiles of p-HBA formation during the oxidation of benzoic acid (BA) in the Cr(III)/H2O2 system. [BA]0 = 10 mM, [Cr(III)]0 = 2 mM, and [H2O2]0 = 20 mM. (b) Effect of methanol (OH radical scavenger) on the degradation of 4-CP in the Cr(III)/H2O2 system. [4-CP]0 = 100 μM, [Cr(III)]0 = 2 mM, [H2O2]0 = 20 mM, [CH3OH]0 = 100 mM, and pHi = 7.
pH leading to complete degradation in 6 h at pH 7. However, with further increase in pHi, the 4-CP removal rate decreased. The complete absence of 4-CP oxidation in acidic condition (pH < 4) can be attributed to the kinetic inertness of the Cr(III) aquocompexes. At pH < 4, Cr(III) exists as the hexaaquo complex [Cr(H2O)6]3+ (pKa = 4, see Supporting Information Figure S1) and its reaction with any organic or inorganic species (H2O2 in the present case) involves the substitution of the coordinated water molecules by the reactant. However, the extremely low frequency of water exchange within Cr(III)-aquocomplex (∼106.3 s1 or half-life ∼40 h)23 makes the aquo-complex substitutionally inert and unreactive toward H2O2 in the present experimental time scale. For comparison, the corresponding water exchange frequencies for Al(III) and Fe(III) are 100.8 s1 and 103.5 s1, respectively.23 However, when pHi increased to 7, a complete removal of 4-CP was obtained. Cr(III) at neutral pH is neither an oxidant nor a reductant and exists solely as insoluble Cr(OH)3(s). To confirm whether 4-CP was removed from aqueous solution 9334
dx.doi.org/10.1021/es2021704 |Environ. Sci. Technol. 2011, 45, 9332–9338
Environmental Science & Technology
ARTICLE
through simple adsorption on Cr(OH)3, control degradation experiments were carried out only in the presence of Cr(III) at pH 7 (without H2O2). The removal of 4-CP was not observed at all in the absence of H2O2 (Figure 1b), which ruled out the possibility of 4-CP removal by adsorption. Moreover, the concurrent production of chloride ions (Figure 1b) accounted for 90% of the removed 4-CP. This implies that a reaction between Cr(III) and H2O2 generates a reactive species that is responsible for the degradation of 4-CP. TOC was also reduced by 33((4)% in 6 h reaction, which indicates that some fraction of 4-CP was actually mineralized. The minor deficit in chloride mass balance (∼10%) may be ascribed to the generation of chlorinated intermediates that were not determined in this work. This clearly demonstrates that 4-CP can be oxidatively degraded at neutral pH in the Cr(III)/H2O2 system. Oxidation of other organic pollutants such as phenol [34((2)% TOC reduction], nitrobenzene [35((1)% TOC reduction], and aniline [25((4)% TOC reduction] was also successfully achieved at neutral pH (Figure 2). To ascertain whether 4-CP is oxidized by hydroxyl radicals generated through Cr(III)-mediated decomposition of H2O2, the oxidative conversion of benzoic acid (BA) to p-hydroxybenzoic acid (p-HBA) was used as a probe reaction. The formation of p-HBA from the reaction of (BA + HO•) has been used as an indirect method to detect HO• formation.20,24 Figure 3a shows the production of p-HBA from BA in the Cr(III)/H2O2 system at different pHi. The formation of p-HBA is completely inhibited at pHi 3 but increases with increasing pH, which is similar to the pH-dependent behavior of 4-CP degradation (see Figure 1). This indicates that HO• generated from the reaction of Cr(III) and H2O2 is initiated only at pH g 5, which corroborates the electron paramagnetic resonance (EPR) study of Shi et al.25 who reported the formation of HO• in neutral (pH = 7.2) solution of Cr(III)/H2O2 but not under acidic (pH = 3) condition. Furthermore, the addition of methanol as a hydroxyl radical scavenger completely inhibited the degradation of 4-CP at neutral pH (Figure 3b). This confirms that H2O2 acts as a precursor of hydroxyl radicals, which are primarily responsible for 4-CP oxidation in the presence of Cr(III). The reaction between Cr(III) and H2O2 system generates HO• through a Fenton-like reaction with the simultaneous formation of intermediate Cr(IV) species.26 CrðIIIÞ þ H2 O2 f CrðIVÞ þ HO• þ OH
ð1Þ
•
Cr(IV) immediately generates another HO from H2O2 (reaction 2)27 or undergoes disproportionation to generate Cr(V) species (reaction 3).27 Cr(V) induces another Fentonlike reaction (reaction 4)28 with further generation of HO•. CrðIVÞ þ H2 O2 f CrðVÞ þ HO• þ OH
ð2Þ
2CrðIVÞ f CrðVÞ þ CrðIIIÞ
ð3Þ
CrðVÞ þ H2 O2 f CrðVIÞ þ HO• þ OH
ð4Þ
The generation of Cr(VI) during 4-CP oxidation via stepwise oxidation of Cr(III) is shown in Figure 1b. Thus, the Cr(III)/ H2O2 system generates HO• via a series of Fenton-like reactions involving intermediate Cr(IV) and Cr(V) species,29 leading to the transformation of Cr(III) into Cr(VI). However, it should be realized that the actual oxidation chemistry can be more complex. It may be possible that the intermediate Cr(IV) and
Table 1. Proposed Chemical Structures of Cr(III)β-CD Inclusion Complexes Identified by MALDI-TOF at Different pHi proposed complex compositiona
pHi
m/z
7
1272.6
[Cr2(μOH)2(H2O)8]4+β-CD
1311.7
[Cr2(μOH)2(H2O)7(OH)]3+β-CDNa+
1273.8
[Cr2(μOH)2(H2O)8]4+β-CD
1312.2
[Cr2(μOH)2(H2O)7(OH)]3+β-CDNa+
1332.6 1358.4
[Cr2(μOH)2(H2O)6(OH)2]2+β-CDNa+ [Cr3(μOH)4(H2O)9]5+β-CD
1359.2
[Cr3(μOH)4(H2O)9]5+β-CD
9
11
Na β-CD (m/z = 1158) adducts were observed in all samples. At pHi e 5, only the Na+ adducts were detected. a
+
Cr(V) species take part in the direct oxidation of 4-CP and its intermediates, which should complicate the overall redox chemistry. The formation of a stable Cr(V)-complex is possible in the presence of organic substrates with ligand groups (e.g., hydroxycarboxylate and 1,2-diol moieties in natural organic matters).30 Moreover, EPR studies have also suggested the formation of stable Cr(V)-peroxo complexes in the Cr(VI)/H2O2 system.31,32 Because these intermediate chromium complexes should influence the oxidation kinetics and mechanisms, the proposed path in Scheme 1 should be taken as a simplified representation of the complex redox process in the Cr(III)Cr(VI)H2O2 system. pH-Dependent Speciation of Cr(III) and the Reactivity for H2O2 Activation. The degradation rate of 4-CP increases with pH above 3 but subsequently decreases beyond neutral pH condition (see Figure 1). When pH increases toward near neutral and alkaline values, the hexaaquo ions are hydrolyzed to hydroxocomplexes (reaction 5). ½CrðH2 OÞ6 3þ f ½CrðH2 OÞ5 ðOHÞ2þ þ Hþ
ð5Þ
This Cr(III) monomeric hydroxo-complex subsequently undergoes hydrolytic condensation to form polynuclear complexes like dimer, trimer, and higher oligomers containing μ-hydroxo bridges between adjacent chromium atoms (see Supporting Information Figure S2). These soluble hydroxo-complexes are eventually polymerized and precipitated as Cr(OH)3(s). However, the hydrolytic conversion of Cr(III)-oligomers into solid Cr(OH)3 occurs sufficiently slowly (>1 yr) to permit separation and isolation of a series of individual oligomers up to a hexamer.33,34 This means that oligomers formed through Cr(III) hydrolysis should be long-lived enough to react with H2O2. The pH-dependent formation of Cr(III) oligomers, however, strongly influences the reaction kinetics with H2O2. Rao et al.35 demonstrated that the reaction rate constant (k) obtained with isolated oligomers decreases as oligomerization proceeds (kdimer > ktrimer). The rate constant for unseparated oligomers in solution (analogous to the present study) was 2 orders of magnitude slower than for the isolated dimer. Thus, the increase in 4-CP oxidation with increasing pH from 3 to 7 indicates that Cr(III) hydrolysis (or oligomerization) initiates the decomposition of H2O2 to generate HO•. On the other hand, the decrease in oxidation efficiency in alkaline condition suggests the formation of higher oligomers, which are less reactive toward H2O2 decomposition.35 To confirm the presence of Cr(III) oligomers and determine the degree of oligomerization at different pH, β-CD was used as a complexing ligand to form inclusion complexes with the 9335
dx.doi.org/10.1021/es2021704 |Environ. Sci. Technol. 2011, 45, 9332–9338
Environmental Science & Technology oligomers. β-CD is a torus-shaped cyclic oligosaccharide with an internal hydrophobic cavity and contains seven α-D-glucopyranose units linked together by α-1,4-glycosidic linkages. It has been extensively used for molecular recognition and as metal ion receptor in hostguest systems.36 Using the oxygen atoms on adjacent pyranose rings as a diol ligand (see Supporting Information, Figure S3), β-CD can form highly stable inclusion complexes with binuclear hydroxy-bridged metal structures (structurally similar to Cr(III) oligomers).37,38 This complexation property of β-CD can be used to isolate different Cr(III) oligomers by using the internal cavity as an efficient trapping site. Table 1 shows the MALDI-TOF analysis of Cr(III) aqueous solutions containing β-CD at different pH. At pH e 5, only the Na+ and K+ adducts of β-CD were detected at m/z = 1158 and 1174, respectively. The absence of β-CD complexes with Cr(III) at pH e 5 can be attributed to the weak interaction between unmodified β-CD and the metal ion. Transition metal cations can form stable complexes only with surface functionalized-βCD, wherein, the metal ion is complexed with ligands situated outside the CD cavity.37 Thus, Cr(III) monomeric species cannot form inclusion complexes with unmodified β-CD and, hence, were not detected in the MALDI-TOF analysis. However, with increasing pH, the degree of oligomerization increased along with the hydrolysis of Cr(III), which is evident from the detection of β-CD inclusion complexes with dimer [Cr2(μOH)2(H2O)8]4+, trimer [Cr3(μOH)4(H2O)9]5+, and intermediate hydroxo-bridged species (see Table 1). At pH 11, only the trimer complex was detected, which indicates that the dimer was involved in formation of higher oligomers (most probably tetramer).39 The absence of MALDI-TOF peaks corresponding to tetramer and/or higher oligomers may be attributed to their larger molecular size, which cannot fit inside the β-CD internal cavity. The MALDI-TOF analysis confirms that Cr(III) oligomerization is initiated at pH > 5 and the subsequent pH increase leads to the formation of higher oligomers. The reactivity of these different Cr(III) hydrolytic species can be correlated to the pH-dependent 4-CP oxidation behavior. In Fenton-like reactions involving H2O2 and transition metal complexes, the oxidation of the metal center is promoted by the formation of a metalhydroperoxo complex intermediate (via ligand exchange), followed by the homolytic cleavage of the peroxo bond to generate HO•.40 In the case of Cr(III)-mediated Fenton-like reaction, the rate of water exchange in Cr(H2O)63+ is too slow (∼106.3 s1) to form a hydroperoxo complex, which explains the absence of 4-CP oxidation at pH 3 (see Figure 1). However, at pH 5, the monohydroxy complex (H2O)5CrOH2+ (pKa = 6.1)34 is the dominant Cr(III) species (see Supporting Information, Figure S1, S2). The complex of (H2O)5CrOH2+ is 75 times more reactive in water exchange reaction41,42 and 605500 times more reactive in anion complexation reaction,43 compared to Cr(H2O)63+. Thus, the hydrolysis of the Cr(III) aquocomplex at pH 5 can facilitate the peroxo ligand substitution, and initiate HO• generation for the oxidation of 4-CP. At pH > 5, the (H2O)5CrOH2+ species can be sequentially deprotonated, and then oligomerized. The rapid oligomerization competes with and mostly dominates stepwise deprotonation reactions.42 At pH 7, the deprotonated dimer [Cr2(μOH)2(H2O)6(OH)]3+ was isolated and identified by MALDI-TOF analysis (see Table 1). The water-exchange rate of this deprotonated dimer is 27 times (for cis form) or 70 times (for trans form) higher compared to the (H2O)5CrOH2+ species.44 Thus, the
ARTICLE
Figure 4. (a) Repeated cycles of 4-CP degradation and the concurrent generation of Cr(VI) in the presence of Cr(III) and H2O2 (pHi = 7). At the end of each cycle, an aliquot of HClO4 (1 N, 1 mL) was added to regenerate Cr(III) (at the point of 1), then 4-CP (3 mM, 0.9 mL) and H2O2 (20 mM) were replenished (at the point of 2), and finally the pH of the solution was readjusted to 7 before initiating the next degradation cycle. (b) The initial cycle of 4-CP degradation ([Cr(III)]0 = 2 mM, [H2O2]0 = 20 mM, pHi = 7) and the subsequent cycle of 4-CP degradation without the regeneration of Cr(III) and without pH readjustment. The time profile of pH change is shown together.
peroxo ligand exchange reaction (hence the generation of HO•) will be enhanced when raising pH from 5 to 7, which is consistent with the faster oxidation of 4-CP at pH 7 than pH 5 (see Figure 1a and Figure 3a). When pH is further increased to alkaline values (pH 9), the OH-ligands are bridged through condensation. As a result, the concentration of oligomers containing nonbridging OH groups (species A and B in Supporting Information, Figure S4) will decrease with retarding the water exchange reaction,41 which subsequently leads to retardation of peroxo complexation and suppression of OH-radical mediated oxidation (see Figure 1). Furthermore, at pH 11, the complete absence of any species with nonbridging OH groups combined with the formation of higher oligomers (trimer and possibly tetramer) lowers the Cr(III) reactivity toward H2O2. Thus, Cr(III) species coordinated with OH ligands are required to catalyze the peroxo complexation mechanism for the generation of HO•. Cr(III)/Cr(VI) Redox Process as AOP. All practical applications of metal-catalyzed Fenton and Fenton-like AOPs are severely 9336
dx.doi.org/10.1021/es2021704 |Environ. Sci. Technol. 2011, 45, 9332–9338
Environmental Science & Technology
Figure 5. Effect of solution aging time on the degradation of 4-CP in the Cr(III)/H2O2 system. [Cr(III)]0 = 2 mM, [H2O2]0 = 20 mM, and pHi = 7.
limited by the fact that the precipitation of metal ions limits working conditions to acidic region,9 and prevents the reuse of the catalyst. The Cr(III)-mediated activation of H2O2 shows the maximum oxidation capacity at neutral and near-alkaline pH. Some hydrolytic oligomers are formed at neutral and alkaline pH and they remain soluble without precipitation and can activate H2O2 with less efficiency. This will reduce the need of adding a large amount of metal salt to compensate for the catalyst loss, and subsequently prevent the problem of sludge disposal. Furthermore, the resulting oxidation product, Cr(VI), is soluble over the entire pH range.10 However, the extreme toxicity of Cr(VI) is a major concern and its complete removal from the treated wastewater is essential. Cr(VI) can be reduced to Cr(III) by using H2O2 as a reductant in acidic condition with the concurrent generation of OH radicals.19,20 Therefore, Cr(III) species can be easily regenerated by simply decreasing pH to acidic values in the presence of H2O2. In this way, we can exploit the pH-dependent dual role of H2O2 as Cr(III) oxidant and Cr(VI) reductant to establish a cyclic redox transformation of chromium along with the generation of HO radicals. This makes the Cr(III)/ Cr(VI)/H2O2 system a new AOP based on the redox cycle of chromium species without the loss of active metal species. We have successfully established the process viability by sustaining the repeated cycles of 4-CP removal at neutral pH using Cr(III) regenerated from Cr(VI) prior to oxidation (Figure 4a). However, the inhibition of 4-CP oxidation under acidic condition requires the pH to be raised back to neutral before each successive oxidation cycle. This repeated addition of acid and base will increase the total ionic strength of the solution and the overall treatment cost. To minimize the salinity increase induced by repeated pH adjustments, Cr(VI)/H2O220 as well as Cr(III)/H2O2 process should be concurrently utilized to generate HO•. That is, as Cr(III) is depleted along with the generation of HO• in the Cr(III)/H2O2 process, the accompanying Cr(VI) can also generate HO• at the same time through the Cr(VI)/H2O2 process. It should be noted that the Cr(VI)/H2O2 and Cr(III)/H2O2 processes have complementary pH-dependence: the former favored at acidic pH but the latter inhibited in the acidic condition. We verified this dual process by achieving consecutive cycles of 4-CP oxidation without regenerating Cr(III) and without pH
ARTICLE
readjustment (Figure 4b). Because the pH decreased to around 4 after the first cycle, the Cr(III)/H2O2 reaction is inhibited in the second cycle. However, the Cr(VI)-induced activation of H2O2 is more efficient at acidic pH20 and hence 4-CP oxidation was achieved through the Cr(VI)/H2O2 process in the second cycle. That is, both Cr(III)/H2O2 and Cr(VI)/H2O2 can be utilized as an AOP that generates HO• and both processes can work either concurrently or sequentially (Scheme 1). The combination of Cr(VI)/H2O2 and Cr(III)/H2O2 processes should reduce the cost for pH adjustments. However, such sequential processes without additional pH adjustments cannot be efficiently sustained for further cycles since the pH gradually converges to a relatively higher pH (>5), at which the Cr(VI)/H2O2 process is very slow. Anyway, despite the additional cost needed for sequential pH adjustments, the Cr(III)/Cr(VI)/H2O2 process works over a wide pH range by recycling the active metal species and provides an advantage over the classical Fenton or Fentonlike process, which needs a strict acidic condition to prevent the iron loss by precipitation. Finally, it should be mentioned that the aging of Cr(III) solution is also an important factor to be considered. Figure 5 shows that the degradation of 4-CP was significantly retarded when using Cr(III) solutions aged at ambient temperature for 1 month. The oligomerization process is dependent on not only pH but also the aging time. Generally, the reactivity of Cr(III) aqueous solutions decreases with the aging time, which is attributed to the transformation of soluble Cr(III) hydrolytic species (monomer and oligomer) into insoluble polynuclear species and/or amorphous chromium oxyhydroxides.45 In the present study, although the 4-CP oxidation efficiency decreased as expected, a significant concentration of 4-CP could be still removed even after 30-day aging period. This AOP based on the redox cycle of Cr(III)/Cr(VI) with H2O2 can be versatilely applied to the degradation of organics through the generation of HO• in chromium-contaminated wastewaters regardless of the oxidation state of the chromium species. However, the practical implementation of this chromium-based AOP and its effectiveness will be influenced by various constituents in wastewaters, which may interfere with the catalytic cycle of Cr(III)/Cr(VI). More thorough studies are required to understand such interfering effects.
’ ASSOCIATED CONTENT
bS
Supporting Information. The pH-dependent speciation of aqueous Cr(III) and H2O2, schematic representation of Cr(III) hydrolysis and oligomerization reactions, chemical structure of β-cyclodextrin and comparative MALDI-TOF spectra at pHi = 7 and pHi = 9. This information is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +82-54-279-2283; fax: +82-54-279-8299; e-mail: wchoi@ postech.edu.
’ ACKNOWLEDGMENT This work was supported by KOSEF NRL program (R0A-2008000-20068-0), KOSEF EPB center (Grant R11-2008-052-02002), 9337
dx.doi.org/10.1021/es2021704 |Environ. Sci. Technol. 2011, 45, 9332–9338
Environmental Science & Technology and KCAP (Sogang Univ.) funded by MEST through NRF (NRF-2009-C1AAA001-2009-0093879).
’ REFERENCES (1) Gogate, P. R.; Pandit, A. B. A review of imperative technologies for wastewater treatment. I: Oxidation technologies at ambient conditions. Adv. Environ. Res. 2004, 8, 501–551. (2) M€agerlein, W.; Dreisbach, C.; Hugl, H.; Tse, M. K.; Klawonn, M.; Bhor, S.; Beller, M. Homogeneous and heterogeneous ruthenium catalysts in the synthesis of fine chemicals. Catal. Today 2007, 121, 140–150. (3) Pignatello, J. J.; Oliveros, E.; MacKay, A. Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Crit. Rev. Environ. Sci. Technol. 2006, 36, 1–84. (4) Laine, D. F.; Cheng, I. F. The destruction of organic pollutants under mild reaction conditions: A review. Microchem. J. 2007, 85, 183–193. (5) Nam, S.; Renganathan, V.; Tratnyek, P. G. Substituent effects on azo dye oxidation by the FeIII-EDTA-H2O2 system. Chemosphere 2001, 45, 59–65. (6) Collins, T. J. TAML oxidant activators: A new approach to the activation of hydrogen peroxide for environmentally significant problems. Acc. Chem. Res. 2002, 35, 782–90. (7) Lin, T. Y.; Wu, C. H. Activation of hydrogen peroxide in copper(II)/amino acid/H2O2 systems: Effects of pH and copper speciation. J. Catal. 2005, 232, 117–126. (8) Shah, V.; Verma, P.; Stopka, P.; Gabriel, J.; Baldrian, P.; Nerud, F. Decolorization of dyes with copper(II)/organic acid/hydrogen peroxide systems. Appl. Catal., B 2003, 46, 287–292. (9) Pestunova, O. P.; Ogorodnikova, O. L.; Parmon, V. N. Studies on the phenol wet peroxide oxidation in the presence of solid catalysts. Chem. Sustain. Dev. 2003, 11, 227–232. (10) Beverskog, B.; Puigdomenech, I. Revised Pourbaix diagrams for chromium at 25300 °C. Corros. Sci. 1997, 39, 43–57. (11) Owlad, M.; Aroua, M. K.; Daud, W. A. W.; Baroutian, S. Removal of hexavalent chromium-contaminated water and wastewater: A review. Water, Air Soil Pollut. 2009, 200, 59–77. (12) Wang, W. X.; Griscom, S. B.; Fisher, N. S. Bioavailability of Cr(III) and Cr(VI) to marine mussels from solute and particulate pathways. Environ. Sci. Technol. 1997, 31, 603–611. (13) CRC Handbook of Chemistry and Physics, 85th ed.; CRC Press: Boca Raton, FL, 2004. (14) Rai, D.; Eary, L. E.; Zacharia, J. M. Environmental chemistry of chromium. Sci. Total Environ. 1989, 86, 15–23. (15) Fendorf, S. E.; Zasoski, R. J. Chromium(III) oxidation by deltaMnO2 (I) Characterization. Environ. Sci. Technol. 1992, 26, 79–85. (16) Zhang, H.; Bartlett, R. J. Light-induced oxidation of aqueous chromium(III) in the presence of iron(III). Environ. Sci. Technol. 1999, 33, 588–594. (17) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry; Wiley-Interscience: New York, 1999. (18) Richard, F. C.; Bourg, A. C. M. Aqueous geochemistry of chromium: A review. Water Res. 1991, 25, 807–816. (19) Pettine, M.; Campanella, L.; Millero, F. Reduction of hexavalent chromium by H2O2 in acidic solutions. Environ. Sci. Technol. 2002, 36, 901–907. (20) Bokare, A. D.; Choi, W. Chromate-induced activation of hydrogen peroxide for oxidative degradation of aqueous organic pollutants. Environ. Sci. Technol. 2010, 44, 7232–7237. (21) APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater, 20th ed; APHA: Washington, DC, 1988; pp 3-653-68. (22) Pettine, M.; La Noce, T.; Liberatori, A.; Loreti, L. Hydrogen peroxide interference in the determination of chromium(VI) by the diphenylcarbazide method. Anal. Chim. Acta 1988, 209, 315–319. (23) Hunt, J. P. Metal Ions in Aqueous Solutions; Benjamin, Inc: New York, 1963. (24) Joo, S. H.; Feitz, A. Z.; Sedlak, D. L.; Waite, T. D. Quantification of the oxidizing capacity of nanoparticulate zero-valent iron. Environ. Sci. Technol. 2005, 39, 1263–1268.
ARTICLE
(25) Shi, X.; Mao, Y.; Knapton, A. D.; Ding, M.; Rojanasakul, Y.; Gannett, P. M.; Dalal, N. S.; Liu, K. Reaction of Cr(VI) with ascorbate and hydrogen peroxide generates hydroxyl radicals and causes DNA damage: Role of a Cr(IV)-mediated Fenton-like reaction. Carcinogenesis 1994, 15, 2475–2478. (26) Tsou, T.-C.; Yang, J.-L. Formation of reactive oxygen species and DNA strand breakage during interaction of chromium(III) and hydrogen peroxide in vitro: Evidence for a chromium(III)-mediated Fenton-like reaction. Chem.-Biol. Interact. 1996, 102, 133–153. (27) Haight, G. P., Jr.; Huang, T. J.; Shakhashiri, B. Z. Reactions of Cr(IV). J. Inorg. Nucl. Chem. 1971, 33, 2169–2176. (28) Shi, X.; Dalal, N. S. ESR spin trapping detection of hydroxyl radicals in the reactions of Cr(V) complexes with hydrogen peroxide. Free Radical Res. Commun. 1990, 10, 17–26. (29) Shi, X.; Dalal, N. S.; Kasprzak, K. S. Generation of free radicals from hydrogen peroxide and lipid hydroperoxides in the presence of Cr(III). Arch. Biochem. Biophys. 1993, 302, 294–299. (30) Codd, R.; Dillon, C. T.; Levina, A.; Lay, P. A. Studies on the genotoxicity of chromium: From the test tube to the cell. Coord. Chem. Rev. 2001, 216, 537–582. (31) Zhang, L.; Lay, P. A. EPR spectroscopic studies on the formation of chromium(V) peroxo complexes in the reaction of chromium(VI) with hydrogen peroxide. Inorg. Chem. 1998, 37, 1729–1733. (32) Perez-Benito, J. F.; Arias, C. A kinetic study of the chromium(VI)-hydrogen peroxide reaction. Role of the diperoxochromate(VI) intermediates. J. Phys. Chem. A 1997, 101, 4726–4733. (33) St€unzi, H.; Spiccia, L.; Rotzinger, F. P.; Marty, W. Early stages of the hydrolysis of chromium(III) in aqueous solution. 4. Stability constants of the hydrolytic dimer, trimer, and tetramer at 25 °C and I = 1.0 M. Inorg. Chem. 1989, 28, 66–71. (34) St€unzi, H.; Marty, W. Early stages of the hydrolysis of chromium(III) in aqueous solution. 1. Characterization of a tetrameric species. Inorg. Chem. 1983, 22, 2145–2150. (35) Rao, L.; Zhang, Z.; Friese, J. I.; Ritherdon, B.; Clark, S. B.; Hess, N. J.; Rai, D. Oligomerization of chromium(III) and its impact on the oxidation of chromium(III) by hydrogen peroxide in alkaline solutions. J. Chem. Soc., Dalton Trans. 2002, 267–274. (36) Szejtli, J. Introduction and general overview of cyclodextrin chemistry. Chem. Rev. 1998, 98, 1743–1753. (37) Norkus, E. Metal ion complexes with native cyclodextrins. J. Inclusion Phenom. Macrocyclic Chem. 2009, 65, 237–248. (38) McNamara, M.; Russell, N. R. FT-IR and Raman spectra of a series of metallo-β-cyclodextrin complexes. J. Inclusion Phenom. Mol. Recogn. Chem. 1991, 10, 485–495. (39) St€unzi, H.; Rotzinger, F. P.; Marty, W. Early stages of the hydrolysis of chromium(III) in aqueous solution. 2. Kinetics and mechanism of the interconversion between two tetrameric species. Inorg. Chem. 1984, 23, 2160–2164. (40) Ensing, B.; Buda, F.; Baerends, E. J. Fenton-like chemistry in water: Oxidation catalysis by Fe(III) and H2O2. J. Phys. Chem. A 2003, 107, 5722–5731. (41) Lay, P. A. Recent developments on the mechanisms of substitution reactions of octahedral coordination complexes. Coord. Chem. Rev. 1991, 110, 213–233. (42) Xu, F.-C.; Krouse, R.; Swaddle, T. W. Conjugate base pathway for water exchange on aqueous chromium(III): Variable-pressure and temperature kinetic study. Inorg. Chem. 1985, 24, 267–270. (43) Espenson, J. H. Formation rates of monosubstituted chromium(III) complexes in aqueous solution. Inorg. Chem. 1969, 8, 1554–1556. (44) Crimp, S. J.; Spiccia, L.; Krouse, H. R.; Swaddle, T. W. Early stages of the hydrolysis of chromium(III) in aqueous solution. 9. Kinetics of water exchange on the hydrolytic dimer. Inorg. Chem. 1994, 33, 465–470. (45) Pettine, M.; Gennari, F.; Campanella, L.; Millero, F. J. The effect of organic compounds in the oxidation kinetics of Cr(III) by H2O2. Geochim. Cosmochim. Acta 2008, 72, 5692–5707.
9338
dx.doi.org/10.1021/es2021704 |Environ. Sci. Technol. 2011, 45, 9332–9338
