Environ. Sci. Technol. 2009, 43, 8332–8337
Influence of Different Nominal Molecular Weight Fractions of Humic Acids on Phenol Oxidation by Permanganate D I H E , † X I A O H O N G G U A N , * ,†,‡ J U N M A , * ,†,‡ A N D M I N Y U † State Key Lab of Urban Water Resource and Environment (HIT), Harbin Institute of Technology, Harbin, P. R. China, and National Engineering Research Center of Urban Water Resources, Harbin Institute of Technology, Harbin, P. R. China
Received June 9, 2009. Revised manuscript received September 1, 2009. Accepted September 8, 2009.
The effects of humic acid (HA) and its different nominal molecular weight (NMW) fractions on the phenol oxidation by permanganate were studied. Phenol oxidation by permanganate was enhanced by the presence of HA at pH 4-8, while slightly inhibited at pH 9-10. The effects of HA on phenol oxidation by permanganate were dependent on HA concentration and permanganate/phenol molar ratios. The high NMW fractions of HA enhanced phenol oxidation by permanganate at pH 7 more significantly than the low fractions of HA. The apparent second-order rate constants of phenol oxidation by permanganate in the presence of HA correlated well with their specific ultraviolet absorption (SUVA) at 254 nm and specific violet absorption (SVA) at 465 or 665 nm. High positive correlation coefficients (R2 > 0.72) implied that π-electrons of HA strongly influenced the reactivity of phenol towards permanganate oxidation which agreed well with the information provided by fluorescence spectroscopy. The FTIR analysis indicated that the HA fractions rich in aliphatic character, polysaccharidelike substances, and the amount of carboxylate groups had less effect on phenol oxidation by permanganate. The negative correlation between the rate constants of phenol oxidation by permanganate and O/C ratios suggested that the oxidation of phenol increased with a decrease in the content of oxygencontaining functional groups.
Introduction Humic substances (HS) are ubiquitous in the environment and comprise the most abundant pool of nonliving organic matter (1). As one of the major constituents in natural water, the presence of HS has great influence on the performance of drinking water treatment and groundwater remediation. The adsorption of HS on colloid surfaces results in an increase in colloidal stability and a decease in coagulation efficiency (2). The presence of HS may also reduce adsorption rates and equilibrium capacities of target micropollutants on porous adsorbents through competitive interactions with * Corresponding authors phone: +86(451)8628-3010; fax: +86(451)82368074; e-mail:
[email protected] (X. Guan), majun@ hit.edu.cn (J. Ma). † State Key Lab of Urban Water Resource and Environment (HIT). ‡ National Engineering Research Center of Urban Water Resources. 8332
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 21, 2009
target compounds (3, 4). Moreover, the competition between HS and target pollutants for available oxidants can reduce the oxidation and disinfection efficiency (5). However, some researchers reported that the presence of HS could enhance the efficiency of some oxidation processes such as ozonation, photocatalytic oxidation, photo-Fenton reaction, biomimetic catalytic system, and oxidation with manganese oxides. Ma and Graham (6) showed that the presence of a small amount of HS could initiate and promote the formation of hydroxyl radicals during Mn-catalyzed ozonation, which enhanced the destruction of atrazine. Lee and Choi (7) found that the addition of humic acids (HAs) at pH 3 increased arsenite oxidation by photocatalytic oxidation in a TiO2 suspension, which could be ascribed to the enhanced superoxide generation through sensitization. Fukushima et al. (8) elaborated that the degradation of aniline was facilitated by the photo-Fenton reactions in an aqueous solution containing HS, which could be attributed to the incorporation of the majority of aniline into the polymeric structure of HS. Fukushima et al. (9) also investigated the effects of HS on the removal of pentachlorophenol (PCP) using biomimetic catalysts and reported that HS of a lower degree of humification would be predicated to be more useful in enhancing the disappearance of PCP in an iron(III)-porphyrin complex reaction system. The study of Xu et al. (10) demonstrated that HA substantially enhanced 17β-estradiol (E2) removal by synthetic manganese oxide (MnO2) due to the complexation of HS with metal ions. The above results show that HS, as one of the major components in natural waters, may compete with target micropollutants for available oxidants, resulting in a decrease in contaminant degradation, or likely promote the removal of micropollutants by changing the mechanisms of oxidation processes. Compared to other oxidants, e.g., ozone, chlorine, chlorine dioxide, and potassium ferrate, permanganate is sometimes preferred because of its relatively low cost, ease of handling, effectiveness over a wide pH range, and comparative stability in the subsurface (11). More importantly, the oxidation of organic matters using permanganate does not lead to the formation of chlorinated or brominated byproducts (12). As a green oxidant, permanganate has received more and more attention and has been widely used in portable water treatment for enhancing coagulation and removing micropollutants (12-16) and for remediation of contaminated groundwater (17). However, most of the previous studies mainly focused on the oxidation of micropollutants using permanganate in pure water and neglected the influence of HS, which are considered as one of the most important organic components in natural waters. Hence, the effects of HS on the permanganate oxidation processes are unknown and need to be elucidated. In order to obtain information of HS structural features, the separation of HS into different nominal molecular weight (NMW) fractions to reduce their heterogeneity is a necessary step. Usually, ultrafiltration (UF) or size exclusion chromatography (SEC) techniques are applied to achieve this (18). Studies performed on different NMW HA fractions showed substantial differences between high NMW and low NMW fractions, suggesting the importance of size fractions in the metal complexing phenomenon and activities of biomimetic catalyst and photoinduction (18-20). Therefore, to clarify the key moieties of HS that influence the permanganate oxidation processes, bulk HS were separated into different NMW subcomponents with ultrafiltration (UF) in the study. The objectives of this study were to investigate the influence of HA, as a surrogate of HS, and its different NMW 10.1021/es901700m CCC: $40.75
2009 American Chemical Society
Published on Web 09/23/2009
fractions on permanganate oxidation of micropollutants under various conditions. Phenol, listed as a priority pollutant by the U.S. Environmental Protection Agency, was employed as a model organic pollutant in the present study. Moreover, spectroscopic techniques, including ultraviolet-visible (UV-vis), fluorescence and Fourier-transform infrared (FTIR), and elemental analysis were employed to characterize HA and its different NMW fractions to elucidate the key moieties of HA that affect the permanganate oxidation process.
Experimental Section Materials. The phenol of 99% purity and potassium permanganate (primary standard reagent grade) were purchased from Tianjin Chemicals Reagent Co., Ltd. (Tianjin, China) and used without further purification. All solutions were prepared with Mill-Q water. The KMnO4 crystals were dissolved in Milli-Q water to make a 10 mM stock solution, which was stored in the dark until use (1-3 days). The stock solution of phenol (2 mM) was freshly prepared for each set of experiments by dissolving a measured quantity of phenol in Milli-Q water to avoid oxidation by air and volatilization. The stock solution of sodium thiosulfate (0.1 M) as a scavenger of oxidants was prepared by dissolving a certain quantity of Na2S2O3 crystals in Milli-Q water. A commercial humic acid purchased from Shanghai Reagent Co., Ltd., China, was used as a surrogate of HS and purified by repeated pH adjustment, precipitation, and centrifugation to remove ash, humin, and fulvic acid, completely following the procedure described by Kilduff and Weber (21). The ionic strength of all the solutions was kept at 300 mM with KCl. Fractionation of HA. After extraction of the HA, HA was separated into five NMW fractions using the UF technique as described by Kilduff and Weber (21) and Francioso et al. (18). The following molecular weight cut-offs of HA >300 kDa (HA>300K), HA 100-300 kDa (HA100-300K), HA 50-100 kDa (HA50-100K), HA 10-50 kDa (HA10-50K), and HA 300K, HA100-300K, HA50-100K, HA10-50K, and HA300K was added, suggesting that most reaction sites may be located in the high NMW fractions of HA. It is well-known that permanganate is an electrophilic reagent, whose oxidation rate is increased with the density of the electron cloud of the target aromatic compounds (28). As reported by Nanny and Maza (29) and Smejkalov et al. (30), noncovalent interactions of HA with phenols were related to the formation of π-π interactions between the monoaromatic ring of substrates and HA aromatic components. The π-π interaction with HA could enhance the density of the electron cloud of phenol, which could result in the enhancement of phenol oxidation by permanganate. In addition, the binding was found to be favored with increasing aromaticity, which corresponded well with CdC content in aromatic compounds (29, 30). From Figure 4, it is clearly seen that the CdC content of HA fractions increased with the NMW of HA fractions. Thus, the π-π interaction of HA with phenol could be one of the important
FIGURE 4. Correlation between apparent second-order rate constants of phenol oxidation by permanganate with (a) SUVA at 254 nm and (b) SVA at 465 nm (E4) and 665 nm (E6) of unfractionated HA and different NMW fractions of HA. Experimental conditions: pH 7.0 ( 0.1. factors contributing to the improvement in phenol oxidation by permanganate in the presence of HA, especially in the presence of high NMW fractions. Previous studies drew the conclusion that the reaction of permanganate with phenol was second order and first order with respect to phenol and permanganate (31). Therefore, it can be assumed that the reaction of phenol with permanganate in the presence of HA also follows a secondorder reaction and first order in each reactant. When the concentration of permanganate is present in excess, the rate of phenol oxidation can be simplified to eqs 1 and 2. -
d[phenol] ) kapp[MnO4-][phenol] ) kobs[phenol] dt (1) kobs ) kapp[MnO4-]0
(2)
where kobs is the pseudo-first-order rate constant (s-1) at pH 7 and [KMnO4]0 is the initial permanganate concentration, which can be considered constant, and kapp is the apparent second-order rate constant (M-1s-1). The values of kapp in these experiments were calculated by nonlinear regression analysis (R2 >0.96) and summarized in Table 1. Table 1 shows that the apparent second-order rate constants of phenol oxidation by permanganate follow the order of HA>300K > HAtot > HA100-300K > HA50-100K > HA10-50K > HA300K > HAtot > HA100-300K > HA50-100K > HA10-50K > HA300K 100-300K 50-100K 10k-50K 300K, (c) HA 100-300K, (d) HA 50-100K, (e) HA 10-50K, and (f) HA < 10K. and the bending vibrations of aliphatic groups. The first band shifted from 1587 cm-1 to 1629 cm-1, and the second band shifted from 1374 to 1385 cm-1, with a decrease in the NMW of HA from >300 kDa to
