Environ. Sci. Technol. 2005, 39, 4647-4654
Fractionation of UV and VUV Pretreated Natural Organic Matter from Drinking Water W . B U C H A N A N , †,§ F . R O D D I C K , * ,†,§ N . P O R T E R , ‡,§ A N D M . D R I K A S § School of Civil and Chemical Engineering, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia, School of Applied Science, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia, Australian Water Quality Centre, Salisbury, South Australia 5108, Australia, and Cooperative Research Centre for Water Quality and Treatment, Salisbury, South Australia 5108, Australia
Recent studies have examined the potential of ultraviolet (UV, 254 nm) and vacuum ultraviolet (VUV, 185 nm + 254 nm) irradiation as either a pretreatment for a biological process or as a sole treatment for the removal of natural organic matter as dissolved organic carbon from drinking water. To understand the potential of UV and VUV irradiation followed by subsequent biological treatment, treated water was fractionated into four components: very hydrophobic acid (VHA), slightly hydrophobic acid (SHA), hydrophilic charged (CHA), and hydrophilic neutral (NEU). The VHA fraction was found to be very susceptible to both UV and VUV irradiation, and the fragmentation products of the high molecular weight VHA and SHA molecules contributed to the CHA and NEU fractions to form a pool of biodegradable, non-UV-absorbing, low molecular weight moieties. The NEU fraction was the most difficult to remove, as most of the components in this fraction were refractory to both the biological and photo-oxidative processes. Therefore, enhanced removal of the NEU fraction is required to increase the effectiveness and potential of the treatment process.
Introduction Recent studies have investigated the use of UV (254 nm) and VUV (185 nm + 254 nm) radiation for the removal of natural organic matter (NOM) as dissolved organic matter (DOC) from drinking water (1-3). NOM removal during irradiation is attributed to mineralization via a complex sequence of photochemical and subsequent oxidation reactions between NOM, reactive species, and rapidly reacting species (4). The biodegradability of NOM has been shown to increase with exposure to artificial sunlight (5), UV-A, UV-B, and UV-C (1, 2, 6), and VUV (7, 8). These studies suggest that the addition of a biological treatment following irradiation is a potentially useful water treatment option. Buchanan et al. (8) confirmed the indicative results of Thomson et al. (2) in a systematic investigation of the biological removal of UV and VUV pretreated NOM. The * Corresponding author e-mail:
[email protected]; tel: (+613) 9925-2080; fax: (+613) 9925-3746. † School of Civil and Chemical Engineering, RMIT University. ‡ School of Applied Science, RMIT University. § Australian Water Quality Centre and Cooperative Research Centre for Water Quality and Treatment. 10.1021/es048489+ CCC: $30.25 Published on Web 05/11/2005
2005 American Chemical Society
results of this study found VUV irradiation was more effective than UV irradiation prior to biological treatment for the removal of NOM due to more rapid formation of biodegradable compounds and mineralization. The greater effectiveness of VUV irradiation is a consequence of the in situ formation of hydroxyl radicals via the photolysis of water at 185 nm (7). Other processes which involve the formation of hydroxyl radicals, such as combined UV/H2O2 photo-oxidation, exhibited similar NOM breakdown and removal behavior to VUV photo-oxidation (9, 10); however, VUV photooxidation has the advantage of being an additive-free process. To further understand the effects of irradiation on NOM, irradiated samples (with and without subsequent biological treatment) were fractionated and analyzed for DOC, UV absorbance, and molecular weight distribution. The dosages delivered (16-233 J‚cm-2) were much larger than typical UV disinfection dosages (40-100 mJ‚cm-2), hence the results shown in the current study would not be expected in typical UV disinfection applications. This paper identifies the fractions of NOM which are most susceptible to photo-oxidation and those most susceptible to biological treatment. The fractionation system designed by Chow et al. (11) was specifically designed for studying water treatment processes and was based on the full scale fractionation systems of Croue´ et al. (12) and Bolto et al. (13). The relative concentration of the four fractions of NOM (very hydrophobic acid (VHA), slightly hydrophobic acid (SHA), hydrophilic charged (CHA), and hydrophilic neutral (NEU) species) can be obtained: It is recognized that these fractions are more operationally than structurally defined subgroups (14). It has been shown that NOM fractions contain a mixture of different compounds and that the types of compounds from each fraction are dependent upon the water source (15). The typical classes of compounds which have been observed in each fraction are listed in Table 1. The functional groups observed in each fraction give clues to the possible behavior of the fraction; for instance, the hydrophobic fractions tend to have greater aromaticity than the hydrophilic fractions (Table 1). The greater the molecular weight and aromaticity of the NOM, the greater its hydrophobic character and the more reactive it is with oxidants such as chlorine and ozone (16). Therefore, these hydrophobic moieties would be expected to react preferentially on exposure to UV or VUV irradiation due to the presence of unsaturated bonds. The effect of UV and VUV irradiation on the individual NOM fractions was investigated to identify which were the most susceptible to photo-oxidation and biodegradability. The biodegradable dissolved organic carbon (BDOC) method was used as a surrogate for biological treatment as it is comparable to DOC removal by industrial scale biofiltration (23) and hence should indicate the likely success of biological treatment.
Materials and Methods The NOM used in this study was collected as a surface water sample in April 2002 from the East Moorabool system at She Oaks, located between the Upper and Lower Stony Creek Reservoirs in Victoria, Australia. The sample was filtered through a 0.45-µm hydrophilic membrane (Durapore PVDF) and stored at 4 °C prior to treatment and analysis. The characteristics are detailed in Table 2. Irradiation experiments were conducted using an annular reactor with a centrally mounted lamp, which facilitates both UV and VUV irradiation. During irradiation experiments, samples were mixed and aerated by humidified air. The VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Proposed Composition of NOM Fractionsa fraction
class of organic compounds
hydrophobic acid hydrophobic base
hydrophobic neutral
hydrophilic acid hydrophilic base hydrophilic neutral
a
ref
Hydrophobic Fractions (VHA + SHA) soil fulvic acids, C5-C9 aliphatic carboxylic acids, 1- and 2-ring aromatic carboxylic acids, 1- and 2-ring phenols portion of humic substance retained by XAD-8 resin at pH 7 which can be eluted by HCl; 1- and 2-ring aromatics except pyridine, proteinaceous substances a mixture of hydrocarbons; >C5 aliphatic alcohols, amides, esters, ketones, aldehydes; long chain (>C9) aliphatic carboxylic acids and amines; >3 ring aromatic carboxylic acids and amines Hydrophilic Fractions (CHA + NEU) >C5 aliphatic carboxylic acids, polyfunctional carboxylic acids, mixtures of various hydroxy acids amphoteric proteinaceous materials containing aliphatic amino acids, amino sugars, peptides, and proteins; NEU > CHA (Figure 2). The hydrophobic fraction has greater aromaticity than the hydrophilic fractions (Table 1) and has high reactivity toward oxidation (16). Consequently, due to the dominance of the hydrophobic fraction in East Moorabool NOM, it was anticipated that the hydrophobics would account for the majority of DOC removed by UV photo-oxidation. The NOM was subjected to UV or VUV irradiation for varying periods and then fractionated to identify the fractions susceptible to irradiation (Figures 3 and 4). As expected, after irradiation at 254 nm, the greatest DOC removal occurred in the VHA fraction (Figure 3). The concentration of the VHAs was reduced in an almost linear
(1)
The behavior observed for VUV-irradiated NOM was different for the SHA, CHA, and NEU fractions. The VHA fraction was markedly reduced by VUV irradiation, particularly within the first 15 min (Figure 4). Mineralization of DOC with VUV irradiation has been reported as following pseudofirst-order kinetics (eq 2) (R2 ) 0.983, k2 ) 0.016 min-1) (8). Similarly, DOC removal of the VHA fraction fitted first-order kinetics (R2 ) 0.964, k(VUV-VHA) ) 0.049 min-1). It has been suggested that the DOC was probably removed by reaction with an oxidant at a constant concentration, presumably the hydroxyl radical produced by water photolysis (7).
d[DOC] ) -k2[OH•][DOC] dt
(2)
where [OH•] is considered constant. During the initial 15 min of VUV irradiation, the concentration of the SHA fraction increased. However, doses greater than this resulted in rates of removal similar to the VHA fraction (R2 ) 0.956, k(VUV-SHA) ) 0.048 min-1). The concentration of the CHA fraction also rapidly increased during the initial 30 min of irradiation and only decreased after the VHA and SHA fractions were largely removed. The concentration of the NEU fraction slightly increased in the first 75 min of irradiation and then reduced to lower than the initial DOC after the VHA and SHA fractions had been almost totally removed. Removal of Chromophores (A254) with UV and VUV Irradiation. The absorbance at 254 nm was measured to determine the effects of irradiation on the chromophores of each of the four fractions, thus yielding information on specific organic groups rather than the total dissolved organic species. VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. A254 of each fraction after varying doses of UV or VUV radiation. (Number of replicate experiments, n ) 4; error bars not included as variation was less than (0.005 au.) Fractionation before and after irradiation determined that the VHA fraction was the most intensely UV absorbing (Figure 5), which is consistent with findings for other sources of Australian NOM (13). The chromophore content of the VHA fraction was markedly reduced by both UV and VUV irradiation, with VUV irradiation resulting in more effective removal. At an equivalent dose of 48 J‚cm-1, 88% of VHA chromophores were removed by VUV irradiation (45 min), while only 47% was removed with UV irradiation (60 min). A similar dose resulted in reduction of the SHA chromophores with both UV and VUV irradiation. Both the CHA and NEU fractions have a low absorbance at 254 nm and hence do not have a high aromatic nor conjugated content; this is consistent with the typical functional groups found within this fraction (Table 1). Molecular Size Distribution of the NOM Fractions. HPSEC was used to measure the molecular size distribution of the UV-absorbing species after varying periods of irradiation. The apparent molecular weight (AMW) was theoretically determined using polystyrene sulfonate standards. However, it is recognized that since the standard is a more flexible polyelectrolyte than NOM the AMW reported may not be truly representative. Thus the results have been interpreted in terms of trends. The chromatograms given in Figure 6 are of individual fractions as obtained by difference; the data for each of the resin effluents were subtracted to achieve a plot which is indicative of the fraction of interest, after varying periods of irradiation. Both UV and VUV irradiation led to a significant decrease in the VHA fraction (Figure 6, plots A and B), and there was a general shift to lower apparent molecular weight (LAMW) compounds. However, differences were observed in the degradation pattern of chromophores when subjected to irradiation by the two different lamps. VUV irradiation resulted in markedly more effective chromophoric removal across the range of molecular weights, whereas UV irradiation seemed to preferentially degrade the high apparent molecular weight (HAMW) chromophores. The untreated VHA fraction consisted predominantly of HAMW chromophoric moieties, although the peak at an AMW of 400 Dalton indicates that there may be some conjugated LAMW moieties. The SHA fraction contained markedly fewer HAMW UVabsorbing molecules than the VHA fraction. The absorbance of the SHA compounds after VUV irradiation was not greatly different from that of the untreated SHA fraction (Figure 6, plot C). The plots indicate a slight decrease in absorbance 4650
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coinciding with an increase in the HAMW compounds; however, it is recognized that the absorbance of these are very low and the appearance of a HAMW group could be an artifact of the data subtraction process. The same occurrence was observed after 30 min of UV exposure (Figure 6, plot D) except that there appears to be a slight accumulation of LAMW (150-200 Dalton) SHA chromophores. Increasing the UV dose seemed to result in the removal of the newly formed LAMW UV-absorbing chromophores. Because the absorbance of the CHA and NEU fractions was very low, they were analyzed as one hydrophilic fraction. Again there appears to be an apparent HAMW group present. After small doses of UV and VUV radiation, the absorbance of the hydrophilic fractions (CHA and NEU) effectively remained relatively unchanged (Figure 6, plots E and F). Fractionation of Biologically Treated Raw Water. The raw East Moorabool water was fractionated after a BDOC determination to identify the biodegradability of the fractions (Figure 7). Overall, 1 mg/L of DOC (approximately 10% of the total) was removed by the sand-attached bacteria, with a standard deviation of (0.1 mg/L for the six replicate BDOC experiments. As the error associated with the DOC analysis is (0.1 mg/L and the DOC contents of the fractions were calculated by difference, there is an error of (0.2 mg/L for each of the fractions, with the exception of NEU as it was the only pure fraction obtained. Taking errors into account, there was no significant difference between the initial and final DOC concentrations for the VHA, SHA, and CHA fractions. Significant DOC removal occurred only for the NEU fractions, where 0.4 mg/L of the DOC was biologically removed. The fractions responsible for the enhanced biodegradability following irradiation are shown in Figures 8 and 9. Similar radiation doses were chosen to give a direct comparison between the effectiveness of UV and VUV irradiation on each fraction. Biological removal of DOC from the VHA fraction was small. In contrast, there was an increase in biodegradability with increasing radiation dose for the SHA fraction. UV irradiation of raw water resulted in the accumulation of CHA components, which became increasingly biodegradable with radiation dose. Similarly, the NEU fraction was found to increase in concentration and in biodegradability with increasing dose. The increased biodegradability caused by UV irradiation can be attributed predominantly to the CHA and NEU fractions, followed by the SHA fraction, with the VHA fraction contributing to a lesser extent.
FIGURE 6. The AMW of the UV-absorbing species in the fractions of raw and irradiated water. Numerical values in legend represent radiation dosage (J‚cm-2). VUV irradiation led to a decrease in the DOC concentration of the VHA fraction, but the biodegradability of the fraction was not enhanced. This is similar to the results for UV irradiation. The initial increase in DOC for the SHA fraction contributed to some increase in biodegradability; however,
the CHA and NEU moieties were again more susceptible to biological removal. As with UV treatment, the VUV-treated NEU fraction contained moieties resistant to biological degradation. These moieties accounted for 0.8 mg/L of DOC after VUV treatment and 1.0 mg/L after UV treatment. This VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. DOC contents of the fractions of NOM before and after BDOC test. (Number of replicate experiments, n ) 6). identifies the NEU fraction as containing some components which are refractory to both photo-oxidation and biological treatment. Overall, the CHA fraction was the major contributor to the enhanced biodegradability as a result of VUV irradiation, followed by the NEU, SHA, and VHA fractions.
Discussion VHA Fraction. UV irradiation reduced the DOC of the VHA fraction in an almost linear fashion (Figure 3) and therefore must be largely responsible for the overall zero-order DOC removal kinetics reported elsewhere (8). The relative A254 of the fractions is consistent with the data of Marhaba et al. (14) and Table 1, as the VHA fraction was found to have a higher degree of aromaticity than the hydrophilic fractions. The AMW as detected by HPSEC with UV detection of the VHA fraction was markedly altered by UV irradiation, which is not surprising given the size and conjugated character of the components. However, only slight diminution of the small AMW UV-absorbing VHA compounds was observed, supporting the theory that the HAMW compounds are preferentially degraded by UV irradiation (7, 8). In contrast, VUV irradiation resulted in the removal of chromophores across the entire range of molecular weights and markedly reduced the DOC and A254 of the VHA fraction, particularly within the first 15 min of irradiation. The biological removal of DOC from the VHA fraction was small, even after treatment with either UV or VUV radiation. This was to be expected as the VHA fraction contains compounds which are high in aromatic character and high molecular weight (Table 1) and are presumably not readily digestible by bacteria. Fractionation demonstrated that although irradiation rapidly reduced the degree of conjugation and the size of the remaining VHA compounds, these structural changes did not significantly enhance the biodegradability of the remaining VHA fraction. SHA Fraction. During UV irradiation, the DOC concentration of the SHA fraction remained effectively constant. There are two possible explanations for this: either the compounds in the SHA fraction were not affected by irradiation at 254 nm or compounds in the SHA fraction were produced (probably from the fragmentation of VHA compounds) and removed simultaneously. The latter is more likely since the absorbance at 254 nm was rapidly reduced and the AMW decreased, indicating significant structural changes. These structural changes are likely to have been responsible for the simultaneous increase in biodegradability of this fraction. VUV irradiation resulted in an initial rapid DOC increase in the SHA fraction. This corresponded to the rapid removal of VHA material over the first 15 min, further supporting the notion that the increase in DOC for the SHA can be attributed to the fragmentation products from the 4652
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VHA fraction. Since there was a simultaneous rapid decrease in absorbance at 254 nm, the fragmentation products contributing to the pool of SHA compounds were not aromatic in character. After small VUV doses, some increased biodegradability was displayed, but longer exposure times resulted in mineralization of almost the entire hydrophobic fraction (VHA and SHA). A VUV dose of 96 J‚cm-2 (90 min) led to the breakdown products which accounted for approximately 80% of the biodegradability of the remaining DOC (8), thus the CHA and NEU hydrophilic fractions must be responsible for this biodegradability. CHA and NEU. In contrast to the hydrophobic fractions (VHA and SHA), the hydrophilic fractions (CHA and NEU) contained negligible concentrations of conjugated compounds, and during both UV and VUV irradiation the conjugated character and AMW remained relatively unchanged (A254 ≈ 0.01). Therefore, the observed DOC increase in both CHA and NEU fractions was likely to be a consequence of non-UV-absorbing breakdown products from the hydrophobic fractions contributing to the pool of CHA and NEU fractions. The DOC of the CHA fraction increased during both UV and VUV radiation, although after a VUV dose of 48 J‚cm-2 (45 min), slight reductions were observed. It is probable that initially the conjugated bonds in the VHA and SHA compounds absorbed the radiation and so provided a screening effect. Furthermore, VHA and SHA fractions contain larger molecular weight compounds and as such have a higher probability of interaction with both the incoming irradiation and any hydroxyl radicals produced. However, after a VUV dose of 48 J‚cm-2 (45 min), the VHA and SHA fractions were greatly diminished, unlike the CHA (and NEU) fraction, and so were unable to significantly screen these compounds. Thus, it would be expected that the CHA fraction would rapidly diminish on depletion of the higher MW compounds, and this was observed. The most obvious explanation for these observations is that components of the CHA fraction were simultaneously removed and formed. The DOC of the NEU fraction increased with both UV and VUV irradiation. However after the VHA and SHA fractions were almost completely depleted by VUV irradiation, the DOC of the NEU fraction was reduced to below the initial concentration. Complete removal of the NEU fraction was not achieved even with the maximum dose (128 J‚cm-2 or 120 min) used, indicating that some NEU compounds are very resistant to VUV irradiation. Nevertheless, the resultant DOC after VUV irradiation was less than that for UV irradiation. Difficulty in removing the lower MW forms of NOM has been observed with other types of treatment, including coagulation (13). In fact, irradiation displayed similar results to coagulation with alum in that the NEU fraction is not removed (11). The enhanced biodegradability of UV- and VUV-irradiated NOM can be attributed mostly to the CHA and NEU fractions formed from the accumulation of the biodegradable compounds as a consequence of the sequential breakdown of HAMW-conjugated compounds from the VHA and SHA fractions. Similar observations have been made with ozonation of NOM. Ozone preferentially reacts with aromatic structures leading to a proportional increase in hydrophilic content and BDOC (16). It has also been observed that oxidative techniques such as ozonation, peroxonation, and even chlorination lead to partial decomposition of high molecular mass structures (such as those identified in the VHA and SHA fractions) to low molecular hydrophilic compounds, which are more bio-available for micro-organisms (28). LAMW non-UV-absorbing refractory NEU compounds were observed after UV or VUV irradiation followed by
FIGURE 8. DOC contents of fractions of UV-treated NOM before and after BDOC test. (Number of replicate experiments, n ) 4.)
FIGURE 9. DOC content of fractions of VUV-treated NOM before and after BDOC test. (Number of replicate experiments, n ) 4.) biological treatment, regardless of the radiation dose. Since there was no significant reduction (from 1.1 to 0.7-1.0 mg/ L) in the DOC of the NEU fraction after either UV or VUV irradiation, respectively, and subsequent biological treatment, it would seem that the refractory NEU components may have been present in the raw water and were not generated by irradiation. The hydrophobic fractions (i.e., VHA and SHA) have been found to be more readily removed than hydrophilic components by conventional treatment processes (14). This finding was also observed in the results of the present study, which demonstrate that the hydrophobic compounds contained in the VHA and SHA fraction are much more susceptible than the hydrophilic components contained in the NEU fraction to removal by UV and VUV irradiation. This is due to the structure (i.e., larger aromatic compounds) of the hydrophobic fractions. These results show that fractionation of NOM could be used as a predictive tool in the evaluation of the effectiveness of this type of process on NOM with different characteristics. If a water body contained high concentrations of VHA and very low concentrations of NEU fractions, it is likely that
VUV irradiation would effectively remove the VHA components, while biological treatment would successfully remove the biodegradable fragmentation products. The VHA fraction of East Moorabool NOM was reduced to a greater extent than the other fractions by UV irradiation (254 nm) due to the higher degree of aromaticity and conjugation. The breakdown of the VHA fraction resulted in non-UV-absorbing fragmentation byproducts which contributed to the increased DOC content of the CHA and NEU fractions. VUV irradiation demonstrated a similar pattern, with the VHA fragmentation byproducts contributing to the pool of compounds in other fractions. The VHA fraction was the first to be diminished, followed by the SHA fraction. After almost complete removal of these fractions, the removal of the CHA fraction became much more rapid and was complete after a dose of 128 J‚cm-2. VUV irradiation was found to be superior in terms of the rate of DOC removal and loss of conjugated character for the VHA, SHA, and CHA fractions; however, the original NEU compounds were found to be resistant to both photo-oxidative processes. VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The hydrophilic fraction (CHA and NEU) produced by the breakdown of HAMW compounds originating from the hydrophobic fraction (VHA and SHA) was the major contributor to the increase in biodegradability. The NEU fraction prior to irradiation comprised non-UV-absorbing LAMW compounds which were extremely refractory to both photooxidative and biological treatment. Thus this study further supports the notion that the NEU fraction is the most problematic in terms of removal for the water treatment industry. The information from this study may be used to predict the effectiveness of irradiation for the removal of NOM with different characteristics.
Acknowledgments The authors acknowledge the Cooperative Research Centre for Water Quality and Treatment for financial support and the Australian Water Quality Centre, Bolivar, South Australia, for HPSEC analyses.
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Received for review September 27, 2004. Revised manuscript received January 28, 2005. Accepted April 7, 2005. ES048489+
