Photodegradation of high molecular weight kraft bleachery effluent organochlorine and color

Wat. Res. Vol. 29, No. 2, pp. 661-669, 1995

Pergamon

0043-1354(94)00152-9

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/94 $7.00 + 0.00

P H O T O D E G R A D A T I O N OF HIGH MOLECULAR WEIGHT KRAFT BLEACHERY EFFLUENT ORGANOCHLORINE A N D COLOR FREDERICK ARCHIBALDand LINE RoY-ARCAND Pulp and Paper Research Institute of Canada, 570 St John's Boulevard, Pointe-Claire, Quebec, Canada H9R 3J9 (First received September 1993; accepted in revised form May 1994)

Abstract--Previously it was shown that the high molecular weight (HMW) fractions of C and E-stage kraft bleachery effluents are very resistant to microbial degradation but are decolorized and dechlorinated by abiotic sunlight-dependent and sunlight-independent processes. In this work, to better understand the importance and mechanisms of this light-mediated effluent decolorization and AOX mineralization, two artificial light sources with defined outputs and spectra were used to irradiate hardwood and softwood HMW effluent fractions. Both longwave u.v. and blue-green visible light were effective in mineralizing a large fraction of the AOX to chloride ions, and in decolorizing the effluent over several days. Photo-decolorization showed a strong oxygen dependence while AOX mineralization did not. Visiblelight at an intensity comparable to the sun at the Earth's surface in northern latitudes mineralized a substantial proportion of the "stable" HMW AOX, in both fresh and salt water, and at low and high effluent dilutions. Such light also mineralized an appreciable fraction of the HMW carbon to CO2 by abiotic mechanisms. The hardwood HMW fraction showed insignificant toxicity as measured by a bacterial toxicity test, both before and after extensive photolysis. Key words--organochlorine, toxicity, photolysis, light, color, chlorine, kraft, bleach, effluent,degradation, adsorbable organic halogen

INTRODUCTION Among the common pulp and paper mill liquid wastes, kraft mill bleachery effluents are one of the most voluminous, and their high molecular weight (HMW) ( > 1 kD) fraction one of the most refractory. Treatments of bleachery effluents using lagoons, activated sludge, or anaerobic systems typically remove a large fraction of the biological oxygen demand (BOD) and low molecular weight (LMW) ( < 1 kD) adsorbable organic halogens (AOX) including most of the chlorinated monophenolics over the course of a few days. However, most of the HMW chemical oxygen demand (COD), color, and AOX remain (Bryant and Amy, 1989; Salkinoja-Salonen et al., 1981; Lindstrom et al., 1981). This apparently "stable" HMW material surviving conventional aerobic biotreatment is the principal source of mill AOX, color and COD emissions (Kringstad and Lindstrom, 1984). There appears to be little correlation between the toxicity of an effluent and its HMW molecules or HMW AOX content (O'Connor et al., 1991; Sagfors and Starck, 1988), i.e. HMW AOX alone usually does not show measurable aquatic toxicity (Kringstad and Lindstrom, 1984; Sagfors and Starck, 1988). One group has reported toxicity in the >20 kD fraction, but attributed it to the adsorption of resin acids to larger molecules 661

(Priha and Talka, 1986). Recently Jokela and Salkinoja-Salonen (1992) and Jokela et al. (1993) have presented evidence that much of the HMW AOX material consists of aggregates of smaller molecules which will dissociate upon high dilution or in certain organic solvents. Nevertheless, the total AOX emissions of a mill may become an important regulatory criterion in Europe and North America, and although numerous studies have looked at ways to avoid or remove " H M W " color, COD and AOX, these problems remain for many mills. What is the fate of this material in nature, where time, light, and slow (usually carbon-limited) microbial processes are major factors? The work of McKelvey and Dugal (1975) showed that photochemical decolorization of kraft bleachery and decker effluent chromophores by far ultraviolet (u.v.) light was an important process occurring over times of < 1 h, and which could be further accelerated by a number of free radical generators and ketone "photosensitizers" (McKelvey and Dugal, 1975). Panchepakesan et al. (1989), Meguro et al. (1976) Sameshima et al. (1975) and Shimada (1982, 1986) extended these findings by reporting that HMW bleachery effluent organochlorine and chlorophenolics were also significantly degraded by intense u.v. radiation over periods of less than 24 h, and that mean molecular weight, total organic carbon (TOC)

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FREDERICKARCHIBALDand LINE RoY-ARCAND

and BOD could also be substantially decreased using u.v. light. Caron and Reeve (1992) reported that both light from a xenon-arc lamp, filtered to remove much of its far u.v. and near i.r. energy, were capable of degrading H M W A O X from two softwood kraft bleachery effluents. They also reported that this photolysis was not highly dependent on temperature and that a total irradiation of about 35 "standard sunhours" degraded 50% of the H M W AOX. In earlier work at Paprican, changes occurring in the pH stable fraction of H M W ( > 1 kD) of combined C- and E-stage bleachery effluents from hardwood and softwood kraft mills were followed over 16 weeks (Roy-Arcand and Archibald, 1993). Incubated in water at 20-23°C and neutral pH, this H M W material showed substantial and continuing mineralization of its H M W A O X to chloride ions, via both daylight-dependent and daylight-independent mechanisms. Significant decreases were also seen in H M W color and apparent molecular size. Comparison of filter-sterile H M W material and the same material inoculated with settling pond sludge organisms indicated that, over 16 weeks, microbial processes were not important in the observed H M W A O X mineralization or chromophore bleaching, and that the H M W carbon was largely unavailable, at least to the microbial consortium employed. The microflora did, however, reduce toxicity, as measured with the Microtox assay. Thus the results suggest that H M W kraft bleachery A O X degrades abiotically to release chloride over a relatively short time span (in ecosystem terms) under conditions similar to those typical of many receiving waters (Roy-Arcand and Archibald, 1993). The present work examines the role of light in the decolorization and dechlorination of H M W bleachery effluent species using artificial light sources of known spectral content and photon flux. The photodegradative effects observed previously with an unknown amount of indirect sunlight were quantified and the importance of wavelength, photon flux rate, wood type, H M W molecule concentration, dissolved oxygen, temperature and salinity of the water on the ability of light to degrade kraft H M W A O X and color were examined. The results indicate that both the long wavelength u.v. and visible components of sunlight are major factors in natural H M W chromophore bleaching and A O X degradation and that the light-driven mechanisms for these two effects probably differ substantially.

MATERIALS

AND METHODS

E~uenls

Kraft bleachery effluents were obtained from two eastern Canadian kraft mills, one using a 92% mixed hardwood (predominantly maple and beech), 8% softwood furnish bleached using a (CD)EoDED sequence with 60% C102 substitution; and the other using softwood (100% spruce) in a (CD)EoDEpD sequence with 60% C102 substitution.

Chlorination (C) and alkaline extraction (E) stage effluents were shipped in sealed filled containers, cooled to 4"C and combined in a 2C:IE ratio (v/v) (comparable to their relative production volumes) at the commencement of each experiment. Only fresh effluent (< 3 days old) were used. Samples (0.5-6.01) of combined (C + E) effluents were microfiltered (0.45/~m pore size), adjusted to pH 7 (NaOH), placed in water-washed Spectra-por dialysis bags (mw cutoff = l kD) and dialyzed against deionized, glass distilled water changed 3 times over 72 h (4°C, dilution factor of effluent to water in the dialysis bag = 1370). It was demonstrated (via lack of 280 nm absorbance) that the bags did not bind measurable amounts of effluent organics. Depending on the experiment, the resulting high molecular weight (HMW) retentate was either used as is, diluted in water, or concentrated 3 4 fold using an Amicon UM-l (1000D exclusion) membrane to give 50-200ml HMW effluent with a final color of OD465 = 1.5 2.0 at pH 6.5-7.5. This permitted absorption of most of the incident green to u.v. light within the 1-2 cm light paths employed in parts of this work. The effluent was either used immediately after concentration or stored in the dark at -20°C, shown not to result in chlorine mineralization or chromophore decolorization. Recently it was reported that simple mixing of C- and E-stage effluents resulted in a 9-25% loss of AOX as CI (Dorica, 1991). This dechlorination was usually complete within 5 15min. Here, mixing of the C- and E-stage effluents was followed by 3 days of dialysis, so all the pH-dependent AOX mineralization should have occurred and all the resulting C1 dialyzed away long before the starting HMW material was used. Thus, our initial HMW experimental material was only that HMW fraction not susceptible to easy dechlorination. Light sources

Two high intensity photolysis light sources were employed. The first consisted of a Rayonet RPR-100 Photochemical Chamber Reactor containing up to 16 x 20-W 25.4cm long fluorescent lamps in a hollow cylinder (diameter between lamp centers = 23 cm). A cooling fan passed air through the cylinder and the entire device was placed in an environmental chamber to allow control of the sample temperature. Sets of fluorescent lamps from Rayonet coated with special fluors and designated "350 nm" and "450 nm" types were employed. Samples were placed in 17 x 125 mm Kimax screw-cap tubes (PTFE liners) around the periphery of a cylindrical tube holder placed in the RPR-100. During irradiation, the tube holder was rotated at 10 rpm to ensure that all tubes received equal light flux. The sample tubes were equipped with gas-tight rubber sepia for the oxygenation experiments. These glass tubes were shown to have negligible light absorbance at wavelengths longer than 325 nm. Sample temperatures under irradiation were determined by direct measurement of dummy sample liquid, as light absorption made the samples significantly warmer than the ambient air. In the case of the high effluent dilution experiments where a low absorbance made a long optical path necessary, a 2-1 beaker replaced the rotating cylindrical tube holder in the center of the Rayonet RPR- 100. Doubts about the manufacturer's published spectra for the 350 and 450 nm lamps prompted the measurement of the emission spectra of the 350 nm and 450 nm fluorescent lamps. This involved modification of a Gilford 250 spectrometer to replace the u.v. and visible light sources with the test light source. The fluorescent tube's output was plotted originally in "negative optical density units" (photon emission) at each of several hundred points across the u.v.-visible spectrum then converted to relative emission (photon flux). The essentially flat (equal) response of the photomultiplier tube to photons having wavelengths of 220-700 nm and a tungsten (visible) lamp control (which showed no sharp

Natural organochlorine photolysis peaks) indicated that all observed peaks were functions of the Rayonet fluorescent tubes. A second light source providing nearly monochromatic light consisted of a high-intensity xenon-arc lamp whose light passed through a high power double slit (input and output) tunable u.v.-visible monochromator (1200 lines/mm diffraction grating) (Photon Technology International, Princeton, N.J.) (PTI). This device was set to emit a 13 nm bandwidth beam of light centered on the selected wavelength and to pass it through two standard square (1 cm path length) 3 ml silica cuvettes, in adjustable cooling jackets. A beam-splitter allocated 10% of the light to one cuvette and 90% to the other. A focusing system allowed the beam to exactly fill the width of the sample cuvette. The contents of the cuvette receiving 90% of the light was stirred with a tiny stir-bar. To determine the photon fluxes and total light energy applied by each of the two sets of Rayonet tubes and at each of the irradiation wavelengths used from the PTI monochromator, chemical actinometry was employed. For wavelengths of 450 nm and below, the Fe(III)-oxalic acid chelate ferrioxalate was employed in the standard method (Hatchard and Parker, 1956) using the measurement of nascent Fe +2 with o-phenanthroline at 510nm. The results were calculated using published quantum yield constants (4)Fe +2) for this actinometer (Bunce, 1984). For the wavelengths of 500, 550, and 600 nm Actinochrome N (meso-diphenyl helianthrene) in air-saturated toluene was used and the absorption of visible light photons, resulting in photobleaching was followed at 429 nm. This bleaching value was then converted to total photons absorbed using the conversion factor F(429 ) = 4.1 × 106cm2mol -~ as recommended by Brauer et al. (1983). Rates were obtained by dividing the total photon flux by the actinometer exposure time. Total bleaching of the Actinocbrome N solutions was kept sufficiently low to prevent oxygen limitation.

Assays Effluent color was determined at 465 nm and referenced to a Pt~So standard solution (CPPA method H.5P). Total organic carbon (TOC) was determined by u.v.-persulfate wet oxidation in a Dohrmann DC-180 automated TOC analyzer. Chloride ions were determined using an Orion 9617 single junction combination chloride-specific electrode. This electrode was previously checked against a Wescan 261 ion chromatograph. Adsorbable organic halogen (AOX) was determined using a Euroglas ECS-1000 AOX analyzer and the recommended (DIN 38409 H14) procedure (AOX conversion to HC1 and microcoulometry). Toxicity was estimated using the Microtox Photobacterium luminescence assay following the Microbics Corp. (Carlsbad, Calif.) procedure except for reconstitution and dilutions of Microtox reagent which were done with 10 mM Tris buffer with 2% NaCI. Suspensions of P. phosphoreum cells were added to effluent dilutions and incubated for 15 min at 15c'C. Light emission was in ECs0 units. They represent the concentration of wastewater in dilution water (%v/v) at which the bioluminescence is reduced by 50%. The ECs0 values were obtained through linear and nonlinear regressions. Confidence intervals were calculated from the regression giving the best fit. For evaluation of oxygen effects on photolysis, 17-ml glass tubes were filled with 10ml effluent, equipped with gas-tight "Suba-seals", repeatedly flushed with air, highpurity oxygen or nitrogen, and illuminated. Levels of CO,, 02 and N 2 were determined in 0.5 ml headspace gas injections into a Fisher-Hamilton gas partitioner equipped with sequential molecular sieve/DEHS columns. These columns were run at 23°C using He as the carrier, and the CO2, N 2 and O: detected by thermal conductivity and quantitated using standards and an integrator.

663

RESULTS AND DISCUSSION

(1) Characterizing the light sources Ideally, one would apply known fluxes and amounts of unfiltered sunlight to fresh H M W effluent components just as they are released from the mill. However, sunlight is too dependent on weather and season, difficult both to measure and control, and the relative efficacy of different wavelengths cannot easily be assessed. Therefore, two artificial light sources were employed; (A) a hollow cylinder of fluorescent lamps having fluor compositions giving relatively broad energy outputs centered roughly on either 350 or 450 nm; and (B) an intense broad-spectrum xenonarc light passed through a high power tunable m o n o c h r o m a t o r having both entrance and exit slits to give a narrow (13 nm), almost monochromatic emission spectrum. The spectra of these light sources are shown in Fig. 1. Only one m o n o c h r o m a t o r setting is shown (500 nm) but when it is tuned to any other wavelength the bandpass shape (emission spectrum) is virtually identical, as the same emission bandwidth (13 nm) was always used and the monochromatorsample path length was short enough that wavelength-dependent light dispersion differences were not important. The Rayonet fluorescent lamps had broad, very uneven emission spectra (Fig. !), as would be expected from the lamps' mechanism of light emission; electronic excitation by low pressure mercury ions of a mixture of compounds, each fluorescing at specific wavelengths, coating the insides of each lamp and the subsequent emission of photons when the excited electrons return to their ground states. Many more small emission peaks far from the nominal emission wavelength were present but are not visible in Fig. !. Only peaks >/1% of total output are displayed, lower peaks presumably having little effect on the overall values of photolysis obtained. Initially, sets of "550 n m " and "650 n m " lamps were also assessed,

100

5o

=" .9

.!1:

20 lo

o 5

_I Q n.

2 1

2o0

T

400

S~l

6o0

Wavelength (nm)

Fig. 1. Light emission spectra of 350 and 450 nm Rayonet fluorescent tubes and of the PTI monochromator set to 500 nm. Spectra obtained as described in Methods and Materials. The spectrum does not show emissions having photon fluxes of < 1% of the maximum, as those emissions cannot contribute significantly to the effects these lamps produce on effluents. Legend: • 450 nm lamps; • 350 nm lamps; [] PTI monochromator.

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FREDERICK ARCHIBALD a n d LINE RoY-ARCAND

but these had excessively broad spectra very similar to conventional "grow light" and "cool white" type fluorescent lamps. Table 1 compares the light output of the Rayonet (fluorescent lamp) and PTI (monochromator) light sources to solar output. The Rayonet system's photon output (with 16 lamps) was directly comparable to the total energy flux of direct sunlight incident at 45-90 ° on a surface high in the atmosphere. However, the 450 nm Rayonet lamps emit a 2.5-fold greater photon flux than the 350nm lamps, and this must be taken into account when comparing the response of HMW material to the 2 sets of lamps. It should be noted that the Table 1 comparisons suffer from the fact that neither the precise bandwidths nor the steepness of the bandpass skirts for the solar irradiation values are specified and the total light fluxes measured (Thekaekara, 1970) are directly dependent upon them.

(2) Wavelength dependence of H M W decolorization and A O X photolysis rates For a photodegradative event to occur: (a) a photon must be absorbed by and transfer its energy to a molecule, and (b) the energy transferred must be sufficient to carry out a chemical alteration. It should be noted that both steps are strongly photon wavelength-dependent. It was necessary to use the PTI monochromator for the wavelength studies as the Rayonet lamps were far too spectrally impure (Fig. 1). This limited the data that could be collected, as only 3 ml effluent samples could be irradiated with the PTI, compared to > 300 ml in tubes (or 21 in a central beaker) in the Rayonet. Figure 2 and Table 1 show that narrow bandwidth 450 nm (blue) light is more effective than narrow bandwidth 350 nm u.v. light in decolorizing HMW chromophores. The comparison in Fig. 2 somewhat underemphasizes the effluent decolorizing superiority of 450 nm light over 350 nm light as about 50% more 350 than 450nm photons were absorbed. HMW AOX degradation by 450 and 350nm light was roughly comparable when the differences in the applied photon flux were taken into account (Table 1). Because the flux of the narrow-band PTI light is relatively low compared to sunlight or the Rayonet lamps (Table 1), only a fraction of the photosensitive targets were actually degraded, i.e. the observed photoeffects were not limited by exhaustion of susceptible molecules. While several previous reports indicated that u.v. light dechlorinates HMW AOX, (McKelvey and Dugal, 1975; Shimada, 1982, 1986; Panchepakesan et al., 1989) the efficacy of specific wavelengths of visible light in the absence of u.v. was unknown. As for HMW AOX photolysis by sunlight (Roy-Arcand and Archibald, 1993), the AOX photolyzed by the narrow bandwidth light was all mineralized to chloride ions.

4o 3O / G.

i,o.- ......... i

'°io.

350

\

I

.

400

\

,..

i

450 500 WAVELENGTH (nm}

550

600

Fig. 2. Wavelength dependence of hardwood HMW color bleaching and AOX photolysis. Legend; • AOX mineralization to CI-; • color loss (OD~5nm). Experiment employed high purity monochromator light (13 nm, bandpass) at 4.0-16.6% of the flux intensity produced by the Rayonet lamps. Incident light absorption by the tubes of effluent varied with wavelength as follows: 350 nm, 99.9%; 400 nm 99.8%; 450 nm, 97.3%; 500 nm, 87.0%; 550 nm, 74.0%; and 600 nm 59.9% at the commencement of the 72 h incubation at 20°C, neutral pH. The output of the spectrophotometer was not equal at all wavelengths. Therefore, the open symbols and broken lines are for the values corrected to a standard flux of 0.372 x 108 einsteins/ml/s. In contrast to the shorter wavelengths, the dechlorination efficiencies of 550 and 600 nm photons may be somewhat underemphasized, as not all incident photons were absorbed (see Fig. 2 legend). The higher fluxes determined at 500 to 600nm (Table 1) are probably not due to higher xenon lamp output or monochromator efficiency but to an error associated with the use of the ferrioxalate chemical actinometer. The literature values for the quantum efficiency constants for ferrioxalate at 350-500 nm were determined by a different group than those for the Actinochrome N actinometer used at 550 and 600 nm (see Materials and Methods). Thus, any error in the Table I. Light source photon fluxes (intensities)

x (nm) 350 400 450 500 550 600

Rayonet ~ (xlO s einstein/ml2/s)

Monochromator b ( x l O -s einstein/ml2/s)

Sun ¢ (xlO s einstein/cm2/s)

1.88

0.312 0.335 0.204 0.480 0.493 0.410

2.6 3.0 3.7 3.2 2.6 2.3

5.02

aRate of light absorption (photon flux) by the liquid in a 17-ml tube (100%o absorption) in the carrousel of the Rayonet light source equipped with 16 lamps. bRate of light absorption by the sample from a 13 nm wide beam centered on the mean wavelength from the PTI monochromator (slit set to +6.5 nm beam, focused on a 3 ml sample in a cuvette receiving 90% of light output), with 5.3 A xenon lamp current. ¢Sunlight incident (at 9 0 ) on a surface in a narrow bandwidth centered on the measured wavelength. Data obtained by high-altitude measurements (Thekaekara, 1970). Since the absorption of incident light by the effluent samples was > 9 7 % below 500 nm, and 87% at 550 nm the absorbed photons/ml in the round tubes (where the average effluent path length was about lore) can be compared directly to the incident solar radiation/cm-'.

Natural organochlorine photolysis

665

Table 2. Oxygen effects in 450 nm Rayonet lamp mediated photolysis~ AOX Absorbance decrease % Initial 02 mineralization CO2c (rag/l) pH b (%) 465 nm 280 nm (%) 0.107 _ 0.013 5.36 24.6_+1.7 4.0_+2.9 1.9-+ 3.6 0.65-+0.08 6.30+0.49 5.10 36.5 -+ 1.8 44.0 -+ 1.3 16.4_+ 2.3 3.08 -+0.14 30.0_+2.05 5.08 39.9+0.8 53.4_+ 1.1 23.1 _ 1.2 3.66_+0.31 ~Dialyzed, combined hardwood effluent (13 ml alquots in gas-tight tubes) incubated 48 h, 18"C in the Rayonet apparatus using 16 x450nm lamps (light flux incident on effluent= 5.02 × 10-8 einstein/ml/s). Initial HMW AOX, 465 nm color and OD2~0 were, respectively, 43.7 mg/I, 1510CU and 6.83 ODU. Mean incident light absorbed by effluentin the tubes was >99% at 280nm and 74.6% at 465 rim. blnitial pH was 5.47. CNumbers indicate percent of total carbon mineralized to CO2. To quantify the CO2 production, it was necessary to run parallel tubes and acidify them by injecting HCI before sampling their headspaces.

d e t e r m i n a t i o n of either q u a n t u m efficiency c o n s t a n t would show up as a spectral o u t p u t irregularity. M a x i m u m solar energy o u t p u t o n a clear day is at 450-550 nm, a n d the m a x i m u m transmission wavelength of light in clear water is 470-520 n m (Wetzel, 1975), so the ability of 450 n m blue light a n d n o t just u.v. to decolorize a n d mineralize o r g a n o c h l o r i n e is very significant in evaluating in vivo A O X p h o t o d e g r a d a t i o n . W i t h the PTI m o n o c h r o m a t o r spectrally pure light, while the relative abilities o f different nearly m o n o c h r o m a t i c wavelengths of light can be compared, any p h o t o d e g r a d a t i v e processes which required 2 or more wavelengths to proceed efficiently would be missed, if such processes exist. The results (Fig. 2) suggest that, especially with 450 nm m o n o c h r o m a t i c light, the m e c h a n i s m s of A O X mineralization to C1 a n d H M W c h r o m o p h o r e decolorization are very different, as their light response curves are so different. This m i g h t be due to the visible c h r o m o p h o r e s being relatively unchlorinated, c o m p a r e d to A O X - b e a r i n g species not a b s o r b i n g strongly in the visible light range.

(3) Oxygen effects on visible light (450nm) photolysis Dissolved oxygen has a m a j o r sensitizing effect on the susceptibility of H M W organics to visible light mediated decolorization a n d organic c a r b o n mineralization to CO2 (Tables 2 a n d 3). U n d e r N2, the moles o f CO: evolved substantially exceeded the moles of O~ consumed, suggesting t h a t m u c h of the oxygen in the CO2 originated in the target molecules or from water, with p h o t o d e c a r b o x y l a t i o n being a likely source (Getoff, 1991). In contrast, the dissolved 0 2 concent r a t i o n had a m u c h smaller effect o n H M W A O X

mineralization to C l - . Since it was only possible to remove 9 8 - 9 9 . 5 % of the dissolved 02 from the sealed tubes with the techniques employed, it was unclear h o w m u c h A O X mineralization would occur in the complete absence o f O2, but since the n u m b e r of C l ions released in the N2-flushed tubes greatly exceeded the 0 2 molecules c o n s u m e d (Table 3), the process was either i n d e p e n d e n t of O 2 or required 0 2 only in a catalytic or non-stoichiometric mechanism. Earlier work c o m p a r i n g the effects of higher ( ~ 8 m g O2/I) to lower ( ~ 2 m g O2/1 ) dissolved oxygen showed significant differences in light-mediated H M W color degradation, but not in A O X degradation ( R o y - A r c a n d a n d Archibald, 1993). This suggested that dissolved oxygen variations, c o m m o n in receiving waters, affect decolorization but not A O X mineralization ( R o y - A r c a n d a n d Archibald, 1993; C a r o n a n d Reeve, 1992). The present work, in which photolysis in very low dissolved oxygen was tried, confirms this difference between photodechlorin a t i o n a n d p h o t o b l e a c h i n g in their oxygen requirements (Tables 2 a n d 3). It is interesting to note that in 4 8 h , 3.1% o f the " s t a b l e " H M W c a r b o n was mineralized to CO 2 (appearing as a large peak on a gas partitioning c h r o m a t o g r a p h ) by 450 n m light c o m p a r a b l e in intensity to direct sunlight at the surface o f a receiving water body. This raises the question o f how m u c h of the " n a t u r a l " mineralization o f C O D and B O D to C O 2, normally attributed to microbial activity is in fact due to strictly abiotic photolysis. The m a r k e d increase in light-mediated 02 c o n s u m p t i o n seen in the presence of high 02 (these tubes were filter-sterile) also suggests that light may mediate considerable receiving water B O D and C O D

Table 3. Rayonet 450 nm photolysis sample tube stoichiometry~ Photons 02 Initial absorbed 02 consumed CO2 evolved CI evolved Treatment (#mol) (#mol) (#mol) (#mol) (,umol) N2-flushed 1.08 60 0.10 1.76 4.0 Air-flushed 64.7 < 60b 17.2 8.32 5.9 O2-flushed 308 < 50b 81.0 9.88 6.4 ~Data from the Table 2 experiments, with numbers in #moles per 17 ml photolysis tube (12 ml effluent each) after 48 h at 18C in the Rayonet with sixteen 450 nm lamps. bProgressive bleaching of the chromophores in the oxygen-containingtubes reduced the photon capture efficiency by an unknown amount. The amount shown (60 umol) assumes no bleaching.

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Table 4. Effects of artificial seawater on photolytic HMW AOX dechlorination 450 nm lighP

AOX medium

Final AOX (mg/I)

Water Water ASW ~ ASW

70.7 58.1 (17.8%) b 74.2 57.5 (22.5%) b

+ +

their liquid effluents that AOX photodegradation proceeds as well there as in fresh water.

(5) Physical factors affecting photodegradation

aFiltered, dialyzed concentrated HMW HW C + E stage effluent molecules were prepared as in Materials and Methods, then irradiated in standard 17ml tubes with 16 Rayonet 450rim lamps for 72 h at 20°C. Initial AOX was similar to the water-no light final AOX. bPercentage dechlorination by light. cASW (artificial sea water); to a portion of the effluent was added the following salts (per liter); NaCI 26.7g; KCI 0.725g; CaCI 2 1.108 g; MgCI2 2.35g; MgSO 4 3.69g; NaBr 0.083 g. This provides all the major ions found in typical sea water.

decrease independently of microbial activity and C O 2 release.

(4) Marine AOX photodegradation While the mechanism of photolytic dechlorination of H M W is not known, destabilization of the chlorine group on an AOX molecule by absorption of a photon's energy followed by displacement of the chloride by another anion from the aqueous milieu is plausible. If this mechanism is significant in AOX photodechlorination, then adding 500 mM C1- ions to the medium should decrease the net displacement of chlorine on organic molecules by other anions. Table 4 shows that the presence of all the major anions and cations in seawater at their typical concentrations (including 539/~M C i - ) has little effect on photodechlorination of the H M W effluent organochlorine. This also suggests for those kraft mills using brackish water or the ocean to receive

Data on the effects of effluent concentration and temperature, and the intensity and duration of the light sources used (the 350 and 450nm Rayonet lamps) on AOX mineralization and chromophore bleaching are presented in Table 5. It can be seen that the light with an emission spectrum centered on 350 nm is somewhat better than the 450 nm light for degrading AOX, especially as the 350 nm lamps have a lower flux of photons than the 450nm lamps (Table 1). On the other hand, 450 nm light is more effective in bleaching chromophores. These findings agree with those obtained with the narrow-band, lower intensity light from the PTI monochromator (Fig. 2). When light from a mixture of 350 and 450 nm Rayonet tubes was applied, the effects on AOX and color were intermediate, although there may have been a slight synergism on the degradation of 280 nm absorbing material (Table 5). Temperature had a significant effect on the rate of AOX photolysis and u.v. absorbance, but surprisingly little effect on chromophore bleaching. Thus winter water temperatures might significantly slow photolytic AOX mineralization, but may have less effect on H M W chromophore bleaching. In contrast, Caron and Reeve (1992) reported that a change from 10 to 25°C had little effect on AOX degradation by xenon lamp light. The much higher u.v. energy content of this light source compared to our sources might be responsible for this discrepancy, and if so, suggests somewhat different dominant mechanisms for visible

Table 5. Effect of certain variables on HMW effluent dechlorination and decolorization a Experimental conditions % Decreaseb Lampsc Variable studied Wavelength

Temperature Time Light flux Standard concentration d 5-fold dilution 3-fold concentration Standard concentration c 512-fold dilution

Irradiation

Number

Type (nm)

Time (h)

"C

AOX

Color (465 nm)

OD (280nm)

16 16 8+ 8 16 16 16 16 16 6 16

450 350 350 + 450 450 450 450 450 450 450 450

24 24 24 24 24 24 96 24 64 24

18 18 18 10 18 10 20 18 18 18

26.2 47.5 39.5 19.1 26.2 19. I 43.5 26.2 18.5 26.2

31.3 15.3 23.7 29.4 31.3 29.4 47.5 31.3 38.0 31.3

15.2 19.7 19.1 9.3 15.2 9.3 20.0 15.2 13.8 15.2

16 16

450 450

24 24

18 18

21.9 20.3

29.9 21.8

9.8 5.0

16

450

48

18

22.8

--

--

16

450

48

18

24.8

--

--

~Values are the averages of triplicate samples. Replicates were very close in all cases. bCompared to unirradiated hardwood HMW species. Hardwood C + E effluent was prepared as in Materials and Methods. Initial HMW AOX, color and OD280 were respectively 43.7 mg/I, 1510 CU and 6.83 ODU. ~Rayonet lamps. °Hardwood initial AOX, color (465 nm) and OD2~0 were for the effluent diluted 5-fold: 8.5 mg/I, 348 CU and 1.96 ODU, and for the effluent concentrated 3-fold: 112 rag/I, 3430 CU and 18.10DU. CSoftwood HMW C + E. Initial HMW AOX was 38.5 mg/I.

Natural organochlorine photolysis 5040-

10

0

2

4

;

8

1'0

1"2 l~g 16

TIME (dJ

2,000

t

tD

1, 0 0 0 ¸

:~ 4

6

;

1~) 1'2 14 16

TIME (d)

10-

applications of low intensity light, at least for AOX mineralization. In contrast, the lower rate of photon absorption by the HMW molecules gave slightly better decolorization. This suggests that the rates of AOX mineralization deep in waters where the light levels are low may be longer than calculated from measured fluxes and the rates reported here. On the other hand, the times employed herein are extremely short compared to the more than 60 years that kraft AOX has been entering North American rivers and lakes. Earlier work in which sterile HMW effluent molecules were exposed to indirect sunlight showed that AOX mineralization to C1- continued for at least 16 weeks (Roy-Arcand and Archibald, 1993). In the present work the "stable" fractions of hardwood and softwood HMW AOX molecules showed >50% dechlorination to C1- by visible light over 16 days (not weeks) at 18°C, with the process apparently continuing at a low rate beyond this period (Fig. 3). This trial employed the 450nm Rayonet lamps although the 350 nm (long u.v.) lamps were shown to be somewhat better at AOX mineralization (Fig. 2). Visible color was 60-65% removed over the 16-day period by the light from the 450 nm lamps. Both color and AOX were more readily removed from mixed hardwood than from softwood (spruce) by visible light. Caron and Reeve (1992) reported similar findings using unfiltered xenon lamp light on dialyzed bleachery effluents. They concluded that about half the HMW AOX was "photolyzable". (6) Photodegradation and H M W toxicity

8-

|e-

0

m

ci

667

4-

0 2-

TIME

Idl

Fig. 3. HMW AOX and color photolysis by 16 Rayonet 450nm lamps over 16d at 18°C on duplicate samples. Effluent prepared as in Materials and Methods. Legend: (---) softwood effluent; (--) hardwood effluent.

The complete hardwood effluent used in this study was tested for toxicity using the microbial Microtox assay. In this test toxicity is quantified as the concentration of the test effluent necessary to decrease light emission by the phosphorescent test bacteria by 50% (ECs0). The initial ECs0 values for the undialyzed effluent was 4.5% (v/v). However, after dialysis all toxicity values were greater than 50%, a level generally considered to reflect insignificant toxicity in this test. Thus, photolysis or the accompanying oxidative processes occuring under the Rayonet light did not create any toxicity. This contrasts with an earlier study in which there appeared to be a low level of toxicity created over 16 weeks by sunlight-mediated degradation (Roy-Arcand and Archibald, 1993). (7) Effects of H M W dissociation and dilution on H M W A O X dechlorination

and u.v. light photolysis. Extended photolysis with the Rayonet lamps produced increased degradation of AOX, color, and u.v. absorbing material, but the effect was not linear with time (Fig. 3). Results in Table 5 also indicate that when equal amounts of light are applied at higher and lower fluxes, the intensity (flux density) of the light is important. Shorter applications of high intensity 450nm light were more effective than longer

Recent evidence indicates that the so-called high molecular weight fraction of kraft bleachery effluents bearing the majority of C + E stage color and AOX consists not of covalently bonded macromolecules but of aggregates of small (< 500 D), relatively water insoluble species, readily separable by organic solvents and by high aqueous dilution (Jokela and Salkinoja-Satonen, 1992). Since high dilution always occurs in receiving waters but almost never before lab

668

FREDERICK ARCHIBALDand LINE RoY-ARCAND

analyses of mill bleachery effluents, this property is important but long overlooked. Earlier work showed that even in the absence of either dilution or light there is, over time, significant abiotic fragmentation of the H M W material (Roy-Arcand and Archibald, 1993). Determining whether the dilution- and time-dependent dissociation of the H M W material into much smaller species has an effect on the photolysis of its AOX required AOX measurements in the ppb range. This was achieved by adding the AOX analysis reagents (sodium nitrate, nitric acid, and 1/3 the normal level of activated charcoal) directly to 3 x 100 ml of the undiluted sample, then passing all 3 through a single filter. This resulted in a maximum sensitivity of about 30 ppb (for a 10 mC coulometer response) on the AOX analyzer. Time-dependent H M W breakdown was evaluated using two aliquots of softwood HMW, prepared as usual. These were dialyzed for 6 days against water (10 ml effluent against 200ml water) at 6°C in the dark. One bag was left in its original dialysis water, and the dialysis water of the other changed daily. Daily sampling from each bag indicated similar rates of size decrease, whether the released L M W material was regularly removed by changing the dialysis water, or allowed to accumulate (data not shown). This suggested that degradation of the H M W material bearing the majority of the C + E stage color and AOX was at least partly dependent on timedependent changes, probably oxidative. To see if photolytic dechlorination and decolorization were comparably effective on more dilute and more concentrated H M W material, two experiments were run: (1) 5-fold diluted and 3-fold concentrated hardwood H M W effluents (filtered, mixed, and dialyzed as in Materials and Methods) were compared under the Rayonet (24 h, 18°C, 16 x 450 nm lamps) and (2) 512-fold diluted softwood H M W effluent was exposed in a 21 beaker in the Rayonet cylinder and compared to 10 ml of the same (undiluted) material in the usual 17-ml tubes (48h, 18°C, 16 x 4 5 0 n m lamps). Neither the small dilutions and concentrations of the hardwood material (Table 5) nor the large dilution of the H M W softwood effluent (which should, according to Jokela and Salkinoja-Salonen (1992) produce a marked diminution in the apparent MW of the material) had a large effect on the effluents' sensitivity to light. After 48 h of blue light (450 nm) photolysis at 18°C, mineralization of the undiluted and of the 512-fold diluted H M W AOX was very similar (Table 5). Precise comparison of the stoichiometry or efficiencies of the dilute and concentrated softwood H M W dechlorination reactions was not possible because of the slightly different amounts of light absorbed per unit effluent by the 2 1 beaker and 17 ml tubes, but it is clear that a dilution great enough to substantially decrease the average size of the H M W material did not markedly change the photodegradation of H M W AOX by visible light.

CONCLUSIONS

"Stable" apparent high molecular weight (HMW) fractions of combined C + E stage bleachery effluents were prepared by pH adjustment, microfiltration, and repeated dialyses against distilled water over three days. In earlier work this (HMW) material was shown to be virtually unavailable to a microbial consortium from a mill bleachery effluent settling pond over 16 weeks (Roy-Arcand and Archibald, 1993). This refractory material probably represents the 30-50% of H M W AOX commonly reported to emerge unscathed from aerobic and anaerobic secondary treatment systems applied to kraft mill bleachery effluents. Irradiating this material with defined visible and long u.v. light sources has led to the following conclusions. 1. H M W chromophore photobleaching by visible light is highly dependent on the presence of dissolved oxygen, while AOX mineralization by visible light is much less so. 2. Both longwave u.v. and blue-violet visible light are effective in photobleaching and AOX mineralization. Long-wave u.v. is more effective for AOX removal and visible light for chromophore bleaching. 3. Significant non-biological light-dependent BOD and COD mineralization to CO2 and more oxidized organic molecules probably occurs in receiving waters. 4. It is unclear whether all the refractory H M W AOX can be converted to chloride ions by visible light, but over half is in 1-16 days, at light intensities similar to natural sunlight at the surface of receiving waters. 5. AOX photodegradation occurs at similar rates in marine and fresh water. 6. H M W AOX photodegradation is relatively independent of AOX concentration and the large decrease in apparent size of the H M W material accompanying high dilution in receiving waters. 7. The hardwood H M W material was non-toxic by the Microtox assay, both before and after blue light photolysis and AOX mineralization.

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