Photochemical degradation of dissolved organic substances in freshwater environment

Photochemical degradation of dissolved organic substances in freshwater environment

Abstract: The carbon cycle in water systems is largely influenced by the photochemical degradation of dissolved organic substances. The aim of this study was to measure the rate of the photochemical degradation of DOC in Lake Balaton. Water samples, collected in June 2007 in the mouth of River Zala and in different basins of the lake (Keszthelyi, Szigligeti, Szemesi and Sifoki -at Tihany and at Balatonfzf- basins), were irradiated with the average radiation (UV-B, UV-A and PAR) of a sunny summer day in a sun simulator for 28 days. Samples were taken on day 3, day 7, day 14, day 21, and day 28. Light absorbance (Pt-colour was determined) and fluorescence intensity were measured. The concentration of dissolved organic carbon (DOC) as well as the quantity, degradation rate and half-life of photolitically degradable DOC was measured. The proportion of the DOC contributable to humic substances was measured both at the beginning and at the end of the experiment. The photolysis ran its curse in 21 days. The concentration of photolitically degradable DOC was 3.4 mg l^-1 (24%) at the mouth of River Zala, which was an order of magnitude lower (0.2 mg l^-1 1,6 %) in the Sifok basin. The interval of the degradation coefficient ranged between 0.2 and 0.01 day^-1, which equal 3 and 59 days of half-life, respectively. The brown-coloured Zala-water discoloured to the Pt-colour value of the Keszthely basin, while the fluorescent spectra showed that the Zala water turned into Sifok water as a result of photolysis. Our results suggest that not only the DOC concentration decreases but the quality of organic substances is also transformed during photolysis, whereby they become more resistant to light degradation during residence in the lake.

Key words: Lake Balaton, DOC, photolysis, degradation coefficient, half-life

Introduction

In water systems, so in Lake Balaton to, the amount of organic substances is one of the major characteristic of the water quality. In lakes the dissolved organic carbon (DOC) arises from two well determined sources, from the primary production in the lake (allochton), and from the catchment area. The DOC affects a large scale of physical, chemical and biological processes. The light absorption of dissolved organic carbon strongly influence the penetration depth of photosynthetically active radiation (PAR) and ultraviolet radiation (Morris et al. 1995), they vigorously absorb the short wavelength light so they change the underwater light clime (Kirk, 1976; Bricaud et al. 1981), also have influence on the temperature (heat household???)of the lake (Snucins & Gunn, 2000) The DOC is interfering with the dissolved nutrition and metals hereby can change his concentrations and biological availability (Perdue 1998, Shaw et al. 2000). The DOC is composed mostly of acid compounds, therefore they can affect the pH of the lake and in lakes with low pH can act like buffers (Driscoll et al., 1994).

In Lake Balaton the major part of the allochtons comes with the Zala River (V.-Balogh et al., 2007), were the part of the humic substances is large up to 75% (V.-Balogh et al., 2003), these substances decompose by adsorption, microbial and photochemical degradation. In course of the photochemical degradation that molecules decay which ones can absorb the UV radiation, in consequence of this the penetration depth of the light in the water is increasing. This progression is more accentuated in the UV-B (280-320 nm) region, which wavelength which wavelength region is the most harmful for the biological systems (Karentz et al. 1994). With tapering of the ozone layer the rate of UV radiation increasing therefore the harmful effects is increasing to. The fotolyzis increase the biological availability of the substances (Strome & Miller, 1978), also generate reactive oxygen radicals (O₂^-, H₂O₂), whichs directly damaging the biological membranes- or indirectly by modifying the redox potential of the metals- (Cooper et al., 1989) influence the biological systems.

From the above follows that the photochemical degradation of the dissolved organic substances can modify remarkably the carbon cycle of the waters. The aim of this study was to measure the rate of the photochemical degradation of DOC in Lake Balaton

Methods

Water samples, collected in June 2007 in the mouth of River Zala and in different basins of the lake (Keszthelyi, Szigligeti, Szemesi and Sifoki -at Tihany and at Balatonfzf- basins), were used for the photochemical degradation study. The water samples were filtered (precombusted at 450 C, GF-5 glass filters, 0.45 m pore size) and 280ml from these were placed in clean quartz tubes (height= 35 cm, interior diameter=3,5 cm) , three same sample from each location. The water samples were exposed to artificial PAR and UV (UV-A, UV-B) radiation in a Sun simulator for 28 days, the dark treatment was wrapped in aluminum foil. On the star of the studies the oxygen concentration of the samples were set at 100%. The intensity of the irradiation in the Sun simulator was: UV-B: 0,291 mW cm^-2, UV-A: 0,403 mW cm^-2, PAR: 695 mmol m^-2 sec^-1, this is equal with the average radiation (UV-B, UV-A and PAR) of a sunny summer day. To measure the intensity of the PAR and UV radiation were used LI-COR respectively VLX 3W type radiometer. The presents of the microbes was checked with Nikon Optoiphot epifluorescent microscope. From the quartz tubes samples were taken on the beginning of the study, the day 3, day 7, day 14, day 21, and day 28. The DOC concentration was measured with Elementar High TOC organic carbon analyzer, also we determined the concentration of the photochemical degradable dissolved organic carbon (FDOC), the dissolution speed and the half time period of DOC, whit the following formulas: FDOC=DOC₀-DOC_t, where DOC₀= DOC concentration at the beginning of the study, DOC_t= DOC concentration after t incubation period, dissolution speed of FDOC: FDOC_t=FDOC₀e^kt, where FDOC_t = concentration of FDOC t incubation period, FDOC₀ = the total concentration of FDOC after the treatment, k= decomposition coefficient, t = time in days. FDOC half time period = ln2 k^-1. The absorbance of samples determined using Simadzu UV 160 A spectrophotometer, we calculated the color intensity (Pt-units) of humic substances using the absorbance at 440nm (Cuthbert & del Giorgio, 1992)

Results and discussion

At the start of the study the DOC concentration in the water samples was among 8,7 mg l^-1 (Sifoki-basin) and 14,4 mg l^-1 (mouth of River Zala), in the dark controls the DOC concentration doesnt change. The DOC concentration decreased exponentially to the 21. day of the study and after that did not change. The amount of the photochemicaly degradable DOC differed significantly in the different sites, only 0,2 mg l^-1 was in the Sifoki-basin, more than sixfold (1,2 -1,4 mg l^-1) was decaying on the middle and western area of the lake, and with an order of magnitude bigger quantity , 3,4 mg l^-1 was decaying in the mouth of River Zala (1. table). This DOC quantity 1.6 % was in the Sifoki-basin and 24 % in the Zala estuary. If we demonstrate the decay rate by the half-life period, then considerable difference can be observed from the different sites, the half-life period was the smallest in the Keszthelyi-basin and in the Zala estuary, which one increases gradually and significantly as we recede from the incoming area. The half-life period was more than 17 times bigger in the Sifoki-basin, than at the incoming of the lake (We mention that the half-life period is far underestimated under continuous irradiance). Due to irradiance the DOC decays most quickly in the Zala River and the Keszthelyi-basin, the slowest decaying was detected in the Sifoki-basin.

In the experimental samples the bacteria did not proliferate, so the microbial decomposition did not influence the experimental results. The number of the bacteria was increasing slightly in the in dark control samples, but this did not cause demonstrable decrease in the DOC concentration.

Table 1. The amount and decomposition speed of photochemical degradable dissolved organic carbon from the mouth of River Zala and in different basins of the lake

Due to the irradiation the composition of the dissolved organic matters changed to. In the samples from the mouth of River Zala the concentrations of the humic acids (HA) decreased with 58%, the concentration of fulvic acids (FA) decreased with 31%, while the concentrations of non humic substances (NHS) increased with 1.6%. On the opposite pole of the lake, at Balatonfzf this change was less demonstrable, the concentration of the HA decreased with 10 %, the concentration of the FA decreased with 8 % , while the proportion of the NHS increased with 6,5 %.

The qualitative differences along the longitudinal axis of the lake were observable well on the beginning of the experiment already. The characteristics of the water samples differed drastically on the beginning of the experiment already. The water samples differed drastically on the beginning of the experiment in their absorbance (figure 1.), the absorbance of the Zalas water was multiple of the Sifoki-basins. The absorption of the water samples decreased continuously to the 21. day due to the irradiation, after the 21.day did not change significantly. The decreasing measure of the absorbance in the Sifoki-basin was 60%, in the Zala water this was 60-80%.

1. figure

The changes in the absorbance of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Sifoki-basin at Balatonfzf (B)

The brown colored water from the mouth of River Zala decreased from the value 89.57 mg Pt l^-1 to 14 mg Pt l^-1. This is below the value 16.57 mg Pt l^-1 which belongs to the water collected from Keszthely. There for, regarding the color of the water, the brown colored water from mouth of River Zala became water from Keszthely.

2. figure

The changing in the fluorescent spectra of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Sifoki-basin at Balatonfzf (B)

Conclusion

Our results suggest that not only the DOC concentration decreases but the quality of organic substances is also transformed during photolysis, whereby they become more resistant to light degradation during residence in the lake. The quantity of the fluorophores and chromophores decreased, while the transparency of the water increased.

Acknowledgement

This study was made possible trough financial support of NKFP 3B022_04 BALKO and OTKA K 63296.

sszefoglal

Az oldott szervesanyagok fotokmiai bomlsa jelents mrtkben befolysolja vzi rendszerekben a sznforgalmat, ezrt tztk clul, hogy meghatrozzuk a DOC fotokmiai bomlsnak sebessgt a Balatonban. A Zala torkolatban s a Balaton klnbz medenciben (Keszthelyi-medence, Szigligeti-medence, Szemesi-medence, s a Sifoki-medencben, Tihanynl s Balatonfzfnl) 2007 jliusban vett vzmintkat Nap-szimultorban egy tlagos nyri nap sugrzsnak (UV-B, UV-A s PAR) tettk ki 28 napon t. A 7., 14., 21. s 28. napon vettnk mintt. Mrtk a fnyabszorbancit (meghatroztuk a Pt-szn) s a fluoreszcencia intenzitst. Mrtk az oldott szerves szn (DOC) koncentrcit, meghatroztuk a fotolitikusan bonthat oldott szerves szn mennyisgt, bomlsi sebessgt s felezsi idejt. A ksrlet elejn s vgn meghatroztuk a huminanyagoknak tulajdonthat DOC rszesedst. A fotolzis 21 nap alatt lezajlott. A fotolitikusan bonthat DOC koncentrci 0,2 mg l^-1 (1,6%) volt a Sifoki-medencben, ennl nagysgrenddel tbb (3,4 mg l^-1 - 24%) bomlott a Zala foly torkolatban. A bomlsi koefficiens intervallum 0,2 s 0,01 nap^-1 kz esett, elbbi 3 nap, utbbi 59 nap felezsi idnek felel meg folyamatos besugrzs mellett. A barna Zalavz a fotolzis kvetkeztben a Keszthelyi Pt-szn rtkre fakult, mg a fluoreszcens spektrumok alapjn a Zalavizbl gymond Sifoki-vz lett. Eredmnyeink rmutatnak, hogy a fotolzis sorn nemcsak a DOC koncentrci cskken, hanem a szervesanyagok minsge is talakul, melynek eredmnyeknt perzisztensebb vlnak a fnybontssal szemben a tban val tartzkods sorn.

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1. figure

The changes in the absorbance of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Sifoki-basin at Balatonfzf (B)

2. figure

The changing in the fluorescent spectra of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Sifoki-basin at Balatonfzf (B)