- Email: [email protected]
Mineralization of organic matter in water by u.v. radiation
Water Res. Vol. 17, No. 4, pp. 355-364, 1983 Printed in Great Britain. All rights reserved
0043-1354/83/040355-10503.00/0 Copyright © 1983 Pergamon Press Ltd
M I N E R A L I Z A T I O N OF O R G A N I C MATTER IN WATER BY U.V. R A D I A T I O N P. BLA~KA and L. PROCH~,ZKOV~ Hydrobiological Laboratory, Institute of Botany*, Czechoslovak Academy of Sciences, Vltavsk/t 17, 151 05 Praha 5, Czechoslovakia (Received May 1979)
Abstract--The conditions for the optimum yield of carbon dioxide in the mineralization of organic matter in water were tested and the relative optimum procedure was outlined. Of all substances tested the greatest difficulties were encountered with the purine bases. Their quantitative mineralization was achieved only by the highest irradiation intensities and/or prolonged irradiation. An evident relationship of mineralization and pH was found in urea and thiourea. Adsorbtion of proteins on silica and glass apparatus was suppressed by phosphoric acid, but a correction of the amount of organic carbon recovered in the blank still had to be applied in protein rich samples.
INTRODUCTION Irradiation of water samples by short wave-length u.v. became one of the generally used mineralization procedures after the papers by Beattie et al. (1961) and Armstrong et al. (1966). The aim of this paper is to review and check some basic factors involved in mineralization of dilute solutions of organic compounds and their mixtures and test the efficiency of mineralization by u.v. radiation and compare it with other methods. CHEMICAL AND PHYSICAL BACKGROUND Oxidation by u.v.-radiation is considered to be caused by "OH radicals which are split off from water molecules by short wave u.v. radiation (Beattie et al., 1961). It is therefore likely that the efficiency of u.v. mineralization depends upon the intensity and wavelength of irradiation. Irradiation is directly proportional to the source intensity and indirectly to the distance between the source and the sample. For a short distance from a linear source a relationship close to the first power of the distance is applicable while at longer distances (multiples of lamps length) the relationship approaches a second power function of that distance. Longer arcs yield less irradiation per cm 2 than shorter ones at the identical lamp output (Fig. 1). In constructing a photochemical unit shorter arcs and the shortest possible distance between the source and the sample should therefore be prefered. The pressure of mercury vapours within the lamp is proportional to the temperature and determines the spectral characteristics and total output of the lamp. It may be influced by varying the voltage applied on the lamp or by the cooling rate. *Now Institute of Landscape Ecology. 355 W.R. 17/4---A
In practice this means that the lamp should be run at the optimum current chracteristics suggested by the producer and the optimum cooling rate which is found empirically. A fan is most frequently applied to cool the samples. The lamp is protected by a silica tube which prevents excessive cooling of the lamp, but allows excess of heat to escape. In general most of the published descriptions of apparatus are poorly defined from an optical point of view and any measurement and calculation becomes very complex. Only Armstrong et al. (1966} and Armstrong & Tibbits (1968) have measured the output of their units between 200 and 400 nm using uranium oxalate actinometry. In Table 1, physical data of several units are summarized and their irradiation is estimated from the data of the authors and of the manufacturers of the tubes. The assumption was made that all energy within the range 200-300 nm is radiated between the electrodes uniformly in all directions normal to the axis of the respective tube. It is realized that this is an oversimplification and the calculations are given just to suggest the probable range of irradiation in the experiments. METHODS
Description of the apparatus
The efficiency of u.v. mineralization was tested on an automated apparatus, the outline of which is shown in Fig. 2. Acidified samples of 4 ml total volume in conical quartz test tubes were prepared in a sampler (1) and were then pumped by a peristaltic pump (3) into the upper wash bottle (4). The tubing was Tygon-Acidfex and glass capilaries approx. 0.5 mm i.d. In the upper wash bottle the sample was purged by oxygen and inorganic carbon dioxide was carried through the drying column (I1) filled with magnesium sulphate dried at 180°C, a flow meter (13) and a valve to the i.r. analyzer IREX (Chemick6 z~ivody, Zfilu~i, Czechoslovakia) (14) and released to the air. A peak broadener (12) was put between the drying column and the
:'15~
P 131 ',2K;\ and l
Pt{o( llA/k~ i~. t example, tile Hanovia Engelhardl IgOA I2(1(1,,' ~ith ,m arc length of 30.5 cm iFig. I1. Oxygen was purilied b~ passing througtl a colunm iith:d
with a mixture of finely ground sodium hydroxide, ~duminium oxide (Brockmann) and phenolphthalein (I:I:IL01 weight ratioi(171 and carried through the reference tuhc of the i.r. analyzer. The electrical signal of the i.r. anal,~zcr was recorded il 5) and the peaks integrated (16)(Integrator IT 1. Laboratorni pHstroje, Czechoslovakia). The \~hole apparatus was controlled by 8 electro-mechanical timcretavs connected in series, however a manual modification of the cycle was possible at each step.
L 200
250
300
nm
Fig. 1. The relative emission spectra of the Hanovia-Engelhardt 189 A lamp (A), and the USSR--Svetotechnika Saransk PRK-7 mercury lamp (B). Both spectra were recorded at identical geometry and sensitivity. The emission was recorded by the Beckman DB-G spectrophotometer equipped with a R-136 photomultiplier.
flow meter to ensure enough time for a complete response of the slow i,r. analyzer. This took the form of a vessel in the shape of two cones connected by their bases filled by glass beads 1 m m in diameter. In the next step the sample was brought by oxygen pressure through the open valve (MT 1) into the quartz capillary coil (7), the preceding sample being forced at the same time into the lower wash bottle (8). The samples were separated by a bubble of oxygen. The capillary was coiled with a diameter of 5 cm around a medium-pressure PRK-7 (USSR) mercury u.v. tube (5) and the sample stayed there for 14min. In the lower wash bottle the sample was purged with oxygen once more and the carbon dioxide which resulted from oxidized organic c o m p o u n d s was brought by stream of oxygen to the i,r. analyzer. The magnetic 2-way valve (SV) set the gas flow alternatively through the upper and the lower wash bottle. The PRK-7, a medium pressure mercury discharge lamp produced by Svetotechnika, Saransk, USSR had an input of 1000W and arc length 19cm and tube diameter o f about 2.5 cm. It produced a stronger radiation per unit area at all lines within the 2 0 0 - 3 0 0 n m rahge than, for
Blauks Distilled water was prepared from tap water in a glass still with stainless-steel electrodes as heating unit. It was then redistilled in a whole glass still alter it had been adjusted to approx. 10 '*M with potassium permaganate and 10 -2 M with sulphuric acid. It was prepared fresh not more than 2 days before each series of determinations. All glass was washed with freshly prepared warm mixture of concentrated sulphuric acid and 30°,o solution of hydrogen peroxide 1 : 1 (v/v) and several times with redistilled water. Acids solutions were irradiated for 45 min by the PRK-7 mercury lamp in a silica flask from the distance of 5 cm. The silica test tubes were treated with a wet tube brush, rinsed several times with redistilled water, filled up by water again and irradiated on a turn-table by the same type of u.v. tube in horizontal position for 45 min. The top of the 8 cm long test tubes rotated approx. 3 cm below the lamp. During washing the tubes were not touched by fingers (except during brush cleaning) and water was sucked off. After irradiation water was replaced by the acidified sample. Several blanks, usually four, were run at the beginning of each series of determinations to clean apparatus and it was washed by three sample volumes of redistilled water at the end again. The main source of elevated blanks was laboratory air and a considerable improvement was achieved by placing a cover over the sampler and flushing it with spent oxygen from the analyser. For longer periods polymethacrylate covers were not adequate, they produced high concentrations (up to 10 mg C 1- ~) if blanks are standing covered by them for a week or two. Glass seemed to be the best material for these covers. The second source of high blanks is the apparatus, particularly the flexible tubing. Acidiflex (Technicon) was found to be superior to Tygon (Technicon) and silicone rubber (Kablo Czechoslovakia). Routine values of blanks were about 2 #g of organic carbon per sample (approx. 0.5 mg C 1-~) and the best values obtained were 0.3-O.4,ug C per sample (approx. 0.1 mg C 1-~). High blanks limited useful sensitivity of the determination; detection sensitivity could be easily increased at least by one order of magnitude (Collins & Williams, 1977). Standards A solution of benzoic acid 200,ug C m l - J was used to calibrate the instrument at the beginning and the end of each experimental series; 0.1 ml was added by a Hamilton ® microsyringe. The calibration was linear up to 40 jug C per sample as benzoic acid and passed through the origin when relevant blanks were subtracted from the readings. This was true also for a calibration line done at 5-times increased sensitivity, the linear range in this case was up to 6,ug C per sample. Precision The coefficient of variation o f blanks was 7.6'~'%; for samples at a concentration of 3 mg C 1- ~ it was 5.3°~i and
357
Mineralization of organic matter in water by u.v. radiation ......................................................................................................
I i I !
!
I I
: ................. r"" ; : • I
I I I I i
l----.JI I I ! !
•
•- - ' -
!
~
r.--n18 .,&-~-u--1
!!"
,
e
l]
I
II
' y
L
.
D°
J
T i" ~'~.~....-
]
_
L.
116
D
'v'
MT4 i~ ..... : - . : : . -
. . . . . . . . .
i
.: .-:,~
[
:
..........
- - ~ - ............................ i l ~ l ~
~
JtF :
:
;
,1~1
:
~
?.j:-'.'.'.~:'::...~:--....~..!.:.!.':::~...L...?
,
~
~ ?
Fig. 2. The scheme of the apparatus: l--sampler; 2--sampling capillary with a solenoid; 3--peristaltic pump; 4--upper wash bottle; 5--PRK-7-mercury lamp; 6~protecting quartz tube; 7--quartz capillary coil; 8-lower wash bottle; 9-10 mercury valves; 1l-drying column (MgSO,); 12-peak broadener; 13--flow meter; 14--i.r. analyzer; 15--recorder; 16--integrator; 17--NaOH column; 18--flow regulator; 19--oxygen tank; MT 1, 2, 3, 4,--magnetic valves; SV--two-way magnetic valve. TR 1--8 electromechanical time relays, arrows on their lower side suggest their interconnections. Solid lines indicate the sample route, dashed lines the flow of oxygen, and dotted lines electrical connections. Table 1. Physical data of various irradiation setups
Reference
u.v. Source
d
Irradiation period
Armstrong
Hanovia8.2 2-3 h Engelhardt (1966) 189 A, 1200W Armstrong Hanovia-Engelhardt 4.4 11-16 h & Tibbits (1968) 507/7240W 509/10 380 W Erhardt (1969) Hanau Q 1200 5.1 15 min 900W Soier & Svetotechnika 3.1 48 min Semenov Saransk, USSR (1971) PRK-7,1000 W Baker et Hanovia 5.0 15-23 min al. (1974) 500 W Goulden & Hanovia-Engelhardt 6.1 4-8 min Brooksbank (1975) 189A, 1200W Collins & Hanovia 69326X 6.0 45 min Williams 1000 W (1977) Stainton Westinghouse 3.7 10 min (1977) 700W Gershey et Hanovia 6932X 6.0 20 min 1000W al. (1979) Svetotechnika 2.6 14 min This paper Saransk, USSR PRK-7, 1000W
Intensityt {mW cm -2) 112
et al.
* Distance between source and sample centers (cm). t Estimated by the present authors.
30
61 375
Detection of CO2 Calculated from pH Calculated from 02 uptake Conductometric Coulometric titration
102
i.r.
130
i.r.
110
i.r.
89
Conductometric
110
i.r.
448
i.r.
P. BI xZK,'~ mid l.. t)RO( tIAzKI'P,,A
35S
at a concentration of 10 nag C 1 ~ it was 0.76"o t.\ = 10 for all groups),
Drying Several materials were tested in the drying column. Phosphorus pentoxide was of little use as it very quickly turned to a thick paste and blocked the flow. Silica gel caused a reversible adsorption of carbon dioxide which was related to the hydration of silica gel. Dried magnesium sulphate trapped water efficiently, carbon dioxide was not retained and passed through the column in a sharp peak which the analyzer could not follow, The response was improved by the peak broadener (12),
Procedure Unless otherwise indicated samples were acidified by 1 ml 10% phosphoric acid and the volume adjusted with "'carbon free" water to 4 ml. In a series of experiments, samples always alternated with blanks, unless they comprised a continuous series, like fractions of a column effluent. In such a case the samples followed one after the other. In discontinuous series (with blanks) all samples were determined in duplicates. If the intermediate blank (between samples) was higher than a clean blank (preceded by a blank) by more than 10% of the previous sample value, a correction was applied by adding the value in excess of the 10?i; to the previous sample.
RESULTS
Easily mineralized substances Organic compounds recovered quantitatively (100 _+ 2% of theory) included: glucose, benzoic, tartaric, oxalic, glycolic, citric, adipic, nicotinic acids, nicotinamide, sodium acetate, ammonium acetate, potassium-hydrogen phthalate, phthalic anhydride, glycine, proline, lysine, valine, norvaline, histidine, tyrosine, tryptophan, methionine, cystine, pyridine, starch, phenol, sulphanilamide, asparagine, aspartic acid, l-naptylamine, semicarbazide, monopyrazolone, polyvinylalcohol, casein-hydrolysate and ferricyanide.
Influence of pH Carbonates and carbon dioxide interfere with the determination of organic carbon and samples are to be acidified in the first step to destroy carbonates. In most methods irradiation proceeds at low pH, only Armstrong et al. (1966), who determined carbon indirectly, irradiate alkaline samples. Collins & Williams (1977) started photo-oxidation at pH > 6, but the sample became more acidic as persulphate was decomposed. They reported minimal rate of photodecomposition of an organic '4C preparation from aged algae at pH 4, the rate increased to pH 8, but slightly also to pH 1.5 where it was about half the rate at pH 8. On the other hand they got higher blanks from the two extra reagents (tetraborate, HC1). Gershey et al. (1979) in their comparison of the three methods irradiated at a pH lower than 4 which did not change much during the photodecomposition step as persulphate was also omitted from the original Collins & Williams (1979) procedure.
Soier & Semenov (private communicatiom adjusted the pH of their samples to 2 by sulphuric acid and this was the procedure we have followed, but ornittmg the Hg 2 ~ catalysis. It was satisfactory for most compounds tested except those indicated in Table 2. The reaction rate with "OH radicals is likely to var 3 with the type of compound, but for most tested a 14 min irradiation period sufficied for complete recovery. Some of the reaction rates showed a relationship with pH due to tautomerixation and protonization equilibria {e.g. urea and thiourea), but others with slightly higher basic dissociation constants (within the range 1.05.10- 12-8.4" 10- 11 for arginine, semicarbazide, sulphanilamide, asparagine, 1-naphtylamine) produced only a very slight relationship with pH (Fig. 3). The yield of CO2 was, however, related not only to the reactivity of the compound but also of all the intermediates. These were not known and therefore it was not easy to suggest a simple interpretation of the relationship of the yield to pH. The low yield of thiourea as compared with urea may be interpreted both by a higher oxygen consumption for oxidation of the former and by the interaction of sulffiydryl group with "OH radicals. Cysteine was recovered with a high efficiency and this might suggest that the former interpretation is more likely (see also Figs 5 and %
Adsorption q] proteins With serum albumin and casein the greater part of the unrecovered carbon was found in samples (blanks) following the protein sample (Table 3). The finished protein samples gave low blank values when returned to the cleaned apparatus. Change of pH, prolonged irradiation, or catalysts have not improved the
Table 2. Organic compounds with a lowered carbon recovery at the following conditions: Acidification with H2SO4, pH 2, 14 min mineralization, temperature of lamp surface 360 and 500°C. The carbon content of all these substances was tested by organic elementary analyses by the method of Ve~e~a (1967). 20/~g C per sample. The volatile and nonpolar substances are not included
Substance Urea Arginine Creatine Cysteine Thiourea Uric acid Guanine Adenine Albumin Casein
Recovery C% Temperature of the lamp surface (°C) 360 500 75 95 57 50 54 --
95 100 92 90 75 77 86 87 52* 65*
*The values without correction on the amount of proteins washed out in the following blank.
Mineralization of organic matter in water by u.v. radiation
,, ioo-
--~ . . . . . .
.
x .... ~ . . . . ~ ~ .
.......
>~
359
2:.-,.
........
.,,.,,.,,,, -~
50--
x-\.\.
\.
.a
\
A~x
o
x
\.\
I
I
1
I
I
2
3
4
pH Fig. 3. The relationship between percentage carbon recovery and pH. 20/~g C per sample, lamp surface temperature approx. 500°C. urea (x . . . . . x j; thiourea (A A}; uric acid (+ . . . . +}; sulphanilamide ( ~ - - - - t ) ; semicarbazide (O . . . . . O); arginine (&. . . . . A); l-naphtylamine (D---r~).
delayed recovery. Serum albumin and casein, but not casein hydrolysate, were evidently retained within the apparatus prior to the lower wash bottle. No apparent increase in the residual volume was observed in the sampler, the pumping capillary and the first stripper. Therefore the adsorption on glass and silica surfaces seems to be most likely--see also Cassidy (1957) and Deyl et al. (1975). The adsorption of proteins was also confirmed by passing serum albumin solutions through columns of crushed borosilicate glass and silica (Fig. 4). Several compounds were tested for depressing this adsorption on silica and glass. Only phosphoric acid was found to be useful (Table 3), though even this compound did not supress the adsorption of protein completely. Boric acid was without any effect, NaF and KHSO4 increased the adsorption of protein. Irradiation of blank samples after a protein sample was necessary to recover the rest of the carbon as carbon dioxide. Three minutes irradiation was sufficient for this purpose. Many of the protein analyses performed gave poor reproducibility. Mixing of samples in the test tubes during the addition of protein solutions (e.g. by a stream of air) was found to improve the reproducibility of results.
Oxidizing agents Various oxidizing agents were used in combination with air or oxygen as carrier gases (Table 4). Absorption spectra of oxidants (Fig. 6) suggested, however, that they absorbed some short wave-length u.v. which was probably the part most active in mineralization (Beattie et al., 1961). These oxidants are destroyed by u.v. and the absorption of u.v. above 200 nm ceases. Only then was water split into radicals at the full rate corresponding to the irradiation. Addition of peroxide or persulphate thus reduced the effective time available for mineralization and in compounds with a slow mineralization rate they may have decreased the yield. A positive effect of oxidants may be produced in samples with a high sulphur and nitrogen content, where oxygen may become a limiting factor. Potassium persulphate is converted by u.v. irradiation into potassium hydrogen sulphate. This increased the adsorption of proteins on silica and glass (see previous section).
T
/" i/'
I0 >,
/
Acidification
Recovery C~o Sample 1. Blank 2. Blank Total
c o
4
52 84
25 10
4 4
81 98
The values are means of six parallel determinations, the solutions were thoroughly mixed by bubbling with air after addition of albumin solutions, approx. 20min before mineralization (30 pg C per sample).
X
/I
,,"
2
o ~
/
H2SO4, pH 2.0 H3PO4, pH 1.3
/
i/
Table 3. Adsorption of albumin within the apparatus and its elution in the following blanks
/
X X,,'~*'* I0
Albumin
I
I 20
I
I 30
applied, /zg ml -~
Fig. 4. The absorption of serum albumin on a column (1.5 x l0 cm} of crushed silica (x) and borosilicate glass (O). Dashed line theoretical recovery without any adsorption.
P. Bt,A:;'KAand k. PRO( II.,'~ZK()V~
360
,ool
o ii f
+
+
i
08
0.6
o
5o
g
// i i i / /// //
\
0.4
\\
//
I 15
I 30
I
I 45
Time,
0.2
60
rain
0
Fig. 5. The relation between c/arbon recovery and time of mineralization. Cystine (O); cysteine (@); thiourea (Ah uric acid (+); 20 #g per each sample.
200
250
300
nm Fig. 6. Absorption spectra of hydrogen peroxide (..... ) 0.05% and potassium persulphate ( ~ ) , 0.3°4 solutions.
In our experiments the addition of persulphate improved mineralization of thiourea and cysteine, but decreased the yield of carbon dioxide from uric acid and serum albumin (Fig. 71. Addition of oxidants some ume before the ~rradiation may result in a partial oxidation both in the sampler and the upper stripper (Fig. 8). For very labile compounds this might be the case also with oxygen, but such compounds are unlikely to occur in natural waters at higher concentrations. Oxygen seems to be most useful as oxidant of "H radicals forming a new oxidizing radical "OOH and preventing recombination of water from "OH and "H radicals and was used throughout this work. Soier & Semenov (1971) have tested He as carrier gas and reported a strongly retarded mineralization. Volatile and non-polar compounds
Volatile a n d polar compounds are determined incompletely, while non-polar and volatile compounds are not determined at all in this method. Benzene solution injected directly into the oxidation coil was recovered with a relatively high yield. Also nonvolatile compounds of low polarity, insoluble in
water, as palmitic acid, were not recovered unless a sodium alkyl-arylsulphonate-based ionic detergent was added to the sample (Table 5). For non-ionic detergents Goulden & Brooksbank ~1975) have reported a high adsorption within the analyzer. In general, good solubility m water and low volatility are prerequisites for a complete recovery of orgamc carbon by the technique described. Volatile. particularly non-polar compounds are stripped away with inorganic carbon dioxide, non-volatile compounds of low polarity are adsorbed within the analyzer before they reach the photochemical reactor. Non-specific determinations
Non-specific determination was limited to the inorganic carbon compounds which may produce CO2 within the reactor. Carbon dioxide (carbonate carbon) was stripped off before the determination. Cyanides were oxidized to carbon dioxide, but the hydrogen cyanide ~s. to a great deal, aerated with oxygen. The carbon from complexed cyanides e.g. K3Fe(CN)6 was
Table 4. Gases, oxidants and pH in published procedures Reference Armstrong et al. (1966) Erhardt (1969) Soier & Semenov (1971) Baker et al. (1974) Goulden & Brooksbank (1975) Stainton (1977)t Collins & Williams (1977) Gershey et al. (1979) This paper
Gas Air
Oxidant
pH
Air Air 02
Approx. 0.03% H202 in some samples Approx. 0.286% K2S20 s None, H202 tested None
8 2.4--1.8" 2t 1.9
02
0.305% KzS2Os
4.0-1.9"
Air Air 02
Approx. 0.15% K2520 8 0.7% KzS2Os None
02
None
2* >6* <4 1.3
*pH decreased during irradiation as potassium persulphate was decomposed to potassium hydrogen sulphate. tPrivate communication.
Mineralization of organic matter in water by u.v. radiation
I
=
~
I
. ~.
~
"Or
&
361
A°
I/' ÷
i 50 I,,., ,,. ,,,," '""""" ""X................................................................. X.• i
i
I
5
IO
15
I 20
KzS2Os, mg per sample
Fig. 7. The relationship of persulphate concentration and carbon recovery, 20 #g C in all samples albumin, ( x - • • x ), uric acid ( + - - - + ), thiourea (& AJ, cysteine ( ~ . . . . O). recovered quantitatively and interfered with the determination of organic carbon. Interference by oxidation products other than CO2 was minimized by the selectivity of the i.r. analyzer. Occasional analyses of nitrogen products suggest that most of organic nitrogen was recovered as nitrate and ammonia (see also Manny et al. 1971) and Afghan et al. 1971). Sulphur seemed to be converted to sulphate. None of these products was carried by oxygen to the analyzer.
Beattie et al. (1961) suggested that in the presence of Fe 3 + or Ce 3 + radiation of longer wave-length may produce similar oxidation effects as short wave radiation without catalysis. In our experiments addition of some metalic ions did not substantially improve the recovery of uneasily mineralized substances irradiated at slightly lowered radiation intensity (Table 6). The higher mercury concentration decreased the yield of carbon from polyvinylalcohol, probably by promoting its cross-linking.
Catalysis
Lamp temperature
Soier & Semenov (1971) have suggested that the use of mercury enhances the rate of mineralization. We tested this catalyst particularly with uric acid and no positive effect was found. The recovery of serum albumin was similarly not improved by Hg 2÷ ions. These compounds might flocculate with Hg 2÷ which would balance the positive effect reported by Soier & Semenov (1971)and confirmed in our experiments for urea (Table 6). Mercury also absorbs both resonance lines at 184.9 and 253.7nm respectively (Calvert & Pitts 1966).
By varying the voltage applied to the lamp its temperature was adjusted and this affected mineralization effect (Fig. 9). This resulted in the conclusion that maximum attainable radiation energy gives more reliable mineralization than variation of other parameters. Tiae surface mercury tube temperature in our experiments was approx. 530°C unless otherwise stated. A prolonged irradiation period produced results similar to the maximized lamp output (Fig. 5).
The procedure described here was compared with two other techniques for determination of dissolved
oo!
Table 5. The percentage recovery of carbon in volatile and non-polar substances, approx. 20/ag C per sample
"= eo
Comparison with other techniques
Substance
x 50
I
I 2
I
I 4
I
I 6
h Fig. 8. The oxidation of oxalic acid (20 ~g C per sample) by 29.6 mg K2S208 per sample, at room temperature.
Acetone Octanol Benzene Benzene Octane Palmitic acid Palmitic and detergent
Recovery C ~o 60 36 0 70* 0 3 Approx. 65"i"
*Injected straight into the coil. "i'Detergent carbon substrated (4.8 #g C per sample).
362
P. BLA2.K:, and
Table 6. Catalysts (a) Influence of some catalysts on the mineralization of uric acid (2(1 ,ug C per sample, the surface temperature of lamp 500 C. 14 min irradiation) Catalyst Recovery C ",
k
Pro(H,~ZKo~.~
]able 7. l-hc pcrccntage recovery of organic carbtm iN standard solutions by persulphatc digestion. Beckman Analyzer model 915 and the u.v. method approx. 20/,g ( per sample for persulphate and u.v.. approx, l l~g Ibr Beckman Substance
O Ce($04.)2 , 28 ,ug Ce (UO2)2NO 3, 48 #g U KMnO,~, 22 #g Mn Os04, 76 itg Os
75 75 83 86 81
{bl Influence of Hg a+ on mineralization of several compounds, experimental conditions as above Recovery C o~ 0 48 240 Substance /~g Hg per sample Benzoic acid Urea Uric acid Polyvinylalcohol
100 93 75 92
100 99 72
100 99 78 75
organic carbon, the persulphate oxidation and the high temperature combustion in a Beckman analyzer model 915. The persulphate method was essentially that of Menzel & Vaccaro (1964) with the following modifications: 1.0ml of 10~o solution of ammonium persulphate, instead of solid K2SzOs and 0.2 ml of 3°/;; HaPO4 were added to 4 ml of sample and inorganic carbon was stripped away by oxygen instead of nitrogen. Results of these comparisons are summarized in Tables 7 and 8. They suggest that this u.v.-irradiation procedures and the Beckman analyzer give fairly comparable results. The recovery with persulphate was generally somewhat lower but some loss by premature oxidation before the closing of the ampoules may occur more easily, due to the modifications used here, than in the original procedure. Also Gershey et al. (1979) reported practically identical results by high temperature combustion and u,v. techniques, but approx. 15°0 less recovery by the
Persulphate
Beckman
83 98 9l 80 100 101 96 88 93 92 85 95 93 84 88 95
98 99 73 87 96 97
Potassiumhydrogenphthalate Glucose Uric acid Albumin Tyrosine Tryptophane Urea Gtycotic acid Norvaline Nicotinic acid Proline Methionine Aspartic acid Citric acid Sodium acetate Creatine
ux 100 1~11 ~8 S2* 1()0 l(ll 99 99 1()I 100 9S 99 100 101 1/)4 92
*After correction for the amount of carbon found in the next blank. persulphate method. Their high temperature combustion is however not strictly comparable with the Beckman technique used here. Particulate carbon The procedure described here was designed for analyses of solutions. Samples of water filtered through a 40 ~tm bronze mesh however, wer e analyzed in parallel with samples filtered through Whatman G F / C filters pre-ignited at 480°C. Prolonged irradiation of the coarse mesh filtered samples did not produce any effect and results suggested that 0-5030 of total organic carbon was found in particles within the 1-40 pm size range. The highest percentage of particulate carbon was found in an eutrophic fishpond when the algal bloom was beginning. Also suspensions of rotifers and bacteria were successfully analyzed and no prolongation of the irradiation period was required. DISCUSSION All papers published on carbon mineralization by u.v. so far claim complete recovery of organic carbon
x/
IOO x/ xj
o
Table 8. Dissolved organic carbon in mg C 1- 1 found in fresh water by the Beckman analyzer (model 915) and the u.v. method
~o -X t 0 ~3
50
3
Locality I 300
I 400
I 500
I 600
t~ °C
Fig. 9. The relationship between the lamp surface temperature and the carbon recovery from uric acid (20 ~g C/sample, 14 min irradiation period).
Slapy Reservoir Vltava River (TS'n n. Vlt.) Orlik Reservoir Bla~im village pool Vltava River (Praha) Peat extract
Beckman
u.v:
8.4 24.5 10.5 17.3 10.2 26.6
8.3 25.6 10.4 17.7 10A 27.2
Mineralization of organic matter in water by u.v. radiation in samples of both fresh and marine water. These statements are based upon two types of indices: (1) modification of experimental conditions, particularly prolonged irradiation did not increase the yield of carbon dioxide; and (2) the data obtained by' the u.v. digestion are comparable with those obtained by other techniques, e.g. persulphate digestion and dry combustion. The agreement among all studies on u.v. digestion is however surprising after a comparison of the experimental details is made (Tables 1 and 4). The only conclusion which can be made is that a majority of organic compounds in water are very labile towards u.v. irradiation. In this paper we have shown some organic compounds are not determined quantitatively even under fairly severe conditions, though the occurrence of most of them in freshwater is unlikely, except in sewage and polluted water. Some others, e.g. proteins, may occur in appreciable quantities in eutrophic water analyzed with some of the biota included: These substances cannot be simply classified and several reasons for their difficult treatment in continuous system were suggested by Stainton et al. (1974), Goulden & Brooksbank (1975) and in the present paper. u.v. Digestion compares favourably with the persulphate digestion procedure and with the data from the Beckman analyzer (Table 7 and 8). Some slight underestimates in the persulphate digestion were expected for some time (Williams P. M., 1969; Williams P. J. B., 1969; Gordon & Sutcliffe, 1973; Sharp, 1973), but the comparison may not be final due to some slight modifications we have introduced in the persulphate method and blank problems in the last two methods (Collins & Williams, 1977). More surprising for us w6re some low data from the Beckman analyzer which may possibly be caused by a rapid removal of volatile intermediates from the hot zone of the furnace by the stream of oxygen. This requires a more detailed study. All the three techniques seem to be applicable for the analysis of organic carbon in water. Future tests should include also the dry combustion method with flame ionization detection (Croll, 1972) and the commerical Technicon apparatus (Schreurs, 1978). The main advantage of the u.v. procedure is its applicability for the analysis of phosphorus, metals etc. without the danger of massive contamination by strong acids and other digestion reagents. There is however, not a single u.v. procedure, but as many as there are papers dealing with this technique. SUMMARY The physical background of the u.v. digestion technique was briefly discussed and some published procedures were compared. Low pH enhanced mineralization of urea and thiourea, but other compounds were not significantly influenced by pH variation within the range 0.6-4.0.
363
Oxidants absorb short wave u.v. and may decrease the yield of CO2 in substances with a low mineralization rate, but they may be useful where dissolved oxygen is not sufficient for a complete oxidation of the sample. Non-polar and volatile substances are aerated away with the inorganic carbon dioxide before the determination, while non-volatile substances of low polarity are adsorbed within the apparatus prior to the photochemical reactor. Proteins are partly adsorbed, their adsorption may be suppressed by phosphoric acid, but a correction from subsequent blank must be applied in protein rich samples. Purines had the slowest mineralization rate of the compounds tested. Metal catalysts did not improve their recovery, but prolonged irradiation or increased output from the mercury lamp caused their complete oxidation.
Acknowledgements--The authors are grateful to a number
of persons, but particularly to the following: Drs J. Hrb~i~ek, Z. Brandl, and M. Legner who read the manuscript and suggested comments. Mrs M. Maregovh, Department of Water and Environment Technology, Polytechnics of Chemical Technology, Praha, supplied data from the Beckman analyzer. Mr P. Piab,ek, Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, Praha made the glass work and Mr L. PrachaL Hydrobiologieal Laboratory, was responsible for the time relay system and M. Buchta for the drawings.
REFERENCES
Afghan B. K., Goulden P. D. & Ryan J. F. (1971) Use of ultraviolet irradiation in the determination of nutrients in Water with special reference to nitrogen. Technical Bulletin No. 40. Department of Energy and Mine Resources, Inland Water Branch, Ottawa. Armstrong F. A. J. & Tibbits S. (1968) Photochemical combustion of organic matter in sea water for nitrogen, phosphorus and carbon determination, d. mar. biol. Ass. U.K. 48, 143-152. Armstrong F. A. J., Williams P. M. & Strickland J. D. H. (1966) Photooxidation of organic matter in sea water by ultraviolet radiation, analytical and other applications. Nature, Lond. 211,481-483. Baker C. D., Bartlett P. D., Farr I. S. & Williams G. I. (1974). Improved methods for the measurement of dissolved and particulate organic carbon in fresh water and their application to chalk streams. Freshwat. Biol, 4, 467-481. Beattie J., Bricker C. & Garvin D. (1961) Photolytic determination of trace amounts of organic material in water. Analyt. Chem. 33, 1890-1892. Calvert J. G. & Pitts J. N. Jr. (1966) Photochemistry. Wiley, New York. Cassidy H. G. (1957). Fundamentals of chromatography. Technique of Organic Chemistry Series (Edited by Weissberger A.), Vol. X, p. 128 and 255. Interscience, New York. Collins J. J. & Williams P. J. leB. (1977) An automated photochemical method for the determination of dissolved organic carbon in sea and estuarine waters. Mar. Chem. 5, 123-141.
g(',4
P. BI x)k', ~itad I. 1:'1~,¢~(IIAZKc~\ ~,
Croll 13. T. (1972) Determination of organic carbon in water. Chem. lmL 1972, 386. Deyl Z., Macek K. & Jan/tk J. (1975) Liquid ('ohmm Chromatoyraphy. Elsevier, New York. Erhardt M. (I969) A new method for automatic measurement of dissolved organic carbon in seawater. Deep-Sea Res. 16, 393 397. Gershey R. M., M a c K i n n o n M. D., Williams P. J. leB. & Moore R. M. (1979) Comparison of three oxidation methods used for the analysis of the dissolved organic carbon in seawater. Mar. Chem. 7, 289 306. Gordon D. C. Jr & Sutcliffe W. H. Jr (1973) A new dry combustion method for the simultaneous determination of total organic carbon and nitrogen in sea water. Mar. Chem. I, 231 244. Goulden P. D. & Brooksbank P. (1975) Automated determination of dissolved organic carbon in lake water. Amd. Chem. 47, 1943 1946. Manny B. A., Miller B. C. & Wetzel R. G, (1971). Ultraviolet combustion of dissolved organic nitrogen compounds in lake waters. Limnol. Oceano,qr. 16, 71 85. Menzel D. W. & Vaccaro R. F. (1964) The measurement of dissolved organic and particulate carbon in seawater. Limnol Oceam~yr. 9, 138 142.
Schreurs W. !197,";A An automated colorimetric method lot the determination of dissolved organic carbon m sea \~atcr b\ u.v. dcslruction, ftydrohio/ Bull. 12, 137 142. Sharp ,I. (1973) Total organic carbon in sea water cornparison of measurements using persulphate oxidation and high temperature combustion. Mar. Chem I, 21 I 22 t) Soicr V. (i. & Semcnov A. D. 11971)The photochemical method of determining organic carbon. Gidrokhim. Mater. 56, 111 120: Chem. Ahstr. 7$, 12123 Stainton M. P., Capel M. J. & Armstrong F. A. J. (1974) The chemical analysis of freshwater. Miscellaneous Special Publication No 25, Department of the Environment, Fisheries and Marine Service, Research and Development Directorate, Freshwater Institute. Winnipeg, Manitoba, Canada. Ve~e~a M. (1967) Orqanickc) element&hi analysa. Stfitni nakl. techn, literatury, Praha {Organic elementary analysis). Williams P. J. B. (1969) The wet oxidation of organic matter in seawater. Limnol, Oceanogr. 14, 292-297. Williams P. M. (1969) The determination of dissolved organic carbon in seawater: A comparison of two methods. Limnol. Oceam)w. 14. 297 298.
0043-1354/83/040355-10503.00/0 Copyright © 1983 Pergamon Press Ltd
M I N E R A L I Z A T I O N OF O R G A N I C MATTER IN WATER BY U.V. R A D I A T I O N P. BLA~KA and L. PROCH~,ZKOV~ Hydrobiological Laboratory, Institute of Botany*, Czechoslovak Academy of Sciences, Vltavsk/t 17, 151 05 Praha 5, Czechoslovakia (Received May 1979)
Abstract--The conditions for the optimum yield of carbon dioxide in the mineralization of organic matter in water were tested and the relative optimum procedure was outlined. Of all substances tested the greatest difficulties were encountered with the purine bases. Their quantitative mineralization was achieved only by the highest irradiation intensities and/or prolonged irradiation. An evident relationship of mineralization and pH was found in urea and thiourea. Adsorbtion of proteins on silica and glass apparatus was suppressed by phosphoric acid, but a correction of the amount of organic carbon recovered in the blank still had to be applied in protein rich samples.
INTRODUCTION Irradiation of water samples by short wave-length u.v. became one of the generally used mineralization procedures after the papers by Beattie et al. (1961) and Armstrong et al. (1966). The aim of this paper is to review and check some basic factors involved in mineralization of dilute solutions of organic compounds and their mixtures and test the efficiency of mineralization by u.v. radiation and compare it with other methods. CHEMICAL AND PHYSICAL BACKGROUND Oxidation by u.v.-radiation is considered to be caused by "OH radicals which are split off from water molecules by short wave u.v. radiation (Beattie et al., 1961). It is therefore likely that the efficiency of u.v. mineralization depends upon the intensity and wavelength of irradiation. Irradiation is directly proportional to the source intensity and indirectly to the distance between the source and the sample. For a short distance from a linear source a relationship close to the first power of the distance is applicable while at longer distances (multiples of lamps length) the relationship approaches a second power function of that distance. Longer arcs yield less irradiation per cm 2 than shorter ones at the identical lamp output (Fig. 1). In constructing a photochemical unit shorter arcs and the shortest possible distance between the source and the sample should therefore be prefered. The pressure of mercury vapours within the lamp is proportional to the temperature and determines the spectral characteristics and total output of the lamp. It may be influced by varying the voltage applied on the lamp or by the cooling rate. *Now Institute of Landscape Ecology. 355 W.R. 17/4---A
In practice this means that the lamp should be run at the optimum current chracteristics suggested by the producer and the optimum cooling rate which is found empirically. A fan is most frequently applied to cool the samples. The lamp is protected by a silica tube which prevents excessive cooling of the lamp, but allows excess of heat to escape. In general most of the published descriptions of apparatus are poorly defined from an optical point of view and any measurement and calculation becomes very complex. Only Armstrong et al. (1966} and Armstrong & Tibbits (1968) have measured the output of their units between 200 and 400 nm using uranium oxalate actinometry. In Table 1, physical data of several units are summarized and their irradiation is estimated from the data of the authors and of the manufacturers of the tubes. The assumption was made that all energy within the range 200-300 nm is radiated between the electrodes uniformly in all directions normal to the axis of the respective tube. It is realized that this is an oversimplification and the calculations are given just to suggest the probable range of irradiation in the experiments. METHODS
Description of the apparatus
The efficiency of u.v. mineralization was tested on an automated apparatus, the outline of which is shown in Fig. 2. Acidified samples of 4 ml total volume in conical quartz test tubes were prepared in a sampler (1) and were then pumped by a peristaltic pump (3) into the upper wash bottle (4). The tubing was Tygon-Acidfex and glass capilaries approx. 0.5 mm i.d. In the upper wash bottle the sample was purged by oxygen and inorganic carbon dioxide was carried through the drying column (I1) filled with magnesium sulphate dried at 180°C, a flow meter (13) and a valve to the i.r. analyzer IREX (Chemick6 z~ivody, Zfilu~i, Czechoslovakia) (14) and released to the air. A peak broadener (12) was put between the drying column and the
:'15~
P 131 ',2K;\ and l
Pt{o( llA/k~ i~. t example, tile Hanovia Engelhardl IgOA I2(1(1,,' ~ith ,m arc length of 30.5 cm iFig. I1. Oxygen was purilied b~ passing througtl a colunm iith:d
with a mixture of finely ground sodium hydroxide, ~duminium oxide (Brockmann) and phenolphthalein (I:I:IL01 weight ratioi(171 and carried through the reference tuhc of the i.r. analyzer. The electrical signal of the i.r. anal,~zcr was recorded il 5) and the peaks integrated (16)(Integrator IT 1. Laboratorni pHstroje, Czechoslovakia). The \~hole apparatus was controlled by 8 electro-mechanical timcretavs connected in series, however a manual modification of the cycle was possible at each step.
L 200
250
300
nm
Fig. 1. The relative emission spectra of the Hanovia-Engelhardt 189 A lamp (A), and the USSR--Svetotechnika Saransk PRK-7 mercury lamp (B). Both spectra were recorded at identical geometry and sensitivity. The emission was recorded by the Beckman DB-G spectrophotometer equipped with a R-136 photomultiplier.
flow meter to ensure enough time for a complete response of the slow i,r. analyzer. This took the form of a vessel in the shape of two cones connected by their bases filled by glass beads 1 m m in diameter. In the next step the sample was brought by oxygen pressure through the open valve (MT 1) into the quartz capillary coil (7), the preceding sample being forced at the same time into the lower wash bottle (8). The samples were separated by a bubble of oxygen. The capillary was coiled with a diameter of 5 cm around a medium-pressure PRK-7 (USSR) mercury u.v. tube (5) and the sample stayed there for 14min. In the lower wash bottle the sample was purged with oxygen once more and the carbon dioxide which resulted from oxidized organic c o m p o u n d s was brought by stream of oxygen to the i,r. analyzer. The magnetic 2-way valve (SV) set the gas flow alternatively through the upper and the lower wash bottle. The PRK-7, a medium pressure mercury discharge lamp produced by Svetotechnika, Saransk, USSR had an input of 1000W and arc length 19cm and tube diameter o f about 2.5 cm. It produced a stronger radiation per unit area at all lines within the 2 0 0 - 3 0 0 n m rahge than, for
Blauks Distilled water was prepared from tap water in a glass still with stainless-steel electrodes as heating unit. It was then redistilled in a whole glass still alter it had been adjusted to approx. 10 '*M with potassium permaganate and 10 -2 M with sulphuric acid. It was prepared fresh not more than 2 days before each series of determinations. All glass was washed with freshly prepared warm mixture of concentrated sulphuric acid and 30°,o solution of hydrogen peroxide 1 : 1 (v/v) and several times with redistilled water. Acids solutions were irradiated for 45 min by the PRK-7 mercury lamp in a silica flask from the distance of 5 cm. The silica test tubes were treated with a wet tube brush, rinsed several times with redistilled water, filled up by water again and irradiated on a turn-table by the same type of u.v. tube in horizontal position for 45 min. The top of the 8 cm long test tubes rotated approx. 3 cm below the lamp. During washing the tubes were not touched by fingers (except during brush cleaning) and water was sucked off. After irradiation water was replaced by the acidified sample. Several blanks, usually four, were run at the beginning of each series of determinations to clean apparatus and it was washed by three sample volumes of redistilled water at the end again. The main source of elevated blanks was laboratory air and a considerable improvement was achieved by placing a cover over the sampler and flushing it with spent oxygen from the analyser. For longer periods polymethacrylate covers were not adequate, they produced high concentrations (up to 10 mg C 1- ~) if blanks are standing covered by them for a week or two. Glass seemed to be the best material for these covers. The second source of high blanks is the apparatus, particularly the flexible tubing. Acidiflex (Technicon) was found to be superior to Tygon (Technicon) and silicone rubber (Kablo Czechoslovakia). Routine values of blanks were about 2 #g of organic carbon per sample (approx. 0.5 mg C 1-~) and the best values obtained were 0.3-O.4,ug C per sample (approx. 0.1 mg C 1-~). High blanks limited useful sensitivity of the determination; detection sensitivity could be easily increased at least by one order of magnitude (Collins & Williams, 1977). Standards A solution of benzoic acid 200,ug C m l - J was used to calibrate the instrument at the beginning and the end of each experimental series; 0.1 ml was added by a Hamilton ® microsyringe. The calibration was linear up to 40 jug C per sample as benzoic acid and passed through the origin when relevant blanks were subtracted from the readings. This was true also for a calibration line done at 5-times increased sensitivity, the linear range in this case was up to 6,ug C per sample. Precision The coefficient of variation o f blanks was 7.6'~'%; for samples at a concentration of 3 mg C 1- ~ it was 5.3°~i and
357
Mineralization of organic matter in water by u.v. radiation ......................................................................................................
I i I !
!
I I
: ................. r"" ; : • I
I I I I i
l----.JI I I ! !
•
•- - ' -
!
~
r.--n18 .,&-~-u--1
!!"
,
e
l]
I
II
' y
L
.
D°
J
T i" ~'~.~....-
]
_
L.
116
D
'v'
MT4 i~ ..... : - . : : . -
. . . . . . . . .
i
.: .-:,~
[
:
..........
- - ~ - ............................ i l ~ l ~
~
JtF :
:
;
,1~1
:
~
?.j:-'.'.'.~:'::...~:--....~..!.:.!.':::~...L...?
,
~
~ ?
Fig. 2. The scheme of the apparatus: l--sampler; 2--sampling capillary with a solenoid; 3--peristaltic pump; 4--upper wash bottle; 5--PRK-7-mercury lamp; 6~protecting quartz tube; 7--quartz capillary coil; 8-lower wash bottle; 9-10 mercury valves; 1l-drying column (MgSO,); 12-peak broadener; 13--flow meter; 14--i.r. analyzer; 15--recorder; 16--integrator; 17--NaOH column; 18--flow regulator; 19--oxygen tank; MT 1, 2, 3, 4,--magnetic valves; SV--two-way magnetic valve. TR 1--8 electromechanical time relays, arrows on their lower side suggest their interconnections. Solid lines indicate the sample route, dashed lines the flow of oxygen, and dotted lines electrical connections. Table 1. Physical data of various irradiation setups
Reference
u.v. Source
d
Irradiation period
Armstrong
Hanovia8.2 2-3 h Engelhardt (1966) 189 A, 1200W Armstrong Hanovia-Engelhardt 4.4 11-16 h & Tibbits (1968) 507/7240W 509/10 380 W Erhardt (1969) Hanau Q 1200 5.1 15 min 900W Soier & Svetotechnika 3.1 48 min Semenov Saransk, USSR (1971) PRK-7,1000 W Baker et Hanovia 5.0 15-23 min al. (1974) 500 W Goulden & Hanovia-Engelhardt 6.1 4-8 min Brooksbank (1975) 189A, 1200W Collins & Hanovia 69326X 6.0 45 min Williams 1000 W (1977) Stainton Westinghouse 3.7 10 min (1977) 700W Gershey et Hanovia 6932X 6.0 20 min 1000W al. (1979) Svetotechnika 2.6 14 min This paper Saransk, USSR PRK-7, 1000W
Intensityt {mW cm -2) 112
et al.
* Distance between source and sample centers (cm). t Estimated by the present authors.
30
61 375
Detection of CO2 Calculated from pH Calculated from 02 uptake Conductometric Coulometric titration
102
i.r.
130
i.r.
110
i.r.
89
Conductometric
110
i.r.
448
i.r.
P. BI xZK,'~ mid l.. t)RO( tIAzKI'P,,A
35S
at a concentration of 10 nag C 1 ~ it was 0.76"o t.\ = 10 for all groups),
Drying Several materials were tested in the drying column. Phosphorus pentoxide was of little use as it very quickly turned to a thick paste and blocked the flow. Silica gel caused a reversible adsorption of carbon dioxide which was related to the hydration of silica gel. Dried magnesium sulphate trapped water efficiently, carbon dioxide was not retained and passed through the column in a sharp peak which the analyzer could not follow, The response was improved by the peak broadener (12),
Procedure Unless otherwise indicated samples were acidified by 1 ml 10% phosphoric acid and the volume adjusted with "'carbon free" water to 4 ml. In a series of experiments, samples always alternated with blanks, unless they comprised a continuous series, like fractions of a column effluent. In such a case the samples followed one after the other. In discontinuous series (with blanks) all samples were determined in duplicates. If the intermediate blank (between samples) was higher than a clean blank (preceded by a blank) by more than 10% of the previous sample value, a correction was applied by adding the value in excess of the 10?i; to the previous sample.
RESULTS
Easily mineralized substances Organic compounds recovered quantitatively (100 _+ 2% of theory) included: glucose, benzoic, tartaric, oxalic, glycolic, citric, adipic, nicotinic acids, nicotinamide, sodium acetate, ammonium acetate, potassium-hydrogen phthalate, phthalic anhydride, glycine, proline, lysine, valine, norvaline, histidine, tyrosine, tryptophan, methionine, cystine, pyridine, starch, phenol, sulphanilamide, asparagine, aspartic acid, l-naptylamine, semicarbazide, monopyrazolone, polyvinylalcohol, casein-hydrolysate and ferricyanide.
Influence of pH Carbonates and carbon dioxide interfere with the determination of organic carbon and samples are to be acidified in the first step to destroy carbonates. In most methods irradiation proceeds at low pH, only Armstrong et al. (1966), who determined carbon indirectly, irradiate alkaline samples. Collins & Williams (1977) started photo-oxidation at pH > 6, but the sample became more acidic as persulphate was decomposed. They reported minimal rate of photodecomposition of an organic '4C preparation from aged algae at pH 4, the rate increased to pH 8, but slightly also to pH 1.5 where it was about half the rate at pH 8. On the other hand they got higher blanks from the two extra reagents (tetraborate, HC1). Gershey et al. (1979) in their comparison of the three methods irradiated at a pH lower than 4 which did not change much during the photodecomposition step as persulphate was also omitted from the original Collins & Williams (1979) procedure.
Soier & Semenov (private communicatiom adjusted the pH of their samples to 2 by sulphuric acid and this was the procedure we have followed, but ornittmg the Hg 2 ~ catalysis. It was satisfactory for most compounds tested except those indicated in Table 2. The reaction rate with "OH radicals is likely to var 3 with the type of compound, but for most tested a 14 min irradiation period sufficied for complete recovery. Some of the reaction rates showed a relationship with pH due to tautomerixation and protonization equilibria {e.g. urea and thiourea), but others with slightly higher basic dissociation constants (within the range 1.05.10- 12-8.4" 10- 11 for arginine, semicarbazide, sulphanilamide, asparagine, 1-naphtylamine) produced only a very slight relationship with pH (Fig. 3). The yield of CO2 was, however, related not only to the reactivity of the compound but also of all the intermediates. These were not known and therefore it was not easy to suggest a simple interpretation of the relationship of the yield to pH. The low yield of thiourea as compared with urea may be interpreted both by a higher oxygen consumption for oxidation of the former and by the interaction of sulffiydryl group with "OH radicals. Cysteine was recovered with a high efficiency and this might suggest that the former interpretation is more likely (see also Figs 5 and %
Adsorption q] proteins With serum albumin and casein the greater part of the unrecovered carbon was found in samples (blanks) following the protein sample (Table 3). The finished protein samples gave low blank values when returned to the cleaned apparatus. Change of pH, prolonged irradiation, or catalysts have not improved the
Table 2. Organic compounds with a lowered carbon recovery at the following conditions: Acidification with H2SO4, pH 2, 14 min mineralization, temperature of lamp surface 360 and 500°C. The carbon content of all these substances was tested by organic elementary analyses by the method of Ve~e~a (1967). 20/~g C per sample. The volatile and nonpolar substances are not included
Substance Urea Arginine Creatine Cysteine Thiourea Uric acid Guanine Adenine Albumin Casein
Recovery C% Temperature of the lamp surface (°C) 360 500 75 95 57 50 54 --
95 100 92 90 75 77 86 87 52* 65*
*The values without correction on the amount of proteins washed out in the following blank.
Mineralization of organic matter in water by u.v. radiation
,, ioo-
--~ . . . . . .
.
x .... ~ . . . . ~ ~ .
.......
>~
359
2:.-,.
........
.,,.,,.,,,, -~
50--
x-\.\.
\.
.a
\
A~x
o
x
\.\
I
I
1
I
I
2
3
4
pH Fig. 3. The relationship between percentage carbon recovery and pH. 20/~g C per sample, lamp surface temperature approx. 500°C. urea (x . . . . . x j; thiourea (A A}; uric acid (+ . . . . +}; sulphanilamide ( ~ - - - - t ) ; semicarbazide (O . . . . . O); arginine (&. . . . . A); l-naphtylamine (D---r~).
delayed recovery. Serum albumin and casein, but not casein hydrolysate, were evidently retained within the apparatus prior to the lower wash bottle. No apparent increase in the residual volume was observed in the sampler, the pumping capillary and the first stripper. Therefore the adsorption on glass and silica surfaces seems to be most likely--see also Cassidy (1957) and Deyl et al. (1975). The adsorption of proteins was also confirmed by passing serum albumin solutions through columns of crushed borosilicate glass and silica (Fig. 4). Several compounds were tested for depressing this adsorption on silica and glass. Only phosphoric acid was found to be useful (Table 3), though even this compound did not supress the adsorption of protein completely. Boric acid was without any effect, NaF and KHSO4 increased the adsorption of protein. Irradiation of blank samples after a protein sample was necessary to recover the rest of the carbon as carbon dioxide. Three minutes irradiation was sufficient for this purpose. Many of the protein analyses performed gave poor reproducibility. Mixing of samples in the test tubes during the addition of protein solutions (e.g. by a stream of air) was found to improve the reproducibility of results.
Oxidizing agents Various oxidizing agents were used in combination with air or oxygen as carrier gases (Table 4). Absorption spectra of oxidants (Fig. 6) suggested, however, that they absorbed some short wave-length u.v. which was probably the part most active in mineralization (Beattie et al., 1961). These oxidants are destroyed by u.v. and the absorption of u.v. above 200 nm ceases. Only then was water split into radicals at the full rate corresponding to the irradiation. Addition of peroxide or persulphate thus reduced the effective time available for mineralization and in compounds with a slow mineralization rate they may have decreased the yield. A positive effect of oxidants may be produced in samples with a high sulphur and nitrogen content, where oxygen may become a limiting factor. Potassium persulphate is converted by u.v. irradiation into potassium hydrogen sulphate. This increased the adsorption of proteins on silica and glass (see previous section).
T
/" i/'
I0 >,
/
Acidification
Recovery C~o Sample 1. Blank 2. Blank Total
c o
4
52 84
25 10
4 4
81 98
The values are means of six parallel determinations, the solutions were thoroughly mixed by bubbling with air after addition of albumin solutions, approx. 20min before mineralization (30 pg C per sample).
X
/I
,,"
2
o ~
/
H2SO4, pH 2.0 H3PO4, pH 1.3
/
i/
Table 3. Adsorption of albumin within the apparatus and its elution in the following blanks
/
X X,,'~*'* I0
Albumin
I
I 20
I
I 30
applied, /zg ml -~
Fig. 4. The absorption of serum albumin on a column (1.5 x l0 cm} of crushed silica (x) and borosilicate glass (O). Dashed line theoretical recovery without any adsorption.
P. Bt,A:;'KAand k. PRO( II.,'~ZK()V~
360
,ool
o ii f
+
+
i
08
0.6
o
5o
g
// i i i / /// //
\
0.4
\\
//
I 15
I 30
I
I 45
Time,
0.2
60
rain
0
Fig. 5. The relation between c/arbon recovery and time of mineralization. Cystine (O); cysteine (@); thiourea (Ah uric acid (+); 20 #g per each sample.
200
250
300
nm Fig. 6. Absorption spectra of hydrogen peroxide (..... ) 0.05% and potassium persulphate ( ~ ) , 0.3°4 solutions.
In our experiments the addition of persulphate improved mineralization of thiourea and cysteine, but decreased the yield of carbon dioxide from uric acid and serum albumin (Fig. 71. Addition of oxidants some ume before the ~rradiation may result in a partial oxidation both in the sampler and the upper stripper (Fig. 8). For very labile compounds this might be the case also with oxygen, but such compounds are unlikely to occur in natural waters at higher concentrations. Oxygen seems to be most useful as oxidant of "H radicals forming a new oxidizing radical "OOH and preventing recombination of water from "OH and "H radicals and was used throughout this work. Soier & Semenov (1971) have tested He as carrier gas and reported a strongly retarded mineralization. Volatile and non-polar compounds
Volatile a n d polar compounds are determined incompletely, while non-polar and volatile compounds are not determined at all in this method. Benzene solution injected directly into the oxidation coil was recovered with a relatively high yield. Also nonvolatile compounds of low polarity, insoluble in
water, as palmitic acid, were not recovered unless a sodium alkyl-arylsulphonate-based ionic detergent was added to the sample (Table 5). For non-ionic detergents Goulden & Brooksbank ~1975) have reported a high adsorption within the analyzer. In general, good solubility m water and low volatility are prerequisites for a complete recovery of orgamc carbon by the technique described. Volatile. particularly non-polar compounds are stripped away with inorganic carbon dioxide, non-volatile compounds of low polarity are adsorbed within the analyzer before they reach the photochemical reactor. Non-specific determinations
Non-specific determination was limited to the inorganic carbon compounds which may produce CO2 within the reactor. Carbon dioxide (carbonate carbon) was stripped off before the determination. Cyanides were oxidized to carbon dioxide, but the hydrogen cyanide ~s. to a great deal, aerated with oxygen. The carbon from complexed cyanides e.g. K3Fe(CN)6 was
Table 4. Gases, oxidants and pH in published procedures Reference Armstrong et al. (1966) Erhardt (1969) Soier & Semenov (1971) Baker et al. (1974) Goulden & Brooksbank (1975) Stainton (1977)t Collins & Williams (1977) Gershey et al. (1979) This paper
Gas Air
Oxidant
pH
Air Air 02
Approx. 0.03% H202 in some samples Approx. 0.286% K2S20 s None, H202 tested None
8 2.4--1.8" 2t 1.9
02
0.305% KzS2Os
4.0-1.9"
Air Air 02
Approx. 0.15% K2520 8 0.7% KzS2Os None
02
None
2* >6* <4 1.3
*pH decreased during irradiation as potassium persulphate was decomposed to potassium hydrogen sulphate. tPrivate communication.
Mineralization of organic matter in water by u.v. radiation
I
=
~
I
. ~.
~
"Or
&
361
A°
I/' ÷
i 50 I,,., ,,. ,,,," '""""" ""X................................................................. X.• i
i
I
5
IO
15
I 20
KzS2Os, mg per sample
Fig. 7. The relationship of persulphate concentration and carbon recovery, 20 #g C in all samples albumin, ( x - • • x ), uric acid ( + - - - + ), thiourea (& AJ, cysteine ( ~ . . . . O). recovered quantitatively and interfered with the determination of organic carbon. Interference by oxidation products other than CO2 was minimized by the selectivity of the i.r. analyzer. Occasional analyses of nitrogen products suggest that most of organic nitrogen was recovered as nitrate and ammonia (see also Manny et al. 1971) and Afghan et al. 1971). Sulphur seemed to be converted to sulphate. None of these products was carried by oxygen to the analyzer.
Beattie et al. (1961) suggested that in the presence of Fe 3 + or Ce 3 + radiation of longer wave-length may produce similar oxidation effects as short wave radiation without catalysis. In our experiments addition of some metalic ions did not substantially improve the recovery of uneasily mineralized substances irradiated at slightly lowered radiation intensity (Table 6). The higher mercury concentration decreased the yield of carbon from polyvinylalcohol, probably by promoting its cross-linking.
Catalysis
Lamp temperature
Soier & Semenov (1971) have suggested that the use of mercury enhances the rate of mineralization. We tested this catalyst particularly with uric acid and no positive effect was found. The recovery of serum albumin was similarly not improved by Hg 2÷ ions. These compounds might flocculate with Hg 2÷ which would balance the positive effect reported by Soier & Semenov (1971)and confirmed in our experiments for urea (Table 6). Mercury also absorbs both resonance lines at 184.9 and 253.7nm respectively (Calvert & Pitts 1966).
By varying the voltage applied to the lamp its temperature was adjusted and this affected mineralization effect (Fig. 9). This resulted in the conclusion that maximum attainable radiation energy gives more reliable mineralization than variation of other parameters. Tiae surface mercury tube temperature in our experiments was approx. 530°C unless otherwise stated. A prolonged irradiation period produced results similar to the maximized lamp output (Fig. 5).
The procedure described here was compared with two other techniques for determination of dissolved
oo!
Table 5. The percentage recovery of carbon in volatile and non-polar substances, approx. 20/ag C per sample
"= eo
Comparison with other techniques
Substance
x 50
I
I 2
I
I 4
I
I 6
h Fig. 8. The oxidation of oxalic acid (20 ~g C per sample) by 29.6 mg K2S208 per sample, at room temperature.
Acetone Octanol Benzene Benzene Octane Palmitic acid Palmitic and detergent
Recovery C ~o 60 36 0 70* 0 3 Approx. 65"i"
*Injected straight into the coil. "i'Detergent carbon substrated (4.8 #g C per sample).
362
P. BLA2.K:, and
Table 6. Catalysts (a) Influence of some catalysts on the mineralization of uric acid (2(1 ,ug C per sample, the surface temperature of lamp 500 C. 14 min irradiation) Catalyst Recovery C ",
k
Pro(H,~ZKo~.~
]able 7. l-hc pcrccntage recovery of organic carbtm iN standard solutions by persulphatc digestion. Beckman Analyzer model 915 and the u.v. method approx. 20/,g ( per sample for persulphate and u.v.. approx, l l~g Ibr Beckman Substance
O Ce($04.)2 , 28 ,ug Ce (UO2)2NO 3, 48 #g U KMnO,~, 22 #g Mn Os04, 76 itg Os
75 75 83 86 81
{bl Influence of Hg a+ on mineralization of several compounds, experimental conditions as above Recovery C o~ 0 48 240 Substance /~g Hg per sample Benzoic acid Urea Uric acid Polyvinylalcohol
100 93 75 92
100 99 72
100 99 78 75
organic carbon, the persulphate oxidation and the high temperature combustion in a Beckman analyzer model 915. The persulphate method was essentially that of Menzel & Vaccaro (1964) with the following modifications: 1.0ml of 10~o solution of ammonium persulphate, instead of solid K2SzOs and 0.2 ml of 3°/;; HaPO4 were added to 4 ml of sample and inorganic carbon was stripped away by oxygen instead of nitrogen. Results of these comparisons are summarized in Tables 7 and 8. They suggest that this u.v.-irradiation procedures and the Beckman analyzer give fairly comparable results. The recovery with persulphate was generally somewhat lower but some loss by premature oxidation before the closing of the ampoules may occur more easily, due to the modifications used here, than in the original procedure. Also Gershey et al. (1979) reported practically identical results by high temperature combustion and u,v. techniques, but approx. 15°0 less recovery by the
Persulphate
Beckman
83 98 9l 80 100 101 96 88 93 92 85 95 93 84 88 95
98 99 73 87 96 97
Potassiumhydrogenphthalate Glucose Uric acid Albumin Tyrosine Tryptophane Urea Gtycotic acid Norvaline Nicotinic acid Proline Methionine Aspartic acid Citric acid Sodium acetate Creatine
ux 100 1~11 ~8 S2* 1()0 l(ll 99 99 1()I 100 9S 99 100 101 1/)4 92
*After correction for the amount of carbon found in the next blank. persulphate method. Their high temperature combustion is however not strictly comparable with the Beckman technique used here. Particulate carbon The procedure described here was designed for analyses of solutions. Samples of water filtered through a 40 ~tm bronze mesh however, wer e analyzed in parallel with samples filtered through Whatman G F / C filters pre-ignited at 480°C. Prolonged irradiation of the coarse mesh filtered samples did not produce any effect and results suggested that 0-5030 of total organic carbon was found in particles within the 1-40 pm size range. The highest percentage of particulate carbon was found in an eutrophic fishpond when the algal bloom was beginning. Also suspensions of rotifers and bacteria were successfully analyzed and no prolongation of the irradiation period was required. DISCUSSION All papers published on carbon mineralization by u.v. so far claim complete recovery of organic carbon
x/
IOO x/ xj
o
Table 8. Dissolved organic carbon in mg C 1- 1 found in fresh water by the Beckman analyzer (model 915) and the u.v. method
~o -X t 0 ~3
50
3
Locality I 300
I 400
I 500
I 600
t~ °C
Fig. 9. The relationship between the lamp surface temperature and the carbon recovery from uric acid (20 ~g C/sample, 14 min irradiation period).
Slapy Reservoir Vltava River (TS'n n. Vlt.) Orlik Reservoir Bla~im village pool Vltava River (Praha) Peat extract
Beckman
u.v:
8.4 24.5 10.5 17.3 10.2 26.6
8.3 25.6 10.4 17.7 10A 27.2
Mineralization of organic matter in water by u.v. radiation in samples of both fresh and marine water. These statements are based upon two types of indices: (1) modification of experimental conditions, particularly prolonged irradiation did not increase the yield of carbon dioxide; and (2) the data obtained by' the u.v. digestion are comparable with those obtained by other techniques, e.g. persulphate digestion and dry combustion. The agreement among all studies on u.v. digestion is however surprising after a comparison of the experimental details is made (Tables 1 and 4). The only conclusion which can be made is that a majority of organic compounds in water are very labile towards u.v. irradiation. In this paper we have shown some organic compounds are not determined quantitatively even under fairly severe conditions, though the occurrence of most of them in freshwater is unlikely, except in sewage and polluted water. Some others, e.g. proteins, may occur in appreciable quantities in eutrophic water analyzed with some of the biota included: These substances cannot be simply classified and several reasons for their difficult treatment in continuous system were suggested by Stainton et al. (1974), Goulden & Brooksbank (1975) and in the present paper. u.v. Digestion compares favourably with the persulphate digestion procedure and with the data from the Beckman analyzer (Table 7 and 8). Some slight underestimates in the persulphate digestion were expected for some time (Williams P. M., 1969; Williams P. J. B., 1969; Gordon & Sutcliffe, 1973; Sharp, 1973), but the comparison may not be final due to some slight modifications we have introduced in the persulphate method and blank problems in the last two methods (Collins & Williams, 1977). More surprising for us w6re some low data from the Beckman analyzer which may possibly be caused by a rapid removal of volatile intermediates from the hot zone of the furnace by the stream of oxygen. This requires a more detailed study. All the three techniques seem to be applicable for the analysis of organic carbon in water. Future tests should include also the dry combustion method with flame ionization detection (Croll, 1972) and the commerical Technicon apparatus (Schreurs, 1978). The main advantage of the u.v. procedure is its applicability for the analysis of phosphorus, metals etc. without the danger of massive contamination by strong acids and other digestion reagents. There is however, not a single u.v. procedure, but as many as there are papers dealing with this technique. SUMMARY The physical background of the u.v. digestion technique was briefly discussed and some published procedures were compared. Low pH enhanced mineralization of urea and thiourea, but other compounds were not significantly influenced by pH variation within the range 0.6-4.0.
363
Oxidants absorb short wave u.v. and may decrease the yield of CO2 in substances with a low mineralization rate, but they may be useful where dissolved oxygen is not sufficient for a complete oxidation of the sample. Non-polar and volatile substances are aerated away with the inorganic carbon dioxide before the determination, while non-volatile substances of low polarity are adsorbed within the apparatus prior to the photochemical reactor. Proteins are partly adsorbed, their adsorption may be suppressed by phosphoric acid, but a correction from subsequent blank must be applied in protein rich samples. Purines had the slowest mineralization rate of the compounds tested. Metal catalysts did not improve their recovery, but prolonged irradiation or increased output from the mercury lamp caused their complete oxidation.
Acknowledgements--The authors are grateful to a number
of persons, but particularly to the following: Drs J. Hrb~i~ek, Z. Brandl, and M. Legner who read the manuscript and suggested comments. Mrs M. Maregovh, Department of Water and Environment Technology, Polytechnics of Chemical Technology, Praha, supplied data from the Beckman analyzer. Mr P. Piab,ek, Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, Praha made the glass work and Mr L. PrachaL Hydrobiologieal Laboratory, was responsible for the time relay system and M. Buchta for the drawings.
REFERENCES
Afghan B. K., Goulden P. D. & Ryan J. F. (1971) Use of ultraviolet irradiation in the determination of nutrients in Water with special reference to nitrogen. Technical Bulletin No. 40. Department of Energy and Mine Resources, Inland Water Branch, Ottawa. Armstrong F. A. J. & Tibbits S. (1968) Photochemical combustion of organic matter in sea water for nitrogen, phosphorus and carbon determination, d. mar. biol. Ass. U.K. 48, 143-152. Armstrong F. A. J., Williams P. M. & Strickland J. D. H. (1966) Photooxidation of organic matter in sea water by ultraviolet radiation, analytical and other applications. Nature, Lond. 211,481-483. Baker C. D., Bartlett P. D., Farr I. S. & Williams G. I. (1974). Improved methods for the measurement of dissolved and particulate organic carbon in fresh water and their application to chalk streams. Freshwat. Biol, 4, 467-481. Beattie J., Bricker C. & Garvin D. (1961) Photolytic determination of trace amounts of organic material in water. Analyt. Chem. 33, 1890-1892. Calvert J. G. & Pitts J. N. Jr. (1966) Photochemistry. Wiley, New York. Cassidy H. G. (1957). Fundamentals of chromatography. Technique of Organic Chemistry Series (Edited by Weissberger A.), Vol. X, p. 128 and 255. Interscience, New York. Collins J. J. & Williams P. J. leB. (1977) An automated photochemical method for the determination of dissolved organic carbon in sea and estuarine waters. Mar. Chem. 5, 123-141.
g(',4
P. BI x)k', ~itad I. 1:'1~,¢~(IIAZKc~\ ~,
Croll 13. T. (1972) Determination of organic carbon in water. Chem. lmL 1972, 386. Deyl Z., Macek K. & Jan/tk J. (1975) Liquid ('ohmm Chromatoyraphy. Elsevier, New York. Erhardt M. (I969) A new method for automatic measurement of dissolved organic carbon in seawater. Deep-Sea Res. 16, 393 397. Gershey R. M., M a c K i n n o n M. D., Williams P. J. leB. & Moore R. M. (1979) Comparison of three oxidation methods used for the analysis of the dissolved organic carbon in seawater. Mar. Chem. 7, 289 306. Gordon D. C. Jr & Sutcliffe W. H. Jr (1973) A new dry combustion method for the simultaneous determination of total organic carbon and nitrogen in sea water. Mar. Chem. I, 231 244. Goulden P. D. & Brooksbank P. (1975) Automated determination of dissolved organic carbon in lake water. Amd. Chem. 47, 1943 1946. Manny B. A., Miller B. C. & Wetzel R. G, (1971). Ultraviolet combustion of dissolved organic nitrogen compounds in lake waters. Limnol. Oceano,qr. 16, 71 85. Menzel D. W. & Vaccaro R. F. (1964) The measurement of dissolved organic and particulate carbon in seawater. Limnol Oceam~yr. 9, 138 142.
Schreurs W. !197,";A An automated colorimetric method lot the determination of dissolved organic carbon m sea \~atcr b\ u.v. dcslruction, ftydrohio/ Bull. 12, 137 142. Sharp ,I. (1973) Total organic carbon in sea water cornparison of measurements using persulphate oxidation and high temperature combustion. Mar. Chem I, 21 I 22 t) Soicr V. (i. & Semcnov A. D. 11971)The photochemical method of determining organic carbon. Gidrokhim. Mater. 56, 111 120: Chem. Ahstr. 7$, 12123 Stainton M. P., Capel M. J. & Armstrong F. A. J. (1974) The chemical analysis of freshwater. Miscellaneous Special Publication No 25, Department of the Environment, Fisheries and Marine Service, Research and Development Directorate, Freshwater Institute. Winnipeg, Manitoba, Canada. Ve~e~a M. (1967) Orqanickc) element&hi analysa. Stfitni nakl. techn, literatury, Praha {Organic elementary analysis). Williams P. J. B. (1969) The wet oxidation of organic matter in seawater. Limnol, Oceanogr. 14, 292-297. Williams P. M. (1969) The determination of dissolved organic carbon in seawater: A comparison of two methods. Limnol. Oceam)w. 14. 297 298.