Environ. Sci. Technol. 1985, 19, 1188-1 191
Rapid Consumption of Bromine Oxidants in River and Estuarine Waters Donald A. Jaworsket and George R. Helz"
Department of Chemistry, University of Maryland, College Park, Maryland 20742 Fluvial and estuarine waters possess a substantial reductive capacity that rapidly destroys strong oxidants introduced from natural or anthropogenicsources. The rapid reduction of bromine oxidants, which in marine waters are intermediates in the decomposition of both chlorine and ozone, has been studied electrochemically in the field and the laboratory with a rotating ring disk electrode. Patuxent estuary water was found to contain about M of substances that reacted extremely rapidly with bromine. Estimated second-order rate constants were on the order of lo' M-l s-l or higher. Bromine consumption did not correlate with salinity, nor was it altered by addition of M ethylenediaminetetraacetic acid or by ultrafiltration. However, destruction of organic matter by UV photolysis eliminated bromine consumption on the lo-, s time scale of the experiments. Likewise, the bromineconsuming components could be titrated away with HOBr, indicating that they are not catalysts. Commercially available humic acid behaved qualitatively in a similar fashion to the natural reductants. Introduction
Chlorination or ozonation of coastal waters produces bromine and its hydrolysis products as daughter oxidants (1-5). Various bromo carbons are generated subsequently (6-8). When typical doses on the order of M are applied and when mixing is rapid, a significant fraction of the oxidant dose disappears from saline water too rapidly to be observed by conventional analytical techniques (8). This is also true when fulvic acid solutions are treated with chlorine in the laboratory (9). Since it is important to understand the oxidant decay mechanisms that give rise to undesirable products, such as bromocarbons, we explored a rotating ring disk electrode method for studying extremely rapid reactions of bromine (10). With this method, Br, is generated electrochemically at a rotating platinum disk and detected electrochemically at an annular platinum ring. Any chemical reactions that consume bromine during its passage between the disk and ring are detected by a diminished signal at the ring. This paper contains two parts. We will first present some shipboard rotating ring disk electrode (RRDE) measurements that characterize the fast bromine demand in the Patuxent Estuary, MD. Then we will describe some laboratory experiments that shed light on the nature of the components responsible for fast bromine demand. Materials and Methods
The rotating ring disk electrode used in this work (Pine Instrument Co.) consists of a 2.55-mm platinum disk surrounded concentrically by a 0.48 mm wide platinum ring. The ring is separated from the disk by a 1.21 mm wide insulator gap. When the electrode is inserted into a beaker of solution and rotated at a constant rate, liquid is drawn vertically up toward the disk and then spun radially out across the ring surface (11-13). Sample preparation for the rotating ring disk electrode (RRDE) experiments was minimal. All samples, were run at ambient pH (usually around 7.5), except where spe-
-'Present address: NASA Lewis Research Center, Cleveland, OH 44135. 1188
Envlron. Sci. Technol., Vol. 19, No. 12, 1985
cifically mentioned to the contrary. Enough granular NaBr was added to the samples to bring the Br- concentration to 0.1 M. All of the samples were run at ambient temperature (25 f 4 "C), except where noted. In the shipboard experiments, the time between collection of a sample and initiation of a measurement was about 3 min. The measurement procedure consisted of applying a potential to the disk electrode to generate Br2 from the Br- initially added to the sample. During the experiment, the disk potential was gradually shifted, under control of an Apple I1 computer, to steadily increase the anodic current and thus the rate of Br, production. At the ring eIectrode, a fixed potential (+0.4 V vs. colomel) was set to reduce back to Br- all the oxidized bromine that had not been consumed by the sample in transit from disk to ring. Therefore, the cathodic current at the ring provided a measure of unreacted bromine. The transit time of solution from the disk to the ring was controlled by the rotational velocity of the electrode and could be varied in the range lo-, to 1 s. The experiments reported in this paper were performed with a rotation rate of 3600 rpm, at which the transit time was about 35 ms. Scan time for an experiment was about 2 min. Details of the electrochemical instrumentation and software can be found elsewhere (10, 14). Field measurements were made on the Patuxent Estuary, located 50-100 km SE of Washington, DC. Surface samples were collected in a polyethylene bucket, while near bottom waters were pumped onboard through polyethylene tubing. Temperature, salinity, pH, and dissolved oxygen (membrane probe) were also measured onboard. Ammonia [Solorzano method (15)] and dissolved organic carbon [Menzel and Vacarro method (16); Oceanography International Analyzer] were determined later on filtered samples. Results and Discussion
Field Data. Figure 1 shows some representative, titration-like data obtained on shipboard. Whereas the ring current always increased linearly with disk current in blanks (i.e., 0.1 M NaBr made with distilled water), there was initially no change in ring current with increasing disk current in natural waters. Only after a threshold was reached did ring current begin to rise. These results imply that the natural waters contain components which consume all the bromine generated at low disk currents. When the bromine generation rate (proportional to disk current) exceeded the delivery rate of reducing agents to the electrode (proportional to the rotational velocity of the disk), it became possible for the ring electrode to sense bromine. From data such as shown in Figure 1, it is possible to calculate both the concentration of bromine-consuming components and the observed second-order rate constant for the reaction of bromine with these components (10, 11, 17). In Table I, values for the concentration of bromineconsuming components, designated C,, are reported for the suite of Patuxent estuary samples. These values are means of triplicate determinations; individual measurements deviated from the mean by less than 5%. The surface and bottom water samples have been separated in this table. The bottom water samples all came from the southern part
0013-936X/85/0919-1188$01.50/0
0 1985 American Chemical Society
3 0 r 7 1 20
I5t
SURFACE
0
5
IO 15 SALINITY (g/kgi
20
Figure 2. Concentrationof fast bromine-consuming components vs. salinity in the Patuxent estuary. Squares denote surface waters, and triangles denote bottom waters, which were anoxic. All data were obtained at 0.1 M Br-, ambient pH (see Table I), and 3600 rpm. Each point is an average of triplicate measurements.
,*
.::t,-
,' I
1
-
-"0
0
5 IO Salinity (g/kg)
15
20
Flgure 3. Observed second-order rate constant vs. salinity in the Patuxent estuary. The same conditions as in Figure 2 apply. Measurement uncertainty is depicted by the vertical bar.
Table I. Bromine Demand, Cb, and Water Composition in the Patuxent Estuary, July 21, 1983
Cb:
pM
T,"C salinity, g/kg pH
11.5 10.6 12.5 14.2 11.3 11.6 13.5 12.0 13.6 15.0 13.4
30.4 30.1 30.2 34.7 30.7 29.2 29.9 29.1 29.2 28.8 29.0
Surface Waters 0.4 6.60 0.7 6.45 1.9 6.50 7.3 6.68 6.9 7.20 8.5 7.21 8.9 7.71 8.9 7.04 9.4 7.68 10.9 7.59 12.5 8.18
20.1 23.3 23.6
24.8 24.4 24.9
Bottom WatersC 13.4 7.30 15.8 6.65 16.5 7.25
NH3, pM
DOC,* pM
0.2 4.4
