Environ. Sci. Technol. 1982, 16, 791-796
Chlorination of Estuarine Water: The Occurrence and Magnitude of Carbon Oxidation and Its Impact on Trace-Metal Transport James G. Sanders
Academy of Natural Sciences, Benedict Estuarine Research Laboratory, Benedict, Maryland 206 12 The application of chlorine to estuarine water usually resulted in the oxidation of a finite quantity of organic carbon, averaging 35 pmol/L. The oxidation may be salinity dependent; with one exception, all samples of >5%0 salinity showed a loss of carbon. In every case, the quantity of carbon associated with the smallest molecular weight fraction increased after chlorination, likely because of scission of macromolecules. However, copper distributions within various size fractions of natural organics did not change significantly. The quantity of carbon oxidized is small in comparison to the total complexation capacity of estuarine waters. Therefore, significant shifts in metal speciation should not occur due to loss of organic complexes. Coprecipitation with other metal oxyhydroxides does occur, may be enhanced by chlorination, and can represent a significant transport mechanism.
Introduction The addition of chlorine to estuarine systems via sewage effluent or cooling water discharges from power plants affects the quantity and structure of organic compounds present in natural systems. Although the chemistry of chlorine in seawater is still being unraveled, chlorine degradation is accompanied by the formation of volatile halocarbons (1,2),deamination (3), and oxidation of organic carbon to C02 (4-6). Of these reactions, the latter is by far the most important (5, 6 ) . Naturally occurring organic compounds are of great importance in determining the chemical speciation of trace metals (7,8); thus, the oxidation of carbon by chlorination could have significant impact upon the bioavailability and potential toxicity of trace metals to the biota (9-11). For example, chlorination of Biscayne Bay water significantly reduced its copper complexation capacity (12),suggesting that the organic fraction involved in the complexation of Cu may be the same fraction oxidized during chlorine degradation. Because of this possible impact on trace-metal availability, I have focused on the interactions between chlorination, dissolved organic carbon (DOC), and trace metals in the vicinity of a power plant utilizing estuarine water for cooling purposes. This paper describes the effects of chlorine on the concentration and molecular size of natural organic compounds from this system and its impact upon organic-metal associations. Materials and Methods Sample Sites and Collection. The majority of samples were taken from the Patuxent River, a subestuary of the Chesapeake Bay (Figure 1). Samples were collected between Western Branch and the estuary mouth (Solomons) covering a salinity range of 0-20%. Samples were taken in all four seasons; water temperatures ranged from -1 to 28 "C (Table I). Two additional samples were collected from the Potomac River at Sandy Point (5%0salinity) and Mathias Point (10% salinity) during the spring of 1981. Samples were collected by hand just below the surface in acid-washed 20-L polyethylene carboys. The carboys were rinsed with river water before sample collection. 0013-936X/82/0916-0791$01.25/0
On two occasions, samples were collected from the intake and discharge canals of the Chalk Point Power Station (PEPCO) and surrounding areas. Five replicate 200-mL samples were collected at each site. The samples were brought into the laboratory and stored untreated at 20 OC until used (usually less than 3 days). Before the water was used in an experiment, it was decanted from the carboys and filtered through glass-fiber filters (Gelman GF/C) in acid-washed plasticware. The filtered water was then placed in acid-washed 4-L polypropylene jugs for manipulation. Chlorination, After the samples were placed in the jugs, NaOCl (5% stock solution) was added to various levels (Table 11). The actual concentration of the stock solution was analyzed by amperometric titration (13)before each experiment. Secondary standard solutions were prepared according to the actual concentrations measured. A 30-mL sample was removed from each jug before chlorinating for an initial measurement of dissolved organic carbon (DOC) concentration (see below). The experimental solutions were allowed to sit on a lighted laboratory bench for 10-30 days at 20 "C to allow the chlorine to degrade. Degradation times are listed in Table 11. On two occasions, experiments were sampled for DOC while underway. In all experiments, 30-mL samples were removed at completion for DOC analysis. Duplicate samples were removed at the same time for analysis of total copper concentrations (see below). Ultrafiltration. After the completion of the experiment, the control sample and the sample that received the highest level of chlorination were fractionated by using Amicon ultrafilters (1000-100 000 nominal molecular weight). The ultrafilters were handled and stored according to manufacturer's instructions and cleaned before use as follows: an overnight soak in distilled H20, three 1-h soaks in distilled H20, 1-h soak in 10% HN03, four 30-min rinses in distilled H20, and a final rinse in "carbon-free" (Baker HPLC-grade) H20. The pressure cells were all plastic in construction and were rigorously acid cleaned for each use. The conventional silicone rubber O-rings were replaced with acid-washed viton O-rings. The samples were fractionated in the following manner. A 400-mL sample was placed into the cell containing the ultrafilter with the smallest molecular weight cutoff. Only the filtrate was retained; the concentrate was discarded. The first 20 mL of filtrate was discarded, then triplicate samples for DOC and duplicate samples for Cu analysis were taken. The cells were then dismantled, rinsed with carbon-free water, and reassembled with the next largest filter in place. A new 400-mL sample was placed in the cell and the procedure repeated. This reverse fractionation scheme minimizes a number of problems inherent to sequential fractionations and/or analysis of concentrates, most importantly, metal and organic contamination, membrane polarization, and the attraction between organic molecules in concentrated solutions. Each sample was fractionated once. DOC Analysis. The concentration of dissolved organic carbon (DOC) in various samples was analyzed by using a persulfate oxidation technique similar to Menzel and
0 1982 American Chemical Society
Environ. Sci. Technol., Vol. 16, No. 11, 1982 791
Table I. Location of Water Samples Used for Experimental Manipulations sample
river
location
80-1 80-2 81-3 81-4 81-5 81-6 81-7 81-8 81-9 81-10 81-11 81-12 81-CP1 81-CP2
Patuxent Patuxent Patuxent Potomac Potomac Patuxent Patuxent Patuxent Patuxent Patuxent Patuxent Patuxent Patuxent Patuxent
Sol omons
Table 11. Initial Chlorine Concentrations (in mg/L as Chlorine) and Duration of Experiments chlorination levels, mg/L
duration, days
80-1 80-2 81-3 81-4 81-5 81-6 81-7 81-8 81-9 81-10 81-11 81-12
0,0.5, 1, 5, 10 0, 0.5,1, 5, 10 0,1, 5, 10 0,1, 5, 10 0, 1, 5, 10 0, 10 0, 10 0, 10 0,10 0, 10 0, 10 0. 10
10 14 14 3, 10 1, 2, 3, 5, 10 20 20 20 20 10 10 30
Yo
temp, “C
0
fall winter winter spring spring summer summer summer fall fall fall fall summer fall
Mill Creek Solomons Sandy Point Mathias Point Solomons Broomes Island Benedi ct Western Branch Nottingham Chalk Point Solomons Chalk Pt. intake, discharge Chalk Pt. intake, discharge
sample
salinity,
season
19 0 20 5 10 18 16 12 0 3
4 0 -1
12 12 21 27 28 20 22 24 14 28 13
10
20 12 11 a
+
6.0 1
. -
4.0 5’0
F o’
3.0 .
;,\ a t
a
B 2.0
-
1.0
-
0
1
8
12
16
20
SALINITY, %o
+
WESTERN KM 5-0
-
BROOMES ISLAND
CHESAPEAKE
3 OL MILL CREEK-
BAY
-
SOLOMONS-
0
380
375
I 77
4
Figure 2. DOC concentrations vs. sallnlty in the Patuxent (0)and Potomac (0)Rivers, 1980-1981. The equation for the regression Is Y = -0.089X 4.93; r 2 = 0.18.
t NOTTINGHAM
1 0
78
Figure 1. Sampling locations in the Patuxent River. The inset shows the rlver’s location In the Chesapeake Bay area.
Vaccaro (14). The analyses were performed on an Oceanography International 524C carbon analyzer. Usual precision was less than 4%. Each sample was analyzed in triplicate. If the precision for any group was greater 792 Envlron. Scl. Technoi., Vol. 18, No. 11, 1982
than lo%, the analyses were repeated. Standard carbon samples (0-50 pg of C) were prepared in large batches from oven-dried potassium biphthalate and were analyzed along with each batch of samples. Copper Analysis. The concentration of total dissolved Cu in the various samples was determined after preconcentration (chelation and solvent extraction (15)). Duplicate samples were collected in rigorously cleaned (16) Teflon bottles, acidified with 0.1% “Ultrex” “Os, concentrated, and analyzed by graphite furnace atomic absorption spectrophotometry. Limits of detection were approximately 0.2 pg/L; precision of the analysis at 1pg/L was 10%.
Results DOC in the Patuxent and Potomac Estuaries. Dissolved organic carbon concentrations in the Patuxent River ranged from 2.7 to 6.0 mg/L, with an average of 4.1 mg/L (Table 111). DOC concentrations decreased with increasing salinity, but the relationship was not significant (0.10 > p > 0.05; Figure 2). Significantly higher concentrations occurred during summer and fall when water temperatures were greater than 15 “C (samples 81-6 through 81-11 and 81-CP1). Only two samples were taken in the Potomac River. Concentrations in these samples were similar (Table 111). Molecular Weight Distributions. With the exception of samples 80-1 and 81-12, all samples were fractionated by using ultrafiltration. However, due to equipment
Table 111. Initial DOC Concentrations (in mg/L as Carbon) and Molecular Weight Distributions (as % of Total Carbon That Passes Through the Filter of Interest) in Both Control and Chlorinated Water Samples chlorinated sample, % DOC control sample, % DOC salinity, DOC,
