Mathematical Model for Simulation of the Fate of Copper in a Marine

8 Mathematical Model for Simulation of the Fate of Copper in a Marine Environment GERALD T. ORLOB

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: November 1, 1980 | doi: 10.1021/ba-1980-0189.ch008

University of California, Davis, CA 95616 DAVORIN HROVAT Wayne State University, Detroit, MI 48202 FLORENCE HARRISON Lawrence Livermore Lab, Livermore, CA 94550 A mathematical model is formulated to describe the first -order kinetics of ionic copper released into a marine environ ment where sorption on suspended solids and complexation with dissolved organic matter occur. Reactions are followed in time until equilibrium between the three copper states is achieved within about 3 hr (based on laboratory determina tions of rate and equilibrium constants). The model is demonstrated by simulation of a hypothetical slug dis charge of ionic copper, comparable to an actual accidental release off the California coast that caused an abalone kill. A two-dimensional finite element model, containing the copper submodel, was used to simulate the combined effects of advection, diffusion, and kinetic transformation for 6 hr following discharge of 45 kg of ionic copper. Results are shown graphically. lVTuclear power stations, situated along a coastline and using once^ through cooling for main condensers, may have a significant impact on the marine environment i n the vicinity of the discharge. Because flows are large, roughly 45 m /sec (1588 cfs) for a 1000 M W ( e ) unit, the zone influenced by the discharge plume also may be extensive, a function of the degree of dispersion and mixing with ambient seawater. Depending on the design and operation of such power stations, and on 3

0-8412-0499-3/80/33-189-195$05.00/0 © 1980 American Chemical Society

Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

196

PARTICULATES

IN WATER

the metals and alloys used in heat exchangers, there is a potential hazard to the marine ecosystem from emissions of toxic metals and from the products of their interactions with the indigenous constituents in seawater. One such metal of particular concern, because of its common use in fabrication of condenser tubes and because of its high toxicity to aquatic organisms, is copper. The prospect of copper entering solution as a corrosion product and reaching critical concentration levels, perhaps in the form of the acutely toxic cupric ion C u (2), is enhanced when the flow of coolant water is initiated again through the condensers after a period of shutdown. Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: November 1, 1980 | doi: 10.1021/ba-1980-0189.ch008

+ +

In the summer of 1974, intermittent testing of the cooling water system was performed at the Diablo Canyon Nuclear Power Station, a facility located on the central California coast. In July, dead and dying red and black abalones were discovered in Diablo Cove, the site of discharge of effluent waters from the station (15). This discovery prompted an investigation of copper concentrations in the discharge waters during start-up. During one test, a transient "slug" of copper with a peak of 7700 /xg/L was introduced into the near-shore area after start-up (20). Subsequent to this investigation, the copper-nickel ( 9 0 10 alloy) heat-exchange tubing was replaced by noncorrosive titanium alloy tubing. F i e l d sampling by one of the authors at this same station at start-up in the summer of 1977 revealed concentrations of approxi mately 30 jwg/L of copper which declined gradually in a few hours toward seawater background levels of about 1 /xg/L. The source of this small pulse was copper present in part of an auxiliary pumping system. (Following this incident copper heat-exchange tubing was replaced by noncorrosive titanium alloy tubing.) Laboratory experience indicates that some forms of copper can be toxic to sensitive marine biota in natural seawater at concentrations less than 10 /xg/L (9,14,18). Inasmuch as copper may be complexed w i t h dissolved organic matter (5,19) and sorbed onto the surface of naturally occurring suspended particles ( 3 , 5 ) , it is reasonable to presume that i n these forms it may find alternative pathways through the marine ecosys tem, perhaps reaching toxic levels because of accumulation in indigenous biota. Given the acute toxicity of some forms of copper and the prospects for exceeding toxic levels in the discharge from power stations, there is a compelling need to mitigate damage through improved design and/or operation procedures. The research reported here is aimed at developing a methodology to predict the fate of transient copper emissions i n the vicinity of power station discharges and to evaluate mitigation measures. Specific attention is given to the several forms of copper and the kinetic relationships that determine the relative concentrations of each within the zone of potentially damaging impact.


8.

ORLOB E T A L .


Chemical

Fate of Copper in a Marine Environment

197

Considerations

The physiochemical forms of copper that are present in seawater may be numerous. Copper released into the water column is partitioned between the soluble and particulate fractions. The form in the soluble fraction is related to the inorganic and organic constituents in the water; the form in the particulate fraction is related to the kinds of sinks present on the particles. The copper species in the soluble fraction has been classified as "labile" or "bound" (6). Definition of the two groups is determined by the experimental conditions under which the measurement is made. In this report we shall refer to the following as labile species: ions, ion pairs, readily dissociable (labile) inorganic and organic complexes, and easily exchangeable copper adsorbed on either colloidal inorganic or organic matter. Inorganic anions to which the copper may be complexed are hydroxides, carbonates, chlorides, sulfates, phosphates, and nitrates; organic anions are amino acids, amino sugars, alcohol, urea, etc. Equilib rium between these forms would be established very rapidly—so fast, i n fact, that differentiation of specific forms is not possible with conventional analytical techniques. Bound species include stable metal-organic complexes, metals bound to high molecular weight organic material, some inorganic complexes, and metals occluded in, or sorbed tightly on, highly dispersed colloids. In cluded in this group of bound species is copper complexed to humic substances, the refractory organic material that comprises approximately 90% of organic material in water. Laboratory experience in evaluating the partitioning of ionic copper injected into natural seawater, containing dissolved organic matter ( D O M ) and suspended particles (SS), indicates that the complexing to D O M and the sorption of copper to particles from labile forms occur at rates much slower (relatively) than those of the pure chemical reactions between free ionic copper and inorganic constituents (5,10). Designating labile copper by C u , the characteristic equilibria with D O M and SS are given by ++

Cu

+ +

+ D O M i Cu ki 2

+ +

• DOM

(1)

2

and

(2) where C u • D O M is the copper complexed with high molecular weight D O M naturally occurring in seawater, C u • SS the copper sorbed on the + +

+ +


198

PARTICULATES

IN WATER

surface of SS, and the fc's the kinetic coefficients governing the rates of reaction. The possibility also exists, at least conceptually, for exchange of copper between the compartments C u • D O M and C u • SS such that + +

Cu

+ +

• D O M + SS fe C u

+ +

+ +

• SS + D O M

(3)

fc 2


3

The conceptual model for disposition of copper among these three compartments is illustrated i n Figure 1. (Note: F o r the present the model is restricted to these three compartments, although it is recognized that it w i l l be necessary ultimately to include interactions with additional compartments such as the sediment bed and indigenous biota, and perhaps others.) In such a system as illustrated i n Figure 1, w i t h a given complexing capacity determined by the concentration of D O M and a sorption capacity determined by the concentration of SS, equilibrium eventually w i l l be reached so that n

C = ]C Ci,eq i=l s

Figure 1.

Schematic of the dynamics of copper


(4)

8.

ORLOB E T A L .

Fate of Copper in a Marine Environment

199

where C is the total concentration of copper in n compartments (three in our case) and C is the equilibrium (steady state) concentration i n compartment i. The distribution of copper between compartments is described by a distribution coefficient K that can be evaluated experimentally: s

ije(l


d ) i

where V is the volume of solution and Mi is the mass of material with which copper is transferred to compartment i by sorption or complexation. The relative proportion of copper i n the fth compartment to the copper contained in the others is given by

It follows that for D O M and SS, the particular compartments we are considering i n addition to copper i n the ionic state, the corresponding /?'s, are

£ = #d,2XDOM 2

(7)

1% — # d , X S S 3

where D O M and SS are ambient concentrations. The rate of sorption of copper to suspended particles was determined for samples collected at the discharge area of a coastal power station ( 5 ) . Steady-state conditions of sorption were approached within 10 hr after spiking with ionic copper using C u as a tracer. Distribution coefficients range from 11,000 to 52,000 and k values from 0.2 to 0.8 hr" . There was some evidence from the data that part of the copper was sorbed in a very short period (less than about 10 m i n ) , while the remainder was sorbed onto the particles at an exponential rate over the next 10 hr as steadystate conditions were approached. However, for present purposes, con sidering sorption on particles as a single compartment, the transfer of copper from labile forms to those sorbed on suspended particles is fairly well represented as a first-order process i n which k approximately equals 0.75 hr" . Additions of organics, particularly i n the form of high molecular weight D O M naturally occurring i n seawater (5), substantially reduced the distribution of copper; the ionic and suspended sediment compart ments indicate the high complexing capacity of these substances. Natu rally occurring D O M in water discharged from coastal power plants gave 6 4

1

13

1


200

PARTICULATES

IN WATER

K values ranging from 100,000 to 600,000 m L / g at D O M levels of 1 to 2 m g / L (see Appendix I for sample calculations). N o information is available on the number of binding sites per molecule for the D O M present, but it appears that the sites were more than adequate for complexing the copper. The limited data on the kinetics of complexation indicate that the reactions can be described by first-order kinetics if the values fc are in the range of 0.5 to 1.0 hr" (10). The concentration of D O M must be much greater than that of copper because of the limited number of sites per molecule of D O M (molecular weight = 10,000 to 100,000, or more). This is true in our case where we are dealing with copper concentrations of a few /xg/L (less than 40 /xg/L), and D O M concentrations of 1000-3000 /xg/L. The assumption of firstorder kinetics appears to be a reasonable approximation of actual behavior in such a case. d i 2

1


]2

Mathematical Considerations If for conditions in which the number of copper atoms is very much smaller than the number of binding sites on SS or D O M , that is, where the rates of complexation and sorption are independent of the concen trations of complexing or sorption agent, then the rates of reaction can be characterized mathematically as pseudo first order. Accordingly, the physiochemical transformations are described by the following rate equations

k

12

C i + fc i C - fc„ d + k 2

2

(8)

C

n

3

dt dC = —k C + k dt 2

21

6C; TT^ -

dt

- fc

3i

2

12

Ci — k C + k C

(T + fcis c

2S

3

1

- fc

32

2

Cs + k

32

23

(9)

s

(10)

C

2

where C = C — C , i = 1, 2, 3. It is noted that the processes of desorption, decomplexing, and exchange between the D O M ( C ) and SS ( C ) compartments are included for completeness, although the kinetics of these changes in state are not yet fully evaluated. The general statement of these equations is of the form {

t

i>eq

2

3

*

Mathematical Model for Simulation of the Fate of Copper in a Marine

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