Geochemical Behavior and Distribution of Copper and Iron in Marine Waters

Author

James Leo Symes III, University of Rhode Island

Date of Award

1983

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Dana R. Kester

Abstract

This thesis consists of three studies related to trace-metal marine geochemistry. The first investigation deals with the distribution of iron in the Northwest Atlantic. The second portion is a study comprising a series of physicochemical laboratory measurements aimed at characterization of the heterogeneous equilibrium of malachite (Cu₂(OH)₂CO₃). This work is presented in two reports. The first deals with the experimental results in terms of equilibrium constants and related thermodynamic parameters while the second addresses the geochemical significance of these findings. The last portion of the thesis research is an investigation of copper(II) interaction with carbonate species with emphasis on the inorganic speciation of copper in seawater.

Trace-metal samples from eleven stations in the Sargasso Sea, Slope Water, and continental shelf water of the North West Atlantic were analyzed for total dissolvable iron by atomic-absorption spectrophotometry. Vertical profiles for iron and associated temperature and nutrient data were obtained at five open-ocean stations. Data from six stations provided a transect across the continental shelf into the apex of the New York Bight.

At open-ocean stations, iron was depleted near the surface to levels of about 1-2 nmol/kg and increased to levels of about 4-7 nmol/kg in the vicinity of the oxygen minimum. A characteristic bottom-water maximum was observed at stations where closely-spaced near-bottom sampling was conducted. This feature was at least partially due to resuspension of particulate iron associated with the nepheloid layer.

Comparison of total dissolvable iron concentrations to those of filtered samples at one Slope Water station indicated that 38% of the available iron in the water column was associated with the particulate phase. Near the surface, in the vicinity of the chlorophyll maximum, iron was present almost totally in particulate form. The dissolved fraction increased to approximately 50% at the nutrient maximum and approached 100% in the mid-portion of the water column. In deeper waters, the particulate fraction again became important, increasing to approximately 80% of the total iron in the near-bottom maximum.

In the apex of the New York Bight, iron levels in excess of 200 and 500 nmol/kg were observed in surface waters and bottom waters, respectively. In the mid-continental shelf region, the iron distribution does not appear to be greatly influenced by coastal sources; within 65 km of the coastline, iron levels approached open-ocean values. The thermodynamics of the dissolution of malachite was studied for two primary reasons: (1) to determine the chemical conditions necessary to permit inorganic precipitation of copper in seawater and (2) to provide an assessment of copper(II) ion activity and complexation by comparison of malachite equilibrium constants measured in various media. A thermodynamic equilibrium constant, K_sp, was determined at 25°C, 1 atm. (K_sp = (3.47 ± 0.56) x 10^-34). A stoichiometric equilibrium constant, K_sp*, was determined at 0.72 ionic strength (K_sp* = (1.00 ± 0.16) x 10-32). An apparent solubility product, K_sp', was determined in 36.9% 0 salinity seawater (K_sp' = (1.3 ± 0.14) x 10-28). The free energy change for malachite dissolution at 25°C, 1 atm is 45.64 ± 0.07 Kc al/mole and the free energy formation of malachite, ΔG_f, is -215.99 ± 0.07 Kcal/mole. The temperature dependence of a "mixed" equilibrium constant, K_sp^t was measured at .0. 72 ionic strength in 5° increments over the range 5-25°C. The linearity of log K_sp^t versus T^-1 suggests that 6H for the malachite dissolution reaction remains constant between 5-25°C and follows the relationship: log K_sp^t = (9.80 ± 0.03) x 10³ T^-1 + (1.52 ± 0.09). The effect of pressure on the solubility of malachite in water and seawater was estimated from parti al molal volume and compressibility data. For 25°C at infinite dilution K_sp(1900bar)/K_sp(O) = 240 and in seawater K_sp'(l000)/K8p'(O) • 44. A comparison of K_sp* and K_sp' indicates that in seawater (S = 37 ⁰/₀₀ TA = 2.3 meq/kg H₂O, pH = 8.1) free copper(II) ion comprises 3.1% of the total dissolved inorganic copper.

The concentration of copper in equilibrium with malachite, Cu₂(OH)₂CO₃, was calculated for open-ocean seawater. The effects of temperature and pressure along with alkalinity and pH induced variation in copper speciation were considered in determining Ksp’, the apparent solubility product for the malachite equilibrium in seawater. Saturation profiles for total dissolved inorganic copper, T(Cu), were calculated for a North Atlantic and a North Pacific station. T(Cu) levels of about 600 nmol/kg in surface waters to 400 nmol/kg in deep waters are required to reach saturation conditions. Corresponding saturation levels of free copper(II) ion range from 10-60 nmol/kg. The results indicate that copper is at least two orders of magnitude undersaturated in the open ocean and that the occurrence of suspended malachite particles cannot be explain ed by direct precipitation from bulk seawater.

Equilibrium constants for copper(II)-carbonate and bicarbonate species were determined at 25°C from consideration of malachite solubility in solutions of 0.72 m ionic strength. The ratios of total dissolved copper, T(Cu) to free copper(II) ion, [Cu²⁺], in 30 malachite saturated experimental solutions of 1-10 x 10^-3 equiv/kg H₂O initial alkalinity (TA_i) in the pH range 5.0-9.3 were fitted to a copper(II) ion speciation model. The experimental data indicate the existence of CuCO₃°, CuHCO₃₊ and Cu(OH)CO₃- in addition to the hydrolysis products in the range of conditions defined by this study. The stoichiometric equilibrium constants at 25°C and 1 atm are: ^βCuCO₃ = (1.59 ± 0.03) x 10⁶, ^βCuHCO₃ = (2.3 ± 0.3) x 102, and ^*βCu(OH)CO₃ = (7.6 ± 0.3) X 10^-4.

A speciation model employing the constants determined in this study and copper(II) hydrolysis constants from previous work suggest that the inorganic speciation in seawater (pH = 8.2, TA = 2.3 mequiv/kg, 25°C) is dominated by the CuCO₃° complex (82%) and that only 2.9% of the total inorganic copper exists as free copper(II) ion. Hydrolysis products, CuOH⁺ and Cu(OH)₂°, account for 6.5% while CuHCO₃⁺ and Cu(OH)CO₃^- species comprise 1.0 and 6.3% of the total inorganic copper, respectively.

Recommended Citation

Symes, James Leo III, "Geochemical Behavior and Distribution of Copper and Iron in Marine Waters" (1983). Open Access Dissertations. Paper 1343.
https://digitalcommons.uri.edu/oa_diss/1343