Reflections on my career as a marine physical chemist, and tales of the deep.

This is a personal review of how one can apply the principles of physical chemistry to study the ocean and other natural waters. Physical chemistry is the study of chemical thermodynamics, kinetics, and molecular structure. My long-term interest in the chemistry of seawater is an extension of my early work on water and the interactions that occur in aqueous electrolyte solutions, which I began as part of my PhD research on the thermodynamics of organic acids in water. Over the years, I have attempted to apply the tools of physical chemistry to elucidate the structures of seawater, brines, lakes, and rivers. I have developed and continue to work on ionic interaction models that can be applied to all natural waters. Here, I reflect on how my students, postdocs, research assistants, and scientific colleagues have influenced my life, my career, and the field of marine physical chemistry. My hope was and is to use these tools to understand the molecular structures of natural waters.

Dissociation constants of protonated methionine species in NaCl media.

The sulfur-containing amino acid, methionine, has a role in the physiological environment because of its strong interactions with metals. To understand these interactions of metals with methionine, one needs reliable dissociation constants for the protonated methionine species (NH(3)(+)CH(CH(2)CH(2)SCH(3))COOH; H(2)B(+)). The values of stoichiometric dissociation constants, pK(i)*, for protonated methionine species (H(2)B(+) if H(+)+HB, K(1); HB if H(+)+B(-), K(2)) were determined from potentiometric measurements in NaCl solutions as a function of ionic strength, 0.25-6.0 mol (kg H(2)O)(-1) and temperature (5-45 degrees C). The results were extrapolated to pure water using the Pitzer equations to estimate the activity of H(+), H(2)B(+), HB and B(-) as a function of ionic strength and temperature. The resulting thermodynamic values of K(1) and K(2) were fit to the equations (T/K): ln K(1)=69.0013-3496.58/(T/K)-10.9153 ln (T/K); ln K(2)=116.4162-10638.02/(T/K)-18.0553 ln (T/K) with standard errors of 0.003 and 0.033, respectively, for ln K(1)* and ln K(2)*. Pitzer interaction parameters (lambda(HB-Na) and zeta(HB-Na-Cl)) for the neutral HB were determined from literature data. The Pitzer parameters (beta(0)(H(2)BCl), beta(1)(H(2)BCl) and C(phi)(H(2)BCl)) for the interactions of H(2)B(+) with Cl(-) and Na(+) with and B(2-) (beta(0)(NaB), beta(1)(NaB) and C(phi)(NaB)) were also determined. These coefficients can be used to make reasonable estimates of the activity coefficients of methionine species and the pK(i)(*) for the dissociation of methionine in physiological solutions, composed mostly of NaCl over a wide range of temperature and ionic strength.

Adsorption of arsenate and arsenite on titanium dioxide suspensions.

Adsorption of arsenate (As(V)) and arsenite (As(III)) to two commercially available titanium dioxide (TiO2) suspensions, Hombikat UV100 and Degussa P25, was investigated as a function of pH and initial concentration of adsorbate ions. The BET surface area and zeta potential values of TiO2 were also measured to understand the difference in adsorption behavior of two suspensions. Both As(V) and As(III) adsorb more onto Hombikat UV100 particles than Degussa P25 particles. Adsorption of As(V) onto TiO2 suspensions was more than As(III) at pH 4 while the adsorption capacity of As(III) was more at pH 9. The electrostatic factors between surface charge of TiO2 particles and arsenic species were used to explain adsorption behavior of As(V) and As(III) at different pH. The Langmuir and Freundlich isotherm equations were used to interpret the nature of adsorption of arsenic onto TiO2 suspensions. The usefulness of adsorption data in removing arsenic in water is briefly discussed.

The distribution of trace metals in Florida Bay sediments.

The distribution of trace metals based on surface sediments collected at 40 stations across Florida Bay was done in June, November and February 2000-2001. Concentrations of Sc, V, Ba, Cd, Cr, Co, Cu, Pb, Mn, Ni, Zn, Al and Mg were determined by ICP-MS, and the total Fe was determined by spectrophotometry. Organic carbon (OC), nitrogen (N), and calcium carbonate (CaCO3) were also measured. Eleven of 13 metals showed a similar distribution pattern for the various months studied. Maximum concentrations of metals were lower than those found in most estuarine systems and were concentrated in the north-central and western zones of the Bay. The Mn and Fe concentrations, unlike the other metals, gradually decreased from north (Everglades) to south (Florida Keys). Some metals (Ni, Zn, Cu, Cr, Pb and Ba) associated with petroleum use showed high concentrations at stations near the Tavernier marina. Florida Bay sediments are predominately CaCO3 (65.9-92.5%). The greatest value for OC (5.5%) and the lowest value of CaCO3 (65.9%) were found in the western zone. Trace metal distribution patterns are similar to the OC and N in the sediments. There was a strong correlation between most metals (V>Cu>Ni>Cr>Al>Co>Ba>Zn>Pb>Mg) and the percentage of OC. The maximum C/N values (9-12) were observed at the stations with the highest OC, where dense colonies of seagrass are found and most of the metals are concentrated. All metals except Mg, Mn and Co showed a strong correlation with Al and the fine fraction of the sediments (aluminosilicates) associated with continental input and river runoff.

Mechanisms of oxidation of organosulfur compounds by ferrate(VI).

The oxidation of organosulfur compounds requires the transfer of oxygen atoms and is important in decontamination of chemical warfare agents, desulfurization of fossil fuel for high quality, deodorization of wastewater and sludge, and remediation of industrial effluents. The kinetics of the oxidation of organosulfur compounds (sulfur-containing amino acids, aliphatic and aromatic thiols, and mercaptans) by the environmentally-friendly oxidant, ferrate(VI) FeO(4)(2-), was quantitatively examined in this study using a kinetic model considering possible reactions among the species of ferrate(VI) and organosulfur compounds. The ratios of ferrate(VI) to the various organosulfur compounds for the one oxygen-atom transfer were 0.50 and 0.67 for Fe(II) and Fe(III) as final products, respectively. The second-order rate constants for the oxidation of organosulfur compounds by protonated ferrate(VI) HFeO(4)(-) ion were correlated with thermodynamic 1-e(-) and 2-e(-) reduction potentials in order to understand the mechanisms of the reactions. The oxidation of the compounds involved a 1-e(-) transfer step from Fe(VI) to Fe(V), followed by 2-e(-) transfer to Fe(III) as the reduced product (Fe(VI)→Fe(V)→Fe(III)). The 2-e(-) transfer steps resulted in the formation of Fe(II) (Fe(VI)→Fe(IV)→Fe(II)). Conclusions drawn from the correlations are consistent with the experimentally determined stoichiometries and products of the reactions. The calculated half-lives for the oxidation were in the range of ms to s at a dose of 10mg K(2)FeO(4)L(-1) and hence ferrate(VI) has a great potential in treating organosulfur compounds present in water and wastewater.

Partial molal volumes and compressibilities of phosphoric acid and sodium phosphates in 0.725 molal NaCl at 25 °C.

The apparent molal volume V(ϕ) and adiabatic compressibility κ(ϕ) of H(3)PO(4), NaH(2)PO(4), Na(2)HPO(4), and Na(3)PO(4) have been determined from density and sound speed measurements in 0.725 m NaCl solutions at 25 °C. The partial molal volumes V(i)* and compressibilities κ(i)* have been determined by extrapolating the values of V(ϕ) and κ(ϕ) to infinite dilution in the NaCl media. The results have been compared to earlier measurements made on a number of electrolytes in NaCl media. The phosphate results showed correlations between the values of V(i)* and κ(i)* as found for other electrolytes. The values for the volume ΔV(i) and compressibility Δκ(i) changes for the dissociation of H(3)PO(4) in 0.725 m NaCl have been calculated from the measurements. The calculated values of ΔV(i) and compressibility Δκ(i) have been used to estimate the effect of pressure K(P)/K(0) on the dissociation of H(3)PO(4) in the oceans using RT ln[K(P)/K(0)] = -ΔV(i)P + 0.5Δκ(i)P(2), where P is the applied pressure in bars. The results are in reasonable agreement with directly measured values.

The hydrolysis of Al(III) in NaCl solutions--a model for Fe(III).

There is a lot of interest in the behavior of Fe(III) in natural waters because of its importance in the productivity of phytoplankton. The hydrolysis of Fe(III) in natural waters limits the solubility of Fe(III) in aqueous solutions. At the present time, reliable hydrolysis constants for Fe(III) are limited to low temperatures (5-50 degrees C) in NaCl to 5 m. The hydrolysis constants of Fe(III) and Al(III) are linearly related over a wide range of temperatures (0-50 degrees C) and ionic strengths (0-5 m NaCl). The near linear correlations allow one to make reasonable estimates for the values of Fe(III) from 0 to 300 degrees C in dilute solutions and from 0 to 100 degrees C to 5 m in NaCl solutions. In this paper, the stoichiometric (beta(i)) and thermodynamic (K(i)) hydrolysis constants for Al(III) in NaCl have been fit to equations of the form log beta(i) - log K(i) = a0I(0.5) + a1I(0.5)/T + a3I + a4I/T + a5I2. This equation has been used to estimate the hydrolysis constant for Fe(III) in dilute solutions to 300 degreesC and in NaCl solutions to 5 m and 100 degrees C. The cause of the correlation in water is due to the differences in the free energies of deltaG0(Al3+) - deltaG0 (Al(OH)j(3-j)) being almost equal to the values of deltaG0(Fe3+) - deltaG0(Fe(OH)(3-j)). An examination of the activity coefficients of Fe3+ and Al3 and the complexes show that the correlation at higher ionic strengths is due to the fact that the activity coefficients ratios are similar gamma(Fe3+)/gamma(Fe(OH)j(3-j))approximately equal to gamma(Al3+)/gamma(Al(OH)j(3-j) in NaCI solutions. These linear correlations appear to also hold for other trivalent metals (Cr3, As3+). The results of this study should be useful in examining the speciation of Fe(III) in hydrothermal brines.

Stability constants for the formation of lead chloride complexes as a function of temperature and ionic strength.

The stability constants for the formation of lead (Pb2+) with chloride [Formula: see text] have been determined using a spectrophotometric method in NaClO4 solutions as a function of ionic strength (0-6 m) and temperature (15-45 °C). The results have been fitted to the equations: [Formula: see text] with standard errors of 0.05, 0.04 and 0.06, respectively. The thermodynamic values of log ß1, logß2 and logß3 at 25.0 °C and the enthalpies of formation of PbCl+, PbCl20 and PbCl3- are in good agreement with literature values. We have combined our results with the earlier work of Seward (1984) to yield thermodynamic constants that are valid from 15 to 300 °C: [Formula: see text] with standard errors of 0.05, 0.08 and 0.10, respectively.

Oxidation of nanomolar levels of Fe(II) with oxygen in natural waters.

The oxidation of Fe(II) by molecular oxygen at nanomolar levels has been studied using a UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5-8.2), NaHCO3 (0.1-9 mM), temperature (3-35 degrees C), and salinity (0-36) on the oxidation of Fe(II) are presented. The first-order oxidation rates at nanomolar Fe(II) are higher than the values at micromolar levels at a pH below 7.5 and lower than the values at a higher pH. A kinetic model has been developed to consider the mechanism of the Fe(II) oxidation and the speciation of Fe(II) in seawater, the interactions between the major ions, and the oxidation rates of the different Fe(II) species. The concentration of Fe(II) is largely controlled by oxidation with O2 and O2.- but is also affected by hydrogen peroxide that may be both initially present and formed from the oxidation of Fe(II) by superoxide. The model has been applied to describe the effect of pH, concentration of NaHCO3, temperature, and salinity on the kinetics of Fe(II) oxidation. At a pH over 7.2, Fe(OH)2 is the most important contributing species to the apparent oxidation rate. At high levels of CO3(2-) and pH, the Fe(CO3)2(2-) species become important. At pH values below 7, the oxidation rate is controlled by Fe2+. Using the model, log k(i) values for the most kinetically active species (Fe2+, Fe(OH)+, Fe(OH)2, Fe(CO3), and Fe(CO3)2(2-)) are given that are valid over a wide range of temperature, salinity, and pH in natural waters. Model results showthatwhen H2O2 concentrations approach the Fe(II) concentrations used in this study, the oxidation of Fe(II) with H2O2 also needs to be considered.

Reduction of hexavalent chromium by H2O2 in acidic solutions.

The rates of the reduction of Cr(VI) with H2O2 were measured in NaCl solutions as a function of pH (1.5-4.8), temperature (5-40 degrees C), and ionic strength (I = 0.01-2 M) in the presence of an excess of reductant. The rate of Cr(VI) reduction is described by the general expression -d[Cr(VI)]/dt = k2[Cr(VI)](m)[H2O2](n)[H+](z), where m = 1 and n and z are two interdependent variables. The value of n is a function of pH between 2 and 4 (n = (3 x 10(a))/(1 + 10(a)), where a = -0.25 - 0.58pH + 0.26pH2) leveling off at pH < 2 (where n approximately = 1) and pH > 4 (where n approximately = 3). The rates of Cr(VI) reduction are acid-catalyzed, and the kinetic order z varies from about 1.8-0.5 with increasing H2O2 concentration, according to the equation z = 1.85 - 350.1H2O2 (M) which is valid for [H2O2] < 0.004 M. The values of k2 (M(-(n+z)) min(-1)) are given by k2 = k/[H+](z) = k1/[H2O2](n)[H+](z), where k is the overall rate constant (M(-n) min(-1)) and k, is the pseudo-first-order rate constant (min(-1)). The values of k in the pH range 2-4 have been fitted to the equation log k = 2.14pH - 2.81 with sigma = +/- 0.18. The values of k2 are dependent on pH as well. Most of the results with H2O2 < 3 mM are described by log k2 = 2.87pH - 0.55 with sigma = +/- 0.54. Experimental results suggest that the reduction of Cr(VI) to Cr(III) is controlled by the formation of Cr(V) intermediates. Values of k2 and k calculated from the above equations can be used to evaluate the rates of the reaction in acidic solutions under a wide range of experimental conditions, because the rates are independent of ionic strength, temperature, major ions, and micromolar levels of trace metals (Cu2+, Ni2+, Pb2+). The application of this rate law to environmental conditions suggests that this reaction may have a role in acidic solutions (aerosols and fog droplets) in the presence of high micromolar concentrations of H2O2.

Impact of anthropogenic CO2 on the CaCO3 system in the oceans.

Rising atmospheric carbon dioxide (CO2) concentrations over the past two centuries have led to greater CO2 uptake by the oceans. This acidification process has changed the saturation state of the oceans with respect to calcium carbonate (CaCO3) particles. Here we estimate the in situ CaCO3 dissolution rates for the global oceans from total alkalinity and chlorofluorocarbon data, and we also discuss the future impacts of anthropogenic CO2 on CaCO3 shell-forming species. CaCO3 dissolution rates, ranging from 0.003 to 1.2 micromoles per kilogram per year, are observed beginning near the aragonite saturation horizon. The total water column CaCO3 dissolution rate for the global oceans is approximately 0.5 +/- 0.2 petagrams of CaCO3-C per year, which is approximately 45 to 65% of the export production of CaCO3.

The oceanic sink for anthropogenic CO2.

Using inorganic carbon measurements from an international survey effort in the 1990s and a tracer-based separation technique, we estimate a global oceanic anthropogenic carbon dioxide (CO2) sink for the period from 1800 to 1994 of 118 +/- 19 petagrams of carbon. The oceanic sink accounts for approximately 48% of the total fossil-fuel and cement-manufacturing emissions, implying that the terrestrial biosphere was a net source of CO2 to the atmosphere of about 39 +/- 28 petagrams of carbon for this period. The current fraction of total anthropogenic CO2 emissions stored in the ocean appears to be about one-third of the long-term potential.

Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.