Important unresolved problems in marine chemistry

Applied Geochemistry, Vol. 3, pp. 61~52, 1988

0883-2927/88 $3.00 + .00 Pergamon Press plc

Printed in Great Britain

Important unresolved problems in marine chemistry MICHAEL R. HOFFMANN W. M. Keck Laboratories, California Institute of Technology, Pasadena, CA 91125, U.S.A.

SEVERALresearch problems come to mind that would fall into the category of unresolved problems in marine chemistry. The chemistry and biochemistry of deep sea thermal vents and near-shore submarine vents are intriguing problem areas that are currently the focus of great attention at the empirical level. Hydrothermal vents are rich i n H2S (0.2-8 raM), H 2 (0.05-1.7 mM), Fe(II) (0.05-2.4 mM), Mn(II) (0.021.0 mM), $8, and total CO2 (2.3-5.7 mM). The detailed dynamic chemistry to these environments are not well understood. What are the rates of chemical transformation in the range of 100-300°C? What are the rates and mechanisms of oxidation at the anoxic/oxic interface in hydrothermal plumes? What role do micro-organisms play in the catalysis of selected transformations, such as the oxidation of H2S to SO]-? Another intriguing problem involves

the possible reduction of Fe(II) to Fe ° in the hightemperature, high-pressure regime of the thermal vents. We see that overall reaction at pH 7 under conditions of STP 1 S (g) + ~ lEe2+ (aq) ~ ~H2

1 o(s) + ~Fe ~ 0(s) ~S + H+Ape°(w) = - 2 . 7 9

is thermodynamically unfavorable (i.e. vG/n = - 2 . 3 vpr°(w)). However, we see that at the elevated tern-. peratures and pressures of these environments, that this reaction becomes thermodynamically favorable as shown in Fig. 1. AV 0

Ape°(m) = Ape°(p,=l)

- -

2.3RT

(P2-

1)

Ap£°(T2) = ApEO(T,=298K) - - _

2.3R

HzS(g)/FeZ*(aq) Redox Couple

,,o I 6.0

,5.0 40 3.0 2.0

0.0

30

40

P(otm)

50

-I .0

'e

-2.0

m

o E

"~

-3.0 -4.0

-5.0 <~c

AGATp • 6.4 k col/mole e" tronsferred

-6.0

-7.0 -8.0

T.°C

-9.0 25"

-10.0 I

x4iP

-11,0

\67"

-12,0 t

\

100 e

FIG. 1. Effect of temperature and pressure on the energy liberated in the overall electron transfer from HzS (g)-to-Fe2+ (aq). Note cell energy requirement for ADP phosphorylation. 61

62

Michael R. Hoffmann

A variety of other reactions that are thermodynamically unfavorable at STP become thermodynamically possible under the conditions found in thermal vents. We see that the reduction of Fe(II) by H2S is thermodynamically feasible but does this reaction proceed at a measurable rate? Do microorganisms mediate this reaction? From Fig. 1 we see that at 100°C and 50 atm that - 1 2 . 0 kcal/mole/etransferred. This value is sufficient to drive the oxidative phosphorylation of A D P to ATP and, therefore, become a viable source of microbial energy. Many other interesting chemical questions arise when considering these unusual environments. Another emerging problem area involves the

chemistry of ultra-small colloidal particles. These quantum-sized particles range in size from 15 to 250 in diameter. The interracial chemistry and physics of these particles are largely unknown. For example, we have prepared transparent ZnO colloids with an average diameter of 50 A. As a semiconductor ZnO is highly active as a photocatalyst for a wide variety of reactions. In addition, the surface hydroxyl groups of ZnO have been shown to be highly effective catalysts for the hydrolysis of a number of organic esters. Ultra-small colloids have recently been isolated from groundwater aquifers. The potential role of "transparent" colloids in marine chemistry needs to be explored.