An Introduction to Environmental Chemistry

6.2.3 Halmyrolysis and ion exchange in estuaries

The electrochemical reactions that impinge on soil-derived river-borne clay min-

erals carried into seawater do not finish with flocculation of particles and sedi-

mentation of aggregates. The capacity for ion exchange in clay minerals (see

Section 4.8) means that their transport from low-ionic-strength, Ca^2 +- and HCO 3 -

-dominated riverwater, to high-ionic strength, sodium chloride (NaCl)-domi-

nated seawater demands reaction with the new solution to regain chemical

equilibrium (see Box 3.2). The process by which terrestrial materials adjust to

marine conditions has been called ‘halmyrolysis’, derived from Greek roots hali

(sea) and myros (unguent), literally ‘to anoint with the sea’. Halmyrolysis is

imprecisely defined but we will consider it to encompass all those reactions that

affect a particle in seawater before burial in sediment.

Various measurements of cation exchange on river clays in seawater have

shown that clay minerals exchange adsorbed Ca^2 +for Na+, potassium ions (K+)

and magnesium ions (Mg^2 +) from seawater (see Section 4.8), consistent with the

differences in ionic composition between river and seawater. In general, compo-

nents with a high affinity for solid phases, such as dissolved phosphorus (P) or

iron (Fe) (Fig. 6.4a), are removed from solution. Thus the rules of ionic behav-

iour arising from consideration of charge/ionic radius (z/r) ratios (see Section 5.3)

are helpful in understanding chemical behaviour in estuarine environments, as

well as in weathering.

186 Chapter Six

Salinity

Conservative

mixing line

LE line

Dissolved iron (

mmol l

–1

)

0

1

2

3

4

5

(a)

0 5 10 15 20 25 30

Conservative

mixing line

LE line

(b)

0

100

200

300

400

500

Salinity

Dissolved barium (nmol l

–1

)

0 5 10 15 20 25 30

Fig. 6.4(a) Dissolved iron versus salinity in the Merrimack Estuary (eastern USA),

illustrating non-conservative behaviour. Linear extrapolation (LE) of high-salinity iron data to

zero salinity gives an estimate of 60% low-salinity removal of iron (after Boyle et al. 1974). (b)

Dissolved barium versus salinity in the Chesapeake Bay (eastern USA). In this case linear

extrapolation (LE) of high-salinity barium data to zero salinity indicates low-salinity release of

barium (after Coffey et al. 1997), with permission from Elsevier Science.