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Notes
1773
0.022
30
0.020
i b 0.019
c -
5 0.016
E
- 0.017
2
0.016
I
0.015
25
I
2 20
0
c g 15
8
2
10
5
Corrected Velocity _
0.014
u 5 10 15 20 25 30 35 40
Salinity, 20
0 A
0
30 40 50 60
Rodius. cm
Fig. 5. Calculated concentration difference of CaSO,
between the solid-liquid interface and bulk solution of
clod cards in natural seawater sources based on Eq. 7.
Curve A was calculated based on literature values of seawater
chemical content at 32%1 and assuming that seawater
was diluted with distilled water to achieve the lower salinities;
curve B was calculated based on chemical analyses
of Puerto Peiiasco seawater at 40?&0 and Tucson tapwater
used as the diluent in our study.
Ac =
1(
_
[Ca2+lb + W42% 2 + K Oo5
2 1
SP
I
Ka2+lb + W42- lb
2 ) . (7)
[Ca2+]i, [Ca2+lb, [S042-]i, and [S042-]b are calcium
and sulfate ion concentrations at the interface
and in the bulk solution. Ksp is the solubility
product, which can be computed by the
expressions given by Marshall and Slusher
(1968), who allow for the presence of ion pairs
such as MgS040 in their correlations.
The values of [Ca2+16 and [S042-]b used in
Eq. 7 are based on the total calcium and sulfate
measured in solution; we have made no attempt
to correct these concentrations for the
presence of ion pairs containing calcium or
sulfate. Increasing the ionic strength (or salinity)
increases the CaSO, solubility product,
thereby increasing the gradient between the interface
and the bulk solution and accelerating
the dissolution rate of a clod card. However,
this effect is counteracted if the concentration
of CaSO, in the bulk solution increases with
salinity, which is the case with seawater; the
seawater used in our experiments contained
CaSO, at concentrations more than eight times
that of the freshwater supply. The brackish
supply, prepared by dilution, was intermediate.
Figure 5 contains salinity correction curves
Fig. 6. Correction for the induced-water-motion effect
of the rotating arm in the tank. Top line is the velocity of
the clod card in the tank at each position on the arm;
bottom line is the velocity of the water in the tank estimated
by the movement of suspended particles in the
water; middle line is the corrected velocity used to correlate
the card dissolution rates with water motion.
based on Puerto Peiiasco seawater and Tucson
tapwater used in our study (curve B) and on
literature values of normal seawater constituents
(Hansson 1973) assumed to be diluted
with distilled water (curve A). Curve B falls
below curve A because Tucson tapwater and
Puerto Pefiasco seawater contain proportionally
more CaSO, than distilled water and standard
seawater.
The water motion tank was used to obtain
data for velocity-weight loss correlations. The
velocity of the clod card through the water was
easily calculated from the rotation rate of the
arm and the distance of the card from the center
of rotation. However, it was necessary to
correct for the rotary movement imparted to
the water in the tank by the moving arm. This
movement was determined at each of the five
positions by measuring the movement of small
suspended particles in the water. A drag model
was correlated to the data and used to correct
the velocities of the cards into estimates of
their velocities relative to the water at each
position (Fig. 6). The correction was more significant
at low than at high velocities.
Forced convection tests were run at temperatures
from 13 to 35°C in three different
salinities on the rotating arm (Fig. 7). Plots in
the form of Eq. 5 were subjected to regression
analyses (Table 2). In all runs, clod card weight
loss at the lowest test velocity was increased
by free convection, and the points fell - 15%
above the curve of best fit to the other points;