4.1 Effects of Evaporation and Activity Coefficient Models on
Feedwater Options
The evaporation progress of four different potential sources of
irrigation water, three coastal shallow groundwater sources and mean
Arabian Gulf seawater, each having a different composition
(Table 2 ), was modelled as an isothermal water depletion using
two different activity coefficient models, B-dot and PHRQPITZ. The
results were normalized by the total mineral mass in each sample so that
the onset of precipitation could be compared between different samples
and between the two activity coefficient models. The final results from
this exercise are presented in Figures 3 and 4 .
In general, the difference in mineral precipitation between the two
activity coefficient models became more pronounced with increasing ionic
strength and salinity, with the least difference obtained for Arabian
Gulf seawater, which had the lowest salinity of the four potential
feedwater sources. The difference was especially pronounced for the
hypersaline coastal shallow groundwater samples, including Masdar Well 4
water, which has a similar salinity and expected composition as the
shallow groundwater at sites D and E (Figure 1 ). These results
corroborate assertions by other researchers
(Zhang, Zheng, & Wan, 2005) that the
Pitzer ion interaction model is more suitable for activity calculations
in strong electrolytes like hypersaline waters and brines. PHRQPITZ, the
Pitzer virial expansion model, consistently showed an earlier onset of
precipitation (i.e., conversely, less dissolution) for all feedwater
sources. Also, since the B-dot model is a Debye-Huckel variant and
depends solely on ionic strength, it fails to incorporate ion
interactions between different types of ions, which can be better
modelled using PHRQPITZ. For this reason, the salt composition of the
precipitate showed variation between the two models (Figure 3 ),
especially for the groundwater samples. The evaporation modelling
exercise clearly demonstrated that the coastal groundwater is a poor
candidate for use as irrigation water in saline water farming of
halophyte plants because of its hypersaline characteristics, presumably
owing to a significant salt flux from underlying evaporite deposits
(Sanford & Wood, 2001) and up wicking of
shallow groundwater resulting in rapid evaporation
(Imes & Wood, 2007). The results also
demonstrated that Arabian Gulf seawater is unlikely to undergo rapid
precipitation until almost 90% of its water mass has been evaporated.
In addition, this exercise displayed the least deviations in
precipitation modelling predictions between the two activity coefficient
models for seawater, confirming that B-dot could be used to model
seawater evaporation with limited error. B-dot also allows for
temperature correction, a feature unavailable with PHRQPITZ, which can
be useful in the future to model the summer months in Abu Dhabi when the
ambient temperatures can approach 50 °C.