Remote community drinking water supply : mechanisms of uranium retention and adsorption by ultrafiltration, nanofiltration and reverse osmosis
View/ Open
Date
29/11/2012Author
Schulte-Herbruggen, Helfrid Maria Albertina
Metadata
Abstract
Worldwide, around 884 million people lack access to safe drinking water. To address
this, groundwater sources such as boreholes and wells are often installed in remote
locations especially in developing countries. However, the natural chemical
composition of groundwater may be a source of toxicity to human health. Uranium is
naturally present in the environment, and concentrations above the World Health
Organisation (WHO) drinking water guideline (15 μg/L) are found in various parts of
the world. Uranium has a complex aqueous chemistry and its speciation, which
varies according to pH and available ligands, determines its behaviour (e.g. mobility,
reactivity or sorption tendency). Nanofiltration and reverse osmosis have proved
effective in removing uranium from water, although fundamental removal
mechanisms are not well understood. Even the more porous ultrafiltration (UF) has
been shown to remove uranium when used in combination with
complexation/coagulation methods.
To address the water purification needs of remotely located communities with no or
unreliable access to energy, a renewable energy powered membrane system was
designed using UF as pre-treatment to remove particles, bacteria and viruses and
NF/RO to remove ions. The system was trialled in the Australian outback, using
natural groundwater high in uranium (>300 μg/L). Results showed that pH had a
large effect on the uranium behaviour in the system and, curiously, interaction by
sorption or precipitation to the membranes was observed at certain pH values.
However, due to the complexity of the water and the combination of UF and NF/RO
membranes, the mechanisms of the uranium retention and interaction with the
membrane were not clear. Further systematic study was needed to investigate the
uranium behaviour with the membranes.
Laboratory studies were carried out with one membrane type at a time: UF, NF and
RO. It was postulated that pH, organic matter and inorganic ions such as calcium
have an important influence on uranium retention and interaction with membranes. Results show that uranium behaviour in the membrane systems was highly pH
dependent. During the UF experiments, increased adsorption of uranium occurred in
uranium-only solutions at pH 5-7. From the UF experiments with organic matter it
could be concluded that organic matter did not increase retention (size exclusion) of
uranium, however it did increase the adsorption. Humic acid increased adsorption to
80-95% at pH 3-5, alginic acid at pH 3 while tannic acid caused a nearly 100%
adsorption at pH 10-11.
Further investigating uranium behaviour with NF and RO membranes, it was found
that uranium showed the same increase in affinity to the membrane at pH 5-7, with
about 50% being taken up by NF and 30% by RO membranes. The effect of pressure
on uranium-membrane interaction was investigated for NF and RO at pH 6 and 8.5.
Pressure and consequent concentration polarisation only increased uranium affinity
to the NF membrane at pH 8.5 where the uranium species and MWCO of the
membrane were similar. There was no or little effect of pressure on the affinity of
uranium to the NF membrane at pH 6 or to the RO membrane.
At pH 6, STEM-EDX results showed that uranium was distributed through-out the
polyamide active layer of the NF membrane while FTIR results confirmed that
uranium bound to carboxyl groups in the polyamide. At pH 8.5 however, FTIR
results showed that uranium did not form chemical bonds with the membrane, but
was rather attracted to the surface through hydrogen bonding and loosely forming a
layer on top of the membrane visible in SEM. It was concluded that at least three
different characteristics of the uranium species and membranes played a role for the
interaction: 1) uranium species valency and membrane charge, 2) uranium species
size relative to the membrane pore size, and 3) the reactivity of the uranium species
towards the membrane functional groups.
The effect of calcium on uranium retention and uranium-membrane interaction in NF
and RO was also investigated. Calcium affects uranium speciation by forming a
neutral complex with uranium at pH 8-9, causing a decrease in adsorption to the
membrane. Calcium also precipitates at pH 10. SEM and TEM images showed that the precipitation of calcium carbonate (CaCO3) as calcite caused co-precipitation of
uranium, trapping it on the surface of the membrane. About 48-55% of the calcium
precipitated which caused a 26-35% co-precipitation of uranium, compared to <5%
adsorption in the absence of calcium at pH 10.
Finally the chemical drinking water quality of mainly boreholes and wells across a
West African country, Ghana, was investigated (199 samples in total from
“improved” sources). In addition, the user water costs were documented and the
scope for advanced treatment explored. The WHO guidelines for chemical water
quality were exceeded in 38% of the samples. The main contaminants were nitrate
(21%), managanese (11%) and fluoride (7%), while heavy metals such as lead,
arsenic and uranium were localised to mining areas. It was concluded that when
taking the cost of unsuccessful borehole development into account, alternative
treatment may be a suitable option where inorganic contamination is high.
The findings from this study show the importance of the water quality conditions
(pH, organic matter and calcium) on the behaviour of contaminants such as uranium
in membrane systems and explain the mechanisms of adsorption and co-precipitation
of uranium to the membranes at certain pH values. These are important
considerations when selecting appropriate membranes for water treatment and also
for the maintenance of membranes. The study also showed that there is need for
advanced treatment of drinking water in e.g. Ghana, but highlights the importance of
strategies on local and national level to ensure long-term sustainability and
integration of any such treatment.