Efficiency of small–scale water purification units to reduce

XII JORNADAS DE INVESTIGACIÓN
Revista Investigación Científica, Vol. 4, No. 2, Nueva época. Mayo - Agosto 2008
ISSN 1870-8196
Efficiency of small–scale water purification units
to reduce radioactive content
Juan Carlos Quiroga Cifuentes
Hugo López del Río
J. Ignacio Dávila Rangel
Fernando Mireles García
Unidad Académica de Estudios Nucleares
Universidad Autónoma de Zacatecas
E–mail: [email protected]
Abstract
In order to asses the efficiency of small-scale water purification units to remove
radioactive elements, gross alpha and beta activities were determined on preand purified water samples from four treatment plants in Zacatecas and
Guadalupe cities. In general, the treatment plants were more efficient to remove
gross alpha activity. The percentage of removed alpha activity ranged from 27
percent to near 100 percent. Fraction of removed beta activity varied between
0% and 77%. Radioactive levels of water samples were found to be lower than
maximum permissible radioactive levels.
Keywords: gross activities; bulk purified water; small treatment plants.
Introduction
Recently, and as an alternative to the commercial bottled water, a number of
small-scale water purification units (SSWP) has begun operating in Zacatecas and
Guadalupe cities. In general, they provide bulk purified water at low cost to public
compared to commercial brands of water. Although water quality is subjected to
national sanitary regulations (DOF, 1995), sanitary surveys are focused on
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Revista Investigación Científica, Vol. 4, No. 2, Nueva época. Mayo - Agosto 2008
ISSN 1870-8196
organoleptic, physicochemical, and microbiological properties (DOF, 1993).
However, no assumption has been made regarding radioactive content of water.
As it has been shown in some papers, high radioactive level can be found both
natural and purified waters (Dávila et al., 2002; Dueñas et al., 1997), which would
be in violation of national regulations. Mexican regulations for radioactive content
in drinking water set permissible limits of 0.56 Bq/L for gross alpha- and 1.85 Bq/L
for gross beta-activity (DOF, 2000). Gross activity concentrations provide rapid and
valuable information about the radiological quality of water and let authorities
take the appropriated intervention level. In this context, the aim of this study was
to evaluate the efficiency of some SSWP units located in Zacatecas and
Guadalupe cities, to remove radioactive elements by measuring gross alpha and
beta activities in pre- and purified water.
Experimental
Water samples were collected in four SSWP plants. Splash and Xel-Ha SSWP plants
were located in Zacatecas and El Oasis and Santa Clarita in Guadalupe city. The
water treatment process includes ultraviolet light, deep bed, membranes,
activated carbon, ozonisation, and softener. The inlet water was always potable
water from municipal supplies. A reasonable volume of pre- and purified water
was transferred to polyethylene container. After collection, the samples were
immediately sent to laboratory where conductivity were measured. Aliquots of 1L
each were evaporated to dryness. After evaporation, the dry solid residue was
dissolved in 10 mL of concentrated hydrochloric acid, evaporated, and
redissolved in 8 mL of water. Finally, solution was transferred to the counting vial.
The same procedure was applied to background samples. Counting mixtures
were prepared by mixing 12 mL of liquid scintillation cocktail OptiPhase HiSafe 3
(Wallac) and the treated sample in a 20 mL volume polyethylene vial. Alpha and
beta radiation was measured in a Liquid Scintillation Counter Wallac 1411 with
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Revista Investigación Científica, Vol. 4, No. 2, Nueva época. Mayo - Agosto 2008
ISSN 1870-8196
pulse shape analysis feature by 21600 s. A previously optimized PSA level of 49
was used to ensure the best alpha/beta pulse separation (Dávila Rangel et al.,
2001).
Results
Conductivity and solid residue of pre- and purified water from El Oasis, Santa
Clarita, Spalsh, and Xel-Ha were 432 uS/cm – 0.3097 gr/L, 62 uS/cm – 0.035 gr/L,
425 uS/cm – 0.3160 gr/L, 293 uS/cm – 0.2103 gr/L, 425 uS/cm – 0.2911 gr/L, 376
uS/cm – 0.2725 gr/L, 492 uS/cm – 0.3157 gr/L, and 501 uS/cm and 0.4041 gr/L,
respectively. Clearly, SSWP unit El Oasis was the most efficient to reduce dissolved
ions (about 85%). Special attention deserves Xel-Ha unit which had a null effect to
remove some dissolved ions. It may be a consequence of poor maintenance
what is supported by the low price of water. About 30% and 10% of total
dissolved ions were eliminated by Santa Clarita and Splash unit, respectively.
Gross alpha activity concentrations of prepurified waters were 448.0±7.8
mBq/L (El Oasis), 419.4±8.2 mBq/L (Santa Clarita), 388.6±6.8 mBq/L (Splash), and
137.9±6.2 mBq/L (Xel-Ha). Next, gross alpha activity concentrations of purified
waters were 200.9±5.2 mBq/L (Santa Clarita), 285.1±7.2 mBq/L (Splash), and
67.5±7.1 mBq/L (Xel-Ha). Alpha activity of El Oasis water was below the minimum
detectable activity (MDA). Therefore, SSPW unit El Oasis was the most efficient
plant to remove alpha emitting radioelements. In fact, the gross alpha activity in
fresh purified water sample collected after replacing a purification membrane
was found to be below MDA.
Xel-Ha and Santa Clarita plants removed
approximately 50% of total alpha activity, and Splash unit eliminated about 30%.
The gross beta activities in prepurified waters were 263.5±13.0 mBq/L (El
Oasis), 311.6±12.6 mBq/L (Santa Clarita), and 296.5±10.1 mBq/L (Splash). Gross
beta activity for Xel-Ha water was below MDA. On the other hand, gross beta
levels in purified waters were 61.5±11.7 mBq/L (El Oasis), 277.3±8.5 mBq/L (Santa
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ISSN 1870-8196
Clarita), 84.5±9.3 mBq/L (Splash), and 187.0±12.4 mBq/L (Xel-Ha). In general,
SSPW units were less efficient to remove beta emitting radioelements. The
anomalous result of Xel-Ha water could be explained on the base of
40
K
desorption from saturated membrane and deep bed. However, additional
analysis will be required in order to test this hypothesis.
Mexican limit values for gross alpha and beta activity were not exceeded by
the eight water samples. According to World Health Organization guidelines (WHO,
2004), maximum allowable concentrations for gross alpha and beta activities in
drinking water of 100 mBq/L and 1000 mBq/L, respectively, ensure a safe exposure
via water ingestion. It can be seen that seven water samples exceeded the limit
for alpha radioactivity content and none of the waters surpassed the beta limit.
Conclusions
Gross alpha and beta activities were measured in pre- and purified water samples
of four small-scale water purification units located in Zacatecas and Guadalupe
cities. Water purification systems were more efficient to remove alpha activity than
beta activity. Although the radiological quality of waters was verified, the
radioactive content depends on factors such as inlet water quality, operating
time, and level of maintenance.
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Revista Investigación Científica, Vol. 4, No. 2, Nueva época. Mayo - Agosto 2008
ISSN 1870-8196
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