Environmental Engineering Reference
In-Depth Information
At contaminated sites, uranium leaches into the subsurface, which has become
a widespread problem at mining and milling sites across North America, South
America, and Eastern Europe [5, 6]. There are four different oxidation states of
uranium in aqueous systems: U(III) (highly unstable), U(IV), U(V) (unstable), and
U(VI). U(VI) is known to form complexes with carbonate, chloride, sulfate and
various organic chelating agents such as acetate. Soluble U(VI) species are the pre-
dominant forms of uranium in contaminated groundwater and soils. The movement
of groundwater usually transports the soluble U(VI) contaminant beyond its orig-
inal boundaries, causing a global problem in aquifers, water supplies, and related
ecosystems and posing a serious threat to human health and the natural environment
[7, 8].
Uranium can enter the human body via inhalation (aerosols), ingestion (drinking
and eating), and wounds (embedding) [9], and it poses a threat to human popula-
tions due to its radioactivity and chemotoxicity [10, 11, 12]. Although there are
no conclusive epidemiological data correlating uranium wastes exposure to specific
health effects, studies using cells and animals suggest the possibility of genetic,
reproductive, and neurological effects from chronic exposure [13].
All uranium isotopes present in uranium contaminants are radioactive and chem-
ically toxic [14]. It is generally accepted that when uranium enters the human body,
radiation and chemical toxicity can increase the risk of cancers such as bone can-
cer and lung cancer and that uranium can accumulate in kidneys for a long period
and cause renal dysfunction and structural damage [15, 9]. Milacic's report pub-
lished in 2008 on health investigations of uranium waste clean-up workers in a
DU-contaminated site in Serbia showed that although disease or tumors did not
develop during the investigation period of four years, the total number of DNA
alterations and damaged cells was higher after uranium decontamination [16].
Because of the threat of uranium radioactivity and chemotoxicity to human pop-
ulations, it is very important to ensure that uranium contamination is under control.
The U.S. Environmental Protection Agency (EPA) has established a Maximum
Contaminant Level (MCL) of 30
g/L for uranium in drinking water [17]. Guidance
on implementation of the standard is provided by the EPA Office of Solid Waste
and Emergency Response Directive no. 9283.1-14, “Use of Uranium Drinking
Water Standards Under 40 CFR 141 and 40 CFR 192 as Remediation Goals for
Groundwater at CERCLA Sites.”
In order to meet the EPA standards, extensive efforts have been made to assess
and remediate the uranium-contaminated sites. In the U.S., the total volume of all
radionuclide wastes is 5.5 million m
3
and the volumes of contaminated soil and
water have been reported to be 30-80 million m
3
and 1,800-4,700 million m
3
,
respectively. Uranium is one of the most common radionuclides in soils, sediments,
and groundwater at these contaminated sites [18, 19, 4]. In the first part of this chap-
ter, various uranium remediation technologies are discussed. Emphasis is placed on
the principles and mechanisms of uranium bioremediation and the key factors affect-
ing it. The second part of this chapter focuses on the use of biofilms for uranium
immobilization in groundwater from subsurface environments.
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