**6. Uranium**

Uranium is present in rocks in amounts from less than 2 to a few 100 mg/kg. Lowest content is found in ultrabasic rocks while black shales of marine origin can have hundreds of mg/kg [43]. Uranium is well common in granites and is mobile under oxidising conditions. Uranium has a large number of complexes and their mobility depends on their charge, uncharged species being more mobile. Uranium is a risk from both the radiation point of view as well as a chemical risk point of view. It seems that in many cases, the chemical risk is the one that is most important [44, 45]. Its guideline values have been changed repeatedly over the last 13 years and its current pro‐ visional guideline is now, what concerns the chemical risk, 30 μg/l [46] (**Table 1**). The chemical risk is its effect on the secondary uptake of water and salts from the primary urine formed in the kidney cortex [45, 47]. The radiological guideline is different for 234U and 238U being, respectively,


**Table 1.** Elements exhibiting redox-sensitive behaviour with guideline values. The figures are derived from WHO [5] and regard guidelines due to health reasons. For manganese only a technical guideline is established.

1 and 10 Bq/l [46]. The uptake by humans of different uranium complexes varies consider‐ ably. A speciation of uranium complexes can be calculated from an ICP‐MS (Induced Coupled Plasma combined with Massspectrometer) analysis by Visual MINTEQ [48] or PHREEQC [49]. Uranium in groundwater is high especially in granitic terrains, for example, in Sweden and Finland. Another source of uranium is in the form of phosphate fertilisers as phosphate rocks have elevated uranium contents [50]. In an area in southern central Finland, the total uranium levels in groundwater were, in some samples, above 3000 μg/l while no health effects were seen [51]. This is far above the current WHO health limit of 15 μg/l. A speciation of the uranium in relatively alkaline groundwater (pH > 7.3) showed that the major portion of uranium was present as calcium‐uranyl‐carbonato complexes (CaUO2 (CO3 ) 3 2− and Ca2 UO2 (CO3 ) . 11H2 O). Thus, these complexes do seem to be less toxic or less bioavailable. There seems to be an interaction with the iron status such as the uranium uptake may be higher at iron deficiency [45, 52] which, by these authors, was considered as an action of divalent metal transporter 1 (DMT‐1).

Mobility of uranium in groundwater is affected by a large number of factors. In general, U(IV) is less mobile than the oxidised form U(VI) [53]. This is true for several actinides and has been considered in the search for safe repositories for radioactive waste [54]. As mentioned above, uranium forms numerous complexes among then carbonate complexes. Elevated bicarbonate contents in groundwater form soluble carbonate complexes [55]. When removing the uranium ex-situ by filter, for instance, it is important to know the speciation [56].

The reduction of uranium is in part not only inorganic but also bacteriologically mediated by Moon et al. [57]. The microbial reduction results in isotope fractionation [58]. There is a considerable community of bacteria even at hundreds of meters in hard rock terrains [59, 60]. One of the substrates for uranium sequestration could be acetate [53]. Sulphate‐reducing con‐ ditions are most favourable for the reduction from U(VI) to U(IV) [53, 61]. Species involved in the reduction are *Desulfobacter, Desulphoropalus* and *Desulfovibrio spp.* [53]. Oxidation of Fe(II) to Fe(III) can also reduce U(VI) to U(IV) [61].

An example of the redox behaviour of uranium is sandstone‐hosted uranium deposits formed by groundwater flow with low concentrations of uranium reaching a redox barrier where the U(VI) is reduced to U(IV) and accumulates as an ore‐body. These deposits are now commonly extracted by in situ recovery with injection of an oxidising solution forming U(VI) and prefer‐ ably forming an uncharged complex [62]:

$$\text{UO}\_2(\text{uraninite}) + \text{VO}\_2 + 2\text{H}^+ \rightarrow \text{UO}\_2^- \text{(uranyl)} + \text{H}\_2\text{O} \tag{2}$$

Complexation with calcium:

$$\text{UO}\_2^{\cdot \cdot \cdot} + 2\text{Ca}^{\cdot \cdot \cdot} + 3\text{HCO}\_3^- \rightarrow \text{Ca}\_2\text{(UO}\_2\text{)}\text{(CO}\_3\text{)}\_3^{\cdot \cdot \cdot \cdot} + 3\text{H}^+\tag{3}$$

This technology is considerably more environment friendly compared with conventional mining which leaves tailings containing leftover uranium.

#### **7. Nitrogen**

1 and 10 Bq/l [46]. The uptake by humans of different uranium complexes varies consider‐ ably. A speciation of uranium complexes can be calculated from an ICP‐MS (Induced Coupled Plasma combined with Massspectrometer) analysis by Visual MINTEQ [48] or PHREEQC [49]. Uranium in groundwater is high especially in granitic terrains, for example, in Sweden and Finland. Another source of uranium is in the form of phosphate fertilisers as phosphate rocks have elevated uranium contents [50]. In an area in southern central Finland, the total uranium levels in groundwater were, in some samples, above 3000 μg/l while no health effects were seen [51]. This is far above the current WHO health limit of 15 μg/l. A speciation of the uranium in relatively alkaline groundwater (pH > 7.3) showed that the major portion of uranium was present

**Table 1.** Elements exhibiting redox-sensitive behaviour with guideline values. The figures are derived from WHO [5]

(CO3 ) 3

complexes do seem to be less toxic or less bioavailable. There seems to be an interaction with the iron status such as the uranium uptake may be higher at iron deficiency [45, 52] which, by these

Mobility of uranium in groundwater is affected by a large number of factors. In general, U(IV) is less mobile than the oxidised form U(VI) [53]. This is true for several actinides and has been considered in the search for safe repositories for radioactive waste [54]. As mentioned above, uranium forms numerous complexes among then carbonate complexes. Elevated bicarbonate contents in groundwater form soluble carbonate complexes [55]. When removing the uranium

The reduction of uranium is in part not only inorganic but also bacteriologically mediated by Moon et al. [57]. The microbial reduction results in isotope fractionation [58]. There is a considerable community of bacteria even at hundreds of meters in hard rock terrains [59, 60]. One of the substrates for uranium sequestration could be acetate [53]. Sulphate‐reducing con‐ ditions are most favourable for the reduction from U(VI) to U(IV) [53, 61]. Species involved in the reduction are *Desulfobacter, Desulphoropalus* and *Desulfovibrio spp.* [53]. Oxidation of Fe(II)

An example of the redox behaviour of uranium is sandstone‐hosted uranium deposits formed by groundwater flow with low concentrations of uranium reaching a redox barrier where the U(VI) is reduced to U(IV) and accumulates as an ore‐body. These deposits are now commonly

authors, was considered as an action of divalent metal transporter 1 (DMT‐1).

**Element Guideline value Note** Arsenic 10 μg/l Provisional Chromium 50 μg/l Provisional Manganese (400 μg/l) Technical

Uranium 30 μg/l Provisional

and regard guidelines due to health reasons. For manganese only a technical guideline is established.

ex-situ by filter, for instance, it is important to know the speciation [56].

2− and Ca2

UO2 (CO3 ) . 11H2

O). Thus, these

as calcium‐uranyl‐carbonato complexes (CaUO2

Nitrate 50 mg/l Selenium 40 μg/l

232 Redox - Principles and Advanced Applications

to Fe(III) can also reduce U(VI) to U(IV) [61].

Nitrogen is a major nutrient in soils and its cycle has been affected to a large extent by anthro‐ pogenic industrial nitrogen fixation, fertiliser production. The nitrogen fixation in nature was passed by the anthropogenic in the 1980s [63]. When organic matter like litter degrades, ammonia is formed. Ammonia is strongly adsorbed on clay minerals and to organic matter, and elevated concentrations are only found close to a point source. Nitrate, on the contrary, is mobile unless it is taken up by plants. While nitrate in surface water may be part of eutrophi‐ cation, the main risk with nitrate in groundwater is as the formation of methaemoglobinemia and the decrease of the oxygen‐carrying capacity of the red blood cells from the lungs to peripheral tissues [64]. This affects children below the age of about 1 year that are bottle fed with high nitrate water. Above the age of 1 year, humans develop an enzyme that recovers the normal haemoglobin. Methaemoglobinemia is not common; a few thousands of cases are reported. Nitrate converted to nitrite in the intestinal tract may be carcinogenic [65]. Nitrate is reduced to nitrite in the intestinal tract and nitrite may form nitrosamines and elevate the risk of gastric cancer [66]. Above all, nitrate in groundwater is a resource in a wrong place; it should be present in the root zone to promote crop growth.

#### **8. Interactions between elements**

In a redox reaction, there is an electron donor and an electron acceptor. Among the elements dealt with above, there are interactions. Oxygen is a common electron acceptor under oxidis‐ ing conditions, for example, the oxidation of As(III) to As(V). Ferric oxyhydroxides play a crucial role for the mobility of arsenic in groundwater in two respects, under reducing condi‐ tions, they are dissolved and arsenite is released and under high pH conditions, above pH 8.2, when they are uncharged making arsenate mobile.

Manganese oxides on the surface of mafic minerals [31] and in general in soils [33] can serve as an oxidant of Cr(III), forming mobile chromate.

Selenium is mobile in groundwater under oxidising conditions, and nitrate from agricul‐ ture mobilises selenium from marine shales [67]. Another interaction between agriculture and groundwater might be found in rice cultivation. A new rice irrigation practice by using intermittent irrigation, allowing the rice field to dry up between irrigations, has several posi‐ tive effects, higher yield, lower arsenic content in the rice grains and lesser methane emission [68]. It might also in the long run affect the groundwater redox conditions.
