Factors controlling the mobility of both Arsenic and Uranium in groundwater Florian Krämer Zusammenfassung: Uran und Arsen sind als Spurenelemente in der Erdkruste und auch im Grundwasser nachzuweisen. In Ausnahmefällen (z.b. anthropogene Beeinflussung, Lagerstätten) können die Konzentrationen aber enorme Größenordnungen annehmen. Der entscheidende Faktor, welcher die Mobilität beider Elemente beeinflusst, ist die Adsorption an verschiedenen Festphasen. Aufgrund der existierenden chemischen Bedingungen, wie Redoxpotential, ph-wert und der Konzentration anderer Ionen, welche in der flüssigen Phase vorliegen, kommt es zur Bildung verschiedener Uran- bzw. Arsenspezies, welche unterschiedlich stark, je nach Chemie des Grundwassers, adsorbiert werden. Abstract: Uranium and Arsenic could be found as trace elements in the earth s crust and in ground water. If there are extraordinary conditions (e.g. anthropogenic influence, deposits) the concentrations, will be much higher in order of magnitude. The most important factor controlling the mobility of both elements is adsorption on different solid phases. Based on the existing chemical conditions like the redoxpotential, ph-value and the concentration of other ions in the liquid phase, different species of Uranium and Arsenic are formed and will be adsorbed differently depending on ground water chemistry. 1 Introduction Ground water composition is ultimately derived from rock weathering. The geology, the existing soils and the climatic conditions of a certain area are important factors for making statements about the concentrations and about the transport behaviour of arsenic and uranium in ground water sources. Adsorption is the predominate mechanism controlling transport of both elements in many ground water systems. Average concentrations of Arsenic in ground water are 0, 5 5 µg/l. But potable ground water supplies in many countries (Bangladesh, India, Taiwan, and Mongolia) contain dissolved arsenic in excess of 10 µg/l. The primary source of arsenic is natural (derived from interactions between ground water and aquifer sediments) and not anthropogenic (WELCH & STOLLENWERK 2003). Long-term exposure to arsenic in drinking water has been implicated in a variety of health concerns including several types of cancer, cardiovascular disease, diabetes, Blackfoot disease and neurological effects. Methyl arsenic compounds have a high acute toxicity, which decreases during demethylation. Limiting values for arsenic are included in the TrinkwV (2001) with 10 µg/l, in WHO with 10 µg/l and in the US EPA also with 10 µg/l. Uranium is a radioactive element and is naturally occurring in ground and surface water. It s a heavy metal like Arsenic and has also a long-term toxicity. Although uranium is enriched in granites and gneiss, ground water from these host rocks often shows low to intermediate uranium concentrations, while some ground waters from sandstone and carbonate aquifers show elevated uranium concentrations up to some hundred mg/l without man made impact. (MERKEL & HASCHE-BERGER 2005). Considering the WHO (2004) recommendation for drinking water of 9 µg/l due to the chemical toxicity of uranium. The TrinkwV (2001) doesn t include a limiting value for uranium and the US EPA proposes a limiting value of 30 µg/l. The facts of toxicity of both elements and radioactive radiation of uranium are important issues in environmental research and for human health care. Therefore it s important to do research in chemical behavior of both elements to minimize risks for environment and human health.
2 Aqueous chemistry 2.1 Arsenic speciation Dissolved arsenic speciation is important in determining the extent of reaction with the solid phase and therefore the mobility of Arsenic in ground water. Arsenic is generally present as arsenate [As(V)] or arsenite [As(III)] for Eh conditions prevalent in most ground waters (Fig. 1). Arsenic metal rarely occurs, and the -3 oxidation state is only found in very reducing environmental conditions. As (III) has been considered to be the more toxic oxidation state (WELCH & STOLLENWERK 2003) however, more recent studies have shown that most ingested As (V) can be reduced to As (III). Thus, exposure to both forms of As may result in similar toxicological effects (WELCH & STOLLENWERK 2003). Fig. 1: The Eh-pH diagramm for As at 25 C and one atmosphere with total arsenic 10-5 mol/l and total Sulfur 10-3 mol/l. Solid species are enclosed in parentheses in cross-hatched area, which indicates solubility less than 10-5.3 mol/l (WELCH & STOLLENBERG 2003). Both As(III) and As(V) form protonated oxyanions in aqueous solutions, the degree of protonation depends on ph. Arsenate is stable is stable in oxidizing environments. For ph values common in groundwater, the predominant As(V) species in solution are H 2 AsO 4 - between ph 2.2 and 6.9 and HAsO 4-2 between ph 6.9 and 11.5.The solution concentration of H 2 AsO 4 - in soil is controlled primarily by adsorption reactions on oxides and hydroxides of Al, Fe, and Mn. Arsenite is stable in moderately reducing environments. H 3 AsO 3 0 predominates up to ph 9.2 and H 3 AsO 3 - from ph 9.2 12 (WELCH & STOLLENBERG 2003). Methylated Species of As(III) and As(V) can be formed by biomethylation and anthropogenic impact. These species are stable under oxidizing and reducing conditions. 2.2 Uranium speciation As the most abundant actinide element, uranium averages 1.2 to 1.3 µg/g in sedimentary rocks, ranges from 2.2 to 15 µg/g in granites, and from 20 to 120 µg/g in phosphate rocks (LANGMUIR 1997). Ground waters in granite have some of the highest uranium concentrations, although they rarely exceed 20 µg/l. Uranium occurs in 4+, 5+ and 6+ oxidation states, which are usually written U(IV), U(V) and U(VI). Most important in nature are uranous [U(IV)] and uranyl [U(VI)] oxidation states. Uranous ion (U 4+ ) and its aqueous complexes predominate in ground waters of low redoxpotential. U(IV) is the major oxidation state in the most common uranium ore minerals uraninite, pitchblende and coffinite. The U(IV) concentrations in ground water at low Eh are usually less than 10-8 M because of the extremely low solubilities of these solids. In the U(V)
oxidation state, uranium occurs as the UO 2 + ion which forms relatively weak complexes. This species is only found at intermediate oxidation potentials and low ph s and is unstable relative to U(IV) and U(VI). In oxidized surface- and ground water, uranium is transported as highly soluble uranyl ion (UO 2 2+ ) and forms different complexes depending on ph-value and redoxpotential (e.g. with sulphate-, phosphate-, carbonate- and fluorid ions) which are highly soluble (Fig 2.) An Eh-pH diagram for the system U-O 2 -H 2 O at 25 C and a typical ground water uranium concentration of ΣU(aq) = 10-8 M is given in Fig. 2. The plot shows the stability fields of the dominant aqueous species and the large size of the stability field of uraninite [UO 2 (c)]. If instead the stability field of UO 2 (am) is plotted, it almost exactly overlaps the field of U(OH) 4. The diagram shows the predominance of the uranyl-hydroxy complexes at low Eh values in the presence of uraninite, with U(OH) 4 0 only important in waters where the Eh is less than about -100 to -200 mv. At a typical ground water CO 2 pressure of 10-2 bar, the highly stable uranyl carbonate complexes predominate above about ph 5 (Fig. 2). It shows that these complexes are stable relative to U(OH) 4 0 under highly reducing conditions. Accordingly above ph 5, the oxidation of U(IV)(aq) and dissolution of UO 2 (s) can occur at lower Eh values when high carbonate concentrations are present. Fig. 2: The Eh-pH diagramm for U at 25 C and one atmosphere with total uranium 10-8 mol/l. (a) system U-O 2 -H 2 O; (b) system U-O 2 -CO 2 -H 2 O at PCO 2 = 10-2 bar. UC = [UO 2 CO 3 ] 0, UDC = [UO 2 (CO 3 ) 3 ] -2 2, -4 UTC =[UO 2 (CO 3 ) 3 ] 3 (LANGMUIR 1997). 3 Adsorption/Desorption of arsenic 3.1 Mechanism Oxides of iron, aluminum (Al) and manganese are potentially the most important source/sink for As in aquifer sediments because of their chemistry, widespread occurrence and tendency to coat other particles. There are two widely accepted mechanisms for adsorption of solutes by a solid surface. Outer-sphere surface complexation or non-specific adsorption involves the electrostatic attraction between a charged surface and an oppositely charged ion in solution. The adsorbed ion resides at a certain distance from the mineral surface. Inner-sphere complexation, also termed specific adsorption, involves the formation of a coordinative complex with the mineral surface. Inner-sphere complex bonds are more difficult to break than outer-sphere complex bonds and result in stronger adsorption of ions.
3.2 ph value Fig. 3: Effect of ph value on As(V) adsorption (WILLIAMS ET AL. 2003). Arsenate is primarily adsorbed at low ph values. From ph 3 to 7 the percentage of As(V) adsorbed decreases slightly from approximately 95% to 85%. As the ph increases from 7 to 10 the percentage of As(V) adsorbed dropped dramatically, decreasing to approximately 40 to 50% between ph 9 and 10. This is typical of anion adsorption onto variably charged surfaces and results from the ühdependent surface charge and speciation of As(V). At lower ph values (ph < 7) As(V) exists predominantly as anion in the form H 2 AsO 4 - and is attracted to the positively charged soil surfaces (e.g. Fe oxides). At high ph values (ph > 7), As(V) exists as anion in the form H 2 AsO 4 2- and the oxide surfaces become increasingly negatively charged. This results in a decrease of As(V) adsorption with increasing ph (Fig. 3) 3.3 Clay minerals Results from experiments have shown that the adsorption per gram of solid of both As(III) and As(V) increased with initial solution concentration of As. Maximum adsorption of As(V) by kaolinite, montmorillonite, illite, halloysite and chlorite occurred up to ph 7, then decreased with further ph increases. Adsorption of As(III) by these same clay minerals was a minimum at low ph and increased with increasing ph. Arsenate adsorbed to a greater extent than As(III) on all clay minerals at ph < 7. At higher ph values, adsorption of As(V) and As (III) were more identical and in some cases As(III) adsorption exceeded that of As(V) (WELCH & STOLLENWERK 2003). Surface area can be an important factor in adsorption of As by clay minerals. Montmorillonite adsorbed about twice as much As(III) and As(V) as kaolinite. The surface area of montmorillonite was 2.5 times greater than kaolinte. Halloysite and chlorite were found to adsorb As(V) to a much greater extent than kaolinite, illite and montmorillonite. The reason of this fact is the greater surface area. But surface area is not always a good indicator of As(V) adsorption. E.g. kaolinite (9.1 m²/g) adsorbed 30 % more As(V) than illite (18.6 m²/g) and 50 % more than montmorillonite (24.2 m²/g). There are adsorption maximum for arsenate around ph 5 and for arsenite around ph 8-9 in contrast to its behaviour to oxides (Fig. 9). As(V) adsorption on clay minerals increases with increasing solution ph from 3 to 5 and decreases with increasing solution ph from 5 to 9. The renewed increase in arsenate adsorption observed when ph increased above ph 9 is most likely an artefact from dissolution of clay minerals at elevated ph. Adsorption affinity for As of the clay minerals is less than the adsorption affinity of the oxides. 100 % adsorption of As(V) was only reached by kaolinite around ph 5. For all other clay systems adsorption was significantly below 100 %, especially for arsenite. The competitive effect of the presence of equimolar concentrations of both As redox states is also shown in Fig. 4. The discrepancy in arsenate adsorption on Al oxide between the binary and the single ion system was the result of differences in amount of total As added. A competitive effect of the presence of arsenate on adsorption of arsenite was observed on kaolinite and illite (Fig. 4) in the intermediate ph range 6.5 to 9. No comparable competitive effect was observed for montmorillonite in the ph range 5 to 9. The apparent increase in arsenite adsorption below ph 5 resulted because of the oxidation of arsenite to arsenate (Fig. 4).
Fig. 4: Arsenic adsorption in kaolinite, montmorillonite and illite as a function of ph and redox state (GOLDBERG 2003). 3.4 Influence of competing ions 3.4.1 Phosphate In addition to ph, the presence of other ions also affects the adsorption of As(V). One of the most significant of these ions is PO 4. Phosphate exhibits similar chemical behavior and is often used in fertilizers in agricultural areas where As may have been used as a pesticide or herbicide. However, it has been concluded that PO4 greatly enhanced the downward mobility of As(V) in soil columns.po 4 amended soils exhibited an increase in mobility of As relative to non-po 4 amended soils and noted the potentially important role of physical nonequilibrium on As(V) transport. In addition to time and As(V) concentration, several other parameters have been shown to influence As(V) adsorption, including ph, ionic strength, and the presence of competing anions will show the effect on adsorption of competing ions. The influence of phosphate on adsorption of As(V) and As(III) by ferrihydrite is a function of ph. Adsorption of both As(V) and As(III) decreased with increasing phosphate concentration. For As(V), the decrease was significant over the entire ph range. Phosphate has the greatest effect on As(III) adsorption at lower ph values. At ph 9, adsorption of As(III) decreased by only a few percent, even at the highest phosphate concentration. Apparently, the neutral H 3 AsO 3 0 was better able to compete for surface-complexation sites with HPO 4 - ² at higher ph. Phosphate competition with As(V) has been observed for other adsorbents. Experiments showed an 85 % decrease in As(V) adsorption by goethite when phosphate/as(v) ratio was increased from zero to 12:1. A phosphate/as(v) ratio of 1:1 caused a decrease of 30 % in As(V)adsorption by both goethite and gibbsite at ph < 8, compared to phosphate free solutions (Fig 8.) Similar effects of phosphate on As(V) adsorption were observed for kaolinite, montmorillonite and illite (MANNING & GOLDBERG 1997). Adding phosphate to natural systems has also been found to increase the mobility of As. 3.4.2 Carbonates There is evidence that carbonate minerals could be important in controlling the aqueous concentration of As, especially at higher ph values. Measurements showed that there is no adsorption of As(III) by CaCO 3 at ph 7. Adsorption of As(V) by calcite increased from a minimum of 0.7 mm/kg at ph 6 to a maximum of 2.0 mm/kg at ph 11. The effect of CO3 on As(V) adsorption was examined because carbonate is one of the most ubiquitous and important aqueous anions in the environment. The presence of CO3 decreased the extent of As(V) adsorption slightly.
3.4.3 Oxides of aluminum and iron The Al(III) atom has the same charge and a nearly identical radius as the Fe(III) atom. As a result, the common hydrous Al oxide phases are structurally similar to hydrous Fe oxides. They have also significant adsorption capacity for As, and their ph dependent adsorption isotherms are similar to those for Fe oxides and hydroxides. As, CH 3 AsO(OH) 2 0 and (CH 3 ) 2 AsOOH 0 were strongly adsorbed up to ph about 7 by amorphous Al(OH) 3, crystalline AL(OH) 3 (gibbsite), α- Al 2 O 3 and γ-al 2 O 3. Adsorption significantly decreased at higher ph values. Fig. 5: Adsorption of As on amorphous Al oxide as a function of ph and As redoxstate (GOLDBERG 2002). As(III) adsorption increased from ph 3 to a maximum at ph 8 then decreased at higher ph values (Fig. 5). 100 % of As(V) is adsorbed from ph 3-9 and then decreases with increasing solution ph. In all done experiments, the amount of adsorption As species increased as initial aqueous concentration increased until sites become saturated. Experiments that compared As adsorption by Fe and Al minerals showed that there is a slightly higher adsorption of As(V) by hydrous Fe oxide than hydrous Al oxide at ph values of 5-8. Fig. 6: Adsorption of As on amorphous Fe oxide as a function of ph and As redoxstate (GOLDBERG 2002). As(V) adsorption on amorphous Fe oxide shows 100 % adsorption from ph 3 to ph 7 and decreasing adsorption with increasing ph above ph 8. In contrast to its behaviour on Al oxide, arsenite adsorption on Fe oxide was virtually 100% throughout the entire ph range from ph 2.5 to ph 10.5 (Fig. 6). Thus it appears that amorphous Fe oxide had a grater affinity for adsorption of arsenite than amorphous Al oxide.
3.5 Organic compounds Organic compounds such as humic acid can adsorb on aquifer material or be present in aquifers as a result of depositional history. These compounds contain surface functional groups which can adsorb ions from solution (WELCH & STOLLENWERK 2003). Adsorption of As by two humic acids is a function of ph, As speciation and the humic acid composition. Experiments have shown that the adsorption of As(V) was slightly greater than As(III). However, the ph effect differed with humic acid composition. Arsenate and arsenite adsorption is differently depending on ash and Ca content. As(V) has a maximum at ph 6, whereas As(III) adsorption has a maximum at ph 8.5. Humic acids with lower ash and Ca content, both As(V) and As(III) exhibited broad adsorption maxima between ph 5.5 and 7.5. 4 Factors controlling adsorption/desorption of uranium 4.1 ph value Despite the influence of ionic strength on adsorption, (Fig. 11) shows the effects of ph at a constant ionic strength of 0.1 M. At low ph values, the adsorption of U(VI) approximately zero. The adsorption increases rapidly with ph over a relative narrow ph range of 4.5 to 5.5. With increasing ph the adsorption decreases also over a narrow ph range of 7.5 to 8.5. The reason for this could be the increase in the dissolved carbonate concentration with ph at constant carbon dioxide partial pressure or a concurrent increase in the concentration of U(VI)-carbonate complexes. There are three ph regions where carbonate exhibits different effects on U(VI) adsorption. The first ph region is at ph<5.0, the second region is between 5.0<pH<8.0, and the third region is at ph >8.0. At ph <5.0, carbonate does not have a negative effect on U(VI) adsorption on iron hydroxides. There is similar U(VI) adsorption behavior in the presence of carbonate at low ph. When ph is between 5.0 and 8.0, aqueous U(VI) is present as uranyl monocarbonate, uranyl dicarbonate, and uranyl tricarbonate. As total carbonate concentration increased, aqueous multicarbonate U(VI) complexes became more predominant and started forming at lower. Furthermore, carbonate attains maximum adsorption at ph 5.5. The reduced U(VI) adsorption could be attributed to the low affinity of uranium(vi) carbonate complexes to the surface sites and the competitive adsorption of carbonate on the surface. In the third region at ph>8.0, carbonate does not adsorb to the iron hydroxides and thus does not compete for the adsorption sites but rather competes with the surface to complex U(VI) as aqueous uranyl tricarbonate. At ph >8.0 and in the presence of carbonate, uranium(vi) tricarbonate is the predominant aqueous complex which prevents the adsorption of U(VI). 4.2 Clay minerals/humic acids Batch experiments for kaolinite and U(VI) are shown in Fig. 7. Fig. 7: Adsorption of U(VI) from solution as a function of ph and U(VI) concentration in the a) presence and b) absence of CO 2 (KREPELOVA, SACHS & BERNHARD 2006) In the presence of CO 2, the percentage of the total U(VI) adsorbed onto kaolinite increases from nearly zero at ph 3 to 97% between ph 6 and ph 8. Above ph 8, the U(VI) sorption decreases. The highest U(VI) adsorption occurs in the ph range, where the U(VI) hydroxyl complexes are important. The low adsorption rate at low ph values suggests the formation of relative strong inner-sphere complexes. The U(VI) sorption with and without CO 2 is comparable in the ph range between ph 3 and ph 8.
At ph > 8, however, no sorption decrease was observed in the absence of CO 2. This behavouir is a result of U(VI) speciation in the solution. In the presence of CO 2, U(VI) forms negatively charged uranyl-carbonato complexes. (e.g. UO 2 (CO 3 ) 3 4- ). Under these conditions the kaolinite surface is also negatively charged. Therefore, the electrostatic repulsions between uranyl-carbonato complexes and kaolinite result in the low U(VI) adsorption in this ph range. An increase of U(VI) initial concentration from 10-6 M to 10-5 M causes a shift of the sorption ph edge by one ph unit to higher ph values. With a higher intial concentration more U (VI) is adsorbed in the maximum of sorption curves but in general the percentage of U(VI) adsorbed onto kaolinite decreases due to higher initial concentration of U(VI) (KREPELOVA, SACHS & BERNHARD 2006). Fig. 8: Adsorption of U(VI) from solution as a function of ph, humic acid (HA) and U(VI) concentration in the a) presence and b) absence of CO2 (KREPELOVA, SACHS & BERNHARD 2006). The presence of HA influences significantly the adsorption of U(VI) onto kaolinite in the entire studied ph range (Fig. 8). At ph < 5, an increase in the U(VI) uptake was observed compared with the HA-free system. HA is almost 100% adsorbed on kaolinite surface in this ph range. It was expected that HA plays a competitive role under these conditions and results in reduction of U(VI). But the adsorbed HA offers additional binding sites for U(VI), therefore the U(VI) can rise up. At ph 5, desorption of HA from the kaolinite surface starts and leads to a decrease of U(VI) adsorption until ph 8.5. At ph > 8.5 in the presence of CO 2, the sorption of U(VI) again increases in the presence of HA. As HA is nearly completely desorbed from kaolinite surface (10 % remain adsorbed at ph 8.5) instead of decreasing adsorption of U(VI), adsorption is enhanced. Without CO 2,there is no decrease of U(VI) adsorption in the presence of HA. That s why carbonate has to play an important role in U(VI) sorption onto kaolinite. It is possible that uranyl-carbonato-humate complexes are formed, which can interact with the kaolinte surface and therefore, enhance U(VI) sorption onto kaolinite in the presence of HA in alkaline ph region. A higher concentration of HA causes lower adsorption of U(VI) on the kaolinite surface (KREPELOVA, SACHS & BERNHARD 2006). 4.3 Iron hydroxides/carbonate Carbonate dramatically affects the adsorption of uranium (U(VI)) onto iron hydroxides and its mobility in the natural environment. The amount of adsorbed U(VI) decreased substantially with increasing carbonate concentrations. Several factors could contribute to the reduced capacity of iron hydroxides to adsorb U(VI) in the presence of carbonate. It is well known that carbonate adsorbs onto hydroxides surfaces. As a result, fewer adsorption surface sites become available to other adsorbates. Carbonate uptake on the hydroxides surfaces will also decrease the surface charge. Aqueous carbonate also forms strong U(VI) complexes, which will have low affinity for the surface sites. Overall, carbonate competes with U(VI) for the adsorption sites and it also competes with the surface sites to complex U(VI). There are three ph regions where carbonate exhibits different effects on U(VI) adsorption. The first ph region is at ph<5.0, the second region is between 5.0<pH<8.0, and the third region is at ph >8.0. At ph <5.0, carbonate does not have a negative effect on U(VI) adsorption on iron hydroxides. When ph is between 5.0 and 8.0, aqueous U(VI) is present as uranyl monocarbonate, uranyl dicarbonate, and uranyl tricarbonate. As total carbonate concentration increased, aqueous
multicarbonate U(VI) complexes became more predominant and started forming at lower ph. Furthermore, carbonate attains maximum adsorption at ph 5.5. The reduced U(VI) adsorption could be attributed to the low affinity of uranium(vi) carbonate complexes to the surface sites and the competitive adsorption of carbonate on the surface. In the third region at ph>8.0, carbonate does not adsorb to the iron hydroxides and thus does not compete for the adsorption sites but rather competes with the surface to complex U(VI) as aqueous uranyl tricarbonate. At ph >8.0 and in the presence of carbonate, uranium(vi) tricarbonate is the predominant aqueous complex which prevents the adsorption of U(VI) (WAZNE ET AL. 2003). 4.4 Iron oxyhydroxide/phosphate Phosphate strongly adsorbed to goethite, and the extent of adsorption decreased with increasing ph. The adsorption of U(VI) onto the surface of goethite in the absence of phosphate is shown in Fig. 15. The extent of U(VI) adsorbed was strongly dependent on solution ph. Below ph 4.0, at total Fe concentration of 3.15*10-4 M, almost all U(VI) remained in the aqueous phase. The percentage of U(VI) adsorbed increased sharply between ph 4 and ph 6, and more than 99% of the U(VI) was adsorbed above ph 6. The ph edge shifted to the left with an increase in total Fe concentration from 3.15*10-4 to 3.15*10-3 M, indicating more U(VI) adsorption at the same ph due to a higher concentration of available surface sites (CHENG ET AL. 2003). In the low ph range, the addition of phosphate greatly increased U(VI) adsorption. Higher phosphate concentration generally caused a greater effect (Fig. 9). Fig. 9: U(VI) adsorption on goethite in the absence of 5 Summary (CHENG ET AL. 2003). This table should show an over all overview about the main factors controlling the transport of uranium and arsenic in ground water. Tab. 1: Summary of controlling factors fort the mobility of arsenic factor redox conditions arsenic arsenite (As(III)) arsenate (As(V)) mobile under low Eh-conditions less adsorbed at oxides or hydroxides predominant under oxidizing conditions complexes are negatively
ph value clay minerals oxides competitive ions phosphate carbonate organic compounds (adsorption differs depending on humic acid composition) because of zero charge of the complex minimum adsorption under low ph high mobile and increasingly adsorbed at high ph values Low adsorption at low ph values but increases with increasing ph high adsorption from ph 3 to ph 8 then decreases at high ph values (Al oxides) more constant adsorption by iron oxide over the entire ph range Greatest effect at low ph values decreased adsorption No adsorption by CaCO 3 Higher adsorption maximum charged and adsorbed on surfaces low mobility adsorbed previously at low ph, mobile under high ph values highest adsorption up to ph 7 then decreasing mobile at ph > 7. Much more adsorbed than As(III) at ph < 7 adsorbed from ph 3-9 and then decreases with increasing solution ph (Aloxides) more adsorbed by iron oxides over the whole ph range Low adsorption over the entire ph range with increasing phosphate concentration Increased adsorption by Ca CaCO 3 from ph 6 to ph 11 higher adsorption than As(III) Tab. 2: Summary of controlling factors fort the mobility of uranium factor redox conditions uranium U(IV) low soluble under reducing conditions no transport, high adsorption (e.g. minerals) U(VI) mobile under oxidizing conditions low sorption of uranyl-carbonato complexes with
ph value high soluble under oxidizing conditions ph doesn t play an important role, because U(IV) is often quickly oxidized to U(VI). U(IV) is primarily bound in minerals increasing solubility of these complexes Higher ph values uranylcarbonato complexes with increasing solubility of these complexes Low ph: uranylion with high solubility and mobility clay minerals No data High adsorption from ph 4 untill ph 7 low mobility Higher initial concentration of U(VI) increases the adsorption oxyhydroxid No data High adsorption between ph 4 to 6 and over ph 6 Higher iron concentration increases U(VI) adsorption competitive ions carbonate No data Increasing carbonate concentration higher adsorption phosphate No data Increasing phosphate concentration higher adsorption of U(VI) at low ph values 6 Literature cited WELCH, A.H. & STOLLENWERK, K.G. (2003): Arsenic in Ground Water Geochemistry and Occurrence: P. 27 67, P. 67-101 MERKEL, B. J. HASCHE-BERGER, A. (2005): Uranium in the environment: mining impact and consequences: P: XVII Preface LANGMUIR, D. (1997) Aqueous Environmental Geochemistry: P. 495 512 WILLIAMS, L. E., BARNETT, M.O., KRAMER, T. A. & MELVILLE, J. G. (2003): Adsorption and Transport of Arsenic(V) in Experimental Subsurface Systems In: Journal of Environmental Quality, Vol.32, P: 841-850 MANNING, B. A. & GOLDBERG, S. (1997): Adsorption and Stability of Arsenic(III) at the Clay Mineral Water Interface In: Environmental Science & Technology (1997), Vol. 31, No. 7, P: 2005 2011
GOLDBER, S. (2002): Competitive Adsorption of Arsenate and Arsenite on Oxides and Clay Minerals In: Soil Science Society (2002), Vol. 66, P: 413-421 KREPELOVÁ, A., SACHS, S. & BERNHARD, G. (2006): Uranium(VI) sorption onto kaolinite in the presence and absence of humic acid In: Radiochimica Acta (2006), Vol. 94, P: 825-833 WAZNE, M., KOFIATIS, G. P. & MENG, X. (2003): Carbonate Effects on Hexavalent Uranium Adsorption by Iron Oxyhydroxide In: Environmental Science & Technology (2003), Vol. 37, P: 3619-3624 CHENG, T., BARNETT, M. O., RODEN, E. E. & ZHUNG, J. (2004): Effects of Phosphate on Uranium(VI) Adsorption to Goethite-Coated Sand In: Environmental Science & Technology (2004), Vol. 38, P: 6059-6065