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Analysis of Uranyl Carbonate Complexes in an Open and Close System

ANALYSIS ofU RANYL CARBONATE COMPLEXES INan OPEN AND CWSE

ENVIRONMENTAL SYSTEM by ANALYTICAL CBlMISTRY

SABAT SIMBOWN*

ABSTRAKKOMPLEKS URANIL KARBONAT DI DALAM SISTEM LINGKUNGANTERBUKA DAN TEKTUTUP DITINJAU DARI SUDUT KIMIA ANALITIK.Diternngkan perubahaJl konsentmsi ion ~ 2- di dalam air alam pada lingkungan terbuka daD

terbltup berasal dari gas C~ yang te~ di udaIa teIbadap pH Jarutan. Reaksi antarn kationUO!+ dengan anion c~ 2- juga diternngkan dengan menggunakan perhitungan, peibandingan

antara masing-masing species kompleks manil kaIbonat terbadap niJai totalnya. Pernngkatlunak Lotus 123 release 2.4 digunakan untuk menghitung masing-masing molfiaksi darimasing-masing spesies kompleks manil kaIbonat terhadap pH lamtan. Didapatkan bahwaspesies kompleks urnnil kaIbonat di dalam air alam pada lingkungan terbuka pada kondisiasam lebih sederhana dibandingkan dengan di dalam air alam pada lingkungan tertutup.Sebaliknya kompleks manil kaIbonat di dalam air pada lingkungan telbuka daD daIamsuasana basa kandungannya lebih sederhana daripada di daIam air alam pada lingkunganterbltup.

ABSTRACTURANll- CARBONATE COMPLEXES IN OPEN AND CWSED ENVIRONMENTALSYSTEM FROM ANALYTICAL CHEMISTRY VIEWPOINT. Variation of C~ 2- ion

concentration from C~ gas originated from abDosphere, in nabJral water in an open andclose system versus pH of the solution is described. Reaction between u~ 2+ cation with

CO32-anion is also described by using calcuJation, comparison of each species of Ul3Dylcarbonate complexes to its total values. Each mol fiaction of uranyl corbonate complexversus pH of the solution is calculated using Lotus 123 release 2.4. It is found that Ul3Dylcarbonate complexes species in natural water which is slightly acid in an open system issimpler than that in a close syStem. On the other hand, uranil carbonate complexes species innaturnl water which is slightly base is simpler in an open system than that in a close system.

atmosphere because chemical andtransportation induStly are burning fossilfuel in large amount The huge amount ofC~ in abnosphere can be understood fromthe ~n house effect and corrosion of

INTRODUCTIONCarbon dioxide is an important element for

various processes, such as biological processof respirntion, biosynthesis and other.Recently C~ gas is rising rapidly in

.Researcher in the Re..c;earch and Development Center for Advanced Technology (p3TM)BAT AN

26

Widyanuklida No.3 Vol2, Agust. 2000:26-32

1'31, log 132 and log 133 of U~CO3,UO2(CO3)22-., UOi~)34-. Theconcentrntion of uranyl ion or metal andcomplex ion in water is govemoo by pH ofthe solutiOll

This paper will discuss of uranylcarbonate and its complexes in an open andclose system in VaJying pH value by use ofLotus 123 as a tool for the calculationwhich is based on 131, !32and 133 values.

THEORYMost commonly used equilibrium

between carbonate species and hydrogen isHenry's constant, which is written asfollowsCO2 (gas) --:;. Caz (sol) k=~=lO-I.5Where khis Henry's constantCOz(gas) + H2O --:;. H2C~ Km=lO-2.8H2CO3 --:;. W+HC~- K1=lO-3.5H2CO3 --:;. W + HCO3- Kal=IO-6.3HCO3- _-:;.W+C~2- K.z= 10-1 0,3

steels which can become serious problems,especially in oil and petrochemical industries.

Compared with other gasses, C~ gas isquite soluble in water. Some of soluble CO2gas in water will react with water to formcarl>onic acid. Concentrn1ion of caIbondioxide in caIbonic acid solution depends onthe partial pressure of caIbon dioxide gas inatmosphere. A higller partial pressure ofcarl>on dioxide gas in atmosphere willproduce higher cocentration of CO2 insolution. So the chelnical equilibrium of thecarl>onate system pblYS an important role inenvironments, for it will react with othercation in naturnl water, such asCa2+, U~ 2+ ,&2+ ,Fe2+ etc.

Carbonic acid in water will dissociate intoits anions and cations, in two steps. Theamount of carl>onate ion in solution stronglydepends on the pH of the solution. H2COJ,HC~ + and C~ 2- are not found in solution of

the same pH. The amount of [C~r- insolution is not detennined only by pH of thesolution, but it is also detennined byproperties of the system. In an open system,the concentration of [CO3r- is higher thanthat in a close system, because CO2 gas isalways supplied by the atmosphere. On theother hand CO2 gas in a close system islimited.

Uranium, is an element wich has manyoxidation states (+3,+4,+5 and +6), in which+3 and +5 states are unstable, +4 is less stablein the air, and +6 stile is stable. Uranyl ion,UO22+ with oxidation state +6, is alwaysfound in uranium minerals, such as incarnotite -K2(u~h(VO4)2.3H2O,rutherfordine U~CO3 etc. RutherfordineU02C~ mineral is slightly soluble in water.

Sea and ground water always containurnnyl ion or uranyl carl>onate complex.These are caused by uranyl ion which has astrong tendency to form carl>onate complexesin caIbonate media, uranyl carl>onatecomplexes such as U02(C~h2-, UO2(CO3 h 4-

The relatively strong complexes of urnnylion with caIbonate can be seen ftom thevalues of log

Figure 1 shows the effect pH variation onmole fraction of each carbonate ions. It canbe seen that H2CO3 predominantly exists ina solution with pH less than 6. For pHUIKler 4, the amount of [HCO3]- and[~f- in solution are very low and theycan not be seen in mole fraction as seenfigure I above. [HC~)- existspredominantly in a range of 6 to 8,5. In thisrange, H2CO3 is decreasing drastically. In arange of 10 to 14, [CO3f- is predominant,while [HCO3]- is decreasing and H2CO3 isalmost nothing.

The amount of [CO3f- is necessary toknow because it will affect the results ofreaction of uranyl and carbonate ion. Forcalculating [C~f- concentl"dtion in openand close systems, the value of normalatmosphere is used. In normal atmosphere,the air contains 10-3,5 atm C~, while itsequilibrium with ~ (sol) can be obtainedfrom Henry's constant above, [H2C~] =[~ (sol)] =~ co2 = 10-1,5 X 10-5 M

27

Analysis of Uranyl Carbonate Complexes in an Open and Close System

[C~]As mentioned above that the amount of[CO3f- in solution is not detennined only bypH of the solution, bllt also by properties ofthe system. Figure 2 shows that in an opensystem, carbonate species in solution arealways in equilibrium with C~ gas, so it isreasonable to take the concentration ofH2CO3 to be 10-5 M. For others species, suchas CO3 and HC03, they can be calculated asfollows.

= CTCO3 (l/(H)2/Kal Ka2+

(H)/Ka2+ 1 .(5)

From Figure 2, it can known as following.The amount of [CO3]2- in an open systemincreases linearly and rapidly for tile rangeof pH 1-11, while for pH > 11 it remainsconstant If tilere are some uranyl ion in anopened and close system, uranyl carbonatesand its complexes would be produced, foruranyl and carbonates ions will react toform uranyl carbonate first. Since uranylcarbonates will also react witil oiliercarbonate ion to form uranyl complexes, tileresult of tilose reactions are not simple. Asmentioned above, tile form and tileconcentration or mole fraction of carbonateion depend on tile pH of tile solution, tilereaction between uranyl and carbonate ionsdepends on pH of tile solution too. Forforming complex reaction between uranyland carbonates ions, it has to have enoughuranyl and carbonates ions in tile samesolution. The amount of each complex canbe calculated and it is based on tilechemical equilibrium of tile reaction below.Chemical reaction between uranyl andcarbonate ions can be written as follows.

(1)H2C~ ~HC~- +HI-

kal = (H1 [HC~]/ [H2C~]HCO3 ~ C~2- + H-

ka2 = (H1 [CO3]/ [H2C~] (2)

Since in an open system, the concentrntion ofH2CO3 is always constant at 10-5 M, then theconcentrntion of [HCO3J will be 10-5 X Kal /[H1 and log [HCO3J = -5 -pKal + pH = pH

-11,3. This equation produces a straight linefor [HCO3J versus pH as shown in figure 2.Similary, for [CO3J concentrntion which canbe calculated from eq 2 gives log [CO3J = -5 -pKal- PKaz + 2pH = 2pH -21,6. This

equation also produces a straight line for log[CO3J versus pH as shown in figure 2.

In a close system, carbonate system followsthe normal acid which dissociaties into 2species, HCO3- and CO32-respectively.

u~ 2+ + C~ 2- -+ U~CO3

~I = 1010 (6)

U~CO3 + CO32- -+UO2(C~)22-132 = 1017 (7)

U~(C~) 2 2- + C~ 2- -+ UO2(C~) 3 4-

133= 1021,6 (8)

H2C~ -+ HCO3 + ~tal = H1 [HC~]! [H2CO3]

HCO3 -+ CO3 + Wka2 = H1 [CO3]! [H2CO3]

If the total concentration of carbonates is 10-5M, the amount of (CO3r- , (HCOJJ- andH2CO3 then can be c.1.lcuJated. Based on thetnaSs balance, the total [CO3J is CTCO3 =(H2CO3J + (HCO3J + (CO3J = 10-5.

where 13 = Summed Fonnation ConstantK == Stepwise Formation Constant,

and131 = kl

132 = kl X k2

133 = kl X k2 X k3The lines for each of the C~ species can

be drawn by using the following expression.

.(3)

[H2CO3] = CTCO3 (1/(I+K..\/[H) +

2K..\ ~/[HI [HC~] = CTCO3 (1/[H] / K..\ +

1+~/[H))

RESULT AND DISCUSSIONThe effect of pH on the H2Co3 amd itsions:Natural water containing carbonates andnuanyl ions will react to foOD uranyl.(4)

28

Widyanuklida No.3 Vol.2, Agust. 2000:26-32

H2C~ and the uranyl ion predomint in thesolution. In a solution of the pH less than 4,there is no reaction between H2Co3 anduranyl ion. Uranyl ion reacts with carl>onateion, if there is enough (c~f- in thesolution. In addition, if the (C~)2-concentration is very low, the concentrationof (U02)2+ will be governed by thesolubility of U02CO3. However, at higheJr(C~)2- concentration, reaction of eq (6,7,.&)above are driven to