Determination of Ultra Trace Level of Uranium in Ground Water

Determination of Ultra Trace Level of Uranium in Ground Water of Different Geo-Chemical Environmentby Adsorptive Stripping VoltammetryR.K.Singhal&V.M.Joshi&J.Preetha&Rupali Karpe&Ajay Kumar&A.G.HegdeReceived:7December2006/Accepted:20March2007/Published online:21April2007#Springer Science+Business Media B.V.2007Abstract The mineral mobilisation in ground water due to rock water interaction largely depends on the localized subsurface geo-chemical environment.Dur-ing this work,traces of uranium were measured by adsorptive stripping voltametry(AdSV)in groundwa-ter having various level of dissolved minerals. Experimental solutions were prepared in the laboratory by spiking natural groundwater with standard solutions of major cations(Ca2+,Mg2+,K+),anions(Cl−1,F−1, and PO3À4),trace metal ions(Cu and Pb)and DOC (Dissolved Organic Carbon).The concentrations of these anions,cations and DOC in subsurface water changes due to changes in the geo-chemical environ-ment at different locations.Experimental results show that major anions do not have any pronounced impact on the analysis of uranium.In the case of cations, only Cu2+shows strong interferences.However addition of2%Ethylene Diammine Tetra-acetic Acid (EDTA)solution in the ratio of1:25removes interfer-ence due to copper upto a concentration of<0.2ppm. Interference with copper was also studied by varying the accumulation potential,pH and concentration of chloroanilic acid.Though no interferences were observed in ground water having dissolved organic carbon(DOC)in the range of0.01–15ppm,between 15–16ppm of DOC,the ability to detect uranium by AdSV decreases sharply.Further,if DOC exceeded 16ppm it was not possible to do the analysis of uranium by AdSV without destruction of DOC,as DOC is a surface active organic compound and accumulates on Hg electrode preferentially over uranium-chloroanilic complex.Keywords Ground water.Minerals.Adsorptive Stripping V oltametry(AdSV).Chloroanlic acid1IntroductionAs per the United States,Environmental Protection Agency(USEPA),the chemical toxicity of uranium is more important than the radiological toxicity and the limit of uranium in various sources of drinking water system was reduced from100ppb to30ppb(USEPAWater Air Soil Pollut(2007)184:17–27DOI10.1007/s11270-007-9395-zR.K.Singhal(*):V.M.Joshi:J.Preetha:R.Karpe: A.Kumar:A.G.HegdeEnvironmental Studies Section/Health Physics Division, Bhabha Atomic Research Center,Trombay,Mumbai400085,Indiae-mail:rsinghal@.inV.M.Joshie-mail:vmj@.inJ.Preethae-mail:pjay@.inR.Karpee-mail:chakorkarpe@yahoo.co.inA.Kumare-mail:ajaykls@.inA.G.Hegdee-mail:aghegde@.in2001).Hence efforts are being made to accurately determine uranium concentrations in drinking water. There are several sources of uranium in groundwater such as rainwater,dissolution of soluble minerals and rock-water interactions.The water entering the ground through precipitation has essentially the same chemical composition as in the precipitation.As the water moves through the vadose zone its chemical composition changes due to addition of organic material and soluble salts.In the saturated zone,the water is isolated from the atmosphere and as the water moves in the ground water,oxygen is consumed by the oxidation of DOC.This changes the oxidation–reduction potential of the system and leads to a variety of rock-water interactions.The origin of major aqueous species in ground water is given in Table1 (Berner and Berner1996).Uranium occurs in rocks and sediments as(1) individual primary and secondary minerals(90%are secondary minerals),(2)isomorphous mixtures in the crystal lattices of other minerals,(3)in the dispersedstate in the form of the uranyl group(UOþ22)and(4)as dissolved compounds in ground water and soil moisture(Banks et al.1995;Copenhaver et al.1992; Porcelli and Swarzenski2003;Zouridakis et al. 2002).The levels of naturally occurring radionuclides in ground water is very low and in general are of no concern from the point of view of radiological impact on human health(Singhal et al.2004).In some of the ground water samples,direct determination of uranium in water sample is not possible by adsorptive striping voltametry due to interferences from other dissolved elements.The present work was performed with a view to identify the interfering elements in the direct analysis of uranium as UO2þ2in ground water.2Materials and Methods2.1Sample CollectionThe ground water samples were collected from three different locations spread over a distance of1.6km at Trombay,Mumbai having latitude19°00′498(N)and longitude72°55′136.Seawater samples were collect-ed at1km off-shore in Mumbai Harbour Bay situated between19°N,72.5°E and19.5°N and73°E.The samples were collected in1l polypropylene bottles.Various physicochemical characteristics including the concentration of uranium was measured in these samples.2.2Sample PreparationThe experimental solutions were prepared,by spiking samples collected from the ambient subsurface envi-ronment and sea water.The samples were collected in pre-washed polypropylene bottles.Various cations, anions and DOC was measured in the collected sample after filtering through0.45μm filter using suction filtration.Total Organic Carbon(TOC)analyzer (Shimadzu,TOC-V CSH)was used and the system was calibrated for the determination of TOC with standard solutions of sucrose.Thereafter samples were spiked with varying concentrations of cation,anions and DOC.To see the impact of variation in the concentration of uranium on the analysis,collected solution from the ambient environment was spiked with varying concentration of uranium.The samples were stored in acid washed200ml polypropylene bottles.The cation and anion in the samples were analysed by using Ion Chromatography(Dionex500). Table1Origin of major aqueous species in groundwater (Berner and Berner1996)Aqueous species OriginNa+NaCl dissolution(some pollution)Plagioclase weatheringRainwater additionK+Biotite weatheringK-feldspar weatheringMg+2Amphibole and pyroxene weatheringBiotite(and chlorite)weathering)Dolomite weatheringOlivine weatheringRainwater additionCa+2Calcite weatheringPlagioclase weatheringDolomite weatheringHCOÀ3Calcite and dolomite weatheringSilicate weatheringSO2À4Pyrite weathering(some pollution)CaSO4dissolutionRainwater additionCl-NaCl dissolution(some pollution)Rainwater additionH4SiO4(aq)Silicate weatheringThe calibration was carried out by using Dionex mixed standard solution for cations and anions Table2.2.3Determination of UraniumTraces of uranium in aquatic medium was very efficiently determined by electrochemical methods by Djogic et al.(1986,2001);Sander et al.(1995); Van den Berg and Huang(1984)and Wang and Setiadji(1992).During this work,ultra trace concen-trations of uranium(VI)has been determined using adsorptive stripping voltammetry of the uranium-chloroanilic acid complex(2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone).The detection limit was calculated as<100ng/l(<0.1ppb).V oltammetric measurements were performed using a Metrohm Autolab30in conjunction with the663V A stand.A multi-mode electrode,also from Metrohm,was used for the reproducible production of hanging mercury drop electrode(HMDE)as the working electrode.The three-electrode system was completed by Ag/AgC1 (3mol/l KCl)reference electrode and a platinum auxiliary electrode.All the AdSV measurements were carried out in the differential pulse(DP)mode using a pulse amplitude of−50mV,a pulse time of30ms and a potential step of4mV(Table3).All chemicals used were analytical reagent grade(Merck and Fluka).A stock solution with1g/l uranium was prepared by dissolv-ing1.78g(CH,COO)2UO22H2O in1l of water containing1ml HNO3(65%).Further dilutions were made as and when required.The concentration of the chloroanilic acid solution was1×10−4mol/l. The pH of the sample in the reaction vessel was adjusted as2using supra-pure7.5M HNO3.3Results and DiscussionAdSV is known to be a very sensitive method for the determination of metals ions which form a strong complex with organic molecules.These organic molecules are surface active and are adsorbed on to the mercury drop which acts as a working electrode in AdSV(Thomas and Henze2001).These adsorptions are preferred rather than charge transfer at the electrode.Uranium forms a1:1complex with chlor-oanilic acid and gets adsorbed on the working electrode.A wide range of metal ions present in the water bodies also form metal chelate complexes.These complexes also get adsorbed on mercury electrode and may interfere in the analysis of uranium.When the adsorbed compounds can be oxidised or reduced,they may be estimated by stripping them from the electrode surface using differential pulse voltametry.Other metals which form complexes with chloroanilic acid do not inter-fere with the uranium determination either because their chloroanilic acid complexes have different accumulation potential or they have different strip-ping peak potential.The formation of the complexes, their stability,and the reduction potentials are strongly dependent upon the pH of the solution.In general,in ground water having pH between4.6and 6.5,uranium is present as carbonate complexes (UO2(CO3)3)−2along with Na+,Ca+2and Mg+2and at pH between7and8it occurs as(UO2(OH))+ (Singhal et al.2005b).In solutions having pH<4it occurs as sulfate compounds UO2(SO4)m.Sander Sylvia(1999)observed that the amount of current generated during the process of reduction of UO2þ2on the working electrode is highly pH dependent.Optimization of current due to reduction potential of uranium-Chloroanalic acid complex was done by studying the current vs pH variation.The variation in the peak height at different pH is shown in Fig.1.This figure shows that the maximum peak height is observed at pH2.It is known that uranium forms1:1complex with chloroanilic acid at pH2. This fact is in good agreement with the observations that chloranilic acid peak at0mV almost disappears when uranium is present in approximately equimolar concentrations.This would mean that a1:1uranium-chloranilic acid complex is formed.Table2gives the initial concentration of anions(Cl−1, F−1,and PO3À4),cations(Pb2+,Cu2+,Ca2+,Mg2+,Na+), DOC and uranium present in the water samples used in the present work.The variations in concentration of cations and anions in the groundwater is due to rock(or sediment)–water interactions which in turn is controlled by the minerals present in the rocks(or sediment).3.1Influence of Anions(F−1,Cl−1PO3À4)and Cations(Ca2+,Mg2+,K+)on the Uranium Determination Water samples,having different concentrations of F−1, Cl−1,PO3À4,Ca2+,Mg2+K+,Cu2+and Pb2+were spiked with varying concentration of uranium in therange of 10–100ppb.The uranium in these samples were measured after 15days and the results are shown in Fig.2.From this figure,it is clear that there is no influence of these anion and cations on the determination of uranium and the minor variations are statistically insignificant.To see the impact of increase in concentration of F −1,Cl −1,PO 3À4,Ca 2+,Mg2+K +,Cu 2+and Pb 2+the experimental solution were spiked by the addition of standards of these anions and cations.The concentra-tion of these anion and cations was increased by 30%from the original concentration in their respective water samples.The samples were spiked with uranium so that the total uranium in all the samples were 10ppb after taking into account the naturally dissolved uranium.The increase in ionic strength doesnot have any impact on the reduction of UO 2þ2i.e.U(VI)during the process of stripping.Reduction of U (IV)at various ionic strengths were also studied in details by Djogic and Branica (1993).Figures 3and 4shows that there is no impact on the uranium peak at various concentrations of anions(F −1,Cl −1,PO 3À4)and cations (Ca 2+,Mg2+K +).Small variations found are within statistical fluctuations.Therefore the maximum accumulation potential for uranium-chloroanilic acid is independent of the concentration of these anion and cations,although there is a possibility of forming Hg 2Cl 2layer at the Hg electrode at higher concentration of Cl −1(Thomas and Henze 2001).This accumulation,in turn affects the adsorption of uranium-chloroanilic complex on the electrode surface but this was not observed in the Cl −1concentration range of 20–24,000ppm.Direct determination of uranium in sea water where average Cl −1concentration in the range of 17,000–19,000ppm were made by Singhal et al.(2005a );Table 2Mean value of various physicochemical characteristics of the well water and sea water used for the experiment Sample (ID)Ground water-1Ground water-2Ground water-3Sea Water F −1(mg/l)0.19±0.050.15±0.040.67±0.030.11±0.01Cl −1(mg/l)20±1.621.3±1.268.0±1.818,000±450PO 3À4(mg/l)0.64±0.03NDNDNDNa +(mg/l)26.0±2.129.7±2.240.2±2.311,200±300K +(mg/l)0.82±0.01 1.74±0.05 2.41±0.14325±25Mg 2+(mg/l)16.2±1.223.2±2.124.5±1.81,150±53Ca 2+(mg/l)42.6±3.249.3±3.464.9±2.1300±20Cu 2+(μg/l) 6.1±0.8 5.1±0.95±0.85±0.8Pb 2+(μg/l) 3.2±0.65±0.94±0.74±0.7DOC (mg/l)10±0.8912±0.1914±1.027.3±0.9U (ppb)1.8±0.042.6±0.123.3±0.814.2±0.09567891011P e a k h e i g h t [-n A ]p HFig.1Dependance of peak height on pH {conditions uranium:2ppb,chloroanilic acid:1×10−4M with initial potential 50mV ,end poten-tial −100mV and step po-tential 0.6mV}Djogic and Branica (1991,1995);Kim and Zeitlin(1971).Similarly in the case of F −1and PO 3À4there is strong tendency for the uranium either to form its fluoride complex or uranyl phosphates (UO 2)3(PO 4)2.The experimental results however show that upto100ppb of uranium as UO 2þ2and F −and PO 3À4in the concentration range of 0.01–2.5ppm and 0.15–1.3ppm do not effect its determination and all theUO 2þ2are available for complexation with chloro-anilic acid.The major cations Ca 2+,Mg 2+,K +in the concen-tration range of 55–450,21–1,500and 1.2–425ppm do not affect the accumulation of uranium-chloroa-nilic acid complex (Fig.4).In the presence of these cations there is a possibility of accumulation of these ions on the surface of the mercury electrode by charge transfer rather than adsorption.3.2Influence of PbLead (Pb)To study the impact of Pb as Pb 2+on uranium determination by AdSV,experiments were conducted by spiking the solution about 65times more than the average concentrations of lead in drinking water i.e.3ppb.Figure 5shows that there is no major impact of lead on uranium determination by AdSV using chloroanilic acid.Only small varia-tion in peak height is observed i.e.peak height is about 10%less in presence of 192ppb lead with EDTA (2.73×10−4M)peak height is increase by 10%compare to solution having no lead.There is no peakTable 3Various optimum conditioned used for the analyis of uranium by adsorptive striping voltametry Sr.No.Pre-treatment1.Purge time (s)1202.Conditioning potential (V)03.Duration (s)04.Stirrer off during conditioning –5.Deposition potential (mV)1006.Duration (s)1807.Stirrer off during deposition –8.Equilibration time (s)5Measurement 9Cell off after measurements –10.Modulation time 0.0511.Interval time 0.15Potentials 12.Initial potential (mV)5013.End potential (mV)−10014.Step potential (mV)615.Modulation amplitude (V)−0.0499Chemical composition of the solution16.pH 1.817.Chloroanilic acid (CAA)concentration9.2×10−7102030405060708090100* Initial concentration of uranium taken into accountC o n c e n t r a t i o n o f u r a n i u m (p p b )Type of water samplesFig.2Impact of physico-chemical characteristics of water on the analysis of uraniumformation for lead alone with chloroanilic acid in the potential range set for the experiment.Similarly,no effects were observed by masking Pb by complexing with EDTA To verify the interferences due to these metal ions,experiments were performed in the presence of Ethylene Diamine Tetraacetic Acid (EDTA).EDTA has been used to mask the interfer-ence of Pb and Cu as EDTA is a strong complexing agent having a stability constant (Log β1)of 16.2and 18.5respectively for Pb and Cu.3.3Influence of CuCopper (Cu)In many places like Olympic dam at Australia and Jadugada at India where uranium is mined,the concentration of copper is very high,hence the interference of copper was studied in detail.The experiments were also conducted to see the effect of chloroanilic acid (CAA),pH and accumulation potential variation on the determination uranium in the presence of copper and EDTA.The experimental observation are as follows.3.3.1Impact of Variation in Concentration of Copper In the case of copper,strong interference was observed in the analysis of uranium if the concentra-tion of copper is more than 200ppb.From Fig.6,it is clear that Cu alone does not give any peak with chloroanilic acid.From the experimental observation,it is clear that no uranium reduction peak were observed in the potential range of −250–0mV .T y pe -1T y pe -2T y pe -3T y pe -4T y pe -5T y pe -6T y pe -7100100010000C l-1S a m p l e s T y p ep p m-1.0-0.50.00.51.01.52.02.53.0F-1p p m0.00.20.40.60.81.01.21.4p p m9.09.39.69.910.210.510.8p p bFig.3Impact of C −1,F −1,and PO 3À4on uranium de-termination by AdSVHowever complexing Cu by the addition of 1ml of2%EDTA solution in reaction mixture of 25ml i.e.final concentration of EDTA in the reaction mixture is 2.73×10−3M,uranium reduction peak reappear.But by further increasing the concentration from 0.2–77ppm,EDTA also was also not able to mask the Cu and a broad peak appeared and made it impossible to measure the concentration of uranium in the presence of copper.Van Den Berg and Huang (1984)had observed a similar interference of Cu in case of uranium determination by complexing with Catechol.They were also able to remove the interference by using EDTA.3.3.2Influence of Variation in Concentrationof Chloroanilic Acid (CAA)Variation in concentration of chloroanilic acid was carried in the range of 9.5×10−8M to 2.6×10−6M,keeping the concentration of uranium as 3.8ppb,EDTA as 2.73×10−4M,pH 1.8and accumulation potential as 100mV .Observed variation in concen-tration of uranium/current was shown in the Fig.7.From this figure its clear that solution having CAA concentration upto 4.7×10−7M have peak position at −20mV and peak height is little low but measurement of U having CAA in the range of 9.2×10−7M to 2.6×10−6M have almost negligible impact on uranium measurement giving constant current and peak posi-tion at −40mV .Therefore in all experiment optimum CAA concentration of 9.2×10−7M was used.3.3.3Influence of Variation in Accumulation Potential Variation in accumulation potential was carried out in the range of 10mV to 400mV ,keeping the concen-tration of uranium as 3.8ppb,EDTA as 2.73×10−4M CAA as 9.2×10−7M and pH as 1.8.Figure 8showsT y pe -1Ty p e -2T y p e -3T y p e -4T y p e -5T y p e -6T y p e -711010S a m p le T y p ep p m10100100p p m100200300400500p p m7.07.58.08.59.09.510.010.5p p bFig.4Impact of K +1,Mg +2,and Ca 2+on uranium determination by AdSVthe variation in current/uranium concentration at dif-ferent accumulation potential.From this figure it is clear that optimum potential is 100/150mV as on other potential either no peak was observed otherwise the peak height is quite low.Therefore in all experiments optimum accumulation potential of 100mV was used.3.3.4Influence of variation in pHVariation in pH was carried out in the range of 0.5to 2.5,keeping the concentration of uranium as 3.8ppb,EDTA as 2.73×10−4M,CAA as 9.2×10−7M and deposition potential as 100mV .Figure 9shows the0.00E+0005.00E-0091.00E-0081.50E-0082.00E-0082.50E-0083.00E-0083.50E-0084.00E-008I /AE/VFig.5Variation in the cur-rent with applied voltage at different concentration of uranium and lead-0.25-0.20-0.15-0.10-0.050.00-2.00E-0080.00E+0002.00E-0084.00E-0086.00E-0088.00E-0081.00E-0071.20E-0071.40E-0071.60E-0071.80E-0072.00E-007I /AE/VFig.6Variation in the cur-rent with applied voltage at different concentration of uranium and coppervariation in peak current position at different pH.From this figure it is clear that optimum pH of the solution should be 1.8.Although pH 1.5also give a similar peak current but the position of the peak was shifted to −20mV.In view of this,observation,optimum pH of the solution was maintained at 1.8in all the experiments.3.4Influence of Dissolved Organic Compound(DOC)on the Uranium Determination DOC is ubiquitous in the aquatic subsurface and surface environment.Variation in the DOC content depends on many localized geo-chemical parameters like oxic and anoxic conditions,pH etc.Determina-tion of uranium in water by AdSV is based on adsorption of surface active compound on a hanging mercury electrode.In natural water samples,surface active substances such as surfactants,humic and fulvic acids are known as organic contaminants (DOC).By maintaining a constant potential on the working electrode,not only the metal complex is adsorbed but also organic surface active compounds could also be accumulated at the electrode surface.Depending on the strength of the adsorption,a competition of both analyte and organic surfactant takes place (Sander et al.1995).The influence of organic surfactants on the uranium determination was tested by Sander and Henze (1996)using alternating current investigations to explain the competitive1E-91E-81E-7I /AE/VFig.8Evaluation of 3.6ppb of U at accumulation poten-tial in therange of 10–400mV and in the presence of 5ppm of copper and 2.7×10−3M EDTA and 9.2×10−7M CAAI /AE/VFig.7Evaluation of 3.8ppb of uranium in the presence of 5ppm of copper and 2.7×10−3M of EDTAadsorption process on the electrode surface.The initial content of DOC was destroyed by refluxing the solution with H 2O 2in acidic medium and initial content of DOC in the experimental was 0.01ppm.By adding different concentrations of humic acid solution in the range of 0.02–16ppm,for a solution having 10ppb of uranium,it can be seen in Fig.10,that the peak height remains almost constant up to a humic acid concentration of 15ppm and thereafter the peak height decreases with the increase in concentrationof humic acid.There was no uranium peak at humic acid concentration of 16ppm.Therefore,destruction of DOC is essential for the analysis of uranium if sample have concentration of DOC more than 15ppm.4ConclusionsThe AdSV determination of uranium with chloro-anilic acid as complexing reagent is a very sensitive0.010.11100102030405060Concentration of DOC (ppm)P e a k h e i g h t (n A )246810Uranium concentration (ppb)Fig.10Influence of dis-solve organic compound on uranium analysis by AdSV-0.10-0.08-0.06-0.04-0.020.000.020.040.060.081E-91E-81E-7I /AE/VFig.9Evaluation of 3.6ppb of U at different pH in the range of 0.5–2.5and in the presence of 5ppm of copper and 2.7×10−3M EDTA and 9.2×10−7M CAAand a very selective method.Almost no interferences are caused either by major cations nor by anions in ground water(Singhal et al.2004).Observed inter-ference caused by deposition of surface active dissolved substances like DOC on Hg electrode is taken care of by the destruction of organic compound by wet digestion.This interference is observed only in the case of organic rich water samples.Sample pretreatment is not necessary in most of the cases as water having more than15ppm DOC is very rare except in samples collected from organically rich areas.The interference due to copper is mainly due to deposition of Cu-chloroanilic acid complex other than UO2þ2chloroanilic acid complex and to a limited extent,can be masked by the addition of EDTA which complexes Cu2+with EDTA.Acknowledgement The authors sincerely acknowledge the guidance and help provided by Shri M.L.Joshi,Head,Health Physics Division.Thanks are also due to Shri H.S.Kushwaha, Director,H,S and E Group,for constant encouragement. 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