Advanced Techniques for Measurement of Heavy Metal Concentrations From Seawater

8/3/2019 Advanced Techniques for Measurement of Heavy Metal Concentrations From Seawater

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Advanced Techniques for measurement of Heavy metal concentrations from seawater

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ABSTRACT

One of the major challenges in recovery of uranium from seawater was to assure quality assurance byaccurate measurement of heavy metal ion concentrations from lean solutions. To address the issue, nuclearanalytical techniques play a key role. Nuclear Analytical Techniques (NATs) are based on nuclear properties of thetrace level isotopes and have high sensitivity and accuracy and are important tools for chemical analysis. In this

paper, the application of Nuclear Analytical Techniques such as Neutron Activation Analysis (NAA) and Solid StateNuclear Track Detection (SSNTD) methods for determining the concentrations of heavy metal ions from leansolutions like seawater is presented. The procedures and time required for analysis are standardized and comparisonof results with non nuclear analytical techniques is made.

Keywords:Seawater; Radiation processing; Nuclear Analytical Techniques; Uranium pick up; Neutron Activation Analysis ;

Nuclear Track detection;

1.0 INTRODUCTION:

The rapid growth of Industry and commerce will lead to expanding demand for energy. We have no choicebut to widen our options for energy availability and develop viable strategies for energy security. India seeks

international co-operation in the field of civilian nuclear energy including China [1]. Uranium is important startingmaterial for nuclear route. In view of the anticipated exhaustion of terrestrial uranium reserve in the future, researchhas been directed towards the recovery of uranium and other heavy metal ions from non conventional sources suchas phosphoric acid and seawater. With its lean but clean resource, oceans can serve as constant potential source ofuranium and other valuable heavy metals for a long run. Uranium is one of the most valuable metals in seawater.The total volume of the oceans has been estimated to be 2x109 km3 and uranium content of oceans must be of theorder 4.5x109 tonnes [2-4]. Thus the oceans are virtually limitless reservoir of dissolved uranium in a well definedchemical environment.

Various inorganic and organic adsorbents have so far been synthesized and evaluated at laboratory level [4-6]. With the development of radiation processing technology, special adsorbents are synthesized by radiationgrafting of amidoxime groups in the form of leaflets, which provides efficient contact patterns with water body inthe sea. Bench scale in-field trials were initiated to develop process technology [3-6]. One of the major challenges

was to assure quality assurance by accurate measurement of heavy metal ion concentrations from lean solutions [5-8].

To address the above issues, nuclear analytical techniques are being developed. In this paper, theapplication of NATs such as Neutron Activation Analysis (NAA) technique and Solid State Nuclear TrackDetection (SSNTD) methods for analyzing the concentrations of heavy metal ions from lean solutions like seawateris presented [8-10]. Researchers from Japan, which is another leading country, taken active interest in this researcharea of uranium recovery, has adopted non nuclear analytical techniques such as ICP-AES for measurement of U+6

concentrations in feed and other solutions. Comparison of results both by nuclear analytical techniques and non-nuclear techniques are presented along with statistical analysis.

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2.0 NUCLEAR ANALYTICAL TECHNIQUES:

The NATs are based on nuclear properties of the trace level isotopes and have high sensitivity and accuracyand are important tools for chemical analysis. Suitable nuclear radiations ( , , and n ) are measured to estimatethe concentration of radiation emitting isotope which in turn is used to determine the elemental concentration incontrast to non nuclear techniques. Non nuclear techniques such as AAS and ICP-MS utilize the properties of theatom as a whole. Nuclear analytical techniques are broadly classified as direct methods and indirect methods.

Nuclear analytical techniques are quite useful and effective in view of capability of multi element analysis in avariety of samples and with good detection limits for a number of elements. These techniques are contamination free[7-10].

Uranium and thorium are naturally occurring elements which can be judiciously exploited to produceelectricity for centuries without the threat of global warming. Monazite (major naturally occurring source of Th)contains 8-9% Th, whereas uranium ores are sometimes quite less. There has been wide interest to exploit other lean

but abundant sources like rock phosphate and sea water for U recovery. Thus analysis of samples of ores, mineralsand seawater for U is important for prospecting these resources. NATs like Neutron Activation Analysis (NAA), X-ray Fluorescence (XRF), Passive Gamma ray Spectrometry (PGS) and Solid State Nuclear Track Detectors(SSNTD) play important role for qualitative as well as quantitative estimation of U in such important samples [7-10]. Whereas Instrumental NAA (INAA) , XRF and PGS are non-destructive in nature and capable of measuringconcentrations of U and Th in ppm range, SSNTD, on the other hand, is very sensitive technique for ultra lowconcentrations of uranium (in ppb range).

2.1 Nuclear Activation Analysis(NAA):

This technique is based on irradiation of a sample with neutrons available from nuclear reactors andsubsequent measurement of the induced radioactivity (b,g) for determination of the concentration of an element.

Neutron fluxes in the range of 1011

to 1015

cm-2

.s-1

are used. In NAA, the steps involved are: (i) Sampling (ii)Preparation of actual sample reference material (SRM) and control sample reference material (CRM) (iii)Preparation of the standards or single comparator (iv) Irradiation of samples and comparator in a reactor (v)Radioactive assay by high resolution -ray spectrometry (vi) Gamma spectra analysis for peak areas (vii) Evaluationof detection efficiency (for ko method) (viii) Calculation of elemental concentrations and (ix) Interpretation of theresults.

The koNAA method uses simultaneous irradiation of sample and a neutron flux monitor such as gold andthe use of a composite nuclear constant called ko . The ko factor is defined as

*

0

0 * * *

0

Mk

M

=

Where 0 is the (n, ) cross section for thermal neutrons; is the abundance of the isotope ofinterest; M is the average atomic mass of the element; is the absolute gamma-ray abundance; The symbol *refers to the corresponding parameters of comparator (Au197). The concentration of element is calculated using

( )

( )

* *0

*

0 0

/1. . .( / )

/

. . .

p

p

N LT

f QS D C W C g gk f QN LT

S D C w

+=

+

2


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Where pN - is the net counts; W and w - are masses of sample and single comparator; f-is sub cadmium to epi-

thermal neutron flux ratio; - is epi-thermal neutron flux ratio; - is efficiency of the detector; 0k , 0Q -

are two nuclear constants; ( 1 )tS e= - is the Saturation factor; ( )dtD e = - is the Decay factor;* / *

( (1 ))CL LT

C e

= - is the Correction factor; CL clock time of counting; LT live time of counting ; - isdecay constant (s-1); td is decay period; t is the duration irradiation

Instrumental NAA (INAA) is a very good technique to analyze solid samples with lower mass range [9-10]. INAA is an isotope specific technique for simultaneous multielement determination in diverse matrices. Itexperiences negligible matrix effect and needs small sample size (50-100 mg) for analysis. In the present work, solidadsorbents (PAO) used for extraction of uranium from seawater were analyzed for U estimation. Three sets ofsolidsamples (before and after elution) were analyzed by INAA using neutron irradiation at Apsara reactor at a neutronflux of 5x1011 cm-2.s-1 and radioactive assay by high resolution gamma ray spectrometry. Samples of about 100 mgmass and U-standards were packed in polythene and were irradiated for 7 hours in Apsara reactor. Radioactive assayof the daughter products of 239U (239Np, 2.27d, 106, 228 and 277 keV) were done by high-resolution gamma rayspectrometry using a 40% HPGe detector. Peak areas were determined by peak-fit software PHAST developed byElectronics Division, BARC. The relative method of INAA was used for calculation of concentration of heavy

metal, the expression which is given below.

sample

std

stdx

samplex

stdxsamplexD

D

cps

cpsmm ..

,

,

,,

=

Where mx, std and mx, sample are the masses of element, . ., x sample x std cps cps are count rates and Dstd and Dsample are decayfactors of activation products of standard and sample respectively.

2.2Nuclear Track Detection method(NTD):

Here the uranium target with the track detector placed on it is exposed to neutron flux. Polyester pieces of 2x2cm2 are used as nuclear track detectors for registration of fission fragment tracks. In SSNTD, fission trackregistration is found to be much more sensitive compared to alpha track registration due to low alpha specificactivity of uranium and high thermal neutron fission cross section.

Solid State Nuclear Track detectors ( SSNTDs) are insulating solids both naturally occurring and man-made.There are several types of these detectors including inorganic crystals, glasses and plastics. When a heavily ionisingcharged particle (alpha, fission fragment, etc) passes through such insulating solids, it leaves a narrow trail ofdamage about 50 in diameter along its path. This is called 'Latent Track' as it cannot be seen with the naked eye. Itis possible to view this latent track with an electron microscope. The exact nature of the physical and chemicalchanges occurring at the damage site depends on the charge (Z) and velocity ( = /c, where is the particlevelocity and c is the velocity of light) of the particle, on the chemical structure of the detector material and also onthe environmental conditions like temperature and pressure. These latent tracks can be enlarged / developed so thatthey can be viewed under an optical microscope by etching with some chemicals such as sodium hydroxide andhydrofluoric acid.

The Pneumatic Carrier facility of Dhruva was standardized for the estimation of uranium in uranyl nitratesolution samples in ppb range by using solid state nuclear track detection method [7,8]. For this purpose, uranylnitrate solutions in the concentration range ~10-1000 ppb were prepared from uranium SRM standard. The standarduranium solutions taken in polypropylene tubes along with the Lexan track detectors inside them were irradiated inthe Pneumatic Carrier Facility (PCF) of Dhruva, research reactor at BARC Mumbai for 1 min. A blank of 2MHNO3 was also irradiated along with the uranium solutions. The irradiated detectors were then etched in 6N NaOHat 60 o C for 1 hr to reveal fission tracks. The fission tracks were counted under an optical microscope to get trackdensity. A linear curve obtained between the track density and uranium concentration indicates the possibility ofusing this facility for the estimation of uranium in ppb range by track method. It has been found that this facility can

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be used for the uranium estimation upto 5 ppb for 1 min irradiation time. The detection limit of uranium can befurther lowered if the irradiation time is increased. The standardized method was then applied to the feed and elutessamples for uranium estimation. For the U estimation in the feed and elute samples, a U standard was alwaysirradiated along with the samples and the U concentration was calculated with respect to the standard. The trackdensity obtained by scanning a small representative detector area is related to concentration C (g/cc) of the element

by equation

wetd

K CXN t T

A

= where wetK is defined as the track registration efficiency in solution and is

expressed in cm; is the neutron flux in reactor and is the thermal neutron cross section. When simultaneousirradiation of a standard and sample is carried out, then only track densities and concentrations of standard andsample come into picture as shown in equation below:

( )

( tan ) tan

d sample sample

d s dard s dard

T C

T C=

The above method makes error independent arising due to small variations in the flux. The PCF facility ofDhruva was standardized for the estimation of uranium in uranyl nitrate solution in ppb levels using SSNTD for thefirst time [7]. The standardized method was then applied to the feed and elute samples of RUSWapp facility,routinely generated from Desalination Division for uranium estimation.

3. APPLICATION FOR MEASUREMENT OF URANIUM IN SEAWATER:

Analysis of feed and elute samples:

During lab-scale experiments, many types of fiber cross sections and geometry were evaluated for theirefficacy for grafting purposes. Polyester and polypropylene fiber materials were short listed for further experiments.Finally experiments are done using polypropylene fiber of 1.5 denier cross section as stem material in non-wovenfelt form. Electron Beam Radiation induced grafting of acrylonitrile on the stem fibre was carried out at ILU-6 tooptimize the parameters for maximized % grafting. The solution viscosity and temperature were also found to beimportant factors during conversion of grafted acrylonitrile into amidoxime. Irradiation of substrate sheets

was carried out using departmentally available 20 kW ILU-6 accelerators under the followingconditions: Beam energy = 1.8 MeV, current = 1.06 mA and variable conveyor speed.

Initial trails were carried out on bench scale and semi pilot scale at Trombay estuary to establish theprocess and material parameters under actual marine conditions. The submerged samples are retrieved from sea anddefouling is done before carrying out elution operation. Analysis of feed and elute samples were carried out by solidstate nuclear track detection method and results are shown in Table-1 below.The concentration factors of up to 100have been achieved.

Sl.

no

Token size Avg.

Graftinglevel

Duration of

submergence

Location of

submergence

Type of

elution

U pick up

(RChD lab)

Remarks/

Method/Technique

1 100 mm x

100mm

50% 360 hrs Cirus jetty Lab scale 30 ppb SSNTD

2 do Do Do Do Do 32 ppb Do3 do do Do Do Do 42 ppb Do4 1000mm x

1000 mm

102% 410 hrs Do Pilot scale 386 ppb do

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Table-1 Analysis of Liquid samples by SSNTD technique


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To ensure repeatability and reproducibility of results, the sample of pilot scale elution is evaluated byvarious techniques as listed in Table-2 below and the data shows an average value of 555 ppb and standard deviationof 115.

Laboratory name U

(ppb)

Remarks/Method/Technique adopted

ACD/BARC 771.7 ICP-MS

EAD/BARC 511 Anodic splitting voltametry

HPD/BARC 543.22 Do

IDD/BARC 499 Alpha-spectrometry

RChD/BARC 437.5 Solid state nuclear track detectionAvg of two readings (386 & 489)

REDS/BARC 570 ICP-AES

Statstics 555.4115.06

Avg. valueStd.deviation

Analysis of solid sample

In order to evaluate the elution efficiency, the solid samples were analysed by Instrumental NeutronActivation Analysis technique and results are as shown in Table-3 below. The solid sorbent samples were analysed

before and after elution. Elution was carried out using 0.5M Hydrochloric acid at temperature of 60C for a durationof four hours. Based on concentration values observed elution efficiency of around 80% is observed for bench scaleoperations.

Table-3 Neutron Activation Analysis of RUSWapp adsorbent samples (correlation of solid and liquid samples)

Sample

id

Concentratio

n (mg/kg)

Avg

value

(mg/g)

Quantity of

adsorbent

taken for

elution

(gms)

Volume of

initial

elution

solution

(ml)

Volume

of final

elution

solution

(ml)

Total

U on

solid

adsorbent

(mg)

Expected

uranium

concentratio

n (mg/l) Remarks

PAO-1 3

PAO-2 2.2

PAO-3 2.9 2.7 94.69 1900 1500 0.2556 0.10299

Reportedvalue for thesolution is0.016 mg/l

PAO-4 0.78

PAO-5 0.64

PAO-6 0.48 0.6333 94.69 0.0599

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Table-2 Comparison of sample analysis by various techniques


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4.0 CONCLUSIONS:

Nuclear Analytical Techniques such as Neutron Activation Analysis (NAA) technique and Solid StateNuclear Track Detection (SSNTD) methods for analyzing the concentrations of heavy metal ions from lean solutionslike seawater is established and standardized. These techniques are found quite useful and effective in view ofcapability of multi elemental analysis in a variety of samples. It has given good detection limits for a number of

elements. The relative standard deviation of the SSNTD method for the Uranium concentration in the range ~ 5 to40 ppb is 25%. The Minimum Detection Limit (MDL) for this method is 5 ppb of uranium at 3 level. Efforts areunderway for further improvement of MDL with longer time irradiation and usage of supra pure solutions. Timerequired to estimate has been reduced from hours to minutes. Both solid samples and eluted liquid samples have

been analysed by nuclear analytical techniques and comparison of results both by nuclear analytical techniques andnon-nuclear techniques are satisfactory and lies within the accuracy limits described by IAEA.

Acknowledgemtns

The authors wish to thank Sri. SK Ghosh Director Chemical Engg Group for giving constantencouragement to the programme. Thanks are also due to AChD, HPhD, IDD, REDS and EAD divisions of BARCfor their technical support in sample analysis. We acknowledge the support received from RUSWapp team and Staffof CIRUS/Dhruva reactors.

References1. Manmohan singh, Inida-China relations Economic Developments in India, Academic Foundations continuing series-

125 , 20082. Generation IV Roadmap crosscutting fuel cycle R&D scope report, issued by Nuclear Energy Research Advisory

committee and the Generation IV International forum, December 20023. Prasad, T.L., Radiation Grafting Of Adsorbent For Recovery Of Uranium From Seawater Divisional

colloquim held at Desalination division of Bhabha Atomic Research Centre Mumbai, India September 26,2006

4. Prasad, NK, Pathak,K, Kumar, M, Matkar, AW, Prasad, TL and Saxena, AK (2009) Challenges in thedesign of a prototype contactor assembly for the recovery of uranium from seawater, Int Jounal of NuclearDesalination, Vol 3, No.3, pp241-246

5. Saxena, A.K., Uranium from seawater: a new resource for meeting future demands of Nuclear reactors, Indian

Chemical Engineering, Section C, Vol 43, No. 3, July September 20016. Prasad TL., Tewari PK., Sathiyamoorthy D., Recovery of uranium from reject streams of Desalination plants was presented at Eighteenth annual conference of Indian Nuclear Society held at Nuclear FuelComplex Hyderabad during November 21-24, 2007.

7. Kalsi, P.C., Chhavi Agarwal, Prasad, T.L., Manchanda, V.K., Uranium estimation in ppb levels by solid state nucleartrack detectors using the pneumatic carrier facility of Dhruva reactor Presented at NUCAR-2009 (Nuclear andRadiochemistry symposium) Organised by Board of Research in Nuclear Sciences Dept of Atomic Energy in associationwith Chemistry department of SVKMs Mithibai college of Arts, Chauhan Institute of Science and Aruthben Jivanlalcollege of commerce and economics held at Vile Parle (W) Mumbai, January 7-10, 2009

8. Kalsi, P.C., Pramila, D.S., Ramaswami, A., Manchanda, V.K., Track etching characteristics of polyester detector andits application to uranium estimation in seawater samples Jpurnal of Radioanalytical and Nuclear Chemistry, Vol 273,

No.2, (2007), pp 473-477.9. Acharya, R.N., Nair, A.G.C., Reddy, A.V.R., and Manohar, S.B., Validation of a Neutron

Activation

Analysis Method using k0 standardization Applied Radiation Isotope, 57 (2002) 391. (For multielementNAA, results of reference materials)10. Tiwari, S., Nair, A.G.C., Acharya, R., Reddy, A.V.R., and Goswami, A., Analysis of

Uranium BearingSamples for Rare Earth and Other Elements by k0-Based Internal Monostandard INAA Method, J. Nucl.Radiochem. Sci., 8 (2007) 25.

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Advanced Techniques for Measurement of Heavy Metal Concentrations From Seawater