Gross alpha activities in the drinking water samples

1. GROSS ALPHA ACTIVITY MEASUREMENT PROCEDURES (Ref: Indian Standard: IS 14194 (Part 2): 1994 "Radio-nuclides in Environmental Samples - Methods of Estimation”) Objective: To estimate Gross - Alpha activity levels in environmental samples, Including air, water etc., Principle: The gross alpha activity of the sample, either particulate collected on an air filter, or water (concentrated or precipitated) and dry deposited on metal Planchet (stainless steel) is directly measured in an alpha counter, having Zinc Supplied (Ag) scintillation detector. Apparatus: i) Alpha counting system having a Zinc Supplied (Ag) detector equipped with a sample holder to accormnodate stainless steel planchets placed in an aluminum planchet holder. The background of the system shall be less than 5 counts per minute (CPM) and counting efficiency, >20%. ii) Stainless steel planchets: 0.5mm thick and 25mm in diameter, (placed in an aluminum planchet holder). iii) Infra-Red Lamp: about 500 Watts, (used for drying precipitates before counting). PROCEDURE: AIR FILTER: Working atmosphere or ambient atmosphere may contain man–made redionuclides or natural radioactivity arising from technological operations or purely from natural environmental processes. Although a certain level of radioactivity in the air we breathe may not be harmful but at levels beyond the permissible limits, this could pose potential hazards to individuals at such locations. Hence, the need to monitor and evaluate the radioactivity levels in air in such environments. In this procedure, we are generally talking of the natural–rationuclides in the atmosphere, of which some have short half-life e.g., Radon and daughters while some have long half-lives (e.g. nat. U nat. Th, Ra-226, Po–2lO etc). Presently we are concerned only with the long- lived component of the alpha activity. The procedure for estimation of long-lived alpha activity in air filters is as follows: a) Place a glass fiber or Whatman-40 or Millipore filter sample (25mm dia) in the suction head of an air sampling pump, and secure it firmly on the suction head without allowing any leakage. b) Note the time and switch on the pump. Not the air flow rate in the flow meter. Filter about 5M3 of air. c) Stop the pump, and note the time and calculate the volume of air passed (in M3) through the filter paper, using the flow meter and pump running time. d) Remove the filter containing the particulates in the sampled air, from the suction head, and store for 300 min. (to allow short lived random daughters to decay) e) Subsequently, count the filter paper containing the particulates for alpha counting, by placing it in an aluminum planchet holder, in the Alpha-Counter, which is kept on for about an hour, prior to counting. f) Counting for a reasonable period, say around ~lO0 min., and note the activity in counts per minute (CPM) and calculate the Gross, long lived alpha activity of the sampled air, by using the counting efficiency of the alpha counting system (in %) and volume of air sampled (M3).

2. The Gross alpha activity (AB) of the sample is calculated as follows: Count rate of the sample mounted on the planchet (Cs) in counts per minute (CPM) = NS– NB. Where Ng = Count Rate in CPM of the sample + count rate of background in CPM. NB Background count rate in CPM. The Gross alpha activity of the sample (As) in Bq/ M3 of air is = Cs x 100/E x l/V, Where, Cs = Net count rate of the sampling in CPM. E = Counting efficiency of the alpha counter in percent. V = Volume of air filtered in M3. Water: Environmental waters, whether they belong to fresh water (e.g., rivers, lakes, ponds, ground waters) or saline/brackish water category (e.g., seawater, estuarine waters), all contain some dissolved radionuclide components, arising from ecosystem processes, due to physico – chemical, geo-chemical and biological activities taking place amongst ecosystem components. Some of the natural alpha redionuclides of interest to human health & safety are nat. Uranium, nat. Thorium, Ra-226, Rn-222 (this is a gaseous one, and hence not significant, for drinking water, considerations). Po–2l0 etc. Amongst these, generally nat. Uranium, Ra– 226 & nat. Thorium may be considered of interest, as their levels have been found to be spread over a wide range of concentrations in potable waters, world wide including in bottled mineral waters and packaged drinking waters. In the light of the above, it becomes highly relevant to assay the levels of the above alpha emitting redionuclides in water samples, commonly grouped under the term “gross–alpha activity". The procedure for the analysis of gross alpha activity is given below. Procedure: l. For water samples, which has, a low Total Dissolved Solids (TDS) content; say less than 100 ppm. a) Check the TDS content of the sample under test, by standard laboratory methods. b) If the test results, demonstrate the TDS level at <l00 ppm, then transfer 10ml of the sample to a small beaker (50ml) and evaporate to near dryness, slowly on a hot plate. c) Cool, and wet with a few drops of dist. Water, stir and transfer the whole contents to a clean, previously counted stainless steel planchet, with the help of a dropper. (Repeat the transfer till the complete residue is transferred to the S.S. planchet. d) Dry the S.S. Planchet with the transferred residue under an infra-red lamp to a stage, when a thin film of residue appears on the S.S. Planchet. e) Remove the planchet and flame it carefully, on a burner to red hotconditions (to remove any organic matter residue, which also reducingthe degree of self- absorption of alpha particulates by the solid residue.Caution: Ensure that the residue does not flake off during flaming andget lost. f) Allow the S.S. Planchet to cool to room temperature and place the planchet in the alpha counting system and count the alpha activity for a known period gsay 60 - 120 minutes). 2. Many water samples particularly those of ground water origin have high Total Dissolved Solids (TDS) content which make them unsuitable for direct evaporation and counting due to high degree of self - absorption of alpha activity, within the solid residue matrix after evaporation, (as given above). Hence the need to apply a different

3. technique to isolate the desired alpha activity containing radionuclides from the samples followed by their counting in an alpha counting system as above. Regents required: i) Bismuth Carrier, prepared from a standard A.R. grade Bismuth Nitrate, Bi (NO3) 3, at a concentration level of 50mg Bi/ml. ii) Lanthanum Carrier, prepared from a standard A.R. grade Lanthanum Nitrate, La (NO3) 3 at a concentration level of lmg La/ml. iii) Phosphoric Acid, (Cone.)H3PO4. iv) Hydrofluoric acid - Nitric acid wash soln. : Mix.5ml Hydrofluoric acid and 5ml nitric acid and dilute to l litre with dist-water. Procedure: l. Alpha background activity determination: Although we use the best of chemical reagent standards, whose chemical composition and assay may not be questionable, but they may still carry traces of natural radioactive nuclides. For e.g., lanthanum reagent is known to carry traces of radio activities due to nat. Thorium, nat. Uranium etc., because they generally co-occur in their geological origins and sources. Hence, it becomes essential to check and evaluate the extent of nat. Uranium, nat. Thorium presence in the lanthanum carrier, when we are employing the same for gross alpha determinations. The procedure for carrying out the same is as follows: a) Transfer accurately by pipette 1 ml lanthanum carrier into a polythenecentrifuge tube (50ml capacity), dilute to 5ml with 4N Hydrofluoric acid and stir well. b) Add 1 ml Conc. Hydrofluoric acid slowly and stir well with a Teflon (or PVC) rod and centrifuge for about 5 minutes. c) Pour out the supernatant and reject the same. Wash the residual precipitate in the centrifuge tube with Hydrofluoric - Nitric acid wash soln., centrifuge again for about 5 - 10 minutes and reject the supernatant. d) Wash the precipitate in the centrifuge tube with 5ml dist. Water and centrifuge again. Reject the supernatant. Note: The supernatant should be free from acidity e) Add a few drops of dist. Water to the precipitate residue in the centrifuge tube, stir well and completely transfer the precipitate - slurry on to a clean stainless planchet, with the aid of a dropper. f) Dry the precipitate slurry on the S.S. Planchet under an infrared lamp, taking care to uniformly spread the slurry on the planchet, (to minimize self- absorption errors of alpha activity). g) Flame carefully the dried residue on S.S. Planchet, on a laboratory burner. Cool and count the same in the alpha counting system, and record the lanthanum Fluoride background count rate NlaF3). Caution: Ensure that during flaming the residue does not flake off and get lost.

4. Alpha activity determination in Water Samples a) Transfer l litre of filtered water sample to a beaker, add 5ml Conc. HNO3, and 500mg Bismuth carrier (i.e., ml of Bi carrier soln) with stirring. b) Add lg of Hydroxylamine Hydrochloride (NH2OH.HCI) and stir again. c) Now, warm on a hot plate and add 1 ml phosphoric acid with stirring, when Bismuth Phosphate (BiPO4) starts precipitating. Allow to cool, when (BiPO4) precipitation is complete, (about an hour may be required). The BiPO4 precipitate will carry most of the alpha emitters, under study. d) Now slowly siphon off the supernatant, and transfer the precipitate as slurry, to a polythene centrifuge tube (50ml capacity) e) Centrifuge the above for a minute and discard the Supernate. f) Dissolve the BiPO4 precipitate in the centrifuge tube in Conc. HCI, of adequate quantity. Now add 1 mg lanthanum carrier (i.e., 1 ml of Lanthanum Carrier soln), followed by slow addition of Conc. Hydrofluoric Acid (HF). Slowly stir with a polythene or PVC or Teflon rod, when Lanthanum Fluoride (LaF3) precipitates. Allow some time (10-15 minutes) for completeness of precipitation. g) Centrifuge and discard the supernate. i. Wash the precipitate thoroughly in the centrifuge tube with Hydrofluoric Nitric acid wash solution. Centrifuge and discard supernate. ii. Wash the precipitate in the centrifuge tube with 5ml distilled water, centrifuge and discard supernate. Note: The supernatant should be free from acidity. iii. Transfer the precipitate from the centrifuge tube completely with few drops of dist. Water as slurry onto a stainless steel planchet using a dropper. iv. Dry the S.S. Planchet (containing the LaF3 precipitate) under an Infrared lamp, taking care to uniformly spread the slurry on the planchet (to minimize self- absorption errors of alpha activity). v. Flame carefully the dried residue on the S.S. Planchet on a laboratory burner, cool and count the same in the Alpha Counting System. h) Record the count rate of the sample (together with background count rate), (Ns). i) Now subtract the count rate of background (NLaF3 ;) from the gross count rate of the sample plus background (Ns) count rate, to get true count rate of the sample. The true count rate of the sample mounted on the planchet (Cs) in counts per minute (CPM) is as follows: Cs = Ns - (NB + NLaF3) (CPM) Where, Ns = Count Rate (in CPM) of the sample plus count rate of Background (in CPM) NB = Background count rate of counter (in CPM) NLaF3 = Blank LaF3 count rate (in CPM) j) Calculate the gross - alpha activity of the sample (As) under study as follows: As = Cs x 100/E x 1/S, (Bq/1) Where Cs = Net count rate of the sample (in CPM) E = Efficiency of the alpha counting systems (in o/o) S = Sample size taken for the analysis (1 litre)