Studies on chemical analogy of calcium and beryllium in soil

Transcription

1 Indian Journal of Chemical Technology Vol. 12, September 2005, pp Studies on chemical analogy of calcium and beryllium in soil P N Bhat *, S Soundararajan & D K Ghosh Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai , India Received 25 October 2004; revised received 24 June 2005; accepted 11 July 2005 This study was specifically aimed to determine the levels of beryllium and calcium in the soil collected near the vicinity of a beryllium processing facility (BPF) and to assess the effect of calcium addition, if any, to the soil. Atomic Absorption Spectrophotometer (AAS) technique was used to estimate the total beryllium and calcium. The beryllium and calcium contents per gram of soil samples were in the range of and µg respectively. The loading of collected samples with salts of calcium revealed analogy with beryllium. This was supported by the rejection of beryllium by calcium-loaded soils. Keywords: Beryllium, calcium, chemical analogy IPC Code: C09K17/06 Soil is one of the key components of terrestrial ecosystems, both natural and agricultural, being essential for the growth of plants and recycling of dead biomass. It is a dynamic system, subject to shortterm fluctuations such as variations in moisture, ph and redox conditions; and it also undergoes gradual alterations in response to changes in soil management and environmental factors. These changes in soil properties could affect the form of metals and need to be considered in decisions on the management of polluted soils. Beryllium (Be) is a useful material because of its unique properties. Its natural abundance is low. Although the presence of beryllium in natural sources is reported in the literature 1,2, the data available on distribution of beryllium in environment near beryllium production plants are scarce. It is reported that beryllium is strongly fixed in soils and clays and may displace other divalent cations that share common sorption sites in the exchange complex 3. The effects of the metal is determined more by its form than by its concentration. The free Be +2 ion is more likely to be sorbed on the surfaces of soil than other species such as neutral or anionic species. The concentration of beryllium in soil being low, the presence of other metal ions in larger quantities, particularly the other Group II A elements that are chemically similar to beryllium, may influence the *For correspondence separation/distribution of beryllium in soils. In the Group II A of periodic table beryllium is placed at the top followed by Mg, Ca, Sr, Ba and Ra. If any of the elements should influence beryllium in the soil it should be mainly calcium as it is the most abundant alkaline earth element. The effect of calcium on the mobility/immobility of beryllium in the soils was studied and it revealed pronounced analogy between both calcium and beryllium in soil. The study was carried out around a beryllium processing facility. Experimental Procedure Sample collection Soil samples were collected from different locations around the beryllium processing facility up to a radial distance of 5 km from the plant. The soil samples at each of the locations were collected from an undisturbed open area of 1 1 m. The area was cleared off grass and debris and the top layer soil up to a depth of 5 cm was dug out with a scoop. The soil ( m) was mixed thoroughly at site and 2 kg of the soil was collected as sample from each location. Samples were collected from thirteen locations. Sample preparation and analysis for Be and Ca The soil samples from various sites were pounded lightly in a ceramic mortar and pestle, sieved through 500-mesh sieve (32 µm), and stored in airtight plastic containers. The samples, which contained moisture of %, were dried at 105 o C.

2 BHAT et al.: STUDIES ON CHEMICAL ANALOGY OF CALCIUM AND BERYLLIUM IN SOIL 535 Weighed quantities (0.1 g each) of four specimens each of the soil samples collected from the thirteen locations were transferred into four PTFE beakers. To each of the beaker, known quantities of beryllium standard solution were added (0, 250, 500, 750 ng Be respectively). These samples were prepared in triplicates. To each of them concentrated nitric acid (10 ml), hydrochloric acid (5 ml) and perchloric acid (2 ml) were added and these preparations were digested on hot plate. The addition of nitric acid was repeated to clear the organic matters and finally the sample was treated with 3 ml of hydrofluoric acid. The residue was dissolved in 2 % HNO 3 solution. Volume of the sample was made upto 100 ml. Beryllium in the sample matrix was measured by Varian make graphite-furnace atomic absorption spectrophotometer (GFAAS). Beryllium concentration in the soil sample was estimated by the standard addition technique 4-6. In the case of determination of calcium, 0.1 g soil of each sample, in triplicate, was analysed without addition of calcium standard. The same procedural steps given for beryllium were followed for sample treatment of these samples also. After appropriate dilutions calcium measurements were carried out by Flame AAS 7-9. Calibration and analytical reproducibility The absorbance of beryllium standard solutions was read out using the peak height mode at the optimum furnace parameters of the GFAAS at a wavelength nm. The standard calibration graph showed the linearity up to 3.0 ng ml -1. The sensitivity of the method was assessed with respect to reagent blank and the lowest detectable concentration found to be 0.1 ng Be ml -1. The relative standard deviation was better than ±6.4 % at the lowest concentration of 0.1 ng Be ml -1 and at higher concentration it was ±2.3 %. Calcium standard solutions were read out at the peak height mode of the nitrous oxide-acetylene flame AAS at a wavelength nm. The standard calibration graph was linear up to 5.0 µg Ca ml -1. The relative standard was better than ±5 % at the lowest concentration of 1 µg Ca ml -1 and at higher concentration it was ± 2.5 % Leachable beryllium in calcium loaded soil Dry, sieved and weighed (2 g each) soil samples were transferred into six containers in duplicate and were subjected to leaching with distilled water containing 0, 100, 200, 300, 400, 500 mg Ca as calcium chloride. The ph of the calcium standard solution was made to 7.0 using ammonia solution (1:1, v/v). Mechanical shakers were used for stirring and the samples were retained in the containers for a period of seven days to attain equilibrium. Everyday, mechanical shaking of the solutions was resorted to for 30 min. After 7 days, the supernatant was filtered. The filtrate was acidified with concentrated nitric acid to ph 1. This acidified sample was analysed for beryllium. Along with this set, a matrix blank (sample matrix without addition of the calcium) was also analysed for beryllium. Similarly, same soil matrix was processed in duplicate for the known addition of Ca as calcium nitrate, following the procedure used for calcium chloride as given above. The average beryllium concentration released from soil with increasing concentration of calcium is presented in Table 1. Adsorption and inhibition of beryllium in calcium loaded soil The above soil samples treated with calcium salts were subjected to the adsorption of beryllium by adding known concentration of beryllium. To each sample 5.0 µg Be was added and stirred with mechanical shakers for 2 h. Samples were allowed to stand for 7 days to attain equilibrium. Supernatants were filtered, acidified to ph 1. These acidified leachates were analysed for beryllium by GFAAS. Data on beryllium adsorbed by calcium treated soil are given in Table 2. Results and Discussion Total Be in soil Total beryllium in soil samples is given in Table 3. The beryllium concentration ranges between 0.8 and 3.02 µg Be g -1. Individual values varied within 25 %. Beryllium concentration in the soil near the beryllium Table 1 Chemical analogue of beryllium in soil Weight of soil taken for each set: 2 g Sample location: Near BPF Added Ca (mg) Be released in leachate (ng/g soil)* CaCl 2 Ca(NO 3 ) *Average Be concentration of soil analysis in duplicate

3 536 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2005 Table 2 Analogue of Be and Ca in soil Amount of soil taken: 2 g Ca +2 added as Be added Be adsorbed (ng) by soil * Ca salts in (mg) (ng) CaCl 2 Ca(NO 3 ) *Average Be concentration of soil analysis in duplicate Overall variation in values: 5 % metal plant is 2.2 µg Be g -1. The soil samples in the radial areas of 5 km also have similar concentrations of beryllium. The lowest of the values is at Turbhe village with concentration of 0.8 µg Be g -1 and the highest is found to be near the creek with a value of 3.02 µg Be g -1. Although the variations are between 0.8 and 3.02 µg Be g -1 (by a factor of about 4), the mean value of these 13 samples of 1.79±0.66 µg Be g 1 can be described as the average concentration of beryllium in the soil in this area. The median value is found to be 1.71 µg Be g -1, which is close to the determined mean value. The difference in beryllium values is probably due to the exchange capacity of the soil and the chemical changes of the environment 10. Table 3 Natural Be in soil near vicinity of BPF Sr. No. Locations Be conc. * (µg/g) 1 Near BPF 2.2 (outside the complex) 2 MAFCO Modern college ESIS Hospital Sanpada Vashi village Near NMMT depot Koper-khairne Near church Turbhe village Near creek ICL School Near Vashi Rly. Station 2.2 Mean : 1.79 ± 0.66 *Average Be conc. of soil analysed in triplicate Chemical analogy of Ca with respect to Be in soil The distribution pattern and the chemical behaviour of beryllium were studied in soil samples collected from the vicinity of the beryllium processing facility. Release of Be by calcium Beryllium released by calcium in soil is given in Table 2. The amount of beryllium released was found to be directly proportional to the amount of calcium added to the soil as CaCl 2 and Ca(NO 3 ) 2 as shown in Figs 1 and 2 respectively. Both chloride and nitrate salts of calcium released beryllium effectively. The soil used for this experiment contains 2200 ng Be g -1 of the soil (Tables 3 and 4; sample 1). The total calcium in this soil is 8756 µg Ca g -1. With no addition of Ca only about 57 ng of Be is released (Table 1). This amounts to only about 2.6 % of the total beryllium. Upon addition of Ca more and more beryllium was released. The addition of Ca varied from 10 to 50 times the amount of Ca present in the soil. The total Be released from the soil by increments of Ca addition from mg of Ca is ng Be Fig. 1 Release of Be from soil by CaCl 2 Fig. 2 Release of Be from soil by Ca(NO 3 ) 2

4 BHAT et al.: STUDIES ON CHEMICAL ANALOGY OF CALCIUM AND BERYLLIUM IN SOIL 537 Table 4 Total concentration of Ca and Be in various soils near the vicinity of BPF Sr. No. Location Total Total Ca Be (µg/g)* (µg/g)* 1 Near BPF (outside the complex) 2 M.A.F.CO Near NMMT Depot Koper-khairne Near church Near Creek Near I.C.L. School Near Vashi Rly. Stn *Average Be concentration of soil analysis in triplicate in the experiment of Ca(NO 3 ) 2. This amounts to 30 % of the total beryllium in soil. The remaining 70 % is not mobilised by calcium under the experimental conditions. This non-mobilisable beryllium may be the one, which has got into the structure by the replacement of silicon through isomorphous replacement. No experimental proof has been found for this. This process of beryllium release, by the addition of Ca is pictorially presented in Figs 1 and 2. These relationships of added Ca versus released beryllium yield perfect straight lines. The coefficients of correlation (r) are 0.96 and 0.98 for CaCl 2 and Ca(NO 3 ) 2, respectively; closeness of these r values means that the release is independent of counter ions (chloride or nitrate) of calcium. The individual equations obtained are y = , (r = 0.96), for CaCl 2, and y = , (r = 0.98) for Ca(NO 3 ) 2. The intercept values in both the cases given by the equations are 51 and 61 ng/g respectively, giving the beryllium released in absence of the added calcium. The slope values differ roughly by a factor of two. The rate of beryllium release is higher by a factor of 2.2 for NO 3 - than by Cl -. This indicates the variation in the process of release. It is very significant that calcium releases beryllium from the soil matrix. The observation that calcium releases beryllium significantly proves that calcium is a good chemical analogue of beryllium. The soil sample, collected near beryllium processing facility, studied in this experiment contains 2.24 µg g -1 Be and µg g -1 Ca (Table 2). Column 1 of the Table gives the amount of added Fig. 3 Adsorption of Be with Ca calcium in the soil, which ranges from mg for 2 g of the soil. In all these six samples, 5 µg of beryllium was added. The adsorbed beryllium contents in the soils treated with CaCl 2 and Ca(NO 3 ) 2 are given in columns 3 and 4. In both cases the adsorbed beryllium by calcium loaded soil indicate a decreased uptake of beryllium with increased calcium content in the soils. This can be seen in Fig. 3. The least square fit lines for the chloride and nitrate loaded soils are fairly similar, the equations being y = , (r = 0.78) for CaCl 2 loaded soil and y = , (r = 0.67) for Ca(NO 3 ) 2 loaded soil The above relationships indicate a negative correlation between adsorbed beryllium versus added calcium. In this there is a clear suggestion that increased calcium concentration may prevent the entry of beryllium into soil. However, as could be seen experimentally in this study, after an initial significant effect of calcium on adsorption of beryllium, further incremental increases of calcium do not result in any significant change in adsorption of beryllium by the soil. These two types of experiments of beryllium release and prevention of beryllium uptake by the increasing amounts of calcium in the soil bring out very significant results. While addition of Ca to the soil releases beryllium in proportion to the added calcium, the presence of increased quantity of calcium

5 538 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2005 total calcium versus total beryllium in soil is presented in Fig. 4. The points in the graph are scattered. Obviously, there is no clear relation between the two but the bestfit line shown in the graph has a negative correlation, although weak. Fig. 4 presents the environmental evidence of a possible correlation of Ca versus Be in soils. Fig. 4 Relation between Ca and Be in soil in the soil inhibits or decreases the uptake of beryllium. Both these experiments imply that Be and Ca are chemical analogues. The process seems to be very specific. Whereas addition of calcium promotes the release of beryllium, Ca inhibits the entry of beryllium into the soil. This has tremendous environmental significance. The study revealed that about 57 ng Be g -1 of soil got leached out by water (Table 1). But when beryllium is added externally the soil uptake of beryllium is observed to be about 4000 ng Be g -1 of soil (Table 2). This is about 70 times more than the quantity of beryllium leached out by water. The effect of addition of calcium on beryllium uptake by soil was studied. Marginal decrease of beryllium uptake by soil is observed with increase in added calcium. Distribution of beryllium and calcium in soils In these experiments, two clues were obtained for the possible environmental relations between beryllium and calcium. Increased addition of calcium released increased quantity of beryllium from the soil and increased calcium concentration also reduced the uptake of beryllium; this relation is positively indicative of a quantitative correlation between beryllium and calcium. Does a situation like this indeed exist in the environment? In order to obtain an answer for this eight soil samples were analysed around the beryllium processing facility for Be and Ca. These results are given in Table 4 and a graph of Conclusion The linear regression coefficient value of shows an excellent correlation between the added Ca and the released Be from soil. The experiments of the uptake of Be by Ca loaded soil yield a weak correlation indicating the possible inhibition of Be uptake by added Ca in soil but the quantities of Be taken up far exceed the amount of Be released by Ca. Thus, calcium does work as a chemical analogue of beryllium. The natural distribution of Ca and Be in some soils also yield a weak negative correlation supporting the concept of chemical analogy of Ca and Be. Acknowledgement The authors thank Dr. D.N. Sharma, Head, Radiation Safety Systems Division, Bhabha Atomic Research Centre (BARC), for his support and valuable suggestions during the course of this study. Thanks are also due to Shri B.P. Sharma, Head, Powder Metallurgy Division, BARC for providing facility to conduct the study. References 1 Anderson M A, Bertich P M & Miller W P, J Environ Qual, 19(2) (1990) Parker S P, Encyclopedia of Chemisty, 1983, Romney E M & Childress J D, Soil Sci, 100 (1965) APHA, AWWA, WPCF. Standard Methods for the Examination of Water and Wastewater, 16 th edn, 1985, Rothery E (Ed), Analytical Methods for Graphite Tube Atomizers (Publication No ), Australia (1982). 6 Bhat P N & Pillai K C, J Water Air Soil Pollut, 95 (1997) Scmidt W F. Dietl, Anal Chim, 326(1) (1987) USEPA. Control Techniques for Beryllium Air Pollutants (EPA Publication AP-116, Washington, DC), Varion Techtron, Analytical Methods for Flame Spectroscopy (Publication No ) Australia (1979). 10 Kaplan D I, Sajwan K S, Adriano D C & Gettier S, J Water Air Soil Pollut 53 (1990) 203.