Subscribe Now Subscribe Today
Research Article
 

Measurement of Natural Radioactivity in the Clays Consummated in Côte d’Ivoire using Gamma-ray Spectrometry



Coulibaly Vamoussa, Sei Joseph, Kouame N`Dri, Koua Aka Antonin, Oyetola Samuel and Brun Stephane
 
Facebook Twitter Digg Reddit Linkedin StumbleUpon E-mail
ABSTRACT

The presence of radionuclides in natural materials was one of the major causes of external and internal exposure to ionizing radiation (gamma rays). The knowledge of the radioactivity levels in these commonly used materials was of great importance in the assessment of possible radiological risks to human health. The purpose of this study was to determine the natural radioactivity due to 226Ra, 232Th and 40K of some clay samples consummated in Côte d’Ivoire for their therapeutic virtues by using gamma spectrometry. The activity concentration of 226Ra, 232Th and 40K ranged from 19.0±8.2 to 54.8±20.8 Bq kg-1 (with a mean of 33.6±13.2 Bq kg-1), from 12.8±3.5 to 42.3±12.1 Bq kg-1 (with a mean of 31.3±8.5 Bq kg-1) and from 16.0±59.2 to 790±245 Bq kg-1 (with a mean of 314.1±101.7 Bq kg-1), respectively. Radium equivalent activities and various hazard indices were also calculated to assess the radiation hazard. The Radium equivalent activity (Raeq) ranged from 90.2±28.4 to 130.3±43.8 Bq kg-1 (with an average of 102.6±33.1 Bq kg-1), was lower than the limit of 370 Bq kg-1. The calculated values of external hazard index (Hex) 0.24±0.08-0.35±0.12 and internal hazard index (Hin) 0.32±0.11-0.50±0.18 were also lower than unity. The annual effective dose ranged from 52.2±16.3 to 73.8±24.8 μSv y-1 with a mean of 59.6±19.1 μSv y-1 whereas the effective dose by ingestion varied from 17.1±6.2 to 30.8±11.1 μSv y-1 with an average of 21.3±7.6 μSv y-1. These values were lower than the limit of 1 mSv y-1. From this study, it was found that the use of the investigated clay samples for their therapeutic virtues did not induce significant radiation hazards.

Services
Related Articles in ASCI
Similar Articles in this Journal
Search in Google Scholar
View Citation
Report Citation

 
  How to cite this article:

Coulibaly Vamoussa, Sei Joseph, Kouame N`Dri, Koua Aka Antonin, Oyetola Samuel and Brun Stephane, 2013. Measurement of Natural Radioactivity in the Clays Consummated in Côte d’Ivoire using Gamma-ray Spectrometry. Journal of Applied Sciences, 13: 140-146.

DOI: 10.3923/jas.2013.140.146

URL: https://scialert.net/abstract/?doi=jas.2013.140.146
 
Received: October 30, 2012; Accepted: December 10, 2012; Published: February 01, 2013



INTRODUCTION

Clay minerals being soil colloids are very widespread in the terrestrial crust. Their basic structural elements are similar: sheets of corner-sharing oxygen tetrahedral with Si4+ and generally some Al3+ as central cations and sheets of edge-sharing octahedral in which oxygen and hydroxyl surround a variety of possible di or trivalent cations (mainly Mg2+, Fe2+, Al3+ and Fe3+).

In addition to these major elements, one meets in their bosom many chemical elements including the alkali, alkaline-earth, metals of transition, lanthanides and actinides. These elements are either in isomorphous substitutions in their network, either in the inter layer space where they play the role of compensatory ions, either to the adsorbed state at the surface of the clay particles. They can also belong to other associated minerals. Some of these chemical elements notably uranium, thorium, potassium have unsteady cores and confer to the clay minerals a natural radioactivity whose importance depends on their concentration. The population whom is exposed to them directly via the ingestion or inhalation pathways can receive external and internal dose.

The knowledge of concentration and distribution of the radionuclides in these minerals are of interest because it provides useful information in the monitoring of environmental radioactivity. Importance of this subject has given rise to numerous studies on the rodioactivity in soils (Pourcelot et al., 2003; Laubenstein and Magaldi, 2008; Al-Hamarneh and Awadallah, 2009; Montes et al., 2012), in the building materials (Hewamanna et al., 2001; Petropoulos et al., 2002; Pavlidou et al., 2006; Turhan, 2008; Kumar et al., 1999; Ravisankar et al., 2012), in foodstuffs (Santos et al., 2002; Bolca et al., 2007) and in groundwater (Amin et al., 2011).

In Côte d’Ivoire, the “Société pour le Développement Minier” (SODEMI) listed several clays deposits through the country. These materials are used in several domains such as pottery, building materials (tiles, bricks...), ceramics, health.

The object of this study was to determine the chemical composition as well as the radioactivity of some consummated clay samples in Côte d’Ivoire for their therapeutic virtues namely by pregnant women. In order to appreciate the risks bound to this practice and to develop standards concerning their exploitations and uses, different parameters relative to radiation hazards have been calculated and compared to the standard limits.

MATERIALS AND METHODS

Clay sampling: The studied eight clay samples come from Anyama and Bingerville, two commons of the district of Abidjan (Côte d’Ivoire). Anyama is situated at 10 km North-West while Bingerville is located at 12 km South-West of Abidjan.

These localities are localized in the sedimentary basin and the clays are supposed to be formed after sedimentation of alluvial products eroded from the uppermost layer of the Precambrian shelf (Le Bourdiec, 1958).

Anyama is situated in a plateau area and the studied green clay (labelled AVA) is dominated by chlorite, illite and quartz with minor amounts of other minerals including smectite (Coulibaly et al., 2012). It is commercialized by the local population for its therapeutic virtues. It is used by internal way and also by external way to treat many diseases such as whitlow, athlete foot and stomach disease. This green clay is also used for soap production. For this study, the only one sample has been collected to about 2 m of depth from mineral deposit and dried in air as that sold.

Bingerville is situated in the coastal lagoon and the seven clay samples are labeled as follows:

LBF : White lokpo
LRF : Red lokpo
LMF : Brown lokpo
LVF : Purple lokpo
LJPF : Yellow lokpo
LJFF : Dark yellow lokpo
LNF : Gray lokpo

They are dominated by kaolinite with relative important amounts of quartz, illite and goethite (Coulibaly et al., 2012) and have the particularity to be eatable (generally by pregnant women) after manning. They are also used for soap preparation and other beauty care. The samples were taken from the site of exploitation according to their color; they suffer the same heat treatment as those sold, commonly called “Lokpo”. This treatment involves cleaning of the clays to rid them of the impurities and heating the sample in an oven built in clay and powered by firewood for 3 to 5 days. This operation often gives the clay material a pleasant and smells appetizing.

For the different analyzes, the samples have been dried at 40°C during 24 h in a steam room, ground and sieved through 80 μm mesh. A portion of each sample was used for chemical composition analysis while the remaining part was used for natural radioactivity determination.

Chemical analysis: The chemical analysis in total rock was carried out by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) for major elements and Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) for trace elements after fusion with LiBO2 and dissolution in HNO3.

The total adsorbed and structural (structural OH) water contents were determined by thermogravimetric analysis at 1200°C.

Radioactivity measurement: The activity measurements were made using a typical high resolution gamma-ray spectrometer. The system comprises a high-purity n-type coaxial germanium (HPGe) detector with a relative photo peak efficiency of 60%. About 100 g of each sample were placed in cylindrical plastic container 75 mm in diameter and 15 mm in height. They were hermetically sealed and stored for 3 weeks before counting, allowing time for 226Ra and 232Th to reach equilibrium with their respective progenies. The accumulation time for gamma ray spectra measurement was 18 h. Prior to sample measurement, the laboratory background was determined and peak corrections were performed.

The concentrations of various nuclides of interest were determined in Bq kg-1 using the counted spectra.

The activity concentration of 40K was measured directly by its own gamma ray at 1460.7 keV, while activities of 226Ra and 232Th were calculated based on the weighted mean value of their respective decay products in equilibrium.

RESULTS AND DISCUSSION

Chemical analysis
Major elements: The major elements expressed in weight percentage of oxides are given in Table 1. All samples contain qualitatively the same elements. Sample AVA differs from those of Bingerville by its high SiO2 content and its low Al2O3 content. It also contains significant quantity of MgO, Fe2O3 and K2O.

Excepted LJFF, the samples of Bingerville have relatively homogenous composition with SiO2 content ranging from 45.73 to 57.05%, Al2O3 from 25.2 to 34.4%, Fe2O3 from 2.3 to 4.92%. Sample LJFF is characterized by its very high Fe2O3 (26%) content and low concentration of silica (42.7%) and alumina (18.33%).

Trace elements: The trace elements expressed in ppm are given in Table 2. They are in the majority constituted of alkali, alkaline-earth, metals of transition, lanthanides and actinides.

Table 1: Chemical composition of the major elements in weight percent
nd: Not determined, AVA: Green clay of Anyama, LBF: White lokpo, LRF: Red lokpo, LMF: Brown lokpo, LVF: Purple lokpo, LJPF: Yellow lokpo, LJFF: Dark yellow lokpo, LNF: Gray lokpo of Bingerville

Table 2: Chemical composition of trace elements in ppm
nd: Not determined, AVA: Green clay of Anyama, LBF: White lokpo, LRF: Red lokpo, LMF: Brown lokpo, LVF: Purple lokpo, LJPF: Yellow lokpo, LJFF: Dark yellow lokpo, LNF: Gray lokpo of Bingerville

Their uranium content ranges from 0.39 to 3.98 ppm and the high content is observed in the sample LJFF while the lowest is observed in the sample LMF. The thorium content ranges from 0.81 to 13 ppm and the high content is observed in the sample LVF while the lowest is observed in the sample LMF.

Activity concentration: The most important source of natural radiation exposure is caused by the gamma rays emitted from members of the uranium and thorium decay chains and radioactive potassium (40K) occurring naturally in soil colloids including clay minerals. In the 238U series, the decay chain segment starting from radium (226Ra) is radiologically most important and therefore, reference is often made to 226Ra instead of 238U (Ravisankar et al., 2012).

The activity concentrations of 226Ra, 232Th and 40K in Bq kg-1 dry weight are given in Table 3 while in Fig. 1, the activity concentrations of the examined samples are compared. The radioactivity concentrations of the three nuclides are relatively homogenous in the different samples of Bingerville but they are different from those of the sample of Anyama. The latest sample is characterized by very low concentration in 226Ra, 232Th and a very high concentration in 40K, in agreement with the chemical analysis. The activity concentrations of 226Ra and 232Th are in the same ranges and are lesser than the activity concentration of 40K.

As can be seen from the results, the lowest value of 226Ra concentration is 19.0±8.2 Bq kg-1 measured in sample AVA while the highest value for the same radionuclide is 54.8±20.8 Bq kg-1 measured in sample LBF.

The observed minimum and maximum 232Th activity concentration were respectively 12.8±3.5 Bq kg-1 in sample AVA and 42.3±12.1 Bq kg-1 in sample LVF.

Activity concentration of 40K varies from 160.0±59.2 Bq kg-1 in sample LVF to 790±245 Bq kg-1 in sample AVA.

For building materials including clay materials, the world average values are 35, 30 and 400 Bq kg-1, respectively for 226Ra, 232Th and 40K (UNSCEAR, 2000). The activity concentration of 40K 790±245 Bq kg-1 in sample AVA exceeds the normal level and is due to the presence of illite in the sample.

The average values of activity concentration of the investigated samples for 226Ra (33.6±13.2 Bq kg-1), 232Th (31.3±8.5 Bq kg-1) and 40K (314.1±101.7 Bq kg-1) are in the ranges of the corresponding typical world averages for the said nuclides, respectively.

The natural radioactivity levels measured in the investigated samples are comparable and within the range reported for other countries.

Table 3: Activity concentration due to 226Ra, 232Th and 40K in the investigated samples
AVA: Green clay of Anyama, LBF: White lokpo, LRF: Red lokpo, LMF: Brown lokpo, LVF: Purple lokpo, LJPF: Yellow lokpo, LJFF: Dark yellow lokpo, LNF: Gray lokpo of Bingerville

Fig. 1(a-c): Different clay samples (a) 40K, (b) 226Ra and (c) 232Th accounted for their activity concentration

Righi and Bruzzi (2006) found 23-24 Bq kg-1 for 226Ra, 30-36 Bq kg-1 for 232Th and 540-590 Bq kg-1 for 40K in red clay roofing tiles (regions of northern Italy). In mean, 33±20 Bq kg-1 for 226Ra, 37±17 Bq kg-1 for 232Th and 511±158 Bq kg-1 for 40K are found in clay bricks (Upper Egypt) (Ahmed, 2005).

However, the activity concentrations for 226Ra, 232Th and 40K measured in the investigated samples are higher than those found in the clays used in building materials of Namakkal (Ravisankar et al., 2012).

Radium equivalent activity; As the distribution of natural radionuclides in samples is not uniform, a common radiological hazard index has been introduced in order to compare the specific activities of materials containing different amounts of 226Ra, 232Th and 40K. This index used to obtain the sum of those activities, is called Radium equivalent activity (Raeq) and is given by the relation below (Beretka and Mathew, 1985):

Raeq (Bq kg-1) = ARa+1.43ATh+0.077AK

where ARa, ATh and AK are respectively the activity concentration of the radionuclides 226Ra, 232Th and 40K.

The above formula is based on the estimation that 370 Bq kg-1 of 226Ra, 259 Bq kg-1 of 232Th or 4810 Bq kg-1 of 40K produce the same gamma dose rate (Stranden, 1976).

The calculated values of Radium equivalent activity (Raeq) for the different samples are shown in Table 4. These values vary from 90.2±28.4 Bq kg-1 (sample LRF) to 130.3±43.8 Bq kg-1 (sample LBF) with an average of 102.6±33.1 Bq kg-1. This estimated value is lower than the recommended maximum value of 370 Bq kg-1 (UNSCEAR, 2000).

The external and internal hazard indices: The external hazard index (Hex) is a criterion to access the radiological suitability of a material.

Table 4: Radium equivalent activity, external and internal hazard indices values of the studied clay samples
AVA: Green clay of Anyama, LBF: White lokpo, LRF: Red lokpo, LMF: Brown lokpo, LVF: Purple lokpo, LJPF: Yellow lokpo, LJFF: Dark yellow lokpo, LNF: Gray lokpo of Bingerville, Raeq: Radium equivalent activity, Hex: External hazard index, Hin: Internal hazard index

The model, proposed by Krieger (1981), defines the external hazard index as follows:

where, ARa, ATh and AK are respectively the activity concentration of 226Ra, 232Th and 40K. The value of Hex must not exceed the limit of unity to keep the radiation hazard insignificant.

The internal hazard index (Hin) gives the internal exposure to carcinogenic radon and its short-lived progeny. It is given by the following formula:

The values of Hin must also be less than unity to have negligible hazardous effect of radon and its short-lived progeny to the respiratory organs (Al-Hamarneh and Awadallah, 2009).

From the results (Table 4), the calculated values for Hex and Hin vary from 0.24±0.08 to 0.35±0.12 and from 0.32±0.11 to 0.50±0.18, respectively. The lowest value is found in sample LRF for Hex and in sample AVA for Hin while the highest values are in sample LBF.

The average values (0.28±0.09) for Hex and (0.37±0.13) for Hin, being lower than unity indicated that the hazardous effects of these radiations are negligible.

External gamma absorbed dose rates and annual effective dose: The external absorbed dose rate, Dex (nGy h-1) contribution from 226Ra, 232Th and 40K at 1 m above the ground was estimating using the equation (Farai et al., 2006; Jibiri and Biere, 2011):

Dext = 0.427 ARa+0.662 ATh+0.043 AK

The annual effective dose rate Eext(μSv y-1), resulting from the absorbed dose rate values and taking into account the conversion coefficient from absorbed dose in air to effective dose and the outdoor occupancy factor, was calculated using the following relation (Al-Hamarneh and Awadallah, 2009):

Eext(μSv y-1) = Dext (nGy h-1)×24 h×365.25 d×0.2× 0.7 Sv Gy-1×10-3

The conversion coefficient from Gy h-1 to Sv h-1 gives the equivalent human dose in Sv h-1 from the absorbed dose rate in air (Gy h-1) while the occupancy factor gives the fraction of the time an individual is exposed to outdoor radiation. The conversion coefficient recommended by the UNSCEAR (2000) is 0.7 Sv Gy-1 and the occupancy factor is 0.2 which suggests that people spent 20% of their time outdoors.

The results of these calculations are given in Table 5. The external absorbed dose rate in air outdoors Dex and the annual effective dose Eext values vary from 42.6±13.3 to 60.1±20.2 nGy h-1 and from 52.2±16.3 to 73.8±24.8 μSv y-1, respectively. The lower values are registered in sample LRF and the highest are in sample LBF. The average values of the absorbed dose rate in air and the calculated annual effective dose are found to be 48.6±15.6 nGy h-1 and 59.6±19.1 μSv y-1, respectively.

The absorbed dose rate in air outdoors from terrestrial gamma ray in normal circumstances is about 57 nGy h-1 while the worldwide average annual effective dose is approximately 70 μSv (UNSCEAR, 2000). The values calculated for the investigated samples are in good agreement with the average worldwide limits.

Annual effective dose by ingestion: The consumption by ingestion of the materials is a source of exposure to the rays emitted by the radionuclides. The effective dose by ingestion of the materials is given by the formula (Cazala et al., 2006):

Where:

Eing (Sv y-1) = Annual effective dose by ingestion
q = Quantity of matter ingested (kg y-1)
hj,ing = Effective dose engaged by unit of incorporation of the radionuclide j (Sv Bq-1) ingested

For 226Ra, 232Th and 40K, the coefficients of effective dose engaged by unit incorporated by ingestion (Sv Bq-1) are 2.8×10-7, 9.2×10-8 and 6.2×10-9, respectively.

While admitting a yearly consumption of 1.5 kg, the relation becomes:

Eing = 1.5×(2.8×10-7 Ara+9.2×10-8 Ath+6.2x10-9 AK)

Table 5: External gamma absorbed dose rates, annual effective dose and annual effective dose by ingestion of the studied clay samples
AVA: Green clay of Anyama, LBF: White lokpo, LRF: Red lokpo, LMF: Brown lokpo, LVF: Purple lokpo, LJPF: Yellow lokpo, LJFF: Dark yellow lokpo, LNF: Gray lokpo of Bingerville, Dext (nGy h-1): External absorbed dose rate, Eext (μSv y-1): Annual effective dose rate, Eing (μSv y-1): Annual effective dose rate by ingestion

The values calculated for the investigated samples are given in Table 5. The observed values vary from (17.1±6.2 μSv y-1 in sample AVA to 30.8±11.1 μSv y-1 in the sample LBF. The average value calculated is 21.3±7.6 μSv y-1. This value is lower than the admissible value of 1 mSv y-1.

CONCLUSION

The radioactivity concentrations of 226Ra, 232Th and 40K measured in the investigated clay samples of Côte d’Ivoire have been determined by gamma ray spectrometry. The specific activity, radium equivalent activity, radiation hazard indices, external annual dose and dose by ingestion have been determined for each sample in order to assess the radiological hazard.

These different parameters calculated in the present study are within the recommended safety limits. The clays studied do not pose any significant radiation hazard and hence can be consummated.

This investigation can serve as model for more extensive studies of the same subjects.

REFERENCES
Ahmed, N.K., 2005. Measurement of natural radioactivity in building materials in Qena city, Upper Egypt. J. Environ. Radioact., 83: 91-99.
CrossRef  |  Direct Link  |  

Al-Hamarneh, I.F. and M.I. Awadallah, 2009. Soil radioactivity levels and radiation hazard assessment in the highlands of Northern Jordan. Radia. Meas., 44: 102-110.
CrossRef  |  Direct Link  |  

Amin, R.M., F.A. Khalil and M.A.K. El Fayoumi, 2011. Natural radioactivity and chemical concentrations in Egyptian groundwater. Environ. Monit. Assess, 173: 29-35.
CrossRef  |  

Beretka, J. and P.J. Matthew, 1985. Natural radioactivity of Australian building materials, industrial wastes and by-products. Health Phys., 48: 87-95.
PubMed  |  Direct Link  |  

Bolca, M., M.M. Sac, B. Cokuysal, T. Karali and E. Ekdal, 2007. Radioactivity in soil and various foodstuffs from the Gediz River Basin of Turkey. Radia. Meas., 42: 263-270.
CrossRef  |  

Cazala, C., B. Cessac and D. Gay, 2006. Methodological guide for the acceptance of waste with natural radioactivity in classified installations for disposal. Report DEI/SARG/2006-009, pp: 1-52.

Coulibaly, V., J. Sei, S. Oyetola, M.T. Sougrati and J.C. Jumas, 2012. Iron speciation in the clays consummated in Cote d'Ivoire: A transmission mossbauer spectroscopy study. Asian J. Applied. Sci., 5: 460-472.
CrossRef  |  

Farai, I.P., R.I. Obed and N.N. Jibiri, 2006. Soil radioactivity and incidence of cancer in Nigeria. J. Environ. Radioact., 90: 29-36.
CrossRef  |  

Hewamanna, R., C.S. Sumithrarachchi, P. Mahawatte, H.L.C. Nanayakkara and H.C. Ratnayake, 2001. Natural radioactivity and gamma dose from Sri Lankan clay bricks used in building construction. Applied Radiat. Isot., 54: 365-369.
CrossRef  |  

Jibiri, N.N. and P.E. Biere, 2011. Activity concentrations of 232Th, 226Ra and 40K and gamma radiation absorbed dose rate levels in farm soil for the production of different brands of cigarette tobacco smoked in Nigeria. Iran. J. Radiat. Res., 8: 201-206.
Direct Link  |  

Krieger, R., 1981. Radioactivity of construction materials. Betonwork Fertigteil-Techn., 47: 468-473.

Kumar, V., T.V. Ramachandran and R. Prasad, 1999. Natural radioactivity of Indian building materials and by-products. Applied Radiat. Isot., 51: 93-96.
CrossRef  |  

Laubenstein, M. and D. Magaldi, 2008. Natural radioactivity of some red Mediterranean soils. Catena, 76: 22-26.
CrossRef  |  

Le Bourdiec, P., 1958. Contribution to geomorphological study of sedimentary basin and coastal regions of Cote d'Ivoire. Eburnean Stud., 7: 7-97.

Montes, M.L., R.C. Mercader, M.A. Taylor, J. Runco and J. Desimoni, 2012. Assessment of natural radioactivity levels and their relationship with soil characteristics in undisturbed soils of northeast of Buenos Aires province, Argentina. J. Environ. Radioact., 105: 30-39.
CrossRef  |  

Pavlidou, S., A. Koroneos, C. Papastefanou, G. Christofides, S. Stoulos and M. Vavelides, 2006. Natural radioactivity of granites used as building materials. J. Environ. Radioact., 89: 48-60.
CrossRef  |  

Petropoulos, N.P., M.J. Anagnostakis and S.E. Simopoulos, 2002. Photon attenuation, natural radioactivity content and radon exhalation rate of building materials. J. Environ. Radioact., 61: 257-269.
CrossRef  |  

Pourcelot, L., D. Louvat, F. Gauthier-Lafaye and P. Stille, 2003. Formation of radioactivity enriched soils in mountain areas. J. Environ. Radioact., 68: 215-233.
CrossRef  |  

Ravisankar, R., K. Vanasundari, A. Chandrasekaran, A. Rajalakshmi, M. Suganya, P. Vijayagopal and V. Meenakshisundaram, 2012. Measurement of natural radioactivity in bulding materials of Namakkal, Tamil Nadu, India using gamma-ray spectrometry. Applied Radiat. Isot., 70: 699-704.
CrossRef  |  

Righi, S. and L. Bruzzi, 2006. Natural radioactivity and radon exhalation in building materials used in Italian dwellings. J. Environ. Radioact., 88: 158-170.
CrossRef  |  Direct Link  |  

Santos, E.E., D.C. Lauria, E.C.S. Amaral and E.R. Rochedo, 2002. Daily ingestion of 232Th, 238U, 226Ra, 228Ra and 210Pb in vegetables by inhabitants of Rio de Janeiro City. J. Environ. Radioact., 62: 75-86.
PubMed  |  

Stranden, E., 1976. Some aspects on radioactivity of building materials. Phys. Norv., 8: 167-173.

Turhan, S., 2008. Assessment of the natural radioactivity and radiological hazards in Turkish cement and its raw materials. J. Environ. Radioact., 99: 404-414.
CrossRef  |  Direct Link  |  

UNSCEAR, 2000. Sources and effects of ionizing radiations. Report to the General Assembly with Scientific Annexes. United Nations, New York, USA.

©  2020 Science Alert. All Rights Reserved