تفكيك شيميايي عناصر و بررسي شدت آلودگي در رسوبات رودخانه سياهرود

Chemical Partitioning & Pollution Intensity of Heavy Metals in Siahrud River Sediment

In the recent years, sediments are used for the assessment of pollution. Since sediments have been in contact for a long time with over lying water, therefore they have been able to absorb pollution on to their surface. In the present investigation, the surficial sediments of Siahrud River have been analyzed. Grab samples were taken from ten samplings sites. The samples were immediately sealed and stored at 4°c until their arrival at the laboratory. Grain size fraction less than 63 //m was chosen for chemical analysis. The total metal content was determined by digesting the samples with a mixture of HN03 & HC104. The chemical partitioning of metals was determined by means of the sequential extraction scheme proposed by the European Unionʹs Standards, Measurements and Testing Program (SM and T, formerly BCR). This scheme consists of three successive extractions that make it possible to determine the association of the metals in three phases: acid-soluble, reducible and oxidizable. The most resistant bonds were digested by hot 50% HC1 (phase 4). Furthermore, a fourth fifth phase, within lattice was determined as the difference between the total metal content and the sum of the contents in the four previous phases. The analysis of metals in the solutions was carried out by an inductively coupled plasma mass spectrometer (ICPAES). The accuracy of the analytical procedures for total metal determinations was checked using CRM 320 (sediment reference material). Replicate analysis of this CRM showed good accuracy, with recovery rates for metals between 97 and 101%. A standard reference material (CRM-601) was used to verify the accuracy of the sequential extraction method. Similarities between the behavior and origin of the metals studied were established by cluster analysis using the weighted pair group method. The loose, sulphide and organic bonds of metals with various sedimentary phases has been assessed by chemical partitioning technique. The sum of these three bonds is indicative of anthropogenic source of metals into the river. The lithogenic portion of metals was obtained by a two step chemical partitioning technique. The anthropogenic portion of metal contents in river sediments shows the following pattern: Pb(85%)>Zn(84%)>Cd(62%)>As(47%)>Mn(35%)>Ni(34%)>Cu(21%) The results show that bio-availability of metals such as Cd, Zn and as are highly dependent on anthropogenic portion and especially loosely bonded ions of such metals. The overall pollution intensity was obtained by various pollution intensity indices. The pollution intensity of Zn and Pb in the sediments of Siahrud River is 3.572 and 3.148, respectively that are indicative of high pollution intensity. Other studied metals fall within "no pollution" to "low pollution" intensities. Table 1 shows the results of pollution intensity of various studied metals that are computed based on the followings: IpoLL=Log2 {Cn/Bn} Where Ipoll= Pollution Intensity Cn= Present concentration of metal in sediment Bn= Background concentration of metal obtained from chemical partitioning studies Table 1: Pollution intensity of Elements in Siahrud River sediment based on Ipoll Element Obtained Ipoll Range of Ipoll Scale of Pollution Intensity (IpoU) Scale of Pollution in River Pollution Intensity in River Ni 0.69 0-1 1 1 No Pollution Mn 0.65 1-2 2 1 No Pollution Pb 3.15 2-3 3 4 High Pollution Cd 1.42 3-4 4 2 Low Pollution Cu 0.34 4-5 5 1 No Pollution Zn 3.57 5-6 6 4 High Pollution As 1.07 - - 2 Low Pollution In general, the bulk chemical analysis show very high concentrations of Ni, Mn, Pb, Cd, Cu, Zn and As in comparison with mean crust and world sediments. However, the chemical partitioning studies clearly show that except for Pb and Zn the other studied elements are governed by the geology of area of study. Therefore, higher concentrations of metals alone cannot be indicative of anthropogenic sources in the area of study. It should also be pointed out that cluster analysis can show the inter relationship amongst the studied metals to a very good extent but such analysis can not quantify the anthropogenic and lithogenic portions. The results of present investigation showed that many of the industries are closed down along the river side since last five to ten years. However, indication of elemental contribution at low energy parts of river can be found. In spite this fact, it should be mentioned that presently the main source of pollution into the river are residential sewage as well as hospital sewage that are being directed into the river without any pre-treatment. To promote the environmental conditions of the river, we propose injection of liquefied oxygen once the residential and hospital sewages are treated. It can also be suggested that in the future studies a better pollution index should be developed. We opine that Ipoll can further be improved by incorporating biological accessibility of metals into the formulae. At the present Ipoll uses background levels of trace metals that are obtained from chemical studies. Though this method is rather superior to the previous ones that use mean crust or shale values, but the presence of pollution may not all alone be considered as a real treat to the environment. Therefore, we have to know about the bio-accessibility of heavy metals and the excess of metals in bio-availability form should somehow be incorporated into the formulae of pollution intensity. On the other hand, other riverine and estuarine processes must be studied to know the fate of heavy metals. In these processes, we emphasis on the adsorption/desorption capacity of estuaries. The ultimate fate of heavy metals in marine environment under oxic and an-oxic conditions must also be studied. The present study has revealed that in spite of higher concentrations of heavy metals, most of them are governed/originated from lithogenous sources. Therefore, it would be interesting to know about the behavior of these elements under anoxic conditions of the Caspian Sea. Changes are made to the pollution indices but it seems that the on-going researches are mainly oriented towards the use of such new formulae than development of new ones. We suggest researchers to focus on the bio-availability and bio-accessibility of metals that can leave harmful impact on fauna and flora in excess concentrations. Such values should be used to revise the pollution intensity formulae. It should be pointed out that grain size of sediment can also alter the over-all concentration of metals. Therefore for obtaining uniform results, researchers should use a common grain size fraction in their future studies. Normally, grain size less than 63 micron meter can be used as standard one. If chemical partitioning technique cannot be used in the study, it would be essential to collect a sediment core to know about the background levels of metals.