Chemical partitioning and pollution intensity of heavy metals in siahrud river sediment

ABSTRACT

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 micro m was chosen for chemical analysis. The total metal content was determined by digesting the samples with a mixture of HNO[3] and HClO[4]. The chemical partitioning of metals was determined by means of the sequential extraction scheme proposed by the European Union's 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% HCl [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-60l] 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