Contamination assessment and chemical speciation of potentially toxic elements in surface sediments of the Karoon River, Ahwaz city district

Abstract

AbstractThe present study aims to evaluate the concentration and speciation of potentially toxic elements in surface sediments of Karoon River in Ahwaz district. For these purposes, 14 surface samples were collected. The total concentration of major and trace elements was determined by an ICP-OES instrument. A five-stage sequential extraction procedure was applied to investigate the fractionation of potentially toxic elements. The pH, organic matter, carbonate content, and cation exchange capacity of the sediments vary between 7- 8.9, 0.4 to 4.9 %, and 5.4 to 12.6 meq/100 g, respectively. Geo-accumulation index values show that the studied samples are classified as non-polluted to slightly polluted sediments. The sequential extraction analysis indicates that 78 %, 77.6%, 61.8 %, 73 %, 65.8 %, 85.8 %, and 60.9% of total Cu, Pb, Cd, Zn, Ni, As, Cr content, respectively, are present in the non-residual phases; therefore, these elements are mainly from anthropogenic sources. Considering the chemical fractionation of the studied elements in the sediments of the Karoon River, Cd has the highest bioaccessibility, whereas Cu and Cr are not bioaccessible.Keywords: Karoon River, Sediment, Toxic Elements, Chemical Speciation. IntroductionRivers are one of the most important aquatic ecosystems that are highly prone to be polluted through sewage discharge. Potentially toxic elements are typically considered the most significant inorganic pollutants, since they are not degradable through natural processes and, finally, are accumulated in the food chain (Nemati et al. 2011). Potentially toxic elements enter the various compartments of the environment through natural (i.e., erosion and weathering processes) and anthropogenic sources (the most important of which is the discharge of industrial, agricultural, and municipal wastewater) (Krishna and Govil 2005; Wang and Qin 2005). Bed river sediments are considered as a main pollutant sink that accumulates toxic metals, and therefore, the composition of river sediments is considered as an important index for the assessment of river pollution status (Yuan et al. 2004; Bermejo et al. 2003).The total concentration of potentially toxic elements in sediments is not a proper criterion for assessing the bioaccessibility, mobility, and toxicity of elements. In order to evaluate the pollution level of a river system, both total and bioaccessible concentrations of elements must be considered. For this purpose, the chemical form of elements must be determined using sequential extraction procedures (Zhang et al. 2020a) in which different forms of a target element (i.e., exchangeable, acid-soluble, reducible, oxidizable, and residual forms) are determined using successive extraction stages.The Karoon River is the longest river flowing in Iran (950 km) that passes through Ahwaz city. During the past years, untreated wastewater from agricultural, industrial and municipal sources has been discharged into the Karoon River. The present study aims to assess the pollution of the Karoon River sediments, and to investigate the bioaccessibility of toxic elements using a sequential extraction method. Materials & MethodsFourteen surface sediment samples (0-5 cm) were collected using a Van Veen Grab sampler. The samples were air-dried at room temperature. After sample crushing, the samples were divided into two portions: one portion was sieved through a stainless steel sieve (230 mesh) for major and trace metal extraction. The second part was kept untreated to measure some physico-chemical properties (i.e., pH, carbonate content, organic matter content, and cation exchange capacity). The pH was determined using the standard method of USEPA (USEPA 1998, METHOD 9045D) by AZ-8601 pH-meter. Organic matter content, carbonate content, and CEC were determined using oxidation with K2Cr2O7 – H2SO4 (Chopin and Alloway 2007), the reverse titration method, and the USEPA method (USEPA 1998, method 9081A), respectively.The total concentration of major and trace elements in the studied samples was measured using an ICP-OES instrument (Model: Varian-735) after a strong acid digestion method (HF+HCl+HClO4+HNO3). The detection limit of the ICP-OES instrument for Cr, Cu, Ni, Pb, Zn, As and Cd was 1, 1, 1, 1, 1, 0.5, and 0.1 mg/kg, respectively. To evaluate the accuracy of the obtained data, international reference materials and blank samples were applied, whereas to assess the precision of data, four measurements were recorded for each element in the sediment sample.In order to investigate the chemical form of the target elements in the sediment samples, the sequential extraction method introduced by Tessier et al. (1979) was applied in which five successive extractions were carried out to obtain exchangeable, bound to carbonates, oxides, organic matter, and silicates. Discussion of Results & ConclusionsSediment’s pH plays a pivotal role in the mobility and toxicity of target elements (Liang et al. 2017). The pH of the studied samples varied between 7 and 8.9 (average value of 7.6). The neutral-alkaline pH of the samples may result in lower accessibility of metals, whereas the accessible fraction of metalloids (e.g., arsenic) usually increases at higher pH values. The organic matter enhances the sorption capacity of sediments. The organic matter content of the samples varied from 0.4 to 4.9 %. The highest organic matter was recorded at sampling site S8, in which a high amount of household and agricultural wastewaters are discharged into the river. Cation exchange capacity is influenced by the presence of clay minerals, Fe-Mn oxides, and organic matter in the sediments. Higher CEC values will result in a higher concentration of bioaccessible metals. CEC values of the studied samples varied between 5.4 and 12.6 (average value of 8.7) meq/100g. The relatively low values of CEC are perhaps due to the high content of sand particles and carbonate phases. The carbonate content of the samples varied between 22.5 and 49 % (average value of 40.3 %), which is in accordance with the neutral to alkaline pH of the samples. The high content of carbonate phases may enhance the sorption of Cd, Pb, and Mn (Sungur et al. 2015).The average content of toxic elements in the studied sediments followed a decreasing order (mg/kg): Cr (108.1) > Zn (67.1) > Ni (52.1) > Cu (31) > Pb (19.1) > As (4.7) > Cd (0.2).In the studied sediment samples, the concentration of Cr was higher than the average concentration of crust sediment, probably due to the discharge of industrial wastewater and /or natural inputs. In order to assess the environmental risks imposed by toxic elements in the studied sediment samples, the obtained results were compared with sediment quality guidelines (SQGs) provided by Macdonald et al. (2000). The Zn, Cd, and As content in all samples (except for sampling site S13 for Zn) was lower than Threshold Effect Concentration (TEC) and Probable Effects Concentration (PEC) values, indicating the lack of negative impacts of toxic elements on aquatic organisms (Xu et al. 2017; Zhang et al. 2020b). At all sampling sites (except S1, S10, and S14), Ni concentrations were higher than TEC at sampling sites S1, S10, and S14, and higher than PEC at sampling sites S3-S9, S11, and S13. The concentration of Cu in all sampling sites was lower than PEC, and its value was lower than TEC in sampling sites S1, S4-S6, S10-S12, and S14. The concentration of Cr in all sampling sites was higher than TCE, and in sampling sites S1, S8, S9 and S12-S14 was lower than PEC. Based on SQG values, the sediments of the Karoon River are polluted with Cr and Ni. The obtained results indicate that the sampling site S13 has the highest concentration of Cu, Zn, Ni, and As, probably due to the discharge of different industrial, municipal and agricultural wastewater into the river. Based on the geoaccumulation index values, the studied samples were classified as unpolluted with As, Cu, and Zn, and are classified as unpolluted to slightly polluted with Ni, Pb, Cd, and Cr. The Potential ecological risk index (PERI) of the samples showed that the studied sediments were classified as low levels of pollution with Cu, Pb, Zn, Cr, As and Ni, whereas they were classified as moderately polluted with Cd.Sequential extraction analysis indicated that Cu, Cd, Zn, Ni, Pb, and As were mainly present as non-residual fractions. Therefore, the bioaccessibility of these elements was high, indicating the impact of anthropogenic sources on the pollution of sediments. Moreover, the high concentration of these elements in non-residual fractions showed that they were mainly from anthropogenic sources. The fractionation trend indicated that Cr was considerably associated with residual phases, which showed the natural source and low bioaccessibility of this metal. In general, fractionation results indicated that the chemical affinity of the studied elements, as well as their source, influenced the chemical speciation of metals and metalloids in the sediment samples of the Karoon River. Individual and Global Contamination Factors (ICF and GCF) were measured based on sequential extraction data. ICF values of the potentially toxic elements followed the decreasing trend as: As (6.5) > Pb (3.5) > Cu (3.3) > Zn (2.8) > Cd (2) > Ni (1.8) > Cr (0.6). Therefore, As and Cr have the lowest and highest pollution hazard, respectively. GCF values indicated that samples S3, S5, and S8 were the most polluted sediments in the study area. Considering the Risk Assessment Code values (Pan et al. 2013), the studied samples were classified as moderately polluted with Cr and Cu, highly polluted with Ni, Zn, Pb, and As, and extremely polluted with Cd. In conclusion, Cr concentration in all studied samples was higher than mean sediments, crust, and shales compositions. The most polluted sampling site was located near the discharge points of industrial wastewaters into the River. The results of sequential extraction analysis indicated that the As, Cd, Ni, Cu, Pb, and Zn were mainly from anthropogenic sources, whereas Cr was dominantly from natural inputs. Based on the obtained results, Cd, Pb and Zn were mobile and bioaccessible, and may induce environmental risks to the aquatic system of the Karoon River.

Keywords

• karoon river
• sediment
• toxic elements
• chemical speciation