Determination of heavy metal content in surface sediments and soil from the River Nile Khartoum City (Sudan)

The surface sediment and surface soil samples were taken from the River Nile (Abroof, Shambat, AL-Halfaia, and AL-Hetana), Khartoum, to determine the available levels of Lead (Pb), Zinc (Zn), and Cadmium (cd) to establish a baseline concentration of each of these available metals, and to investigate their possible correlations with soil and sediment properties. The content of each of the three heavy metals in soil and sediment samples was determined using atomic absorption spectroscopy, AAS. The values of Zinc in the soil and sediment samples, were found to be between (0.192 to 0.294 ppm) and (0.101 to 0.181) respectively, showing the highest content in soil and sediment samples in Shambat (S) and Abroof (A), and the lowest values recorded for samples in AL-Hala (H) and AL-Hetana (E). While Lead content was ranged between (0.105 to 0.136 ppm) for soils samples (AL-Hetana(E) and Shambat(S)), and (0.078 to 0.141) for sediment samples (Abroof(A) and shambat (S)). Whereas, content for Cadmium ranged between (0.010 to 0.015 ppm) for soil samples Shambat, AL-Hetana and Abroof and (0.011 to 0.016) for sediment samples Shambat, AL-Hala and Abroof. Soil and sediment physicochemical properties (pH, temperature & conductivity) were found to be correlated with the available heavy metals content, suggesting that the enhanced mobility of heavy metals are related to anthropogenic activities, the high percentage of organic carbon in soil (11.6%) and sediment (15.0%).


Introduction
River Nile is the longest river in the world and it is a source of life to millions of people, owing 6.825 km (4.238 miles) from south to north [1].
Soil is the mixture of minerals, organic matter, gases, liquids, and the countless organisms that together support life on earth [2]. Sediment is the loose sand, clay, silt and other soil particles that settle at the bottom of a body of water, it can come from soil erosion or from the decomposition of plants and animals. Wind, water and ice help carry these particles to rivers, lakes and streams. The Environmental Protection Agency lists sediment as the most common pollutant in rivers, streams, lakes and reservoirs [3]. One source de nes heavy metal as one of the common transition metals, such as copper, lead, and zinc. These metals are a cause of environmental pollution from sources such as leaded petrol, industrial e uents, and leaching of metal ions from the soil into lakes and rivers by acid rain [4]. Living organisms require varying amounts of heavy metals. Iron, cobalt, copper, manganese, molybdenum, and zinc are required by humans [5]. All metals are toxic at higher concentrations and all soils and sediments contain some concentration (usually low) of trace and toxic metals from natural sources. However, these background levels can vary widely depending on a number of factors such as parent material [6]. It is usually a result of human activities that levels of metals increase and due to this pollution in soils and sediments can rise to the point where they represent a potential health or ecological risk [7].
The most common types of soil pollutants are heavy metals such as cadmium, chromium, copper, zinc or mercury, pesticides or herbicides, organic chemicals, oils and tars, explosive or toxic gases, combustible or radioactive materials, biologically active compounds and asbestos [8].
The objectives of this study are to determine the of the heavy metals concentrations; zinc (Zn), lead (Pb), and cadmium(Cd) in the sediment and soil of Abroof, Shambat, AL-Hal a, AL-Hetana Khartoum, Sudan, in order to assess the impact of past anthropogenic activities on soil and sediment quality, to give suggestions of methods to analysis heavy metal.

Study area and sample collection
In the River Nile and its tributaries there are many activities like burning of coal for the production of bricks and modern constructions buildings [10].

Experimental
Electrical conductivity (EC), temperature, organic carbon content and pH were measured. The pH for the samples was determined using Geotechnical Test Method (GTM-24) method by dissolved the samples in the deionized water with ratio of (1:5) (sample: water) and pH measured after 30 minutes using pH meter. An alternate current electrode was used to measure electric conductivity at 25.0ºC.

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The organic matter determined using Standard Test Methods (ASTM D2974) by heating sample at 550°C in a furnace for 6 hours, the samples were weighted many times until reached to constant weight.
The samples were digested in heating mantle inside a Fume Hood (Missunshi 64132) using HNO 3 (65%) & HCLO 4 (60%) in the ratio 2:1 respectively, then the digested samples were transferred into a beakers(100ml) and ltrated with nitric acid (20%), then the ltrate was placed in a volumetric ask (100ml), ready for analysis by AAS spectrophotometer.
The solutions of the digested samples were analyzed using air/acetylene atomic absorption spectroscopy model (210 VGP) (CAT#28750.15) with the use of prepared standards for Pb, Zn & Cd. Nitric acid (65%) was used in the preparation of standard solutions (20%). Calibration curves were used to calculate the concentration of the metals. A reagent blank was used to zero the instrument. This was followed by aspiration of standard solution and nally the soil sample extract was aspirated. Soil and sediments samples were subjected to heavy metal analysis and also for physicochemical parameters measure including pH, organic matter and electrical conductivity.  [12] showed that the mineralogical results reveal the riverine origin of the sediments of the Gda.sk Basin, with the coarser mineral grains deposited near the river mouth and the nest terrigenous particles transported further from the coast. This ne material is the best carrier for the external inputs of organic matter, which is then transported and accumulated in surface sediments far from the coast.

Results And Discussion
In all soil and sediments samples concentration of all heavy metals was recorded below the permissible limits worldwide table (3) and (4). Concentration of lead ranged between 0.078 to 0.141ppm, zinc ranged between 0.101 to 0.294 ppm and concentration of cadmium was recorded to range between 0.011 to 0.016 ppm. Heavy metal concentrations in sediment and soil are affected by particle size and composition of sediments and concentrations generally increased with decreasing particle size of sediments and concentrations generally increased with decreasing particle size of sediments [10].
Figures (1) and (2) show the comparison of elements concentration between soil and sediment samples for study, Cd was shown same distribution in all sites, but Zn and Pb were shown different, the presence of a point source of contamination may cause a sharp increase, for the sediment and soil samples and may be the source of Zn in soil from sediment deposition.
Mohamed (2007) [1] reported that exhausts from vehicles might contribute to increased concentration observed downstream the city Khartoum because exhausts are regarded as the major source of environmental contamination by lead and Ni. The concentration of Zn in soil was higher compared to sediment gure (4), the concentration of Zn difference between all sites in river, this indicate the difference of elements such sources and the concentration of Cd in E was higher compared to other sites gure (5), the strong association of Cd indicates common sources, and these metals may have been derived from anthropogenic sources, especially the paint industry, the application of phosphate fertilizers to the agricultural soil has led to increase. The gure (6) and (7) showed that the concentration of Pb and Cd elements in all study areas is greater than local study, the source of Pb element is automobile exhausts. Higher levels of Pb often occur in water bodies near highways and large cities due to high gasoline combustion. The concentration of Zn for all study areas is greater than local study except in site A ( gure 8), the variation of heavy metal concentration in the study area was due to irrigation of land by industrial waste water and other agronomic practices.

Conclusions
The content of metals varied across the study areas.
1. The highest concentration of Zn was found in Shambat and Abroof sites (0.294, 0.181ppm), this high level cannot be accounted for naturally and is the result of pollution from out site anthropogenic activities.
2. In comparison, higher concentrations of Pb are shown in Shambat soil and sediment (0.136, 0.141 ppm) respectively, and high concentrations of Cd in Abroof soil (0.015 ppm) and AL-Hetana sediment (0.016 ppm).
3. These metals are not added to the soils and sediments through anthropogenic processes instead their concentrations on the area are dominated by natural sources, weathering, geology and pH.
4. The pH in all samples were alkaline, ranging between (7.15-7.55) that is due deposition of mineral salt.

Declarations
Availability of data and materials