Speciation of chromium in water and sediments from Mokolo and Blood rivers, Limpopo Province

Abstract

Rivers provide the main water sources for domestic, industrial, and irrigational purposes; however, they could be polluted by receiving wastes from municipal and industrial sources as well as runoff from agricultural land. This could leave rivers contaminated with chromium (Cr) and other potentially toxic elements. Chromium can be either essential or carcinogenic depending on the chemical form. The aim of this study was to assess and quantify trivalent Cr [Cr(III)] and hexavalent Cr [Cr(VI)] in water and sediment samples, collected from Mokolo and Blood Rivers in the Limpopo province. Water and sediment samples were collected from ten sampling sites from each river. Microwave assisted acid digestion and microwave assisted extraction methods were used to obtain a simple, rapid and safe method for the determination of total Cr and Cr(VI) in sediments. Water samples were acidified with ultra-pure HNO3 and analysed directly to quantify total Cr. The accuracy of the method was evaluated by employing a sediment standard reference material (SRM 8704) and trace elements in water reference material (SRM 1643f). Total Cr and Cr(VI) in water and sediment samples were quantified using flame atomic absorption spectrometry (FAAS) and graphite furnace-atomic absorption spectrometry (GF-AAS). The Cr concentrations in water and sediment samples collected from Blood River were found in the range 1.56 to 6.11 μg/L and 129.2 to 252.9 μg/g, respectively. The concentrations of Cr in water and sediment samples obtained from Mokolo River ranged from 1.34 to 3.53 µg/L and 25.7 to 156.4 μg/g, respectively. A new solid phase extraction method was developed using chromabond-NH2 column to determine Cr(VI) in water. In order to achieve separation of Cr(VI), the sample was pre-concentrated and passed through a conditioned chromabond-NH2 column at a flow rate of 3 mL/min. Hexavalent Cr was selectively adsorbed onto the column and the contents were dried under vacuum. The retained Cr(VI) was subsequently eluted with two column volumes of 2 M HNO3 and diluted to a final volume of 10.0 mL. The limit of detection was 0.105 µg/L and the relative standard deviations were less than 2%. The validation of the procedure was performed by spiking standard solutions containing Cr(III) and Cr(VI) and the percentage recoveries were higher than 88%. The concentrations of Cr(VI) in BloHexavalent Cr in sediment samples was determined by employing leaching reagents of 0.1 M Na2CO3 and 0.01 M Na3PO4. Determination of Cr(VI) by using 0.1 M Na2CO3 as leaching reagent followed by atomic spectrometric measurements provided satisfactory results with percentage recoveries of 94.9 to 105%. This was achieved by extracting the sediment–reagent solution mixture for a period of 15 min at maximum pressure and temperature of 700 psi and 120 ºC, respectively. The leached Cr(VI) was then quantified by GF-AAS after filtration of the sample solutions through a hydrophilic Millipore PVDF 0.45 µm filter. The quantified levels of Cr(VI) leached by 0.1 M Na2CO3 in sediment samples of Blood and Mokolo Rivers were in the range 0.41 to 1.32 μg/g and 0.17 to 0.82 μg/g, respectively. The concentrations of Cr(VI) obtained employing the leaching reagent of 0.01 M Na3PO4 were found to be below LOD value of 0.06 µg/g in Mokolo River in all the sites and were in the range of 0.07 to 0.09 µg/g in Blood River. The low values of Cr(VI) in water and sediments show that Cr(III) did not oxidise to Cr(VI) during sample preparation. In Blood and Mokolo Rivers, although the concentrations of Cr(VI) are low, their values suggest a link with the industrial and agricultural activities in the area. od and Mokolo Rivers ranged from 0.13 to 1.99 µg/L and 0.13 to 0.55 µg/L, respectively