Speciation of selenium in water and sediments from Mokolo and Blood Rivers, Limpopo Province

Abstract

Surface water from the rivers serves as a source of water for many purposes including drinking, irrigation and animal farming. The quality of surface water deteriorates due to human, animals and industrial waste. Amongst these wastes, selenium and other trace elements contaminants are included. Selenium can either be essential or toxic depending on the concentration and oxidation state. The aim of this current study was to determine the concentrations of inorganic selenium [Se(IV) and Se(VI)] in water and sediment samples collected from Blood and Mokolo Rivers in Limpopo Province, South Africa. Water and sediment samples were collected from 10 different sampling sites from down to upstream of each river. Water samples were acidified with 1% ultra-pure HNO3 and analysed directly for total selenium concentration. The accuracy of the method was validated using SRM1643f (trace elements in water reference material). Sediment samples were digested using microwave assisted acid digestion for the determination of total concentration of selenium. The accuracy of the method was evaluated using SRM 8704 (sediment standard reference material). Total concentration of selenium in both water and sediment samples were quantified using ICP-MS. The total concentration of selenium in water samples from Blood and Mokolo River were found to be in the range of 0.0682 to 2.72 μg/L and 0.0851 to 25.4 μg/L respectively. The selenium concentrations in all sediment samples were found to be below instrument detection limit of 0.0571 ng/g in both rivers. An adopted SPE method using Dowex 1 x 2 resin (chloride form) as an adsorbent material to preconcentrate and separate Se(IV) and Se(VI) was used for the speciation in water samples. Both Se(IV) and Se(VI) were retained on the column. The retained Se(IV) and Se(VI) were eluted using 15 mL 1 M HNO3 and 3 M HNO3 respectively at a flow rate of 2 ml/min and diluted to the final volume of 20 mL. The instrument detection limit was 0.192 μg/L and 0.108 μg/L for Se(IV) and Se(VI) respectively. The validation of the method was performed by using SRM 1643f and solutions of known concentrations. The water samples were adjusted to an optimum pH of 6 throughout the speciation analysis. The Se(IV) had higher percentage recoveries of 95 – 114% than Se(VI) with 53%. vi Concentrations of Se(IV) and Se(VI) in Blood River ranged from 0.0411 to 0.820 μg/L and 0.0811 to 1.75 μg/L respectively. Concentrations of Se(IV) and Se(VI) in Mokolo River ranged from 0.135 to 2.79 μg/L and 0.0961 to 14.8 μg/L respectively. The inorganic selenium species in water samples were also determined by using the adopted online mode of HPLC-ICP-MS with Hamilton PRP-X100 column. The separation of the two species was achieved by using isocratic elution of 100 mM NH4NO3 at pH 8.5 in 8 min. The method was successfully validated using SRM 1643f. The LOD of 0.842 μg/L and LOQ of 2.81 μg/L for Se(IV) were achieved. The LOD of 0.690 μg/L and LOQ of 2.30 μg/L for Se(VI) were achieved. The Se(IV) and Se(VI) concentrations determined using HPLC-ICP-MS were found to be in good agreement with Se(IV) and Se(VI) concentrations obtained using SPE in both rivers. The presence of Se(IV) and Se(VI) in water samples in Blood and Mokolo Rivers indicates that industrial and agricultural activities taking place near the rivers have an effect on the quality of the water. The selenium in water may be due to wastes from industrial, municipal and agricultural runoffs. The absence of selenium in sediment samples suggests that the area where the rivers are located is not rich in selenium.