Determination of elemental concentrations in edible seaweeds, sea sediments and seawater samples from the Kenyan Coast using X-ray Fluorescence techniques

Abstract

Since ancient times, seaweeds have been used as human food, animal feed, fertilizers, salt extractors and pharmaceuticals in orient countries but not to any great extent in Kenya. Currently, world seaweed industry is estimated at over US$ 6 billion annual income and there is steady growing demand for consumption and direct uses of seaweeds as food across the globe. This makes Kenya a potential producer of edible seaweeds in the region especially the Eucheumoids and Gracilaroids used for Agar and Carrageenan hydrocolloids production in food industries. Despite the growing consumption, marine pollution remains a major threat at the Kenyan coast due to increasing populations and industrial developments at the coastal city of Mombasa. Previous studies at the coast on heavy metal pollution remain implicit especially with regard to the use of biological indicators such as edible seaweeds to monitor trace element contamination and pollution of the marine ecosystem. On the other hand, presence of trace elements in edible seaweeds is an aspect that can compromise its use as human food. Therefore, this study sought to establish concentration levels for total and bio-available trace metals in the edible seaweed samples using X-ray Fluorescence (XRF) technique to specifically determine the trace element content and to address the safety of seaweeds as food product as well as for their use as biological indicators of marine pollution. In this study, a total of 40 seaweeds samples, 20 sea sediment samples and 20 sea water samples were collected by random method from the sampling sites at the Kenyan coast that included: Kibuyuni, KWS (Shimoni), Gazi Bay, Makupa and Tudor Creek. Seaweed samples were prepared by digestion method for TXRF analysis. This analytical method was also applied for seawater samples. The sea sediments were oven dried crushed, grounded and pelletized then Page | xi analyzed using ED200EDXRF machine comprised of cadmium radioactive source and Canberra Si-Li detector with a resolution of 190 eV at 5.9 KeV line. For seaweed samples, the concentration levels (in μg g-1) obtained varied as follows: Ca [217- 1253; 36.1 - 398], Mn [1.0 – 10.1; 0.1 - 3.9], Fe [7.0 - 123; 1.8 – 16.6], Cu [0.3 – 4.6; 0.0 – 0.6] and Zn [0.3 - 7.6; 0.0 – 1.6], for total and bioavailability contents in seaweeds respectively. In sediment and seawater, concentrations obtained were: Ca [<962 - 2,555; 54.8 - 175], Mn [<99; 4.7 - 43.8], Fe [<62 - 225; 7.1 - 61.2], Cu [14 – 41.2; 2.9 - 19.8] and Zn [19.0 – 78.4; 3.7 - 52] in μg g-1 and μg ml-1 respectively. From the findings, concentrations of trace elements in seaweeds were relatively high while the toxic elements such as As and Pb were below detection limits hence within EPA/WHO regulations. Therefore, the direct consumption of edible seaweeds as food should be encouraged. On the other hand, measurement of trace elements in sediments and seawater presents marked inconsistencies that studies have proved to be associated with changes in organic composition of the sediments and seasonal changes affecting dissolved matter in the sea. This makes seaweeds the preferred option as pollution indicators in the marine environment.