Heavy metal analysis of sewage sludge by x-ray Fluorescence technique and the environmental Implications

Abstract

The use of sewage sludge as a manure has for a long time been considered as the most economical method for its disposal. However, while serving as a good source of plant nutrients, application of sludge on agricultural soils also increases the heavy metal content of the soils. Excessive amounts of some of these heavy metals can be phytotoxic and may, in some cases, enter the food chain and thus end up being toxic to animals or human beings feeding on such foods. In this project, sewage sludge from Kariobangi Sewage Treatment Works, in Nairobi, has been analyzed for its heavy metal content. These analyses have been performed using x-ray fluorescence technique. Atomic absorption spectroscopy method has also been used, for some of the elements, for comparison. Samples analyzed included the sludge itself, soils on which the sludge has been used, and plants grown on such soils. For comparison purposes, soil samples, plants from areas where sludge is not used, fertilizers, and manures were also analyzed. The following metals have been shown to be present in all the sludge samples: 'i'; Titanium (2220 - 8550 ppm); Chromium (90 - 530 ppm); Iron (28000 - 61270 ppm); Nickel (26 - 97 ppm); Copper (296 - 490 ppm); Zinc (1350 - 2400 ppm); Gallium (11 - 24 ppm); Lead (248 - 580 ppm); Rubidium (16 - 50 ppm); Strontium (79 - 170 ppm); yttrium (29 - 67 ppm); Zirconium (210 - 460 ppm) ; and Niobium (53 - 116 ppm). Soils and manures were found to contain the same heavy metals at slightly different levels. In addition to determining the total heavy metal contents in the soil samples, the available amount to the plant was also determined for manganese, copper, iron, and zinc. The available amounts were determined by atomic absorption spectroscopy. The results obtained in this work indicate that the use of sewage sludge as a·manure results in accumulation of s~me of these heavy metals in the soil. The high levels of available contents of these metals especially manganese and copper in sludge - enriched soils may be phytotoxic. Analysis of fertilizer samples indicated that some metals, such as strontium, may accumulate in soils if extensive use of fertilizers is practised. Analysis of plant materials grown on sludge - enriched soils showed that these plants, for example, cowpeas, spinach, sugarloaf had high levels of heavy metals. While usage of sludge as a manure may still be the best method of disposal there is a great need to monitor regularly the levels of heavy metals in the sludge, soils, and plants. Results of such monitoring may then be used to decide when and where sludge may be used as well as the type of plants that may be grown on sludge - enriched soils.