Evaluation of several digestion methods in the extraction of heavy elements from different organic sources

In recent years, the increased water, soil, and environmental degradation, due to the excessive use of chemicals, has encouraged researchers into organic farming. Despite all the positive effects associated with the consumption of sewage sludge and municipal compost on the soil physical and chemical properties, there is still a great concern in terms of environmental and health. Depending on the source, composts often contain relatively large amounts of heavy metals, so accurate measurement of heavy metal contents in these compounds is important.

In this study, 20 compost samples from different sources, including manure, municipal waste and sewage sludge sources were selected. Five methods of digestion in three replicates were used to extract the heavy metals of the compost samples, including nitric acid, dry ashing, nitric–perchloric acid, sulfuric acid and sulfosalicylic acid methods. The elements wrer Cd, Pb, Ni, Cr, Co, Cu, Zn, Mn and Fe.

Analytical results indicated that the nitric–perchloric acid procedure was the most efficient for recovering Cd, Pb and Cr from the organic samples. After that dry ashing method extracted the highest amount of Cd, Pb and Cr from all compost samples (on average).Since perchloric acid is potentially hazardous during digestion procedure, dry ashing was recommended as an alternative method. The recovery of Cd and Pb in the organic compounds is affected not only by the digestion method, but also by the type of compost. For example, the nitric–perchloric acid procedure recovered more Cd and Pb from municipal waste and sewage sludge than manure and Poultry fertilizers. In the case of other elements including nickel, cobalt, copper, manganese, zinc and iron, sulfosalicylic acid has the highest efficiency in extracting these elements from compost samples. Extraction of these elements was only affected by the type of extraction method and the type of organic composition had no effect on it

Different digestion methods used in this study and also different compost sources that were tested had a very significant effect at the level of (P < 0.05) on the extraction of all heavy metals. Despite the fact that the organic compounds used in this experiment had a relatively wide range in terms of various structural and chemical properties; the digestion methods used for the heavy and micronutrient elements in these compounds had a relatively specific trend in terms of the extraction potential of these elements. In general, in the case of heavy metals, cadmium, lead, and chromium, nitric acid + perchloric acid, and then dry digestion method, had the best efficiency in extracting these elements. For the other elements including nickel, cobalt, copper, manganese, zinc, and iron, sulfosalicylic acid was the most effective in extracting these elements from compost samples.