WATER-SOLUBLE FRACTIONS OF HEAVY-METALS DURING COMPOSITING OF MUNICIPAL SOLID-WASTE

The need to assess the limitations of compost use on agricultural soils has created a demand for information on the availability of metals in municipal solid waste compost. This study was done to determine the water-extractable fractions of Cd, Zn, Cu, and Pb and investigate changes in water-soluble organic C during the complete composting of a pile of ground municipal solid waste. Samples were collected every 3 to 5 d for 60 d. The progressive increase in total heavy metals concentrations during the composting period was not accompanied by a corresponding increase in the water-extractable amounts of metals. The water-extractable fractions of Pb and Zn decreased rapidly with composting time, and a more complex behavior was observed for Cu and Cd. No relationship was found between pH and extractable fractions of all metals, even if higher amounts of Pb and Zn were extracted when the pH of the composting material was below neutrality. Total soluble C concentration of extracts rapidly decreased during the first days of composting. There was a significant correlation between water-extractable Pb (r = 0.99***) and Zn (r = 0.98**), and water-soluble C of nonhumic decomposition products. The water-extractable fraction of Cu was significantly correlated only with water-soluble humic C (r = 0.94**).1 No correlation was found for the water-extractable fraction of Cd. The degree of stabilization reached by organic matter appears to be important in determining potential mobility of heavy metals.