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TECHNICAL REPORTS
Heavy Metals in the Environment

pH-Dependent Release of Cadmium, Copper, and Lead from Natural and Sludge-Amended Soils

^a Dep. of Soil Science, Kasetsart University, Phaholyathin, Chatujak, Bangkok, 10900, Thailand
^b Dep. of Agricultural Sciences, The Royal Veterinary & Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark

* Corresponding author (haha{at}kvl.dk)

Received for publication October 16, 2001. The pH-dependent release of cadmium, copper, and lead from soil materials was studied by use of a stirred flow cell to quantify their release and release rates, and to evaluate the method as a test for the bonding strength and potential mobility of heavy metals in soils. Soil materials from sludge-amended and nonamended A horizons from a Thai coarse-textured Kandiustult and a Danish loamy Hapludalf were characterized and tested. For each soil sample, release experiments with steady state pH values in the range 2.9 to 7.1 and duration of 7 d were performed. The effluent was continuously collected and analyzed. Release rates and total releases were higher for the Hapludalf than the Kandiustult and higher for the sludge-amended soils than the nonamended soils. With two exceptions the relative release rates (release rate/total content of metal in soil) plotted vs. steady state pH followed the same curves for each metal, indicating similar bonding strengths. These curves could be described by a rate expression of the form: relative release rate = k[H⁺]^a, with specific a (empirical constant) and k (rate constant) parameters for each metal demonstrating that metal release in these systems can be explained by proton-induced desorption and dissolution reactions. With decreasing pH, pronounced increases in release rates were observed in the sequence cadmium > lead > copper, which express the order of metal lability in the soils. The flow cell system is useful for comparison of metal releases as a function of soil properties, and can be used as a test to rank soils with respect to heavy metal leaching.

Abbreviations: CEC, cation exchange capacity • DOC, dissolved organic carbon

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