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TECHNICAL REPORTS
Heavy Metals in the Environment

Cadmium Binding by Fractions of Dissolved Organic Matter and Humic Substances from Municipal Solid Waste Compost

^a Institute of Plant Nutrition (330), University of Hohenheim, 70593 Stuttgart, Germany
^b Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, P.O.B. 12, Rehovot 76100, Israel

* Corresponding author (yonachen{at}agri.huji.ac.il)

Received for publication July 24, 2001. The agricultural practice of amending soils with composted municipal solid waste (MSW) adds significant amounts of organic matter and trace metals, including Cd. Under these conditions, soluble organic complexes of Cd formed in the compost may be more significant than previously thought, due to Cd bioavailability and mobility in the soil environment. To study the relative importance of different types of organic ligands in MSW compost for the binding of Cd, six fractions of the dissolved organic matter (DOM) in addition to humic acid (HA) and fulvic acid (FA) were extracted and their complexation of Cd quantified at pH 7 using an ion-selective electrode (ISE). The highest complexing capacities (CC) for Cd were found for the most humified ligands: HA (2386 µmol Cd g^-1 C of ligand), predialyzed FA (2468 µmol Cd g^-1 C), and HoA, a fulvic-type, easily soluble fraction (1042 µmol Cd g^-1 C). The differences in CC for Cd of the various organic ligands were not directly related to total acid-titratable or carboxylic groups, indicating the importance of sterical issues and other functional groups. The strength of association between Cd and the organic ligands was characterized by calculating stability constants for binding at the strongest sites (pK_int) and modeling the distribution of binding site strengths. The pK_int values of the DOM fractions ranged between 6.93 (HiN: polysaccharides) and 8.11 (HiB: proteins and aminosugars), compared with 10.05 for HA and 7.98 for FA. Hence, the highly complex and only partially soluble organic molecules from compost such as HA and FA demonstrated the highest capacity to sequester Cd. However, strong Cd binding of organic ligands containing N-functional groups (HiB) in addition to a high CC of soluble, humified ligands like HoA indicated the relevance of these fractions for the organic complexation of Cd in solution.

Abbreviations: ATG, acid-titratable groups • BS, binding site • CC, complexing capacity (for Cd) • Cd_B, bound cadmium • Cd_F, free (ionic) cadmium • Cd_T, total cadmium • DOC, dissolved organic carbon • DOM, dissolved organic matter • FA, fulvic acid • FA > 1000, fulvic acid after dialysis with a molecular weight cutoff of 1000 daltons • HA, humic acid • HoA, HoN, HoB, HiA, HiN, and HiB, dissolved organic matter fractions • HS, humic substances • ISE, ion-selective electrode (for Cd) • L, ligand • L_C, ligand concentration expressed as dissolved organic carbon • MSW, municipal solid waste • OM, organic matter • pK, stability constant • pK_i, incremental stability constant • pK_int, intrinsic stability constant for binding at the strongest sites • , bound cadmium/ligand concentration (Cd_B/L_C) • ', bound cadmium/ligand concentration (Cd_B/L_C) values lying equidistant between neighboring data points on the axis of the Scatchard plot

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