Adsorption of cadmium on biosolids-amended soils

Debate exists over the biosolid phase (organic or inorganic) responsible fo r the reduction in phytoavailable Cd in soils amended dth biosolids as comp ared with soils amended with inorganic salts. To test the importance of the se two phases, adsorption isotherms were developed for soil samples (nine b iosolids-amended soils and their five companion controls) and two biosolids samples from five experimental sites with documented histories of biosolid s application. Subsamples were treated with 0.7 M NaClO to remove organic c arbon. Cadmium nitrate was added to both moist soil samples and their soil inorganic fractions (SIF) in a 0.01 M Ca(NO3)(2) solution at three pH level s (6.5, 5.5, and 4.5), and equilibrated at 22 +/- 1 degreesC for at least 4 8 h. Isotherms of Cd adsorption for biosolids amended soil were intermediat e to the control soil and biosolids. Decreasing pH did not remove the diffe rence between these isotherms, although adsorption of Cd decreased with dec reasing pH level. Organic matter removal reduced Cd adsorption on all soils but had little influence on the observed difference between biosolids-amen ded and control soils. Thus, increased adsorption associated,vith biosolids application was not limited to the organic matter addition from biosolids; rather, the biosolids application also altered the adsorptive properties o f the SIF. The greater affinity of the inorganic fraction of biosolids-amen ded soils to adsorb Cd suggests that the increased retention of Cd on bioso lids-amended soils is independent of the added organic matter and of a pers istent nature.