Adsorption of Cadmium on Biosolids‐Amended Soils

Abstract

Debate exists over the biosolid phase (organic or inorganic) responsible for the reduction in phytoavailable Cd in soils amended with biosolids as compared with soils amended with inorganic salts. To test the importance of these two phases, adsorption isotherms were developed for soil samples (nine biosolids-amended soils and their five companion controls) and two biosolids samples from five experimental sites with documented histories of biosolids application. Subsamples were treated with 0.7 M NaClO to remove organic carbon. 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 levels (6.5, 5.5, and 4.5), and equilibrated at 22 +/- 1 degrees C for at least 48 h. Isotherms of Cd adsorption for biosolids-amended soil were intermediate to the control soil and biosolids. Decreasing pH did not remove the difference between these isotherms, although adsorption of Cd decreased with decreasing pH level. Organic matter removal reduced Cd adsorption on all soils but had little influence on the observed difference between biosolids-amended and control soils. Thus, increased adsorption associated with biosolids application was not limited to the organic matter addition from biosolids; rather, the biosolids application also altered the adsorptive properties of the SIF. The greater affinity of the inorganic fraction of biosolids-amended soils to adsorb Cd suggests that the increased retention of Cd on biosolids-amended soils is independent of the added organic matter and of a persistent nature.

The role of organic and inorganic fractions of cow manure and biosolids on lead sorption

“…easily reducible Fe and Mn) in Pb sorption is greater in the manure compared to the biosolids. This result is in agreement with the observations of Li et al (2001), Hettiarachchi et al (2003), Lombi et al (2002), and Scheckel and Ryan (2004) that showed the significant role of the inorganic fractions of organic amendments in Pb sorption. Additionally, a 0.8 unit decrease in pH of the manure after removing easily reducible Fe and Mn could explain reduction (28.1%) of Pb sorption of this amendment.…”

“…This is in agreement with the finding of Li et al (2001) that indicated removal of OC reduced sorption of Cd for biosolids-amended soil samples. However, in that research the increased Cd sorption attributed to biosolids application was still present compared to the control soil that indicates the role of the inorganic fraction of the biosolids.…”

“…As illustrated by these isotherms, addition of cow manure or biosolids increased the slope of the sorption isotherm. These observations were in agreement with the previous studies conducted with different biosolids amended soils (Li et al 2001;Hettiarachchi et al 2003). Additionally, cow manure and biosolids add extra adsorptive phases to soil systems and therefore alter adsorptive characteristics of soil.…”

“…Lead sorption capacity of cow manure and biosolids before and after removing OC, easily reducible Fe and Mn oxides phases and untreated sample was determined using the procedure used by Li et al (2001) for Cd.…”

“…However, it has been suggested that organic amendments including biosolids (sewage sludge), composts, and manures can immobilize lead (Pb), cadmium (Cd), and zinc (Zn) in contaminated soil. This result has been interpreted as indicating that organic matter adds sorption phases to soils, which reduce metal availability to plants (Li et al 2001).…”