Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/15/2003
Publication Date: 10/15/2003
Citation: Hettiarachchi, G.M., Ryan, J.A., Chaney, R.L., Lafleur, C.M. 2003. Sorption and desorption of cadmium by different fractions of biosolids-amended soils. Journal of Environmental Quality. 32(5):1684-1693. Interpretive Summary: Plant uptake of Cd from biosolids-amended soils could increase food-chain transfer of Cd, hence much research has been conducted to characterize whether plant Cd will be increased when biosolids are applied to cropland. Examination of soils from long-term field plots with high Fe biosolids products suggested that the ability of biosolids-applied Fe to adsorb or occlude Cd might be the reaction which controlled Cd phytoavailability. Thus Cd adsorption and desorption tests were conducted on soils collected from plots established 20 years ago. To prevent soil pH shifts from affecting relative adsorption results, all soil suspensions were adjusted to pH 5.5 during sorption tests. To characterize the relative role of organic matter vs. hydrous oxides of Fe and Mn, organic matter was removed by heating the soil with hypochlorite, and Mn or Fe+Mn were removed by selective extraction reagents. These tests showed that although soil organic matter can adsorb Cd, that the more significant difference between control and biosolids-amended soil was the increase in Fe and Mn oxides. And such oxides are important because they can persist long after the added organic matter is biodegraded. Examination of desorption of soil Cd supported the same model, that hydrous Fe oxides were holding Cd in a form much more difficult to remove by exchange with 0.01 M Ca2+. These findings strongly support the concept that adding Fe during sewage treatment can significantly increase Cd adsorption by biosolids-amended soils and hence reduce the potential for food-chain transfer of the applied Cd.
Technical Abstract: To test the hypothesis that not only the organic but also the inorganic fraction in biosolids affects metal phytoavailability in biosolids amended soil systems, a series of Cd sorption and desorption experiments were conducted on different fractions of soils which had previously been amended with biosolids, Cd-salt, or remained unamended (control). Soils were collected from a long-term field experiment established in Beltsville, MD in 1976. The sorption and desorption behavior were studied at pH 5.5 by batch experiments in 0.01 M Ca(NO3)2. Cadmium sorption increased with increasing biosolids rate and the increase was different for the different biosolids. Removal of organic carbon (OC) using hypochlorite reduced Cd adsorption but did not account for the observed differences between biosolid-amended soils and control. After removal of both OC and Fe/Mn, the slopes of Cd sorption isotherms observed in the biosolids-amended and control soils were no longer different. Desorption experiments showed that substantial proportion of Cd sorbed by soils cannot be readily desorbed. This apparent hysteresis effect was higher for biosolids amended soils than the control soil. In the case of control soils, 37.1% of the sorbed Cd was recovered during desorption whereas only 15.8 and 10.5% were recovered from two different biosolids-amended soils. As observed for sorption removal of both organic and Fe/Mn fractions were more effective in removing the observed differences between the biosolids-amended and control soils than either of them alone. Results show that Cd added to biosolids-amended soils behaves differently than Cd added to soils without biosolids and support strongly the hypothesis that the addition of Fe and Mn in the biosolids increased the retention of Cd in biosolids-amended soils.