Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/1/2004
Publication Date: 1/10/2005
Citation: Basta, N.T., Ryan, J.R., Chaney, R.L. 2005. Heavy metal and trace element chemistry in residual-treated soil: a review of impacts on metal bioavailability and sustainable land application. Journal of Environmental Quality. 34(1):49-63. Interpretive Summary: This paper is a review of the present understanding of the fate and potential transfers of trace elements present in biosolids, composts, manure, and by-products when used on agricultural land. Research has been conducted intensively on these issues for over 30 years and progress in understanding has been significantly advanced. Industrial pretreatment of industrial wastes, separation of compostable materials for processing, and evaluation of a wide range of by-products has identified those which research has indicated can be used on cropland without affecting adversely the sustainable use of that land. A multi-pathway risk assessment for elements in biosolids has been used to characterize whether 1000 t/ha of any material would comprise risk to humans, livestock, crops, and the general environment in order to develop limits in the US. Research has characterized that most elements are so insoluble in soils or roots that they do not move into crops in significant amounts even when present in soils. Indeed, many comprise no risk to animals which eat soil, including children. Others may become phytoavailable at acidic soil pH and cause phytotoxicity to sensitive crops, but for nearly all of these, injured crops do not comprise risk to livestock which consumed the crop as 100% of diet. When applied in excess to alkaline soils, Mo and Se are accumulated by plants and can cause toxicity in grazing livestock. Thus Mo and Se are limited to protect against this risk. Cd can be accumulated by crops, but Zn is usually about 100-times higher concentration than Cd, and limits Cd uptake except in rice where flooded soil management allows Cd to move to rice grain in amounts which can harm humans in their lifetime. Because subsistence rice diets cause Fe and Zn deficiency of consumers and these deficiencies promote Cd absorption, and because Zn is not increased in rice grain even when soils are highly Zn contaminated, these consumers are susceptible to Cd in land applied residuals. Under regulated conditions with pretreatment, residuals have not caused Cd risks. Another aspect of land-applied residuals has been found to limit transfer and risk of trace elements, the inorganic adsorption surfaces of Fe and Mn oxides, phosphates, etc., in the residual. Although organic matter may be biodegraded over time, elements applied in residuals remain strongly adsorbed on these inorganic adsorption sites persistently, limiting plant uptake and bioavailability of elements in ingested soils. In particular Fe and Mn oxides and phosphates can improve soil fertility and metal binding by soils and increase the utility of residuals for remediation of industrially contaminated metal rich soils. Ecosystem risks are also summarized indicating that soil organisms and food-chains are protected under normal agricultural management of regulated quality biosolids, composts and many other residuals.
Technical Abstract: Heavy metal solubility and availability in land-applied residuals is governed by fundamental chemical reactions between metal constituents, soil, and residual components. Iron, Al, and Mn oxides, organic matter, and phosphates, carbonates, and sulfides are important sinks for heavy metal in soil-residual systems. The pH of the soil-residual system is often the most important chemical property governing heavy metal sorption, precipitation, solubility, and availability. Heavy metal phytoavailability in residual-treated soils is often estimated using soil extraction methods. However, spectroscopic studies show sequential extraction methods may not be accurate in perturbed soil-residual systems. Plant bioassay is the best method to measure the effect of residuals on phytoavailability. Key concepts governing phytoavailability are (1) the salt effect, (2) the plateau effect, and (3) the soil-plant barrier. Metal availability is greater from metal-salt-spiked soil than from metal residual-treated soil. Plant metal content plateaus at high residual loadings corresponding to the residual's sorption capacity. The soil-plant barrier limits transmission of many heavy metals through the food chain. Cadmium, an important human health concern, pass through the soil-plant barrier. After decades of research to clarify potential Cd risk to humans from land-applied residuals, it has become clear that only one population group has experienced Cd disease from food, the subsistence rice consumers who ate home-grown rice produced on contaminated paddy soils. Research is needed to (1) determine heavy metal retention of residual-soil systems, (2) determine the effect of residuals on ecological receptors and the ability of residuals to reduce ecotoxicity in metal-contaminated soil, and (3) predict the long-term bioavailability of heavy metals in residual-soil systems.