Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/1998
Publication Date: N/A
Interpretive Summary: Copper is added to swine feed to reduce disease and increase weight gain, hence, relatively high levels of copper are found in swine waste. High copper levels are also found in sewage, municipal composts, and industrial wastes. We are concerned about the possible buildup of copper to toxic levels in soils receiving annual applications of copper rich waste over a period of many years. We found that copper is tightly held by both organic matter and minerals in soils. We also discovered that copper is strongly held by a relatively small fraction of the soil organic matter and that copper tends not to come back off soil once it has been adsorbed. This study provides basic information needed by scientists and environmentalists to assess the long term sustainability of repeated applications of copper rich wastes to agricultural fields. This study will help design better waste management systems benefitting both farmers on whose land the waste is applied and the general public by reducing the chances of copper damage on crops.
Technical Abstract: The objective of this study was to quantify sorption and desorption of Cu on various clay and clay-organic matter fractions separated from a Zook soil located in central Iowa. The clay was separated into four particle size fractions, and portions of each fraction were treated with H2O2 and/or dithionite-citrate-bicarbonate (DCB) for removal of organic matter and free-iron compounds, respectively. A batch-equilibration technique was employed to evaluate sorption of Cu at pH 6.0 in the presence of 0.01M CaCl2. The sorption isotherms show that Cu was preferentially sorbed on organic matter associated with the coarse clay fraction of the soil. After removal of organic matter, the fine clay (less that 0.02 um) exhibited higher Cu retention than did the coarse (0.2-2.0 um) and medium clay (0.02-0.2 um). The DCB treatment created or exposed high-affinity sites for Cu on the fine clay, but had little effect on Cu sorption for the coarse clay. Copper appears to be specifically sorbed on the surfaces of silicate clays in excess of that which can be accounted for by variable charge sites. The results suggest that Fe oxides may coat the surfaces of layer silicates, blocking access of Cu to potential sorption sites. Substantial sorption-desorption hysteresis was observed for all samples, suggesting that Cu forms high-energy bonds with both organic matter and layer-silicate surfaces.