Reactivity of fly ash from copper smelters in an Oxisol: implications for smelter-polluted soil systems in the tropics
Abstract
Soils near non-ferrous metal smelters are often highly polluted by metal(loid)s. Compared to temperate soils, tropical soils are particularly affected by this type of contamination, because they generally exhibit low organic matter contents and contain low-activity clays and less reactive Fe-oxides.
This study was performed to provide information on the reactivity of Cu-smelter fly ashes in a tropical soil and to assess contaminant release and subsequent distribution and mobility. Laboratory pot experiments using wetted Oxisol (representing an acidic soil typical for tropical areas) were performed with polyamide experimental bags with Cu-smelter fly ashes inserted in the topmost part of the soil column.
We monitored the loss in fly ash mass and the soil pore-water chemistry during fly ash incubation in soil over 504 h of the experiment. In addition, the distribution and availability of the contaminants (As, Co, Cu, Pb, Zn) in the soil after the fly ash incubation were evaluated using bulk chemical analyses and ethylenediaminetetraacetic acid (EDTA) extractions.
Smelter fly ashes were highly reactive in the Oxisol (mass loss in the range 8-36 %). Whereas Pb and As were released from the fly ash samples in lower amounts and were efficiently retained in soils, other contaminants (Co, Cu, Zn) were released to a greater extent and were found to be highly mobile, partly due to the formation of uncharged sulphate complexes, which are not bound to the surfaces of kaolinite and Fe-oxides.
Especially Cu occurred in the EDTA-extractable fraction and was found to move downward in the soil column. The smelter fly ash incubation experiments indicated that the assumption of the vulnerability of tropical soils with respect to smelter-related contaminants is justified, especially under low pH conditions, where retention mechanisms for metals are limited.