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Retention of chromated copper arsenate (CCA), a wood preservative, in soil

University of British Columbia

Chromated copper arsenate (CCA) is a biocidal chemical used by the wood preservation industry to extend the service life of wood products. CCA is currently the most commonly used wood preservative in British Columbia. Environmental concerns regarding the fate of CCA solutions accidentally released or chronically spilled to unpaved ground at wood preservation facilities has led to the present investigation on the retention and release of CCA constituents in soils. Batch and column tests were used to evaluate the adsorption and desorption of arsenic, chromium and copper in several B.C. soils. Batch tests were carried out using varying concentrations of CCA and individual arsenic, chromium and copper solutions. Migration of arsenic, chromium and copper in a soil column following application of a single slug dose of 2% CCA solution was investigated. Continuous leach column experiments were also conducted to evaluate adsorption of CCA constituents in soils. The column soils were sectioned and subjected to a sequential extraction procedure following influent CCA solution breakthrough to determine the partitioning, mobility and availability of As, Cr and Cu. Results from the adsorption experiments were used to identify important soil characteristics influencing the attenuation of CCA in the subsurface environment. Adsorption isotherm data was found to be well described by the Freundlich equation. The Fe and Al amorphous and crystalline hydrous oxide component of the soils studied was highly correlated with arsenic adsorption while the percent organic carbon was significantly correlated with chromium and copper retention in the soils. The clay content of the soils was poorly correlated with retention of CCA constituents in the soils studied. Desorption of arsenic, chromium and copper from soils previously equilibrated with CCA solutions showed that the extent of metal release is dependent on the amount of metal retained. At high initial CCA solution concentrations chromium was desorbed to a greater extent than arsenic or copper. This result suggests the following order for mobility of CCA constituents in the soils: chromium > copper > arsenic. Chromium, and to a lesser extent copper, adsorption was lower in CCA equilibrated batch tests than in single solute batch tests, particularly at high initial metal solution concentrations. Arsenic adsorption was not affected by the presence of codissolved solutes in CCA solution. The migration of arsenic, chromium and copper in a soil column following application of a single dose of 2% CCA revealed that chromium was the most mobile of the CCA constituents. Breakthrough curve plots showed that arsenic was the most strongly retained CCA constituent in both soils tested. Copper was also strongly retained in the soil columns however complete breakthrough was achieved indicating that the soils had reached a finite capacity for copper retention. Chromium was again the most weakly held CCA constituent in the continuous leach column test. Partitioning of the metals in the column soils showed that arsenic, chromium and copper were largely associated with the ammonium oxalate extractable fraction. Only copper was found to have a significant amount of retained metal in the exchangeable pool. The results suggest that environmental monitoring at CCA wood preservation facilities should focus on the potential for chromium migration in the subsurface.

Text

2010-08-16

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http://hdl.handle.net/2429/27455

Soil Science

University of British Columbia

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