Stabilization of arsenic and chromium polluted soils using water treatment residues

Chromated Copper Arsenate (CCA) is a mixture of arsenic, chromium and copper salts which have widely been used for impregnation of wood. World-wide many contaminated sites and brownfields exist, where wood impregnation with CCA has taken place, resulting in soil contamination and leaching of contaminants. Arsenic, chromium and copper cannot be degraded and existing methods for cleaning the soil are rarely used as they are expensive and technically demanding.
Chemical stabilization of polluted soil is an alternative method for soil remediation, especially metal contamination, and consists in adding an amendment to immobilize the contaminants. Cement is often used, because it, in addition to chemical stabilization, also improves the geotechnical strength as well as reducing hydraulic conductivity, but the stabilization can be purely chemical and done by amending the soil with iron containing substances or other sorbents. Iron water treatment residues mainly consist of ferrihydrite, an oxidized iron oxy-hydroxide with a high reactivity and a large specific surface area with a high capacity for adsorption. Iron water treatment residues (Fe-WTR) are a by-product from treatment of groundwater to drinking water and can be used as a soil amendment to decrease the mobility of CCA in contaminated soil. Stabilization with Fe-WTR was tested at the Collstrop site in Hillerød, Denmark. The site has been polluted with a wide range of wood impregnation agents including CCA during 40 years of wood impregnating activities at the site. Since activities ceased at the site more than 30 years ago it has been a brownfield with ongoing monitoring of arsenic contaminated groundwater. The first 1 m2 smallscale field experiment was amended with 2.5% Fe-WTR and monitored for 3 consecutive years, during which the amendment showed a remarkable effect on the porewater. Porewater concentrations of arsenic decreased by two orders of magnitude in the amended soil compared to an undisturbed soil profile. A full scale field experiment was then initiated, where mixing of soil and Fe-WTR was done with an excavator mounted with a rotary screening bucket. In two plots of 100 m2 soil was homogenized with the screening bucket to 1 meter below ground and one of those plots was simultaneously amended with Fe-WTR.
An unexpected high water content of the Fe-WTR made the amendment only 0.6% dry weight of the soil and subsequent analysis of Fe concentrations in the amended soil showed an uneven distribution of the amendment. Analysis of the porewater from June 2011 to July 2012 showed that arsenic, chromium and copper in porewater was reduced more than 90% in the part of the field that received the most Fe-WTR amendment, even at this low addition ratio. In a series of batch leaching tests, where polluted soil from the Collstrop site was amended with 5% dry weight Fe-WTR, the leaching of arsenic in strongly polluted soil was decreased by 98% and 91% for chromium compared to unamended soil. The concentration of pollutants in the leachate from an amended, slightly polluted soil (255 mg/kg As and 27 mg/kg Cr) did not at any time exceed 50 μg/L, which means that the soil can be reused for construction e.g. roads and baffle walls as described by the Danish Reuse Act. Ageing of ferrihydrite, the main constituent of Fe-WTR, is of concern as the retention of contaminants may decrease during its transformation to other iron phases. To study the transformation of ferrihydrite, permeable bags containing fresh Fe-WTR were buried at the field site for 4 years. Reactivity as a measure of the degree of transformation was determined by reduction in 10 mM ascorbate at pH 3. As transformation products are much less reactive, this method can be used to quantitatively determine the transformation and reduction rates which were found to be up to one order of magnitude lower in the aged Fe-WTR compared to fresh Fe-WTR. Oxalate-extractable iron decreased from 95% in fresh samples to 40-50% in the aged samples and transformation products characterized by XRD were primarily goethite. During burial Fe-WTR has scavenged the soil porewater for especially As and Cu, increasing contaminant fractions from trace amount to up to 9.2 mmolAs/molFe and 1.5 mmolCu/molFe. Contaminants were equally associated with the oxalate-extractable iron fraction and the remaining iron fractions, suggesting that sorption capacity does not decrease dramatically with transformation. Increased leaching of contaminants with time was not observed in field experiments as natural variability was too large to for this effect to be observed, but indications of a decrease of As retention was observed after 103 days in the controlled environment of the batch experiment. Increased porewater concentrations of arsenic were observed in the small-scale experiment during winter, where increased precipitation floods the soil and creates possible iron reducing conditions in the lower parts of the amended plot. In the field scale experiment measurements of the secondary groundwater table proved, that the soil was periodically flooded and iron, but not arsenic, concentrations increased during flooding in the unamended field.