Sorption of acidic organic contaminants to carbonaceous sorbents – mechanisms and prediction
O 15.14 in Forum of Young Hydrogeologists
13.04.2016, 16:15-16:30, Plank Hörsaal, Geb. 40.32
The remediation of low concentration diffuse contaminated sites can be cost intense and is therefore often not feasible, but may nevertheless pose risks to both soil and groundwater resources. Recently the potential use of sustainable low cost carbonaceous sorbents to immobilize neutral organic contaminants and heavy metals showed promising results, and has therefore received increasing attention in the scientific community . In addition, biochar, a carbonaceous sorbent, is used in agriculture to increase soil pH and water holding capacity. As a side effect, agricultural biochar implementation may prevent the leaching of agrochemicals to the groundwater and is thought to have a positive CO2 footprint.
Many contaminants of concern, however, are organic acids that dissociate under environmental conditions, and may thus exist as neutral and/or anionic species. The sorption of dissociated organic acids to carbonaceous sorbents is typically lower than that of neutral species, but is driven by complex processes that are not yet fully understood. Approaches to predict sorption developed for neutral compounds and based on Kow, are unlikely to be suitable for organic acids, due to the effects of dissociation on sorption. Previous studies on the sorption of organic acids to soils have demonstrated that Dow, which describes the decrease in hydrophobicity of acids as a result of dissociation, is a useful alternative to Kow .
The aim of the present study was to test the applicability of a Dow based concept to describe the sorption of acidic organic contaminants to carbonaceous sorbents. Batch experiments were performed with a series of 15 materials including carbon nanotubes, activated carbon, and a variety of biochars and four acidic compounds commonly used for pesticidal and biocidal purposes (i.e., 2,4-D, MCPA, 2,4-DB, and triclosan). The wide range of properties considered allows (i) discussing the mechanisms driving the sorption of neutral and anionic species to carbonaceous sorbents, and (ii) their dependency on sorbate and sorbent properties.
Results showed that the sorption of the four acids was influenced by factors that are usually not considered for neutral compounds. Dissociation affected the sorption of the four compounds, and sorption of the anions ranged over five orders of magnitude, thus substantially contributing to sorption in some cases. For prediction purposes, most of the variation in sorption (>80%) could be well described with a two-parameter regression equation including Dow and specific surface area. The proposed model may serve as a base to estimate the environmental fate of organic acids in the presence of carbonaceous sorbents.
 M. Ahmad, A. U. Rajapaksha, J. E. Lim, M. Zhang, N. Bolan, D. Mohan, M. Vithanage, S. S. Lee, and Y. S. Ok, “Biochar as a sorbent for contaminant management in soil and water: a review.,” Chemosphere, vol. 99, pp. 19–33, 2014.
 M. Kah and C. D. Brown, “LogD: lipophilicity for ionisable compounds,” Chemosphere, vol. 72, pp. 1401–1408, 2008.
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