Degradation of surface-active compounds in a constructed wetland determined using high performance liquid chromatography and extraction spectrophotometry

Abstract

Degradation of anionic and nonionic surfactants in a constructed wetland with horizontal subsurface flow was studied using high performance liquid chromatography and extraction spectrophotometry. The ratio of individual homologues of linear alkylbenzene sulfonates (LAS) and the efficiency of their removal were studied. Tridecyl-, dodecyl-, undecyl-, and decylbenzene sulfonates were removed with efficiencies of 92.9%, 84.3%, 64.7%, and 41.1%, respectively. These differences are due to sequential shortening of the alkyl chain in homologues during degradation (the higher homologue can provide the lower one). The formation of sulfophenyl carboxylic acids during ω-oxidation of the alkyl chain followed by successive α- and/or β-oxidation is also a possible mechanism for removal of LAS. Solid phase extraction using Chromabond® HR-P columns was used for preconcentration of the analytes prior to their determination by HPLC. Methylene blue active compounds were determined using extraction spectrophotometry. The average efficiency of their removal was 84.9% in this case. The efficiency of nonionic surfactant removal (98.2%) was significantly higher in comparison to that for anionic surfactants. The concentration of the endocrine disruptor nonylphenol (a product of nonylphenol polyethoxylate surfactant degradation) determined in the profile of the wetland was beneath the limit of detection (0.4 μg/L). The average outflow concentrations of anionic and nonionic surfactants determined by spectrophotometry were 0.54 and 0.021 mg/L, respectively. The average outflow concentrations of decyl- and tridecylbenzene sulfonates determined by HPLC were 0.195 and 0.015 mg/L. Efficiencies of 86.4% and 92.2% were obtained for removal of organic compounds as indicated by chemical and biochemical oxygen demand (COD_Cr and BOD₅). These results demonstrate the suitability of the constructed wetland for degrading surface-active compounds.

Keywords:

• Municipal wastewater
• organic pollutants
• anionic and nonionic surfactants
• seasonal monitoring
• reed bed
• ion pair chromatography
• environmental analytical chemistry

Acknowledgments

This work was supported by Ministry of Education, Youth, and Sports of the Czech Republic, research project MSM 6007665801 and by CzechGlobe – Centre for Global Climate Change Impacts Studies, Reg. No. CZ.1.05/1.1.00/ 02.0073. The authors are also much obliged to Jitka Šímová for many helpful comments and her significant assistance in COD_Cr and BOD₅ determination and English Editorial Services Ltd for editing and improvement of English.