Impacts of particulate organic carbon and dissolved organic carbon on removal of polycyclic aromatic hydrocarbons, organochlorine pesticides, and nonylphenols in a wetland
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Background, aim, and scope
The potential of wetlands for controlling point- and nonpoint-source pollution in surface water has attracted increasing interest. The partitioning process of organic contaminants between water, particulate organic carbon (POC) and dissolved organic carbon (DOC), impacts their behaviors in the aquatic environments. Meantime, the partitioning process of organic contaminants is closely related to their physicochemical properties, such as hydrophobicity (or Kow), and their fates in wetlands may vary greatly depending on physicochemical properties. The aim of this study was to examine fates and removals of polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs) and nonylphenols (NPs) in a wetland in Beijing, China, and provide useful information for ecological remediation.
Materials and methods
Water samples, collected at five sites from inlet to outlet of the wetland once a month in summer 2006, were immediately filtered within 2 days through 0.45-μm glass fiber prefilters and enriched by solid-phase extraction. The filtered particulates were collected as the total suspended particulates (TSPs), freeze-dried, and Soxhlet-extracted. After extraction, samples were purified following a clean-up procedure and analyzed by GC-MS.
TSPs could be removed efficiently with a removal rate of 97.4%, and DOC could be moderately removed with a removal rate of 44.7% from inlet to outlet. The total removals of target contaminants varied widely from null to 82.0%. A good correlation between logKow and logKoc (organic-carbon-normalized suspended-particulate partition coefficient) was observed (r2 = 0.84 for PAHs and r2 = 0.86 for OCPs, p < 0.01). Ratios of the POC-bound fraction of target contaminants (or DOC-bound fraction) to the freely dissolved fraction increased with their Kow values. The removal of the POC fraction contributed more than 50% to the total removal for the contaminants with logKow > 5.0. Only a small portion of the removal was attributed to the removal of the freely dissolved fraction.
Hydrophobic compounds such as PAHs and OCPs with higher Kow values would show stronger POC or DOC preference. Their removal depended greatly on their Kow values and the removal of total suspended particulates. On the other hand, concentrations of NPs decreased little in the wetland, probably due to their production through degradation of their precursors and relatively low hydrophobicity.
POC and DOC play essential roles on the fates and removals of hydrophobic organic contaminants in the wetland. The removal of target contaminants with a high Kow should be mainly through association with the suspended particulates which were precipitated and retained in the wetland. The fates of the organic contaminants in the wetland greatly depended on their hydrophobicities.
Recommendations and perspectives
Further work should be done to study the influence of hydraulic retention time and some other environmental factors, e.g., temperature, on removals and fates of organic contaminants. Behavior of NPs and their precursors in the wetland should also be investigated more thoroughly.
KeywordsDissolved organic carbon Fate Organic contaminants Particulate organic carbon Wetland
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