Effect of natural dissolved organic carbon on phosphate removal by ferric chloride and aluminum sulfate treatment of wetland waters

The use of wetlands for the removal of excess N and P has become widespread. Some sensitive P‐limited ecosystems, however, may require additional reductions in the concentration of P entering the system. It has been proposed that the treatment of wetlands through addition of ferric chloride or alumi...

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Bibliographic Details
Published in:Water resources research 2009-09, Vol.45 (9), p.n/a
Main Authors: Qualls, Robert G, Sherwood, Lindsay J, Richardson, Curtis J
Format: Article
Language:English
Subjects:
aluminum sulfate
citric acid
colloids
dissolved organic carbon
ecosystems
ferric chloride
phosphate removal
phosphates
phosphorus
provenance
stoichiometry
water treatment
wetlands
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Summary:The use of wetlands for the removal of excess N and P has become widespread. Some sensitive P‐limited ecosystems, however, may require additional reductions in the concentration of P entering the system. It has been proposed that the treatment of wetlands through addition of ferric chloride or aluminum sulfate can augment the natural P removal mechanisms. However, high concentrations of natural dissolved organic matter may interfere with the removal of P by metal addition. We evaluated the doses of ferric chloride and aluminum sulfate necessary to reduce total P concentrations below 0.32 μM (10 μg/L) in water from the Northern Everglades, and we determined the effect of various concentrations (21, 38, and 60 mg/L) of natural dissolved organic carbon (DOC) on the removal of PO4 and total P. High concentrations of natural DOC inhibited both the short‐term removal of PO4 and the longer‐term removal of total P from the water column. Similar results were observed using 15 μM citric acid in an experiment to determine whether citric acid could effectively mimic the inhibition of phosphorus removal associated with natural DOC. Stoichiometry of these experiments indicates that the mechanism of natural DOC interference was not complexation of the metal ions by the DOC; we hypothesize that it could be adsorption to the terminal hydroxyl groups on a polynuclear Fe or Al colloid, effectively blocking the adsorption sites from a phosphate molecule. Also, the ability of citric acid to mimic the inhibitory effects also suggests that the results of the study are broadly applicable to wetland and other waters with high natural organic acid concentrations.
ISSN:0043-1397
1944-7973
DOI:10.1029/2008WR007287