Lanthanum-modified bentonite (LMB) is increasingly applied to eutrophic lakes to reduce orthophosphate (PO4) concentrations in the water column and mitigate PO4 release from sediments. In environmental waters, dissolved organic matter (DOM), such as humic and fulvic acids, has been shown to interfere with PO4-binding to LMB and exacerbate lanthanum (La)-release from bentonite. This evidence served as motivation for this study to systematically determine the effects of DOM, solution pH, and bentonite as a La carrier on PO4 removal. We conducted both geochemical modeling and controlled-laboratory batch kinetic experiments to understand the pH-dependent impacts (pH range 6-8.5) of DOM (Suwannee River fulvic acid, and Suwannee River natural organic matter, Leonardite coal humic acid, Elliot soil humic acid; 0-28 mg C L-1) on PO4-binding to LMB and La release from LMB. The role of bentonite was studied by comparing PO4 removal obtained by LMB and La3+ (added as LaCl3 salt to represent the La-containing component of LMB). Our results from both geochemical modeling and batch experiments indicate that, in the absence of DOM, PO4 removal is essentially complete when La and PO4 are present at equimolar levels and that aqueous PO4 is primarily removed via precipitation of LaPO4(s). LMB’s PO4-binding ability decreased in the presence of DOM, and the decrease was more pronounced at pH 8.5 than at pH 6. At the highest evaluated DOM concentration (28 mg C L-1), PO4-removal by La3+ was substantially lower than that by LMB, implying that bentonite clay in LMB shielded La from interactions with NOM, while still allowing PO4 capture by La. Finally, the presence of NOM promoted La-release from LMB, and the amount of La released depended on solution pH and both the type (i.e., fulvic/humic acid ratio) and concentration of NOM. Overall, these results provide an important basis for management of P in lakes and eutrophication control that relies on LMB applications.

Effects of dissolved organic matter and pH on orthophosphate removal by lanthanum-modified bentonite

ACS Fall 2021

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