Natural Ferrihydrite as an Agent for Reducing Turbidity Caused by Suspended Clays

Authors: Rhoton, Fred; BIRHAM, JERRY - OHIO STATE UNIVERSITY

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2009
Publication Date: 7/1/2009
Citation: Rhoton, F.E., Bigham, J.M. 2009. Natural Ferrihydrite as an Agent for Reducing Turbidity Caused by Suspended Clays. Journal of Environmental Quality. 38(5): 1887-1891.

Interpretive Summary: Sediment concentrations of 100 mg L-1 is considered the breakpoint between unimpaired and impaired aquatic ecosystems. Most of the water bodies in the Mississippi Delta exceed this concentration due primarily to high concentrations of suspended fine clays. Since these fine particles also carry sorbed chemical contaminants, we evaluated the effectiveness of a naturally occurring ferrihydrite mineral at flocculating the suspended clays obtained from representative soils clays in the region, because this mineral is an efficient scavenger of phosphorus, arsenic, mercury, and lead. Ferrihydrite effectiveness was based its ability to lower turbidity measurements at arrange of pH conditions. Our data indicate that ferrihydrite concentrations as low as 10 mg L-1 added to pH 5 suspensions significantly reduced turbidity levels. These results show ferrihydrite obtained as a byproduct from water treatment plants will improve water clarity to the extent that the health of aquatic ecosystems will be improved both from a physical and chemical point of view.

Technical Abstract: The turbidity of water can be reduced by the addition of positively charged compounds which coagulate negatively charged clay particles in suspension causing them to flocculate. This research was conducted to determine the effectiveness of the Fe oxide mineral ferrihydrite as a flocculating agent for suspended clays similar to those found in high turbidity waters of the Mississippi delta. Clay concentrations of 100 mg L-1 from the the Dubbs silt loam (fine silty, mixed, active, thermic Typic Hapludalfs), Forestdale silty clay loam (fine, smectitic, thermic Typic Hapludalfs), and Sharkey clay (very fine, smectitic, thermic Chromic Epiaquerts) were suspended in 0.001 N CaCl2 solutions at a pH of 5, 6, 7, or 8. Ferrihydrite was mixed with the suspension at concentrations of 0, 10, 25, and 50 mg L-1. After settling periods of 24 and 48 h, % transmittance (T) was measured at a wavelength of 420 nm using a 3 ml sample collected at a depth of 2 cm. The greatest reductions in turbidity after 24 h equilibration were recorded for the pH 5 suspensions of the Dubbs (31%) and Forestdale (37%) clays at a ferrihydrite concentration of 10 mg L-1, relative to the 0 ferrihydrite treatment. These results indicate that the effectiveness of ferrihydrite, as a means of reducing turbidity associated with soil clays, is greatest at pH values below its zero point of charge (5.77) where this mineral develops a net positive charge.