Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2004
Publication Date: 6/1/2005
Citation: Rhoton, F.E., Bigham, J.M. 2005. Phosphate adsorption by ferrihydrite amended soils. Journal of Environmental Quality. 34(3)891-896(2005).
Interpretive Summary: Many soil resources in the United States are subject to excessive runoff and erosion losses. The transport of phosphorus by runoff from such upland areas in either sorbed or dissolved forms creates eutrophication-related problems in offsite aquatic environments. Ferrihydrite is a highly reactive iron oxide mineral that can be prepared synthetically for purposes of adsorbing phosphorus, however, the cost of such material is prohibitive on a large scale basis. Instead, we identified a natural source of ferrihydrite, as a byproduct of a water treatment process, which was applied to agricultural soils to assess its ability to adsorb phosphorus in soil solutions. The data indicated that this naturally occurring ferrihydrite adsorbed substantial quantities of phosphorus from soil solutions, and that it was most effective under acid soil conditions (below pH 7.0). These results suggest that ferrihydrite added to soils at rates exceeding 1.5 tons/acre will effectively reduce phosphorus losses in runoff.
Technical Abstract: New technology and approaches for reducing P in runoff from high sediment yield areas are essential due to implementation of increasingly rigorous water quality standards. This research was conducted to determine the ability of ferrihydrite (Fe5 HO8 ' 4 H2O) to adsorb P from soil solutions using agricultural soils with a range of particle size, pH, organic carbon, and Fe oxide contents. A naturally occurring ferrihydrite, collected as an Fe oxide sludge byproduct from a water treatment facility was equilibrated with these soils at rates of 0, 0.34, 3.36, 16.80, and 33.60 Mg ha-1 for a 60 d period. Individual 2 g subsamples of each soil were then equilibrated with 0, 5, 10, 20, and 40 mg kg-1 P in 20 ml of 0.01 M CaCl2 on a reciprocating shaker for 24 h. Phosphorus in solution after 24 h was measured by colorimetric methods, and designated as final P concentrations. The data indicated that the unamended soils with a pH < 6.0 adsorbed, in some cases, 50 times more P than soils with a pH > 7.0. The final P concentrations, averaged for all initial P concentrations and ferrihydrite rates ranged from 0.09 to 4.63 mg kg-1, were most highly correlated with pH (r = 0.844; P < 0.01), Fe0 (r = - 0.699; P < 0.10), and Fed (r = - 0.639; P < 0.10) contents of the unamended soils. In terms of individual soils, correlation coefficients (r) for final P concentrations versus ferrihydrite amendment rates indicated a statistically significant (P < 0.001) negative relationship at all initial P concentrations for most A horizons. The r values for the high Fe oxide content B horizon soils did not show a statistically significant response to ferrihydrite additions. These results indicate that effectiveness of this ferrihydrite material, in terms of adsorbing P, is dependent on incipient Fe oxide contents and soil pH due to its dependent charge which is negative above pH 5.77.