Phosphate adsorption by ferrihydrite-amended soils

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. The objectives of this research were to characterize ferrihydrite (Fe5HO8.4H2O) in terms of its ability to adsorb P from soil solutions and relate its P adsorptive capacity to several soil properties that influence P mobility. A naturally occurring ferrihydrite, collected as an Fe oxide sludge by-product from a water treatment facility, was equilibrated with soil samples at equivalent 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. After 24 h, P in solution was measured by colorimetric methods, and designated as final P concentrations. The data indicated that the unamended soils with a pH of 7.0. The final P concentrations, averaged for all initial P concentrations and ferribydrite rates, ranged from 0.09 to 4.63 mg kg(-1), and were most highly correlated with pH (r = 0.844; P less than or equal to 0.01), oxalate-extractable Fe (r = -0.699; P less than or equal to 0.10), and dithionite-extractable Fe (r = -0.639; P less than or equal to 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 less than or equal to 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. The results indicate that P adsorption, in soils amended with ferrihydrite, will be greatest under acid pH conditions below the ferrihydrite zero point of charge (pH 5.77), and low incipient Fe oxide contents.