Development and evaluation of a kinetic model to describe phosphate sorption by hydrous ferric oxide gel |
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| Authors: | JR McLaughlin JC Ryden JK Syers |
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| Institution: | Department of Soil Science, Massey University, Palmerston North New Zealand |
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| Abstract: | Isotherms for the sorption of inorganic phosphate (P) by hydrous ferric oxide gel (Fe gel) were described by a three-equation Langmuir sorption model. Each equation described sorption within a distinct concentration range or region (I, II, and III) of the overall isotherm. Regions I and II involved chemisorption, whereas region III involved a more physical sorption type. With increasing sorption time between 0.7 and 28.7 days, the extent of sorption in region I increased by more than 30%. In contrast, the extent of sorption in regions II and III remained essentially constant. An equation was developed, based on the change in the sorption maximum of region I (bI) with increasing sorption time, which described the change in solution P concentration with time. The increase in bI with time, evaluated by the closeness of fit of this relationship to experimental data, was found to depend on two factors: first, the extent to which P was chemisorbed, and this was affected by pH and ionic strength; second, the batch of Fe gel used. For two different levels of P addition, the proportion of sorbed P which remained extractable in 0.1M NaOH, decreased with increasing sorption time. After 30 days only 88% of the sorbed P remained NaOH-extractable. The data obtained indicated that the increasing chemisorption of P with increasing sorption time involves the diffusion of sorbed P into the bulk of the Fe gel particles. This concept is discussed in relation to mechanisms proposed by previous workers to explain the time-dependence of P sorption. |
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