SULFATE RETENTION BY SPDOSOLS IN THE PRESENCE OF ORGANIC-LIGANDS

The aim of this work was to study the influence of low molecular weigh t organic acid concentration (0 to 1600 mumol C L-1) and type (oxalic, citric (CI), salicylic) and of solution pH (3.2 to 4.7) on the kineti cs of SO4 sorption by the B horizon of two Spodosols (HER and CON) fro m the Lower Laurentians, Quebec. Using a miscible displacement techniq ue, the soil materials were leached at a flow rate of 0.2 ml min-1 for 6 h at 25-degrees-C. The slope parameter (1/beta) of the Elovich equa tion was used as an apparent SO4 sorption rate. The presence of organi c ligands in solution reduced SO4 sorption by the HER B and the CON B horizons from 543 to 365 mumol SO4 kg-1 and from 1053 to 642 mumol SO4 kg-1, respectively. For both soils, 1/beta values were always higher (97 to 51 mumol kg-1 ln min-1 (HER) and 256 to 135 mumol kg-1 ln min-1 (CON)) in systems that were free of organic acids; however, the relat ionships between the properties of the acids and their relative SO4 so rption inhibition capacity was not clear. The extent of SO4 sorption i nhibition increased considerably as CI concentration was augmented fro m 100 to 1600 mumol C L-1. The efficiency of CI in reducing SO4 sorpti on by the HER B horizon was pH dependent, with SO4 sorption during the experiment decreasing by 23% at pH 3.2 and by 355% (desorption) at pH 4.7. The presence of CI led to the desorption of native exchangeable SO4 when the pH was above 4.2 or when the total C concentration was hi gh (1600 mumol C L-1). The data thus reveal that temporal changes in C cycling can affect SO4 mobility in acidic forest soils with potential implications on the transport of nutrient cations if the total anioni c equivalent charge of the soil solution is increased.