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Geochemistry International, 2013, Volume 51, Issue 4, pp 290-305
A. Yu. Kudeyarova
19414. Institute of Physicochemical and Biological Problems of Soil Science, Russian Academy of Sciences, Institutskaya ul. 2, Pushchino, Moscow oblast, 142290, Russia
Abstract
It is widely believed that the phosphate anions bound by aluminum and iron compounds (sorbents) of acid soils lose their migration capability. Such an opinion is in contradiction with the increasing influx of phosphorus compounds (from fertilized soils) into basins. This fact suggests the study of properties of metal-phosphate complexes forming at different phosphate loads on soil sorbents. Among them, the loads corresponding to the location of phosphorus fertilizer granules deserve special consideration. Experimental data showed that phosphate loads determine charge of metal-phosphate complexes and their capability to pass into liquid phase. The formation of retained on sorbent surfaces metal-phosphate complexes occurs only at small (approximating to natural) phosphate loads when water and/or hydroxyl ligands of soil metal complexes-sorbents are subjected to phosphate substitution. Subsequent phosphate binding on sorbent surfaces coated with metal-phosphate complexes (of the type of ordinary phosphate salts) resulted in the formation of anionic metal-phosphate complexes capable to pass into liquid phase. Charge of anionic complexes increased with the increasing phosphate loading on soil sorbents and determined the intensity of sorbent dissolution and the formation of new solid phosphate phase. As shown, dissolution of gibbsite (aluminum hydroxide) in the 1 M phosphate solution resulted in the formation of ammonium salt of complex alumophosphoric acid (taranakite). The presence of alumopyrophosphate anionic complexes in liquid phase of the system gibbsite−1 M phosphate solution permits to suppose that phosphate anions could be coordinated to the positively polarized P atoms in ligands of alumoorthophosphate complexes. It seems likely that formation of P−O−P-groups was responsible for the enhancement of gibbsite dissolution and conversion to taranakite. As for iron-humus sorbents in overphosphatized soils, the reactions at P atoms of humic macroligands generated their destruction with release of P-containing fragments in liquid phase. According to obtained data, a decrease of the efficiency of sorption barriers in acid overphosphatized soils is due to the formation of soluble P-containing products of destructive transformation of natural aluminum and iron compounds.
