Precipitation of aluminium and phosphate affected by acidification

Acidified waters often show elevated concentrations of Al (with up to 6 mg L-1 being not unusual). A pH increase resulting e.g. from mixing with non-a cidified water or from biological activities may be linked with Al precipit ation. Up to now, this phenomenon was described for acid mine drainages. Th is investigation focuses on a whitish precipitate naturally formed in a bro ok of an atmospherically acidified catchment in the Ore Mountains, Germany. Based on infrared spectra the precipitate was identified as an Al-hydroxos ulfate with crystal water. A simulation of natural conditions in the laboratory showed that Al precipi tated only if sulfate or phosphate ions were added to the solution. In the case of sulfate being added, the infrared spectrum of the precipitate was s imilar to the natural precipitate. Al-27 NMR spectroscopy revealed tetrahed rally coordinated Al in some precipitates which evidences the participation of the tridecameric [Al13O4(OH)(24)(H2O)(12)](7+) cation beside other poly meric Al cations. Precipitation experiments subjected to the given conditions showed that the phosphate elimination from solution with Al was much higher than with Fe. With Al and Fe added together, the P elimination rate was likewise high, an d phosphate was bound onto Ar in the precipitate. This was demonstrated by SEM-EDX spectroscopy. Based on these results we present a possible reaction mechanism. The precipitation of Al together with P allows a significant retention of b oth elements in sediments because in contrast to Fe, Al immobilizes phospha te even under anoxic conditions.