Conversion of AIPO₄-P to Fe-bound P in Sea Sediments

Abstract

Fe-bound P in the sediment was suspected to be the cause of the frequent red tide in Hakata Bay in summer, because it releases PO₄^3- into the upper water when H₂S is generated in the sediment. Thus, the contents of Fe-bound P were measured by applying “H₂S-extraction method” to the summer sediments in Hakata Bay and Ariake Bay where red tide scarcely occurs. Their contents were 34±20 ppm in Hakata Bay and 42±21 ppm in Ariake Bay, only 7% and 8% to their total P, respectively. The contents of Al-bound P, Ca-bound P, total P and residual organic P were also measured in these sediments. The contents of Al-bound P were 47±36 ppm in Hakata Bay and 190±211ppm in Ariake Bay, 10% and 35% to their total P, respectively. The contents of residual organic P were 255±109 ppm in Hakata Bay and 140±101 ppm in Ariake Bay, 53% and 25% to their total P, respectively. There was not much difference observed in the con-tents of total P and Ca-bound P between these two bays. The distributions of Fe-and Al-bound P revealed that Fe-bound P does not decrease so quickly as AI-bound P does in the sediments. Then, the solubilities of FePO₄-P and AlPO₄-P which are the representatives of Fe-bound P and AI-bound P were checked and it was found that the solubility of FePO₄-P is smaller than that of AIPO₄-P at seawater pH. This suggested that AIPO₄-P may convert to FePO₄-P in these sediments. This conversion was soon confirmed experimentally. Fe-bound P does not seem statically to be the source of PO₄^3- as the cause of red tide because of their low contents in sediments. However, AIPO₄-P from river water converts to Fe-bound P in sea sediments and newly produced Fe-bound P supplies PO₄^3- into the upper water. Therefore, we can say that Fe-bound P is dynamically the causal substance to red tide.