Optimized digestion methods: organic phosphorus sequential extraction, total phosphorus, and nitrogen simultaneous determination in sediments

Abstract

Purpose: The organic phosphorus (OP) that is found in sediments influences the water quality of the overlying water. Accurate methods for OP extraction are important to identify phosphorus fractions in the sediments. We optimized the following: (1) the pH range of solutions after digestion; (2) the amounts of TP, H2SO4, and K2S2O8 in the digestion; and (3) the digestion efficiency between acid K2S2O8 and alkaline K2S2O8 in total phosphorus (TP) and total nitrogen (TN) measurements. Moreover, we explained the related experimental phenomena and the results with an in-depth mechanism. Materials and methods: Sediments samples were taken from three locations (Yellow Sea, Jiehe River, and Jiaolai River, China). Each sample was analyzed to optimize sequential OP extraction based on the Ivanoff method. Results and discussion: We focused on the digestion step and digestion efficiency between acidic K2S2O8 and alkaline K2S2O8, total phosphorus (TP), and total nitrogen (TN) measurements. The results indicate that the optimal pH range of solutions after digestion is from 3 to 5.5. The TP digestion efficiency in combination withH2SO4 + K2S2O8 (acid) is, on average, 8% higher than that of NaOH+ K2S2O8 (alkaline). However, the results for TN suggest the opposite. Optimal amounts of TP, H2SO4, and K2S2O8 for digestion were also clearly determined. Conclusions: The pH of solutions after digestion affects the color development necessary for effective phosphorus (P) determination. The TP and TN could be determined simultaneously using the alkaline K2S2O8 digestion system under the experimental conditions. Finally, the optimal orthogonal combinations and mass ratios of P, K2S2O8, and H2SO4, respectively, were determined to be ~ 1.5 × 10−5:1:8 (NaHCO3, 16 h), ~ 5.6 × 10−5:1:3 (HCl, 3 h), ~ 1.6 × 10−5:1:5 (NaOH, 16 h), and ~ 1.4 × 10−5:1:8 (NaOH, 16 h, pH = 0.2). We improved the accuracy and precision of the Ivanoff method using an optimized sequential extraction method.