Potential for sediment phosphorus release in coal mine subsidence lakes in China: perspectives from fractionation of phosphorous, iron and aluminum

Abstract

Few studies have been conducted on how the quantitative relationship between phosphorus (P) and iron (Fe) and aluminium (Al) compounds in the sediments of coal mine subsidence lakes influence sediment P release. Four representative lakes, characterized by sedimentary environments of soil inundation, were selected in the Huainan and Huaibei coal mine areas of China. Their ages, pollutant loading patterns and nutrient levels were assessed to evaluate the potential for sediment P release based on the fractional composition of P, Fe and Al. Sediment P, Fe and Al were extracted sequentially using ammonium chloride (NH4Cl), bicarbonate-dithionite (BD) and sodium hydroxide (NaOH) at 25 °C, followed by HCl, and then NaOH at 85 °C. The resulting fractions were considered as environmental indicators for P, Fe and Al, including ion-exchangeable forms (NH4Cl-P, NH4Cl-Fe, NH4Cl-Al), associated fractions with reducible metal hydroxides (BD-P, BD-Fe, BD-Al) and amorphous hydroxides (NaOH25-P, NaOH25-Fe, NaOH25-Al), acid-soluble fractions (HCl-P, HCl-Fe, HCl-Al) and residual species (NaOH85-P, NaOH85-Fe, NaOH85-Al), respectively. The potential for sediment P release was related to the concentrations of Al and Fe compounds in the presence of soil inundation. Calcareous soils in the Huaibei area were influential in regulating sediment P release, whereas soil Fe and Al were influential in the Huainan area. The results agreed with a common empirical model that predicts low P flux if the molar ratio of [NH4Cl-Al + BD-Al + NaOH25-Al]:[NH4Cl-Fe + BD-Fe] > 3 or [NaOH25-Al]:[NH4Cl-P + BD-P] > 25 in sediments when anoxia develops. Increased loading of oxidizable matter (OM) or enriched P bound to Fe oxides tends to change these ratios and increase the potential for sediment P release.