Stability and Speciation of Phosphorus in River Sediments.

Sediments are heterogeneous mixtures of assorted soil separates and organic matter that serve as repositories of many pollutants. This study investigated phosphorus (P) stability in river sediments as controlled by P speciation and environmental conditions, such as temperature (T), dissolved oxygen (DO), redox conditions (Eh), and pH. It attempted to characterize the chemical and physical parameters that affect P speciation in sediments, and to evaluate the experimental condition under which sediment-bound P could potentially become available in river waters. Sediment cores samples were collected from James River, in Virginia, near a former Dairy farm and analyzed for T, pH, Eh, carbon (C) and metal ions. In a laboratory experiment, samples were suspended in deionized water and equilibrated under different pH and aerobic/anaerobic conditions to assess the most stable P species present under a controlled environment. High P concentration correlated well with Fe and had non-uniform correlations with clay, Al and Ca, which changed with depth of cores. At low pH, P concentration was higher in anaerobic than aerobic sediments and aluminum precipitation was highest at low pH, which indicated Al phosphate stability in anoxic (reduced) conditions. Beyond pH 7, in aerobic environments, the Minteqa2 speciation model predicted a high stability for Al-OH species.  Elemental P distribution assessment using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) inferred that P at pH 4 could be bound more to C and Al, than to Fe and Mn.