The chemistry of aquatic phosphate: inorganic processes in rivers

Phosphate levels in turbid rivers with low calcium concentrations are controlled by a solid ferric hydroxide-phosphate solution present in colloidal suspensions or suspended particulates. A chemical model, based on this behavior, is consistent with data from dialyzed suspensions of iron and phosphorus prepared in the laboratory as well as from the Amazon, Zaire, Orinoco, Sepik, Delaware, Hudson, Negro, and Mullica rivers. Data indicate that solid Fe/P ratios are related to solid activity coefficients by an exponential parameter, y, which represents the deviation of solid-solution from ideality. The model is mathematically consistent with Langmuir and Freundlich sorption isotherms under equilibrium conditions, and demonstrates that the isotherm parameters consist of a combination of selected constants and variables defined by solution theory. The reciprocal of the model parameter-y is shown to be equivalent to the exponential parameter in a Freundlich isotherm. The Langmuir parameter and Freundlich exponential parameter are related through the model parameter-y in systems at constant pH and ionic strength.From a presentation given at the Third International Workshop on Phosphorus in Sediments, Woudscholten/Utrecht, The Netherlands, September 30, 1991, under the auspices of: International Association of Theoretical and Applied Limnology, Limnological Institute (Royal Netherlands Academy of Arts and Sciences), Institute for Inland Water Management and Waste Water Treatment, and the Netherlands Institute for Sea Research.