Humic acids coagulation

Nathalie A. Wall and Gregory R. Choppin

Florida State University

Department of Chemistry, Tallahassee, FL 32306, USA


The Waste Isolation Pilot Plant (WIPP), the first certified deep geological nuclear repository in the world is now open for permanent transuranic (TRU) waste repository; it has been created within a natural bedded salt formation at a depth of c.a. 655 m. WIPP is required to isolate TRU wastes for over 10,000 years, over which period of time humic acids may be generated via degradation of organic substances present in the wastes. Humic acids (HA) have a strong capacity to bind metals, so it is important to assess the humic solubility to model the effect on metals in the repository. HA coagulation varies with parameters such as the ionic strength and the presence of divalent cations, due to their colloidal and polyelectrolyte characteristics. This work presents experimental data on the effects of WIPP parameters (ionic strength, electrolyte, pH, divalent cations concentration) on the HA coagulation. Results show that, in the absence of divalent cation, humic acids are most soluble at low ionic strength and the alkali cations have an increasing efficiency in coagulating the humic acids in the following order: Li+ < Na+ < K+. The effect of calcium on HA coagulation varies with the ionic strength of the solution: only 0.001 M Ca2+ is needed at low ionic strength (0.1 m NaCl) to increase the humic coagulation, but 0.05 M Ca2+ is needed at high ionic strength (3 m and 5 m, NaCl). Mg2+ is less efficient in coagulating HA than Ca2+ is: HA coagulation increases in the presence of 0.1 M Mg2+ in 0.1 m NaCl. The experimental results can be interpreted by the DLVO theory criteria. This work was funded at Florida State University by Sandia National Laboratory, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL8500.