Optimizing the removal of carbon phases in soils and sediments for sequential chemical extractions by coulometry

Abstract

We have developed a coulometric technique to optimize the removal of the carbonate and organic fractions for sequential chemical extractions of soils and sediments. The coulometric system facilitates optimizing these two fractions by direct real-time measurement of carbon dioxide (CO ₂ ) evolved during the removal of these two fractions. Further analyses by ICP-MS and alpha-spectrometry aided in interpreting the results of coulometry experiments. The effects of time, temperature, ionic strength and pH were investigated. The sensitivity of the coulometric reaction vessel/detection system was sufficient even at very low total carbon content (<0.1 mol kg ^–1 ). The efficiency of the system is estimated to be 96% with a standard deviation of 8%. Experiments were carried out using NIST Standard Reference Materials 4357 Ocean Sediment (OS), 2704 Buffalo River Sediment (BRS), and pure calcium carbonate. Carbonate minerals were dissolved selectively using an ammonium acetate-acetic acid buffer. Organic matter was then oxidized to CO ₂ using hydrogen peroxide (H ₂ O ₂ ) in nitric acid. The carbonate fraction was completely dissolved within 120 min under all conditions examined (literature suggests up to 8 h). For the OS standard, the oxidation of organic matter self-perpetuates between 45 and 50 °C, a factor of two less than commonly suggested, while organic carbon in the BRS standard required 80 °C for the reaction to proceed to completion. For complete oxidation of organic matter, we find that at least three additions of H ₂ O ₂ are required (popular methods suggest one or two).