Impact of Hydrofluoric Acid Treatment on Humic Acid Properties Extracted from Organic Soils and an Organic Amendment: A Technical Evaluation

The chemical and physical characteristics of humic acids (HA) may differ depending on their source, and the ideal extraction method should not modify the characteristics of HA. Hydrofluoric acid (HF) is often used in HA extraction methods to remove inorganic substances that are often present in the sample in addition to the organic molecules of interest. Organic soils contain up to 90% of organic matter and some may think that the necessity of the HF treatment is not crucial when extracting HA from organic soils. In this study, HA were extracted from turkey litter compost (TLC), an agricultural organic soil (AOS) and a riparian soil (RS), and the impact of HF on the properties of the extracted HA was assessed. HF decreased the ash content and the concentration of inorganic components in extracted materials, especially in the HA from RS, which had higher concentrations of aluminosilicates and amorphous Si. For TLC, no significant difference in either the total charge or charge distribution was observed with HF treated samples compared to untreated samples, while the HF treatment decreased the proton binding capacity at alkaline pH (>= 7) in the HA from the two soils. We assume that aluminosilicates, amorphous Si and iron oxides left in the samples interacted with protons at alkaline pH, thus increasing the proton binding capacity. Therefore, HF treatment should be included when samples containing these mineral constituents even for materials rich in organic matter.