Use of leonardite humic acids for metals extraction in mine soils

Mine soils usually exhibit high levels of metal contamination. Phytoextraction is an environmental friendly and cost-effective technique for soil remediation, which consists of removal of metals from soil by plant roots and their translocation to aerial parts. Humic substances used as soil amendments could enhance metal mobility and uptake by plants due to formation of soluble complexes. This study aimed to evaluate the effects of a commercial humic acid derived from leonardite added at different rates and pH to a contaminated soil from an abandoned copper mine in Colmenarejo (Madrid, Spain) on metal mobility. The objective was to assess its potential as soil amendment for phytoextraction. Soil samples (10 g) were sequentially extracted 6 times with 20 mL of solution containing humic acid at: 0 (control), 0.25, 1, and 5 g L-1. Solutions were previously adjusted to pH: 4, 6.1 (natural soil pH) and 8. Extracts were analyzed for pH, electrical conductivity, metal concentrations (Cu and Zn) and ratio of absorption at 465 to 665 nm (E4/E6) as an indirect measure of soluble organic matter content. Results showed that addition of higher doses of humic acid increased soluble organic matter content, and therefore extracted a higher concentration of Zn and particularly Cu, due to formation of soluble humic complexes. At higher pH humic acid became more soluble and managed to extract increasing amounts of metals, except in the highest humic acid treatment due to its precipitation or flocculation. The use of this humic acid as soil amendment could promote metal phytoextraction by tolerant plants in contaminated mine soils, but it should be applied at proper pH and other soil conditions to avoid risk of metal leaching to groundwate