Soil washing with solutions of humic substances from manure compost removes heavy metal contaminants as a function of humic molecular composition.

Humic Substances (HS) from Leonardite and two different composts were used as biosurfactants to wash heavy metals (Cu, Pb, Zn, Cd, Cr) from a soil added with two metals concentrations and aged for 4 and 12 months. Composts were obtained by mixing manure with either 40 (CM-I) and 20 (CM-II) % of straw as structuring material. For both aging periods and both metal concentrations, HS from CM-I removed more metals than from Leonardite, whereas the washing capacity of HS from CM-II was negligible. 13C-CPMAS-NMR spectra of HS indicated that while aromatic moieties for CM-I and Leonardite were more abundant than CM-II, HS from CM-I was largest in carboxyl and phenolic carbons. Hence, HS from CM-I had a greater complexing capacity than from both Leonardite and CM-II and effectively displaced heavy metals from soil during the washing treatment. Moreover, the amount of metals removed by solutions of ammonium acetate (AA) and diethylenetriaminepentaacetic acid (DTPA), was found invariably smaller than by HS from CM-I, thereby indicating that HS removed more than one metal specie. The combined washing with HS from CM-I before and after soil treatment by either AA and DTPA revealed significant larger metal removals than by single solutions alone. This shows that humic soil washing also renders residual metals potentially more available to subsequent soil remediation approaches, such as phytoextraction. These results suggest a novel, efficient, and molecularly-based technology to remediate soils from heavy metals can be based on a low-cost and sustainable humic matter produced from recycled biomasses.