Soil Washing with Humic Acid: A toxicity constraintment technique for a successful soil remediation

Natural organic matter (NOM), can be multifunctional, since besides its application in agriculture to increase soil fertility, it can be also used in the soil remediation from either heavy metals, or from persistent organic contaminants. Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are major organic contaminants of polluted soils and represent a considerable health and environmental hazard due not only to their carcinogenic, mutagenic, and teratogenic properties, but also to their great persistence in soils conferred by a high hydrophobicity. The use of synthetic surfactants to remediate contaminated sites is limited by their intrinsic toxicity towards soil microorganisms that hinders further biodegradation of pollutants and a balanced biological activity in the washed soils. Alternatively, the use of biosurfactants in washings of polluted soils have been increasingly proposed in recent years. Among biosurfactants, NOM fractions have been recognized to be capable of incorporating PAHs and PCBs, increasing aerobic degradation of recalcitrant soil contaminants and effectively washing away pollutants from soils. Herein a soil sample collected in 1999 from a highly contaminated site around the former chemical plant of ACNA (near Savona) in Northern Italy was used. Humic acid (HA) was isolated from a North Dakota Leonardite (Mammoth, Chem. Co., Houston, Texas), and purified. 50g of soil were suspended with 500ml of HA (10g L^-1) at pH7 for 24h. The suspension was then centrifuged at 3000rpm and the HA solution was filtered and collected. 200ml of milli-Q water were added to the residual soil and after suspension, centrifugation and filtration were added to the HA collected before, and freeze-dried. Off-line pyrolysis of 1g of soil and of 50mg of HA before and after the soil washing was conducted and the materials extracted were subjected to GC-MS analysis. Aliivibrio fischeri Bioluminescence Inhibition Test was used to assess the ecotoxicity of the HA before and after the treatment, using Microtox Model 500 Toxicity Analyzer (Azur Environmental) following the 81.9% basic test protocol. Bioluminescence was recorded after 15 min of exposure. GC-MS results of ACNA soil resulted in 1464±206mg kg^-1 PAHs before soil washing (BSW) and 2.95±0.4mg kg^-1 after soil washing (ASW), and in 151±14mg kg^-1 PCBs BSW and 0.18±0.02mg kg^-1 ASW. At the same time no PAHs or PCBs were identified at the HA BSW, but in HA ASW (100mg) 1656±175mg PAHs and 163±17mg PCBs were spotted. Atomic Adsorption Analysis (AAS) on HA and soil BSW and ASW, showed no heavy metal toxicity. The ecotoxicological results of the HA BSW shown a 0.89±0.01% inhibition, which was slightly increased ASW (1.85±0.11%). In both cases the HA was found to not cause ecotoxicity effects. This stunning result, indicates that when the aromatic pollutants come in contact with the humic supramolecular matrix, they are incorporated in the humic hydrophobic domains by forming stable π-π bonds with the humic aromatic constituents and, thus, are no-longer bioactive. This finding can be a milestone on the use of humics as materials for environmental remediation.