Introduction

Malfunctioning of industrial activities often causes contamination of soils and waters by toxic and persistent organic pollutants (POP), such as polychlorobiphenyls (PCB) and/or heavy metals (HM). PCB are among the most hazardous pollutants (Borja et al. 2005), and, as highly hydrophobic contaminants, are adsorbed in soils on surfaces of clay and other oxides and on organic matter (Wang et al. 2005; Sannino et al. 2013).

The soil washing of PCB by HA solutions may result not only more efficient but also much faster than the bioremediation processes. In fact, although microorganism by anaerobic and aerobic treatments have been used to promote PCB biodegradation in soil, this process is not only time consuming but also poorly efficient as it depends on several chemical and environmental factors (Furukawa and Fujihara 2008) such as nutrients availability, temperature, pH (Borja et al. 2005; Wiegel and Wu 2000), and type of bacteria (Sannino et al. 2016). Moreover, aerobic microorganisms degrade prevalently lower chlorinated PCB congeners while anaerobic microbes are known to transform highly and ortho-chlorinated PCB (Adebusoye et al. 2008; De et al. 2006). In addition, in many cases PCB dechlorination occurs only after a lag period that varies from few days to several months and is also liable to generate in the process further toxic by-products (Wiegel and Wu 2000; Chen et al. 2001).

Similarly, the soil washings with humic surfactants appear much more efficient than the phytoremediation progressively called upon to reclaim polluted soils, since not only the latter method explores a very thin soil depth but also its contribution to the microbial degradation of PCB is extremely slow (Van Aken et al. 2010; **g et al. 2018). In fact, it has been estimated that the phytoremediation of an industrial soil contaminated by relatively biolabile PCB, such as tri- and tetra-CB congeners, would take more than 20 years to be completed (Schwitzguébel 2017). Conversely, this work has shown that a single and rapid washing with HA solutions at a L/S ratio of 10:1 was equally effective in removing either low- or high-chlorinated PCB congeners and either planar or no planar PCB. In fact, our findings do not reveal any relation between number and position of chlorine atoms and amount of PCB removed from soil, thereby indicating that the soil decontamination by HA washing is substantially independent on type of PCB congeners.

Conclusions

Soil washing by solutions of humic surfactants can be a promising technology for rapid, efficient, and simultaneous remediation of soils contaminated by both heavy metals and very recalcitrant pollutants, such as PCB. In fact, the HA washing removed from soil an average of 47% of all HM, with a peak of 57 and 67% for highly toxic metals such as Hg and Cu. The much larger efficiency of HA than water alone in removing HM from soil is due to the considerable chelating capacity of the heterogeneous acidic functional groups present in the humic molecules.

Concomitantly, the HA surfactant property leads to formation of pseudo-micellar domains when the supramolecular structure of humic matter is arranged in aqueous solutions, and favors the repartition of highly hydrophobic pollutants like PCB from the soil surfaces into the humic pseudo-micelles. In fact, the same soil washing experiment that efficiently removed HM, also enabled the removal of up to 75% of the total PCB present in the industrially polluted soil that was investigated here.

We thus believe that the use of natural humic surfactants represents an efficient, environmentally friendly, and cost-effective alternative to common methods for soil remediation and may well replace commercial synthetic surfactants in soil washing technologies. In fact, a washing procedure based on humic solutions can be applied to remove simultaneously from soils large amounts of both heavy metals and recalcitrant organic compounds and the resulting contaminated HA easily disposed by an ex situ incineration. Moreover, the washing by humic surfactants would leave behind in soil enough metabolic carbon to speed up the natural attenuation of the organic pollutants remaining in soil.