Colloid-facilitated release of polybrominated diphenyl ethers at an e-waste recycling site: evidence from undisturbed soil core leaching experiments

Polybrominated diphenyl ethers (PBDEs), a class of persistent organic pollutants, have been frequently detected in soil at e-waste recycling sites. However, the key factors controlling the transport of PBDEs from surface soil to the vadose zone and groundwater are unclear. Here, colloid-enhanced leaching of PBDEs from undisturbed soil cores collected at an e-waste recycling site in Tianjin, China, is reported. Spatially heterogeneous release of colloids and PBDEs was observed in all the tested soil cores under chemical and hydrodynamic perturbations, indicating the presence of preferential flow paths. Colloid concentration in the effluent significantly increased as ionic strength decreased (from 10 to 0.01 mmol/L), probably due to the stronger electrostatic repulsion between colloidal particles and the soil matrix at lower ionic strength. In contrast, colloid mobilization was not significantly affected by the changes in pH of the influent (from 4.0 to 10.0) and flow rate (from a Darcy velocity of 1.5 to 6.0 cm/h). The concentrations of 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether (BDE-209), the predominant PBDE congener at the site, detected in the leachate (ranging from 1.09 to 3.43 ng/L) were much lower than previously reported results from packed column leaching tests, and were positively correlated with colloid concentrations. This indicates that remobilization of colloids at e-waste recycling sites can promote the leaching and downward migration of PBDEs from surface soil. The findings highlight the potential risk of surface soil PBDE contamination to groundwater quality and call for further understanding of colloid-facilitated transport for predicting the fate of PBDEs at e-waste recycling sites.

Effects of transient flow conditions on colloid-facilitated release of decabromodiphenyl ether:Implications for contaminant mobility at e-waste recycling sites

Polybrominated diphenyl ethers(PBDEs)are ubiquitous contaminants,especially in the soil and groundwater of contaminated sites and landfills.Notably,2,20,3,30,4,40,5,50,6,60-decabromodiphenyl ether(BDE-209),one of the most frequently and abundantly detected PBDE congeners in the environment,has recently been designated as a new pollutant subject to rigorous control in China.Colloid-facilitated transport is a key mechanism for the release of PBDEs from surface soils and their migration in the aquifer,but the effects of hydrodynamic conditions,particularly transient flow,on colloid-facilitated release of PBDEs are not well understood.Herein,we examined the effects of typical transient flow conditions on the release characteristics of colloids and BDE-209 from surface soil collected from an e-waste recycling site by undisturbed soil core leaching tests involving multiple dry–wet cycles(with different drying durations)and freeze–thaw cycles.We observed significant positive correlations between BDE-209 and colloid concentrations in the leachate in both the dry–wet and freeze–thaw leaching experiments,highlighting the critical role of colloids in facilitating BDE-209 release.However,colloids mobilized during the dry–wet cycles contained higher contents of BDE-209 than those in the freeze–thaw cycle tests,and the difference was primarily due to the more intensive disintegration of soil aggregates and elution of newly formed inorganic colloidal particles(mainly primary silicate minerals such as quartz and albite)with low BDE-209 content during the freeze–thaw process.These findings underscore the necessity of considering transient flow conditions when assessing the fate and risks of PBDEs at contaminated sites.