Enhanced soil washing process for the remediation of PBDEs/Pb/Cd-contaminated electronic waste site with carboxymethyl chitosan in a sunflower oil-water solvent system and microbial augmentation

An innovative ex situ soil washing technology was developed to remediate polybrominated diphenyl ethers (PBDEs) and heavy metals in an electronic waste site. Elevated temperature (50 degrees C) in combination with ultrasonication (40 kHz, 20 min) at 5.0 mL L-1 sunflower oil and 2.5 g L-1 carboxymethyl chitosan were found to be effective in extracting mixed pollutants from soil. After two successive washing cycles, the removal efficiency rates for total PBDEs, BDE28, BDE47, BDE209, Pb, and Cd were approximately 94.1, 93.4, 94.3, 99.1, 89.3, and 92.7 %, respectively. Treating the second washed soil with PBDE-degrading bacteria (Rhodococcus sp. strain RHA1) inoculation and nutrient addition for 3 months led to maximum biodegradation rates of 37.3, 52.6, 23.9, and 1.3 % of the remaining total PBDEs, BDE28, BDE47, BDE209, respectively. After the combined treatment, the microbiological functions of washed soil was partially restored, as indicated by a significant increase in the counts, biomass C, N, and functioning diversity of soil microorganisms (p<0.05), and the residual PBDEs and heavy metals mainly existed as very slow desorbing fractions and residual fractions, as evaluated by Tenax extraction combined with a first-three-compartment model and sequential extraction with metal stability indices (I-R and U-ts). Additionally, the secondary environmental risk of mixed contaminants in the remediated soil was limited. Therefore, the proposed combined cleanup strategy is an environment-friendly technology that is important for risk assessment and management in mixed-contaminated sites.