Chemically assisted 2.45 GHz microwave irradiation for the simultaneous removal of mercury and organics from contaminated marine sediments

Abstract

This work aims to investigate the simultaneous mercury (Hg) and polycyclic aromatic hydrocarbons (PAHs) removal from seabed sediments by means of 2.45 GHz microwave irradiation with chemical enhancers. Decontamination kinetics were assessed applying Tween® 80, methylglycinediacetic acid (MGDA) and citric acid as enhancers. Results clearly showed that sediment dielectric features allowed a large conversion of microwaves (MW) irradiated energy into temperature increase (405 °C within 7 min). The combined use of Tween® 80 and MGDA in a 650 W MW irradiation treatment brought a final residual Hg concentration of 2.2 mg kg−1, whereas Tween® 80 and citric acid allowed a residual concentration less than 1 mg kg−1 (R = ~ 99%). Lower residual concentration was found also for total PAHs (< 1 mg kg−1) already after 1-min irradiation. Modelling revealed for all treatments that exponential decay has a very good fit with experimental points. For the unenhanced MW treatment, the decay rate (k) was 0.259. When Tween® 80 and acid citric were contextually used, they brought an almost doubled k value of 0.493. The co-presence of PAHs decreased the Hg removal kinetic only without citric acid addiction. A simultaneous and very rapid Hg and PAHs total removal is very difficult to be achieved by other cleanup techniques, which operate in a selective way. Desorption parameters calculated are useful for Hg-PAH co-contamination removal kinetics prediction and for scaling-up studies that, at the moment, are essential to meet the great challenge of applying MW at the full scale.