Chelate-assisted phytoextraction of mercury in biosolids

Mercury contaminated stockpiles of biosolids (8.4 mg kg− 1 Hg) from Melbourne Water's Western Treatment Plant (MW-WTP) were investigated to evaluate the possibility of their Hg chelate-assisted phytoextraction. The effects of ammonium thiosulphate (NH4)2S2O3, cysteine (Cys), nitrilotriacetic acid (NTA), and potassium iodide (KI) were studied to mobilize Hg and to increase its uptake in plant shoots. Three plant species were selected for this study, one herbaceous and two grasses: Atriplex codonocarpa, Austrodanthonia caespitosa and Vetiveria zizanioides. KI proved to be the best candidate for Hg phytostabilization in biosolids because it facilitated the concentration of this metal mainly in roots. (NH4)2S2O3 was shown to be the most effective chelating agent among those tested for Hg phytoextraction as it allowed the highest translocation of Hg into the above-ground tissues of the selected plant species. The phytoextraction conditions using A. caespitosa as the best performing plant species were optimized at an (NH4)2S2O3 concentration of 27 mmol kg− 1 and contact time with biosolids of seven day. Monitoring of the Hg concentration in biosolids and in leachate water during a 9-day treatment revealed that the biosolids Hg concentration decreased significantly after the first day of treatment and then it decreased only slightly with time reaching a value of 5.6 mg kg− 1 Hg at the end of the 9-day period. From the corresponding results obtained for the leachate water, it was suggested that a relatively large fraction of Hg (0.7 mg kg− 1 Hg) was promptly mobilized and consequently the plants were able to take up the metal and translocate it into shoots.

Research highlights
► A. codonocarpa, A. caespitosa and V. zizanioides phytoextract Hg from biosolids.
► KI, ammonium thiosulphate, cysteine, and nitrilotriacetic acid mobilize Hg.
► Most Hg was phytoextracted when ammonium thiosulphate and А. caespitosa were used.