Sulfur-aided phytoextraction of Cd and Zn by Salix smithiana combined with in situ metal immobilization by gravel sludge and red mud.

As phytoextraction implementation may be limited by metal toxicity and leaching, we investigated the idea of in situ metal immobilization in bulk soil, while increasing metal bioavailability in the rhizosphere. Salix smithiana was grown in a pot experiment on two Cd/Zn polluted soils. Treatments with or without willows included: no additives; gravel sludge + red mud kg(-1); acidification with S to pH 3.5; and metal immobilization combined with soil acidification. Salix smithiana removed up to 0.78 ± 0.06% total Cd and 0.34% (±0.02) total Zn from the non-treated soils. The phytoextraction efficiency in the S treatments was enhanced by up to ~50% in response to metal solubility that was magnified by reductive co-dissolution from Mn (IV) and Fe(III) (oxy)hydroxides during microbial S oxidation in the willow rhizosphere. The proposed technique proved to enhance phytoextraction efficiency while controlling the risk of metal leaching from the root zone and phytotoxicity.