Removal of phenanthrene and cadmium from co-contaminated alkaline soil by carpet grass, Siam weed and winged bean

Aim: Phytoremediation of soil co-contaminated with polycyclic aromatic hydrocarbon (PAH) and heavy metal is rarely reported. The aim of this study was to investigate the ability of carpet grass (Axonopus compressus), Siam weed (Chromolaena odorata) and winged bean (Psophocarpus tetragonolobus) to remove cadmium and phenanthrene concurrently from contaminated soil. Methodology: Soil was spiked with phenanthrene alone or phenanthrene plus cadmium to give initial concentration of phenanthrene in soil with and without cadmium were 44.9 and 87.8 mg kg(-1) dry soil, respectively. Initial concentration of cadmium in soil spiked with phenanthrene plus cadmium was 6.2 mg kg(-1) dry soil. Carpet grass, Siam weed and winged bean were planted separately in phenanthrene-spiked soil or phenanthrene and cadmium-spiked soil for 60 days. Growth of each plant, phenanthrene remaining in soil, cadmium remaining in soil, phenanthrene and cadmium in biomass of each plant were measured on day 30 and 60 of transplantation. Results: Carpet grass, Siam weed and winged bean grew normally in soil spiked with phenanthrene alone or phenanthrene+cadmium over the 60-day experiment. The presence of plants did not result in cadmium removal, as the amount of this metal in soil remained unchanged after 60 days. Negligible amounts of phenanthrene and cadmium were accumulated by Siam weed, carpet grass and winged bean after 60 days. All three plants could increase the removal of phenanthrene from soil. Around 6.3-12.4% and 5.127.1% of phenanthrene remained in planted soil in the absence or presence of cadmium in soil on day 60, respectively. Interpretation: The results suggest that phytostimulation may be the main mechanism of phenanthrene removal from contaminated soil. The simultaneous removal of phenanthrene and cadmium in planted soil was not observe. A mild alkaline soil may decrease the accumulation o cadmium by plants.