Conditioning of Heavy Metal-Polluted River Sediment by Cannabis sativa L.

We are currently developing a process for the remediation of heavy metal-polluted sediment by solid-bed bioleaching. Unfortunately, freshly dredged sediment is impermeable to water, unsuitable for solid-bed leaching, and therefore needs preliminary conditioning to improve its structure. A pilot-scale conditioning plant consisting of several basins (each with a base of 6 m² and a depth of 0.7 m) was filled with freshly dredged, slightly predewatered sediment from the Weisse Elster River. Hemp (Cannabis sativa L.) was chosen for conditioning because it develops quickly from seeds, produces large amounts of biomass, and forms commercially useful fibers. Irrespective of the plant cover, the sediment dried out, was oxidized and turned acidic. However, its structure and permeability only improved when the sediment was covered with plants and not when all the vegetation appearing was removed regularly. Although hemp germinated completely regularly on sediment, 1 week after seeding the leaves became partially discolored, deformed, and dry, and ultimately about 95% of all the hemp plants died. The surviving hemp produced shorter shoots and much lower amounts of biomass than hemp on unpolluted soil. Studies in pots and hydrocultures indicated that the inhibition of hemp growth is caused by the synergistic action of a low pH and several mobile toxic metals. Hemp on sediment incorporated much higher quantities of heavy metals (especially Zn, Cd, and Ni) than hemp on unpolluted soil, which calls the commercial use of the former seriously in question.