Conditioning of sediment polluted with heavy metals using plants as a preliminary stage of the bioremediation process: a large-scale study

PURPOSE: A bioremediation process for sediments contaminated with heavy metals has been developed based on two core stages: (1) conditioning of dredged sludge using plants; and (2) solid-bed bioleaching of heavy metals from the resulting soil-like material using microbially produced sulfuric acid. In laboratory and pilot-scale tests, reed canary grass (Phalaris arundinacea) was found to be best suited for the conditioning process. To demonstrate the feasibility of conditioning in practice, a study on a larger scale was performed. MATERIALS AND METHODS: The sediment originated from a detritus basin of the Weisse Elster River in Leipzig (Saxony, Germany) and was polluted with heavy metals, especially with zinc and cadmium. The dredged sludge was a muddy-pasty, anoxic, and had a high organic matter content. The experimental basin (base area of 50 × 23 m) was filled with 1,400 m³ of sludge to a height of 1.2 m. Conditioning was carried out in five segments that were planted with pre-cultivated Phalaris plants at two plant densities, sowed with Phalaris seeds using two different seeding devices, and grown over by vegetation. Plant development and changing sediment characteristics were analyzed during two vegetation periods by harvesting plant biomass every 4 weeks and sampling sediment material at two different depths every 2 weeks over a total duration of 475 days. RESULTS AND DISCUSSION: At the end of the second vegetation period, the pre-cultivated Phalaris plants had reached a height of 2 m, compared to 1.8 m for the sowed Phalaris seeds. Regarding root penetration and the degree of sediment conditioning, the less expensive sowing techniques yielded similar results to planting pre-cultivated plants. The content of heavy metals in the Phalaris plants was below the permissible limits for Germany. The vegetation evapotranspirated large amounts of water from the sediment and transported oxygen into the anoxic sludge. The water content was reduced from 68 to 37 %. The muddy-pasty sludge turned into a soil-like oxic material with a high permeability to water. The oxidation of sediment-borne compounds lowered the pH from 7.3 to 6.0. Due to the high total precipitation in Saxony in the summer of 2010, a maximum of 65 % of the sediment was conditioned. CONCLUSIONS: The feasibility of the first core stage of the bioremediation process for sediments was demonstrated in practice by conditioning 1,400 m³ of dredged sludge using reed canary grass. To establish the proposed sediment treatment in practice, the applicability of the central core stage–solid-bed bioleaching of conditioned soil-like sediment–will also be tested at a larger scale.