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The core stages of a sediment remediation processthe conditioning of dredged sludge by plants and the solid-bed leaching of heavy metals using microbially produced sulfuric acidwere tested on a pilot scale using a highly polluted river sediment. Conditioning was performed in 50 m3 basins at sludge depths of 1.8 m. During one vegetation period the anoxic sludge turned into a soil-like oxic material and became very permeable to water. Reed canary grass (Phalaris arundinacea) was found to be best suited for conditioning. Bioleaching was carried out in an aerated solid-bed reactor of 2000 L working volume using oxic soil-like sediment supplemented with 2% sulfur. When applying conditioned sediment, the oxidation of easily degradable organic matter by heterotrophic microbes increased the temperature up to 50°C in the early leaching phase, which in turn temporarily inhibited the sulfur-oxidizing bacteria. Nevertheless, most of the metal contaminants were leached within 21 days. Zn, Cd, Mn, Co, and Ni were removed by 61−81%, Cu was reduced by 21%, while Cr and Pb were nearly immobile. A cost-effectiveness assessment of the remediation process indicates it to be a suitable treatment for restoring polluted sediments for beneficial use.

The remediation of heavy metal-polluted aquatic sediment by solid-bed bioleaching requires a material well permeable to air and water. Freshly dredged sediment is nearly impermeable and needs previous conditioning to make it suitable for solid-bed leaching. This conditioning – in practice carried out by planting sediment packages with helophytes – comprises water removal by evapotranspiration, abiotic and microbial oxidation of sediment-borne reduced compounds, acidification, as well as structural changes improving the sediment permeability. The rate of this process seems to be limited by the transport of oxygen into the sediment bed. For a better understanding of the physicochemical processes occurring during conditioning, sediment oxidation was studied in a stirred suspension to minimize transport limitations. Freshly dredged, silty, anoxic, heavy metal-polluted sediment from the Weisse Elster River (Germany) was suspended in water and then continuously stirred and aerated at 20 °C. Aerobic conditions appeared within a few hours. The redox potential increased from – 400 to + 220 mV, at first very quickly and later more slowly. Sediment-borne inorganic sulfur compounds were oxidized to sulfate (S0 mainly within two days and sulfide within ten days), which reduced the pH from 7.2 to 5.9. A successive oxidation of FeS to Fe(II) sulfate, the oxidation of Fe(II) to Fe(III) followed by Fe(III) oxyhydrate formation caused the dissolved Fe to sharply increase and thereafter rapidly decrease. Ammonium was completely oxidized in a nitrification process to form nitrate, further decreasing the pH to 5.5. The acidification increased the solubility of Mn, Zn, Mg, Ca, and K. The increase in dissolved Mn rules out any oxidation of Mn(II) to Mn(IV) since Mn(IV) would have been insoluble under the prevailing pH and redox conditions. Sediment oxidation did not proceed in a well-defined, redox-potential-directed order, but individual (partly microbially) oxidation processes superimpo

The operation of the core stages of a sediment remediation process - the conditioning of anoxic sludge by plants and the solid-bed leaching of heavy metals using microbially produced sulfuric acid - was tested on a pilot scale using a highly polluted river sediment. Sediment conditioning was performed in 50 m3 basins at sludge depths of 2 m. During one vegetation period the anoxic dredged sludge turned into a soil-like oxic material and became permeable to water. Reed canary grass (Phalaris arundinacea) was found to be best suited for conditioning. Bioleaching was carried out in an aerated solidbed reactor with a 2,000 L working volume using soil-like oxic sediment supplemented with 2% sulfur. When applying conditioned sediment, the heat production by microbial oxidation of organic matter increased the temperature up to · 50°C, which temporarily inhibited the sulfur-oxidizing bacteria. Under optimum process conditions the heavy metals were solubilized within a few weeks. Zn, Cd, Ni, Co and Mn were leached by up to 80%. Cu was partially dissolved, while Cr and Pb proved nearly immobile. Sediments conditioned by plants and long-term stored oxic sediments were leached with the same efficiency. In total, 60-65% of the heavy metal load was removed after 6 weeks.

PAH-contaminated waste wood is a serious environmental problem. As an alternative to incineration and landfill disposal, wood containing PAHs may be detoxified by composting. The efficiency of this process depends on the composting conditions. The aerobic treatment of PAH-containing wood was therefore investigated under varying environmental conditions with particular attention to the kinetics of PAH degradation and wood mineralization. The composting of pine wood spiked with 2 g/kg phenanthrene, anthracene and pyrene each and subsequently artificially aged was studied on a laboratory-scale using a respiration analyzer. The temperature was found to highly influence both PAH degradation and wood decay. The fastest and most extensive PAH degradation and wood mineralization were found at 30°C. Higher temperatures particularly inhibited the degradation of anthracene and pyrene. The addition of urea markedly accelerated both PAH degradation and wood mineralization. Only small amounts of urea were needed to maximize PAH degradation, whereas higher amounts of urea were required to maximize wood mineralization. Urea hydrolyzes to ammonium carbonate, which in turn forms highly volatile ammonia. When more then 2 g/kg urea-N was added to the wood, excessive nitrogen disappeared as ammonia via the gas phase. Using nitrate instead of urea dramatically reduced both PAH degradation and wood mineralization. Although a slightly alkaline pH seemed to promote PAH degradation, it has to be taken into account that this experiment was carried out with nitrate as an N source rather than urea to avoid any N losses at high pH values. Glucose as a cosubstrate neither accelerated PAH degradation nor stimulated wood decay. Molasses as a cosubstrate actually inhibited PAH degradation since it contains much salt and alkalinized the rot material.

The elimination of heavy metals from bioleaching process waters (leachates) by electrolysis was studied in the anode and cathode region of a membrane electrolysis cell at current densities of 5–20 mA/cm2 using various electrode materials. The leaching waters containing a wide range of dissolved heavy metals, were high in sulfate, and had pH values of approx. 3. In preliminary tests using a rotating disc electrode the current density-potential curve (CPK) was recorded at a rotation velocity of 0, 1000 and 2000 rpm and a scan rate of 10 mV/s in order to collect information on the influence of transport processes on the electrochemical processes taking place at the electrodes. The electrochemical deposition-dissolution processes at the cathode are strongly dependent on the hydrodynamics. Detailed examination of the anodic oxidation of dissolved Mn(II) indicated that the manganese dioxide which formed adhered well to the electrode surface but in the cathodic return run it was again reduced. Electrode pairs of high-grade steel, lead and coal as well as material combinations were used to investigate heavy metal elimination in a membrane electrolysis cell. Using high-grade steel, lead and carbon electrode pairs, the reduction and deposition of Cu, Zn, Cr, Ni and some Cd in metallic or hydroxide form were observed in an order of 10–40 % in the cathode chamber. The dominant process in the anode chamber was the precipitation of manganese dioxide owing to the oxidation of dissolved Mn(II). Large amounts of heavy metals were co-precipitated by adsorption onto the insoluble MnO2. High-grade steel and to some extent lead anodes were dissolved and hence were proven unsuitable as an anode material. These findings were largely confirmed by experiments using combination electrodes of coal and platinized titanium as an anode material and steel as a cathode material. With both electrode combinations and current densities of 5 or 10 mA/cm2, in the cathode region low depositions of 10–20

Process waters from waters of the bioleaching process contain a wide range of dissolved heavy metals, are high in sulphate, and have pH values in the range of 2-3. The leachates need to undergo a cost-effective water treatment procedure before they can be released into receiving waters. This study examines the efficiency of separating heavy metals and sulphate by alcalisation with Ca(OH)2, NaOH und CaCO3 on the laboratory scale as well as in a pilot plant using the example of process waters from bioleaching heavy-metal contaminated sediments. Alcalisation with Ca(OH)2 at pH 9-10 led to the precipitation of all the dissolved heavy metals in the form of metal hydroxides or mixed salts. Reaction times of 30 min were sufficient when using milk of lime. The limits for heavy metals in treated wastewaters laid down in German regulations were reached, including on the pilot scale. Sulphate was precipitated as gypsum down to residual concentrations of 1.5-2 g/L. The gypsum matrix facilitated the sedimentation and dewatering of the sludges. NaOH caused at pH 8-9 the total separation of heavy metals as hydroxides with the exception of Mn. However, because the sulphate remains in solution and Na ions are introduced, the salt load of treated waters increases. Another disadvantage is the formation of voluminous precipitates which contain large amounts of salts and which are almost impossible to dewater. Neutralisation with CaCO3 caused up to 98% of the dissolved heavy metals to precipitate. The sulphate content was reduced to just 5.6 g/L. The minimum reaction time needed was 24 h. The limits for Zn, Cd and Co in the treated waters were not reached. Irrespective of the alcalising agent used, the precipitates contained about 6-7% heavy metals in dry matter (predominantly Zn and Mn). Relatively large amounts of Al and Mg were also detected. The main component of the sludges upon alcalisation with Ca(OH)2 was gypsum (up to 70%).

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.

The remediation of heavy metal-polluted sediment by solid-bed bioleaching, using the percolation principle, requires a material well permeable to air and water. Freshly dredged sediment is nearly impermeable, unsuitable for solid-bed leaching, and therefore needs preliminary conditioning. During conditioning, the sediment underwent physicochemical changes such as oxidation and acidification, which were significantly accelerated by the presence of reed canary grass (Phalaris arundinacea L.) in comparison to the processes taking place in sediment without plant cover. P. arundinacea transpired large amounts of water followed by the formation of cavities in the sediment package which were then penetrated by atmospheric oxygen. Furthermore, P. arundinacea actively transported oxygen into the sediment via the roots. Oxygen and root exudates stimulated the growth of microbes, which together with hair roots made mineral sediment particles stick together, forming larger aggregates and changing the sediment structure from muddy-pasty to crumbly and soil-like. The structural changes markedly improved the permeability of the sediment to water by a factor of 5,000. Sediment conditioned with P. arundinacea consisted of larger and more stable particles and therefore it was twice as permeable as unplanted sediment. Comparative solid-bed bioleaching experiments on a pilot scale demonstrated that the removal of heavy metal occurred at nearly the same rate and efficiency with conditioned (for six months with P. arundinacea) and spontaneously ripened (stored for six years in the open) sediment.

Das Umweltforschungszentrum Leipzig-Halle hat ein Sanierungsverfahren entwickelt, bei dem Gewässersedimente durch mikrobielle Laugung von Schwermetallen befreit werden.

The investigations into the membrane electrolysis cell show that electrochemical metal separation from bioleaching process waters can represent a practical alternative for metal separation by alcalization, Coal and platinized titanium material exhibit good anodic resistance at the current densities tested. By contrast, high-grade steel and to some extent lead anodes were dissolved and are hence unsuitable for this purpose. However, for practical application, suitable ways are required to discharge the precipitates containing heavy metals deposited on the electrodes from the electrolysis cell and to prevent membrane clogging. Regarding the main components zinc, manganese, and nickel, the combination electrodes proved to be suitable for eliminating the heavy metals from the aqueous phase.Another way of treating diluted process waters containing sulphuric acidic and heavy metals is to concentrate the sulphuric acid in the anode region and to precipitate the heavy metals in the cathode region. The sulphuric acid recovered could then be returned to the leaching process, hence avoiding wastewater.

When dredged sediments contaminated by heavy metals are exposed to air, the metals become partly soluble and the material turns into an environmental risk. As an alternative to disposal of dredged sludge in landfills, a cleansing process based on solid-bed bioleaching is under development. Solid-bed leaching works only on materials easily permeable by both air and water. Unfortunately, freshly dredged sediment is impermeable and thus unsuitable for solid-bed leaching. Therefore, the structure of the contaminated sediment must be altered in a preliminary conditioning step. The SECON plant, constructed for practical-scale conditioning experiments, can accommodate sediment packages of up to 2 m in thickness. Four treatment basins were each filled with 44 m3 of freshly dredged, silty sediments from the Weisse Elster River. One basin was planted with reed (Phragmites australis), a second one with reed canary grass (Phalaris arundinacea). The sediment in the third basin was exposed to spontaneously developing plants while the sediment in the fourth basin was kept free of vegetation. During one vegetation period, large amounts of water were extracted by evapotranspiration. As a result, cracks and cavities developed within the package through which air penetrated and oxidized the sediment. Plant root and microbe activities transformed the sludge into a crumbly, soil-like material.

Um die Funktionsfähigkeit von Gewässern und wassertechnischen Anlagen aufrechtzuerhalten, müssen sie regelmäßig von sedimentiertem Material beräumt werden. Die Baggerschlämme sind oft mit Schwermetallen belastet und ihre Entsorgung ist kostenintensiv. Durch die Trennung des Materials in eine hochbelastete Feinkornfraktion und in eine nahezu schadstofffreie Kies- und Sand-Fraktion, die einer kommerziellen Verwertung zugeführt werden kann, lassen sich die Entsorgungskosten reduzieren. Die Separation des kontaminierten Sediments setzt jedoch eine genaue Kenntnis der Schadstoffverteilung voraus. Solche Untersuchungen wurden an einem schwermetallbelasteten Sediment aus der Weißen Elster durchgeführt. Das untersuchte Material wurde aus der Geschiebefalle Kleindalzig, 20 km südlich von Leipzig, gebaggert und in benachbarte Absetzbecken gepumpt. Dabei wurden die Baggerschlämme nach der Korngröße klassiert. Die vor allem aus Sand, Kies und Steinen bestehenden Ablagerungen waren schadstoffarm, während das schluffreiche Material stärker mit Schwermetallen kontaminiert war. Untersuchungen an einzelnen Korngrößenfraktionen zeigten, daß die Schwermetalle bevorzugt am Feinkorn und besonders stark an der Organik gebunden sind; die Anreicherungsfaktoren lagen metallspezifisch zwischen 6 und 27 gegenüber der Mineralfraktion gleicher Korngröße. Berechnungen zufolge könnten im untersuchten Fall durch Separation des Sediments in eine organikfreie Sandund Kiesfraktion und in eine die Organik und den Schluff enthaltende Fraktion die Menge des zu entsorgenden Materials auf etwa 20 % reduziert werden.

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We are developing a sediment treatment process based on the removal of heavy metals by sulphur-acidic leaching. Four operations - purely chemical or microbial leaching in suspension or in the solid bed - were compared with each other. The metals were quickly dissolved by H2S04 in suspension, but this process is unsuitable on a larger scale. Leaching with microbial H2S04 formation needed more time, but during solid-bed leaching it led to technological advantages.

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During the microbial treatment of a sandy model soil artificially contaminated with polycyclic aromatic hydrocarbons (PAHs), a large residual pollution was found. The remainig PAHs were sorbed into the micropores of the soil and were therefore not bioavailable. Using a lab-scale precolator, the microbially pretreated soil was subjected to aftertreatment with surfactants with the aim of further degradation of its pollution. Two commercial nonionic surfatants of the polyethoxylate type, Präwozell F1214/5 N and Sapogenat T-300, were used. The surfactants differ both in their physicochemical properties (CMC value, PAH solubilization capacity, adsorption onto soil) and in their microbial degradability. During aftertreatment under permanently aerobic conditions, only a weak PAH accumulation in the liquid phase was observed, which was due to a low solubilization rate as well as to simultaneous microbial degradation of the dissolved PAHs. Temporary anaerobiosis successfully suppressed the microbial degradation of both the surfactant and the solubilized PAHs, resulting in a more intensive PAH accumulation. But the PAH content of the soil - the essential criterion for evaluating the efficiency of surfactant application - was not decreased to a larger extent with surfactants than without them. To find out why the surfactants failed to act, the surfactant and hydrocarbon distribution among the liquid and solid phases was studied in mixtures of phenantherne-spiked solis and Präwozell-containig liquids; at heavy phenanthrene loading, the aqueous phase was saturated with PAH; at weak loading, it was unsaturated. Model-aided data analysis showed that the soil may contain PAH in two fractions: strongly sorbed into soil pores and, in the case of heavy loading, also weakly attached to the soil surface. The latter is easily extractable, resulting in a PAH-saturated liquid, while strongly adsorbed PAH is only partially dissolved due to competition between the micelles and the soil pores

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Schwermetallbelastete Gewässersedimente sind ein großes Umwelt- und Kostenproblem. Kommen die anoxischen Schlamme bei Aufwirbelung oder Ausbaggerung mit Sauerstoff in Kontakt, gehen die Schwermetalle durch mikrobielle Oxidations- und Versauerungsprozesse langsam in Lösung. Ursache sind die in den Sedimenten vorhandenen Thiobacillus-Bakterien. Sie werden unter aeroben Bedingungen aktiviert und nutzen Eisen-(II) und reduzierte Schwefelverbindungen als Energiequelle. Dabei wird Schwefelsäure produziert, welche die Metalle löst. Ziel des von der Deutschen Bundesstiftung Umwelt geförderten Forschungsprojektes ist es, dieses natürliche Solubilisierungspotential im Rahmen eines naturnahen Verfahrens zur Sedimentaufberereitung und -reinigung zu nutzen.