Environ. Sci. Technol. 2004, 38, 1582-1588
Bioremediation Process for Sediments Contaminated by Heavy Metals: Feasibility Study on a Pilot Scale H . S E I D E L , * ,† C . L O ¨ SER,‡ A. ZEHNSDORF,† P. HOFFMANN,† AND R. SCHMEROLD§ UFZ Centre for Environmental Research Leipzig-Halle, Department of Remediation Research, Permoserstrasse 15, D-04318 Leipzig, Germany, and Bauer und Mourik Umwelttechnik GmbH & Co., In der Scherau 1, D-86529 Schrobenhausen, Germany
The core stages of a sediment remediation processsthe conditioning of dredged sludge by plants and the solid-bed leaching of heavy metals using microbially produced sulfuric acidswere 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 sulfuroxidizing 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 costeffectiveness assessment of the remediation process indicates it to be a suitable treatment for restoring polluted sediments for beneficial use.
Introduction The pollution of sediments by heavy metals is still an unsolved environmental problem all over the world. Contaminants may be released under changing physicochemical conditions and pollute water, plants, and livestock. Currently, polluted dredged sediment is mainly disposed of in landfills or underwater sites (1). However, landfilling polluted sediment has several disadvantages and cannot be regarded as a longterm solution (2). Far fewer remediation techniques are available for contaminated sediment than for soil. The wide range of contaminants and the hard properties of muddypasty sludge sediments are more difficult to treat than most * Corresponding author phone: +49 341235 2207; fax: +49 341 235 2492; e-mail:
[email protected]. † UFZ Centre for Environmental Research Leipzig-Halle. ‡ Current address: Institute of Food Technology and Bioprocess Engineering, Dresden University of Technology, D-01062 Dresden, Germany. § Bauer und Mourik Umwelttechnik GmbH & Co. 1582
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other waste materials. The only widely used technique for sediment treatment is the separation of less polluted sand fractions in order to minimize the contaminated portion that requires dumping (3-4). For ecological and economic reasons, economically feasible remediation methods which can treat contaminated sediments such that they can be restored for beneficial use are urgently needed. Much research has been done on the possible application of biological methods to remove heavy metals. The microbial oxidation of sulfidic minerals and elemental sulfur (S0), causing sometimes serious environmental problems, can also serve to remove metals under controlled conditions. The activity of sulfur-oxidizing Acidithiobacilli has been intensively studied as a way of removing heavy metals from contaminated soil (5-8), sewage sludge (9-12), and sediment (13-16). Most of these studies were performed in suspension. The hallmarks of laboratory processing are the aerobic leaching of suspensions with solids contents between 2% and 10%, the addition of high portions of S0 (5-30%, w/w) as a nutrient and acid source, sometimes the initial acidification with acid, and often inoculation with a Acidithiobacillus culture. Under optimum conditions a series of heavy metals could be removed to a high extent. Bioleaching of suspensions with S0 as a substrate was found to be better than purely acid treatment for heavy metal solubilization (9,17) and enables the reduction of the sulfuric acid required for acidification (18). Suspension leaching is not economically feasible for treating large amounts of sediment. An alternative is solidbed leaching similar to the heap leaching of ores. In a solid bed, the density and particle size distribution of the solid matrix are important factors regulating mass transport, sulfur oxidation, and microbial activity (19). However, the solidbed leaching of heavy metals from waste is scarcely reported. A previous study demonstrated heavy metal removal from contaminated soils in a system comprising solid-bed bioleaching and bioprecipitation stages (6). The soils were enriched with 1% S0 and, after inoculation with a sulfuroxidizing bacteria culture, percolated with a liquid medium in a 15 L tank. Approximately 69% of the main heavy metals present were removed from an industrially contaminated soil after 175 days. Unlike soils, dredged anoxic sediment has proven unsuitable for solid-bed leaching due to the low permeability of the sludge, the vigorous activity of the heterotrophic microflora, and the oxygen deficiency (20). However, improving the physicochemical properties and the structure of dredged sediments by pretreatment with plants enabled leaching of heavy metals in laboratory percolator systems with nearly the same efficiency as suspension leaching. This approach forms the basis of a previously presented sediment remediation process consisting of two core stages: the conditioning of anoxic dredged sediment by plants and the solid-bed bioleaching of the resulting soil-like oxic material (21, 22). Nevertheless, good performance in the laboratory does not guarantee similar efficiency on a large scale since important process parameters can be carefully controlled in the laboratory and fluctuations are usually avoided. Some influences on process operation under practical conditions such as the effects of solid inhomogeneity, channeling, or pH and temperature gradients can only be studied reliably on a pilot scale. To be applicable in a commercial decontamination plant, the process must be scaled up without loss of efficiency. Using a highly polluted river sediment, we describe the sediment conditioning in a pilot plant consisting of a number 10.1021/es030075d CCC: $27.50
2004 American Chemical Society Published on Web 01/23/2004
TABLE 1. Physicochemical Parameters of Weisse Elster River Sediment after Dredging and Conditioning for 24 Weeks by Plants (Uncertainity of Analysis Typically (5%) sediment conditioned parameter water content (%) pH value buffer capacitya (g/kg) sulfur oxidation degreeb (%) mobile Zn (mg/kg) portion of the fractionc 1000 mm (%) water permeabilityd (m3/m2/h)
sediment freshly dredged
without vegetation
with spontaneous vegetation
with P. australis
with P. arundinacea
68 7.2 48.0 8.5 0.6
54 6.6 21.6 27.4 78
50 6.1 15.7 40.6 75
52 6.1 19.6 35.7 39
49 6.2 14.7 39.1 88
96.4 0.6 3.0 0.0
94.8 3.1 2.1 1.2
91.7 7.1 1.2 1.8
93.1 5.9 1.0 1.6
80.9 10.9 8.2 1.9
a Buffer capacity ) consumption of H SO to reach pH 2.8. 2 4 from 0.5 m depth. d Mixed sample from oxidized zone.
b
Sulfur oxidation degree ) SO4-S in relation to the total-S content. c Mixed sample
of 50 m3 basins and the operation of a leaching pilot plant containing a solid-bed reactor with a 2000 L working volume, both developed on the basis of above-mentioned laboratory studies (21, 22). The objectives of this study were (i) to evaluate the potential of suitable plants to dewater and oxidize polluted dredged sludge in order to produce a soil-like material at depths of about 2 m, (ii) to examine the course of bioleaching by usual process parameters such as temperature, pH value, sulfate formation, metal solubilization, O2 consumption, CO2 production, and metal removal, and (iii) to estimate the utility and cost-effectiveness of the sediment remediation process on a practical scale.
Experimental Section Sediments. The sediments originated from a detritus trap in the Weisse Elster River near Kleindalzig, 20 km south of the city of Leipzig (Saxony, Germany). This river originates in the Western Ore Mountains in the Czech Republic. The detritus of the river is highly polluted by heavy metals from previous uranium and tin mining (23). The dredged sediment from the trap is occasionally pumped into settling basins on a neighboring sediment deposit. In the basins, which each hold 30 000 m3, the dredged material settles into an unpolluted gravel and sand fraction and a highly polluted fine fraction. Generally, only the sediment fine fraction underwent further treatment. The sludge taken from a settling basin had a water content of 70% and was anoxic, muddy-pasty, and impermeable to water. The fraction >1000 µm was 3%. Its heavy metal content was (in mg/kg dry mass) as follows: Cd ) 9, Co ) 42, Cr ) 270, Cu ) 142, Mn ) 1326, Ni ) 136, Pb ) 135, and Zn ) 1958. The sediment contained 0.7% total S; the sulfur oxidation degree was about 8.5%. The sediment also comprised 20% organic matter (determined as loss on ignition). The pollution of the sediment with hydrocarbons and phenols was negligible. The anoxic sediment was used for conditioning experiments to prepare it for subsequent bioleaching. Oxic sediment from the deposit was taken directly for bioleaching tests without any pretreatment. The sediment, which had been stored for some years in the open deposit, had spontaneously turned oxic and was soil-like and very permeable to water (22). The metal content of this ‘longterm stored sediment’ was about 1.5 times higher than that of the freshly dredged material (see Table 2). The oxic sediment contained 1.4% total S; the sulfur oxidation degree was about 65%. Sediment Conditioning. The pilot plant for conditioning consisted of several basins each with a base of 5 × 5 m and a volume of 50 m3. All the basins were lined with plastic foil, filled with gravel to a height of 0.3 m as drainage covered
with textile fabric, and fitted with acrylic-glass tubes for root examination using the minirhizotron video camera technique (24). Each basin was filled with about 45 m3 sludge to a depth of 1.8 m. Two basins were planted with the helophytes Phragmites australis and Phalaris arundinacea, respectively, and two basins remained unplanted. Vegetation spontaneously arising from the natural seed potential of the sludge was regularly removed in one unplanted basin, while the development of vegetation in the other basin was not disturbed. The duration of the conditioning test was 24 weeks. Plant development and changing sediment characteristics were analyzed every 4 weeks. Two samples were taken from the whole sediment layer of each basin using Riverside drills with a diameter of 7 cm (Ejkelkamp, The Netherlands). The cores were mixed providing one homogeneous sample of each basin. To determine vertical changes in sediment characteristics, additional samples were taken from different depths of the sediment layer. After 24 weeks, the over-ground plant biomass was harvested, prepared, and analyzed as described below. Sediment Leaching and Process Water Treatment. Solidbed leaching was performed in a percolator system consisting of a solid-bed reactor containing the sediment and a liquid tank holding the circulating process water (Figure 1). The solid-bed reactor had a base of 1 × 1.5 m and a height of 2 m and was made of stainless steel, lined by a plastic foil, and fitted with a drainage and sprinkler installation. The liquid tank was an 80 L plastic vessel. Naturally moist oxic sediment of 1000 kg dry weight was mixed with 20 kg of elemental sulfur and placed in the reactor, resulting in a bed height of 1.3 m. During the acidification period the solid bed was permanently aerated with about 600 L/h and sprinkled with 50 L of process water once a day. No inoculation of the sediment with sulfur-oxidizing bacteria was carried out. The process water remaining in the solid bed (the initial water content of the sediment was lower than its waterholding capacity) was completed just before sprinkling by filling the liquid reactor with water to a volume of 50 L. The solid-bed temperature, gas flow, and gas composition were continuously measured (the latter with an Enviromax-C respirometer, Columbus Instruments, Ohio). Sampling of the sediment was carried out in the same manner as described above. After 42 days of leaching, the solubilized metals were washed out from the sediment package 10 times with water (1000 L/run). The leachates were alkalized with Ca(OH)2 in a stirring reactor, and the precipitates containing the heavy metals were separated by a filter press (25). VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(Spectro Inc.) according to the German standard DIN 38406E22 method.
Results and Discussion
FIGURE 1. Diagram of the pilot-scale percolation system for the bioleaching of sediment contaminated by heavy metals consisting of a solid bed and a liquid reactor (working volumes of 2000 and 50 L). During the acidification period, the process water circulation was 50 L/day and the air flow through the solid bed was about 600 L/h (continuous lines). During the washing period, the sediment package was rinsed with 1000 L/day (broken lines). Sediment Revitalization. Revitalization was performed by adding CaCO3 and compost to the leached sediment. To examine the mobility of metals, charges of revitalized sediment weighing 4 kg dry weight were stored in 10-L pots for 450 days. Samples were taken regularly and eluted with water according to the German standard method DIN 38414S4. The influence of the weather on mobility of metals was studied by storing revitalized sediment in 20-L lysimeters which had been exposed in the open for 210 days. The leakage water and sediment samples were taken at intervals of 30 days. Analytical Methods. The water content, pH, and redox potential of sludges and solids were analyzed according to the German DIN 38414 method. The oxidation status of the sediments was assessed by measuring the redox potential and determining the sulfur oxidizing degree. The buffer capacity was estimated by the titration of sediment suspensions with 1 M H2SO4 at 6 mL/h using a DMS-Titrino 716 (Metrohm, Switzerland). The particle size distribution of solids was determined by sieve analysis according to the German DIN 19683 method as well as by laser diffraction spectrometry (21) using a Helos laser diffraction spectrometer (Sympatec, Germany). The water permeability of sediment was tested in a percolator system consisting of a column with a diameter of 0.1 m, a storage vessel, and a peristaltic pump. A 1 kg dry weight amount of sediment was percolated with water at a constant rate. Once a steady state had been reached, the flow rate was raised gradually until the flow exceeded the permeability of the bed. The metal and total-sulfur content of solids such as sediment and precipitation sludge were analyzed by wavelength-dispersive XRF using an SRS 3000 spectrometer (Siemens, Germany), sometimes with prior extraction of the solids with water to remove mobile metals (21). The SO4-S content was determined from the intensity of the S-Kβ 1 satellite peak of the XRF spectrum. Mobile metals in the sediment were determined by extraction samples of 20 g dry weight with a total volume of 0.2 L of water (including the pore water of the sediment) for 1 h on a laboratory shaker. Freshly dredged sediment was eluted under a nitrogen atmosphere to avoid any oxidation. Dissolved metals and sulfate in aqueous samples were determined by ICP-AES using an Elan 500 Spectroflame 1584
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Development of Plants during Sediment Conditioning. Some helophytes and grasses, especially those with a high transpiration rate and an extended root system, have been found to be favorable for sediment conditioning when used on sediment layers of 0.5 m (21). To determine the conditioning potential of the plants, the pilot-scale test was performed with sludge on layers of 1.8 m. Reed (P. australis) was used as precultivated plants (10 plants/m2) to ensure the quick development of vegetation. The sludge was overgrown within 12 weeks. However, the reed hitherto mainly used for activated-sludge dewatering developed poorly above ground was colonized by aphids, became chlorotic, and began to break off after 16 weeks. This damage can be attributed to the excessive supply of mineral nutrients, especially nitrogen (26). The sediment layer was rooted to 0.45 m on average after 24 weeks. Reed canary grass (P. arundinacea), a deep-rooting grass of wet sites and muddy river banks, was also applied as precultivated plants (10 plants/m2). The grass produced plenty of above-ground biomass, suppressed the evolution of other vegetation, and did not show any diseases. The ground was completely covered within 10 weeks, and the sediment was rooted to 0.70 m on average after 24 weeks. Most species that developed spontaneously from the natural seed potential of the sludge in the basin without planting were only found in few specimens with the predominance of knot-weed (Polygonum lapathifolium). The spontaneously developed plants formed nearly groundcovering vegetation within 20 weeks. The sediment was rooted to 0.45 m on average. Changes in Sediment Characteristics during Conditioning. The purpose of conditioning is to improve the sediment properties for subsequent solid-bed bioleaching within a short period. The changes in sediment properties during conditioning are summarized in Table 1. Freshly dredged sediment mainly consists of small particles 30% (36). Stabilization by fixation is also an inexpensive process (Euro30-125/t) and available for almost all sediments but is considered environmentally harmful and therefore not accepted by many authorities. The high costs of incineration (Euro200-1000/t) make this method economically unacceptable. At present on the German market, the costs of any sediment treatment mostly exceed the costs of disposal (Euro20-200/t). Soon, land disposal of polluted dredged material will be restricted by legal regulations. and so technologies for the pretreatment and treatment of sediments will have to be increasingly used. Regarding the disadvantages of traditional treatment methods, such as limitations of sediment washing by particle size and organic matter or risks of disposal and fixation due to the long-term release of contaminants, the remediation method described here offers a suitable alternative for isolating toxic metals and restoring the treated material for beneficial use. The treatment is environmentally friendly VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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since it works at ambient temperatures, consumes little energy, requires no toxic chemicals, and produces low amounts of waste. The costs are expected to decrease with the scaling-up of treatment plants and commercialization on the market. However, to establish the proposed sediment treatment in practice, further work is necessary to improve the efficiency and applicability of the process.
Acknowledgments We gratefully acknowledge the kind financial support for this work provided by the foundation Deutsche Bundesstiftung Umwelt (A.Z. 12099). We would like to thank Mrs. C. Pietsch for her technical assistance and Dr. R. Wennrich and Dr. P. Morgenstern from the UFZ Department of Analysis for providing the heavy metal analysis data.
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Received for review June 16, 2003. Revised manuscript received October 20, 2003. Accepted November 28, 2003. ES030075D
