Environ. Sci. Technol. 1998, 32, 2154-2161
Utilization of Biomass Residues for the Remediation of Metal-Polluted Soils K L A U S F I S C H E R , * ,† H A N S - P E T E R B I P P , ‡ PETER RIEMSCHNEIDER,§ PETER LEIDMANN,⊥ DIETER BIENIEK,‡ AND ANTONIUS KETTRUP† GSF-National Research Center for Environment and Health, Institute of Ecological Chemistry, Ingolsta¨dter Landstrasse 1, D-85764 Neuherberg, Germany, and Ecological Chemistry and Environmental Analytics, Technical University of Munich, D-85350 Freising-Weihenstephan, Germany
The utilization of biomass residues as sources for natural chelates is a new approach to improve the ecological and economical balance of leaching techniques for the remediation of metal-polluted soils. Residues, such as molasses, blood meal, and silage effluents, containing various aliphatic carboxylic acids, sugar acids, and amino acids or their precursor compounds were selected, hydrolyzed and oxidized, if required, and analyzed for their organic constituents. Soils that were contaminated with metals via sewage sludge amendment were extracted in batch and column experiments at various pH values. Grass silage effluent removed ∼75% of Cd and >50% of Cu and Zn at pH 4.4. The neutralized effluent was less effective except for Cu (69% leached). The neutral blood meal hydrolysate extracted primarily Cu (55-66%) and Ni (38-67%). Metal bonds attacked by this extractant were identified using a sequential leaching procedure. Hydrolysates containing sugar acids mobilized Cu and Pb under alkaline conditions. The actual results support the conclusion that biomass residues have a potential to serve as extractants in remediation techniques.
Introduction The extraction of heavy metals with organic chelating agents is one of the key techniques for the physicochemical remediation of polluted soils. Whereas investigations on a bench-scale level have been conducted since the early 1970s, pilot-scale feasibility studies go back to the mid 1980s (1). Despite the intensive knowledge gained meanwhile in this field, further efforts were undertaken to optimize existing treatment methods and to enlarge the potential area of application (2-5). New perspectives open up with developments to utilize metal solubilization by chelates as a process element within other remediation techniques such as phytoremediation (6, 7), extraction by surfactants (8) or electrokinetic remediation (9). * Author to whom correspondence should be addressed (phone: +4989-31872777; fax: +4989-31873371; e-mail:
[email protected]). † Technical University of Munich. ‡ GSF-National Research Center for Environment and Health. § Present address: SAKOSTA Gmbh, D-81249 Mu ¨ nchen, Germany. ⊥ Present address: Labor Wilhelm, D-86529 Schrobenhausen, Germany. 2154
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 14, 1998
The majority of the soil extraction studies tested the leaching capability of synthetic chelating agents such as aminopolycarboxylic acids (EDTA, NTA, DTPA) with increasing focus on EDTA. Arguments for their utilization are high leaching efficiencies, high thermodynamic stabilities of the formed complexes, good solubilities of metal chelates, normally low adsorption of the chelating agents and of their metal compounds on soils, and high stabilities against chemical and biological transformation. Despite these practical benefits, the utilization of these compounds in soil treatment processes is controversial, especially with regard to in-situ applications. Due to the minimal or slow degradation of EDTA and related compounds in various soil environments (10), long-term influences on speciation and bioavailability of toxic and essential elements cannot be excluded. The risk of groundwater pollution by penetrating chelates and their metal compounds has to be taken into account. Additionally, human toxicological objections against a higher environmental input of NTA, derived from mutagenesis studies (11), are not dispelled completely. One strategy to reduce the potential environmental impact of extraction solutions aims at the utilization of natural or nature-identical chelating agents, belonging to various groups of organic acids, that is, hydroxy carboxylic acids, di/ tricarboxylic acids, sugar acids, and amino acids. Despite lower complex formation constants, their leaching efficiencies are not always essentially lower than those of synthetic aminopolycarboxylic acids, as confirmed by several studies (12, 13). Natural substances for soil cleanup are advantageous because they are generally very biodegradable, integrated in soil carbon cycles and compatible with environmental self-regulation processes (low risk of detrimental effects), and partially generated by plants, fungi, and soil microorganisms to enhance the bioavailability of essential elements and to detoxify heavy metals. To improve the ecological balance as well as the economics of soil remediation by natural chelates, we pursued the idea to utilize biomass residues from agriculture and food engineering as a source for the required compounds. For this purpose, a variety of residues can be considered, which contain either chelates in free chemical form or components easily transformable into the target compounds by chemical or enzymatic reactions. At the same time, the utilization of residues can offer an opportunity for the mitigation of waste disposal problems and for cost reduction of process chemicals. Therefore, our selection of residues was guided by the following criteria: (i) chemical composition and content of utilizable components; (ii) amount and availability; (iii) free supply. Consequently, the objectives of this study were (i) to analyze selected groups of organic ligands in untreated and chemically modified residues; (ii) to examine the capability of residue formulations to extract Cd, Cr, Cu, Ni, Pb, and Zn from polluted soils under various physicochemical conditions; and (iii) to determine the bonding-type specific depletion of the metals from soils.
Materials and Methods Soil Characterization. The soil samples, each a humus, sandy silty loam, were collected at 0-30 cm depth from a former agricultural site north of Munich, Germany. The metal burden was caused by decades of sewage sludge amendment. After collecting, the samples were air-dried and mixed thoroughly in polyethylene screw-top tubes clamped into a reciprocating shaker. The samples were characterized acS0013-936X(97)00620-2 CCC: $15.00
1998 American Chemical Society Published on Web 06/04/1998
TABLE 1. Physical and Chemical Soil Properties soil sample parameter soil type sand (63-2 mm) (%) silt (2-63 µm) (%) clay (
