Biogeochemistry of Chelating Agents - ACS Publications - American

Chapter 23

Soil Washing Using a Biodegradable Chelator

Downloaded by UNIV OF ARIZONA on May 4, 2013 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch023

Domen Leštan andBoštjanKos Agronomy Department, Biotechnical Faculty, University of Ljubljana, 1000 Ljubljana, Slovenia

Remediation of heavy metal contaminated soils with soil washing methods involves addition of water with chelators, or acids in combination with chelators. This can be done in situ or ex situ in reactors with soil slurry, and on site as a heap leaching. Chelators are used to enhance heavy metal solubility in soil solution from the soil solid phases, where the major part of soil heavy metals usually resides. A novel method of soil washing using biodegradable chelator [S,S]-stereoisomer of ethylenediamine-disuccinate (EDDS) and a horizontal permeable reactive barrier was proposed, and evaluated for Pb and Cu contaminated soil. During laboratory studies of EDDS enhanced in situ soil washing and heap leaching, biodegradable heavy metal-EDDS complexes were microbially degraded and the released heavy metals chemically immobilized in the barrier. After remediation the barrier material enriched with heavy metals was removed from the soil.

© 2005 American Chemical Society In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

383


384 Heavy metals make a significant contribution to soil contamination. The contamination has resulted from a combination of industrial, urban, and modern agronomical practices. The primary sources of this pollution are the burning of fossil fuels, mining and smelting activities, and the use of fertilizers, pesticides and sewage sludge. Some heavy metals: V , Cr, Mn, Fe, Cu, Zn, Mo and N i are considered to be essential micronutrients for at least some forms of life; others have no known biological function. A l l heavy metals at high concentrations have strong toxic effects and are an environmental threat. Heavy metals enter the body in food (i.e. crops grown on heavy metal contaminated soil), ingestion of soil, or inhalation of dust. A n increasing body of evidence suggests that soil organisms, vitally important for soil health and fertility, are sensitive to heavy metal stress (/) and that the biological diversity of the soil is reduced by heavy metal contamination (2). In future, the availability of arable land may decrease because of stricter environmental laws limiting food production on contaminated land. Already national farmers and consumer organizations in European Union and Associated Countries do not recognize organic/ecological farming on soils contaminated with heavy metals. Soil remediation is just one, but absolutely necessary step in rehabilitation of contaminated land (Figure 1). It is often the most expensive measure and as such crucial for a final decision on soil rehabilitation.

Soil survey Risk assessment » Remediation Ex situ:

In situ:

• Soil excavation/disposal * Soil capping • Soil washing (soil slurry) * Soil washing • Soil washing (heap leaching) * Solidification/stabilization • Elektrokinetic remediation * Phytoextraction Land management for prevention of recontamination: • Permanent plant cover • Wind barriers • Improvement of soil structure —

—

—

Accompanying measures: . Monitoring • Education

Figure 1. Steps in rehabilitation of heavy metals contaminated soil Remediation costs are often too high and outweigh environmental impact, health hazard, legislation requirements and public sensitivity towards contaminated lands.

Remediation of Heavy Metal Contaminated Soil Remediation of sites contaminated with toxic metals is particularly challenging. Unlike organic compounds, metals cannot be degraded, and

In Biogeochemistry of Chelating Agents; Nowack, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


385 cleanup requires their immobilization and toxicity reduction or removal. The selection of the most appropriate soil remediation method depends on the site, soil characteristics and heavy metals concentration. At present, various approaches have been suggested for remediating heavy metal contaminated soils (3). However, no methods are available that give satisfactory results in dealing with the problem. Solidification/stabilization contains the contaminants in an area by mixing or injecting agents such as cement and lime, and for Pb contaminated soil various phosphate sources (i.e. apatite), which form insoluble salts with Pb (4). Heavy metals are less available for plants and the bioconcentration through the food chain is reduced. However, toxic metals remain in the soil and can be harmful through soil ingestion or inhalation of soil dust. Electroremediation is proposed as an in situ method for remediation of blocks of heavily contaminated soil. Current designs are not appropriate for decontamination of surface soil layers. Electroremediation methods mostly involve electrokinetic movement of charged particles suspended in soil solution. It is therefore questionable whether heavy metals are removed from solid organic and inorganic fractions, where they predominantly reside (Pb in particular) in most soils (5). Soil washing currently involves ex situ extraction of heavy metals from soils with chelators or acids, in reactors or as heap leaching. The separation of metals from waste chelator/acid solution after extraction has not yet been adequately addressed. Soil washing in reactors involves stringent physical treatments and is harsh for the soil structure and flora. Chemical agents involved can dramatically inhibit soil fertility, with subsequent negative impacts on the ecosystem. Phytoextraction is a * soil-friendly', publicly appealing alternative. However, for Pb, one of the most widespread contaminants, no real

Table 1. Cost of heavy metal contaminated soil remediation Soil Treatment Asphalt capping Soil capping Excavation/ heavy metal stabilization/ disposal Heavy metal imobilization Soil washing Soil vitrification Phytoextraction with hyperaccumulating plants Chelator induced phytoextraction SOURCES: References 5, 7,8.

Cost ($)

600 270 450 400-800 2-10

Cost ($/ha) 160.000 140.000 1.500.000

200-100.000 260.000


386


hyperaccumulating plant, with high Pb uptake and high biomass essential for effective phytoextraction, has been reported so far. This is presumably because only a small portion of the Pb in soil is present in soil solution or exchangeable from soil colloids and thus phytoavailable (5). There have been reports of high Pb phytoaccumulation when Pb in soil was mobilized by a chelator addition. However, chelator mobilized heavy metals are leached through the subsoil and pose a threat of groundwater contamination (6). Many of these technologies have been used full-scale but can be prohibitively expensive (Table 1).

Soil Washing Using Biodegradable Chelator and Horizontal Permeable Reactive Barrier The literature to date reports a number of chelators that have been tested for cheiator-induced soil washing. These include ethylenediamine tetraacetate (EDTA) and its structural analogues: trans-l,2-diaminocyclohexane-iV;MiV ^tetraacetic acid (CDTA), dietyienetriamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA) (9, 10). In most cases, the EDTA treatment was superior in terms of soil washing efficiency. However, side-effects related to the soil addition of chelators cause health, safety and environmental concerns. Many synthetic chelators and their complexes with heavy metals are toxic (//) and poorly photo-, chemo- and biodegradable in soil environments (12). A combined widespread use of fertilizers and slow decomposition has led to background concentrations of EDTA in European surface waters in the range 10-50 mg V (13). Recently biodegradable chelator [S,S]-stereoisomer of ethylenediaminedisuccinate (EDDS) was proposed for remediation of heavy metal contaminated soils with soil washing (14, 15). The use of horizontal permeable reactive barriers to prevent leaching of heavy metals (Figure 2) was proposed in a novel in situ soil washing method (15). EDDS was first isolated as a metabolite of the soil actinomycete Amycolatopsis orientalis (16). It is therefore naturally present in soil, where it is easily decomposed into benign degradation products (17). The [S,S]-isomere of EDDS is readily biodegradable using the criteria stipulated by the organization for economic co-operation and development (OECD). The OECD criteria state that 60% of the compound must biodegrade within 28 days. For EDDS the final C 0 yield exceeded 80% after 20 days, assessed by the modified Sturm test (1719). Mineralization of EDDS in sludge-amended soil was rapid and complete in 28 days. The reported calculated half-life was 2.5 days. Jaworska et al. (20) r

1

2


387 assessed environmental risks for use of EDDS in detergent applications. The toxicity to fish and daphnia was low (EC > 1000 mg/L). Presently, it is the only commercially available chelator that is naturally present in soil, where it is readily decomposed into benign degradation products. Theoretically, chelator extraction efficiency depends on the stability constant (logK) of metal-chelator complex formation. Extensive database compiled by Martel et al. (21) indicate that logK of EDDS complexes with most heavy metals is comparable to logK of benchmark chelators such is EDTA. However, for Pb (one of the most ubiquitous soil contaminants) reported logK of EDTA (18.0 at 25 °C and ionic strength (μ) = 0.1) is substantially higher than of EDDS complex (12.7 at 20 °C, and μ = 0.1). Nevertheless, the comparison of logK data needs to be considered with caution. Speciation in a metal-chelator system is controlled by the concentration of all metals and chelators, the logK of all complexes and by the kinetic of coordination reactions. In addition, other chelator reactions, adsorption in the soil solid phases, mineral dissolution and chelator degradation, are substantially affected by the chelated metal ion (22). For example: Vandevivere et al. (23) investigated EDDS for its applicability for the ex situ washing extraction of Pb, Zn, and Cu from soil, sewage sludge, and harbor sediments. They reported that extraction efficiency of EDDS for Zn, Cu and also for Pb was equal or superior to those obtained with EDTA and NTA.


50

Figure 2. Flowsheetfor in situ soil washing using biodegradable chelator and horizontal permeable reactive barrier.

In the proposed method a horizontal permeable reactive barrier is placed below the layer of contaminated soil. It is composed of substrates for enhanced microbial activity and sorbents for heavy metal immobilization. Excess water flows through the barrier as essentially heavy metals-clean soil water. When the


388 targeted level of soil cleansing is reached, after one or several cycles of chelator addition, the barrier material can be excavated and deposited, and the contamination thus removed from the soil (Figure 2). We borrowed the concept of permeable reactive barrier from groundwater remediation, where the barrier is constructed below ground as a vertical underground wall, filled with reactive materials. The barrier is built by digging a long, narrow trench in the path of the polluted groundwater. Clean groundwater flows out of the other side of the wall. Various reactive materials have been investigated and include zeolite, elemental iron and limestone. Reactive materials in the barrier trap harmful chemicals or change the chemicals into harmless ones. For example, zero-valent iron can be used for reduction of toxic C r to harmless Cr , and limestone for Pb precipitation. Downloaded by UNIV OF ARIZONA on May 4, 2013 | http://pubs.acs.org Publication Date: July 21, 2005 | doi: 10.1021/bk-2005-0910.ch023

6+

3+

Chelator Induced Soil Washing of Pb Lead contamination of soils is one of the world's most prevalent public health problems, especially through Pb intake in concentrations regarded as non-toxic over extended periods. Once released from industrial and agricultural sources of contamination into the soil matrix, Pb forms strong bonds with the solid soil fractions and is generally retained in the surface soil layer (24). The adverse impact of Pb on environmental quality and on human health thus persists for long periods. Pb is linked with failures of human reproduction (4). It also causes metabolic disorders and neurophysiological defects in children, and affects the haematological and renal systems. The feasibility of in situ soil washing using EDDS and horizontal permeable reactive barriers was examined by simulating the process in 15 cm diameter soil columns, equipped with trapping devices for leachate collection. Barriers were positioned 20 cm deep in the soil (except for control columns with no barrier). Soil was collected from the 0-30 cm surface layer at an industrial site of a former Pb and Zn smelter in the Mezica Valley in Slovenia. The following soil properties were determined: pH (CaCl ) 6.8, organic matter 5.2%, total Ν 0.25%, sand 55.4%, coarse silt 12.0%, fine silt 18.9%, clay 13.7%, total available Ρ 81.4 mg kg" , total available Κ 38.2 mg kg" , C 0 " 125.6 g kg" , Pb 1400 mg kg' , Zn 800 mg kg" . The soil texture was sandy loam. 2

1

1

2

1

3

1

1

Functioning of Horizontal Permeable Reactive Barrier Bio-degradability of the heavy metal-chelator complex and immobilization of released heavy metals is essential for horizontal permeable reactive barrier to function. The functioning of the barrier was examined by using biodegradable


389 chelator EDDS and non-biodegradable EDTA. EDDS and EDTA were used in concentration of 10 mmol kg" of soil and applied in 200 mL of deionized water with pH 9.35 and 4.25, respectively. Soils were irrigated 2 times a week with 50 mL kg" soil tap water for 9 weeks after chelators application. Barriers were composed of 1.5 cm wide substrate layer of nutrient enriched peat, acrylamide hydrogel or vermiculite, followed by a sorption layer of apatite/soil mixture (EDDS treatment only). The mass balance of Pb leached is shown in Table 2. In columns with no chelator addition and no barrier installed, the concentrations of Pb in leachates were under the detection limit of the instrument (

Biogeochemistry of Chelating Agents - ACS Publications - American

Recommend Documents