Phytostabilization of a metal contaminated sandy soil. I: Influence of compost and/or inorganic metal immobilizing soil amendments on phytotoxicity and plant availability of metals

Abstract

In a lysimeter set-up, compost addition to an industrial contaminated soil slightly reduced phytotoxicity to bean seedlings. The “Phytotoxicity Index” (on a scale from 1 to 4) decreased from 3.5 to 2.8. The same treatment also reduced metal accumulation in grasses: mean Zn, Cd and Pb concentrations decreased respectively from 623 to 135, from 6.2 to 1.3 and from 10.7 to <6 mg kg⁻¹ dry weight. When combined with inorganic metal immobilizing amendments, compost had a beneficial effect on plant responses additional to the inorganic amendments alone. Best results were obtained when using compost (C) + cyclonic ashes (CA) + steel shots (SS). The “Phytotoxicity Index” decreased to 1.7, highest diversity of spontaneously colonizing plants occurred, and metal accumulation in grasses reduced to values for uncontaminated soils. Based on the first year evaluation, C + CA + SS showed to be an efficient treatment for amendment assisted phytostabilization of the contaminated Overpelt soil.

Introduction

Metal-contaminated soils contribute to human and animal metal exposure, through food chain transfer, inhalation of wind blown dust, or direct ingestion of soil (Pierzynski, 1997). At the most severely contaminated sites, where biodiversity is extremely reduced, and plant growth is strongly inhibited, there is a significant risk of off-site migration of the contaminated soil and leaching of contaminants into the groundwater (Vangronsveld and Cunningham, 1998). This dispersion of metals increases the likelihood of human and animal exposure. In response to a growing concern for human health and environmental quality, many technologies have been developed to treat and remediate metal-contaminated soils, in an attempt to replace or complement the very expensive and invasive excavation and soil washing scenarios (Cunningham et al., 1995). One of the remediation options gaining considerable interest over the last decade is the in situ immobilization of metals using metal immobilizing agents (van der Lelie et al., 2001, Vangronsveld and Cunningham, 1998). In contrast to the more classical remediation techniques such as solidification, vitrification, or soil washing (Iskandar and Adriano, 1997), in situ immobilization techniques aim to reduce contaminant exposure, without being destructive to structure and biological activity of soils. Moreover, no by-products are generated. In situ metal immobilization can be used in combination with phytostabilization approaches. Sorption or precipitation reactions induced by the soil amendments decrease the concentration of contaminant mobile pools in the amended soil. A corresponding reduction in the plant available metal fraction allows revegetation and ecosystem restoration on heavily contaminated sites. The vegetation reduces or even prevents the dispersion of the contamination through wind and water erosion, and improves the aesthetic value of formerly bare areas (Vangronsveld and Cunningham, 1998). The vegetation itself may contribute to metal immobilization processes through biological activities and the production of organic matter (Bouwman and Vangronsveld, 2004).

Cyclonic ashes (“Beringite”) are described as modified aluminosilicates originating from the fluidized bed burning of coal refuse from the former coal mine in Beringen (Belgium) (Vangronsveld et al., 1999). These ashes have been successfully applied to reduce metal mobility and phytotoxicity in metal contaminated soils in greenhouse and field experiments (Vangronsveld and Clijsters, 1992, Vangronsveld et al., 1995a, Vangronsveld et al., 1995b). A healthy and sustainable vegetation cover established at an abandoned barren zinc smelter site with soil metal contents up to 13,250 mg kg⁻¹ Zn (sandy soil; pH-H₂O 5.5) (Vangronsveld et al., 1999, Bouwman et al., 2001). Twelve years after treatment the nematode fauna was still improving (Bouwman and Vangronsveld, 2004); metal availability remained low, and no increase of phytotoxicity was observed (Ruttens et al., unpublished results). Several other studies have indicated that steel shots are an effective treatment to reduce metal mobility and its accumulation in crops (Mench et al., 1994, Mench et al., 1997, Mench et al., 2003, Sappin Didier et al., 1997). Steel shots (iron grit) are an industrial, iron bearing material (97% α-Fe) intended for shaping surfaces (Mench et al., 1998). Basically the working mechanism of steel shots is based on Fe and Mn release, followed by the formation of Fe and Mn oxides, and sorption and/or co-precipitation reactions of metals with these oxides; the action mechanism of cyclonic ashes is based on alkaline properties, sorption on clay particles and (co-)precipitation reactions (Boisson et al., 1999b, Mench et al., 1998).

The present study evaluates the potential of the combined application of cyclonic ashes and steel shots (CA + SS) to phytostabilize a contaminated substrate located at an old Zn smelter site in Overpelt (Belgium). Since 1993, only secondary Zn production occurred at this plant. Boisson et al., 1999a, Boisson et al., 1999b have shown, based on chemical extractions with Ca(NO₃)₂, that the combined use of cyclonic ashes and steel shots increased the metal-immobilizing capacity of these amendments compared to the application of each of the compounds separately. These authors investigated 6 contaminated soils of different origin, among them also a soil from the site in Overpelt, which is subject of the present study (OV1). The combination of CA (5% w/w) + SS (1% w/w) generally reduced exchangeable fractions of Zn, Cu, and Ni more than treatment with phosphates applied as hydroxyapatite (HA 5% w/w). Cadmium and Pb exchangeability on the contrary were reduced more by HA (5%) than by CA (5%) + SS (1%). For all these elements, CA (5%) + SS (1%) was more efficient than HA (1%). The CA + SS treatment had the advantage that it also diminished water extractable As concentrations, whereas this concentration was strongly increased after HA treatment. Meanwhile other studies have confirmed the effectiveness of the combined CA + SS treatment (Geebelen et al., 2003, Mench et al., 2003), which is particularly promising to remediate mixed arsenic and metal contaminated soils. The use of combinations of treatments has been shown to be effective also in other studies (Brown et al., 2004, Hettiarachchi and Pierzynski, 2002, Hettiarachchi et al., 2000).

The soil in Overpelt has a sandy texture and is, like many other industrial sites, characterized by a low fertility and a low water-holding capacity (WHC), which may hamper revegetation success. Soil fertility and WHC can be ameliorated by compost addition (Brady, 1984). Organic matter additions may also increase soil pH and add exchange capacity which both contribute to a decrease of metal availability. Different types of organic matter have been evaluated for their capacity to immobilize metals (Basta et al., 2001, Berti and Cunningham, 1997, Brown et al., 2003a, Brown et al., 2003b, McBride and Martinez, 2000, Shuman et al., 2002). On the other hand, organic matter additions can increase the amount of soluble organic ligands and eventually lead to the opposite effect (Martinez and Mc Bride, 1999; Shuman, 1998). The objective of the present study was to investigate the success or failure of the CA + SS treatment on a semi-field scale and to evaluate the effect of a combined treatment with compost and mineral soil amendments. The C + CA treatment was added because it has resulted in a successful and sustainable revegetation at another site in the same contaminated area (Bouwman et al., 2001, Bouwman and Vangronsveld, 2004; Vangronsveld et al., 1995a, Vangronsveld et al., 1996). The experiments were conducted outdoors using lysimeters, allowing us to study plant responses and metal leaching. The results on metal leaching are reported in a next paper (Ruttens et al., 2006).