Water Air Soil Pollut (2012) 223:6059–6067 DOI 10.1007/s11270-012-1340-0
Hexavalent Chromium Dynamics and Uptake in Manure-Added Soil K. Molla & A. Dimirkou & V. Antoniadis
Received: 19 April 2012 / Accepted: 26 September 2012 / Published online: 7 October 2012 # Springer Science+Business Media Dordrecht 2012
Abstract The soil dynamics of hexavalent Cr, a particularly mobile and toxic metal, is of a great environmental concern, and its availability to plants depends on various soil properties including soil organic matter. Thus, in a pot experiment, we added 50 mg Cr(VI) kg−1 soil and studied Cr(VI) soil extractability and availability to spinach, where we applied both natural (zeolite), synthetic adsorptive materials (goethite and zeolite/goethite) and organic matter with farmyard manure. We found that, compared to the unamended control plants, dry matter weight in the Cr(VI)-added soil was greatly decreased to 17 % of the control, and height was decreased to 34 % of the control, an indication of Cr toxicity. Also, exchangeable Cr(VI) levels in soil decreased back to the unamended control even in the first soil sampling time. This was much faster than the exchangeable Cr(VI) levels in the mineral-added soil, where Cr(VI) levels were decreased to the levels of the unamended control in the third sampling time. The positive effect of organic matter was also indicated in the Cr quantity soil-to-plant transfer coefficient (in grams of Cr in plant per kilogram of Cr added in soil), a phyto-extraction index, which was significantly higher in the manure-amended (1.111 gkg−1) than in the mineral-added treatments (0.568 gkg−1). Our findings K. Molla : A. Dimirkou (*) : V. Antoniadis Department of Agriculture, Crop Production and Rural Environment, University of Thessaly, Fytokou Street, Volos 384 46, Greece e-mail:
[email protected]
show that organic matter eliminates the toxicity of added Cr(VI) faster than the mineral phases do and enhances the ability of spinach to extract from soil greater quantities of Cr(VI) compared to mineral-added soils. Keywords Spinach . Transfer coefficient . Chromium reduction . Phyto-extraction
1 Introduction Soil contamination by heavy metals has been an increasing concern due to various anthropogenic inputs (including disposal of industrial wastes, sewage sludges, agrochemicals, etc.) (Papafilippaki et al. 2007; Golten 2011; Zoffoli et al. 2012). Especially Cr is a very toxic element, without any known physiological functions in plant or animal kingdom organisms, and its origins in soil may also derive from the weathering of ultramafic rocks (Economou-Eliopoulos et al. 2012). Chromium is mainly found in two stable oxidation states, Cr(III) and Cr(VI). The hexavalent form has long been recognized as toxic, soluble, bioavailable, and carcinogenic, while the trivalent chromium is rather less mobile in soil (Donatella et al. 2012). Thus the behavior of Cr(VI) has been of a great concern. In recent years, pressures for increasing agricultural production has created the need to utilize soils even in the vicinity of industries, where water streams and soils may well have been affected by a degree of contamination, as is the case of the Thiva basin in South Greece (Economou-Eliopoulos
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et al. 2012). Especially vegetable plants with edible parts are sensitive to Cr(VI) contamination, and the risks of increased transfer of toxic metals into the human food chain considerably increases in case of leafy vegetables, which are to be consumed as raw salads (EconomouEliopoulos et al. 2011). Spinach (Spinacea oleracea) is among the most sensitive vegetables because of the raised concerns over the fate of Cr transfer through it (Patra et al. 2001; Srikanth and Reddy 1991). The risk is even greater when such crops are sprinkle-irrigated with water contaminated with Cr, because Cr-containing substances are likely to stay on leaf surfaces and then to be directly consumed. The greater risk is associated with the fact that one link of the “soil–plant–human” food chain is bypassed, and this effect has long been established for Cr(VI) (e.g., Sheehan et al. 1991), as well as other toxic elements (e.g., the case for Pb in Brady and Weil 2002, p. 820). Over the years, several techniques have been used in order to minimize Cr contamination risks. These include the use of various materials with high metal retention capacity. One of the most widely used soil amendments is natural-occurring zeolite, which has been found to retain Cr both physically (by entrapping metals into its pores) and chemically (by electrostatic attraction onto its negative-charged surfaces) (Leyva-Ramos et al. 2008). In recent years, several attempts have been made to synthesize adsorbents with high cation exchange capacity (CEC) from raw natural soil minerals. Among others, such synthetic materials are zeolite–goethite systems, which have been successfully used in batch sorption tests as metal retention phases (Ioannou et al. 2009), but there is a void in the literature concerning research on their effect on agronomic characteristics and Cr uptake of vegetable plants. Moreover, Cr(VI) is found to be readily reduced to Cr(III) in soil, especially in the presence of abundant reducing soil agents, such as organic matter phases. Thus, the application of organic matter-containing amendments may also have beneficial effects. However, to our knowledge, the effect of added organic matter in accelerating the reduction of Cr(VI), as compared with other high-CEC soil amendments, has not been addressed. The hypothesis tested in this study was that inorganic phases will facilitate added organic matter in reducing Cr(VI) toxicity to soil and plant. The aims were (a) to study Cr(VI) bioavailability to spinach, (b) to assess the role of added organic matter in affecting the reduction of Cr(VI) in soils, and (c) to
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compare the added mineral phases in relation to Cr (VI) uptake behavior by spinach in soils both amended and unamended with manure.
2 Materials and Methods A greenhouse pot experiment was established in Volos area (central Greece). The soil used in this study was obtained from the Farm of the University of Thessaly, located at Velestino. The soil (uncultivated for at least 2 years before the beginning of the experiment) was a sandy clay loam (54 % sand, 30 % clay, 16 % silt), with pH 7.82, organic matter content of 2.51 %, and total (aqua regia) Cr of 298 mgkg−1. A composted sample of farmyard manure (63 % moisture, 41 % organic matter content, pH 8.32, electrical conductivity of 3.55 dS m−1, Kjeldahl-N of 18.36 gkg−1, and trace element content (units in mgkg−1) of Cr02.33, Cu025.6, Mn0445.7, Ni017.2, and Zn0692.3) was collected from a farm near Volos. Samples of zeolite (clinoptilolite) were obtained from the S & B Company and had a cation exchange capacity of 2.35 molc kg−1, specific surface area of 31 m2 g−1, and an average particle size of less than 2 mm. Goethite was synthesized in the laboratory using the method proposed by Schwertmann and Cornell (2000). Zeolite–goethite system was also prepared in the laboratory according to the method reported by Dimirkou et al. (2009). A bulk quantity of the soil was air-dried and passed through a 2-mm sieve. It was then divided into two equal parts of 80 kg of each. The first part was thoroughly mixed with 6.35 kg of moist farmyard manure, equal to 2.35 kg of dry matter manure, equal to 7.00 g of organic carbon per kg soil, and this mixing resulted in the increase of organic matter content to 3.09 %. These two parts (both the manureamended and the unamended) were further divided into four equal parts of 20 kg each. One was added with zeolite at 1 gkg−1 soil (a treatment hereafter called “Z”), another was added with goethite at 1 gkg−1 soil (a treatment hereafter called “G”), a third part was added with the zeolite–goethite system at 0.25 gkg−1 soil (a treatment hereafter called “Z–GY”), and the last part was left without any of these additions. These resulted in eight treatments (two manure rates×three adsorptive materials+two controls) thus far, which were replicated four times and were placed into 4-kg pots. No fertilizer was added to the mixtures. The pots were placed in a greenhouse at constant temperature environment of 20–
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22 °C. The pots were watered at 65 % of the soil field capacity and were left to equilibrate before any further treatment for 2 days. Then each pot was sown with ten seeds of spinach (S. oleracea) on April the 11, 2011. At day 22 after the sowing (on the 23rd of April), the four replicates of each treatment received 1 L of a solution containing 200 mgL−1 of Cr(VI) as CrO3 (an addition equal to 50-mg Cr kg−1 soil), which resulted in the increase of the total Cr soil content to 355 mgkg−1. Apart from these, another four pots containing soil– manure mixture with no Cr addition and other four pots containing soil alone (no manure, no Cr) were also sown with spinach. Thus, the experimental design had a total of ten treatments (the previous treatments+one control of soil-and-manure mixture+one control of soil alone) each replicated four times, resulting in a total of 40 pots. Hereafter, the control without Cr addition is named “M” and the Cr-added control “M-Cr(VI)”. The pots were irrigated with deionized water every second day and were moved every week to compensate for possible light and temperature differences. The plants were harvested at day 56 (on June 6). During the experiment, exchangeable Cr(VI) was measured in 10-g moist soil samples which were obtained three times, on 24 April (1 day after Cr(VI) addition), on 15 May (middle of experiment), and 6 June (end of experiment). The samples were extracted with 0.01 M KH2PO4, and to the extractants, we developed color by the diphenyl carbazide method (Gheju et al. 2009). The extractants were then analyzed using a spectrophotometer (Shimadzu UV–VIS 120–01) at 540 nm. At the end of the experiment, airdried soil samples were sieved through a 2-mm sieve and analyzed for total (aqua regia-digested, according to Ure 1995) and available Cr(III) content, extracted with diethylene triamine pentaacetic acid (DTPA) (according to Lindsay and Norvell 1978). Chromium content (both total and DTPA-extractable) was analyzed with atomic absorption spectrophotometer (Perkin Elmer 3300), an analysis which accounts for the Cr(III)+Cr(VI) content. Trivalent chromium was then calculated by subtracting the Cr(VI) concentration (already measured with the diphenyl carbazide method) from the total (i.e., Cr(III)+Cr(VI)) Cr content. During the experiment, plant height was recorded on 23 April, 9 May, 25 May, and 6 June. On 6 June (end of the experiment), plants were harvested, washed with dilute 1 M HCl, then with deionized water, and were dried in an oven at 70 °C for about
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48 h. After that, the aboveground biomass weight was recorded, and the samples were milled into fine pounder and stored in plastic bags until analysis. For the plant Cr analysis, 1 g of plant sample was dried at 520 °C for 24 h into porcelain crucibles, the ash was washed with 20 mL of 20 % HCl (Jones et al. 1990), and the extractants were analyzed for total (Cr(III)+Cr (VI)) Cr and Cr(VI), according to the methods mentioned above. Chromium availability was assessed both with soil and plant analyses and with the use of the soil-to-plant transfer coefficient (Tc), which was calculated as follows: Tc0(PQt−PQc)/Added Cr(VI), where PQ is plant quantity of Cr(VI) (in milligrams per pot), added Cr(VI) is equal to 200 mg per pot, and “t” and “c” denote “treatment” and “control,” respectively. The values of PQ were calculated as the product of Cr(VI) concentration in plant (in milligrams per kilogram in plant) multiplied by the aboveground plant dry matter weight (in grams per pot). The data were analyzed for ANOVA, and the differences among treatments were compared according to the LSD test for a level of significance of 95 % using the Statgraphics Plus 5.1 package.
3 Results 3.1 Spinach Agronomic Characteristics With Cr addition, dry matter weight decreased significantly at M-Cr(VI) compared to the M control in the treatments without manure (Fig. 1a). In the manureamended soil, dry matter weight was significantly higher at M-Cr(VI) compared to the M-Cr(VI) in the soil without manure (significant differences between the two are indicated with an asterisk, Fig. 1b). In the manure-amended soil, although there was a slight decreasing trend, there were no differences between any of the Cr treatments (M-Cr(VI), Z, G, or Z–GY). In the soil without manure, plant height in the M control was the highest among all the treatments (Fig. 2a). In the manure-amended soil, there were no differences between the M-Cr(VI) and any other Cradded treatment (Fig. 2b). 3.2 Chromium in Soil DTPA-extractable trivalent Cr was increased significantly in all Cr-added treatments compared to M in the
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(A) 6 5
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Fig. 1 Aboveground dry matter weight (DW) of the plants grown in the experiment in soils a without and b with manure addition. Within each graph, treatments with different letters have significant differences at p
