EDTA-enhanced phytoremediation of contaminated calcareous soils: heavy metal bioavailability, extractability, and uptake by maize and sesbania Vishandas Suthar, Kazi Suleman Memon & Muhammad Mahmood-ul-Hassan
Environmental Monitoring and Assessment An International Journal Devoted to Progress in the Use of Monitoring Data in Assessing Environmental Risks to Man and the Environment ISSN 0167-6369 Volume 186 Number 6 Environ Monit Assess (2014) 186:3957-3968 DOI 10.1007/s10661-014-3671-3
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Author's personal copy Environ Monit Assess (2014) 186:3957–3968 DOI 10.1007/s10661-014-3671-3
EDTA-enhanced phytoremediation of contaminated calcareous soils: heavy metal bioavailability, extractability, and uptake by maize and sesbania Vishandas Suthar & Kazi Suleman Memon & Muhammad Mahmood-ul-Hassan
Received: 13 August 2013 / Accepted: 28 January 2014 / Published online: 11 February 2014 # Springer International Publishing Switzerland 2014
Abstract Natural and chemically enhanced phytoextraction potentials of maize (Zea mays L.) and sesbania (Sesbania aculeata Willd.) were explored by growing them on two soils contaminated with heavy metals. The soils, Gujranwala (fine, loamy, mixed, hyperthermic Udic Haplustalf) and Pacca (fine, mixed, hyperthermic Ustollic Camborthid), were amended with varying amounts of ethylenediaminetetraacetic acid (EDTA) chelating agent, at 0, 1.25, 2.5, and 5.0 mM kg−1 soil to enhance metal solubility. The EDTA was applied in two split applications at 46 and 60 days after sowing (DAS). The plants were harvested at 75 DAS. Addition of EDTA significantly increased the lead (Pb) and cadmium (Cd) concentrations in roots and shoots, uptake, bioconcentration factor, and phytoextraction rate over the control. Furthermore, addition of EDTA also significantly increased the soluble fractions of Pb and Cd in soil over the controls; the maximum increase of Pb and Cd was 13.1-fold and 3.1-fold, respectively, with addition of 5.0 mM EDTA kg−1soil. Similarly, the maximum Pb and Cd root and shoot concentrations, translocation, bioconcentration, and phytoextraction efficiency were observed at 5.0 mM EDTA kg−1 soil. The results V. Suthar : M. Mahmood-ul-Hassan (*) Land Resources Research Institute, National Agricultural Research Center, Islamabad 45500, Pakistan e-mail:
[email protected] V. Suthar : K. S. Memon Department of Soil Science, Sindh Agriculture University, Tandojam, Pakistan
suggest that both crops can successfully be used for phytoremediation of metal-contaminated calcareous soils. Keywords Phytoremediation . Heavy metals . EDTA . Maize . Sesbania
Introduction Soil irrigation with untreated wastewater in peri-urban areas is a common practice in many developing countries including Pakistan (Nazif et al. 2006; Mekala et al. 2008; Ahmad et al. 2010; Nasir et al. 2012), and hence, wastewater-derived heavy metals such as cadmium (Cd), chromium (Cr), lead (Pb), nickel (Ni), and copper (Cu) are causing soil contamination. Repeated application of untreated effluents is elevating the concentrations of soil metals (Mahmood-ul-Hassan et al. 2012; Akhtar et al. 2013) which not only affects ecosystem functioning but also poses potential health risks to human beings due to the transfer of these contaminants into the food chain. Remediation of soils polluted with heavy metals is, therefore, vital for their safe use for food and fodder production. There are several techniques for the remediation of soils polluted with heavy metals including chemical remediation, bioremediation, and phytoremediation. Phytoextraction, the use of plants for extracting heavy metals from contaminated soils, is considered an environmentally safe and cost-effective remedial technique. It involves the use of plants to translocate and
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accumulate high quantities of metals from soil into the harvestable parts of roots and aboveground shoots (Song et al. 2005; Zhuang et al. 2007; Mahmood 2010). This process facilitates the removal of metal contaminants from a soil matrix to such a level that the soil can be used without danger for arable purposes. The harvested plant parts, rich in accumulated metals, can be safely processed by drying, ashing, composting, storage at a landfill, gasification, anaerobic digestion, pure plant oil production, and microbial, physical, or chemical means (Ghosh and Singh 2005; Ginneken et al. 2007). Early phytoextraction research focused on hyperaccumulating plants which have the ability to accumulate high amounts of heavy metals in their plant tissues. However, hyperaccumulators often accumulate only a specific element and are, as a rule, slow growing, low biomass-producing plants with little known agronomic characteristics (Cunningham et al. 1995). With time, scientists have developed chemically enhanced phytoextraction techniques with fast growing and high biomass crops (Zea mays, Helianthus annuus, Sorghum vulgare, Amaranthus spp., Sesbania aculeata etc.) to overcome limitations due to low metal solubility and bioavailability (Tandy et al. 2006; Komárek et al. 2007a, b; Melo et al. 2008). In this technique, soil is amended with synthetic organic chelates, such as DTPA (diethylenetriaminepentaacetic acid), EDTA (ethylenediaminetriacetic acid), EDDHA (ethylenediamine-di (o-hydroxyphenylacetic acid), N TA ( n i t r i l o t r i a c e t i c a c i d ) , H E D TA (hydroxyethylendiaminetriacetic acid), EDDS (ethylenediamine-disuccinic acid), citric acid, oxalic acid, malic acid, and elemental sulfur to increase the bioavailability of the heavy metals in the soil and thus enhance phytoextraction by high biomass plants (Evangelou et al. 2007; Jean et al. 2008). Chelating agents increase desorption of heavy metals from the soil matrix to the soil solution, thereby facilitate metal transport into the xylem, and increase metal translocation from roots to shoots (Blaylock et al. 1997; Huang et al. 1997; Song et al. 2005). In view of the high solubility and persistence of chelates like EDTA in soils, there is a possibility that heavy metals could leach down the soil profile and consequently pose a risk to groundwater quality (Römkens et al. 2002; Zhuang et al. 2007). It is therefore essential to choose the proper application rate and time and select the appropriate combination of chelating agent and crop to maximize the concentration of the
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metal–ligand complex (Alkorta et al. 2004). Wenzel et al. (2003) reported that gradual application of small doses of the chelating agent during the growth period minimized the phytotoxicity and environmental problems associated with the use of chelating agents for phytoextraction. Among others, EDTA has been proved to be the most efficient chelate used for Pb phytoextraction from polluted soils (Kos and Lešten 2003; Nascimento and Xing 2006). Incubation studies showed that EDTA was most effective in removing Pb, Cu, Zn, and Cd from infected soils, while extraction efficiency depended on several factors such as the stability of heavy metals in soil, EDTA strength, electrolytes, pH, and soil matrix (Papassiopi et al. 1999; Coscione et al. 2009). The objectives of this research work were to evaluate the Pb and Cd phytoextraction potential of locally available high biomass maize and sesbania plant species and to compare natural and chemically enhanced phytoextraction using synthetic chelators.
Materials and methods Soil sampling and sample preparation Bulk surface samples of two soils used for arable agriculture that were polluted with heavy metals, Gujranwala (fine–loamy, mixed, hyperthermic Udic Haplustalf) and Pacca (fine, mixed, hyperthermic Ustolic Camborthid), were collected from peri-urban areas of Gujranwala (N 32°–06.262; E 74°–10.236) and Mirpurkhas (N 24°–57.212; E 69°–17.329). Formed from alluvial sediments, both soils have been irrigated with untreated wastewater for at least 30 years. The climate of the Gujranwala area is subtropical continental, subhumid with a hot summer (maximum temperature 40.56 °C and at times the temperature reaches 48.33 °C), and mild winter. The mean annual rainfall is 750 mm, and about two thirds of the total rain is received during the monsoon season (July and August) in the form of high-intensity downpours. In contrast, the Mirpurkhas site is arid, marine–tropical with mean summer temperature of 105 °F, and a mild winter. The mean annual rainfall varies from 150 to 175 mm and most of it is received during the monsoon season (July and August) in the form of heavy showers. Soil samples were air-dried, crushed, and sieved through a 2-mm stainless steel sieve. A portion of≈
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200 g was drawn from the
