Phytoextraction Efficiency of Cadmium and Zinc by Arum (Colocasia esculenta L.) Grown in Hydroponics

Heavy metals reach soils through natural pedogenic (or geogenic) processes and anthropogenic activities. Often the concentrations of heavy metals released into the soil system by pedogenic processes are low and are largely related to the origin and nature of the parent material. However, anthropogenic activities primarily associated with industrial processes, manufacturing and the disposal of domestic and industrial waste materials are the major sources of metal enrichment in soils (Adriano, 2001). Unlike pedogenic input, metals added through anthropogenic activities often have high bioavailability. Metal uptake and accumulation from soils by plants are influenced by such factors as plant species, soil metal concentration, soil properties, rapid transport within the plant, the proliferation of roots in metal hotspots within the soil etc. (Adriano, 2001). Among the heavy metals, cadmium (Cd) is non-essential to biota, more mobile and bioavailable, potentially toxic to humans at lower concentrations than those toxic to plants (Kabata-Pendias and Pendias, 1992; Singh and McLaughlin, 1999). Zinc (Zn) is essential in trace amounts for plants but its concentrations found in contaminated soils frequently exceed those required by the plant and soil organisms, and thus create danger to animal and human health (Greenland and Hayes, 1981; Alkorta et al., 2004). Cd and Zn concentrations in some industrial sites of Bangladesh are found to range from 0.1 1.8 and 53 477 mg kg -1 , respectively (Kashem and Singh, 1999) which are above the background level for Cd (0.01 0.2) and Zn (68 mg kg -1 ) in soil (Domingo and Kyuma, 1983; Singh and Steinnes, 1994). However, the concentrations of Cd and Zn in vegetables grown in agricultural soils adjacent to the industrial areas of Bangladesh were observed in the range of 1.0 4.7 and 16.5 67.1 mg kg dry weight, respectively by Ahmad and Goni (2010) and 0.4 0.8 and 98 244 mg kg -1 dry weight, respectively by Kashem and Singh (1999). These values exceed the acceptable tolerance level for FAO/WHO standard of 0.3 mg Cd kg dry weight and 60 mg Zn kg dry weight (Codex Alimentarious Commission, 1984). It is therefore, important to develop methods to cleanup Cd and Zn contaminated soils. Phytoremediation, where hyperaccumulators are used to take up large quantities of pollutants from contaminated soils has been touted as a promising alternative for the generally expensive and disruptive conventional remediation techniques to reduce environmental health risks posed by Cd and Zn contaminated sites (McGrath et al., 2002). To date, about 700 species of plants have been reported to be hyperaccumulators of different contaminants (Xi et al., 2010), of which a good number of species have been considered as Cd and Zn hyperaccumulators (Raskin and Ensley, 2000). However, successful phytoextraction requires that these plants are capable of producing high biomass while accumulating large amounts of contaminants in the biomass from the soil (Liu et al., 2015). In the present investigation, we select a common and locally popular plant species arum (Colocasia esculenta L.). This plant is widely distributed in Bangladesh and can grow in both dry and marshy conditions. It has deep roots and long shoots. It possesses the