A comparative study of cadmium phytoextraction by accumulator and weed species

Abstract

Phytoextraction has shown great potential as an alternative technique for the remediation of metal contaminated soils. The objective of this study was to investigate cadmium (Cd) phytoextraction ability of high biomass producing weeds in comparison to indicator plant species. The pot study conducted with 10 to 200 mg Cd kg⁻¹ soil indicated that Ipomoea carnea was more effective in removing Cd from soil than Brassica juncea. Among the five species, B. juncea accumulated maximum Cd, but I. carnea followed by Dhatura innoxia and Phragmytes karka were the most suitable species for phytoextraction of cadmium from soil, if the whole plant or above ground biomass is harvested. In the relatively short time of this experiment, I. carnea produced more than 5 times more biomass in comparison to B. juncea. There were significant differences (p < 0.05) between the shoot length and shoot mass of control and treated plants.

Introduction

Cadmium (Cd) is a heavy metal naturally present in soil; it is non-essential and highly toxic to most organisms, having toxicity 2 to 20 times higher than many other heavy metals (Vassilev et al., 1998). Cadmium is the fourth most toxic metal to vascular plants (Jones et al., 1993, Oberlunder and Roth, 1978). It is placed in seventh position in the top ten priority hazardous substances list as provided by the American Agency for Toxic Substance and Disease Registry (Kamnev and Lelie, 2000), and therefore is considered a very serious pollutant. Total Cd levels exceeding 8 mg kg⁻¹, or soluble (bioavailable) levels exceeding 0.001 mg kg⁻¹, are considered toxic to plants (Kabata-Pendius and Pendius, 1992, Bohn et al., 1985). The primary risk pathway associated with Cd contaminated soils has been identified as the soil–plant–human pathway and the consumption of the crop or byproducts grown on these soils leads to its biomagnification in the food chain (Page et al., 1982). Cadmium content in soil has dramatically increased from anthropogenic sources including smelters, agricultural applications of fertilizer, and sewage sludge. Since Cd in soil is available for plant uptake and subsequent human uptake, Cd in the environment poses a significant health risk.

Plants can extract Cd from the soil and transport it via the xylem into shoots and leaves where it accumulates. Cadmium absorption occurs because of the chemical similarity to zinc (Blaylock et al., 1997). It has been shown that pollutants can be removed from a contaminated site by harvesting the plant biomass containing the pollutant; this is referred as phytoextraction (Chaney, 1983). The two major factors that determine the total amount of metal extracted by plants are: (a) the concentration of the pollutants in dry biomass; and (b) the total biomass produced by the plant. Plants used for phytoextraction should be fast growing, deep rooted, easily propagated and accumulate the target metal. Ideally the species should have a high bioconcentration factor (BCF), which is defined as the plant/soil metal concentration.

As per their ability to absorb, accumulate, and tolerate metal within their tissues, plants exhibit three major responses and can be classified into three categories: hyperaccumulators, indicators and excluders (Wagner and Yeargan, 1986, Alloway, 1995). Plants with extreme levels of metal tolerance are called as hyperaccumulators. In 1983, Chaney reported that several hyperaccumulator species have a high BCF, but due to very low biomass the amount of phytoextraction is less; additionally, harvesting of these species commercially is difficult as per the present agronomic practices (Chaney et al., 1995). Indicator plants such as Brassica juncea (Indian mustard) (Wagner and Yeargan, 1986, Alloway, 1995), in comparison to hyperaccumulators, have a lower metal bioaccumulation but have at least 10 times the biomass production, so that the actual amount of extraction is higher. Indicators regulate metal uptake so that the internal concentration reflects the external levels.

Excluders maintain low and constant metal concentration in their shoots. High biomass species like willow and poplar fall under this category and accumulate small amount of metal (9–167 μg g⁻¹ by willow; 6–75 μg g⁻¹ by poplar) per unit of dry weight. Excluders produce more biomass, up to 30 tones per hectare (Robinson et al., 2000). However, a very long time is needed for remediation by excluder plants; a period of 12 years was calculated for removal of 0.6 mg kg⁻¹ of cadmium, based on realistic willow tree biomass production rates and experimentally-determined cadmium uptake rates (Greger and Landberg, 1999).

The aim of this study was to investigate the phytoextractive potential of high biomass producing, commonly found weed species that can grow in both dry land and marshy conditions. An efficient method for testing the potential of commonly found weed species would be to grow an accumulator plant, such as B. juncea in comparison to hardy species like Ipomoea carnea, Phragmytes karka, Dhatura innoxia, Cassia tora, and Lantana camara. Use of edible species increases the risk of heavy metals being introduced into the food chain. Species like B. juncea are accumulators and are edible; to restrict the passage of heavy metals into the food chain, non-edible species were chosen and the phytoextraction was compared with respect to total biomass and Cd uptake. I. carnea, P. karka, D. innoxia, C. tora and L. camara were chosen, but only the results of the first three were reported. Although they may have a lower accumulation capacity, their potential of high biomass and ability to grow in deficient conditions makes them ideal candidates for this study.