Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

286j 15 The Use of Rhizospheric
wastewater is below 100 mg l 1 and the effluent contains less than 1 mg l 1 of metal
ions [52]. However, microbe-based technologies have a number of disadvantages
such as small particle size, low mechanical strength and low density, which make
biomass-effluent separation difficult. The problem can be mediated using immobilized
cells. The cost involved in immobilization is high because a well-adjusted
chemical and physical environment is important to maintain the adhesion of cells
and metal ion removal capacity of microbial cells. In addition to the cost of cell
culture, the system should attain more than 99% metal ion removal efficiency with
loading capacity greater than 150 mg metal ions g 1 for a competitive niche [13].
As some metal ions are highly toxic in nature, the wastewater containing metal
must be polished after conventional treatment to meet discharge standards [53].
Thus, there is a growing interest in developing a reliable and inexpensive technology
that can reduce toxic concentrations of metal ions to environmentally acceptable
levels or to recover natural metal resources [53–56].
15.3.3
Phytoremediation
15.3.3.1 An Overview of Phytoremediation
Phytoremediation is defined as the use of green plants to remove pollutants such as
metal ions from the environment [41,42,57–61]. This plant-based remediation technology
mainly depends on the metal ion hyperaccumulating properties of certain
plants [39,62,63]. The term hyperaccumulator is used to describe a plant with a
highly abnormal level of metal ion accumulation [64]. Baker and Brooks [65] have
defined hyperaccumulators as plants that contain more than 1 mg g 1 (0.1%) of Co 2
+ ,Cu 2+ ,Cr 6+ ,Pb 2+ or Ni 2+ or 10 mg g 1 (1%) of Mn 2+ or Zn 2+ in dry matter [60].
Studies on hyperaccumulator species by collecting plants in metal ion contaminated
areas have been initiated. A detailed review of this project can be found in Reeves
and Baker [64].
The goal of current phytoremediation efforts is to develop innovative, economical
and environmentally compatible approaches to remove metal ions from the environment.
Many studies have revealed the feasibility and importance of phytoremediation
[40–42,60,61,66,67]. The most important features of phytoremediation
include lower costs for treatment and the generation of a potentially recyclable metal
ion rich plant residue [39]. Its cost, which includes the entire capital and operating
expenses, is far below those of many competing technologies (Table 15.1). Phytoremediation
also offers a cost advantage in wastewater treatment, because plants can
remove up to 60% of their dry weight as metal ions, thus markedly reducing the
disposal cost of the hazardous residue [39]. Ideally, one can use a good metal ion
accumulator with a high accumulation rate, fast growth and high biomass production;
however, there is no single species that fulfils all these requirements. Phytoremediation
has its disadvantages (Table 15.2); the greatest limitation is that it needs a
long time and large area. The time to remediate a site depends on the life cycle of
plants and their growth requirements. Phytoremediation also needs a large surface
area, which depends on the uptake rate of the plants, to accommodate the treatment