Phytoremediation of Cadmium-Polluted Water/Sediment by Aquatic Macrophytes: Role of Plant-Induced pH Changes

Cadmium (Cd) is one of the most readily absorbed and rapidly translocated heavy metal in soil-plants systems. Aquatic plant species play an important role in the removal of Cd from waste waters passing through wetlands. Wetlands' macrophytes directly take up the metals, or indirectly affect the removal efficiencies, by increasing the residence time and reducing water velocities, which in turn enhances sedimentation. Different categories of wetland plants, i.e., emergent, surface floating, free floating, rooted leaves, submerged macrophytes, and trees have different ways of taking up the contaminants. In aquatic plants, Cd evokes a number of parallel and/or consecutive events at molecular, physiological, and morphological levels. The appropriate understanding of the mechanisms employed by macrophytes may optimize their use for remedial purposes. This chapter, therefore, focuses on summarizing the responses of aquatic macrophytes to Cd, with and their possible implementation in phytoremediation techniques. The principle components reviewed include Cd-induced growth inhibition, changes in water relations and ion metabolism, effects on photosynthesis, changes in enzyme activities, and formation of free radicals. Efforts have also been made to highlight the defense mechanisms such as cell wall immobilization, phytochelatin-based sequestration and compartmentalization processes, plasma membrane exclusion, stress proteins, peroxidases, metallothioneins, and so forth. In addition, understanding the role of Cd in modifying the rhizospheric pH also has great importance in understanding remediation, and hence, will be the subject of this chapter.