Zinc and Chromium removal mechanisms from industrial wastewater by using water hyacinth, eicchonia crassipes

2.2.1 Systematic position

a. Taxonomy

Cronquist (1988), Thorne (1992) and Takhtajan (1997) suggest following water hyacinth taxonomic placement (Center et al. 2002):

Division: Magnoliophyta

Class: Liliopsida

Subclass: Commeinidae

Superorder: Commelinanae

Order: Pontederiales

Family: Pontederiaceae

Genus: Eichhornia

Specific epithet: crassipes (Martius) Solms-Laubach.

b. Morphology

Water hyacinth (Eichhornia crassipes) is a perennial, floating macrophyte, freshwater aquatic vascular plant with rounded, upright, shiny green leaves and spikes of lavender flowers (Reed et al., 1997). The petioles of the plant are spongy with many air spaces and contribute to the buoyancy of the hyacinth plant. When grown in wastewater, individual plants range from 0.5 to 1.2 m from the top of the flower to the root tips (Reed et al., 1997).

The plants spread laterally until the water surface is covered and then the vertical
growth increases. Hyacinths are very productive photosynthetic plants. Their rapid

growth is a serious nuisance problem in many slow flowing southern waterways. These same attributes become an advantage when used in a wastewater treatment system.

Figure 2.2: Morphology of water hyacinth plant (source: Aquatics, 2005)

2.2.2 Ecological factors

Water hyacinth is heliophyte plant growing best in warm waters rich in macronutrients. Optimal water pH for growth of this aquatic plant is neutral but it can tolerate pH values from 4 to 10. This is very important fact because it points that E. crassipens can be used for treatment of different types of wastewater. Optimal water temperature for growth is 28-30^oC. Temperatures above 33^oC inhibit further growth (Center et al., 2002). Optimal air temperature is 21-30^oC. So if aquatic systems with water hyacinth are constructed in colder climates it would be necessary to build greenhouses for maintaining optimal temperature for plant growth and development. Low air humidity from 15% to 40% can also be limiting factor for undisturbed growth of water hyacinth (Allen, 1997). E. crassipens tolerates drought well because it can survive in moist sediments up to several months (Center et al., 2002).

2.2.3 Potentials and constraints in using of water hyacinth

Water hyacinth is plant with many advantages firstly because it can be used for many
purposes, but it has one major consequence. E. crassipes is one of the most invasive
weeds that can destroy precious aquatic ecosystems in a short time which can lead to

series of other problems. Because of that it is very difficult to answer the question - Is E. crassipes the golden plant or the world's worst aquatic weed?

One can often read that people have a moral imperative to think about potential utilization of abundantly available biomass of water hyacinth in tropical countries for the benefit of people for whom E. crassipes has created many problems or even has destroyed their lives. There are many examples around the world of how communities or individuals have used water hyacinth to great advantage. In regions where it can be found in abundance water hyacinth can be used like food for people because its leaves are rich in proteins and vitamin A. It can be also utilized as green fertilizer or as mulch, compost and ash in regenerating degraded soils. (Lindsey and Hirt, 1999).

In African countries like Uganda water hyacinth has also influenced on much frequent occurrence of diseases (dysentery, malaria, and schistosomiasis) related to content of different pathogens in water. It has been discovered that water hyacinth's quest for nutrients can be turned in a more useful direction. The plant has been shown to accumulate trace elements such as Ag, Pb, Cd and Zn.

The focus on water hyacinth as a key step in wastewater recycling is due to the fact that it forms the central unit of a recycling engine driven by photosynthesis and therefore the process is sustainable, energy efficient and cost efficient under a wide variety of rural and urban conditions.

2.3 Heavy metals

The designation «Heavy metals» is applied to a group of metals and metalloids with a specific density greater than 5 g / cm³. They are frequently associated with pollution and toxicity in the environment. In general «trace metals» is the term used because they occur in low concentrations in the earth's crust. Element like As, Cd, Cr^VI, Hg and Pb are known to be very toxic. However some metals like Co, Cu, Mn, Se and Zn are essential for living organisms at low concentrations and are vital components of enzymes. Heavy metals occur naturally in the environment, usually at relatively low concentrations as a result of weathering and other pedogenic processes acting on the

rock fragments on which soils develop (Rulkens et al., 1995). These metals are then transported to the aquatic ecosystem through leaching and run-off phenomena.

Human activities have increased the amount of heavy metals released to the environment. Metals have been exploited at an alarming rate because of their economic value. The negative consequences of this situation have only been realized within the last decades. Because of that, many researches on the interaction of heavy metals with various components of the environment have been conducted and still going on with the main objective of finding suitable ways of solving and avoiding heavy metals pollution on the surrounding environment.

The existence of heavy metals in the environment represents a very significant and long-term environmental hazard. Even at low concentrations these metals can be toxic to organisms, including humans. In particular, chromium is a contaminant that is a known mutagen, teratogen and carcinogen (Chang, 1996; Young et al., 2006).

The removal of heavy metals from aqueous solutions has therefore received considerable attention in recent years. However, the practical application of physicochemical technology such as chemical precipitation, membrane filtration and ion exchange is sometimes restricted due to technical or economical constraints. For example, the ion exchange process is very effective but requires expensive adsorbent materials (Lehmann et al., 1999; Volesky, 2001).

The use of low-cost waste materials as adsorbents of dissolved metal ions provides economic solutions to this global problem and can be considered an eco-friendly complementary (Volesky et al., 1995; Mullen et al., 1989). At present, emphasis is given to the utilization of biological adsorbents for the removal and recovery of heavy metal contaminants. (Young et al., 2006).

Aquatic macrophytes are known to remove metals by surface adsorption and/or absorption and incorporate them into their own system or store them in a bound form (Rai et al., 1995). The uptake of trace metals by the root systems of aquatic plants depend both on the kind of metal and on the species of plant absorbing the metal (Samecka-Cymermann and Kempers, 1996).