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

2.4 Wastewater

Wastewater is a general term that encompasses a myriad of wastes in the water medium originating from diverse sources. Normally, the two major sources of concern are of domestic and industrial origin but also agriculture.

Several authors shown results from Iron bridge water hyacinth system in Florida, USA demonstrated that phosphorus removal was from 35 to 80 % (U.S. EPA, 1988). The same facility successfully removed about 60% of BOD5 and 43% of suspended materials from wastewater. These systems can also remove heavy metals like chromium, cadmium, copper, zinc and other effectively. In their experiments Maine et al. (1999) have shown that 72% of cadmium was removed from wastewater by water hyacinth. Accumulated nutrients and heavy metals are removed from aquatic systems by plant harvesting and sediment dredging (Reddy and Sutton, 1984; U.S. EPA, 1988).

There are many speculations on the use of water hyacinth upon harvesting. According to some authors (Lindsey and Hirt, 1999) it can be use like food for people or fodder. But it is not recommended to consume water hyacinth if it was used for removal of heavy metals, rare earth elements or other toxic substances that can cause problems if they enter food chain (Chua, 1998). Upon harvesting water hyacinth can be used for composting, anaerobic digestion for production of methane, and fermentation of sugars into alcohol (U.S. EPA, 1988). These operations can help in recovering expenses of wastewater treatment.

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. 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

Table 2.1 shows that the effective response of water hyacinth after exposed to cadmium and zinc in different concentrations is different depending on metal. As it shown, zinc was more adsorbed and taken up by the plant than the cadmium.

Table 2.1: Maximum growth response of water hyacinth exposed to Cd and Zn

Parameter	Cd	Zn
Relative growth	0.85^a	0.89^c
Metal accumulation (mg/kg) shoot	113.2^a	1926.7^c
root	2044^a	9652.1^c
Residual concentration (mg/L)	0.185^a	6.29^c
BCF	622.3^b	788.9^d

a: 4 mg/L Cd, b: 2 mg/L Cd, c: 40 mg/L Zn, d: 5 mg/L Zn.
(Source: Xiaomei et al., 2004)
BCF : bioconcentration factor

2.5 Foliar absorption

In addition to root absorption, plants can also derive low amounts of some elements through foliar absorption. Foliar absorption of solute depends on the plant species, its nutritional status, the thickness of its cuticle, the age of the leaf, the presence of stomata guard cells, the humidity at the leaf surface and the nature of the solutes (Marschner, 1986). Metal antagonism, such as Cu and Zn, can occur in foliar absorption as well as in the root (Channel, 1986). Aerosol deposited lead does not penetrate the cuticle of higher plants, but tend to adhere to the surface of leaves. They can however be absorbed through the cuticle of some bryophytes (Alloway, 1990).