Removal of heavy metals from aqueous solution by water hyacinth (Eichhornia crassipes)

Abstract

Water hyacinth ( Eichhornia crassipes ) is a fast growing, free-floating aquatic weed. Floating aquatic plants are capable of assimilating large quantities of trace elements and heavy metals, some of which are essential for plant growth. The uptake of these elements is often increased when plants are cultured in wastewater containing high levels of macronutrients. They have the ability to absorb heavy metals. Heavy metals and other trace contaminants enter surface and groundwater in various ways and adversely affect flora and fauna. Hence, the removal of such impurities is necessary. Batch studies were conducted and the uptake of arsenic, chromium, mercury, nickel, lead and zinc from the aqueous solution for six different concentrations ranging from 5 mg/l to 50 mg/l was studied. The daily uptake of heavy metals for all concentrations was recorded and the results analysed. Efficiency of removal was determined when each metal was present separately. Samples were analysed by using a UV visible spectrophotometer. Results indicated that at lower concentrations i.e. 5 mg/l of heavy metals, the plant growth was normal and removal efficiency was greater. At higher concentrations, greater than 10 mg/l, the plant started wilting and removal efficiency was reduced. It was observed that in aqueous solutions containing 5 mg/l of arsenic, chromium and mercury the maximum uptake was 26 mg/kg, 108 mg/kg and 327 mg/kg of dry weight of water hyacinth respectively. The heavy metal removal efficiency was also checked by evaluating the uptake rate constant (k) for water hyacinths. A generalized polynomial model was proposed for kinetics of nickel removal. Both models were verified and found to work satisfactorily. Finally, it was concluded that by using water hyacinth, heavy metals could be effectively removed from wastewater when their concentrations were less than 10 mg/l. Lead and zinc were removed more efficiently at higher concentrations than other metals, but plants started to discolour at high concentrations of metals and this luxury uptake leads to destruction of plants in most cases.