Biosorption of heavy metals using water hyacinth Eichhornia crassipes (Mart.) solms-Laubach: adsorption properties and technological assessment

Abstract

The adsorption of heavy metal ions (Cd2+), Cu2+), Pb2+) and Zn2+) by untreated water hyacinth Eichhornia crassipes obtained from Lake Victoria, Kenya was investigated under batch mode. A The effects of sample part, particle size, contact time, solution pH, and temperature, shaking speed, adsorbent dosage and initial metal ion concentration of aqueous solutions were investigated. The roots showed highest ability for the uptake of heavy metals (Cu, Zn, Cd and Pb) from aqueous solutions, therefore were used. for all subsequent sorption studies in this research. Maximum removal of metal ions took place at a pH of 4-6. The adsorption of the heavy metal ions increased with increase in the ratio of the sample dose of the adsorbent to concentration of the heavy metal ions and decreased with increasing adsorbent particle size. Increase in shaking speed was observed to increase the amount of ions that adsorbed on the ,J d&orbent. However agitation rate beyond 400 rpm led to a decrease in percentage adsorption. Increase in temperature led to a decrease in the percentage adsorption. Heavy metals adsorption was considered a fast process with almost all the adsorption taking place in the first 5 minutes upon contact of the adsorbent with the aqueous solutions of metals. Twenty minutes of equilibration time was found to be adequate for adsorption of the metal ions, yielding 100 % adsorption at low concentration of metal ions. In order to understand the mechanism of adsorption, Fourier Transform Infra Red (FT-IR) analysis and ion exchange experiment were carried out. Ion exchange accompanied by the. ~lease of light metal ions (K+, Na+, Mg2+ and Ca2+) was observed to be the main mechanism of heavy metal adsorption. Complexation also occurred as demonstrated by FTIR spectra involving hydroxyl, caboxylate, phosphate, ether and amino functional groups. The equilibrium data were correlated with Langmuir and Freundlich Isotherm models. Based on the Langmuir model the maximum adsorption capacities were found to be 3.200. 17.568. 16.166 and 12.820 mg g' for cadmium, copper, lead and zinc respectively. The correlation with I Langmuir showed that uptake of the heavy metals in aqueous systems occurred by monolayer sorption with the eventual formation of a plateau of saturation. Several kinetic models were applied to the adsorption data arid it was found that pseudo-second order fitted well with the adsorption data. These results indicated that Eichhornia crassipes can be used as a low cost adsorbent for the removal of heavy metals from aqueous solutions.