In the environmental treatment field, if the water hyacinth root is to be considered as a viable treatment method for metal-bearing waste streams, sorption data and the corresponding isotherm parameters must be generated and made available to the scientific and engineering communities. The maximum sorption uptake value is an important feature of the sorbent characterizing its performance at high residual metal concentrations. The potential of this biosorbent material for being regenerated after each metal uptake cycle is extremely important. It is the desorption part of the biosorption process which requires a thorough and quantitative examination leading to establishment of the biosorbent regeneration potential. In addition to equilibrium studies, kinetics of the biosorption has to be determined in order to establish the time course of the heavy metal uptake. The necessary contact time eventually determines the size of the contact equipment which in turn directly affects both the capital and the operating costs of the process. Both equilibrium and kinetic characterizations of the biosorbent material are crucial for quantitative assessment of its performance and for the process design. Finally, localizing the metal deposition site within the biosorbent biomass and understanding the metal-sequestering mechanism are all crucial aspects of the quest for an efficient biosorption process which should feature high metal selectivity and uptake.

Experimental

The water hyacinth plants, collected in Rio Grande do Sul state, Brazil, were rinsed well with tap water and allowed to air dry; then, the roots were removed and pulverized in a laboratory bladed mixer to minus 0.59 mm. The heavy metal containing solutions were made by using reagent grade of Pb(NO3)2, Cd(NO3)2 and ZnSO4·7H2O with distilled deionized water. The pH of the solution was adjusted with diluted HNO3 and NaOH.

Batch sorption experiments were conducted at room temperature by continuous agitation of a mixture of root and solution using a magnetic stirrer. 100-ml of a heavy metal containing solution was combined with a set quantity of the roots in these experiments. After an appropriate time, the water hyacinth roots were separated by vacuum filtration using Milipore 0.45-µm-pore size membrane filters, and the filtrates were analyzed for residual appropriate metal ions. The effect of pH on heavy metals uptake was studied by equilibrating the sorption mixture at different initial pH values by the addition of 0.1 M HNO3 or NaOH solution before the addition of preweighed root.

For desorption studies, several types of eluting solutions were selected and tested in the course of the present work. The water hyacinth roots, previously exposed to a heavy-metal-containing solution, had sequestered a certain amount of the metal at optimum uptake conditions. These materials were used in all elution experiments which were performed by using a batch technique in 100-ml beaker with magnetic stirring at room temperature. The ratio of the loading biomass weight (in milligrams) to the eluant volume (in milliliters) was kept at 0.5. For sorption-desorption series tests, following the elution of the lead-laden roots with 0.05 M CaCl2, the eluted roots retained by the filter membrane were washed with distilled deionized water, dried at below 50 °C, and reloaded with lead in another sorption cycle.

Cadmium, lead and zinc were analyzed by flame atomic adsorption spectrophotometry using a Perkin-Elmer 3280 Atomic Absorption Spectrophotometer. Wavelengths were 228.8, 283.3 and 213.9 nm for Cd, Pb and Zn respectively. The electrokinetic properties (zeta potentials) of the water hyacinth roots in various solutions were studied as a function of pH using a Lazer Zee Meter, Model 501, Manufactured by Pen Kem, Inc., NY. The BET surface areas for the natural water hyacinth roots and H2SO4 prewashed roots were measured using a Micromeritics Flowsorb II 2300, manufactured by Micromeritics Instrument Co., Georgia.

Summary and Conclusions

From the above investigation of biosorption of heavy metal ions from aqueous solution by the nonliving water hyacinth roots, the following observations can be made.

1. The rate of sorption of heavy metal ions by the water hyacinth roots is very fast and reaches equilibrium between 20 and 30 minutes with 0.4 mg/ml root concentration. The apparent first-order reversible kinetic model gives satisfactory fits of the uptake of Cd and Pb. Film diffusion appears justified to be the rate limiting step for heavy metal sorption.
2. The uptake of heavy metal ions by the water hyacinth roots is strongly pH dependent. The fraction of metal removal increases significantly over a relatively-narrow pH range. The sorption of metal ions by the roots reaches a maximum around pH 3.0 and then remains unchanged thereafter.
3. It is demonstrated that the water hyacinth roots are able to remove the metal ions effectively from aqueous solutions over a wide concentration range. At exposure concentrations in the sub-ppm range, the heavy metals can be almost completely removed from solution within 30 minutes. Adsorption fits the generalized Langmuir equation very nicely for the studied water hyacinth root – heavy metal ion systems. The relative affinity of the roots for lead sorption is greater than for cadmium. The maximum sorptions of the roots are 33 and 86 mg/g approximately for Cd and Pb, respectively.
4. The uptake of heavy metal ions is increased by an increase in the root mass. Acid pretreatment decreases the amount of the metal uptake by the roots, which is believed to be due to reduction of the exchangeable metal ions in the roots, such as Ca, Mg and Mn, etc., after this pretreatment.
5. The uptake of metal ions by the roots are noticeably affected by the presence of the other metal ions, and competition among various metal ions is found to be very dependent on the sorption affinity of the roots.
6. The desorption of heavy metal ions from the loaded water hyacinth roots was studied with various eluants. Acidic CaCl2 solution appears to be most effective eluant capable of stripping off 100 pct Cd and Pb from the roots within 30 minutes. A series of sorption-desorption (10 cycles) for Pb result in no significant loss of sorption activity of the roots. The biosorption of metal ions by the inactive water hyacinth roots is clearly an easily reversible physicochemical process with a considerable potential for technological process exploitation.
7. Because this water hyacinth is widely spread in tropical and temperate areas of the world, it is believed that this material would be a cheap and low cost source of a metal sorbent. The encouraging results of heavy metal uptake on the water hyacinth roots should also stimulate further studies focusing on the potential of applying this naturally-abundant material in developing a new metal recovery process.