versión On-line ISSN 0717-3458
Electron. J. Biotechnol. v.9 n.2 Valparaíso abr. 2006
Recovery of lead and cadmium ions from metal-loaded biomass of wild cocoyam (Caladium bicolor) using acidic, basic and neutral eluent solutions
Michael Horsfall Jnr*
Fred E. Ogban
Eyetemi E. Akporhonor
Financial support: This project was sponsored by International Foundation for Science (IFS) in collaboration with COMSTECH (The Committee on Scientific and Technological Cooperation of the Organization of Islamic Conference,
Keywords: cocoyam, desorption, heavy metals removal, metal recovery, water treatment.
The effects of acidic, basic and neutral reagents on the recovery of Pb2+ and Cd2+ from metal-loaded biomass of wild cocoyam (C. bicolor) were investigated by eluting the biomass in five successive cycles using
Metal ion recovery studies from spent biomass are an important aspect of the waste management and environmental remediation goals. These studies may help to elucidate the nature of recovery of metal ions from wastewater and the recycling of adsorbent. The possibility of regeneration of spent biosorbent is crucially important to keeping the process costs of remediation down and to opening the possibility of recovering the metal(s) extracted from the liquid phase. The deposited metals are washed out (recovered) and biosorbent regenerated for another cycle of application. The recovery process should result in (1) -high-concentration metal effluent; (2) -undiminished metal uptake upon re-use; (3) -no biosorbent physico-chemical damage. The recovery of metal ions and sorbent regeneration studies might require somewhat different methodologies. Screening for the most effective regenerating solution is the beginning. Different affinities of metal ions for the biosorbent result in certain degree of metal selectivity on the uptake. Similarly, selectivity may be achieved upon the elution-recovery operation which may serve as another means of eventually separating metals from one another if desirable.
A number of studies have demonstrated the feasibility of using plant biomass to remove heavy metal ions from aqueous solutions (sago waste; Quek et al. 1998, cassava waste; Abia et al. 2003, banana pith; Low et al. 1995, Medicago sativa (Alfalfa); Gardea-Torresdey et al. 1998; and Spagnum Moss Peat; Ho et al. 1995) just to mention a few. Each of these studies have also mentioned the importance of recovery of metal ions from the biomass and eventual regeneration of the spent biomass. However, very little has appeared in the literature as a major study on the recovery of adsorbed metals from the biomass (Gardea-Torresdey et al. 1998; Zhou et al. 1998; Chu and Hashim, 2001). C. bicolor biomass has been used as an effective adsorbent for metal ions from aqueous solution (Horsfall and Spiff, 2004a; Horsfall and Spiff, 2004b; Horsfall and Spiff, 2005a; Horsfall and Spiff, 2005b). However, most biomass in its native form may not be suitable for process applications as they may disintegrate under the harsh conditions of wastewater processing, especially in cases where the biomass is exposed to a stronger reagent to recover the adsorbed metal ions and regenerate the biomass for reuse. It is because of this reason we have used several desorption reagent with different strength in order to assess the reusability of the biomass after recovery. The purpose of this study is therefore to examine the effect of acidic, basic and neutral eluents in the recovery of Pb2+ and Cd2+ from biomass of wild cocoyam (C. bicolor).
Biomass preparation and surface characterization. C. bicolor (wild cocoyam) biomass was used in this study. The preparation and estimation of the surface characteristics of the biomass has been reported elsewhere (Horsfall and Spiff, 2004a; Horsfall and Spiff, 2004b; Horsfall and Spiff, 2005a; Horsfall and Spiff, 2005b), which are outlined below. Before the batch adsorption experiment, the wild cocoyam corms were washed with deionised water, cut into small pieces, air-dried for two days in the laboratory temperature and then dried in an oven (Gallen Kamp, model OV-160, England) at
FTIR characterization of biomass. The finely divided biomass was analyzed to determine their functional groups using a Fourier Transform Infrared Spectroscopy (Shimadzu IR Prestige - 21, FTIR - 84005) using KBr as the rock salt. The finely divided solid biomass was mixed with a disc using a hydraulic press and mould. The mixture on the disc was inserted in the path of the IR beam and held in position.
Batch adsorption experiment. Batch experiments were first conducted to load the Caladium bicolor biomass with metal ions separately. In this experiment 500 mg of the biomass samples with particle size 100 µm was weighed and placed in pre-cleaned test tubes in triplicates. An initial metal ion concentration of 100 mg/L were made from spectroscopic grade standards of Pb2+ obtained from Pb(NO3)2 and Cd2+ obtained from Cd(NO3)2.4H2O). The two metal solutions made separately were adjusted to pH 5.0 with concentrated HCl solution. Fifty millilitres of each metal solution were added to each tube containing the biomass and equilibrated for 2 hrs by shaking at
Batch recovery experiment. 250 mg of metal-laden C. bicolor biomass were placed in several flasks containing 50 mL of
The amounts of metal ion remaining on the biomass as a function of time (qt) were estimated after determining the amount of metal ion in the recovery reagents and subtracting from the initial amount of metal ion on the biomass (qe) by using a mass balance equation (Equation 1):
where qt isthe actual metal ion concentration recovered from the biomass (mg/g) and Ct is concentration of metal ion remaining in solution after recovery (mg/L) at time t (min), respectively, m is mass of biomass used (g) and v is the volume of eluent solution used (mL).
The surface area was determined to be 32.91 ± 1.22 (m2 g-1), while the bulk density, porosity and pore volume were 1.63 ± 0.11 (g cm-1), 59.31 ± 1.14 (%) and 0.61 ± 0.03 (cm3 g-1) respectively. Furthermore, the cation exchange capacity (25.69 ± 0.58; meq g-1) and surface charge density (0.78 ± 0.04; meq m-2) were also determined. The FTIR spectra of the investigated C. bicolor biomass are shown in Figure 1. Inspection of these spectra reveals the presence of the following peaks at wave no. 3400 cm-1 representing - OH stretching frequency; several peaks at wave no 1650 cm-1 and 1050cm-1 representing - C = N of amides and C - O of alcohol, and peak at 1709 cm-1 for C = O group of ketones. The peak at 2300 cm-1 represents a C - H saturated hydrocarbon of chain. On the overall the spectra indicated the presence of hydroxyl, carboxyl, amide and possibly carbonyl group.
The applicability of plant biomass for metal ion recovery from waste stream requires that the biomass be regenerated efficiently so that the bound metal can be recovered in concentrated form and the biomass reused. The data as presented in Figure 2 gives the percent recovery of Pb2+ and Cd2+ as a function of time. It is apparent from the figure that recovery in the acidic media was quite rapid with equilibrium recovery achieved within the first 15 - 25 min of contact time. It is also noticeable from the figure that increased contact time does not significantly increased recovery after 25 min. From the metal-laden biomass, over 90% and 75% of Pb2+ sorbet were recovered by acidic media concentrations of
Although complete desorptions were not achieved, it is clear from the results that the HCl acidic media is a better eluent than the NaOH basic media. This is because acidic media contain high concentrations of protons present in the recovery reagent which may displace bound metal ion from the active sites on the biomass than hydroxonium ions. Macroscopic observations indicate that the initial biomass weight and colour changed in the acid and basic media with these changes increasing with increase in eluent concentration. Although distilled water is ineffective in recovering the metal ions on the biomass, it leaves the biomass for several recyclability. Generally, the influence of the recovery media tested for the recovery of Pb2+ and Cd2+ from the biomass is of the order
The recovery with dilute hydrochloric acid solution was found to be better option in all the reagents tested because it leaves the biomass for further reuse. Since a single cycle of adsorption-desorption equilibrium studies is not enough to sufficiently access the recovery capacity of an efluent, hence, five cycles was tested. The results of the investigation are quite useful for metal ion recovery eluent selection using batch or stirred flow reactors. Additional research is on-going to elucidate the recovery efficiency of this and other adsorbent with respect to volume of eluent, biomass reloading efficiency and biomass regeneration/recycling.
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