INTRODUCTION:
Heavy metals are known to progressively accumulate in our ecosystem once they are introduced, from very low levels to levels that exceed the threshold in the environment1. Most of these metals such as cadmium, lead, arsenic, mercury, copper, chromium, cobalt and nickel are very toxic. They are carcinogenic and at very high concentrations, may lead to brain damage2. So their removals from industrial waste waters remain an important challenge. In spite of the hazards associated with these metals, a number of industries indiscriminately dispose of their waste containing metals directly into the environment, especially in the water bodies (streams, lake, river etc.).
Also, through the use of domestic antiseptic soaps and pesticides on our farms, these poisonous substances are washed into the water bodies3. Consequently, these metals accumulate in aquatic biotas which inhabit these water bodies. Feeding on these aquatic animals such as fishes and use of the contaminated water for drinking and cooking purposes can lead to poisoning of humans4.
Various methods have been used to remove heavy metal from waste water such as chemical precipitation, coagulation, floatation, adsorption, ion exchange, reverse osmosis and electrodialysis5. The production of the sludge in the precipitation methods poses challenges in handling treatment and hand filling of the solid sludge. Ion exchange usually requires a high capital investment for the equipment as well as high operational cost. Electrolysis allows the removal of metal ions with the advantage that there is no need for additional chemicals and also there is no sludge generation. However, it is inefficient at a low metal concentration. Membrane processes such as reverse osmosis and electrodialysis tend to suffer from the in-stability of the membranes in salty or acidic conditions and fouling by inorganic and organic substances present in waste water6. A search for a low-cost and easily available adsorbent has led to the investigation of materials of biological origin as potential metal biosorbents. Biosorption is becoming a potential alternative to the existing technologies for the removal and recovery of toxic metals from wastewater. The main advantage of biosorption is that it leads to the significant amount of energy saving from a more efficient wastewater treatment system operating for fewer hours, it is economically attractive because waste biomass is inexpensive and widely available7.
Chestnut shell is one of the agricultural waste products. Its constituents include; lignin-30.25%, cellulose-31.42%, pentosans-36.24%, ash-0.678. The other constituents are hemicelluloses and extractives. These extractives contain tannins, pectins, with phenol, carboxyl and hydroxyl groups. Lignins and cellulose in the shell affords its adsorptive/ion exchange properties. Acid mine drainage (AMD) occurs when metal sulfides, most commonly pyrite, are exposed to and react with air and water. This study was carried out using chestnut (castanae sativa) shell as a biomaterial for the removal of heavy metals from acid mine drainage (AMD).
MATERIALS AND METHODS:
Adsorbent:
Chestnut fruits (Castanae sativa) used for this work were obtained from a horticultural garden in Ondo State, Nigeria. The endosperms were separated from the shells. The shells were soaked for one hour in water, scrubbed using a sponge and tap water to remove dust and other impurities, soaked in hot water to remove soluble and coloured components and then sun dried for three (3) hours. The dried shells were ground into smaller particles using the manual grinding machine and sieved to obtain finer particles using 120mm mesh. The finer dust particles were re-introduced into an oven at a temperature of 105oC for 24hours and then preserved in an air tight glass bottle to protect it from moisture. The concentration of Pb and Cd ions in the adsorbent were determined by soaking 5g of the adsorbent in 100ml deionized water for 60min in a 250ml beaker. Aliquot portions of the eluate were decanted into 100ml plastic bottle and analyzed for the heavy metals present in the adsorbent using Atomic Absorption Spectrophotometer (AAS) (AA-7000)
Adsorbates:
All reagents used for this study were analytical reagent grade. Stock solution of Pb2+ions was prepared by dissolving 1.60grams of lead (II) nitrate in 700mL of distilled water and topping up to 1000mL in a one litre volumetric flask. The solution was then diluted to obtain the desired standard solutions. Cd2+ions were prepared by dissolving 1.63grams of cadmium chloride in 700mL of distilled water and topping up to 1000mL in a one litre volumetric flask. The solution was then diluted to obtain the desired standard solutions. 0.1M Nitric acid and aqueous ammonia were used throughout to adjust the solution pH whenever needed.
Adsorption experiments:
Batch adsorption experiments were carried out to study the effect of initial metals ion concentration, contact time, PH and temperature on the adsorption of Pb2+ and Cd2+ on chestnut shell. Adsorption studies were carried out using 25ml of each metal ion solution and 0.5g of the adsorbent. At the end of each experiment, the content of each tube was filtered using a filter paper after which the concentration of residual metal ions in each filtrate was determined using Atomic Adsorption Spectrometer (AAS) (AA-7000). All experiments were carefully conducted in triplicates to acquire good reproducible result. The adsorption of Pb2+ and Cd2+ ions from Acid Mine Drainage on chestnut shell were studied using the various optimum conditions obtained from the effect of pH, contact time, temperature and initial metal concentration on the removal of Pb2+ and Cd2+ ions from wastewater.
Different adsorption kinetic models were used to determine the adsorption parameters: Pseudo-first-order kinetic model, pseudo-second-order kinetic model, Elovich equation and intra-particle diffusion model.
The pseudo-first-order kinetic model is expressed as:
t (1)
Where qe and qt are amounts of adsorbate adsorbed (mg/l) at equilibrium and at contact time t (min) respectively and k is rate constant.
The pseudo-second-order kinetic model is expressed as:
(2)
Where qe and qt are the amounts of adsorbate adsorbed (mg/l) at equilibrium and at contact time t (min) respectively and k is the pseudo-second-order rate constant.
The Elovich equation was developed to describe the kinetics of chemisorption of gases on to solids and it is generally expressed as:
(3)
Assuming the initial boundary condition q = 0 at t = 0, equation (1) on integration becomes
(4)
Where
qt = Amount of adsorbate adsorbed at time ‘t’,
β= Initial adsorption rate (mg g-1min-1) and
Β = Desorption constant (g mg-1) during any experiment.
The intra-particle diffusion can be estimated by using the Weber-Morris intra-particle diffusion model.
(5)
where q is the amount of metal adsorbed at any time, t, kid is the intra-particle diffusion rate coefficient and C gives an idea about the thickness of the bounding layer i.e. the larger the intercept, the greater the contribution of surface sorption in the rate determining step.
Isotherm studies:
Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among liquid and adsorbent, based on a set of assumptions that are mainly related to the heterogeneity/homogeneity of adsorbents, the type of coverage and possibility of interaction between the adsorbate species. The analysis of equilibrium data for the adsorption of Cd and Pb on chestnut shell was done using the Langmuir isotherm.
The Langmuir isotherm is given by:
(10)
Where qm and Kl are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption respectively.
RESULTS AND DISCUSSION:
Effect of concentration:
The effect of initial concentration of metal ions on metal ion adsorbed was studied at 30oC. It was found that the concentration of metals ions adsorbed increased with increase in initial metal ion concentration, reached equilibrium and started decreasing. The initial increase was due to the fact that as the concentration increased, more metal ions were available in the solution for the adsorption process and most of the binding sites on chestnut shell were free, which allowed quick binding of both ions at increased rate on the biomass. As the binding sites became occupied, the uptake reached equilibrium and started decreasing as shown in Figure 1, due to poorer uptake at higher metal concentration as a result of the increased ratio of initial number of moles of the ions to the vacant sites available. The two-phase metal uptake process had been reported in previous reports on the biosorption of heavy metal with different sorbents.
Figure 1: Variation of percentage of ion absorbed with concentration of metal ions at 30oC
Effect of contact time:
The effect of contact time on the adsorption of Pb2+ and Cd2+ by chestnut shell is shown in Figure 2. It was observed that the concentration of metal ions adsorbed on the chestnut shell increased with time. This was due to the migration of higher fraction of the metal ions from the bulk solution through the adsorbent boundary layer onto the active sites of the adsorbent as time progresses. This enhanced sorption of the metal ion with increase in agitation time may be due to the decrease in boundary layer resistance to mass transfer in the bulk solution and an increase in kinetic energy of the hydrated metal. The initial faster rate may be due to the availability of the uncovered surface area of the adsorbents, since the adsorption kinetics depends on the surface area of the adsorbents, as these sites were progressively filled, the more difficult the sorption becomes, as the sorption process tend to be more unfavorable.
Figure 2: Variation of quantity of ion absorbed by chestnut shell with time at 30oC
Effect of temperature on the removal of Pb2+ and Cd2+ ions:
Figure 3 show that the removal of Pb2+ and Cd2+ from aqueous solution by chestnut shell is temperature dependent. Increase in temperature from 30oC to 70oC was found to result in a steady increase in the removal efficiency of the adsorbent for both Pb2+ and Cd2+ ions. This was probably due to the effect of temperature on the interaction between the shell surface and the metal ion in solution. Increase in temperature probably weakened the bond formed between the metal ions and the adsorption sites on the adsorbent, thereby resulting in an increase in the amount of metal ions adsorbed on the adsorbent. This implies that increase in temperature creates a wider surface area for adsorption at the adsorbent. However, the magnitude of such increase reached equilibrium between 600C and 700C for Pb2+ and between continues to 500C and 600C for Cd2+ and started declining on further increase in temperature. This is because with increasing temperature, the attractive forces between biomass surface and metal ions are weakened and the sorption decreases.
Effect of pH
The effect of pH on the adsorption of Pb2+and Cd2+ ions was studied at a concentration of 0.5 mg/l for 30 minutes and at 30oC. Figure 4 shows that the concentration of the metal ions increased as the pH moves from the acidic pH 2 to alkaline pH 10. This shows that alkaline medium tends to support adsorption more than acidic medium.
When the pH of the adsorbing medium was increased from pH 2 to 10, there was a corresponding increase in the deprotonation of the adsorbent surface, leading to a decrease in H+ ion on the adsorbent surface. This creates more negative charges on the adsorbent surface, which favours adsorption of positively charge species and the positive sites on the adsorbent surface. The optimum pH for Pb2+ ion removal was 6, and 8 for Cd2+ ion.
Effect of optimum conditions on the removal Pb2+ and Cd2+ ions from AMD:
The removal of Pb2+ ion from Acid Mine Drainage was carried out at pH 6, 300C temperature and 120 minutes contact time. The percentage absorbed with an initial concentration of 461.500 mg/l, was found to be 97.94 %. For Cd2+ ions, the removal was carried out at pH 8, 600C temperature and 180 minutes contact time. The percentage absorbed with an initial concentration of 0.52 mg/l, was found to be 95.96 %. The results obtained showed that chestnut shell is an efficient absorbent for the removal of Pb2+ ion and Cd2+ ion from Acid Mine Drainage.
Figure 3: Variation of quantity of ion absorbed by chestnut shell with temperature
Figure 4: Variation of quantity of ion absorbed by chestnut shell with pH
Adsorption kinetics:
For pseudo-first- order kinetic model, the plot of log (qe-qt) against time should give a straight-line graph where the following parameters, ka and qe can be deduced from the slope and the intercept respectively. The plot of log (qe-qt) against time in Figure 5 shows that the data did not fit to pseudo-first- order kinetic model. The pseudo-second-order equation is based on the sorption capacity of the solid phase. It predicts the behavior over the whole range of data. The plots of t/q against t (min) for the adsorption of Pb2+ and Cd2+by chestnut shell in Figure 6 indicated that the experiment data for the adsorption of Pb2+ and Cd2+ion by chestnut shell fitted into the pseudo-second-order adsorption kinetic model. The k and q values determined from the slopes and intercept of the plots are presented in Table 1 along with the corresponding values of the correlation coefficients. From the results, it can be seen that the values of correlation efficient (R2) implies that the adsorption of Pb2+ and Cd2+ion by chestnut could be best described by the pseudo-second-order model.
Figure 5: The plot of pseudo first order kinetics
Figure 6: The plot of pseudo second order kinetics
