Heavy metal uptake by agro based waste materials
Anwar R. Saleemi
Muhammad Mahmood Ahmad
Financial support: Merit scholarship for PhD to Suleman Qaiser by Higher Education Commission Pakistan.
Keywords: biosorption, ficus religiosa leaves, hexavalent chromium, kinetics, lead, wastewater.
Presence of heavy metals in the aquatic systems has become a serious problem. As a result, there has been a great deal of attention given to new technologies for removal of heavy metal ions from contaminated waters. Biosorption is one such emerging technology which utilized naturally occurring waste materials to sequester heavy metals from industrial wastewater. The aim of the present study was to utilize the locally available agricultural waste materials for heavy metal removal from industrial wastewater. The wastewater containing lead and hexavalent chromium was treated with biomass prepared from ficus religiosa leaves. It was fund that a time of one hr was sufficient for sorption to attain equilibrium. The equilibrium sorption capacity after one hr was 16.95 ± 0.75 mg g-1 and 5.66 ± 0.43 mg g-1 for lead and chromium respectively. The optimum pH was 4 for lead and 1 for chromium. Temperature has strong influence on biosorption process. The removal of lead decreased with increase in temperature. On the other hand chromium removal increased with increase in temperature up to
The application of biosorption in environmental treatment has become a significant research area in the past ten years. Heavy metal ions are reported as priority pollutants, due to their mobility in natural water ecosystems and due to their toxicity (Volesky and Holan, 1995). The discharge of heavy metals into surface waters has become a matter of concern in Pakistan over the last two decades. These contaminants are introduced into surface waters through various industrial operations. The pollutants of concern include lead, chromium, zinc, and copper. Heavy metals such as zinc, lead and chromium have number of applications in basic engineering works, paper and pulp industries, leather tanning, petrochemicals fertilizers, etc. The hexavalent and trivalent chromium is often present in electroplating wastewater (Kratochvil et al. 1998). Other sources of chromium pollution are leather tanning, textile, metal processing, paint and pigments, dyeing and steel fabrication. Lead is used as industrial raw material in the manufacture of storage batteries, pigments, leaded glass, fuels, photographic materials, matches and explosives (Raji and Anirudhan, 1997).
Lead and chromium are toxic metal contaminants in water. According to Pakistan standards the maximum discharge limits for lead and chromium in wastewater are respectively 0.5 mg l-1 and 1.0 mg l-1. Maximum limit in drinking water is 0.05 mg l-1 for both metals. In fact there is no safe level of these metals in drinking water and even a very dilute content can cause adverse health effects. Lead is toxic to living organisms and if released into the environment can bio accumulate and enter the food chain. Lead is known to cause mental retardation, reduces haemoglobin production necessary for oxygen transport and it interferes with normal cellular metabolism. Lead has damaging effects on body nervous system. It reduces I.Q level in children. Strong exposure of hexavalent chromium causes cancer in the digestive tract and lungs and may cause gastric pain, nausea, vomiting, severe diarrhoea, and haemorrhage (Mohanty et al. 2005).
The conventional methods for treatment of lead and chromium wastes include: lime and soda ash precipitation, adsorption with activated carbon, ion exchange, oxidation and reduction, fixation or cementation. These methods are economically unfavourable or technically complicated, and are used only in special cases of wastewater treatment (Kratochvil et al. 1998; Sharma, 2003).
Biosorption of heavy metals from aqueous solutions is a relatively new technology for the treatment of industrial wastewater. Adsorbent materials derived from low cost agricultural wastes can be used for the effective removal and recovery of heavy metal ions from wastewater streams. The major advantages of biosorption technology are its effectiveness in reducing the concentration of heavy metal ions to very low levels and the use of inexpensive biosorbent materials (Holan and Volesky, 1994; Kratochvil and Volesky, 1998).
Removing metals from wastewater requires development of new sorbents. A wide range of commercial sorbents including chelating resins and activated carbon are available for metal sorption, but they are relatively expensive. In recent years, numerous low cost natural materials have been proposed as potential biosorbents. These include moss peat, algae, leaf mould, sea weeds, coconut husk, sago waste, peanut hull, hazelnut, bagasse, rice hull, sugar beet pulp, plants biomass and bituminous coal. (Lee and Volesky, 1997; Singh and Rawat, 1997; Gupta et al. 1998; Quek et al. 1998; Brown et al. 2000; Chong and Volesky, 2000; Dakiky et al. 2002; Johnson et al. 2002; Reddad et al. 2002; Babel and Kurniawan, 2003; Pagnanelli et al. 2003; Sekhar et al. 2003). In this research adsorbent prepared from ficus religiosa leaves was used for treatment of lead and chromium wastes. Effect of operating conditions like temperature, pH and initial metal concentration, on lead and chromium biosorption were investigated.
Leaves of different trees are very versatile natured chemical species as these contain a variety of organic and inorganic compounds. Cellulose, hemicellulose, pectins and lignin present in the cell wall are the most important sorption sites (Volesky, 2003). Leaves have chlorophyll, carotene, anthocyanin and tannin which contribute to metal biosorption. The important feature of these compounds is that they contain hydroxyl, carboxylic, carbonyl, amino and nitro groups which are important sites for metal sorption (Volesky, 2003). Fourier transform infrared (FTIR) spectra of ficus religiosa leaves also indicated the presence of these functional groups (Figure 2).
Cr(VI) is present in solution as CrO4-2 and Cr2O7-2 at normal pH values but when pH values are reduced below 3 then Chromium exists in the form of HCrO4- (Cimino et al. 2000; Demirbas et al. 2004; Park et al. 2006b). When adsorbent developed from ficus religiosa leaves is intimately mixed with chromium solution at low pH values then OH- group present in biomass are replaced by chromate ions in the solution. At pH values close to five the adsorbent surfaces are negatively charged due to release of H+ ions, therefore these attract lead cation (Pb+2). Leaves also have Ca, Mg, Na ions. These are present in the structure of complex organic compounds in leaves and exchange with Pb+2 cations during sorption process (Kratochvil et al. 1998). Leaves have considerable amounts of CaO, MgO, Na2O, K2O etc. When leaf powder is mixed in water these oxides are converted into hydroxides. These hydroxides precipitate the metal cations (Schneider et al. 2001).
All chemicals used were of analytical reagent grade. Lead solution of 1000 mg l-1concentration was prepared by dissolving
Ficus religiosa leaves were collected from local environment of University of Engineering and Technology Lahore, Pakistan. These leaves were washed with tap water and dried in shadow. Dried leaves were ground and sieved to 50 mesh sizes. This powder was soaked in 0.1 molarity (M) HNO3 for 24 hrs (50 g leaves powder was soaked per litre). It was filtered and washed with distilled water to remove acid contents. The washing was continued till the pH of the filtrate became near neutral. It was first dried at room temperature and then in an oven at
FTIR spectroscopy was used to identify the chemical groups present in leaves. The samples were examined using JASCO FTIR 4000 spectrometer within range 400-4000 cm-1. KBr was used as background material in all the analysis.
Batch experiments were carried out to find the equilibrium time for sorption of chromium and lead on ficus religiosa leaves. All experiments were performed three times and average values were used in all calculations.
Where C0 and Cf are the initial and final concentrations of metal in solution, V is the volume of solution and m is the mass of adsorbent. As shown in Figure 3 about 80% removal was attained in first 15 min and concentration became almost constant after 45 min. The fast initial uptake was due to the accumulation of metal ions on surface of adsorbent which is a rapid step. More time was consumed on diffusion of ions to binding sites. It was concluded that one hr was sufficient for sorption to attain equilibrium. The equilibrium capacity obtained after one hr of sorption was 5.66 ± 0.43 mg g-1 and 16.95 ± 0.75 mg g-1 for chromium and lead respectively.
Keeping all other parameters constant adsorbent dose was varied from 1.0 to 50 gm l-1. It can be seen from Figure 4 that an adsorbent dose of 10 gm l-1 is sufficient for optimal removal of both metals. Increasing the dose further did not affect the percentage removal. The removal capacity was low at high dose rate and vice versa. This was due to metal concentration shortage in solution at high dose rates.
Keeping the same operating conditions as mentioned previously, pH of solution was varied from 0.5 to 8 by the addition of
Keeping all other parameters constant temperature was varied from
Biomass contains more than one type of sites for metal binding. The effect of temperature on each site is different and contributes to overall metal uptake. The effect of temperature on biosorption also depends on the heat of sorption. Usually for physical sorption heat of sorption is negative; sorption reaction is exothermic and preferred at lower temperature. For chemisorption the overall heat of sorption is combination of heat of various reactions taking place at sorption sites. It depends on type of metal and adsorbent. That is the reason for having different behaviour of lead and chromium uptake with temperature.
Initial concentrations of both metals were varied from 10 to 1000 mg l-1and quantity of adsorbent was kept constant at 10 gm l-1. It was observed that removal capacity decreased with decrease in metal concentration.
Langmuir adsorption model was applied to data:
This equation was rearranged to get:
Where qe is the amount of metal sorbed per unit weight of biomass at equilibrium, Ce is the residual equilibrium metal concentration left in solution after binding, qmax is the maximum possible amount of metal ion adsorbed per unit weight of biomass and b is the equilibrium constant related to the affinity of the binding sites for the metals, lower is b more is the affinity of metal to biomass.
Equilibrium concentration Ce and equilibrium capacity qe were calculated for each initial metal concentration. Ce was plotted against Ce/qe and a straight line was fitted in the data. Correlation coefficient of 0.996 for chromium and 0.972 for lead indicate that sorption followed Langmuir model (Figure 7 and Figure 8). Values of Langmuir constants qmax and b were calculated from slope and intercept of lines in Figure 7 and Figure 8 and are shown in Table 1.
Low values of parameter b indicate that ficus religiosa leaves have high affinity for chromium and lead. The values of equilibrium relation parameter, RL were calculated for various initial concentrations for both metals. As shown in Table 2, RL values lie between 0 and 1 which indicate favourable sorption isotherm for both metals.
In order to explain the kinetics of biosorption pseudo second order kinetics model was applied.
This equation was integrated and arranged to get the following equation.
Where k is the sorption coefficient, qe is the equilibrium capacity and qt is the sorption capacity at any time t.
Time, t was plotted against t/qt and straight lines having correlation coefficient of 0.99 were fitted in the data for both metals. The sorption coefficient k and equilibrium capacity qe were calculated from the slope and intercept of lines in Figure 9 and are shown in Table 3.
The values obtained for equilibrium capacity for both metals were very close to those obtained experimentally after one hr of sorption. It confirmed that one hr was sufficient for sorption to attain equilibrium and sorption followed pseudo 2nd order kinetics
Ficus religiosa leaves powder was found to be a very good adsorbent for hexavalent chromium and lead. It has good sorption capacity for both metals. The sorption capacity for hexavalent chromium was 5.66 ± 0.43 mg g-1 and for lead was 16.95 ± 0.75 mg g-1. The sorption was pH dependent and optimal pH was 4 and 1 for lead and chromium respectively. Biosorption of metals was temperature dependent. Optimal temperature was
First author is thankful to Dr. Shaiq A Haq for his valuable suggestions. Special thanks to Dr. Nadeem Feroz and Muhammad Umar for their cooperation and support.
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