چكيده لاتين :
In this research, rice husk and activated carbon adsorbents were used for removal of lead ion from aqueous solution at pH= 6. In the batch tests experimental parameters were studied, including contact time and initial metal ions concentration. Survey of the equilibrium time at different concentration showed that equilibrium time is reduced with decrease of initial concentration of solution from 50 mg/1 to 1 mg/1 for both of the adsorbents, and they are closed to each other. In this state is obtained equilibrium time 45 and 60 min for activated carbon and rice husk, respectively. With the increase of initial concentration of solution from 50 mg/1 to 500 mg/1, the equilibrium time is also decreased and they converge. In this condition, is obtined equilibrium time 60 min for activated carbon and rice husk and and are closed to each other. Maximum equilibrium time is obtained 120 and 90 min for rice husk and granular activated carbon adsorbent. Study of the adsorption efficiency at different concentrations indicated that adsorption efficiency is increased with decrease of initial concentration of solution for both the adsorbents, and they are closed to each other. The maximum adsorption efficiency for rice husk and activated carbon at initial concentration of 1 mg/1 was found. The attained adsorption efficiency in this concentration was approximately %98 and %99.7 for rice husk and activated carbon adsorbents.
In this study, kinetic adsorption models were also studied and Ho et al. (1996) model rather than the lagergren (1898) described data better. Also, fitting langmuir and freunlich adsorption isotherm models on experimental data showed that freundlich model described experimental data better.
The pollution from heavy metals not only threatens human health, but also has an irrevocable damages on natural ecosystems. In 1984 World Health Organization (WHO) suggested the maximum concentration of lead 0.05 mg/1 in drinking water. According to these harmful effects and also accumulative property of this metal in human body and because of not being biodegradable, removal of this metal from sewage is considered an essential task.
Different methods have been suggested for industrial wastewater treatments, such as chemical precipitation, reverse osmosis, ion exchange and adsorption by activated carbon are useful methods in industry. In 1883 activated carbon has been used for wastewaters treatment in America. Powdered activated carbon was also used in Chicago to control the smell due to chlorophenol in 1920. However, the high cost of activated carbon preparation had people think about using other cheap and available absorbents in recent years. Although other absorbents havenʹt the capability of activated carbon in adsorbing pollutants, but the low cost of their preparation has attracted scientistʹs attention to use these substances. Study about the heavy metals adsorption, using plant residue was generally started from 1970 to 1980.
One of the most important agricultural residues is rice husk which is attained from rice milling industry. It is estimated that annual rice production in worlds is about 100 million tones. Approximately, a large amount of rice husk is burnt in the area, generating CO2 and other forms of pollution.
Because of the existing a lot of abundant floristic fiber, protein and some functional groups such as carboxyl, hydroxyl and amidogen, etc, in rice husk, it makes adsorption processes possible.
Thus the use of rice husk not only is cheap and easily available, but also it reduces the environmental pollutions.
This study is purposed to investigate rice husk usage possibility as a replacement for activated carbon adsorbent for removal of lead ions from aqueous solution at low concentrations. In this article adsorption efficiency and equilibrium time convergence are studied at low concentration. Kinetic and isotherm adsorption models are also considered.
Fresh rice husk was obtained from a local rice mill and was passed through different sieve size. In this study granular activated carbon (which itʹs raw substance is wood) is used. The diameters of carbon adsorbent particles were about 1.5 mm. Stock solutions of 1000 mg/1 were prepared by using metal nitrate salts, Pb(N03)2, which were diluted with distilled water to prepare working solutions.
For providing the kinetic adsorption experimental, 12 numbers of 250 ml erlenmeyer flask is used in order to pour 1 g weighted rice husk adsorbent in it. Then 100 ml of lead solution with 1 mg l"1 concentration is added to each erlenmeyer flask and the pH is adjusted at 6. The solutions are put on the reciprocating shaker with the speed of 180 rpm for different time such as: 5 to 240 minutes. The mixture was then filtered on qualitative filter paper and the residual lead ion concentration in the solution was analyzed by Atomic Absorption Spectrophotometer. These processes were carried out for initial lead concentration of 10, 20, 50, 100, 200 and 500 mg l1. Also this process for activated carbon adsorbent is repeated.
Adsorption isotherms were determined by shaking various weights of adsorbents (0.1 to 1.2 g) with 100ml lead solutions using a fixed concentration of 1 mg L"1 at the predetermined equilibrium time. With the beginning of adsorption, the uptake of metal ions on both the adsorbents increased. The adsorption process of lead on adsorbents was speedy and for rice husk and activated carbon in 60 and 45 min, the process of adsorption reached equilibrium. At equilibrium stage, 0.098 mg/g and 0.0997 mg/g of lead ion were removed from the solution by rice husk and activated carbon, respectively (efficiency adsorption at equilibrium time for rice husk and activated carbon adsorbents were %98 and %99.7, respectively). The adsorption efficiency in all concentrations increased with increase in agitation time. The results demonstrate that for both the adsorbents, adsorption efficiency increased with decreasing initial concentration of solution from 500 to 1 mg/L. The equilibrium time for 1,10, 20, 50, 100, 200 and 500 mg/1 of initial lead concentration was found to be 60, 90, 105, 120, 90, 75 and 60 min for rice husk adsorbent and 45, 60, 75, 90, 75, 70 and 60 min for activated carbon adsorbent, respectively.
In high concentrations due to much aggregation of metal ions in adsorbent around and the increase of encounter chance between metal ions and adsorbent, a little time spends to saturate of adsorbent capacity, and process reaches to equilibrium. Due to decrease the number of metal ions in solution and also decrease of encounter chance between metal ions and adsorbent, Equilibrium time of adsorption process increased with decrease of solution initial concentration from 500 mg/1 to 50 mg/1. It is expected that due to decrease of encounter chance between metal ions and adsorbent, equilibrium time increased with decrease of initial concentration solution from 50 mg/1 to 1 mg/1, but considering that at low concentrations, metal ions quantity in solution is low, this limited quality of ions will adsorbed to the adsorbent surface in little time. Thus it is observed that equilibrium time reduced with decrease of initial concentration solution from 50 mg/1 to 1 mg/1 for both adsorbents and obtained 60 and 45 min for rice husk and activated carbon, respectively.
It is observed that with decreasing initial concentration of the lead from 500mg L"1 to 1 mg L"1, the adsorption efficiency increases from %33.44 to %99.7 for activated carbon and %33.21 to %98 for rice husk. Also results demonstrate that maximum adsorption efficiency at equilibrium time is obtained at 1 mg L"1 of lead initial concentration. The number of existed metal ions in solution reduced with decrease in initial concentration; therefore adsorbents have been high removal percentage at low concentration, with decreasing initial concentration of lead from 100 mgLʹ1 to 1 mg L"1, adsorption efficiency difference between rice husk and activated carbon adsorbents will reduces, and equals at lmg L"1 initial concentration of lead, approximately.
The calculated qe values obtained from the lagergren (1898) kinetic model do not agree with the experimental qe values (Table 1). This shows that the adsorption of Lead on rice husk and activated carbon does not follow the Lagergren (1898) kinetic model. The calculated values of q. from the Ho et al. (1996) kinetic model agree very well with the experimental data (Table 1). Indicates that the adsorption of lead onto rice husk and activated carbon adsorbents follows Ho et al. (1996) kinetic model at initial concentration 1 mg L~l. The comparison of Ho et al. (1996) model coefficients for activated carbon (k2= 13.13) and rice husk (k2= 10.856) shows that adsorption speed of lead on activated carbon is quicker than rice husk adsorbent.
Table 1: lagergren and Ho et al. rate constants and calculated and experimental qe values.
adsorbents Qe exp (mg/g) Lagergren models Ho et al. models
qecal (mg/g) ki (1/min) R1 Qe cal (mg/g) k2 (g/mg/min) R2
Rice Husk Activated carbon 0.098 0.0997 0.0148 0.0149 0.0642 0.0882 0.8744 0.8673 0.0991 0.101 10.856 13.13 0.999 8 0.999 8
The fitted constants for two models with regression coefficients are summarized in Table 2. The n values between 1 and 10 indicate good adsorption efficiency. This demonstrates that the rice husk and activated carbon adsorbents have good adsorption efficiency for Pb removal. It was found that the Freundlich adsorption isotherm fitted the experimental data better than that of the Langmuir adsorption isotherm for both the adsorbents.
Table 2: Freundlich and Langmuir constants
adsorbents Langmuir Freundlich
b (mg/g) a(L/g) R2 n k R2
Rice Husk 0.413 12.89 0.9765 1.828 0.76 0.9969
Activated carbon 0.476 33.99 0.971 1.741 1.621 0.9907
The equilibration time of adsorption process and also adsorption efficiency as economical parameters have been the most important for technology improvement of wastewater treatment based on natural adsorbent. The solution concentration is an effective factor for equilibration time and adsorption efficiency at low concentrations. Therefore equilibrium time reduces with decrease of initial concentration of solution from 50 to 1 mg/1 for both the adsorbent, and they close to each other. Also adsorption efficiency increase with decrease of initial concentration solution from 500 to 1 mg/1 for both the adsorbents, and they converge. Considering the convergence of equilibrium time and lead (II) adsorption efficiency by rice husk and activated carbon adsorbents at low concentrations, the difference between the strong adsorbent (activated carbon) and weak adsorbent (rice husk) fairly vanished and rice husk can be used instead of activated carbon for lead adsorption from aqueous solution at low concentrations.
