Since heavy metal ions are not biodegradable, they are usually removed from the contaminated water by physical or chemical treatment processes. Conventional treatment methods (precipitation, membrane separation, ion exchange, reverse osmosis and electrolysis etc.) are not often feasible because of high treatment cost, the need for continuous input of chemicals, and the production of toxic sludge (Akar et al. 2006). However these techniques proved ineffective in remaining within Cr(III) discharge limits (1-2mg/dm3) of industrial effluents (Geundi et al 1997). As a result, the use of alternative treatments such as ion exchange, carbon adsorption, membrane filtration, electroseparation, and bioaccumulation has been applied in "polishing" these effluents (Alves et al 1993). However, such processes may be ineffective and extremely expensive. Bioadsorption, on the other hand, is an emerging technology that also works to overcome the selectivity disadvantages of traditional decontamination processes. (Unz et al 1996).
Biosorption technology is based on the interaction between toxic metals and the binding functional groups on the cell wall structure of the microorganisms or plants. These are mainly composed of polysaccharides, lipids and proteins. Biosorption has recognized as a potential alternative method over the conventional separation techniques. This process utilizes live, dead, pretreated and immobilized forms of biological cells like bacteria, fungi, yeast, algae and agriculture waste as sorbent materials (Gadd, 1990).
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Removal of heavy metals from wastewaters through adsorption, particularly biosorption, has emerged as an alternative technology. A variety of biomaterials and microorganisms have been explored by researchers for biosorption and bioaccumulation including fungi and agriculture waste materials (Fourest et al 1994). Biosorption may occur actively through metabolism or passively through some physical and chemical processes. A biosorbent's immobilization procedure is necessary for the industrial application of biosorption .Immobilization of the agriculture waste in some suitable matrix like silica gel, polyurethane or alginate has proved useful in industrial application. The physical entrapment of the agriculture waste inside a polymeric gel in the form of beads is one of the most widely used techniques for immobilization which not only tackles the above problem but also provides mechanical strength, rigidity and porosity characteristics to the biosorbents. Further, the metal can be recovered from the loaded beads using appropriate desorption techniques, thereby, minimizing the possibilities of environmental contamination (Lu et al 1995).
Everywhere in the world corn are cultivated as important crop. Corn is Pakistan's third most important cereal after wheat and rice. The use of corn in Pakistan for direct human consumption is declining, but its utilization in the feed and wet milling industry is growing at a much faster pace than anticipated. Currently, sufficient corn is grown in Pakistan for domestic needs and there is neither a surplus nor deficit in corn grain supplies. Currently except potato maize is the most profitable, stable and dependable agricultural crop in Pakistan. (Tariq et al 2010). Corn cob and corn leaves are one of the most plentiful and important agriculture waste in maize cultivation accounts for up to 50% of the total corn seed production. Immature cobs are boiled and eaten as corn on the cob or the grains may be removed and eaten as vegetable, or it may be canned. More mature cobs are roasted. The cobs are used for fuel, smoking pork products, and are also as source for charcoal (Sultana et al. 2007).
, c The present study is designed to study the usefulness of immobilized corn cob and corn leaves agriculture waste biomass as a biosorbent for Cr(IIl) and Cr(Vl) from aquous industrial waste from tanneries to evaluate the effect of different experimental variables like pH, initial metal concentration and contact time. After biosorption the morphology of the surface of corn cob and corn leaves biomass with a scanning electron microscope (SEM) will be observed.
Krishna et al. (2005) studied the possibility of using moss (Funaria hygrometrica), immobilized in a polysilicate matrix as substrate for speciation of Cr(III) and Cr(VI) in various water samples has been investigated. Experiments were performed to optimize conditions such as pH, amount of sorbent and flow rate, to achieve the quantitative separation of Cr(III) and Cr(VI). During all the steps of the separation process, Cr(III)was selectively sorbed on the column of immobilized moss in the pH range of 4-8 while, Cr(VI) was found to remain in solution. The retained Cr(III) was subsequently eluted with 10 ml of 2 mol lâˆ’1 HNO3. A pre-concentration factor of about 20 was achieved for Cr(III) when, 200 ml of water was passed. The immobilized moss was packed in a home made mini-column and incorporated in flow injection system for obtaining calibration plots for both Cr(III) and Cr(VI) at low ppb levels that were compared with the plots obtained without column. After separation, the chromium (Cr) species were determined by inductively coupled plasma mass spectrometry (ICP-MS) and flame atomic absorption spectrometry (FAAS). The sorption capacity of the immobilized moss was found to be 11.5 mg gâˆ’1 for Cr(III). The effect of various interfering ions has also been studied. The proposed method was applied successfully for the determination of Cr(III) and Cr(VI) in spiked and real wastewater samples and recoveries were found to be >95%.
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Anjan et al.(2007). analysed Biosorption of Cr(VI) using native strains of cyanobacteria from metal contaminated soil in the premises of textile mill. Biosorption reported as a function of pH (1-5), contact time (5-180 min) and initial chromium ion concentration (5-20 mg/l) to find out the maximum biosorption capacity of alginate immobilized Nostoc calcicola HH-12 and Chroococcus sp. HH-11. The optimum conditions for Cr(VI) biosorption are almost same for the two strains (pH 3-4, contact time 30 min and initial chromium concentration of 20 mg/l) however, the biomass of Chroococcus sp. HH-11 was found to be more suitable for the development of an efficient biosorbent for the removal of Cr(VI) from wastewater, as it showed higher values of qm and Kf, the Langmuir and Freundlich isotherm parameters. Both the isotherm models were suitable for describing the biosorption of Cr(VI) by the cyanobacterial biosorbents.
Gao et al. (2008) analysed the removal of Cr(VI) from aqueous solution by rice straw, a surplus agricultural by product . The optimal pH was 2.0 and Cr(VI) removal rate increased with decreased Cr(VI) concentration and with increased temperature. Decrease in straw particle size led to an increase in Cr(VI) removal. Equilibrium was achieved in about 48 h under standard conditions, and Cr(III), which appeared in the solution and remained stable thereafter, indicating that both reduction and adsorption played a part in the Cr(VI) removal. The increase of the solution pH suggested that protons were needed for the Cr(VI) removal. A relatively high level of NOâˆ’3 notably restrained the reduction of Cr(VI) to Cr(III), while high level of SOâˆ’24 supported it. The promotion of the tartaric acid modified rice straw (TARS) and the slight inhibition of the esterified rice straw (ERS) on Cr(VI) removal indicated that carboxyl groups present on the biomass played an important role in chromium remediation even though were not fully responsible for it. Isotherm tests showed that equilibrium sorption data were better represented by Langmuir model and the sorption capacity of rice straw was found to be 3.15 mg/g.
Li et al. (2008) investigated the comparative study on adsorptions of Pb(ll) and (CrVl) ions by free cells and immobilized cells of Synechococcus sp. ,in which different aspects including Zeta potential of the cells, the influence of pH, temperature and initial concentration of metal ions, as well as adsorption kinetics and mechanism were referred. The lyophilized free cells have a surface isoelectric point at pH 3, and the correlative experiment indicates that there is an electrostatic adsorption feature of Cr(Vl) and Pb(ll). The immobilization of the free cells by Ca-alginate does not significantly modify the adsorption features of the biosorbent. The absorption processes of Cr(Vl) and Pb(ll) on both free and immobilized cells are apparently affected by pH and the initial concentration of metal ions in the bulk solution, but are much weakly affected by temperature in the test range of 10â-¦C-50â-¦C. The slow course of biosorption follows the first order kinetic model, the adsorption of Pb(ll) obeys both Langmuir and Freundlich isotherm models, while the adsorption of Cr(Vl) obeys only Freundlich model. FT-IR results indicate that carboxylic, alcoholic, amide and amino groups are responsible for the binding of the metal ions, and reduction of Cr(lll) to Cr(Vl) takes place after Cr(Vl) adsorbs electrostatically onto the surface of the biosorbents.
Park et al. (2008) analysed agricultural biowastes such as banana skin, green tea waste, oak leaf, walnut shell, peanut shell and rice husk, banana skin screened as the most efficient biomaterial to remove toxic Cr(VI) from aqueous solution. X-ray photoelectron spectroscopy (XPS) study revealed that the mechanism of Cr(VI) biosorption by banana skin was its complete reduction into Cr(III) in both aqueous and solid phases and partial binding of the reduced-Cr(III), in the range of pH 1.5-4 tested. One gram of banana skin could reduce 249.6 (Â±4.2) mg of Cr(VI) at initial pH 1.5. Namely, Cr(VI)-reducing capacity of banana skin was four times higher than that of a common chemical Cr(VI)-reductant, FeSO4.7H2O. To diminish undesirable/serious organic leaching from the biomaterial and to enhance removal efficiency of total Cr, its powder was immobilized within Ca-alginate bead. The developed Cr(VI)-biosorbent could completely reduce toxic Cr(VI) to less toxic Cr(III) and could remove almost of the reduced-Cr(III) from aqueous phase. On the basis of removal mechanisms of Cr(VI)and total Cr by the Cr(VI)-biosorbent, a kinetic model was derived and could be successfully used to predict their removal behaviors in aqueous phase. In conclusion, our Cr(VI)-biosorbent must be a potent candidate to substitute for chemical reductants as well as adsorbents for treating Cr(VI)-bearing waste waters.
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Alez et al. (2009) investigated the biosorption of Cr(III) onto packed columns of Agave lechuguilla using an advective-dispersive (AD) model and its analytical solution. Characteristic parameters such as axial dispersion coefficients, retardation factors, and distribution coefficients were predicted as functions of inlet ion metal concentration, time, flow rate, bed density, cross-sectional column area, and bed length. The root mean square error (RMSE) values 0.122, 0.232, and 0.285 corresponding to the flow rates of 1, 2, and 3 (10âˆ’3)dm3 minâˆ’1, respectively,indicated that the AD model provides an excellent approximation of the simulation of lumped breakthrough curves for the adsorption of Cr(III) by lechuguilla biomass. Therefore,the model can be used for design purposes to predict the effect of varying operational conditions.
Chen et al. (2010) analysed Cr(III) ionic imprinted membrane adsorbents Cr(III)-PVA/SA) by blending sodium alginate (SA) with polyvinyl alcohol (PVA). In these new membrane adsorbents, polyethylene glycol was used as porogen, and glutaraldehyde was the cross-linking agent. Our new developed membrane adsorbents can be used without centrifugation and filtration. To investigate the adsorption kinetics of Cr(III) ions from aqueous solution onto this newly developed Cr(III)-PVA/SA, we performed a batch of experiments under different conditions by changing the concentration of Cr(III) ions in the Cr(III)-PVA/SA, pH value of the solution, adsorbent dose, initial Cr(III) ions concentration, adsorption temperature and contact time. Our Cr(III)-PVA/SA exhibited the maximum Cr(III) ions uptake capacity of 59.9 mg/g under the following conditions: 0.078 wt% of Cr(III) ions in the Cr(III)-PVA/SA, solution pH value of 6.0, adsorbent dose of 0.5 g/L, the initial Cr(III) ions concentration of 50 mg/L, at 25 â-¦C. To study the mechanism of adsorption process, we examined the intra-particular diffusion model, Lagergren pseudo-first-order kinetic model and pseudo-second-order kinetic model, and found pseudo-second-order kinetic model
exhibited the best correlation with our experimental data. Furthermore, our adsorption equilibrium data could be better described by the Langmuir equation. Competitive adsorption studies of the binary system of Cr(III)/Cu(II), Cr(III)/Cd(II) and the ternary system of Cr(III)/Cu(II)/Cd(II) were also investigated using Cr(III)-PVA/SA, the results indicated that selectively adsorbed amount of Cr(III) ion on Cr(III)- PVA/SA is significantly higher than that of Cu(II) and Cd(II) ions. We also used five times consecutive adsorption-desorption experiments to show that the Cr(III)-PVA/SA has high adsorption and desorption efficiencies.
Huang et al.( 2010) studied a new approach for the preparation of tannin-immobilized adsorbent by using mesoporous silica bead as the supporting matrix. Bayberry tannin-immobilized mesoporous silica bead (BT-SiO2) was characterized by powder X-ray diffraction to verify the crystallinity, field-emission scanning electron microscopy to observe the surface morphology, and surface area and porosity analyzer to measure the mesoporous porous structure. Subsequently, the adsorption experiments to Cr(III) were applied to evaluate the adsorption performances of BT-SiO2. It was found that the adsorption of Cr(III) onto BT-SiO2 was pH-dependent, and the maximum adsorption capacity was obtained in the pH range of 5.0-5.5. The adsorption capacity was 1.30mmolgâˆ’1 at 303K and pH 5.5 when the initial concentration of Cr(III) was 2.0mmolLâˆ’1. Based on proton nuclear magnetic resonance (HNMR) analyses, the adsorption mechanism of Cr(III) on BT-SiO2 was proved to be a chelating interaction. The adsorption kinetic data can be well described using pseudo-first-order model and the equilibrium data can be well fitted by the Langmuir isothermal model. Importantly, no bayberry tannin was leached out during the adsorption process and BT-SiO2 can simultaneously remove coexisting metal ions from aqueous solutions. In conclusion, this study provides a new strategy for the preparation of tannin-immobilized adsorbents that are highly effective in removal of heavy metals from aqueous solutions.
Kathiravan et al. (2010) studied the external mass transfer effects on the reduction of hexavalent chromium (Cr(VI)) using calcium alginate immobilized Bacillus sp. in a re-circulated packed bed batch reactor (RPBR). The effect of flow rate on the reduction Cr(VI) was studied. Theoretically calculated rate constants for various flow rates were analyzed using external film diffusion models and compared with experimental values. The external mass transfer coefficients for the bioconversion of Cr(VI) were also investigated. The external mass transfer effect was correlated with a model of the type JD = K, Re (1-n). The model was tested with various K values and the mass transfer correlation JD = 5.7, Re=0.70 was found to predict the experimental data accurately. The proposed model would be useful for the design of industrial reactor and scale.
Ansari et al. (2011) immobilized Rosa centifolia and Rosa gruss an teplitz distillation waste biomass using sodium alginate for Pb(II) uptake from aqueous solutions under varied experimental conditions. The maximum Pb(II) adsorption occurred at pH 5. Immobilized rose waste biomasses weremodified physically and chemically to enhance Pb(II) removal. The Langmuir sorption isothermand pseudosecond- order kinetic models fitted well to the adsorption data of Pb(II) by immobilized Rosa centifolia and Rosa gruss an teplitz. The adsorbed metal is recovered by treating immobilized biomass with different chemical reagents (H2SO4, HCl and H3PO4) and maximum Pb(II) recovered when treated with sulphuric acid (95.67%). The presence of cometals Na, Ca(II), Al(III), Cr(III), Cr(VI), and Cu(II), reduced Pb(II) adsorption on Rosa centifolia and Rosa gruss an teplitz waste biomass. It can be concluded from the results of the present study that rose waste can be effectively used for the uptake of Pb(II) from aqueous streams.