Adsorption of hexavalent chromium from aqueous solutions by low cost biosorbents.
The various metal finishing processes result in the generation of heavy metal pollutants, which are toxic and non-biodegradable. Electroplating is the most commonly adopted metal finishing process. Hexavalent chromium [Cr (VI)] emanating from chrome plating is carcinogenic to human (Karthikeyan et al., 2005). Chromium has been considered as one of the top 16th toxic pollutants and because of its carcinogenic and teratogenic characteristics on the public, it has become a serious health concern (Torresdey et al., 2000).Chromium can be released to the environment through a large number of industrial operations, including metal finishing industry, iron and steel industries and inorganic chemicals production (Gao et al., 2007). Extensive use of chromium results in large quantities of chromium containing effluents which need an exigent treatment. Chromium (VI) is known to cause various health effects such as skin rashes, upset stomach and ulcers, respiratory problems, weakened immune system, kidney and liver damage, alteration of genetic material, lung cancer and also death (HDR Engineering Inc., 2001). World health organization recommended the maximum allowable concentration of 0.05 mg/L in drinking water for chromium (VI). According to the Indian Standard Institution, the desirable limit for chromium as chromium (VI) in drinking water is 0.05 mg/L. There are various methods to remove Cr (VI) including chemical precipitation, membrane process, ion exchange, liquid extraction and electrodialysis (Verma et al., 2006). These methods are non-economical and have many disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal or treatment. In contrast, the adsorption technique is one of the preferred methods for removal of heavy metals because of its efficiency and low cost (Li et al., 2007). The property of biosorption for dead biomass is independent of life functions since dead substrate is not affected by toxic waste. In addition to the ease of use and storage dead biomass can be easily regenerated and reused (Spinti et al., 1995). Several low cost adsorbents such as xanthated saw dust, peanut skins, rice straw and hulls, peat, activated charcoal, rice husk, coconut husk and rice bran, silica, Albizzia lebbeck pods, lime, steel wool, lignin and Ablesmoschus esculentus etc. have been used for removal of chromium. The effectiveness of low cost agro based materials like Tamarindus indica seeds (TS), crushed coconut shell (CS), almond shell (AS), groundnut shell (GS) and walnut shell (WS) for Cr (VI) removal were investigated. Batch tests indicated sorption capacity [q.sub.e] followed the sequence TS > WS > AS > GS > CS (Agarwal et al, 2005). Different variables such as pH, agitation rate and metal concentration affect the biosorption process (Churchill et al., 1995; Chen and Yiacaumi, 1997; Guibalt et al., 1992; Veglio and Beolchini, 1997; Yetis et al., 2000). The present research work includes the use of a suitable biosorbent (living or dead) contacted to aqueous solutions containing a metal ions (Cr / Ni). This contacting process is allowed to proceed for a sufficient time in the form of a packed bed reactor (PBR) for the biosorbent to sequester the metal ion after which the biosorbent is separated from the liquid phase. The liquid phase is then discharged and the metal containing biosorbent is either regenerated (by eluting the metal as a concentrated solution) or disposed in an environmentally acceptable manner.
Material and Methods
This study was accomplished in Environmental biotechnology laboratory, Department of Biotechnology, Punjabi University. The details of materials and methods of the study are discussed as below:
Procurement of Electroplating Industry Effluent
Electroplating Industry effluent has been procured from ME Fine chemicals, Ludhiana. The initial concentration of Cr (VI) ions was determined by following the procedures of APHA, (1992).
Procurement of Biosorbents
Albizzia lebbeck pods, Groundnut shells have been procured from Thapar Institute of Engineering & Technology campus and local market respectively.
Column Equilibration Experiments
Preparation of Biosorbents and loading the Packed Bed Reactor (PBR)
The selected biosorbents i.e. Groundnut shell, Albizzia lebbeck pods are pretreated to improve the physical characteristics of the adsorbents to prevent color leaching as well as to insolubilise the tannins. Endocarp was removed from Prunus dulcis & ground to small pieces. Each of the adsorbents was sun dried and then kept in incubator at 37[degrees]C for 2-3 hours to remove the moisture before grinding. Albizzia lebbeck and Groundnut pods were ground to small pieces,
powdered and sieved through 36mm sieve The mixture (3:1 i.e. 3 parts of Groundnut and 1 part of Albizzia lebbeck) have been loaded into glass column of height 57cm and diameter 1.7cm to promote even distribution of packing, the ends of which were closed by glass wool to prevent the sorbent particles from separating and floating to design PBR. Three columns i.e. Pure GNSP, Hybrid columns were prepared. Treatment with acidified aqueous formaldehyde and 0.2N [H.sub.2]S[O.sub.4] was done to condense the tannins & to remove the color-leaching problem. The column was equilibrated with a slow down flow of water to completely wet the adsorbent and to eliminate air bubbles before introduction of metal solution. The column was washed with distilled water and finally acetone to relieve yellow color of both the biosorbents.
A glass column with an inner diameter of 3 cm and 45 cm in length was used for continuous flow experiments. Two centimetre layer of glass wool was placed at the top and bottom of column. Stock solution of chromium metal ion were prepared in double distilled water using analytical reagent grade of potassium dichromate as per standard methods (APHA, 1992). A series of standard solutions of chromium (20, 40, 50,100 and 200ppm) and industrial effluent (200ppm) have been passed through Hybrid, Pure GNSP Reactor for determining their removal efficiency at optimized pH and flow rate. The fractions have been collected through fraction collector by making use of peristaltic pump so as to achieve continuous treatment technology. Cr (VI) ions concentration were determined in the filtrate using spectrophotometer by colorimetric techniques i.e. diphenyl carbazide method. The binding capacity of all the columns had been calculated to check the adsorption capacity of the biosorbents. The effects of various parameters on the rate of adsorption process were observed by varying initial concentration of chromium ion, [C.sub.0] (20, 40, 50, 100 and 200ppm), adsorbent concentration, W (5 and 10 g/100 mL) and initial pH of solution (2, 3, 5, 7, 9). The solution volume (V) was kept constant (100 mL). The chromium removal (%) at any instant of time was determined by the following equation:
Chromium removal (%) = [C.sub.0] - [C.sub.f]/[C.sub.f] x 100
where, [C.sub.0] and [C.sub.f] are the concentration of chromium at initial and final i.e. before and after passing through the column, respectively. To increase the accuracy of the data, each experiment was repeated 3 times. Adsorption isotherm studies were carried out with different adsorbent doses ranging from 1 to 6 g/100 mL while maintaining the initial chromium concentration at 5 mg/L.
Effect of pH
pH has been termed as the master variable controlling the surface charge of the adsorbent as well as degree of ionization and speciation of aqueous adsorbates. pH has been documented to control the surface properties of several adsorbents. The experiments of this stage were done under the conditions of constant temperature (25 [degrees]C), adsorbent dose (10 g/100 mL) and initial chromium concentration (5 ppm). pH of solution was changed and the chromium removal was investigated. The experimental results of this stage are presented in Fig. 1. As it is shown, the optimum pH of solution was observed at pH of 2 and by increasing pH, a drastic decrease in adsorption percentage was observed. This might be due to the weakening of electrostatic force of attraction between the oppositely charged adsorbate and adsorbent that ultimately lead to the reduction in sorption capacity (Bayal et al., 2006). The adsorptive phenomenon of Cr (VI) to various biosorbents can be attributed to various mechanisms such as electrostatic attraction and repulsion, chemical interaction and ion exchange which are responsible for adsorption of Cr (VI) on sorbent surfaces. The most prevalent form of Cr (VI) in aqueous system are acid chromium (HCr[O.sup.-.sub.4]), chromates (Cr[O.sub.-2.sub.4]) and dichromate (Cr[O.sup.-2.sub.7]) ions and other oxyanions (Bennefield et al., 1982). At lower pH acid chromate ions (HCr[O.sup.-.sub.4]) are the dominant species. Because of proteolysis of surface sites by [H.sup.+]/[H.sub.3][O.sup.+] on the surface of biosorbents by adsorbing [H.sup.+]/[H.sub.3][O.sup.+] which are of smaller size and more mobile at pH 2-3 results in strong electrostatic force of attraction between sorbent and acid chromate ions (HCr[O.sup.-.sub.4]) consequently resulting in higher adsorption capacity.
[FIGURE 1 OMITTED]
As the pH increases from 3 to 7, there occurs decrease in Cr (VI) removal. This may be due to decrease in net positive centres on the surface of the sorbent due to adsorbed Cr (VI) species, resulting in weakening of electrostatic forces between sorbate and sorbent which ultimately leads to the reduction in the sorption capacity.
Effect of Flow rate
Flow rate is one of the important characteristics in evaluating sorbents for continuous-treatment of metal laden effluents on an industrial scale. The flow rate at which the synthetic as well as the effluent containing Cr (VI) ion is passed through Packed bed reactor is optimized to 0.3 ml/min. At this flow rate the wastewater has sufficient contact time with biosorbent to show maximum removal. With the increase in flow rate, the removal efficiency goes on decreasing (Fig. 2).
[FIGURE 2 OMITTED]
Effect of chromium concentration on adsorption process
From the Table 4.1 it was observed that the Cr (VI) adsorption increases with the increase in initial metal ion concentration for hybrid column (3parts GNSP & 1part A.lebbeck). The extent of adsorption from solution increases with concentration but it is probable that a limit is attained in adsorption by a solid surface from solution. This was due to higher probabilities of collision between metal ion and adsorbent (Cetinkaya et al., 1999). On applying this developed technology to industrial effluent Cr (VI) removal was suppressed from 99.4 % to 96.7% at 200ppm concentration. In case of Pure GNSP column, the percentage removal for Cr (VI) goes on decreasing with the increase in metal ion concentration (Table 4.2). The binding capacity for hybrid column and Pure GNSP column was found to be 61 mg and 41 mg respectively.
Although Albizzia lebbeck pods had potential for biosorption but being seasonal cannot be exploited for commercialization. Moreover, % Cr (VI) removal by employing pure A.lebbeck pods was 83.4% for 100m1 of 200ppm concentration as studied by Verma & Rehal, 1996. Groundnut shell waste is generated as an agriculture waste in plenty. In the present study, by replacing pure A. lebbeck column with hybrid column of A.lebbeck and GNSP the % removal efficiency for Cr (VI) was very much improved almost 96.7%. From the results it had been shown that pure GNSP column proved less effective bioscavenger for Cr (VI) ions than hybrid column.
Industrial effluent (200ppm) = 96.7% removal
Amount of Biosorbent = 10gm (3parts GNSP: 1part Albizzia lebbeck), pH = 2.5, Flow Rate = 0.3 ml/min., Total Binding capacity of Hybrid column = 61mg
Adsorption phenomenon conforms to the Freundlich adsorption isotherm in the range 50-200ppm for hybrid column.
Industrial effluent (200ppm) = 89.9% removal
Amount of Biosorbent= 10gm, pH = 2.5, Flow Rate = 0.3 ml/min, Total Binding capacity of Pure GNSP column = 41mg
Optimized parameters and application of developed treatment technology
The optimum mesh size was 36 mesh (BS Sieve) for maximum adsorption capacity for both the columns. 96.7% Cr (VI) was removed with lOgm of biosorbent for 200ppm of Cr (VI) solution at pH 2.5 and flow rate of 0.3m1/min.
The experimental data obtained has been analyzed to examine the biosorptive ability of both the natural resins i.e. Albizzia lebbeck and Groundnut shell in combination and of pure Groundnut shell column. The percent removal of Cr (VI) in case of aqueous solutions for hybrid column comes out be in the range of 95-99.5% and for Groundnut column decreases to 91%. In case of industrial effluent Cr (VI) removal was 96.7% and 89.9% in hybrid and Pure GNSP column respectively. Plot of percent removal against the pH indicates the maximum removal in the range 2-3. The flow rate was adjusted to 0.3 ml/min.
The adsorption phenomenon of Cr (VI) to the selected biosorbents can be attributed to various mechanisms such as electrostatic attraction and repulsion, chemical interactions and ion exchange responsible for adsorption of Cr (VI) acid chromium (HCr[O.sub.4]), chromates (Cr[O.sup.2-.sub.4]) and dichromate ([Cr.sub.2][O.sup.2-.sub.7]) ions and other oxyanions (Bennefield et al., 1982).
The equilibrium biosorption data were modeled by using simple adsorption model such as Freundlich equation to obtain sorption isotherm. The experimental data obtained for the adsorption of Cr (VI) ions on modified hybrid & pure GNSP column conformed with the Freundlich adsorption isotherm (Fig. 4.3 and Fig. 4.4). The Freundlich equation (Freundlich, 1907) is an empirical equation employed to describe the heterogeneous systems. So the presentation of the data is in terms of the residual metal concentration left in solution after binding ([C.sub.e], typical units mg metal/litre) vs, the amount of metal bound to the biosorbent (X/M usually determined by difference, typical units mg metal/g of dry weight). Such a plot is fitted to Freundlich equation (X/M = k[C.sup.1/n.sub.e]) where k and n are Freundlich constants characteristic of the system. The equation can be linearised by taking logarithms of both sides of the equation:
log X/M = log k + (1/n) log [C.sub.e],
A plot of log X/M vs. log Ce yields a straight line with log k as the intercept and 1/n as the slope. Log k is the sorption capacity when [C.sub.e] = 1 and 1/n is the sorption intensity (Faust and Aly, 1985). The Freundlich isotherm parameters K and 1/n were found to be 1.535 and 1.416 for hybrid column and 0.1754 and 0.809 for Pure GNSP column.
Table 4.3 and 4.4 shows the data for interpretation by Freundlich isotherm for hybrid and Pure GNSP column. From the graphical representations (Fig. 4.3 and 4.4) the values of K and 1/n were obtained. From the Fig. 4.3 & 4.4 it can be inferred that Hybrid and Pure GNSP column follows Freundlich adsorption isotherm in the concentration range 50-200 ppm.
[FIGURE 4.3 OMITTED]
[FIGURE 4.4 OMITTED]
Regeneration of columns
Reusability of sorbent is of crucial importance in industrial practice for metal removal from wastewater. This can be evaluated by comparing the sorption performance of regenerated biosorbent with the original biosorbent. The recovery percentage can be obtained from the following relation (Zhao, et al., 1999 and Arica, et al., 2003):
Recovery (%) = (Desorbed /Adsorbed) * 100
that the "desorbed" is the concentration and/or the mass of metal ions after the desorption and the "adsorbed" is equal to (Co-Ce) and/or (mo-me) for each recovery process. mo and me are the heavy metals mass in the aqueous solution, before and after the biosorption, respectively..
The column regeneration studies were carried out using 0.2M [K.sub.2]S[O.sub.4] at 10 mL/min flow and found that the recovery was in the range of 80-85%. 200ppm industrial effluent was passed through hybrid column at the same flow rate and pH. The percent removal comes out to be 96.7% in case of Hybrid column and 89.9% in case of Pure GNSP column. 0.1 M [H.sub.2]S[O.sub.4] and 0.1M KOH were also tried for re-regeneration.
Hybrid column (96.7%) proved to be much more efficient than Pure GNSP column (89.9%) for the removal of 200ppm Cr (VI) from industrial effluent. The developed treatment technologies depend upon pH, flow rate, initial concentration of metal ion. It was inferred that lower pH appeared to be the most favorable range for adsorption of Cr (VI) showing peak adsorption at pH 2.5. The increase in pH resulted in decrease of percentage adsorption. The biosorbents utilized for removal of chromium were the waste products so convenient to use and readily available. Regeneration of biosorbents for hybrid column was successful and hence there is no problem of solid waste disposal. The total binding capacity of Hybrid and Pure GNSP column came out to be 61 mg & 41 mg at an optimized flow rate of 0.3m1/min and pH 2.5 respectively.
Adsorption phenomenon conformed to the Freundlich adsorption isotherm in the range 50-200ppm for hybrid and Pure GNSP column. A plot of log X/M vs. log [C.sub.e] yields a straight line with log k as the intercept and 1/n as the slope where log k was the sorption capacity when [C.sub.e] = 1 and 1/n was the sorption intensity. The Freundlich isotherm parameters along with the K and 1/n were found to be 1.535 and 1.416 for hybrid column and 0.1754 and 0.809 for Pure GNSP column.
The author is thankful to Council of Scientific and Industrial Research (CSIR), New Delhi for providing financial assistance for the present research venture.
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Simmi Goel * * Corresponding Author: Dr. Simmi Goel, Lecturer, Department of Biotechnology, Mata Gujri College, Fatehgarh Sahib-140406, Punjab (INDIA) E-mail: firstname.lastname@example.org
Table 1: Removal of Chromium using Hybrid column. Conc. (ppm) Conc. (ppm) % Removal Before After Treatment Treatment 20 0.40 98.00 40 0.64 98.40 50 3.35 93.30 100 7.50 92.5 200 18.0 91.0 Table 2: Removal of Cr using Pure GNSP Column. Conc. (ppm) Conc. (ppm) % Removal Before After Treatment Treatment 20 0.88 95.60 40 1.24 96.90 50 0.45 99.10 100 0.60 99.40 200 1.20 99.40 Table 4.3: Effect of concentration on sorption of Cr (VI) on hybrid column. Amount of Ion Amount of Ce Log Ce Log X/M adsorbent (g) conc. Cr (VI) (mg/l) (M) (ppm) adsorbed (mg) (X) 10 20 1.912 0.88 -0.0555 -0.71815 10 40 3.876 1.24 0.0934 -0.41162 10 50 4.955 0.45 -0.3468 -0.30496 10 100 9.940 0.60 -0.22185 -0.1340 10 200 19.880 1.20 0.07918 0.2984 Amount of % removal adsorbent (g) (M) 10 95.60 10 96.90 10 99.10 10 99.40 10 99.40 Table 4.4: Effect of concentration on sorption of Cr (VI) on pure GNSP column. Amount Ion Amount of Ce Log Ce Log X/M % of conc. Cr (VI) (mg/l) removal adsorbent (ppm) adsorbed (g) (mg) (M) (X) 10 20 1.960 0.40 -0.3979 -0.70774 98.00 10 40 3.936 0.64 -0.1938 -0.40494 98.40 10 50 4.665 3.35 0.52504 -0.33114 93.30 10 100 9.25 7.50 0.87506 -0.03385 92.50 10 200 18.2 18.0 1.25572 0.26007 91.00 100ml of Cr (VI) solution, 10gm of biosorbent, pH 2.5, flow rate 0.3ml/min