CHAPTER 4: BIOSORPTION OF Cr(VI)

Transcription

1 CHAPTER 4: 4.1 Introduction Apart from microbial Cr(VI) reduction, biosorption is another cost effective and feasible approach for removal of Cr(VI) from industrial wastewaters (Wang and Chen, 2009). There are three principle advantages of using biosorption as an alternate treatment process; first, it can be carried out in-situ at the contaminated site, second this technology is environmentally benign and third the process is cost effective. These advantages have served as the primary incentives for developing full scale biosorption processes to clean up Cr(VI) pollution. Biosorption is defined as the passive uptake of toxicants by inexpensive dead/inactive biological materials or by materials derived from biological sources (Chojnacka, 2010). Biosorption is due to a number of metabolism-independent processes that essentially take place in the cell envelope, where the mechanisms responsible for pollutant uptake will differ according to the biomass type (Gupta et al., 2000; Baldrian, 2008; Farooq et al., 2010). Although, many biological materials bind to Cr(VI) but only those with sufficiently high metalsequestering capacity and selectivity for Cr(VI) would be suitable for removal from industrial effluents Biosorbents from microbial origin The large number of micro-organisms belonging to various groups, viz bacteria, fungi and algae have been reported to bind Cr(VI) to varrying extents (Bishnoi et al., 2007; Han et al., 2007; Liu et al., 2007; Prakasham et al., 1999; Srivastava et al., 2008). These biosorbents can effectively sequester dissolved Cr(VI) out of complex solutions with high efficiency. In general, the biosorbent behaviour for metallic ions is a function of the chemical nature of the microbial cells (Gupta et al., 2000). Bacteria: Bacterial biomass represents an efficient and potential class of biosorbents for the removal of Cr(VI) in particular and any metal ions/dyes in general. The advantage of using bacteria as biosorbent for Cr(VI) removal is due to small size of bacteria, their ubiquity, their ability to grow under controlled conditions, and their resilience to a wide range of environmental situations (Vieiral and Volesky, 2000). The cell envelope of all bacteria is not identical. Infact, the cell-wall composition is one of the most important factor which plays vital role in Cr(VI) biosorption. Several functional groups are present on the bacterial cell wall, including carboxyl, phosphate, amine and 78

2 hydroxyl groups. Of these, amine groups are responsible for binding to negatively charged chromate ions via electrostatic interaction (Volesky, 2007). Fungal biosorbents: Another important biosorbent that has gained momentum in recent years are fungal biosorbents (Baldrian, 2008). In general fungal cell walls are mainly % polysaccharides, with proteins, lipids, polyphosphates and in-organic ions. Chitin is a common constituent of fungal cell walls. Chitin is a strong but flexible nitrogen containing polysaccharide consisting of N-acetyl-glucosamine residues. All these biopolymers offer many functional groups such as carboxyl, hydroxyl, sulphate, phosphate and amino groups that can bind several metal ions (Volesky, 2007). Important fungal biosorbents include Aspergillus, Rhizopus, Neurospora, Mucor etc. (Aksu and Balibek, 2007; Khambhaty et al., 2009; Liu et al., 2007; Mungasavalli et al., 2007; Prakasham et al., 1999; Tunali et al., 2005). The ongoing research on Cr(VI) biosorption suggests that fungal biomass can passively bind metal ions via various physicochemical mechanisms. Biosorption of metals may involves one or combination of several phenomenon viz. ion exchange, complexation, co-ordination, adsorption, electrostatic interaction and chelation (Volesky, 2001; Wang and Chen, 2009). Fungi are of special interest in search for and the development of new biosorbent materials due to their high biosorption capacity. Fungal biomass for Cr(VI) biosorption offer several advantages over any other microbial biomass (Baldrian, 2008; Gupta et al., 2000). Fungi are easy to grow and produce high yield of biomass Alternately, fungi are widely used in a variety of large-scale industrial fermentation processes, and thus fungal biomass can be easily procured in substantial quantities (as source of potential biomaterials for removal of Cr(VI) from wastewaters) as a by-product from industrial fermentation processes, with low cost. The fungal cultures are also amenable to genetic and morphological manipulations that may result in better raw biosorbent materials. All these advantages together has made fungi of primary interest as a raw material for development of efficient Cr(VI) biosorbents Preparation of microbial biosorbents As biosorption process mainly involves cell surface sequestration, increasing/activating the binding sites on the cell surface has been considered as an effective approach for enhancing the biosorption 79

3 capacity (Farooq et al., 2010; Parvathi and Nagendran, 2008; Yan and Viraraghavan, 2000). It should be noted that, Cr(VI) behaves as an oxy-anion in aqueous medium. Hence, Cr(VI) may not bind to negatively charged functional groups such as carboxylate, phosphate and sulphate present on the microbial cell wall due to respective charge repulsion. On the other hand, positively charged amino groups present in the cell wall components (hexoseamines and proteins) can electrostatically attract the negatively charged chromate ions (Bai and Abraham, 2002; Mungasavalli et al., 2007; Tunali et al., 2005). In the light of certain recent studies, a number of methods can be employed for the modification of microbial cell surfaces in order to enhance the Cr(VI) binding capacity of biomass and to elucidate the mechanism of biosorption. These modifications can be introduced either during the growth of micro-organisms or in the pre-grown biomass. The pre-grown biomass could be given several physical and chemical treatment to tailor the Cr(VI)-binding properties of biomass. The physical treatment includes heating/boiling, freezing/thawing, drying and lyophilization. Various chemical treatments used for biomass modification include washing the biomass with detergents, cross-linking with organic solvents and treatment with alkali or acid (Bai and Abraham, 2002; Gupta et al., 2000; Yan and Viraraghavan, 2000). The development of pretreated fungal biomass is practically feasible because the techniques required are relatively simple and technologically preferable. The pre-treatment have been suggested to modify the surface characteristics/groups either by removing or masking the groups or by exposing more metal binding sites (Bayramoglu et al., 2005). The selective acid pre-treatment of Lentinus sajor-caju mycelia revealed the involvement of amino groups in Cr(VI) biosorption (Bayramoglu et al., 2005). The acid pre-treatment can cause degradation of protein and hexoseamine in the cell-wall structures and can increase number of amino group binding sites (Bai and Abraham, 2002). On the contrary, treatment of Rhizopus nigricans with NaOH affected the Cr(VI) biosorption unfavourably (Bai and Abraham, 2002). Alkali treatments of the biomass may cause hydrolysis of protein constituents and de-acetylation of chitin. Both of these would reduce the number of amino groups from the biomass surface, thereby affecting Cr(VI) biosorption efficiency. Biosorbents with low density of amino groups show low Cr(VI) biosorption capacity and various procedures which increase the number of amino groups, result in enhanced Cr(VI) biosorption capacity Factors affecting microbial Cr(VI) biosorption Batch Cr(VI) biosorption experiments are usually influenced by various chemical factors. Studies demonstrating the effect of such factors are important in the evaluation of full biosorption potential of any biosorbent. The important factors include: 80

4 I. Solution ph: The ph affects the solution chemistry of Cr(VI) as well as activity of the functional groups associated with the biomass (Kumar et al., 2008). The effect of ph on biosorption of Cr(VI) has been investigated by various researchers using a variety of different types of biomass (Chen et al., 2006; Kiran et al., 2007; Tunali, et al., 2005). The experimental results by Khambhaty et al. (2009) indicated that biosorption capacity of dead biomass of Aspergillus niger improved with an increase in the solution acidity up to ph 1.0. The biosorption capacity of Cr(VI) reduced when the ph of the solution was increased from 1.0 to 5.0. At very low ph values, the surface of biosorbent would be surrounded by the hydronium ions and overall surface charge of the fungal cell wall will be positive, thereby enhancing the Cr(VI) interaction with binding sites on the biosorbent with greater attractive forces. As the ph increases, the COOH groups would undergo deprotonation, resulting in accumulation of negative charges on the surface of biosorbent. The overall negative charge on biosorbent would result in reduced Cr(VI) sorption efficiency mainly due to repulsion of negative charged chromate ions (Bai and Abraham, 2002). II. Temperature: Temperature is another influential factor for Cr(VI) biosorption by microbial biomass. The Cr(VI) biosorption reactions are generally endothermic, therefore biosorption capacity increases with increase of temperature. Tewari et al. (2005) found that the biosorption of Cr(VI) by Mucor hiemalis increases with increasing temperature in the range of 27 ºC to 50 ºC. Similarly, Mungasavalli et al. (2007) reported that biosorption efficiency as well as rate of Cr(VI) by A. niger increased with increase in temperature from 5 ºC to 30 ºC. The enhanced biosorption efficiency of Cr(VI) has been attributed to the increased surface activity and kinetic energy of the solute (Aydin and Aksoy, 2009; Nityanandi and Subbhuraam, 2009). The high energy of the system increases the affinity of binding sites for Cr(VI) thereby facilitating the contact between Cr(VI) and cell surface. III. Initial Cr(VI) concentration: The higher initial concentration of Cr(VI) in the solution remarkably influences the equilibrium uptake of Cr(VI). Kiran et al. (2007) demonstrated that with increase in initial Cr(VI) concentration from 10 mg/l to 50 mg/l, the Cr(VI) uptake capacity of Lyngbya putealis increased from 8.0 to 48 mg/g of the biomass. It should be noted that higher initial concentration provides driving force to overcome all mass transfer resistance of Cr(VI) ions between the aqueous and solid phases, resulting in higher probability of collision between Cr(VI) ions and biomass (Liu et al., 2007). IV. Biosorbent dose: Contrary to initial Cr(VI) concentration, the Cr(VI) biosorptive capacity of microbial biomass is inversely proportional to the biosorbent dose (Bishnoi et al., 2007; 81

5 Liu et al., 2007). An increase in the biomass concentration generally increases the amount of solute sorbed owing to increased surface area and number of binding sites. However, the quantity of biosorbed solute per unit weight of biosorbent decreases with increasing biosorbent dosage. Liu et al. (2007) and several other authors reported that the Cr(VI) uptake capacity of biomass (mg/g biomass) is found to decrease with increasing biomass dose. An important factor which leads to decrease in Cr(VI) uptake at high biomass dose is the limited availability of solute (Cr(VI) ions) to occupy all the accessible binding sites on the biosorbent, thereby leading to low Cr(VI) uptake (Khambhaty et al., 2009) Biosorption Mechanism: Three different models for biosorption of Cr(VI) have been proposed: Anionic adsorption, anionic adsorption coupled reduction and reduction coupled cationic adsorption. I. Anionic Adsorption: According to this model of Cr(VI) biosorption, negatively charged chromium species bind through electrostatic attraction to positively charged functional groups on the surface of biosorbents (Bai and Abraham, 2002). This model is based on the observation that at low ph Cr(VI) biosorption increases and at high ph Cr(VI) biosorption decreases. At low ph functional groups of the biosorbent become protonated and easily attracts negatively charged chromate ions, however at high ph, the negative charge of deprotonated functional groups would repel chromate ions, causing reduced binding (Tunali et al., 2005). Adsorption coupled Reduction Reduction coupled Adsorptio Figure 4.2 Mechanism of Cr(VI) biosorption by biomaterials (Park et al.,2005 a, 2007). 82

6 II. Anionic adsorption coupled reduction (Park et al., 2007): This mechanism consists of three steps, (i) the binding of anionic Cr(VI) to the positively charged groups (amino and carboxyl groups) present on the biomass surface, (ii) the reduction of Cr(VI) to Cr(III) by adjacent electron donor groups, and (iii) the release of the reduced Cr(III) into the aqueous phase due to electronic repulsion between the positively charged groups (Figure 4.2). III. Reduction coupled cationic adsorption (Park et al., 2005 a ): Here, Cr(VI) is directly reduced to Cr(III) in the aqueous phase by contact with the electron donor groups of the biomaterials and the reduced Cr(III) may either form complexes with biomaterial or remains in the aqueous phase (Figure 4.2) Modelling Biosorption: Mathematical models can describe the behaviour of the biosorption processes operating under different experimental conditions. They are very useful to understand the mechanisms of Cr(VI) biosorption and to evaluate performance of bio-sorbents for Cr(VI) removal. The solid-liquid Cr(VI) biosorption systems have been described by number of models with varying degrees of complexity. These are usually based on two types of investigations: kinetic models and equilibrium models. 1. Kinetic model Kinetic studies offer information on the mechanism of biosorption and potential rate controlling step such as mass transport and chemical reaction processes. The kinetic models also give information about reaction pathways, time to reach equilibrium and validation of experimental data (Vijayaraghavan and Yun, 2008). Different models have been used to investigate the mechanism of Cr(VI) biosorption. The degree of closeness between experimental data and model predicted values shows the better fit of the model. Additionally, a relatively high R 2 value (R 2 values close to or equal to 1.0) indicates that the model successfully describes the kinetics of Cr(VI) biosorption. Generally, two models are used to demonstrate the kinetics of Cr(VI) biosorption. i. Pseudo First-Order or Lagergen kinetic model: It is the first equation for metal ions biosorption of liquid-solid system based on capacity of solid. The pseudo-first order equation is generally expressed as: 83

7 log( q eq q ) log q t eq k 1 t Where, q t and q eq is sorption capacity at time t and at equilibrium (mg/g) respectively and k 1 is pseudo-first order rate constant. In case the biosorption follows pseudo-first order rate equation, a plot of log(q eq - q t ) vs t should yield a straight line with intercept of log q eq and slope of k 1 /2.303 (Lagergren, 1898). ii. Pseudo-Second order kinetic model: The pseudo-second order kinetic model predicts the behaviour over the whole range of biosorption and is in agreement with biosorption mechanism being the rate-controlling step. The linear form of pseudo-second rate equation is expressed as t q 1 2 t k2 ( qeq ) t q eq 4.2 Where, k 2 is pseudo second-order rate constant. In case the biosorption follows pseudo-second order rate equation, a plot of t/q t vs t should yield a straight line with intercept of 1/k 2 q eq 2 and slope of 1/q eq (Ho and McKay, 1999). The shape (linearity) of graph and comparison of experimental and calculated q eq values can help in deciding which kinetic model is followed by biosorption process. Another important factor which influences the kinetic model is the value of R 2. A value of R 2 > 0.9 or equal to 1 shows the suitability of model for describing the kinetics. 2. Equilibrium model: The equilibrium data of metal ion biosorption, commonly known as adsorption isotherms are used to analyse the interaction of metal ions with the biosorbents and also, provide information on the capacity of the adsorbent (Wang and Chen 2009). The classical single-solute adsorption isotherm models of Langmuir and Freundlich that are widely used to analyse equilibrium data for water and wastewater treatment applications have been shown to describe the biosorption equilibrium. Such models are usually preferred 84

8 since they are simple, give good description of experimental behaviour in a large range of operating conditions. i. Langmuir isotherm model: The Langmuir isotherm model was originally developed from studies of gas-solid phase adsorption on activated carbon (Langmuir, 1916). The Langmuir isotherm model is used to estimate the sorption capacity as well as the maximum uptake values of any biosorbent. The empirical Langmuir equation is valid for saturated monolayer sorption onto a completely homogeneous surface with a finite number of identical sites and negligible interaction between adsorbed molecules. In addition, the model assumes uniform energies of adsorption onto the surface and no trans-migration of the adsorbate molecule on the surface plane takes place. This model is mathematically described as: q eq qmax bc 1 bc eq eq.4.3 Here, q eq is the metal concentration (mg) adsorbed onto biomass (per gram), C eq is the residual metal concentration in solution (mg/l), q max is the maximum specific uptake (mg/g biomass) corresponding to saturation of binding sites, and b is the ratio of adsorption/desorption rates. The q max and b can be determined from the linear plot of C eq /q eq versus C eq. The linear derivatives of Langmuir equation is given below: C q eq eq 1 C q b q max eq max ii. Freundlich Isotherm model: The Freundlich model, first proposed in 1906, is characteristic of multi-layer heterogeneous surfaces with uniform energy and reversible adsorption. In addition (Freundlich, 1906), the Freundlich equation suggests that the binding sites are not equivalent or independent i.e the adsorption energy of a metal binding to a site on an adsorbent depends on whether or not the adjacent sites are already occupied and adsorption energy exponentially decreases with engagement of the active binding centres of an adsorbent. Freundlich isotherm is expressed by following equation: 85

9 q eq k f C 1/ n e Here, k f and n are the Freundlich constants characteristic of the system. k f and n are indicators of biosorption capacity and biosorption efficiency respectively. The Linear form of Freundlich isotherm is expressed as: 1 ln qeq ln k f ln Ceq.4.6 n A plot of ln q eq versus ln C eq should yield straight line with 1/n as slope and ln K f as intercept. In summary, microbial biomass represents an efficient and potential class of biosorbents for the removal of hexavalent chromium from industrial and municipal wastewater. Although several attempts have been made to develop biosorption as an effective bioremediation process, progress is very slow especially considering the screening and selection of most cost effective biomaterial. In this context, the present chapter demonstrates the biosorption efficiency of Cr(VI) tolerant oleaginous fungal isolate, Pythium sp. The whole investigation is categorised into three parts: i. Influence of physico-chemical parameters on Cr(VI) biosorption by Pythium sp. biomass. ii. iii. Analysis of the biosorption behaviour of Pythium sp. biomass for its successful application in Cr(VI) removal from an electroplating industry effluent. Development of low-cost biosorbent for Cr(VI) biosorption, using spent biomass of Pythium sp. left upon extraction of oil. 86

10 4.2 Materials and Methods Fungal strain and chemicals A laboratory strain of Cr(VI) tolerant Pythium sp. isolated from chromium contaminated soil was used. It was grown and maintained on Potato dextrose agar medium. All chemicals used were of analytical grade (AR) and purchased either from Qualigens Fine Chemicals, India or Hi Media Laboratories, India or Ranbaxy Fine Chemicals Limited, India Preparation of Biosorbents For biosorption studies, three kind of biomass prepared from mycelia of Pythium sp. viz. i) Untreated mycelia of Pythium sp.; ii), Acid-treated mycelia of Pythium sp.: iii) Spent biomass obtained after extraction of oil from Pythium sp. cultivated under solid-state condition using boiled rice as substrate, were investigated. The biomass was dried, powdered using mortar and pestle. This powdered biomass was preserved in airtight polyethylene containers for further use Various pre-treatments to biomass The 5 g of Pythium sp biomass was subjected to various pre-treatments in order to study their effect on Cr(VI) adsorption. The biosorbent was autoclaved for 15 min at 121 ºC (referred as heat inactivated biomass) or boiled for 15 min in 500 ml of NaOH (1N) or treated with o-phosphoric acid (10 %, v/v) or with hydrochloric acid (1 M) or treated with glutaraldehyde (2 %, v/v) or formaldehyde (15 %, v/v). Following pre-treatment, biomass was collected by filtration and washed with deionized water until the ph of the washing solution was close to ph 7.0 ± 0.1. The effect of pre-treatments on Cr(VI) biosorption efficiency of biomass was compared to that of untreated fungal mycelium Effect of initial Cr(VI) concentration on Cr(VI) biosorption 10 mg/ml Pythium sp biomass (as per the requirement) was mixed with 100 ml of test Cr(VI) solution. Test solutions containing Cr(VI) ions were prepared from analytical grade potassium dichromate. The concentration of Cr(VI) ranged from mg/l. After mixing the biomass, the experimental set was kept on shaker (150 rpm) at 30 C. 1 ml samples were collected from conical flasks at regular time intervals and filtered through Whattman No. 1 filter paper. The filtrates were analyzed for residual Cr(VI). 87

11 4.2.5 Effect of ph on Cr(VI) biosorption The fungal biomass (10 mg/ml) was added to 100 ml Cr(VI) solution (100 mg/l) with varying ph (ph 1.0 to 8.0). The ph of the solution was adjusted using 0.1 N HCl /0.1 N NaOH. At all ph values, controls without biomass addition were kept in order to compensate for effect of ph on Cr(VI). It should be noted that ph of the solution did not influence the concentration of Cr(VI). The Cr(III) was not detected experimentally in the aqueous phase during any of the biosorption experiments performed (data not included) Effect of adsorbent concentration on Cr(VI) biosorption The varying amount of Pythium sp biomass (2 to 20 mg/ml) was used in biosorption experiment and residual Cr(VI) was monitored at regular time intervals Kinetics of Cr(VI) biosorption The commonly studied model for understanding the kinetics of Cr(VI) biosorption includes, Lagergren-first order and pseudo-second order rate equation (Lagergren, 1898). Linear form of Lagergren -first order rate equation was expressed as follows: log( q eq k q ) log qeq 1t t Where, q t and q eq represents sorption capacity at time t and at equilibrium respectively and k 1 is pseudo-first order rate constant. In case the biosorption followed Lagergren -first order rate equation, a plot of log( q eq - q t ) vs t would generate straight line with intercept of log q eq and slope of k 1 / Similarly, linear form of pseudo-second rate equation was expressed as (Ho et al., 1999): t q t 1 2 k2 ( qeq ) t q eq Where, k 2 is pseudo second order rate constant. In case the biosorption followed pseudo-second order rate equation, a plot of t/q t vs t would 2 generate a straight line with intercept of 1/k 2 q eq and slope of 1/q eq. The shape (linearity) of graph, comparison of experimental and calculated q eq values helped in deciding which kinetic model was followed by biosorption process. 88

12 4.2.8 Thermodynamic Studies Effect of temperature on Cr(VI) removal was studied by agitating biomass in 100 ml of 100 mg Cr(VI)/L at different temperatures (25 to 50 ºC) for different agitation times till equilibrium was attained and then the results were analysed to determine the initial rate of biosorption at different temperatures. The activation energy of the biosorption process was calculated by employing Arrhenius equation as follows: ln k = Ea/R T + lnao Where Ea is activation energy and Ao is constant called the Frequency factor. Value of Ea were determined from the Slope (-Ea/R) of ln k versus 1/T plot (Aksu, 2005) The thermodynamic parameters of the biosorption i.e. the enthalpy change (ΔH) and entropy change (ΔS) were calculated using the Van t Hoffs plot (ln Kc Vs 1/T) given as: S H ln Kc. 4.8 R RT Where, T is the temperature in Kelvin, R is the universal gas constant ( KJ/mol) and Kc is equilibrium constant calculated as: Q Kc C eq eq 4.9 Here, Q eq was the amount of Cr(VI) adsorbed per unit biomass (mg/g biomass) and C eq was the Cr(VI) concentration in solution at equilibrium (Aydin, 2009; Ucun, 2008) Equilibrium model for Cr(VI) biosorption Adsorption isotherm, based on equilibrium data are basic requirements for the design of adsorption systems. The classical adsorption models (Langmuir and Freundlich isotherms) were used to describe the equilibrium between adsorbed metal ions (Cr(VI)) on the spent biomass of Pythium sp. (q eq ) and metal ions in solution (C eq ) as a function of varying initial Cr(VI) concentration Characterization of Cr(VI) biosorption The characterization of the untreated, acid treated and chromium laden acid treated biomass was done using FTIR analysis. The spectra were collected by a Perkin Elmer Spectrum GX in the range of cm -1. The specimen of various biosorbents were first mixed with KBr in an approximate ratio of 1:100, then ground in an agate mortar, and pressed at 10 tons for 5 min in 89

13 order to form pellets. The presence of Cr in the unloaded and chromium laden biosorbent was analyzed by scanning electron microscopy (Philips XL30 ESEM) coupled with energy dispersive X-ray analysis (EDAX) Characteristics of effluent sample to investigate bioremediation potential of acid treated biomass of Pythium sp. The effluent was obtained from Prakruti Environment Consultant from an unit located in industrial estate of Vadodara, Makarpura, Gujarat, India. The characteristics of electroplating effluent sample are listed in Table 4.1 (As given by the consultants). The major contaminants of wastewater were: Cr(VI), Ni and Fe. In addition, ph of the wastewater was highly acidic i.e, 1.5. Table 4.1 Physical and chemical characteristics waste water collected from effluent of electroplating industry. Parameters mg/l COD 32 Ammonium Nitrate 61.6 Total solids Total Dissolved salts 10.1 Chlorides 350 Sulphate Sulphides ND ND Nickel 32.8 Iron Oil and grease Hexavalent Cr(VI) 5,000 ph 1.5 Colour Yellow to brown Analysis of Cr(VI) ions The concentration of the Cr(VI) ions was determined spectrophotometrically after complexation of the Cr(VI) ion with 1, 5-diphenylcarbazide (Anon, 1998). The absorbance was recorded at 540 nm 90

14 and concentration was determined from the calibration curve. The amount of chromium adsorbed was monitored by determining residual Cr(VI) in the solution at different time intervals and subtracting it from the initial chromium. 4.3 Results and Discussion: Part A: Hexavalent chromium biosorption by biomass of Pythium sp.: Batch and Kinetic studies Effect of pre-treatment on Cr(VI) adsorption by fungus Table 4.2 shows that biomass of Pythium sp. pre-treated with hydrochloric acid (HCl) and high temperature exhibited 1.7 and 1.4-fold higher Cr(VI) biosorption (2.9 mg/g and 2.5 mg/g), respectively in comparison to untreated biomass (1.6 mg/g). Notably, HCl plus heat treated biomass adsorbed to the same extent as HCl pretreated biomass. The pre-treatment of biomass with 1.0 N NaOH resulted in reduced Cr(VI) biosorption efficiency (0.8 mg/g). The alkali pre-treatment is known to cause hydrolysis and de-acetylation of protein constituents. It also causes drastic effects like swelling of biomass, probably due to polymer chain breakage, thereby reducing biosorption potential (Bayramoglu et al., 2005; Yang and Chen, 2007). Formaldehyde and glutaraldehyde pretreated biomass too exhibited lower efficiency of Cr(VI) biosorption. Both formaldehyde and glutaraldehyde act as a fixative by causing cross-linking of hydroxyl (-OH) group of glucose in the cell wall. Hence, reducing the accessibility of specific binding sites for Cr(VI) ions (Yang and Chen, 2007). The increase in adsorption capacity after acid treatment could be attributed to the fact that acid hydrolysis improves exposure of more amino sugar moieties on the biomass surface, which gets more easily protonated at adsorption ph, thereby enhancing the binding of Cr(VI) through electrostatic charge attraction (Bai and Abraham, 2001; Tunali et al., 2005). 91

15 Table 4.2 Effect of physical and chemical pre-treatments on Cr(VI) uptake ( g/g) by Pythium sp. biomass. Cr(VI) Biosorption (µg/g) Before Autoclave After Autoclave Untreated NaOH O-Phosphoric Acid Hydrochloric Acid Formaldehyde Gluteraldehyde FTIR and EDAX analysis In order to elucidate the mechanism of Cr(VI) biosorption by Pythium sp, FTIR analysis of untreated, acid treated and chromium laden acid treated biomass was carried out (Figure 4.3). The intense broad absorption bands at cm -1 represent -OH groups of glucose and the NH stretching of proteins and acetamide groups of chitin. The strong bands at cm -1 were attributed to the amide bonds in chitin or protein. The absorption band around 1741 cm -1 represent C = O stretch of acetamide group. The moderately strong absorption band around 1032 cm -1 and 1154 cm -1 may be assigned to CN stretching vibration of chitin, chitosan and protein components of fungal cell wall. A short absorption band at 1456 cm -1 seems to be due to asymmetric bending of acetyl moiety. The bands at 2925, 1549, 1377 and 1032 cm -1 represents C-H stretching, vibrations, N-H bending (scissoring), -CH3 wagging and C-OH stretching vibrations, respectively and are due to several functional groups present in fungal cell wall components. The FTIR spectrum of acid treated biomass shows a significant difference in absorption bands in the frequency range of 1659 to 1032 cm -1, thus indicating the chemical alterations in the cell wall of acid treated biomass (Bai and Abraham, 2001). The FTIR spectrum of acid treated biomass loaded with chromium reveals a shift in broad absorption bands from 3323 to 3303 cm -1 and decrease in intensities of bands associated with NH bonds. It should be noted that Cr(VI) behaves as an oxy-anion (CrO 2-4, or Cr 2 O 2-7 ) in aqueous medium, according to aqueous solution chemistry of chromium. Therefore, it may not bind to negatively charged functional groups on the biomass surface such as carboxylate, phosphate and sulphate, 92

16 because of the respective charge repulsion. Thus, it can be suggested that the amino groups, the major cell wall components (i.e. chitin, chitosan and proteins) gets protonated at low ph (i.e. ph 2.0) due to acid pre-treatment of the biomass and thereafter, negatively charged chromate ions become electrostatically attracted towards the positively charged amino groups of fungal cell wall (Kumar et al., 2008; Tewari, et al., 2005; Tunali et al., 2005). Figure 4.4 shows the EDAX spectra obtained before and after Cr(VI) biosorption onto HCl treated fungal biomass. These spectra clearly indicated the presence of Cr(VI) ions over the surface of metal loaded HCl treated fungal biomass whereas Cr(VI) was not detected in the acid treated biomass before biosorption (control). This observation was similar to that reported by Tunali et. al. (2005) for adsorption of Cr(VI) by Neurospora crassa. Figure 4.3 FTIR spectra of untreated (A), acid treated unloaded biomass (B) and acid treated Cr(VI) loaded biomass (C) of Pythium sp. 93

17 A B Figure 4.4 EDAX spectra of acid treated biomass (A); acid treated Cr(VI) loaded biomass (B) of Pythium sp Effect of biosorbent dose on Cr(VI) biosorption. The increase in biosorbent dose from 2 to 10 mg/ml resulted in rapid adsorption of Cr(VI) (Figure 4.5). Further increase in the biomass dose didn t affect the Cr(VI) removal. Thus, rest all experiment were performed at a biosorbent dose of 10 mg/ml. The increase in Cr(VI) removal with increasing dose of biomass may be attributed to increase in the number of adsorption sites. 94

18 Cr(VI) Removed (mg/l) Biomass (mg/ml) Figure 4.5 Effect of acid pre-treated biosorbent dose of Pythium biomass on Cr(VI) removal (mg/l). Initial Cr(VI) concentration: 100mg/L, Contact time: 360min Effect of ph on Cr (VI) biosorption. Figure 4.6 shows that the extent of Cr(VI) biosorption by the acid treated biosorbent increased with decrease in ph. The Cr(VI) biosorption capacity of fungal biomass was maximum at ph 1.0 (12.5 mg/g biomass) and markedly decreased at ph 2.0 (10.15 mg/g biomass), it remained constant there after till ph 6.0 and then decreased gradually with further increase in ph. This increase in adsorption with decrease in ph may be due to protonation of functional groups involved in biosorption of negatively charged chromate ions. At alkaline ph the overall charge on the biosorbent surface would become negative and consequently due to respective charge repulsion of negatively charged Cr(VI) ions like HCrO - 4, Cr 2 O 2-7, CrO 2-4, result in lower adsorption efficiency (Kumar et al., 2008). Hence, electrostatic attraction probably plays an important role in biosorption of negatively charged chromium ions at low ph. Additionally, the dominant form of Cr(VI) at ph 1.0 is the acid chromate ion species (HCrO - 4 ), which would shift to other forms, CrO and Cr 2 O 7 with increase in ph. Since there is an increase in sorption of Cr(VI) as ph decreases to 1.0, it may be suggested that HCrO 2-4 is the active form of Cr(VI) which is being absorbed by the acid treated fungal biomass. 95

19 Cr(VI) Biosorption (mg/g) ph Figure 4.6 Effect of ph on biosorption of Cr(VI) by fungal biomass. Biomass dose: 10 mg/ml, Initial Cr(VI) concentration: 100mg/L, biomass dose: 10 mg/ml, contact time: 360 min. Park et al. (2005 b ) have demonstrated the adsorption-coupled biomass reduction of Cr(VI) to Cr(III) on the surface of dead biomass of A. niger. They found that upon incubation of Cr(VI) with dead biomass of A. niger, Cr(VI) was completely removed from aqueous solution, however it was accompanied with appearance of corresponding amount of free Cr(III) in solution as well as increase in ph from 2.00 to In present study, we did not find the appearance of Cr(III) in aqueous solution concomitant with removal of Cr(VI) (data not shown) by biomass of Pythium sp. In addition, we monitored the ph of solution during the course of all biosorption experiments and no significant change in ph was observed. Thus, in present study, Cr(VI) removal from aqueous solution by Pythium biomass seems to follow anionic adsorption mechanism (Park et al b ) Effect of initial Cr(VI) concentration on Cr(VI) biosorption by untreated and acid treated biomass of Pythium sp. The biosorption of Cr(VI) was carried out as a function of the initial concentration of Cr(VI) ions (from mg/l), at ph 1.0, under agitated conditions (150 rpm) with 1440 minutes of contact time using 10 mg/ml of untreated and acid treated biomass of pythium sp. 96

20 Cr(VI) Biosorption (mg/g) Cr(VI) Biosorption (mg/g) A Time (min) mg/l 200 mg/l 300 mg/l 400 mg/l 500 mg/l B Time (min) mg/l 400 mg/l 200 mg/l 500 mg/l 300 mg/l Figure 4.7 Effect of initial Cr(VI) concentration on biosorption efficiency of untreated fungal biomass (A) and acid treated fungal biomass (B). Biomass dose: 10mg/mL, contact time: 1440 min. It was found that the Cr(VI) biosorption increased from 2.76 to 7.8 mg Cr(VI)/ g of untreated biosorbent and from 12 to 50.6 mg Cr(VI)/g of acid treated biosorbent when the initial Cr(VI) concentration increased from 100 mg/l to 500 mg/l (Fig. 4.7). The maximum Cr(VI) uptake was found to be 50.6 mg Cr(VI)/g by acid treated biomass at an initial concentration of 500 mg Cr(VI)/L. The enhancement in the Cr(VI) biosorption from concentrated Cr(VI) solution may be attributed to an increase in the number of Cr(VI) ions competing for the available functional groups 97

21 present on the surface of biomass (Kumar et al., 2008). This in turn would increase the higher probability of collision between chromate ions and biosorbents thereby providing the driving force to overcome all mass transfer resistance experienced by metal ions between the aqueous and solid phase. Table 4.3 Pseudo-second order kinetic parameters for biosorption of Cr(VI) by untreated and treated biomass of Pythium sp. at varying initial Cr(VI) concentration. Cr(VI) (mg/l) Untreated Biomass Treated biomass q eq, mg/g (Expt) q eq, mg/g (Cal.) K 2 R 2 q eq, mg/g (Expt) q eq, mg/g (Cal.) K 2 R To analyze the mechanism of Cr(VI) biosorption, the adsorption data at different concentration of Cr(VI) by both pre-treated and un-treated biomass were fitted to various kinetic models such as, Lagergren first-order and pseudo-second order kinetic model. Figure 4.8 shows a plot of linearized form of the pseudo-second order model therefore, proves the linear relationship between Cr(VI) ion concentration and Cr(VI) biosorption. The pseudo-second order kinetic model assumes that the metal biosorption process is dependent on the number of metal ions present in the solution as well as the free metal binding sites on the biosorbent surface. The second order rate constant K 2 and q eq calculated from the intercept and slope of the plots are summarized in Table 4.3. It is clear from the table that with increase in Cr(VI) ion concentration, value of q eq increases, which suggests the linear relationship between Cr(VI) ion and Cr(VI) biosorption. The calculated q eq value shows very good agreement to the experimental q eq values. The correlation coefficient (R 2 ) for the second order kinetics was greater than 0.9 (R 2 > 0.9) for all Cr(VI) concentration tested. Several investigations on the kinetics of Cr(VI) adsorption onto various biosorbents have also reported higher correlations for pseudo-second order model (Arica and Bayramoglu, 2005; Muench, 1996; Ye et al., 2010). 98

22 t/q(g.min/g) t/q (g.min/g) A t (min) 100 mg/l 200 mg/l 300 mg/l 400 mg/l 500mg/L B t (min) 100 mg/l 200 mg/l 300 mg/l 400 mg/l 500 mg/l Figure 4.8 Linearized pseudo-second order kinetic plots at varying initial concentrations of Cr(VI) for untreated biomass (A) and acid treated fungal biomass (B). Biomass dose: 10mg/mL, contact time: 1440 min Effect of temperature on Cr(VI) biosorption Temperature, under certain range can have an influence (positive/negative) on the biosorption of metal ions (Khambhaty et al., 2009). Figure 4.9 shows that the biosorption of Cr(VI) by the biosorbent appears to be dependent over the temperature range of 30 to 40 ºC. This 10 degree 99

23 Cr(VI) Biosorption (mg/g) Cr(VI) Biosorption (mg/g) increase in temperature increased Cr(VI) biosorption capacity (mg Cr(VI)/g biosorbent) from 3.84 to 4.5 and 9.04 to 9.38 for untreated and acid treated biomass, respectively (Fig 4.9) A Time (min) 30ºC 35ºC 40ºC 10 9 B Time (min) 30ºC 35ºC 40º C Figure 4.9 Effect of temperature on biosorption of Cr(VI) by untreated fungal biomass (a) and acid treated fungal biomass (b). Initial Cr(VI) concentration: 100mg/L, biomass dose: 10 mg/ml, contact time: 60 min. The contact time required for complete Cr(VI) removal was found to decrease with increase in temperature. The increase in Cr(VI) biosorption with increasing temperature may be due to either higher affinity of sites for chromate ions or an increase in number of binding sites on biosorbent surfaces as a result of reorientation of fungal cell wall components (Aydin and Aksoy, 2009; Bai 100

24 ln K and Abraham, 2001). It has been reported that rise in biosorption capacity with temperature is accompanied with the rise in the kinetic energy of metal ions, which subsequently increases the collision frequency between biosorbent and sorbate leading to enhanced metal sorption (Bai and Abraham, 2001). 1/T (K) Treated biomass Untreated Biomass Figure 4.10 Arrhenious plot for Cr(VI) biosorption on untreated and acid treated biomass of Pythium sp. The activation energy for the sorption of Cr(VI) by untreated as well as acid treated biomass was determined from Arrhenious plot (Figure 4.10). It was calculated as 44.5 and 50.8 KJ/mol for untreated and HCl treated biomass, respectively. The values of activation energy suggests that the sorption of Cr(VI) on Pythium sp. biomass is a chemical process, since activation energy for chemical sorption is generally more than 4 6 KJ/mol (Shuler and Kargi, 1992). The chemical sorption means that the rate varies with temperature according to finite activation energy in the Arrhenius equation. Bayramoglu et al. (2005) have reported similar observation in their studies on Lentinus sajor-caju for adsorption of Cr(VI) Thermodynamic Studies The increase in Cr(VI) biosorption with temperature may be correlated with the endothermic nature of the biosorption process. To further confirm the temperature dependency of the biosorption process, thermodynamic parameters were calculated using Van t Hoff equation, 101

25 ln Kc y = x R 2 = /T (1/K) Figure 4.11 Van t Hoff plot of ln K c vs 1/T (1/K). Figure 4.11 shows the van t Hoff plot of ln Kc vs 1/T (1/K). The values of enthalpy change (ΔH) and entropy change (ΔS) were calculated from the slope and intercept of the plot (Table 4.4.). The positive value of ΔH (10 KJ mol -1 ) shows the Cr(VI) biosorption process is endothermic. Additionally, positive ΔS (0.040 KJ mol -1 ) indicated, high affinity of the Cr(VI) for the sorbent used. Similarly, Gibb s free energy (ΔG) was also calculated. The magnitude of ΔG (KJ/mol) increases with increase in temperature showing the feasibility of the biosorption process. The negative values of ΔG (Table 4.4) confirmed the spontaneity of the Cr(VI) biosorption process. Table 4.4 Thermodynamic parameters for the biosorption of Cr(VI) on Pythium sp. Temperature (K) Equilibrium constant Gibb s Free Energy Enthalpy (ΔH, KJ mol -1 ) Entropy (ΔS, KJ mol -1 ) (K c) (ΔG, KJ mol -1 )

26 ln qeq ln qeq Analysis of adsorption isotherm The purpose of adsorption isotherm is to relate the metal concentration adsorbed on the biosorbent with metal concentration in the bulk solution. The analysis of the isotherm data is important to develop an equation that accurately represents the biosorption results. The isotherm can be described by several sorption isotherm models, of which Langmuir and Freundlich are the most widely referred models. Figure 4.12 shows typical linearized plot of Freundlich isotherm model for increasing concentration of Cr(VI). The linear plots of ln q eq versus ln C eq confirmed that Cr(VI) biosorption by both untreated and acid treated biosorbent followed Freundlich adsorption model. 2.5 A ln ceq B ln Ceq Figure 4.12 Linearized Freundlich biosorption isotherm for Cr(VI) sorption by untreated fungal biomass (A), acid treated fungal biomass (B) at varying initial Cr(VI) concentration (100 to 500 mg/l). 103

27 The values of regression coefficient (R 2 ) for Freundlich isotherm were found to be and for un-treated and acid-treated biomass, respectively (Table 4.5). The Freundlich isotherm constant K f was calculated as 1.78 and 12.0 for un-treated and acid-treated biomass, respectively. Likewise, the value of n was calculated as 2.9, for both un-treated and acid-treated biomass. The high magnitude of K f and n illustrate high adsorption capacity of the biomass. The experimental value of n is greater than unity, which indicates favourable adsorption (Raji and Annirudhan, 1998). Table 4.5 Isotherm parameters for Cr(VI) biosorption by untreated and acid treated biomass of Pythium sp at varying concentration of Cr(VI) (100 to 500 mg/l). Biomass Freundlich Isotherm Constants Langmuir Isotherm Constants n K f R 2 Qₒ b R 2 Untreated Treated The adsorption data followed Freundlich isotherm model suggesting the heterogeneity on the biosorbent surface. The binding sites are not independent and adsorption energy of a metal binding site on biosorbent depends on whether or not the adjacent sites are already occupied. Thus, the adsorption of Cr(VI) by fungal isolate seems to be a complex process involving multilayer, interactive or multiple site type binding (Freundlich, 1906). Similar observations have been reported by other workers on Cr(VI) biosorption studies using Spirogyra (Gupta et al., 2001), Rhizopus arrhizus (Bai and Abraham, 2001). 104

28 Cr(VI) Biosorption (%) Part B: Removal of Cr(VI) from electroplating industrial effluent by sorption onto Pythium biomass Effect of contact time on Cr(VI) biosorption from electroplating industrial effluent by acid-treated biomass of Pythium sp. Electroplating industrial effluent (ph: 1.5) containing 5000 mg/l of Cr(VI) ions was diluted (without any pre-treatment of effluent) 50 times with glass distilled water to get the final concentration of 100 mg Cr(VI)/L Time (min) Figure 4.13 Time course for Cr(VI) removal by biosorption using Pythium sp. biomass from electroplating industrial waste, diluted to final Cr(VI) concentration of 100 mg/l. Figure 4.13 shows the role of contact time on Cr(VI) biosorption using 10 mg acid treated biomass of Pythium sp. under shaking condition (150 rpm). It was found that biosorption increased from 50 to 80 % as the contact time was increased from 0 to 240 minutes. One gram of Pythium sp. biomass could remove 80% of Cr(VI) at equilibrium. This biosorption efficiency was slightly lower than Cr(VI) removal efficiency of Pythium sp. from pure solution of Cr(VI). This may be attributed to competition between Cr(VI) and other metal ions present in electroplating effluent for functional groups on the surface of biomass. Similar reduced Cr(VI) biosorption efficiency from electroplating industrial waste by A. niger biomass has been reported by Kumar et al. (2008). 105

29 Cr(VI) Biosorption (mg/g) Effect of initial Cr(VI) concentration in effluent on biosorption The biosorption of Cr(VI) ions from electroplating effluent was carried out for 420 minutes at 150 rpm using acid-treated biomass (10 mg/ml) of Pythium sp. with series of dilute effluents with Cr(VI) concentration in the range of mg/l. It can be demonstrated from the experimental results (Figure 4.14) that uptake capacity (q eq, mg/g biomass) increased from 7.0 mg to 44.8 mg Cr(VI)/g acid-treated Pythium sp. biomass with increase in Cr(VI) from 100 to 500 mg/l. The Cr(VI) biosorption capacity of Pythium sp. biomass (43 mg/g) was comparable or better than other biosorbents reported for removal of Cr(VI) from electroplating effluents viz. Padina boergesenli (49 mg/g), Lentinus edodes (21.5 mg/g); C. lypolytica (10 mg/l) and A. niger (65 % from electroplating effluent contaminated with 47 mg/l Cr(VI)) (Chen et al. 2006; Kumar et al. 2008; Thirunavukkarasu and Palanivelu 2007; Ye et. al. 2010) Time (min) 100 mg/l 150 mg/l 400 mg/l 500 mg/l Figure 4.14 Cr(VI) biosorption (mg/g) by Pythium sp. biomass from electroplating industrial waste diluted to final Cr(VI) concentration in the range of mg/l. Kinetic studies based on plot of t/q t vs t (equation 2.) indicated that the biosorption of Cr(VI) ion followed pseudo-second order reaction in the Cr(VI) concentration range of mg/l (Figure 4.15). 106

30 t/q(mg.min/g) t(min) 100 mg/l 150mg/L 400 mg/l 500 mg/l Figure 4.15 Linearized second order kinetic plot of Cr(VI) biosorption by Pythium sp. biomass at varying initial concentration of Cr(VI). The values of experimental/calculated equilibrium uptake capacities (q eq expt and q eq cal), correlation regression coefficient (R 2 ) and second-order rate constants (k 2 ) are presented in table 4.6. Table 4.6 Second order kinetic parameters for biosorption of Cr(VI) by Pythium sp. biomass at various dilutions of electroplating industrial waste water. Effluent Dilution Cr(VI), mg/l q eq, mg/l, (Expt) q eq, mg/l, (Cal) k 2 R : The equilibrium uptake capacity increased (from 7.26 to 44.8 mg/g biomass) whereas second order rate constant (k 2 ) was found to decrease (from to 0.075) with increasing concentration of Cr(VI) (from 100 to 500 mg/l). The calculated uptake capacity values estimated from second-order kinetic model were in agreement to the experimental values. Additionally, correlation regression coefficients of pseudo-second order model were quite high (R 2 > 0.98); very close to unity. 107

31 Therefore, Cr(VI) ion biosorption by acid-treated biomass followed pseudo-second order model. These observations are in agreement with the observations made by Ye et al. (2010) where they used Candida lipolytica and dewatered sewage sludge for biosorption of Cr(VI) ions from electroplating wastewater Adsorption isotherms for Cr(VI) ion biosorption The experimental values of equilibrium uptake capacities of Cr(VI) ions from electroplating effluent (Table 4.7) by acid-treated biomass of Pythium sp. were analyzed by Freundlich and Langmuir isotherm models. Table 4.7 Isotherm parameters for Cr(VI) biosorption by Pythium sp. biomass at various dilutions of electroplating industrial wastewater. Freundlich Isotherm Constants Langmuir Isotherm Constant n K f, mg/g R 2 b Q, mg/g R The Langmuir and Freundlich adsorption constants calculated from the corresponding isotherms (data not shown) are presented in Table 4.7. It can be seen that R 2 value for the Freundlich isotherm is against the Langmuir isotherm R 2 value of Analysis of correlation regression coefficient shows that Cr(VI) biosorption by Pythium sp. from electroplating effluent fits better to Freundlich isotherm. The Freundlich isotherm constants K f and n were calculated as 8.3 and 1.13, respectively. The high magnitude of K f and n illustrate high adsorption capacity of biomass (Bai and Abraham, 2001; 2002). All these results showed that Freundlich isotherm model fitted the results quite well and are in agreement with the heterogenous nature of sorbent (Pythium biomass) surface. 108

32 Part C: Biosorption of hexavalent chromium using spent biomass of oleaginous Pythium sp. : Kinetic studies in batch mode. Hexavalent chromium tolerant Pythium sp. when cultivated on Potato Dextrose Agar medium produced strong coconut aroma. Further, it was found to accumulate oil on the basis of microscopic observation (Figure 4.17). Interestingly the fatty acid composition (using GC-MS) of oil produced by Pythium sp. in solid state fermentation using rice as substrate revealed palmitic acid, stearic acid, oleic acid and linolenic acid (62 %) along with low proportion of eicosa-pentaenoic acid and caryophyllene as major fatty acids (Figure 4.17). Figure 4.16 Lipid staining in cells of Pythium sp. during growth in Basal Salt Medium. Figure 4.17 Gas chromatograph of oil extracted from Pythium sp. 109