Hexavalent chromium removal by gingelly oil cake carbon ... - NOPR

Indian Journal of Chemical Technology Vol. 18, September 2011, pp. 391-402

Hexavalent chromium removal by gingelly oil cake carbon activated with zinc chloride K B Nagashanmugam1† & K Srinivasan2* 1

R & D Centre, JSW Steel Ltd, Salem Works, Salem 636 453, India Department of Chemistry, Govt. College of Engineering, Anna University, Salem 636 011, India

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Received 28 July 2010 ; accepted 3 March 2011 An activated carbon has been prepared from gingelly oil cake (GOC) by zinc chloride treatment and its Cr(VI) removal capacity is compared with that of commercial activated carbon (CAC). The effect of experimental parameters such as pH, initial concentration, contact time and adsorbents dose for Cr(VI) removal has been studied. Langmuir, Freundlich and Temkin models are tested to describe the equilibrium isotherms. The maximum adsorption capacity of the adsorbents calculated from Langmuir isotherm is found to be 62.5 mg/g and 25.13 mg/g for zinc chloride treated gingelly oil cake carbon (ZTGOC) and CAC respectively. R2 values show that both Langmuir and Freundlich models fit well to explain the adsorption phenomenon for ZTGOC and CAC. The kinetic data fits best to pseudo-second order model. FT-IR analysis has been used to obtain information on the nature of possible interaction between carbon adsorbents and metal ions. SEM images confirm the adsorption of Cr(VI) onto these adsorbents through morphological observations. Thermodynamic study shows the feasibility of process and spontaneous nature of the adsorption. The carbon adsorbents have also been tested for the removal of Cr(VI) from chrome plating wastewater and are found to remove Cr(VI) effectively. Keywords: Adsorption isotherms, Commercial activated carbon, Chrome plating wastewater, Chromium(VI), Kinetics, Zinc chloride, Gingelly oil cake carbon

Hexavalent chromium Cr(VI) is one of the most dangerous heavy metals, a major pollutant in wastewater1. It is a strong oxidizing agent, which irritates plant, animal tissues and is both carcinogenic and mutagenic2,3. The recommended limit of Cr(VI) in potable water is only 0.05 mg/L (ref. 4). But the industrial effluents contain much higher concentrations compared to the permissible limit. Thus, treatment of the effluent to reduce or remove the pollutant before discharging into the environment becomes inevitable5,6. A number of methods have been developed for the removal of Cr(VI) from wastewater such as ion-exchange, chemical precipitation, electro deposition and reverse osmosis7-10. But these methods suffer from disadvantages, such as requiring large excess of chemicals, generating volumetric sludges, involving high capital investment and running cost. Adsorption is one such technique that would be comparatively more useful and economical for this aim11. Several adsorbents have been developed and tested, ranging from low cost material, such as moss ——————— Corresponding authors. * E-mail: [email protected] † E-mail: [email protected]

peat12, sawdust13, hazelnut shell14, rice husk carbon15, bagasse fly ash16,17, red mud18, fertilizer waste19,20, green algae21, butter oil cake22, jatropha oil cake23, olive oil cake24 and soya cake25 to more sophisticated adsorbents, such as: metal oxides26, hydrous metal oxides27, hybrid materials28, bio-materials29, activated carbons30, carbon nanotubes31, boehmite32 and maghemite33. The objective of the present study is to investigate the adsorption potential of carbon derived from gingelly oil cake by zinc chloride activation for the removal of Cr(VI) from aqueous solution and chrome plating wastewater and to compare the performance of the carbon with commercial activated carbon. The optimum adsorption conditions are also evaluated as a function of contact time, pH, adsorbent dose and initial concentration. Adsorption isotherm, kinetic studies and desorption studies has been conducted to understand the adsorption phenomenon. Experimental Procedure Instrumentation

The Fourier Transform Infrared Spectroscopy (FT-IR) spectrum was recorded with an FT-IR

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INDIAN J. CHEM. TECHNOL, SEPTEMBER 2011

spectrophotometer (Model 8400S, SHIMADZU, Japan). Surface morphology was studied by using Scanning Electron Microscope (Model JEOL 6360, Japan). The Atomic Absorption Spectrometer (AAS) (Model SL 163, ELICO Ltd, Hyderabad, India) was used for the determination of Cr(VI) concentration. Adsorbents were analysed by sequential X-Ray Fluorescence Spectrometer (XRF) (Model ARLAdvant XP, Thermo Electron Corporation, Switzerland), Carbon & Sulphur analyzer (Model LECO CS 200) and Nitrogen & Oxygen analyzer (Model LECO TC 500) (LECO Corporation, Michigan, USA). Nitrogen adsorption-desorption isotherms were measured on a Surface and Porosimetry analyzer (Model Micromeritics ASAP 2020, USA). The pH was measured with a Digital pH Meter (Model 335, Systronics, India). A thermally controlled mechanical shaker (horizontal shaking type) with a speed of 200 rpm was used for batch studies. Reagents

All the chemicals used in this study were of analytical reagent grade and distilled water was used for the preparation of solutions. Preparation of Cr(VI) solutions

Stock aqueous solution of Cr(VI) having concentration of 1000 mg/L was prepared by dissolving 2.829 g of K2Cr2O7 in 1000 mL of distilled water. Cr(VI) solutions of desired concentrations were prepared by adequate dilution of the stock solution with distilled water. Preparation of adsorbents

Gingelly oil cake (GOC), an agricultural waste byproduct procured from oil industries, was washed several times with distilled water to remove any surface impurities and dried. 20 g of oil cake was soaked in 20% solution of ZnCl2 for 24 h. The solution was then decanted off. The wet oil cake was spread uniformly on a glass plate and dried at 110°C. The dried material was heated slowly to a temperature of 600°C followed by thermal activation under nitrogen atmosphere at 800–850°C for 30 min in a muffle furnace. After cooling, the excess ZnCl2 present in the carbonized material was leached out by immersing in 10% HCl for about 12 h. The carbon was washed with distilled water to remove traces of HCl and ZnCl234 and dried in an air oven at 110°C. Activation of GOC with ZnCl2 generates more interspaces between carbon layers leading to more micro porosity and more surface area. The increase in

porosity with ZnCl2 activation suggests that the porosity created by this reactant is due to spaces left by ZnCl2 after washing with HCl. ZnCl2 activation causes electrolytic action, termed as “swelling” in the molecular structure of cellulose, which leads to the breaking of lateral bonds in the cellulose molecules resulting in increased inter- and intra-voids. ZnCl2 promotes the development of porous structure of the activated carbon because of the formation of small elementary crystallites 34,35. Zinc chloride treated gingelly oil cake carbon (ZTGOC) thus prepared and commercial activated carbon (CAC) (SD Fine Chem, Mumbai, India) procured from the market were powdered and sieved to a particle size of 80-120 mesh (ASTM) and used for further experiments. Batch experiments

In all sets of experiments, 100 mL of Cr(VI) solution with desired concentration adjusted to a desired pH was taken in high density poly(ethylene) (HDPE) bottles of 300 mL capacity and desired dose of adsorbent was added to it. The solution pH was adjusted with 0.1 M HNO3 or KOH. The solutions were agitated for various contact times at 30±1°C and carbon particles were separated by using centrifuge with a speed of 500 rpm and the supernatant liquid was analysed for Cr(VI) by AAS. Before determination of total quantity of Cr(VI) in the adsorption medium, the supernatant liquid after adsorption was digested with KMnO4 in sulphuric acid medium to oxidize Cr(II) and Cr(III) to Cr(VI)20. Solute and adsorbent free blanks were used as control in all the experiments. Adsorption isotherm and kinetic studies were carried out with different initial concentrations of Cr(VI) by maintaining a constant adsorbent dose. Adsorption isotherm studies were made after equilibrating the solution for 24 h. The concentration of unadsorbed Cr(VI) in the supernatant liquid was determined by using an AAS with an airacetylene flame. The hollow cathode lamp was operated at 15 mA and analytical wavelength was set at 358 nm. The removal efficiency (E) of the adsorbents on Cr(VI) was calculated by using the following expression: E(%) =

(Co - Ce ) Co

×100

… (1)

where Co and Ce are the initial and equilibrium concentrations (mg/L) of Cr(VI) solution respectively.

NAGASHANMUGAM & SRINIVASAN: HEXAVALENT CHROMIUM REMOVAL BY GINGELLY OIL CAKE CARBON

Results and Discussion Properties of adsorbents

Tables 1 and 2 show the proximate analysis, characteristics and XRF analysis of ZTGOC and CAC. Proximate analysis was carried out according to Indian Standard Methods of Test36. The low pH value of ZTGOC is due to hydrochloric acid treatment in the preparation of ZTGOC. The high decolorizing power and low phenol number values suggest that CAC may be more suitable for organic adsorption. Table 1—Physico-chemical characteristics of the activated carbons Parameter Moisture (%) Matter soluble in water (%) Matter soluble in acid (%) Bulk density (g/cm3) pH Iron content (%) Decolorizing power (mg/g) Ion-exchange capacity (m equiv / g) Phenol number (mg) Volatile matter (%) Ash (%) Fixed carbon (%) Adsorption-desorption isotherm BET surface area (m2/g) Langmuir surface area (m2/g) Total pore volume (cm3/g) Average pore size (Å) Elemental analysis Carbon (%) Sulphur (%) Nitrogen (%) Oxygen (%)

ZTGOC

CAC

13.20

3.70

12.23

4.80

17.67 0.80 2.80 0.59 75.20 1.15 48.00 37.85 9.52 52.63

9.20 0.74 7.60 0.18 90.00 35.00 12.73 1.19 86.08

333.93 485.80 0.22 25.88

558.02 798.15 0.32 22.88

52.60 1.57 2.74 12.43

85.90 0.22 0.11 4.99

Loss on ignition CaO P2O5 K2O MgO SO3 SiO2 Fe2O3 Cl Al2O3 Na2O ZnO MnO TiO2 CuO

Though the surface area of ZTGOC is marginally lesser than CAC, it is compensated by its high ionexchange capacity and higher average pore size in the removal of metal ions from solutions. Though the carbon content of ZTGOC is lesser than CAC, its Cr(VI) removal capacity has been found to be more, which may be due to the presence of various functional groups on the surface of ZTGOC. The high values of sulphur, nitrogen and oxygen contents in ZTGOC, show that these elements are present in the functional groups, favoring Cr(VI) removal from solutions. Loss on Ignition (LOI) (Table 2) corresponds to carbon content and other volatile materials. The percentage of zinc and chlorides is higher in the case of ZTGOC than CAC due to zinc chloride treatment. Effect of contact time

Table 2—XRF analysis of carbon adsorbents Component

ZTGOC (%)

CAC (%)

90.904 0.930 0.182 0.029 0.290 0.661 0.780 3.412 1.740 0.280 0.018 0.720 0.016 0.017 0.002

98.430 0.183 0.008 0.042 0.032 0.159 0.161 0.870 0.007 0.063 0.064 0.001 0.002 0.002 0.005

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Contact time of adsorbate and adsorbent is of great importance in adsorption, because it depends on the nature of the sorption system used. The sorption process is influenced by several parameters such as the structural properties of the adsorbent (porosity, specific area, particle size, etc.), the properties of metal ions (ionic radius, co-ordination number and speciation), concentration of metal ions, chelates formation between metal ions and the adsorbate, etc37. The adsorption of Cr(VI) onto ZTGOC and CAC was initially very fast and slowly reached equilibrium (Fig. 1). Thus, 120 min and 180 min contact time were considered to be adequate for the maximum adsorption of Cr(VI) onto ZTGOC and CAC respectively. The removal efficiency of ZTGOC for an initial Cr(VI) concentration of 10 mg/L, for a sorbent dose of 0.2 g /100 mL, for an optimum time of 2 h, at pH 2.5 was found to be 93.5%, whereas the

Fig. 1—Effect of agitation time on adsorption of Cr(VI)

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removal efficiency of CAC for a sorbent dose of 0.3 g/100 mL and for an optimum time of 3 h at pH 1.5 was found to be 88.5%. This shows that the removal efficiency of ZTGOC is 1.5 times more than that of CAC. Initially, active binding sites were largely available on these sorbents surfaces and consequently the rates of adsorption for these sorbents were high. However, with increasing coverage, the fraction of the surface rapidly diminished and Cr(VI) ions had to compete among themselves for the adsorption sites. Effect of pH

Aqueous phase pH governs the speciation of metals and also the dissociation of active functional sites on the sorbent. Hence, metal sorption is critically linked with pH. Not only different metals show different pH optima for their sorption but may also vary from one kind of biomass to the other38,39. For an initial concentration of 10 mg/L and for a carbon dose of 0.2 g/100 mL, the maximum removal achieved by ZTGOC at pH 2.5 was found to be 93.5%, whereas, for a carbon dose of 0.3 g/100mL, the maximum removal achieved by CAC at pH 1.5 was found to be 88.5%. It could be observed from Fig. 2 that the uptake of Cr(VI) increases with decrease in pH. A lower pH causes the adsorbent surface to carry a more positive charge and thus would more significantly attract the negatively charged chromium species in solution. This indicates that the physicochemical adsorption due to coulombic attraction was the predominant process of Cr(VI) removal. When the pH of the solution increases, the surface becomes negatively charged and the adsorption of Cr(VI) decreases as negatively charged sites on the adsorbent surface does not favor Cr(VI) adsorption due to electrostatic repulsion. Chromium exists mainly in two oxidation states which are Cr(VI) and Cr(III) and the stability of these forms is dependent on the pH of the system40-42. It is well known that, HCrO4- is the dominant form of Cr(VI) at pH below 3.024,43. Maximum adsorption at pH 2.5 and 1.5 for ZTGOC and CAC indicates that Cr(VI) is the predominant species adsorbed on the ZTGOC and CAC at pH 2.5 and 1.5 respectively. Effect of adsorbent dose

The effect of adsorbent dose on the percentage removal of Cr(VI) for these adsorbents is shown in Fig. 3. A maximum removal of Cr(VI) achieved by ZTGOC at an optimum sorbent dose of 0.2 g/100 mL for an initial concentration of 10mg/L at pH 2.5 was

Fig. 2—Effect of pH on adsorption of Cr(VI)

Fig. 3—Effect of adsorbents dose on adsorption of Cr(VI)

found to be 93.5 %, whereas the maximum removal by CAC at an optimum sorbent dose of 0.3 g/100 mL at a pH of 1.5 was found to be 88.2%. It could be seen from Fig. 3 that initially the percentage removal increases very sharply with increase in adsorbent dose but after a certain value (0.2 g/100 mL for ZTGOC and 0.3 g/100 mL for CAC) the percentage removal was almost constant. This trend is expected because as the adsorbent dose increases the number of active sites/surface area increases and thus more Cr(VI) is attached to their surfaces44. At low adsorbent dose, all the active sites are entirely exposed and the adsorption on the surface is saturated faster. But at higher adsorbent dose, though the number of active sites are more, since the initial concentration of Cr(VI) is constant (10 mg/L), the extent of adsorption is small20. Hence, the curve exhibits a steep increase with an increase in adsorbent dose and after a certain value (0.2 g/100 mL for ZTGOC and 0.3 g/100 mL for CAC), the curve exhibits almost horizontal line parallel to x-axis.

NAGASHANMUGAM & SRINIVASAN: HEXAVALENT CHROMIUM REMOVAL BY GINGELLY OIL CAKE CARBON

Adsorption isotherms

Nitrogen adsorption-desorption isotherms for ZTGOC and CAC were carried out to understand the type of isotherm and are illustrated in Figs 4 and 5. According to the IUPAC classification, the adsorption isotherms of ZTGOC and CAC have type I shapes with H4 hysteresis loops. They show a horizontal plateau parallel to pressure axis. Adsorption at higher relative pressures was small and tending to level off. Type I isotherm is mostly common to chemisorption, as pores being narrow that they can not accommodate more than a single molecular layer. Nitrogen uptake was significant only in the low pressure region (region I) where p/po 1 = unfavorable isotherm RL = 1 = linear isotherm RL = 0 = irreversible isotherm 0< RL < 1= favorable isotherm The RL values for ZTGOC and CAC were found to be between 0 and 1, which shows that the sorption of

Fig. 4—Nitrogen adsorption-desorption isotherm of ZTGOC

Fig. 6—Langmuir adsorption isotherm for adsorption of Cr(VI) onto ZTGOC & CAC

Fig. 5—Nitrogen adsorption-desorption isotherm of CAC

Table 3—Langmuir adsorption isotherm constants Adsorbent a b RL R2 L/mg mg/g ZTGOC 0.0645 62.50 0.3511 0.9925 CAC 0.0822 25.13 0.3020 0.9799

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Cr(VI) onto these adsorbents is favorable. From RL values of Table 3, it is clear that the adsorption of Cr(VI) onto ZTGOC and CAC was favorable at all concentrations investigated. The linear form of Freundlich isotherm is represented by the following equation48: log x/m = log KF + 1/n (log Ce)

... (4)

where Ce is the equilibrium concentration (mg/L) and x/m is the amount adsorbed per unit weight of adsorbent (mg/g). The plot of log(x/m) versus log Ce gives a straight line, which indicates favourable adsorption. Figure 7 shows Freundlich adsorption isotherms for adsorption of Cr(VI) onto ZTGOC & CAC. The KF and n values were calculated from the intercepts and slopes respectively. The values of adsorption capacity (KF), adsorption intensity (n) and correlation coefficient (R2) for ZTGOC and CAC are presented in Table 4. The values of adsorption intensity ranging between 1 and 10 (1