Sorption of chromium(VI) from electroplating effluent onto chitin immobilized Mucor racemosus sorbent (CIMRS) impregnated in rotating disk contactor blades

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JIEC-2155; No. of Pages 14 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Sorption of chromium(VI) from electroplating effluent onto chitin immobilized Mucor racemosus sorbent (CIMRS) impregnated in rotating disk contactor blades S. Anuradha Jabasingh a,*, D. Lalith a, Pavithra Garre b a b

Chemical Engineering Division, Sathyabama University, Chennai 600119, Tamilnadu, India Department of Biotechnology, Sathyabama University, Chennai 600119, Tamilnadu, India

A R T I C L E I N F O

Article history: Received 17 May 2014 Received in revised form 22 July 2014 Accepted 25 July 2014 Available online xxx Keywords: Chromium(VI) Mucor racemosus Electroplating effluent Rotating disc contactor Isotherms Batch studies

A B S T R A C T

The study was aimed at investigating the efficacy of chitin immobilized Mucor racemosus sorbent (CIMRS) sorbent, impregnated in the modified rotating disc contactor (MRDC) blades for the sorption of chromium(VI) contained in the electroplating effluent. The optimum time, pH, temperature and CIMRS dosage were found to be 8 h, 7.0, 323 K and 0.7 g/150 mL, respectively, for MRDC sorption studies. Desorption studies were also carried out in MRDC at 60 8C. Seven isotherms were applied to model the experimental data. The present study reveals highly promising nature of CIMRS for Cr(VI) sorption from electroplating effluent. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Chromium is found in many forms, and the two most prevalent forms are trivalent chromium, Cr(III) and hexavalent chromium, Cr(VI). In small amounts, chromium-3 is a vital nutrient needed for healthy human metabolism, but chromium-6 is a known carcinogen and dangerous even in small amounts. Drinking water can be contaminated by hexavalent chromium released by steel and pulp mills, as well as metal-plating and leather-tanning facilities [1]. The presence of hexavalent chromium in the effluent from electroplating industry causes deleterious effects in the aquatic systems. Electroplating effluent contain Chromium in the form of Cr(VI). They are toxic to fish life and swiftly penetrate the cell walls and once transported through them they are rapidly reduced to Cr(III), which subsequently binds to the macromolecules and accumulates in the liver, kidney, spleen and bone marrow of aquatic living beings. They are mainly absorbed through gills. International Agency for Research on Cancer (IARC) has classified Cr(VI) in group 1 which states carcinogenic threat to human. Because of their toxicity, the US Environmental Protection Agency (EPA) and the European Union have designated Cr(III) as priority

* Corresponding author. Tel.: +91 044 34503141; fax: +91 044 2450 1065. E-mail address: [email protected] (S. Anuradha Jabasingh).

pollutants [2]. The exposure to a mixture of Cr(VI) compounds of different solubility result in the highest risks to humans. The current guideline value of maximum acceptable concentration of Cr(VI) in drinking water is 0.06 ppb. To comply with this limit, effluent should be treated to reduce the Cr(VI) to acceptable levels. Methods including reduction, evaporation, ion exchange, electrodialysis, solvent extraction, reverse osmosis and chemical precipitation have been reported for the removal of heavy metal ions [3]. The selection of treatment method and the mode of treatment are based on the concentration of metal ions in the wastewater system and the cost of treatment. Significant sludge production, increasing cost of the landfill, cost of the process including the material cost and operating cost remain the meticulous frustrating aspects of these methods [4]. Many reports have appeared on the development of low cost sorbents particularly bio-sorbents from several bacterial and fungal species [5–10]. Spent biomass has been used by several researchers around the globe for the removal of the Cr(VI) from effluents. Biomass including spent Pleurotus ostreatus [11], Rosa damascene [12], Termitomyces clypeatus [13], Phanerochaete crysosporium [14], pistachio hull [15], Cyanobacterium Oscillatoria laete-virens [16] and Citrus cinensis [17] have been used for the removal of Cr(VI) ions. So far the removal of heavy metal ions and dyes from aqueous and effluent solutions has been carried out under batch and

http://dx.doi.org/10.1016/j.jiec.2014.07.045 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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Nomenclature a A aL B C0 Ce Cf CIMRS D Deff Ea Ed F h K1 k1 k2 K2 kd KD KE KF KFG KH Kid KK KL KLD Ks KT MWCO M n qe qm qt R R2 RL T t V W

DG

radius of spherical sorbent particle (mm) Arrhenius constant Langmuir equilibrium parameter (mL mg1) rate coefficient in Boyd model initial concentrations of the solute in the bulk solution (mg L1) the equilibrium concentration of the solute in the bulk solution (mg L1) final concentrations of the solute in the bulk solution (mg L1) chitin immobilized Mucor racemosus sorbent particle diffusion coefficient (cm2 s1) effective diffusion coefficient (cm2 s1) activation energy of sorption (kJ mol1) activation energy of desorption (kJ mol1) fractional attainment of equilibrium at time t initial sorption rate (mg Cr(VI) g1 CIMRS min1) equilibrium constant of the formation of complex between sorbed molecules first order rate constant (min1) second order rate constant (g CIMRS mg1 Cr(VI) min1) energetic constant of the interaction between sorbed molecules (kJ mol1) desorption rate constant distribution coefficient (mL mg1) Elovich equilibrium constant (L mg1) Freundlich constant or multilayer sorption capacity (mg g1) Fowler–Guggenheim equilibrium constant (L mg1) Hill–de Boer constant (L mg1) intraparticle diffusion coefficient (mg Cr(VI) g1 CIMRS min0.5) Kiselev equilibrium constant (L mg1) Langmuir constant (mL mg1) kiloliter discharge surface sorption factor ((mg Cr(VI) g1 CIMRS) Temkin equilibrium constant (L mg1) molecular weight cut-off mass of the sorbent (g) sorption intensity equilibrium adsorption capacity (mg Cr(VI) mg1 CIMRS) maximum sorption capacity (mg g1) sorption capacity at any time t (mg Cr(VI) mg1 CIMRS) the universal gas constant (kJ mol1 K1) coefficient of correlation dimensionless constant referred to as separation factor temperature (K) time (min) volume of effluent (mL) interaction energy between sorbed molecules (kJ mol1) Gibbs free energy (kJ mol1)

DH DQ DS

enthalpy (kJ mol1) the variation of sorption energy (kJ mol1) entropy (J mol1 K1)

Greek letter u surface coverage (qe/qm)

column studies [17–19]. In this study, a modified RDC is employed for the removal of Cr(VI) ions from the electroplating effluent. The fungi Mucor racemosus was pretreated, immobilized and impregnated into the MRDC blades for the removal of Cr(VI) ions. Kinetic studies were carried out by varying the initial Cr(VI) ion concentration, pH, sorbent dosage, particle size, and temperature. Studies were carried out under batch operation in a modified RDC. The paper as well aims to study and model the sorption isotherms of Cr(VI) from the effluent onto chitin immobilized M. racemosus sorbent (CIMRS). The representation of the sorption isotherms onto CIMRS is based on various sorption models with two, three, or more parameters [20–26]. Langmuir, Freundlich, Temkin, Fowler– Guggenheim, Kiselev, and Hill–de Boer models are used for the determination of the energy of sorption, interaction energy between sorbed molecules, and complex formation between sorbate. Experimental Electroplating effluent Sample of electroplating effluent was collected from Dindigul town in upper Kodaganar river basin, Tamil Nadu in autoclaved reagent bottles and immediately stored at 277 K. The effluent was determined for its color [27,28], pH (Intech, Model—IN-112, digital pH meter), COD [29], BOD [30], total solids [31] and heavy metals (Philips XL30 Elemental EDAX) within 24 h of sample collection. Culture The fungus used in this study was isolated from a tannery nearby Central Leather Research Institute, Chennai. Samples were dispensed into petriplates and were brought to the laboratory. The isolates were grown on Potato Dextrose Agar at 313 K, 318 K, 323 K and 328 K for 24 h prior to study their sorption capacity using batch sorption test [32]. The isolated fungus was further sub-cultured on the Potato Dextrose Agar at regular intervals and incubated at 313 K. The isolate was identified based on the colony morphology, microscopic observation and molecular identification [33,34]. M. racemosus colonies are fast-growing, whitish to grayish, usually thick owing to the abundant upright sporangiophores [35]. Microscopic observation and molecular testing The morphology of the isolate was examined on potato dextrose agar (PDA) at 313 K, 318 K, 323 K and 328 K in the dark. Colonies on PDA at 313 K after 6 days were fast growing with upright sporangiophores measuring 4.5 to 5.6 mm long and 2.5 to 3.5 mm wide. Microscopic examination revealed the spores (sporangiospores) produced inside spherical sporangia at tips of the sporangiophores are brownish, measuring 30 to 60 mm in diameter [33,35]. Rhizoids and stolons were absent [34]. DNA was extracted from the hyphae of a 48 h culture on PDA slants and suspended in UltraPURE distilled water in 2 ml Eppendorf tubes, each containing one sterile 4.5-mm steel shot pellet.

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Fungal strain identification For the nucleotide sequence analysis, fungal genomic DNA was purified using the Fungi Genomic DNA Isolation Kit (MTK 08) (Modern Science Co., Nasik, India). The fungal primer pairs annealing at the 50 and 30 end of the 18S rRNA, 50 -GTAACCCGTTGAACCCCATT-30 and 50 -CCATCCAATCGGTAGTAGCG-30 , respectively, were used for amplification. The PCR was run for 35 cycles in a DNA thermal cycler (Thermal Cycler Applied Biosystems 2720, USA). Amplified PCR products were then analyzed in a 1% (w/v) agarose gel and purified. Purified products were cloned and subsequently sequenced using automated DNA sequencer (ABI 3130 Genetic Analyzer, USA). The 16S rDNA sequence obtained was compared with the sequence obtained from the nucleotide database of National Center for Biotechnology Information (NCBI). The phylogenetic analysis of the strain M. racemosus using its nucleotide sequence data showed that this strain had the highest homology (98%) with M. racemosus strain ATCC1216b and M. racemosus RPG19. Based on the evolution distance and partial sequencing, this strain isolated was identified as M. racemosus. CIMRS preparation Mucor racemosus sorbent preparation The M. racemosus biomass (100 g) was washed with 500 mL of deionized water and rinsed with 200 mL of 1% HCl. It was dried at 303 K for 24 h. The dried biomass was mixed with modifying agent composed of 3% 0.1 M oxalic acid, 3% 0.1 M malic acid and 1% 0.05 M EDTA and soaked in the same for 48 h. The mixture was drained and dried at 333 K for 12 h [35]. The sorbent prepared was washed repetitively with distilled water and 1% NaHCO3 solution to remove residual acid. It was dried at 313 K for 2 h, powdered and sieved to particle size ranging from 0.5 mm to 3.0 mm. The powered material was named M. racemosus sorbent (MRS). This is modified with chitin and is considered for the initial batch sorption studies. Mucor racemosus—Chitin immobilization A commercial chitin powder (purity 60%) was purchased from Merck, Germany. Chitin was modified by dissolving 5 g of chitin and 40 g MRS in 200 mL of 37% HCl below 278 K with rapid stirring. The temperature was then slowly raised to 310 K. At 310 K, the viscosity of the solution increased then rapidly decreased. At this point, the solution was filtered using glass wool into a beaker containing 3 L of deionized water, below 278 K with moderate stirring. The suspension was stirred for 30 min, and then placed overnight in a cold room below 278 K [36]. The MRS-chitin particles were allowed to settle to the bottom and excess water is decanted. The chitin was filtered out using Whatman no. 40 filter paper. This MRS-chitin was dialyzed overnight using bio-design dialysis tubing 8000 MWCO using a pipette washer with a diameter of 20 cm and a water flow rate of 1 mL s1. The sample was dried overnight at 323 K in a vacuum oven with a vacuum of 400 mm Hg. The BET surface area and total pore volume were determined using nitrogen gas adsorption analyzer at 77 K with an ASAP 2010 instrument (Micromeritics ASAP 2010 adsorption analyzer). Specific surface area and total pore volume of the modified MRS-chitin known as CIMRS were 1013.5 m2 g1 and 0.956 cm3 g1, respectively. This modified MRS-chitin, large, crystalline and brownish-yellow in color was selected for immobilization onto the vanes of modified rotating disk contactor.

3

concentration on Cr(VI) sorption. All reagents used were of AR grade (Sigma-Aldrich, Germany). Interference by Mo, V, Fe and Cu is removed by extracting the ions with chloroform [39]. The collected effluent was found to possess Cr(VI) concentration of 228 mg L1 at pH 3.6. 50 mL of effluent of known Cr(VI) concentration was taken in a 250 mL screw-cap conical flask with 0.3 g of sorbent and agitated in thermostatic shaker bath at a speed of 200 rpm at 298 K for 48 h. Samples of solution were collected at 30 min interval and filtered through a 0.45 mm membrane filter and used for Cr(VI) analysis [35]. Studies in modified rotating disk contactor (MDRC) The rotating disc contactor (10 KLD) considered for the treatment of electroplating effluent containing Cr(VI) ions comprises a semibuoyant rotor mounted on a shaft (2) (Fig. 1). Modification is made in the vanes (7, 8) by redesigning it to house the CIMRS sorbent in a mesh structured packing [37]. The rotor comprises a drum (5) with coaxially mounted discs each sandwiching a series of closely spaced spiral vanes (4). The electroplating effluent is held in the tank (1, 6) in which the drum is partly submerged, and the effluent enters the drum through inlet (9) to pass around the outer periphery of the discs via a passageway as the rotor rotates. The vanes pick up the effluent which is gradually lifted to a duct (3) in the axial region of the rotor communicating between adjacent discs leading to an outlet (10). The pictorial representation of the sorbent incorporation onto the surfaces of Modified Rotating Disk Contactor (MDRC) blades is shown in Fig. 1a and b. The collected electroplating effluent was divided into two portions, on portion having Cr(VI) concentration 228 mg L1; pH 3.6 and other diluted to different concentrations in the range from 50 to 200 mg L1 for sorption studies. 0.7 g of CIMRS sorbent with particle size of 2 mm was impregnated into each MRDC blades using mesh structured packing [38]. The rotor rpm was maintained as 200 rpm at 323 K. The entire operation was carried out for 48 h. Samples of solution were collected at 30 min interval and filtered through a 0.45 mm membrane filter and used for Cr(VI) analysis. Residual chromium(VI) concentration Effluent after the sorption was treated with 0.25% diphenylcarbohydrazide (DPC) in acid medium to determine the final concentrations of Cr(VI). Hexavalent chromium reacts with DPC under acid conditions to form a red–violet color (diphenylcarbazide–dichromate complex) quantified using Perkin-Elmer

Initial prescreening batch studies Initial batch experiments were carried out to test the efficiency of CIMRS as sorbent. 0.3–0.9 g CIMRS was used to treat electroplating to determine the parametric effect of initial sorbate

Fig. 1. Design aspects of modified rotating disk contactor, (a) In-flow and out-flow of effluent (b) MRDC blades with Cr(VI) ions sorbed.

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UV–visible spectrophotometer (model 550S). Absorbance was measured at 550 nm using 1-cm path length cuvettes. The Cr(VI) ions sorbed on the surface of the CIMRS was computed by the following equation [35], C0  Ce V qe ¼ M

(1)

The sorption percentage was calculated from the differences between the concentrations of Cr(VI) in the effluent before and after sorption [36] C0  C f  100 Sorption % ¼ C0

(2)

Dynamics of sorption Sorption dynamics refers to the solute uptake rate which controls the residence time of sorbate uptake at the solid–solution interface. The kinetics of Cr(VI) sorption on CIMRS was analyzed using pseudo first-order [40], pseudo second-order [41] and intraparticle diffusion [42] kinetic models. The sorption kinetic data were treated with pseudo first-order model [40]. dq ¼ k1 ðqe  qÞ dt

(3)

Integrating the above equation between the limits 0 to t from q = 0 to q = qt, the above kinetic expression becomes   k1 (4) t log ðqe  qt Þ ¼ log qe  2:303

F ¼1 F¼



1   2  6 X 1 exp n Bt p2 n¼1 n2

qt qe

(11)

where F is the Fractional attainment of equilibrium at time t, D is the Particle diffusion coefficient (cm2 s1) and Deff is the effective diffusion coefficient (cm2 s1) B¼

Deff p2 a2

The rate controlling mechanism was analyzed using McKay plot of log (1  F) versus t at different initial concentrations [46,47]. Thermodynamic analysis The activation energy Ea for the sorption of Cr(VI) on CIMRS was determined using Arrhenius equation [36]. k ¼ AeEa =RT

qe Ce

KD ¼

(15)

DGo ¼ RT ln K D

dq ¼ k2 ðqe  qÞ2 dt

ln K D ¼

where k2 (g CIMRS mg1 Cr(VI) min1) is the second order rate constant, determined from the plot of t/qt versus t. The initial sorption rates were given by h ¼ k2 q2e

(7)

The prediction of rate limiting step is an important factor to be considered in the sorption process [43], especially when sorption is controlled due to film diffusion at earlier stages and later by particle diffusion. By fitting the data in the intraparticle diffusion plot, the mechanism involved in the sorption process can be identified. The intraparticle diffusion coefficient Kid is given by the equation qt ¼ K id t 0:5

(14)

The Gibb’s free energy, enthalpy and entropy for the sorption process are investigated using

where qt is the sorption capacity of MRS (mg Cr(VI) mg MRS) at any time t and k1 is the first order rate constant (min1). The pseudo second order model [41] was given as

On integration for the boundary conditions q = 0 to q = qt at t = 0 to t = t, we have   t 1 1 ¼ þ (6) t qt k2 q2e qe

(12)

A plot of Bt versus t, is made to distinguish the film and particle diffusion controlled sorption [58]. For longer times of sorption, Eq. (10) reduces to McKay equation      2 q 6 Dp (13) t log 1  t ¼ log 2 þ qe p a2

1

(5)

(10)

DG DH DS ¼ þ RT RT R

(16) (17)

where KD is the distribution coefficient in mL mg1. The values of KD, indicate retentability of CIMRS and the mobility of Cr(VI) in CIMRS as well as in the solution phase [48]. Modeling the sorption data The distribution of Cr(VI) ions between CIMRS and the solution, when the system is at equilibrium is used to determine the maximum sorption capacity of CIMRS. Langmuir equation, used for fitting equilibrium data is given by qeL ¼

K LCe 1 þ aL C e

(18)

The amount Cr(VI) ions sorbed at equilibrium per unit mass of the MRS is given by the above equation, where aL ¼

KL qm

(19)

The linear form of Langmuir isotherm is given by (8)

Further analyses of the sorption kinetic data were made to find out the effect of initial concentration on the rate determining step [41,42,44]. Assuming the sorbent particle to be a sphere of radius ‘a’ and Fick’s law to be governing the entire diffusion process, the relationship between the amount of Cr(VI) ion uptake and time is given as  0:5 " #    1 X Dt na Dt 0:5 p þ 2 ierfc pffiffiffiffiffiffi  2 (9) F¼6 2 a a Dt n¼1

Ce 1 aL ¼ þ Ce qeL K L K L

(20)

where KL (mL mg1) and aL (mL mg1) represent Langmuir constants. The maximum sorption capacity in the Langmuir model is given by qm (mg cellulase mg1 CIMRS) [20]. The essential features of Langmuir isotherm can be expressed in terms of a dimensionless equilibrium parameter RL ¼

1 1 þ aL C 0

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(21)

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Values of RL between 0 and 1 indicate favorable sorption. The Freundlich isotherm gives the relationship between equilibrium liquid and sorption capacity based on multilayer sorption. This isotherm is widely used in studies at low concentrations of solute in solution. The expression for Freundlich equation is given by 1=n

qeF ¼ K F Ce

(22)

The linear form of Freundlich equation is given by ln qeF

1 ¼ ln K F þ ln C e n

(23)

where KF and n are multilayer sorption capacity (mg g1) and sorption intensity, respectively. Values of n between 1 and 10 represent beneficial sorption [21]. According to the Elovich model [22], sorption sites increase exponentially with sorption process, due to the occurrence of multilayer adsorption. It is expressed by   qeE q ¼ K E C E exp  eE (24) qm qm ln

qeE q ¼ ln K E qm  eE Ce qm

(25)

u¼

RT

DQ

Kiselev model is expressed by K K Ce ¼

u

(30)

ð1  uÞð1 þ K 1 uÞ

1 KK ¼ þ KK K1 C e ð1  uÞ u

(31)

Hill–de Boer [26] equation describes simultaneous sorption where both mobile sorption and lateral interaction exists among sorbed molecules. Hill–de Boer model is expressed by the equation, KHCe ¼

u 1u

exp



u

K2u 1  u RT



(32)

where KH is the Hill–de Boer constant (L mg1), u the fractional coverage, R the universal gas constant (kJ mol1 K1), T the temperature (K), and K2 is the energetic constant of the interaction between sorbed molecules (kJ mol1). The attraction between the sorbed species is expressed by K2. A positive K2 signifies attraction and a negative value means repulsion between sorbed species. The latter feature is characterized by the Volmer equation. Desorption of sorbed Cr(VI)

where KE is the Elovich equilibrium constant (L mg1) and qm is the maximum sorption capacity (mg g1). If the sorption obeys Elovich equation, maximum sorption capacity and Elovich constant can be calculated from the slopes and the intercepts of the plot ln (qe/Ce) versus qe. Temkin model [23] assumes that the heat of sorption of all the molecules in the layer decreases linearly with coverage due to interaction between sorbent and sorbate. Further to this, sorption is characterized by a uniform distribution of the binding energies that are the specific individual prospective of the ions sorbed. Temkin model is given by

u¼

5

ln K T C e

(26)

RT RT ln K T þ ln C e DQ DQ

(27)

where u is the fractional coverage, R the universal gas constant (kJ mol1 K1), T the temperature (K), DQ = (DH) the variation of sorption energy (kJ mol1), and KT is the Temkin equilibrium constant (L mg1). If sorption obeys Temkin equation, the variation of sorption energy and the Temkin equilibrium constant can be calculated from the slope and the intercept of the plot u versus ln Ce. Fowler–Guggenheim [24] considered an isotherm equation by taking into account the lateral interaction of the sorbed molecules. The explicit form of the equation is   u 2uW exp (28) K FG C e ¼ RT 1u

0.1 M HNO3 and 0.5 M EDTA were pumped into the MRDC and the shaft was made to rotate at 500 rpm [49]. Desorption was carried out for 4 h at 333 K. The entire batch operation was carried out for 48 h. Samples of solution were collected at 30 min interval and filtered through a 0.45 mm membrane filter and used for Cr(VI) analysis. A recovery of 80% was obtained during desorption. The residual regenerative solution is analyzed for their Cr(VI) content and the experimental data for desorption of measured amount of sorbed Cr(VI) at pH 1.0 using 0.1 M HNO3 and EDTA is investigated. Analytical methods The surface area of CIMRS was measured by BET [45]. Proximate and ultimate analysis of the CIMRS was carried out to determine their characteristics [50]. The sorbents made in this study were characterized by scanning electron microscopy (Philips XL30 Scanning Electron Microscope and EDAX). The surface morphology of MRS before pretreatment, MRS after pretreatment, CIMRS before chromium sorption, CIMRS after ten times of repeated usage was observed by subjecting the samples to scanning electron microscopy (SEM) using Philips XL30 scanning electron microscope with electron acceleration voltage of 15 kV and probe current of 6  1011 A after subjecting them to goldsputtering in a denton vacuum desk I for 2.0 min under a 200 m Torr Argon atmosphere and a current of 30 mA. Results and discussion

The linear form of which is ln

C e ð1  uÞ

u

¼ ln K FG þ

2W u RT

Characterization of electroplating effluent and CIMRS (29)

where KFG is the Fowler–Guggenheim equilibrium constant (L mg1), u the fractional coverage, R the universal gas constant (kJ mol1 K1), T the temperature (K), and W is the interaction energy between sorbed molecules (kJ mol1). W is positive, negative and zero if the lateral interaction is repulsive, attractive, and no interaction, respectively. The measured heat of sorption is directly proportional to interaction energy that in turn is positively related to the loading capacity of the sorbent. At conditions of no interaction Fowler–Guggenheim equation was observed to reduce to the Langmuir equation. Kiselev [25] model predicted the sorption characteristics in the localized monomolecular layer. The

The electroplating effluent collected was studied for identifying the heavy metal content (Table 1). In addition to this the proximate and ultimate analysis of CIMRS is carried out and provided in Table 2. Results from characterization of the effluent are as follows. The pH of the effluent, CIMRS dosage, particle size of the selected sorbent and the temperature of sorption were initially optimized at lab scale in batch studies. The optimum conditions studied were incorporated in the MRDC during the Cr(VI) removal. Experiments in MRDC were carried out using the electroplating effluent to find the effect of variables including the initial concentration of Cr(VI) ion (mg L1) and time of treatment on Cr(VI) sorption.

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Table 1 Examination and heavy metal concentration profile. Source of effluent

Electroplating effluent

Location

Dindigul town in upper Kodaganar river basin

Physical examination Appearance Odor Turbidity NTU Total solid (mg L1) Total suspended solid (mg L1) Total dissolved solid (mg L1) Electrical conductivity (mV cm1) Chemical examination pH Total hardness (as CaCo3) (mg L1) Calcium (as Ca) (mg L1) Magnesium (as Mg) (mg L1) Sodium (as Na) (mg L1) Potassium (as K) Iron (as Fe) (mg L1) Manganese (as Mn) (mg L1) Chromium (as Cr) (mg L1) Cobalt (as Co) (mg L1) Copper (as Cu) (mg L1) Cadmium (as Cd) (mg L1) Free ammonia (as NH3) (mg L1) Chloride (as Cl) (mg L1) Sulphate (as SO4) (mg L1) Phosphate (as PO4) (mg L1) Tidy’s test (as O) (mg L1) Silica (as SiO2) (mg L1) COD (mg L1) BOD (mg L1) Total kjeldhal (as N) (mg L1)

Greenish yellow 2054 Pt/Co Offensive smell 2.4 9520 470 9050 12900 3.62 1275 310 120 2300 950 0.3 0 228.2 0.986 9.5 0.521 13.44 2487 822 4.75 270 38.56 2763 930 71.68

228 mg L1) for the equilibrium time of 8 h at optimum pH 7.0 and at 303 K. The effect of sorbent dosage on the percentage removal is shown in Fig. 2b. It was found that the surface area available for the sorption is limited for a specific dosage of CIMRS. The reduced sorption of Cr(VI) ions at high concentration of solution was mainly due to the unavailability of sorption sites. The sorption of Cr(VI) increased from 54.8% to 75.2%, if the sorbent dosage is decreased from 0.3 g to 0.7 g for 150 mL of Cr(VI) solution at 50 mg L1 concentration. This is due to the availability of more binding sites for complexation of Cr(VI) ions [52]. The same was found to increase from 14.5% to 36.4% for Cr(VI) solution at 228 mg L1 concentration. However, increasing the sorbent dosage above 0.7 g had very slight influence on the percentage removal. This effect may be due to the decline in the Cr(VI) ions in the solution with the increase in the CIMRS dosage. Hence, further addition of CIMRS above 0.7 mg in 150 mL solution was considered to be reasonably inapt. At lower Cr(VI) ion concentration the available sites of sorption on the sorbent of a particular quantity is more, hence percentage removal was found to be high and vice versa at high concentrations of Cr(VI) ions in the solution. The sorption density for Cr(VI) sorption was observed to be 20.55, 15.41, 12.33, 11.53, 8.807, 8.706 and 8.05 mg g1 for CIMRS dosage of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 g, respectively. This indicates that as the sorbent dosage is increased above 0.7 g/150 mL of 100 mg L1 Cr(VI) solution, the sorption density was not significantly altered. Sorption density was observed to be 5.54, 8.807, 8.46, 6.54 mg g1 for 0.7 g sorbent dosage for other Cr(VI) solutions 50, 100, 150 and 200 mg L1. Therefore, sorbent dosage of 0.7 g was taken as optimum for 100 mg L1 solution. Effect of particle size on Cr(VI) sorption

Effect of pH on the sorption of Cr(VI) ions by CIMRS Sorption experiments were carried out using Cr(VI) solutions at different pH values by maintaining the sorbent dosage as 0.7 g/ 150 mL at 303 K for initial concentration of 228 mg L1. The effect of pH on the percentage removal of Cr(VI) ions and sorption capacity were shown in Fig. 2a. Cr(VI) ions sorption was found to increase from pH of 2.0 to 7.0, after which no significant increase in the metal ion uptake was observed. The reduced uptake of metal ion at lower pH was due to the high concentration of H+ and free Cr(VI) ions present in the solution that offered a competitive sorption for the sites on the surface area of the CIMRS. However, above the pH range of 7.0, the chromium hydroxide species started to precipitate and thereby clogged the pores of CIMRS. The percentage removal of Cr(VI) ions increased from 12.31% to 89.69% as the pH of the solution was varied from 2.0 to 9.0. This is due to the surface complexation reaction, which is influenced by electrostatic force of attraction between Cr(VI) ions and surface of CIMRS. Cr(VI) has high charge density 7.19 g cm3, higher electro negativity of 1.6 and larger ionic radius of 0.044 nm which shows favorability to electrostatic force of attraction [51]. The optimum pH was chosen to be 7.0 for further sorption studies. Effect of CIMRS dosage on the on the sorption of Cr(VI) ions Sorbent studies were carried out using CIMRS dosages ranging from 0.3 to 0.9 g in Cr(VI) ion concentration (50 mg L1 to

The effect of particle size on the sorption of Cr(VI) ions on sorption capacity and sorption percentage with different particle sizes of CIMRS, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 mm for 8 h, using 228 mg L1 Cr(VI) solution at the pH of 7.0 for the sorbent dosage of 0.7 g/150 mL is shown in Fig. 2c. The sorption capacity increased from 3.256 mg g1 to 8.967 mg g1 as the CIMRS particle size was increased from 0.5 mm to 2.0 mm, but on further increasing the particle size above 2.0 mm, the sorption capacity decreased to 6.245 mg g1 from 8.967 mg g1 at a particle size of 2.0 mm. The same trend was observed for sorption percentage which increased from 45.26 to 95.32% and then decreased to 64.21% as the particle size of the sorbent increased. The surface area exposure of CIMRS was observed to govern the Cr(VI) ion sorption. Effect of temperature on the sorption of Cr(VI) ions The effect of temperature from 303 K to 343 K on the sorption density of Cr(VI) ions at a sorbent dosage of 0.7 g/150 mL Cr(VI) at pH 7 for the initial concentration of 228 mg L1 is shown in Fig. 2d. Cr(VI) ion sorption was found to increase from 303 K to 343 K. The results revealed an increase in the sorption capacity from 2.236 mg g1 to 9.563 mg g1 as the temperature increased for an initial concentration of 228 mg L1 at an optimum time of 8 h. This indicates the endothermic nature of the sorption process. The increase in temperature increases the mobility of the cation. In addition to this, the endothermic nature of the sorption process involves physical as well as chemical phenomena. The higher

Table 2 Physical and chemical properties of CIMRS. Sorbent

Surface area (m2 g1)

Bulk density (g cm3)

Particle sizes (mm)

Moisture (%)

Ash (%)

Volatile (%)

Carbon (%)

Hydrogen (%)

Oxygen (%)

Nitrogen (%)

CIMRS

1013.5

0.956

0.5-3.0

0.05

13.65

78.42

40.30

3.52

14.52

1.51

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Fig. 2. Effect of (a) pH, (b) sorbent dosage, (c) particle size and (d) temperature on the sorption percentage and sorption capacity of Cr(VI) on CIMRS. Data points in parts (a), (c) and (d) correspond to: *, sorption percentage (%); and *, sorption capacity (mg g1). Experimental conditions: sorbent dosage = 0.7 g/150 mL; pH 7.0; Cr(VI) conc. = 228 mg L1.

temperature is responsible for the bond rupture in the sorbent which subsequently increase in the active sites on the sorbent [53]. Higher temperatures had least effect on the sorption capacity and sorption percentage. The optimum temperature for the sorption was found to be 323 K. After optimizing the initial lab scale conditions including the pH, sorbent dosage, sorbent particle size and temperature, further studies were carried out using the collected electroplating effluent by dividing them into two portions, on portion having Cr(VI) concentration 228 mg L1; pH 3.6 and other diluted to different concentrations in the range from 50 to 200 mg L1 for sorption studies in a 10 KLD MRDC. The pH of the effluent was maintained as 7.0 using NaOH.

The initial concentration of Cr(VI) ion provides an important driving force to overcome mass transfer resistance between aqueous and solid phases, hence sorption experiments were conducted using CIMRS with different initial concentrations of Cr(VI) ions for 8 h, at the pH of 7.0 for the sorbent dosage of 0.7 g/ 150 mL Cr(VI) solution. The percentage sorption of Cr(VI) ions is greater at lower initial concentration than at higher initial concentrations, due to the limited surface area available for a specific dosage of CIMRS for sorption. The excess amount of Cr(VI) ion in the solution is less sorbed due to the unavailability of

Studies on MRDC Experiments on MRDC were carried out under optimized conditions. The temperature of the effluent was increased to 323 K prior to treatment in MRDC. The effect of contact time on the sorption of Cr(VI) ions in the effluent was studied by varying the contact time from 0 to 12 h for chromium solution with initial concentration from 50 mg L1 to 200 mg L1 and 228 mg L1. The amount of Cr(VI) ions sorbed were found to increase with an increase in the contact time and attained equilibrium at 8 h for all the concentrations studied. This behavior is attributed to the relatively less available sorption sites on the surface of the CIMRS as contact time increases. At equilibrium, the maximum sorption percentages were found to be 75.24, 62.00, 49.10, 37.22 and 36.45 for chromium solution with initial concentration of 50, 100, 150, 200 and 228 mg L1, respectively (Fig. 3). Thus the optimum time of sorption was found to be 8 h.

Fig. 3. Effect of time on the Cr(VI) sorption onto CIMRS in MRDC.

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Fig. 4. Application of (a) the pseudo-first-order kinetic model and (b) the pseudo-second-order kinetic (c) intraparticle diffusion plot (d) McKay plot model to the experimental data for Cr(VI) sorption onto CIMRS. Data points in both parts correspond to the following initial Cr(VI) ion concentrations: *, 50 mg L1; , 100 mg L1; &, 150 mg L1; ^, 200 mg L1; and ~, 228 mg L1.

sorption sites [9]. The increase in the initial Cr(VI) ion concentration from 50 mg L1 to 200 mg L1 during each batch experiment decreased the percentage removal from 92.37% to 45.2%. For the effluent at 228 mg L1 concentration, the percentage removal was found to be 46.4%. When the same sorbent impregnation dosage was employed in the MRDC blades, the percentage removal was enhanced to 85.63-95.63%; this is due to the enhanced surface area available for sorption and the continuous movement of the rotor blades, which attracts the Cr(VI) ions toward the blades. The rate of percentage removal is higher in the beginning due to a larger surface area of CIMRS being available for the sorption of Cr(VI) ions. After the sorbed materials forms a thick layer, the capacity of the sorbent gets exhausted and the uptake rate was observed to be controlled by the rate of sorbate transport from the exterior to the interior sites of the sorbent. Sorption kinetics The results of kinetic analysis are shown in Fig. 4a–d. From the slope of log (qe  qt) versus t plot, the first order rate constants k1

are found to be 1.451, 0.527, 0.928, 0.527 and 0.371 h1 for initial Cr(VI) ion concentration of 50, 100,150, 200 and 228 mg L1 at 323 K, respectively (Fig. 4a). The regression coefficients values shown in Table 3 confirm the applicability of the model. Pseudo first order kinetic model fitted the data well and represented the rapid stages of sorption. But from the plot of log (qe  qt) versus t, it was observed that this model fits the data well for the first 60 min and after that the data deviates from the Lagergren theory. Thus, this model represents the initial stages of sorption, which is found to be rapid. This similar kind of trend was observed by Ho and Mc Kay [41]. Hence, the use of Lagergren kinetic model [40] for Cr(VI) sorption on to CIMRS for the entire sorption period was found to be inappropriate; consequently, kinetic data was treated with pseudo second order model (Fig. 4b). It is observed that the pseudo second order rate constant k2 decreased in the order of 0.0171, 0.0041, 0.0037, 0.0017 and 0.001 and the initial sorption rate ‘h’ decreased from 32.26 to 20.83 for the initial Cr(VI) concentrations 50, 100, 150, 200 and 228 mg L1. This is due to the mass transfer resistance offered by the excess ions in the effluent with Cr(VI) ion concentration. Such competitions between the like ions hinder

Table 3 Kinetic parameters for the sorption of Cr(VI) on CIMRS in MRDC. C0 (mg mL1)

50 100 150 200 228

Pseudo first order kinetic model

Pseudo second order kinetic model

Intraparticle diffusion model

k1 (h1)

R2

k2 (g CIMRS mg1 Cr(VI) min1)

R2

h (mg Cr(VI) mg1 CIMRS min1)

Kid (mg Cr(VI) mg1 CIMRS min0.5)

Ks (mg Cr(VI) mg1 CIMRS)

1.450 0.527 0.928 0.527 0.370

0.912 0.905 0.931 0.921 0.911

0.017065 0.004114 0.003789 0.001723 0.001021

0.998 0.997 0.992 0.995 0.998

32.25 28.57 26.31 21.27 20.83

12.27 21.17 26.01 26.00 29.33

31.79 57.42 44.26 58.31 67.26

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the movement of these particles to reach the sorption site and hence the initial sorption rate was found to decrease. This model can be applied for the entire sorption process and confirms the chemisorption of Cr(VI) onto CIMRS. The solute transfer is usually characterized by the external mass transfer (boundary layer diffusion) or by intraparticle diffusion or both for a solid–liquid sorption. Mechanism of sorption involves ion transport from the bulk solution through liquid film to the CIMRS exterior surface. After which, Cr(VI) ion may be transported to the pores of CIMRS or sorbed onto the exterior surface. The equilibrium reaction is the last and the most rapid step. The slowest step determines the rate controlling parameter in the sorption process. The rate controlling parameter might be distributed between intraparticle and film diffusion mechanisms [42,47]. Sorption is controlled due to film diffusion at earlier stages and later by particle diffusion. By fitting the data in the intraparticle diffusion plot, the mechanism involved in the sorption process can be identified. In the diffusion plot of Fig. 4c, the initial curved portion relates to the boundary layer diffusion and the latter linear portion represents intraparticle diffusion. These two regions suggested the ensuing of sorption process by both surface sorption and pore diffusion. From the slope of the second linear portion of the plot, the intraparticle sorption parameter Kid was found to be 12.27, 21.17, 26.01, 26.02 and 29.33 mg Cr(VI) mg1 CIMRS min0.5. The boundary layer effect was depicted by the surface sorption factor ‘Ks’ mg Cr(VI) mg1 CIMRS characterized by the intercept of diffusion plot. Large values of intercept, emphasis more contribution of surface phenomena to the sorption process. In this case, Ks factor was found to increase for the initial Cr(VI) ion concentrations. Surface sorption becomes more predominant as the rate controlling step at higher concentration and pore diffusion was also found to govern the sorption process. The values for the intraparticle sorption coefficient and surface sorption factor are given in Table 3 for all the Cr(VI) ion concentrations studied. McKay plots of log (1  F) versus t at different initial concentrations are shown in Fig. 4d. The rate controlling mechanism was analyzed for different initial concentrations, using the above plot. Linearity was found at high concentrations of 150 mg L1, indicating that it was entirely controlled by film diffusion process and at low concentration, the plots are found scattered, indicating the particle diffusion to be the rate controlling step. The Cr(VI) treatment with CIMRS prepared from M. racemosus biomass was found to be complex and followed both surface sorption and particle diffusion as revealed by the sorption mechanism studies. Sorption isotherms Variation in temperature leads to smaller changes in the sorption capacity of Cr(VI) ions onto CIMRS. Large molecular weight of Cr(VI) ions leads to minor changes in the sorption capacity, since at specific temperatures; molecules with large molecular weight possess smaller velocity. Hence, the equilibrium sorption capacity of Cr(VI) on CIMRS had marked increase during the lowering of the temperature, testifying the hypothesis put forth by the kinetic theory. The sorption capacity increased from 2.236 mg to 9.563 mg per mg of CIMRS as the temperature increased from 293 K to 343 K indicating the endothermic nature of the sorption process. The increase in temperature increased the mobility of the cation. The augmentation of sorption capacity at higher temperatures indicated the involvement of physisorption, chemisorption and an increase in the number of active sites due to the bond rupture [35,50,52]. Plot of ln k versus T1 gives activation energy Ea as 28.733 kJ mol1 and Arrhenius constant as 1.0131 h1 (Fig. 5a). R and k1 represent Universal gas constant and pseudo first order rate constant, respectively. In the process of sorption, Gibbs free energy, enthalpy and entropy play a vital role in determining

9

Fig. 5. (a) Arrhenius plot (b) ln KD versus T1.

the spontaneity of the process. The plot of ln KD versus T1 results in a straight line, whose slope and intercept gives DH8 and DS8 (Fig. 5b). The values of KD were seen to decrease with an increase in temperature. The values of DG8, DH8 and DS8 are given in Table 4. Temperature was seen to adversely affect the process of sorption. This may be due to the effect of a more negative value of Gibb’s free energy which makes the reaction spontaneous [53]. The negative DG8 values indicate the thermodynamic favorability of the reaction toward the sorption of Cr(VI) ions onto CIMRS [47]. Positive DS8 and DH8 values indicate the spontaneity of sorption at high temperatures [49]. Affinity factor plays a major role in determining the sorption capacity of Cr(VI) ions. The increase in temperature increases the mobility of solute through the solution. It also increases the pore diameter of the CIMRS sorbent. Cr(VI) ions are dehydrated and get accessed to the micropores of CIMRS. Modeling the sorption data The equilibrium data were modeled with the Langmuir, Freundlich, Elovich, Temkin, Fowler–Guggenheim, Kiselev, and Hill–de Boer models. The experimental values of qe and Ce are initially treated with the linearized equations in order to Table 4 Thermodynamic parameters for the sorption of Cr(VI) on CIMRS in MRDC. Sorption temperature T (K)

Distribution coefficient KD (mL mg1)

Gibbs free energy DG8 (kJ mol1)

Enthalpy DH8 (kJ mol1)

Entropy DS8 (J mol1 K1)

293 303 313 323 333 343

1.516 0.815 0.482 0.296 0.286 0.284

1398.72 1446.61 1494.50 1542.39 1590.28 1638.17

4.456

4.789

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Fig. 6. Modeling the equilibrium sorption data using (a) Langmuir, (b)Freundlich, (c)Elovich, (d)Temkin, (e) Fowler–Guggenheim, (f) Kiselev, and (g) Hill–de Boer models.

determine the models parameters and the isotherms are reconstituted using the determined values (Fig. 6a–g). Langmuir, Freundlich, Elovich, Temkin, Fowler–Guggenheim, Kiselev, and Hill–de Boer parameters for the sorption of (VI) ions onto CIMRS are shown in Table 5. Langmuir isotherm The amount of Cr(VI) ions sorbed on CIMRS at equilibrium per unit mass of the CIMRS (aL) was found to be 0.051 mL mg1 and KL was found to be 2.415 mL mg1 represent Langmuir constants. From the linear form of Langmuir isotherm, the maximum sorption capacity qm in Langmuir model was found to be 47.62 mg Cr(VI) mg1 CIMRS. The linear plot of Ce/qe versus Ce confirms the applicability of Langmuir model (Fig. 6a). In order to check the validity of the Langmuir model, it is interesting and essential to recalculate the values of RL between 0 and 1 indicated favorable sorption. This constant related to the energy of sorption indicates the sorption nature to be either unfavorable if (RL > 1), linear if RL = 1, favorable if 0 < RL < 1 and irreversible if RL = 0. RL is greater than 0 but less than 1 indicating that Langmuir isotherm is favorable. Closer nature of RL value toward irreversibility has prompted the investigation on Freundlich isotherm to obtain the relationship between equilibrium liquid and sorption capacity based on multilayer sorption. R2 value of 0.99 shows the significance of the model.

Freundlich isotherm This isotherm is widely used in studies at low concentrations of solute in aqueous medium [54]. The equilibrium data were further analyzed using the linearized form of Freundlich equation using the same set experimental data, by plotting ln qe versus ln Ce. The magnitude of the exponent n gives an indication on the favorability of adsorption. It is generally stated that values of sorption intensity, n in the range 2–10 represent good, 1–2 moderately difficult, and less than 1 poor adsorption characteristics [21,54]. Multilayer sorption capacity, KF was found to be 9.402 mg mg1. The calculated Freundlich isotherm constants and the corresponding coefficient of correlation values were shown in Table 5. The coefficients of correlation are high (0.942) showing a good linearity. CIMRS sorbent is an excellent sorbent for Cr(VI) with n = 3.333. A good agreement between the experimental and predicted values suggests the validity of the Freundlich model for the experimental equilibrium data. On the other hand, Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations (Fig. 6b). Hence we assume the exponential covering of sorption sites and use the Elovich model to describe the same. Elovich isotherm The Elovich isotherm constants, KE and qm are obtained using the linear form of the Elovich equation [22]. Elovich isotherm

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Table 5 Langmuir, Freundlich, Elovich, Temkin, Fowler–Guggenheim, Kiselev, and Hill-de Boer parameters the sorption of Cr(VI) on CIMRS in MRDC. Model isotherm

Model parameters

Sorption experiments

C0 (mg mL1) Ce (mg mL1) qe (mg Cr(VI) mg1 CIMRS) qeL(mg Cr(VI) mg1 CIMRS) KL aL (mL mg1) qm (mg Cr(VI) mg1 CIMRS) R2 qeF(mg Cr(VI) mg1 CIMRS) KF N R2 qeE (mg Cr(VI) mg1 CIMRS) KE qm (mg Cr(VI) mg1 CIMRS) R2 qeT (mg Cr(VI) mg1 CIMRS) KT DQ (kJ mol1) R2 qeFG (mg Cr(VI) mg1 CIMRS) KFG W (kJ mol1) R2 qeK (mg Cr(VI) mg1 CIMRS) KK K1 R2 qeH (mg Cr(VI) mg1 CIMRS) KH K2 (kJ mol1) R2

Langmuir

Freundlich

Elovich

Temkin

Fowler–Guggenheim

Kiselev

Hill-de Boer

50 12.40 18.80 18.30 2.415 0.0051 47.35 0.990 20.0 9.402 3.33 0.947 18.80 0.521 13.16 0.924 18.22 0.126 11.66 0.971 18.78 0.0076 1628.46

100 38.0 31 31.35

150 76.30 36.90 37.84

200 125.60 37.20 41.15

228 145.0 41.50 41.91

27.99

34.51

40.04

41.85

31

36.85

37.20

41.50

31.21

37.51

41.04

41.54

12.05

12.45

12.85

13.25

31.32

37.21

40.10

39.89

1795.21

1850.78

38.30

41.51

40.10

38.10

1684.06

18.81

30.90

18.20

30.28

10,167.8

exhibited lower coefficients of correlation (R2 = 0.924), lower than those obtained for Langmuir and Freundlich models (R2 = 0.990 and R2 = 0.947). In spite of the good correlation coefficient, the values of maximum sorption capacity determined using the linear transformation of the Elovich equation are much lower than the experimental sorbed amounts at equilibrium corresponding to the plateaus of the sorption isotherms. Multilayer sorption does not convince the experiment in the studied concentration range (Fig. 6c). The incapability of the Elovich model to describe the sorption scenario of Cr(VI) onto CIMRS is revealed from the sorption isotherm. Temkin isotherm The slope and the intercept of the plot u (qe/qm) versus ln Ce reveals the variation in the sorption energy, DQ and Temkin equilibrium constant, KT. Value of the theoretical maximum sorption capacity (qm theoritical) can be used to calculate the surface coverage, u. The adsorption data were analyzed by a regression analysis to fit the Temkin isotherm model. The parameters of Temkin model as well as the correlation coefficients are determined. The very higher values of the coefficient of correlation (R2 = 0.971) show a good linearity (Fig. 6d). The experimental equilibrium curves are very close to those predicted by the Temkin model [23]. The variation of sorption energy, DQ = (DH), is negative, which indicates that the endothermic nature of the sorption reaction [47,55]. It is evident that the Cr(VI) ions have a positive increment (endothermic effect). Fowler–Guggenheim isotherm The sorption data for the Cr(VI) sorption onto CIMRS were analyzed by a regression analysis to fit the Fowler–Guggenheim isotherm. The coefficients of correlation and the parameters of the Fowler–Guggenheim model, W, KFG are summarized. It is important to notice that the Fowler–Guggenheim isotherm is only

10,514.9

1739.63 0.987 36.90 0.5 0.008 0.994 36.12 0.00579 10,861.9 0.979

11,208.9

11,555.9

applicable for u < 0.6. The interaction energy, W, is negative, indicating the attraction between the sorbed molecules. For the studied sorption, the linearization is good and in order to verify the validity of the Fowler–Guggenheim model, it is interesting and necessary to recalculate the sorbed amounts using the equilibrium concentration values and the Fowler–Guggenheim parameters. It is noticed that the Fowler–Guggenheim model perfectly describes the equilibrium isotherms when the theoretical maximum sorption capacities were used for the calculation of the values of surface coverage (Fig. 6e). Kislev isotherm The equilibrium data were modeled using the linearized form of Kiselev model by plotting 1/[Ce(1  u)] versus 1/u. The calculated Kiselev isotherm parameters and the corresponding coefficient of correlation values were determined and it was observed that for the values of surface coverage (u) calculated using the maximum sorption capacities determined from the Freundlich model, the Kiselev isotherm is found valid. The equilibrium constant of the formation of complex between adsorbed molecules, K1 is positive indicating the formation of complex between the sorbed molecules (Fig. 6f). Hill–de Boer isotherm The Hill–de Boer isotherm constants, KH and K2, as well as R2, for the sorption systems using CIMRS are obtained using the linear form of the isotherm. The Hill–de Boer model allows verifying the assumptions made by the Fowler–Guggenheim equation, in spite of the slightly less R2 = 0.979 compared to R2 = 0.994 coefficient of correlation, obtained by Fowler–Guggenheim (Fig. 6g). The energetic constant of the interaction between sorbed molecules, K2, is positive indicating the attraction between the sorbed molecules. This result is in agreement with that obtained using the Fowler–Guggenheim equation. Table 5 illustrates the variation

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in the maximum sorption capacity determined experimentally and theoretically from the models. The Langmuir equation is not appropriate for the experimental results. The mode of linearization of the equation influences the estimation of parameters [54,55]. The model of Freundlich is more suitable than that of Langmuir, but it slightly diverges with the experimental results for the higher values of equilibrium concentrations [21]. The Elovich representation does not lead to a correct determination of the maximum sorption capacity. The equation of Temkin leads to the determination of the variation of sorption energy [23]. The sorption reaction is found to be endothermic. This model adequately fits the equilibrium sorption data. The Fowler– Guggenheim model effectively describes the sorption isotherms and shows that the interaction among sorbed molecules is attractive. The Kiselev isotherm presents a notable difference between experimental and theoretical curves. The model of Hill– de Boer states the attraction between the sorbed molecules [26]. For these reasons, interactions of Cr(VI) ions onto CIMRS surface is an unconfined multilayer sorption confirming the involvement of indefinite, unlimited sites for sorption [48]. Each site can accommodate more than one molecule. The interaction among sorbed molecules is attractive and there is an association between them. Sorption is carried out on energetically different sites and it is an endothermic process. Scanning electron microscopic images of sorbent The change in the surface morphology of CIMRS during sorption was clearly observed in the scanning electron microscopic images. The morphology of CIMRS during the sorption and CIMRS after ten repeated cycles of usage during sorption is shown in SEM images

(Fig. 7a–d). Fig. 7a shows the smooth surface of M. racemosus without any eruptions on the texture. On subjecting M. racemosus to modification using 3% 0.1 M oxalic acid, 3% 0.1 M malic acid and 1% 0.05 M EDTA, a large number of pores were formed on MRS, possibly due to the penetration of oxalic acid and malic acid into the matrix and the subsequent wearing away of the matrix due to penetration (Fig. 7b) [56]. The pretreated MRS is immobilized into the chitin matrix. MRS immobilized chitin matrix known as CIMRS is shown prior to sorption in Fig. 7c. During sorption, these pores are accumulated with Cr(VI) ions. After 10 times of repeated usage, CIMRS seems to get dilapidated out of the surface of CIMRS, as shown in Fig. 7d. The SEM images clearly picturizes the entire scenario of Cr(VI) sorption onto the CIMRS. These images provide a qualitative confirmation of the change in the morphology of CIMRS due to the treatment. The SEM image and EDAX composition analysis of the CIMRS before and after sorption is shown in Fig. 8a and b. The EDAX spectra for CIMRS before the sorption process confirms the presence of carbon, an indication of the apt methodology ensured to prepare the sorbent. The EDAX spectra for CIMRS after the process of sorption indicates presence of chromium ions in the pores, a positive indication of compatibility in usage of the prepared sorbent for sorption of Cr(VI) ions. Back scattered electrons are used to detect contrast between areas with different chemical compositions. The backscattered electrons employed in SEM consist of high-energy electrons originating in the electron beam. They are reflected or back-scattered out of the CIMRS sample due to the elastic scattering interactions with specimen Cr(VI) ions. Heavy elements backscatter electrons more strongly than light elements, and this cause the Cr(VI) ions sorbed to appear brighter in the SEM images (Fig. 8b) [57].

Fig. 7. (a) MRS before pretreatment (b) MRS after pretreatment (c) CIMRS before Cr(VI) sorption (d) CIMRS after ten times of repeated Cr(VI) sorption.

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Fig. 8. EDAX microcomposition analysis of the CIMRS (a) before sorption and (b) after sorption.

Desorption studies Regeneration of Cr(VI) up to 80% is carried out by using 0.1 M HNO3 and EDTA. Experimental data for desorption of a measured amount of CIMRS with sorbed Cr(VI) at pH 1.0 using 0.1 M HNO3 and EDTA are shown. Plot of ln kd versus T1 gives activation energy for desorption as Ed as 10.168 kJ mol1 in comparison to the activation energy for sorption 28.733 J mol1 and the Arrhenius constant is 76.17 h1 for desorption in comparison to the Arrhenius constant value of 1.0131 h1 for sorption (Fig. 9). R and kd represent Universal gas constant and desorption rate constant, respectively. The release of sorbed Cr(VI) ions was highest at 93.1%. These results suggest a reversible equilibrium process similar to ion exchange involved in Cr(VI) sorption. As the vanes of MRDC swept away the electroplating effluent, the H+ ions in the effluent displaced the sorbed counter ions which are Cr(VI) in this case, in

Fig. 9. Kinetics for Cr(VI) desorption from CIMRS.

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the bed of the same electrical charge depending on the charges and concentration of the displacing H+ ions. These results suggest partial reversibility of the sorption process. The sorption of Cr(VI) besides electrostatic attraction is also due to stronger physicochemical complexation between Cr(VI) and the charged components of the CIMRS. Conclusions In this study, the chitin immobilized M. racemosus sorbent (CIMRS) with particle size in the range of 0.5–3.0 mm was prepared from M. racemosus biomass coupled with chitin and impregnated into the MRDC blades. Chemical morphology and structural analysis of the sorbent were determined using SEM-EDAX. The simplicity, accuracy, efficiency and robustness of the technique adopted in MRDC, encourage its application in similar sorption and desorption studies. The equilibrium time of sorption was found to be 8 h, at the pH of 7.0 for the sorbent dosage of 0.7 g/228 mL effluent Cr(VI) solution at 303 K. The kinetics of the process was predicted by pseudo first order and second order model. Cr(VI) sorption as indicated by the thermodynamic parameters was found to be a spontaneous, feasible, endothermic. Sorption of Cr(VI) onto CIMRS was investigated and modeled using different isotherms. Film and intraparticle diffusional phenomena were found to control the complete sorption. In conclusion, CIMRS is a good supporting material for the sorption of Cr(VI) ions and cheaper when compared to other carriers with the efficacy to be desorbed and recharged after prolonged use. Our work will be continued with sorption experiments on MRDC with additional prudence in its design, aiming at higher sorption capacities at Cr(VI) higher concentrations. Acknowledgements The authors are grateful to the Directors of Sathyabama University, Tamilnadu, India for providing institutional support. The comments and recommendations of the anonymous reviewers and the editor Jonghwi Lee are greatly acknowledged. References [1] L.S. Clesceri, A.E. Greenberg, A.D. Eaton, Standard Methods for Examination of Water & Wastewater, 20th ed., American Public Health Association (APHA), American Water Works Association (AWWA) and Water Environment Federation (WEF), Washington, DC, 1998. [2] USEPA, Federal Register, 52, USEPA, Washington, DC, 1987, p. 25861131. [3] M. Dakiky, M. Khamis, A. Manassra, M. Mer’eb, Adv. Environ. Res. 6 (2002) 533. [4] D. Mohan, C.U. Pittmann Jr., J. Hazard. Mater. B137 (2006) 762. [5] D. Park, Y.S. Yun, J.M. Park, Environ. Sci. Technol. 38 (2004) 4860. [6] A.K. Giri, R. Patel, S. Mandal, Chem. Eng. J. 15 (2012) 71. [7] A. Abdel-Razek, Nat. Sci. 9 (2011) 211. [8] G. Bla´zquez, F. Herna´inz, M. Calero, M.A. Martı´n-Lara, G. Tenorio, Chem. Eng. J. 148 (2009) 473. [9] M.H. Karaog˘lu, S¸. Zor, M. Ug˘urlu, Chem. Eng. J. 159 (2010) 98. [10] A.P.S. Batista, L.P.C. Roma˜o, M.L.P.M. Arguelho, C.A.B. Garcia, J.P.H. Alves, E.A. Passos, A.H. Rosa, J. Hazard. Mater. 163 (2–3) (2009) 517.

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