Acknowledgments:
July, 2012
I would like to thank Md. Mohibul Alam, Assistant Professor, Dept. of Chemical Engineering & Polymer Science, Shahjalal University of Science & Technology, Sylhet, Bangladesh, for giving me an opportunity to work with his group on this project that was both challenging and frustrating.
His guidance helped me to overcome many obstacles, and I have learned much from this experience and have come away with one of the great learning experiences of my life.
His guidance helped me to overcome many obstacles, and I have learned much from this experience and have come away with one of the great learning experiences of my life.
I would like to thank Prof. Dr. Md. Akhtarul Islam, Head, Dept. of Chemical Engineering & Polymer Science, Shahjalal University of Science & Technology, Sylhet, Bangladesh, for his inspiring invaluable advices which always inspired me during this thesis work.
I would like to thank all the staff of Chemical Engineering & Polymer Science department, Shahjalal University of Science & Technology, Sylhet, Bangladesh, for their kind helps during the research work.
Finally I thank to my parents for instilling a strong work ethic in me, which is the key to all success.
Al Momin Jamiul
Author
Declaration:
I am Al Momin Jamiul, hereby declare that the work described in this thesis has been carried out by me under the supervision of Md. Mohibul Alam, Assistant Professor, Dept. of Chemical Engineering & Polymer Science, Shahjalal University of Science & Technology, Sylhet, Bangladesh.
Al Momin Jamiul
Author
Approval:
This is to certify that Al Momin Jamiul, a student of Chemical Engineering & Polymer Science in Shahjalal University of Science & Technology, Sylhet, Bangladesh, of session 2007-08, registration no: 2007332020 has successfully completed the thesis work on the title “Study on Adsorption of Hexavalent Chromium onto Water Hyacinth” under my supervision for the partial fulfillment of the requirements for the degree of B.Sc. Engineering in Chemical Engineering & Polymer Science.
Supervisor
MD. MOHIBUL ALAM
ASSISTANT PORFESSOR
DEPARTMENT OF CHEMICAL ENGINEERING & POLYMER SCIENCE
SHAHJALAL UNIVERSITY OF SCIENCE & TECHNOLOGY
SYLHET, BANGLADESH
Abstract:
Hexavalent chromium can be reduced to trivalent stat using various reductive system. In this work the adsorption capacity of water hyacinth for hexavalent chromium was determined. Water hyacinth was selected because of it’s availability and high adsorption capacity.
The experiment was carried out on the removal of Cr (VI) from dichromate solution, which is a form of chromium in the tannery effluent. To optimize the process, experiment was carried out on the effect of pH, effect of different amount of dosages etc. The amount of Cr (VI) adsorption increased with decrease of pH. The optimum parameter were evaluated as pH = 3.0 and dosages = 1.5 kg/m3. The equilibrium data fit well in both Langmuir and Freundlich isotherm. From the reduction analysis of chromium it was seen that Cr (VI) decreased and Cr (III) increased with increasing time. Three kinetic models (the pseudo-first order, the pseudo-second order and the unified approach) were used to calculate the adsorption rate constants.
Adsorption capacity and Langmuir constant were obtained as 0.055(kg/kg) and 415.45455(m3/kg) respectively. The Freundlich constants Kf and n were found as 0.65564 and 5.41 respectively. It was also found that the rate constants k1and k2 are not the function of the initial concentration C0.
1. Introduction:
Chromium enters the environment primarily through its widespread use in industrial applications, including tanning, metallurgy and plating. Once introduced, it persists as either Cr (VI) or Cr (III). Hexavalent Cr exists in ground water systems as the oxyanion HxCrO4x-2 (Chromate), which exhibits high water solubility and is a mutagen, teratogen and carcinogen (1). In-contrast, Cr (III) is relatively nontoxic and strongly partitions to the solid phase (2).
Hexavalent chromium, which is known as a toxic metal because of its high oxidizing capacity(3). The industrial effluent can contain Cr (VI) at concentrations ranging from tenths to hundreds of mg/L (4). The maximum tolerable limit of Cr (VI) in the effluent of industrial wastewater is 0.1mg/L (5) and the recommended limit of chromium in potable water is 0.05mg/L (6).
Processes to remove the Cr (VI) from the wastewater such as chemical reduction and precipitation, ion-exchange, adsorption etc.(7). In adsorption process a large number of papers report the adsorption properties of naturally appearing low cost materials, as well as, some industrial waste products. They are sand(iron-oxide-coated) (8), saw dust (9), fly ashes(10), waste metallic oxides(11), hydroxides(12) and activated sludge(13) etc. They conduct the experiment in highly acidic medium but not mentioned as reduction occurred in that system or not. Some other authors notified that during the adsorption process some amount of Cr (VI) reduced to Cr (III) (14, 15). Some other authors notified that chromate reduction and retention process within Arid Subsurface Environments. Here, they examine the reduction of chromate by sediment obtained from the Hanford formation beneath the Interim Disposal Facility (IDF) at the Hanford site (16). According to some other authors investigations, Cr(VI) reduction by magnetite at high pH (< 20% of potential reduction capacity(17). We also got some other notifications those Fe (II) (18, 19), organic matter (20), Fe(II) bearing minerals (21, 22), sulfide compounds(23). These reductions all ultimately lead to Cr (III), but the rates and products differ.
In recent years, adsorption process is extensively applied in the treatment of Cr (VI) containing wastewater. At the end of seventies the use of water hyacinth to remove Cr (VI) from wastewater was proposed since they require small solution volumes and no cost (collected from pond). Adsorption by water hyacinth is highly pH dependent and adsorption efficiency increases with decreasing pH. Water hyacinth was selected because of its high capacity for a large number of contaminants in aqueous solutions (26).
In the present year, water hyacinth is used as adsorbent for Cr (VI) removal from aqueous solution and also observed the reduction ability into this system. Protons are consumed during the reduction of Cr (VI) as follows:
Cr2O72- + 14 H+ + 6e- ↔ 2Cr3+ + 7H2O
CrO42- + 8H+ + 3e- ↔ Cr3+ + 4H2O
HCrO4- + 8H+ + 3e- ↔ Cr3+ + 4H2O
H2CrO4 + 6H+ + 3e- ↔ Cr3+ + 4H2O
Adsorption is viewed as a Langmuir type physico-chemical reversible process and a three parameter model is proposed which describes an adsorption system from both equilibrium and kinetic viewpoints.
In this context, the water hyacinth undoubtedly be a cost effective adsorbent and needs no regeneration. Laboratory based batch kinetics and isotherm studies are conducted to determine the adsorption capacity along with reduction ability of water hyacinth. The effect of contact time, initial concentration of Cr (VI) and pH on adsorption and reduction are studied.
1.1 Chromium removal and an overview:
Many models have been employed in the removal of hexavalent chromium. The batch removal of hexavalent Chromium (Cr (VI)) from wastewater under different experimental conditions using economic adsorbents produced from the pyrolysis and activation of the waste tyres (TAC) and from the pyrolysis of sawdust (SPC) was investigated (27). The performance of these adsorbents against other commercial adsorbents has also been carried out. The removal was favored at low pH, with maximum removal at pH=2 for all types of experiments. The effects of concentration, temperature and particle size have been reported. The batch sorption kinetics have been tested for a first-order reversible reaction, a first-order and second-order reaction.
The removal of Chromium (VI) from aqueous solution under different conditions using an adsorbent was also investigated. The adsorbent is Beech (Fagus orientalis L.) sawdust studied by using batch techniques. Batch studies indicated that the percent adsorption decreased with increasing initial concentration of Chromium (VI) (28). A contact time of 80 minutes was found to be optimum. Maximum Chromium (VI) removal was observed near a pH of 1.0. Adsorption conformed to the Freundlich and Langmuir isotherms.
The removal of Chromium (VI) by phytoremidation from soil was studied. Wheatgrass (Triticum aestivum) and Indian Grass (Sorghastrum nutans) grown in contaminated soil were used (29). The fibrous roots of the grasses have been shown to be effective in the removal of metal contaminants in the soil. The remaining Chromium (VI) was extracted from the soil after a growing period of 10 days and analyzed for using the diphenylcarbizide colorimetric method. Extraction methods were also performed on the plants to determine if Chromium (VI) was present.
The study on performance of low-cost adsorbent such as sawdust of Gmelina arborea (Ghambhari tree) in the removal of Chromium (VI) ion from aqueous solution is performed.
The adsorbent material adopted was found to be an efficient media for removal of Chromium (VI) ion in continuous mode using fixed bed column (30). A comparative study has also been done on the adsorption capacity of sawdust of different mesh sizes. The column studies were conducted with a fixed column of diameter 7 cm and a bed height of 0.06-0.12 m. The flow rate of the solution passing through the adsorbent bed was maintained at a fixed value of 1 litre/min. It was found that the metal uptake capacity (amount of removal) of Chromium (VI) ion decreased but the adsorption capacity (percentage of removal) increased with the decrease in the concentration of Chromium (VI) in the initial sample solution. It was also observed that the order of metal uptake capacity and adsorption capacity of sawdust of different ISS mesh size for removal of Chromium (VI) removal was as follows: (-30+10) > (-70+50).
A new biosorbent material marine micro alga Isochrysis galbana, from marine sources Bay of Bengal, was immobilized and used as an adsorbent for removal of Chromium (31). It was found that the increase in aqueous metal concentration increased metal uptake. The presence of acid decreased metal uptake probably due to the preferential adsorption of hydrogen ion compared to the Chromium ion.The immobilized calcium alginate beads were also found to absorb chromium in the absence of biomass as well and presence of biomass increased the metal adsorption by 3-4 folds. This immobilized biomass was very effective for the removal of chromium. Isochrysis galbana was found to absorb chromium more strongly when compared to another marine micro alga, Chaetoceros calcitrans, probably due to presence of polysilicate layers over Chaetoceros cells and correlated by Freundlich type of equation for the range of experimental parameters covered in the present study. An empirical equation was proposed to estimate the equilibrium metal concentration in the immobilized algal beads (CS) as a function of pH and aqueous metal concentration (CA).
Several low cost biomaterials such as baggase, charred rich husk, water hyacinth and eucalyptus bark (EB) were tested for removal of chromium (32). All the experiments were carried out in batch process with laboratory prepared samples and wastewater obtained from metal finishing section of auto ancillary unit. The adsorbent, which had highest chromium (VI) removal was EB. Influences of chromium concentration, pH, contact time on removal of chromium from effluent was investigated. The adsorption data were fitted well by Freundlich isotherm. The kinetic data were analyzed by using a first order Lagergren kinetic. The Gibbs free energy was obtained for each system and was found to be -1.879 kJ/mol for Cr (VI) and -3.885 KJ/mol for Cr(III) for removal from industrial effluent. The negative value of AG0indicates the feasibility and spontaneous nature of adsorption. The maximum removal of Cr (VI) was observed at pH 2. Adsorption capacity was found to be 45 mg/g of adsorbent, at Cr (VI) concentration in the effluent being 250 mg/L. A wastewater sample containing Cr (VI), Cr (III), Mg and Ca obtained from industrial unit showed satisfactory removal of chromium. The results indicate that eucalyptus bark can be used for the removal of chromium.
Water Hyacinth (Eichhornia crassipes) has demonstrated its ability to remove nutrients and other chemical elements from sewage and industrial effluents (33). In this study, 10.00 ppm chromium, 35.00 ppm copper and 14.00 ppm nickel solutions were used and absorption ability of water hyacinth was observed. Due to the positive experimental results on the removal of chromium, copper and nickel, its seen that water hyacinth may be used for industrial effluents under conditions in Turkey.
The removal of Cr (VI) using viable anaerobic granular biomass as a biosorbent was investigated (34). The effect of Cr (VI) concentration on biogas content and COD removal using batch studies indicated that the phase II methanogenic-rich) culture was more sensitive than the phase I (acidogenic-rich) culture. Toxicity indices both cultures using COD removal were developed based on linear-log interpolation. The median inhibition Cr (VI) concentration (IC(50)), for phase II cultures was found to be 263mg/L, while that for phase I cultures was 309mg/L. A sorption study was conducted on viable and non-viable (dried) phase I-rich biomass: both followed the Langmuir model. In addition, the biosorption capacity for metabolically inhibited biomass was 25% less indicating some level of cellular uptake associated with Cr (VI) removal. This study demonstrated the potential for a two-phase anaerobic treatment system for a Cr (VI) contaminated effluent.
The possibility of removing hexavalent chromium from wastewater by electrochemical treatment using a graphite felt electrode and synthetic electrolytes was also investigated (35). It is suggested that the process proceeds in two steps: electrochemical reduction of the hexavalent chromium to chromic ion followed by the formation of an insoluble chromic hydroxide in an electrochemically generated high pH environment. The chromic hydroxide adheres to the electrode surface as a charged colloidal particle. The electrochemical dissolution of the hydroxide layer by potential inversion is also discussed as a possible regeneration procedure.
It is concluded that these steps can be included within a single separating column provided that the feed pH and the column voltage are carefully controlled.
A method has been developed for the removal of chromium using ferrous sulphide generated in situ (36). The effects of experimental parameters such as pH, reagent dosages, interference from cations and chelating agents have been investigated. Under optimum conditions, removal efficiencies of 99 and 97% for synthetic and industrial samples have been obtained. The method offers all the advantages of sulphide precipitation process and can be adopted easily for industrial effluents.
An electrochemical process using carbon aerogel electrodes was developed to treat chromium-contaminated water (37). The operational conditions viz. pH (2-7), initial metal ion concentration (2-8mh/L), and charge (0.3-1.3 A h) were optimized to achieve maximum removal efficiency. The dimensions of the cell and electrode area were 1.8 dm (length) * 0.75 dm (breadth) * 0.95 dm (height) and 0.54 dm2 respectively. In the experiments, chromium concentration dropped from 2mg/L to 0.008mg/L (99.6% removal) under optimized conditions of pH 2 and 0.8 A h. To optimize the flow rate, experiments were carried out at different flow rates (60-600 L/h) in the electrochemical reactor. Batch experiments were designed by response surface methodology using Box-Behnken design, which can be used to optimize the key parameters for maximizing the removal percentage. An R2 value of 09736 was obtained from the regression analysis of the performed experiments which exhibited a close fit between the experimental results and model predictions.
The removal of chromium anions from aqueous solutions by using nanofiltration-complexation consisting of pilot-scale nanofiltration equipment (Osmonics Sepa CF Membrane Cell) and water-soluble p-sulfonated calyx (4) arene ligand was studied (38). For the determination of optimum removal conditions of the chromium anions, the effect of pH, ligand cavity size and foreign anions on the retention of the chromium anions in nanofiltration-complexation system was also evaluated. The results showed that water-soluble p-sulfonated calyx (4) arene was an effective and selective ligand for the chromium anions over Cl- , SO42- and NO3-anions in nanofiltration-complexation system at pH 9.4.
The removal of chromium species from aqueous dilute solutions using polymer-enhanced ultrafiltration (PEUF) process was investigated (39). Three water soluble polymers, namely chitosan, polyethyleneimine (PEI) and pectin were selected for this study. The ultrafiltration studies were carried out using a laboratory scale ultrafiltration system equipped with 500,000 MWCO polysulfone hollow fiber membrane. The effects of pH and polymer composition on rejection coefficient and permeate flux at constant pressure have been investigated. For Cr (III), high rejections approaching 100% were obtained at pH higher than 7 for the three tested polymers. With chitosan and pectin, Cr (VI) retention showed a slight increase with solution pH and did not exceed a value of 50%. An interesting result was obtained with PEI. The retention of Cr (VI) approached 100% at low pH and decreased when the pH was increased. This behavior is opposite to what one can expect in the polymer-enhanced ultrafiltration of heavy metals. Furthermore, the concentration of polymer was found to have little effect on rejection. Permeate flux remained almost constant around 25% of pure water flux.
Because the disposal of CCA-treated waste wood is becoming a serious problem in many countries, it is necessary to develop proper remediation technique. Extraction of CCA-treated wood with bioxalate (BO) solvent was evaluated at various solvent pH levels, BO concentrations, extraction temperature and liquid to solid ratios, as well as with different alkaline metal hydroxides as pH-adjusting reagents (40). It was clarified that approximately 90% of the chromium, copper or arsenic could be extracted effectively under particular pH conditions (2.2-3.2). Extraction efficiency was also improved by increasing solvent concentration from 0.0125 M to 0.125 M. In addition, increasing BO solution temperature from 25 to 750C promoted the extraction of chromium and arsenic; however, that of copper was higher than these metals, irrespective of temperature. The optimum conditions for extraction of chromium, copper and arsenic from treated wood using BO solution were determined to be as follows: pH of 2.2-3.2, solvent concentration of 0.065-0.125 M, solvent temperature of 750C, and solvent to CCA-treated wood mixture ratio of 20:1 (ml : g). Under these conditions, the extraction efficiencies for chromium, copper and arsenic were maximized at 89.4, 88.2 and 94.1% respectively, thus demonstrating the applicability of BO chelation for the remediation of metal contaminated solid waste.
A bacterial consortium with representatives of sulfate-reducing and denitrifying bacteria was selectively enriched. Model experiments under microaerobic conditions showed that it precipitated chromium from Cr (VI) containing waters (area of a former electroplating factory, Leipzig, Germany) by two different mechanisms: by sulfate reduction and precipitation as sulfide and some direct reduction (41). Sulfate reduction needed fatty acids as organic substrates and resulted at the first stage in no sulfide accumulation. In the absence of the fatty acids but with straw as organic substrate, the direct reduction of chromium was observed without sulfate reduction. In this case Cr (VI) reduction rate correlated with that of the denitrification.
The foam separation technique holds great promise especially when the concentrations involved is very low. The basis for the separation is surface absorption phenomena. The success of this technique depends on the stability and characteristics of foam. The operation is simple with less maintenance as there are no moving parts. A fundamental study about the selective foam fractionation of chromium (VI) metal ion in continuous column was carried out with sodium lauryl sulphate (SLS) as foaming agent using synthetic aqueous and industrial effluent containing chromium ion (42). The effect of air flow rate, liquid pool height, pH, concentration of feed solution and foam height were varied to study the performance on enrichment factor in foam separation and the enrichment ratio. The experiments were carried out to optimize the various parameters for effective separation of Cr (VI) ions. From these studies it has been observed that as the height of the liquid pool increases, the separation factor also increases which is due to the fact that the residence time of bubble in the liquid pool is more. As the concentration of the bulk solution is reduced, the surface tensions increases and in turn the separation factor increases. The enrichment ratio is observed to be high when ever the foam is dry. An increase in the feed flow rates resulted in having more available chromium ions leading to increase in the enrichment ratio. The flow rate varied from 2 to 6 LPH. Tannery effluent was used for carrying out these experiments.
A novel, low energy, natural analog based eletrokinetic system called FIRS (ferric iron remediation and stabilization) applies a low-magnitude direct electric potential between two or more sacrificial iron-rich electrodes placed at opposing sides of a mass of contaminated soil or sediment (43). The electric potential generates a strong Eh/pH gradient between the two electrodes, promotes anodic dissolution, Fe(0) and Fe (2+) (aq) migration and forces the precipitation of ferric iron oxyhydroxides, hematite, goethite, magnetite and ZVI at near neutral pH values at the interface of the anodic and cathodic domains. The system uses approximately one tenth of the energy requirements of most conventional electrokinetic systems. In the feasibility trial, FIRS achieved significant reduction of hexavalent chromium in the target soils and performed at least as well as in the bench-scale studies.
1.2 Aim of the work:
A large number of papers report the adsorption properties of naturally appearing low cost materials, as well as, some industrial waste products. They conduct the experiment in highly acidic medium but not mentioned as reduction occurred in that system or not.
Some other authors notified that during the adsorption process some amount of Cr (VI) reduced to Cr (III) (14, 15). Some other authors notified that chromate reduction and retention process within Arid Subsurface Environments.
According to some other authors investigation, Cr (VI) reduction by magnetite at high pH (<20% of potential reduction capacity) (17).
Many of these experiments were conducted by using naturally occurring bioadsorbent.
The aims of the work are as follows:
- To investigate the adsorption of hexavalent chromium with water hyacinth.
- To investigate the reduction of Cr (VI) to Cr (III) by water hyacinth.
- To evaluate the equilibrium and kinetic parameters of Cr (VI) adsorption by water hyacinth.
- To see the effect of solution pH on removal and reduction of Cr (VI).
- To see the effect of adsorbent dosages on adsorption and reduction of Cr (VI).
2.1 Materials:
Hater hyacinth was used as adsorbent. A stock Cr (VI) solution was prepared in distilled water using K2Cr2O7(MARK, Germany) and Cr (III) solution by using Cr (III) chloride (MARK, Germany). The KMnO4 (MARK, Germany) solution was prepared by using distilled water and DFC (MERCEK, Germany) solution was prepared by using both acetone (SIGMA, Germany) and distilled water. All working solutions were prepared by diluting the stock solution with distilled water. The pH adjustments were made with HCl (MERCEK, Germany) or NaOH (SIGMA, Germany) solutions.
2.2 Analytical methods and Procedures:
Cr (VI) Determination:
At first 500ppm of 1000ml of K2Cr2O7mother solution was made.
For making the calibration curve of Cr VI), six 100ml conical flasks were taken and in each 0.4, 0.8, 1.6, 2.4, 3.2 and 4ml of K2Cr2O7 solution were taken respectively and filled with distilled water up to 100ml. That is why 2, 4, 8, 12, 16, 20ppm working solution of K2Cr2O7 were prepared respectively.
For measuring the absorbance in UV meter, water was taken as a blank.
For batch adsorption, 200ml of 75, 100, 125, 150ppm of K2Cr2O7 solution were prepared and in each 0.3gm of water hyacinth were added and pH of the solution was maintained at 5.2.
Then these were subjected to the shaking for 16 hours.
After shaking each solution was centrifuged with the help of centrifugal tubes. Then from each sample 2ml was collected and diluted 50 times by adding water of 98ml. Then these solutions were taken in six separate test tubes and absorbance were measured spectrophotometrically at
Total Cr Determination:
500ppm of 1000ml of Cr (III) chloride mother solution was prepared.
For making calibration curve, 20ml of 50ppm Cr (III) solution added with 20ml of 50ppm KMnO4. Then 2, 3, 4, 5, 6ml of samples were taken separately in five volumetric flasks respectively and diluted each up to 500ml. From that diluted solution 2, 3, 4ml were taken separately in three volumetric flasks and diluted each up to 100ml.
For making the blank solution, 10ml distilled water was taken and 50ml of KMnO4 was added to it. Then 5ml was taken and diluted up to 500ml in a volumetric flask. 10ml of that was taken and 1ml of H2SO4and 1ml of DFC were added to it.
After batch adsorption and centrifugation, 20ml was collected from each of the 75, 100, 125, 150ppm samples respectively and added 20ml of 50ppm solution of KMnO4 with each sample.
Then 5ml was collected from each of the 75, 100, 125, 150ppm samples and diluted up to 500ml.
Then 1ml of H2SO4 and 1ml of DFC were added to each sample.
After that calibration curve data were measured spectrophotometrically at
Study of Adsorption Isotherm:
Adsorption studies were performed by the batch technique. For isotherm studies a series of 200ml conical flask were employed. Each flask was filled with predetermined amount of adsorbate solution of varying concentrations at maintained at desired pH. A known amount of adsorbent was added into each flask and shaken continuously by a mechanical shaker for a maximum period of eight to twelve hours. Afted equilibrium has attained, the supernatant solution was centrifuged in a centrifugal unit taking the solutions in centrifugal tubes. The uptake of the adsorbate was monitored spectrophotometrically and total bulk concentration was also monitored spectrophotometrically.
The experimental variables considered for isotherm study.
Initial solution concentrations were 75, 100, 125 and 150ppm.
PH =3.
Amount of adsorbent =0.3gm.
Volume of solution =200ml.
Contact time = 10 hours.
Adsorption Kinetics Study:
For kinetic investigation the batch technique was selected because of its simplicity. A number of conical flask (200ml) containing predetermined amount of solution of known concentration was placed in a mechanical shaker. Fixed amount of adsorbent was added into each flask and then started to shake. At various time intervals, a few quantities of solution from each flask were taken and centrifuged in centrifugal unit to measure those supernatant solution. After measuring, the solutions were fed back to respective flask. By measuring the absorbance, the concentration of chromium (VI) was found for that particular time. From this data the amount adsorbed was calculated for various time intervals. The actual amount adsorbed was calculated 1, 5-diphenylcarbazide method as per procedure. The chromium (III) concentration was then calculated from the difference between the total chromium and chromium (VI) concentrations. The amount adsorbed q, were calculated by the following equation:
q = ( C0 – Ct ) * V / w
Where, Ct =Bulk concentration
C0 =Initial concentration
V=Volume of the solution
W=Amount of adsorbent.
The experimental variables considered for kinetic study.
Initial concentration of the solution = 75, 100, 125, 150ppm.
pH=3
Amount of adsorbent=0.3gm.
Volume of the solution=200ml.
Contact time=12 hours.
Study of pH Effect:
A number of conical flasks containing chromium VI) solution ( Cr2O72- solution ) of known concentration were taken. At room temperature, fixed amount of adsorbent was added into each flask. The pH of each flask was adjusted in such a way that the volumes of solution remain unchanged. The solutions were then agitated for near about ten hours. The solution of each flask were centrifuged and analyzed to determine the removal percentage at various pH.
The experimental variables considered for effect of pH:
Volume of the solution=200ml.
Contact time=12 hours.
Initial solution concentration=150ppm.
Study of Adsorbent Dosages Effect:
A number of conical flasks containing chromium VI) solution ( Cr2O72- solution ) of known concentration were taken. At room temperature, different amount of adsorbent was added into each flask. The pH of each flask was maintained at a constant value. The solutions were then agitated for about 12 hours. The uptake solutions of each flask were centrifuged and analyzed to determine the percent removal of chromium (VI) for each case variable.
The experimental variables considered for effect of adsorbent dosages study.
Initial solution concentration=150ppm.
Initial pH=5.2.
Amount of adsorbent = 0.2, 0.3, 0.4gm.
Volume of the solution=200ml.
Contact time=12 hours.
3. Result and Discussion:
3.1 Effect of solution pH on Adsorption:
The initial pH of adsorption medium is one of the most important parameters affecting the adsorption process. Figure-3 shows the effect of pH on the adsorption and reduction of chromium (VI). When pH of the chromium (VI) solution was increased from 2 to 5.2, the adsorption percentage decreased from 75% to 40% where as reduction of chromium (VI) to Chromium (III) also decreased from 34% to about 14%. So, from here it is seen that adsorption as well as reduction of chromium (VI) on water hyacinth is highly pH dependent and works at highly acidic medium.
3.2. Effect of Adsorbent Dosages:
As the amount of adsorbent increases the removal percentage of chromium also increases as shown in Figure 4. Increase in amount of adsorbent means increase in number of active sites and thus removal also increases. It is seen there is maximum removal when adsorbent dose is 0.4gm. But from economic point of view 0.3gm of adsorbent dose is selected. All experiments in this paper are carried out at this dosage.
3.3 Sorption Equilibrium and isotherms:
3.3.1 Langmuir Isotherm:
The adsorbent data for wide range of adsorbate concentrations are most conveniently described by adsorption isotherms, such as Langmuir isotherm, which relates equilibrium adsorption density qe ( Kg adsorbate/Kg adsorbent ) to equilibrium adsorbate (Cr (VI)) concentration in the solution, Ce. The Langmuir isotherm is valid for monolayer adsorption onto a surface containing a finite number of ideal sites. The model assumes uniform energies of adsorption onto the surface and no transmigration of adsorbate in the plane of the surface. The Langmuir isotherm is represented by the following equation:
1/qe = 1/(KqαCe) + 1/qα ----------------------------------------- (2)
Where, qe is the amount adsorbed, Kg Cr (VI)/Kg water hyacinth, qα is the maximum adsorption capacity, Kg Cr (VI)/Kg water hyacinth, Ce is the equilibrium Cr (VI) concentration, Kg Cr (VI)/m3 solution and K is the adsorption equilibrium constant. The plot of 1/qe vs. 1/ Ce is linear as shown in Figure 5. Maximum adsorption capacity qα and equilibrium constant K are calculated from the slop and intercept of the plot and are found to be qα=0.055 Kg/Kg and K =415.45455 m3/Kg.
3.3.2 Freundilich Isotherm:
The Freundilich or Classical isotherm is in the form:
Ce = K (qe)n ------------------------------------------ (3)
The Freundilich isotherm is represented by the following equation:
Log qe = n log K + (1/n) log Ce ------------------------ (4)
Where K and n are the Freundilich constants and it is generally stated that values of n in the range 2-10 represent good adsorption. qe and Ce are as previously defined. Beating in mind that the freundilich model is consistent immobile adsorption, attempts to attach significance to all the coefficients in both will procure a paradox. Necessary conditions for the use of fitted isotherms for the interpretation of adsorption interactions include attainment of extremely accurate surface coverage. It is also unlikely for the case where Langmuir model agrees with experimental sites of adsorbent surface or those adsorbed molecules don’t undergo movement in the surface.
Such deviations from the basic assumptions of the Langmuir model limit the interpretation of values for qe and K, although the value of qe would nevertheless represent a practical limiting adsorption capacity. This does not necessarily detract from the practical value of the model for mathematics representation of experimentally observed equilibrium relationships. An accurate mathematical description of equilibrium adsorption capacity is indispensable for reliable predictive modeling of adsorption systems and it is useful to quantitatively compare adsorption behavior for different adsorption systems or varied conditions within any given system.
The Freundilich constants K and n are calculated from the slope of the plot and are found to be K = 0.65564 and n = 5.41.
3.4 Reduction of Chromium at Equilibrium:
Reduction at equilibrium is shown in Figure 7. Plot between bulk concentration, Cb and initial concentration, Co shows that reduction increases with increasing initial concentration.
Experimental variables:
Initial concentration = 75, 100, 125 and 150ppm.
Amount of adsorbent = 0.3 gm.
Volume of the solution = 200ml.
PH = 3.
3.5 Reduction Kinetics:
The Chromium (VI) reduced to Chromium (III) with increasing time. The plot between the bulk concentration, Cb and time (hr) for various initial concentrations is shown in Figures 8(a), 8(b), 8(c) and 8(d).
3.5.1 Reduction Kinetics at Initial Concentration 75 ppm:
.
Experimental variables:
Initial concentration = 75 ppm.
Amount of adsorbent =0.3 gm.
Volume of the solution = 200 ml.
pH = 3.
Time of shaking = 10 hours.
3.5.2 Reduction Kinetics at Initial Concentration 100 ppm:
Experimental variables:
Initial concentration = 100 ppm.
Amount of adsorbent =0.3 gm.
Volume of the solution = 200 ml.
pH = 3.
Time of shaking = 10.5 hours.
3.5.3 Reduction Kinetics at Initial Concentration 125 ppm:
Experimental variables:
Initial concentration = 125 ppm.
Amount of adsorbent =0.3 gm.
Volume of the solution = 200 ml.
pH = 3.
Time of shaking = 11 hours.
3.5.4 Reduction Kinetics at Initial Concentration 150 ppm:
Experimental variables:
Initial concentration = 150 ppm.
Amount of adsorbent =0.3 gm.
Volume of the solution = 200 ml.
pH = 3.
Time of shaking = 12 hours.
3.6 Kinetics of Hexavalent Chromium Adsorption:
Kinetics parameters for chromium adsorption onto water hyacinth were evaluated using intra-particle diffusion, pseudo-first order, pseudo-second order and unified approach models.
3.6.1 Intra-Particle Diffusion Kinetics:
Effect of contact time and initial chromium (VI) concentration on chromium removal was investigated using chromium (VI) concentration 75-150 ppm at pH 3. The experimental courses of kinetics are presented in figure 8 where the experimental data are indicated by the points. More than 30-50% of chromium (VI) was removed at initial 0.5-2 hours of adsorption and thereafter the rate decreased gradually leading to equilibrium. This decreasing removal rate towards the end suggests formation of monolayer coverage of chromium on the outer surface of the adsorbent and pore diffusion onto the inner surface of the adsorbent particles through the film due to continuous agitation maintained during the experiments.
The chromium molecules are most probably transported from the bulk of the solution into the solid phase by intra-particle diffusion which is often the rate limiting step in many adsorption processes. The possibility of intra-particle diffusion is explored by using the following equation.
qt= kidt1/2 + C-------------------------------------------------- (4)
Where Cis the intercept and kidis the intra-particle diffusion rate constant.
Consistent with equation(4), the values of qt correlated linearly with values of t1/2, as shown in figure 9 and the rate constant kid was directly evaluated from the slop of regression line. The R2 values given in Table 1 are all higher than 0.86, Confirming that the rate limiting step is actually the intra-particle diffusion process. The values of intercept C provide information about the thickness of the boundary layer, i.e. the resistance to the external mass transfer. The larger the intercept the higher is the external resistance.
Intra-particle diffusion parameters were found from the slop of amount adsorbed, q (kg/Kg) vs.time1/2, t1/2 graphs. These have been shown in the figure 9(a, b, c & d).
3.6.2 Lagergren Pseudo-First Order Kinetics:
The following Lagergren pseudo-first order rate expression has been applied for the determination of specific rate constant of adsorption for chromium (VI)-water hyacinth system.
ln (qe – q) = ln qe –kad * t
Where qe and q are the amounts of chromium (VI) adsorbed (kg/kg) at equilibrium and at time t (hour) respectively. kad is the rate constant of adsorption. It may be concluded from data fitting in the rate expression that the reaction taking place is of the first order.
A linear plot figure 11 of ln (qe – q) vs. t confirms the applicability of the pseudo-first order rate expression for the whole range of contact time. Adsorption rate constant kad for different initial chromium (VI) concentrations are calculated from the slops of the lines and presented in Table 2. The rate constant is found to be a function of initial concentration. It is important to note that for a pseudo-first order model, the correlation coefficient (R2) is higher than 0.82 (except for 150 ppm) which is indicative of good correlation. Moreover, from Table 2 it can be seen that the experimental values of qe are not in good agreement with the theoretical values calculated from the intercept but they are very close (except 125 ppm). Furthermore, there is no specific trend of kad with different initial concentrations and therefore, the pseudo-first order model is not suitable for modeling the adsorption of chromium (VI) onto water hyacinth.
Lagergren pseudo-first order kinetics parameters were found from the slop and intercept of ln (qe– q) vs. t graphs. These have been shown in the figure 11(a, b, c & d).
3.6.3 Pseudo-Second Order Kinetics:
Ho et al. developed pseudo-second order rate equation and it is applied to analyze the adsorption of chromium (VI) onto water hyacinth. Assuming the adsorption capacity of chromium (VI) on the adsorbent particles to be proportional to active sites on the surface of the adsorbent, then Lagergren equation modified by Ho and McKay is given by equation 6:
dqt/dt= k (qe – qt)2----------------------------------------(6)
Where qe and qt are sorption capacity at equilibrium and at time t respectively(kg/kg). k is the rate constant pseudo-second order sorption (kg/kg.hr). For the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, the integrated form of equation 6 becomes:
1/(qe– qt) = 1/qe + k*t ---------------------------------(7)
Valid for a pseudo-second order reaction. Equation 7 can berearranged to obtain:
qt= t / (1/kqe2 + t/qe) -------------------------------------(8)
That has a linear form:
t/qt= 1/kqe2 + t/qe-----------------------------------------(9)
An adequate pseudo-second order kinetics model shows a linear plot (Figure 14) of t/qtvs t. the value of qe and k for different initial concentrations are deduced from the slope and intercept of the plot of t/qt vs t and represented in Table 3.
Pseudo-Second order kinetics parameters were found from the slop and intercept of t/qtvs. t graphs. These have been shown in the figure 13(a, b, c & d).
The application of a pseudo-second order model leads to a much better regression coefficients, all greater than 0.63 which confirms that the adsorption phenomenon follows the second order kinetics. The experimental and calculated values of qeare not close (Table 3). Although the kinetic data of chromium adsorption onto water hyacinth fit well with both pseudo-first order and pseudo-second order kinetic models, in both cases the rate constants are dependent on initial concentrations which make them difficult for the application in adsorption process modeling. Furthermore these kinetic constants are inconsistent with equilibrium parameters. This inconsistency is due to the application of completely different models for equilibrium and kinetic studies.
3.6.4 Unified Approach Model:
Islam et al. developed a new kinetic model named Unified Approach Model to characterize the adsorbent-adsorbate system using both equilibrium and kinetic concepts. The model is valid for the system where equilibrium data fitted well with Langmuir isotherm model. It is considered that the adsorption process could be described by a physic-chemical interaction of the type:
k1
A + ac ↔ acA, K= k1/k2------------------------------------------- (10)
k2
Where A, ac and acA represent respectively the adsorbate, active sites on adsorbents and active complexes, and k1, k2 and K represent the rate constant for adsorption, desorption and Langmuir respectively. The model is given by the following equation:
dq/dt = k1 (qα-q) (C0-waq) – k2q-------------------------------------- (11)
Where q is the amount of chromium (VI) adsorbed (kg/kg) at tome t, C0 is the initial concentration of chromium (VI) (kg/m3) and wa is the adsorbent dosage (kg/m3). At wquilibrium the following relations are true:
dq/dt = 0 and q=qe=(C0-Ce)/wa-------------------------------------- (12)
Where Ce is the bulk chromium concentration at equilibrium.
At equilibrium equation 11 reduces to the Langmuir equation ( Equation 2). Solving equation 11 we obtain the relation q versus t:
With,
Where,
a=wa, b=C0+waqα+1/K, and c=qαC0
For t , q=qe=β
The calculated value of β directly gives the value of qe for a given initial concentration and adsorbent dosage; k1 is determined from the slope of the equation 13 and k2 is determined from the relation k2=k1/K. The k1 and k2 values are found to be independent of different initial concentrations. Table 4 summarizes the kinetic parameters found from unified approach model. Theoretical q values at different time for different initial concentrations can be calculated from equation 14 using the values of rate constants and qα. In figure 12 the dense line represents the theoretical q line and symbols are experimental points. It is clear that the predicted adsorption data are in good agreement with those found experimentally. Thus it can be seen that the unified approach model describes the equilibrium and kinetics well and is useful for modeling the chromium (VI) adsorption onto water hyacinth.
4. Conclusion:
The removal of chromium (VI) from aqueous solution was studied using water hyacinth as an adsorbent. The adsorption of chromium (VI) onto water hyacinth is highly pH dependent and the percent removal of chromium (VI) decreased from 74% to 30 % with pH from 2 to 5.2. The equilibrium data adsorption are in good agreement with the Langmuir isotherm model. The maximum adsorption capacity was obtained as 0.055 kg/kg at pH 3. Kinetic data fit well with pseudo-first order kinetic model. The equilibrium adsorption density calculated from the model differs significantly from the experimental values. Pseudo-second order kinetic model describes the data well; but the rate constant was found to be a function of initial chromium concentration. Unified approach model was applied to characterize the adsorbent-adsorbate system. Using the equilibrium and kinetic data, the forward and backward rate constants were determined from the model. The theoretical q values are in good agreement with experimental values for all initial concentrations. From the reduction analysis it was seen that the chromium (VI) decreases with increasing time, while chromium (III) increases with increasing time. The percent removal of chromium (VI) increases with increasing adsorbent dosages.
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