Adsorption of chromium(VI) from aqueous solutions by chemically treated water hyacinth Eichhornia crassipes
Sujana M Gude* & S N Das
Environment & Sustainability Department, Institute of Minerals and Materials Technology (RRL), Bhubaneswar 751 013, India
Email: mgsujana@gmail.com
Received 13 November 2006; revised received 24 September 2007; accepted 6 October 2007
The adsorption potential of water hyacinth Eichhornia crassipes for the removal of Cr(VI) from aqueous solutions has been stidied. The influence of various parameters including contact time, temperature, solution pH, initial adsorbate and adsorbent concentrations and effect of competing anions were studied. Results obtained indicate that the treated weed Eichhornia crassipes has remarkable adsorption capacity for Cr(VI) at pH below 4. The rate of adsorption was rapid and equilibrium attained within 2 h. The method followed first order kinetics. The adsorption process does exhibit a Langmuir type behaviour which is affected by the temperature. The maximum Cr(VI) removal was found to be 7.5 mg/g of dry weight weed at pH of 3.0 in 120 min at 25°C. The calculated activation energy for the method studied was around 54.6 kJ/mol.
About 500 mg weed could remove Cr(VI) successfully from 100 mL of chromite mine water containing 2.8 mg/L Cr(VI) at normal conditions.
Keywords: Adsorption, Water hyacinth, Cr(VI) removal, Mine water
The problem of removing pollutants from water is important and is becoming more so with rapid industrialization in developing countries. The Cr(VI), a major pollutant from chromium based industries is highly toxic and carcinogenic. The waste containing chromium compounds in solution may result from a variety of operations like steel works, electroplating, leather tanning and chemical manufacturing.
Chromium is encountered in the environment in the oxidation states of Cr(III) and Cr(VI), whereas Cr(III) is more stable and less toxic, Cr(VI) is highly soluble in water and toxic1. The International Agency for Research on Cancer (IARC) has found that Cr(VI) is carcinogenic to human health. EPA has set the maximum level of total chromium allowed in drinking water at 100 µg/L.
In order to remove chromium from aqueous solutions, different processes have been investigated.
There are two main treatment methods, the first type of method aims to remove Cr(VI) ions directly while the second type employs reduction of Cr(VI) to Cr(III) followed by addition of lime to increase the pH for alkaline precipitation of Cr(OH)3. The reduction-precipitation technique is widely practiced for the treatment of wastewaters containing high Cr(VI) levels. Alternate methods such as adsorption, ion exchange and membrane techniques2-4 have been
studied extensively for Cr(VI) removal from waste streams. Among these methods, the adsorption technique is an economically feasible alternative.
During the last 2-3 decades various substances were used as adsorbents such as rice husk based active carbon5, groundnut husk6 for the removal of Cr(VI) from water and wastewater. The use of plant based materials for heavy metal ions removal has been proposed by many researchers as an alternate inexpensive method for water treatment. The plant cells have a diverse chemical composition, which is expected to vary among species. The metal ion removal capacity of non-living biomaterials is believed to occur through functional groups associated with the proteins, polysaccharides, lignin and biopolymers found in the cell and cell walls7,8. The conventional ion-exchange resins are designed with a single functionality whereas biomass materials can contain numerous functionalities9. Depending on the nature of organism, two categories of binding can occur in biomass based materials10.
(i) Passive binding, which occurs in both living and non-living cells, involves very rapid physical adsorption and/or ion exchange with cell surface.
(ii) Active binding, typical of living cells, is characterized by slower metal uptake as a result of metabolic activity.
A number of studies demonstrated the high metal binding capacity of different weeds11-14. Most of the research publications in this area have been associated with seaweed for the treatment of heavy metals such as Pb2+, Hg2+ and Cd2+. Weed grows rapidly, relatively inexpensive to generate and has adsorption capacities that are comparable to inorganic adsorbents15,16.
Water hyacinth (Eichhornia crassipes) is a floating microphyte, whose appetite for nutrients and explosive growth rate has been put to use in cleaning up municipal and agricultural wastewater17. These studies14 employed living plants, which were shown to be highly effective in removing Cd2+, Cu2+, Ni2+, Zn2+ and Pb2+. Although some studies have reported the use of biomaterial derived from non-living dried water hyacinth roots for the removal of toxic metals18, the removal of Cr(VI) from aqueous solutions using dried water hyacinth has not been reported.
In the present study, dried aquatic weed Eichhorina crassipes was tested for its capability to remove Cr(VI) from aqueous solution by adsorption method.
In order to study the kinetics of adsorption various parameters like contact time (0-6 h), pH (2.5-10.5), temperature (25-45°C), adsorbate concentration (5-30 mg/L), adsorbent dose (0.05-0.5 g/50 mL) and concentration of competing anions (5-30 mg/L) were studied. Results are discussed in terms of adsorption kinetics to understand the variables which influence the adsorption. The adsorption efficiency of Eichhornia crassipes for the removal of Cr(VI) was also tested using chromite mine effluents.
Experimental Procedure
Chemicals
All the chemicals used in the experiments were of pure grade. The stock solution of 100 mg/L Cr(VI) was prepared by dissolving potassium dichromate salt (MERCK) in requisite amount of water and stored the stock solution by keeping in the refrigerator at 4°C.
For experiments, required concentrations were obtained by diluting the stock solution with distilled water. The pH of the suspensions was adjusted with NaOH (MERCK) or HCl (MERCK) to required values. The lab-ware used in the experiments were soaked in diluted HCl solution for 6 h, and then rinsed with distilled water.
Treatment of water hyacinth
Water hyacinth, Eichhornia crassipes plants were collected from the Chilka Lake in Orissa, India. The weeds were washed repeatedly with ordinary water
followed by distilled water and air dried. The dried weeds were blended in a homogenizer into finer particles (0.5-1.0 mm). To prevent the colour leaching and improve the physico-chemical characteristics of weed, sample was treated with HCHO in acidic media19. The treated sample was washed free from HCHO, dried at 50°C for 24 h in oven and stored in desiccators for subsequent use.
Adsorption studies
Batch mode adsorption studies were carried out by agitating 100 mg of treated weed sample with 50 mL of solution containing Cr(VI) at pH 3.0(±0.1). For different studies the predetermined concentrations of adsorbate and adsorbent were placed in flasks, agitated at constant speed (150 rpm) in a thermostatic water bath shaker at designated temperatures over a period of time. The solutions were separated from adsorbent by centrifugation and the supernatant solution was analysed spectrophotometrically20. The final concentrations of Cr(VI) in solution was determined by developing colour with 1,5-diphenyl- carbazide reagent and absorbance was measured on Perkin-Elmer Lambda 35 UV/Visible Spectrophoto- meter. The pH of the solutions at the beginning and end of the experiments were measured and the average values are reported. All the pH measurements were carried out by an Systronic Digital pH meter (model 361) using a combined glass electrode. The pH meter was calibrated with Orian standard buffers before any measurements.
Results and Discussion
Effect of pH
The pH of the solution is an important factor that controls the adsorption of Cr(VI). Figure 1 shows the
Fig. 1 — Effect of pH on Cr(VI) adsorption by treated weed Eichhorina crassipes. Adsorbent dose was 100 mg/50mL; initial Cr(VI) concentration 10 mg/L; temp. 25°C and shaking time 4 h.
extent of Cr(VI) adsorption as a function of pH at constant initial Cr(VI) concentration at 25°C. The treated weed is active in lower pH, removal efficiency decreased gradually after pH 4 and the adsorbed amount was found to be negligible at a pH value
> 7.4. Similar observations were reported for adsorption of Cr(VI) by Fe-modified steam exploded wheat straw21. The cell wall of water hyacinth, contains a large number of surface functional groups.
The variation in adsorption may be due to type and ionic state of these functional groups and metal chemistry in solution22. It is well known that dominant species of Cr(VI) at pH range 2-5 is HCrO4−, as pH increases to higher side the HCrO4− converts to CrO42− form. It can be said that the adsorption of Cr (VI) below pH 4.0 is due to the electrostatic attraction of the positively charged surface functional groups with HCrO4− species. At pH below 4 surface functional groups carry positive charges and as the pH increases to the higher side the weed surface possesses more functional groups carrying a net negative charge which tends to repulse the anions. However, it is clearly shown in the figure, that though there is a removal of Cr(VI) at pH range 4-6, but rate of removal is considerably less. This indicates that other mechanism like physical adsorption is also taking place on the weed surface.
Similar observations were reported for the adsorption of Cr (VI) by Spirogyra species17.
Effect of weed dose
It was observed that increasing the weed quantity in the solution strongly affected the Cr(VI) removal from the aqueous solution. The weed amount was varied from 0.05 to 0.5 g while keeping the other parameters constant, the results are shown in Fig. 2.
The percent of adsorption increased with increase of weed dose whereas loading capacity decreased, this is due to the concentration of surface binding sites. As the weed concentration in the solution increases available surface sites will also increase, which leads to higher adsorption of Cr(VI) ions from solution. The complete removal of Cr(VI) occurred only with a relatively high weed concentration.
Kinetic study
Kinetic studies were carried out to see the effect of initial Cr(VI) concentration and temperature on the Cr(VI) removal. The effect of time factor was studied up to 6 h at different initial Cr(VI) concentrations and temperatures. The results showed in Fig. 3 indicate that the removal process is fast on weed surface and
percent of removal increased with an increase in agitation time and decreased with Cr(VI) concentrations. The equilibrium was attained after 120 min for all Cr(VI) concentration in the solution.
This is due to the ratio of the initial Cr(VI) ions to the available surface sites is low in case of lower concentrations, as the concentration of Cr(VI) increases the available sites on surface become fewer and hence the percentage of Cr(VI) adsorption decreases from 70 to 41% as the initial concentrations of Cr(VI) increases (5-15 mg/L) at pH 3(±0.1). The equilibrium concentration Ce used in this study was taken after 4 h. The faster initial rate may be due to the availability of organic functional groups and as the available surface sites progressively decrease the final rate becomes slower.
Fig. 2 — Effect of weed dose on adsorption of Cr(VI) in 50 mL of solution containing 10 mg/L Cr(VI) at pH 3; temp. 25°C.
Fig. 3 — Effect of time variation on Cr(VI) adsorption for various initial Cr(VI) concentrations (5-15 mg/L), weed dose 100 mg/50mL; pH 3; temp. 25°C.
Figure 4 shows the effect of time on adsorption of Cr(VI) on weed surface at 25, 35 and 45°C. Increase of Cr(VI) adsorption efficiency as the temperatures
increased from 25 to 45°C indicating the chemical nature of the adsorption. For all the temperatures the Cr(VI) adsorption was equilibrated at the end of 120 min. The adsorption rate constant Kads for the adsorption of Cr(VI) on weed surface at different Cr(VI) concentrations (5-15 mg/L) and temperatures (25-45°C) was determined by using Lagergren equation23,
Fig. 4 — Effect of time variation on Cr(VI) adsorption capacity for different solution temperatures (298-318 K). pH 3; weed dose 100 mg/50 mL of solution containing 10 mg/L of Cr(VI).
e e ads
log( ) log
2.303
q −q = q − K t …(1)
where qe and q are the amounts of Cr(VI) adsorbed at equilibrium and at any time in mg/g. The plot of log(qe−q) versus time presented in the Fig. 5 shows the straight line curve indicating the applicability of the Lagargren equation and first order kinetics. The adsorption rate constant Kads values were calculated from the slope of the plot and found to be 2.09, 1.72 and 1.05 × 10−2 min-1 for an initial concentration of 5, 10 and 15 mg/L, respectively. These values suggest that the low concentrations of Cr(VI) promoted the adsorption efficiency. The values of Kads for different temperatures were calculated from the slops of the plots in Fig. 6 and found to be 2.0, 2.46 and 3.66 × 10−2 min-1 for 25, 35 and 45°C respectively, indicating an increase in the rate with increase in temperature.
The activation energy for the Cr(VI) adsorption onto weed was calculated by using Arrhenius equation described below:
k = Ae−Ea/RT …(2)
where k is rate constant at temperature of T (K), A is frequency factor, R is universal gas constant (8.314 Jmol−1 K−1) and Ea (Jmol−1) is activation energy for
the process. When the lnK values were plotted versus 1/T, a linear plot was found (Fig. 7). The Ea value calculated from the slope of the plot is 54.69 kJ/mol, which indicates the chemical nature of the adsorption, and is comparable with the reported value for plant based materials19.
Fig. 5 — Lagergren plot for the adsorption of Cr(VI) on weed at different initial concentrations of Cr(VI).
Fig. 6 — Lagergren plot for the adsorption of Cr(VI) on weed at different solution temperatures.
Isotherm study
The adsorption studies conducted at fixed adsorbent dose and temperature and varying Cr(VI) concentrations (5-30 mg/L) have been fitted into the rearranged Langmuir adsorption isotherm (Fig. 8),
e
e L
1
C C
q = K b+ be …(3)
where Ce is the equilibrium concentration mg/L and qe
is the amount of Cr(VI) adsorbed at equilibrium in mg/g and KL and b are Langmuir constants related to equilibrium constant and adsorption capacity respectively. In the Fig. 9, the linear plot of Ce/qe
versus Ce regression analysis at different Cr(VI) concentrations indicated the applicability of the Langmuir adsorption isotherm. The values of the Langmuir equilibrium constant (KL) and adsorption capacity (b) determined from the slope and intercept of the plot were found to be 7.58 mg/g and 0.322 L/mg respectively.
Fig. 7 — Arrhenius plot.
Fig. 8―Adsorption isotherm of Cr(VI) at different initial Cr(VI) concentrations. pH 3; temp. 25°C and weed concentration 100 mg/50 mL.
Effect of other anions on Cr(VI) adsorption
The drinking water or wastewater contains many co-occurring inorganic anions. Uptake rate of nitrogen and phosphorous in the water by living Eichhornia crassipes is reported in the literature24, no data is available on the dried weed material. If the relative binding ability of various anions on dried weed surface is known, their influence on the adsorption of Cr(VI) can be estimated. Therefore it was thought worthwhile to study the effect of carbonate, sulphate, phosphate and nitrate anion concentrations (5-30 mg/L) on Cr(VI) removal. Varying amounts of
these solutions were added to the test solutions with 10 mg/L Cr(VI), 100 mg of weed at pH 3. From Fig. 10 it is observed that the Cr(VI) removal efficiency decreased with increasing concentrations of phosphate and nitrate while sulphate concentration showed moderate effect. The increase of carbonate concentration in the solution did not have much effect on Cr(VI) removal by weed. However, lower concentrations of these anions showed negligible effect on Cr(VI) removal efficiency by water hyacinth.
Fig. 9 — Langmuir plot for the adsorption of Cr(VI).
Fig. 10 — Effect of competing anions on adsorption of Cr(VI) on weed surface. Cr(VI) 10 mg/L; pH 3 and weed 100 mg/50 mL.
Removal of Cr(VI) from chromite mine water
The Cr(VI) adsorption experiments were conducted to see the Cr(VI) removal efficiency of water hyacinth from chromite mine water at natural conditions. The analysis of the mine water is given in Table 1, the tests were conducted without pH adjustments.
Adsorbent amount was varied from 0.05 to 1 g, the
weed sample was added to 100 mL of mine water and agitated for 4 h. The results are shown in Fig. 11, it was found that about 500 mg of the weed is required to treat 100 mL of mine water containing 2.8 mg/L of Cr(VI).
Conclusions
From the results of the studies the following conclusions can be drawn,
(i) The Cr (VI) removal capability of treated weed from aqueous solution is found to be high. The degree of the removal highly depends on initial pH, adsorbate and adsorbent concentrations, and temperature.
(ii) The adsorption followed first order kinetics.
Studies are significant since the data can be used to develop predictive models for column experiments.
(iii) The adsorption process does exhibit a Langmuir type behaviour which is affected by the temperature. The maximum Cr(VI) removal was
found to be 7.5 mg/g of dry weight weed at pH of 3.0 in 120 min at 25°C.
Table 1 — Characteristics of chromite mine water Parameter
pH
Turbidity, TNU Conductivity (ms/cm) COD (mg/L)Total Total chromium (mg/L)*
Cr(VI) (mg/L)
Value
7.4 81 10.4 145 5.8 2.8
*Total chromium was analyzed by AAS.
Fig. 11 — Effect of weed dose on removal of Cr(VI) from chromite mine water containing 2.8 mg/L of Cr(VI) at pH 7.4, time 4 h and temp. 25°C.
(iv) The amount of chromium adsorbed depends on the presence of other anions like sulphate, phosphate and nitrate, at higher concentrations these ions interfere with the adsorption of Cr(VI) on weed surface.
The production of water hyacinth is inexpensive and the weed is widely available in many tropical and sub tropical parts of the world. This method may be a promising tool to remediate Cr(VI) in industrial and chromite mining wastewater.
Acknowledgements
This work was supported by OEP (Orissa Environment Programme) in the form of a research project. The authors are grateful to the Director, Institute of Minerals and Materials Technology (formerly known as Regional Research Laboratory), Bhubaneswar, Orissa, India, for giving permission to publish this work.
References
1 IPCS (International Programme on Chemical Safety), Environmental Health Criteria 61: Chromium WHO, Geenva, (1988) 11.
2 Yun Y S, Park D, Park J M & Volesky B, Environ Sci Technol, 35 (2001) 4353.
3 Pansini M, Colella C & Gennaro M D, Desalination, 83 (1991) 145.
4 Yinguang Chen & Guowei Gu, Bioresource Technol, 96 (2005) 1722.
5 Yupeng Guo, Jurui Qi, Shaofeng Yang, Kaifeng Yu, Zichen Wang & Hongding Xu.. Mater Chem Phys, 78 (2002) 132.
6 Perez-Candela M, Martinez J M & Torregrosa-Macia R, Water Res, 29 (1995) 2174.
7 Yun Y S & Volesky B, Environ Sci Technol, 37 (2003) 3601.
8 Donghee Park, Yeoung-Sang Yun & Jong Moon Park, Chemosphere, 60 (2005) 1356.
9 Lawrence R Drake & Gary D Rayson, Analytical Chemistry News and Futures, 1 (1996) 22A.
10 Darnall, D W, Greene B, Hosea M, McPherson R A, Henzl M & Allexander M D In Trace Metal Removal from Aqueous Solution edited by R Thompson (Burlington House: London), 1986, 1.
11 Schiewer S & Wong M H, Environ Sci Technol, 33 (1999) 3821.
12 Davis T A, Volesky B & Vieira R H S F, Water Res, 34 (2000) 4270.
13 Deniz V & Volesky B, Water Res, 39 (2005) 239.
14 Kuber C Bhainsa & D’Souza S F, J Environ Sci Health Part A: Toxic/Hazard Subs Environ Eng, 36 (2001) 1621.
15 Lazaridis N K & Asouhidou D D, Water Res, 37 (2003) 2875.
16 Shubo Deng & Renbi Bai, Water Res, 38 (2004) 2424.
17 Gupta V K, Srivastava A K & Neeraj Jain, Water Res, 35 (2001) 4079.
18 Shaban W Al Rmalli, Chris F Harrington, Mohammed Ayub
& Parvez I Haris, J Environ Monitor, 7 (2005) 279.
19 Raji & Anirudhan T S, Indian J Chem Technol, 3 (1996) 49.
20 APHA (American Public Health Association), Standard Methods for the Examination of Water and Wastewater, 19th edn (Washington DC), 1995.
21 Li Chun, Chen Hongzhang & Li Zuohu, Process Biochem, 39 (2004) 541.
22 Matheickal J T, Yu Q & Woodburn G M, Water Res, 33 (1999) 335.
23 Lagergren S, About the theory so-called adsorption of soluble substances K Svenska. Ventenskapsakad Hanl, 24 (1899) 1-39, as cited by Trivedi H C, Patel M & Patel R D, J Eur Polyn, 9 (1973) 525.
24 Petrucio M M & Esteves F A, Rev Brasil Biol, 60 (2000) 229.
