Determination of adsorption isotherm models for the biosorption of chromium using cherry leaves ( Muntingia calabura L.)

Contaminations of industrial metals into the river possess major threat to environment. Chromium is a heavy metal which has the wide applications in tannery and electroplating industries. Above the permitted level of Chromium(VI) into surface water leads to severe health hazards. Therefore, biosorption is a technology used for the sorption of heavy metal. In this present study adsorption isotherm models was studied for the biosorption of chromium by cherry leaves. From the adsorption isotherms it was found that the experimental data fits well with the Langmuir isotherm than the Freundlich isotherm. The monolayer capacity Q m was fond to be 11.98 mg/l and the adsorption affinity was found to be positive which indicates the efficient biosorption of chromium.


INTRODUCTION
An accelerated release of the heavy metals into the aquatic environment poses serious water pollution problems because of their toxicity, persistence and bioaccumulation in food chains. Metals and pesticides, in particular, have an inclination to accumulate and undergo food chain magnification (Klavins et al., 2009). Of the important metals such as mercury, lead, cadmium and chromium(VI) are regarded as toxic; whereas others, such as copper, nickel, cobalt and zinc are not as toxic, but their extensive usage and increasing levels in the environment are of serious concerns reported by Vijayaraghavan and Yeoung (2008). Its atomic number is 24. It is a hard metal of steely gray colour and also it has a high melting point of 1907 °C. It is odorless and tasteless metal. Many of it compounds are intensely coloured. Chromium is important metal due to its high corrosion resistance and hardness. According to the International Agency for Research on Cancer (IARC) Cr (VI) is considered as carcinogenic to both humans and animals. Therefore the permitted level of Chromium (VI) into surface water is regulated to below 0.05 mg/l by the U.S. EPA, while total Cr (including Cr(III), Cr(VI) and its other forms) is regulated to below 2 mg/l reported by Deepa prabhu et al. (2006) and Donghee et al. (2004).

International Letters of Natural Sciences Vol. 25 13
Effluents can also contain inorganic wastes such as brine salts and metals. The increased industrial activities have reduced the availability of good quality water by producing a large amount of effluents to the rivers. Industrial effluents often contain various toxic metals, harmful gases and several organic and inorganic compounds. Babarinde (2011) stated that neem (Azadirachta indica) is a plant that grows across the world and it is used majorly for medicinal purpose. Hossain et al. (2007) states that banana peels is a low cost and readily available materials for preparing bioadsorbent. In this present work adsorption isotherm models were tested for the biosorption of chromium using cherry leaves.

1. Selection of the plant
Cherry leaf was selected as adsorbent for removal of chromium ion in this study. The leaves were obtained from a Cherry tree located in K. S Rangasamy College of Technology (Autonomous), Tiruchengode. The plant was identified and authenticated by Botanical Survey of India Southern Circle, Tamil Nadu Agricultural University, Coimbatore. The binomial name of cherry leaves is Muntingia calabura L.

Preparation of leaf adsorbent
Mature Cherry leaves were collected and it was washed thoroughly under running tap water to remove dust and any adhering particles. The leaves were then dried under sunlight for a few days until it became crisp. The dried leaves were crushed and blended to powder form using a blender. It was stored in an airtight plastic for further use to avoid contact with moisture in atmosphere.

3. Preparation of chromium stock solution
Stock solution of chromium of 1000 mg/L was prepared by dissolving 2.828 g of potassium dichromate in 1 L of distilled water. The working solution was prepared by diluting the stock solution.

4. Chromium analysis
Chromium content was analyzed using spectrophotometric method, as described in the Standard methods for the Examination of Waste and Wastewater (APHA), to measure the concentrations of the Cr. The reaction takes place between Cr and 1,5-diphenycarbazide results in the pink coloured complex. Then it was able to be spectrophotometrically analyzed at 540 nm.

5. Adsorption isotherm model
There are different types of isotherm model from that Langmuir and Freundlich isotherm were tested.

6. Langmuir Isotherm
Maximum adsorption capacity was produced from complete monolayer coverage of adsorbent surface was determined by Langmuir Isotherm. It was given as,

C e /q e = 1/bq m + Ce/qm
where; C e = Concentration of chromium at equilibrium (g/l) q e = amount of chromium bioadsorbed at equilibrium (mg/l) q m = amount of chromium sorbed per unit mass of the bioadsorbent (mg/g) B = Ka/Kb K a = adsorption rate constant K b = desorption rate constant From the plot of (c e /q e ) vs c e , the intercept and slope tends to b and q m value respectively.

7. Freundlich Isotherm Model
The Freundlich Isotherm model was used to describe the adsorption on a heterogeneous surface. It was expressed as log q e = log K f + n C e where; q e = amount of chromium bioadsorbed at equilibrium(mg/l). C e = Concentration of chromium at equilibrium (g/l). K f = Adsorption capacity (l/g) n = Adsorption affinity (Dimensionless). From the plot of log q e vs log c e , the intercept and slope tends to K f and adsorption affinity n.
Pearson's chi-squared test was used to analyze the goodness of fit for pseudo first and second order reactions model. Its properties were first investigated by Karl Pearson in 1900.  shows that the plot of (C e /q e ) vs C e was found to yield a straight line and the slopes and the intercepts of the plots give a the values of q m and b. The monolayer capacity of bioadsorbent was found to be q m 11.98 mg/g which was quite larger than crab shell.  shows that the plot of log q e vs log c e found to yield a straight line and the slopes and the intercepts of the plots gives the values of n and k f . The adsorption affinity '1/n' havingvalues less than 1, which is related to a favourable sorption of chromium.

2. Freundlich Isotherm model
In both the plots there is a good agreement between experimental and predicted behaviour such that the regression coefficient R 2 was close to 1.

CONCLUSION
From the adsorption isotherms the experimental data fits well with the Langmuir isotherm than the Freundlich isotherm. The monolayer capacity Q m was fond to be 11.98 mg/l and the adsorption affinity was found to be positive which indicates the efficient biosorption of chromium.