Spectrophotometric Study on Kinetics and Thermodynamics of Adsorption and Catalytic Transformation of K2Cr2O7 to K2CrO4 by Natural Hermit Crab Shell Powder

Accepted: May 25, 2011; Published: August 30, 2011 Abstract The adsorption and transformation of K2Cr2O7 at natural hermit crab shell powder was studied by UV-visible spectrophotometry. K2Cr2O7 is adsorbed at natural hermit crab shell powder, and follows the Freundlich adsorption model with the adsorption constant of KF=888.8 and adsorption free energy change of 16.83 kJ.mol . The adsorbed K2Cr2O7 is catalytically transformed into K2CrO4 in moderate conditions by natural hermit crab shell powder with the apparent first-order rate constant of 0.0219 s, and equilibrium constant of K=2.27×10 M.


Introduction
Compared to Cr(III), Cr(VI) is a more harmful toxic species to the human normal nervous system, immune system, internal secretion system and a cause of cancers [1], and cannot be directly decomposed by microbes in environment [2]. The removal of Cr (Ⅵ) from environment had been reported with chemical or physical adsorptions [3], or bioadsorption [4][5][6][7][8]. The transformation of Cr (Ⅵ) to Cr(III) may be used to reduce the toxicity of Cr (Ⅵ). Among Cr (Ⅵ) compounds, the toxicity of K 2 Cr 2 O 7 is stronger than K 2 CrO 4 [9]. The transformation from K 2 Cr 2 O 7 to K 2 CrO 4 is commonly catalyzed by strong bases [10]. In acidic solution, Cr (Ⅵ) is mainly in K 2 Cr 2 O 7 form, but in weak base or base solution, the main form of Cr (Ⅵ) is K 2 CrO 4.
The natural hermit crab shell is an important natural polymer mainly composed of calcium carbonate, chitin and chitosan with the twisted lamellar structure in the endocuticle and the ribbon shape pore canal tubule in the exocuticle [11]. The strong bioadsorption of natural hermit crab shell for Cr (Ⅵ) has been studied with a well adsorption model [12]. In our previous work on the electrochemical determination of Cr (Ⅵ) experiments with natural hermit crab shell powder modified electrode [13], it was found that the natural hermit crab shell powder not only adsorbs Cr (Ⅵ) effectively but also shows a catalytic transformation from K 2 Cr 2 O 7 to K 2 CrO 4 . In the present paper, the adsorption and catalytic behavior of natural hermit crab shell to Cr (Ⅵ) were studied by UV-visible spectrophotometry [14], and some interesting results are reported here.

Instruments and materials
The spectrophotometric experiments were carried out on UV-visible spectrophotometer (UV-Lamda 25, Perkin-Elmer Co., USA). The hermit crab shell powder was separated from the system by centrifugal separation with CT6T centrifugal machine (Tianmei Biochemical Device Instrument Engineering Co., Shanghai, China). Crab shells were collected from hairy crabs purchased from Suxiege Co., Shanghai, and cleaned with drinking water and ultrapure water. Potassium dichromate (K 2 Cr 2 O 7 ), potassium chromate (K 2 CrO 4 ), disodium hydrogen phosphate (Na 2 HPO 4 ), citric acid and hydrochloric acid (HCl) were all analytical pure (purchased from Shenyang Chemical Co.), and all solutions were prepared with ultrapure water (18.2 M cm -1 ) prepared with Milli-Q system (Billerica, MA, USA).

Experimental procedure
Dried clean natural hermit crab shell was grounded into powder (about 80-100#) with grinder. The natural hermit crab powder 0.0500g was mixed with 5mL 1.0×10 -4 M potassium dichromate solution in 10mL centrifugal tube for adsorption. After adsorption for certain time, the mixture was centrifuged at 4500 rpm (at 25℃ for 5 min). The obtained supernatant solution was taken for UV-visible spectrophotometric experiments.

UV-visible spectrophotometry of Cr(Ⅵ)
The UV-visible spectra of Cr(Ⅵ) solutions before and after the adsorption and catalysis with hermit crab shell powder were shown in Figure 1. K 2 CrO 4 solution shows a UV-visible spectrum with absorption peaks at 273nm and 370nm (curve 2 in Figure 1). K 2 Cr 2 O 7 solution gives out a spectrum with absorption peaks at 257nm and 350nm (curve 3 in Figure 1). After treatment with hermit crab shell powder, K 2 Cr 2 O 7 solution shows an identical spectrum of K 2 CrO 4 with absorption peaks at 273nm and 370nm (curve 4 in Figure 1). These results indicate that the K 2 Cr 2 O 7 is catalyzed by hermit crab shell powder, and transforms into K 2 CrO 4 during the treatment.
According to the literature [10], the transformation reaction is described as, So, the absorbance of the peak in curve 4 is about 2 times higher than that in curve 2.
Natural hermit crab shell is a lamellar structural material with ribbon shape pore canal tubule in the exocuticle [11]. The canal tubule with elliptical shaped cavities is about 15-25 µm in diameter [15]. The cavity is full of . As we know, the natural hermit crab shell has been used to absorb Cr (Ⅵ) [12]. But the hermit crab shell powder with this kind catalysis behavior has not been found and studied. This catalytic behavior may be a useful property of natural materials beyond the chemical adsorption, which open a new way to explore the natural hermit crab shell and other natural materials with the inside structures with high selectivity and catalytic activity [17][18][19][20].

Thermodynamics of transformation reaction
According to the reaction (1), the reaction equilibrium constant can be expressed as, Here, [species] indicates the equilibrium value of the species. The distribution of species of CrO 4 2in the system can be expressed as, The reaction equilibrium constant can be obtained from the following equation, The transformation between Cr 2 O 7 2and CrO 4 2can also been realized by changing the pH in medium [10]. So, the reaction equilibrium constant can be obtained by spectrophotometry with different solution pH. The From this equation, it was obtained that the middle point of the curve located at

Catalytic kinetics of Cr(Ⅵ) by hermit crab shell powder
The transformation kinetics of Cr 2 O 7 2into CrO

Conclusion
In the present paper, the adsorption and transformation of K http://astonjournals.com/csj