EFFECT OF MODIFICATION OF MONGOLIAN NATURAL ZEOLITES ON ADSORPTION OF CHROMIUM FROM AQUEOUS SOLUTION

Removal of chromium (Cr (III) ) as well as hexavalent chromium (Cr (VI) ) from tannery wastewater at same time by adsorption using Mongolian natural zeolite was studied in terms of the characteristics, modifi cation, and chromium adsorption performance of the zeolite. The cation exchange capacity of the zeolite ranged from 37×10−3 to 144×10−3 eq·g−1. From the results of phase identifi cation and elemental analysis of the zeolite after modifi cation run, the zeolite was not decomposed and was modifi ed successfully. The zeolites modifi ed by Ba2+, Cu2+, and HDTMA-Br could adsorb Cr (III) as well as the unmodifi ed one could. The higher pH gave the higher adsorption ability, similarly to the adsorption of Cr (VI). Consequently, this adsorption method with Mongolian natural zeolite was proposed to remove Cr (III) together with Cr (VI) from tannery wastewater at same time.


Introduction
Chromium is one of the regulated toxic heavy metals in the environment.Chromium compounds are widely used in industries such as leather tanning, electroplating, manufacturing of dye, paint and paper.Chromium exists in the aqueous solution mainly in two states Cr (III) and Cr (VI).Cr (III) is extensively used in the tanning agent.Although Cr (III) is considerably less toxic than Cr (VI), its disposal as a dissolved species in natural waters or as sludge in soils may pose serious health risks because it can be oxidized to Cr (VI) in the environment [1].
In Ulan Bator, water resource pollution by hexavalent and trivalent chromiums (Cr (VI), Cr (III)) in wastewater from tannery, which is one of the most important industries in Mongolia, becomes serious.In the fact, in Mongolia, there are plenty of natural zeolite deposits, although these resources are not suffi ciently exploited and studied yet.The most common, stable and abundant forms are hexavalent (Cr (VI) ) and trivalent (Cr (III) ) chromiums.There has been various works studying the removal of chromium (Cr) from aqueous solution by adsorption using natural zeolite.The zeolite modifi ed by Ba 2+ could adsorb and favorably remove Cr (VI), which exists as chromate anion in aqueous solution, whereas the unmodifi ed one could not [1], due to the zeolite surface is charged negatively.On the contrary, Cr (III) forms cation in aqueous solution.
In this study, removal of Cr (III) as well as Cr (VI) from tannery wastewater at same time by adsorption using Mongolian natural zeolite was studied in terms of the characteristics, modifi cation, and chromium adsorption performance of zeolite.

Experimental
The natural zeolite samples obtained from Tsagaan Tsav and Urgon deposit in Mongolia.In previous study reported that fi ve kinds of natural zeolite used for modifi cation and adsorption run.These samples have different base components in their content and different porous properties.The detailed information of Mongolian natural zeolites is shown in Table 1 [1] The principal conditions of zeolite modifi cation are shown in Table 2.The zeolite samples and the aqueous solution of Ba 2+ [1,2], Cu 2+ [2], or HDTMA-Br [3] were brought into contact with each other to modify the samples.The modifi ed samples were washed twice and dried.Thus obtained samples with and without modifi cation were kept in a desiccator before in characterization and adsorption.The element composition on the zeolite surface was determined by energy dispersive X-ray fl uorescence spectrometry (EDX) and the modifi cation of zeolite samples with Ba 2+ was characterized by XRD to know that after modifi cation zeolite samples based component.The cation exchange capacities (CEC) of the zeolites were measured by the method of US-EPA Method 9081 [4].The principal conditions of batch adsorption run are summarized in Table 3. Cr (NO 3 ) 3 ×9H 2 O was used as a source of Cr (III).The respective aqueous solutions were analyzed by pH meter and ICP spectrometer.The result of XRD spectra of natural zeolite samples were before and after modifi cation shown in Figure 2. The before and after zeolite modifi cation with Ba 2+ , the spectra did not changed almost same as unmodifi ed one.The zeolite were not decomposed with the modifi cation solution.The adsorbed amount of Cr on zeolite, q, was calculated by the material balance relationship.
Figure 3 shows the effects of modifi cations on the adsorption isotherms of Cr (III) on the zeolite.The unmodifi ed zeolite could adsorb Cr (III) as expected.Zeolites modifi ed by Ba 2+ , Cu 2+ , and HDTMA-Br could also adsorb Cr (III), namely, Cr (III) would be removed by the modifi ed zeolite as well as Cr (VI) [1] at same time.While the modifi cations with Ba 2+ and Cu 2+ did not affect the adsorption performances, the performance of the zeolite modifi ed by HDTMA-Br was lower.In the solution of higher pH in this study, Cr(OH) 2+ is dominant [5].The adsorption isotherms were described by Langmuir equation.Figure 4 shows the saturated adsorption amount, q * , and the adsorption coeffi cient, K L , are plotted the average of equilibrium pH.The q * and K L values increased with increasing pH.The K L adsorption parameters of Cr (III) much higher than that of K L of Cr (VI) [1].The q* parameter were adsorption performances almost same as each other.

Conclusions
The cation exchange capacity of the zeolite ranged from 43×10 −3 to 144×10 −3 eq•g −1 .The zeolites modifi ed by Ba 2+ , Cu 2+ , and HDTMA-Br could adsorb Cr (III) as well as the unmodifi ed one could.The higher pH gave the higher adsorption ability.The K L adsorption parameters of Cr (III) are much higher than that of K L of Cr (VI).The q* parameters adsorption performances almost same as each other.Consequently, the adsorption using Mongolian natural zeolite was proposed to remove Cr (III) together with Cr (VI) from tannery wastewater at same time.

Figure 1
Figure 1 Comparison of element contents between surface of zeolite without and with modifi cation by Ba 2+

Figure 2
Figure 2 XRD spectra before and after modifi cation of natural zeolite

Figure 3
Figure 3 Effect of modifi cation on adsorption isotherm of Cr (III) on Mongolian natural zeolite

Figure 4 Figure 5
Figure 4 Effect of the pH of equilibrium solution on the saturated adsorption amount, q * , and adsorption coeffi cient, K L , in Langmuir equation Figure 5 shows the relation between Cr (VI) and Cr (III) adsorption parameters with pH.The K L adsorption parameters of Cr (III) much higher than that of K L of Cr (VI)[1].The q* parameter were adsorption performances almost same as each other.

Figure 5
Figure 5 The relations between Cr (VI) and Cr (III) adsorption parameters with pH Nomenclature K L = adsorption coeffi cient in Langmuir equation [-] L 0 = mass of solution [g] pH = pH in solution q = adsorption mass of chromium atom per unit mass of adsorbent [g-Cr•g-Zeo −1 ] q * = saturated adsorption mass of chromium atom per unit mass of adsorbent in Langmuir equation [g-Cr•g-Zeo −1 ] S = mass of adsorbent [g] x = mass fraction of chromium atom in solution [-]

Table 1
Principal characteristics of Mongolian natural zeolites