Results and Discussion

Effect of agitation time and initial Cr (VI) concentration

The equilibrium time required for biosorption of Cr (VI) on tamarind shells is obtained by studying adsorption of Cr (VI) at various initial concentrations. The data is represented graphically in Fig. 1. The extent of adsorption efficiency increases sharply with time and attains equilibrium at about 60 minutes for all the concentration studied. According to the above results, the agitation/equilibrium time was fixed at  120 minutes for the rest of the experiments so that complete equilibrium is reached. The gradual increase in the rate of biosorption of Cr (VI) and plateau thereon after 60 minutes indicates that the adsorption occurs through a smooth continuous formation of adsorption layer till saturation.

Fig 1: Percentage biosorption of chromium (VI) from solution of different concentration, pH 2.0, by 10gl^-1 biomass as related to the time of contact at 120 rpm

Effect of pH 

The biosorption of Cr (VI) was found to depend on the pH of the metal solution (Fig 2). Biosorption capacity was found to decrease with an increase in the pH, maximum adsorption being observed at a pH of 2. It is well known that the dominant form of Cr (VI) at this pH is HCrO₄^-. Increasing the pH will shift the concentration of HCrO₄^-  to other forms, CrO₄^2- and Cr₂O₇^2-. It can be concluded that the active form of Cr (VI) that can be adsorbed by tamarind pod shell is HCrO₄^-.

Fig 2: Effect of pH on the biosorption of Chromium (VI) at different concentrations, by 10gl^-1 at 120 rpm with equilibrium time of 120 minutes.

Adsorption Isotherms

Among various plots employed for analyzing the nature of adsorbate-adsorbent interaction, adsorption isotherm is the most significant. The results of adsorption studies of chromium (VI) at different concentrations ranging from 20 to 500 ppm on a fixed amount of adsorbent are expressed by two of the most popular isotherm theories viz., Freundlich and Langmuir isotherms. These isotherm equations are as follows:

Freundlich:
                  q = K_f C_eq ^1/n

                  ln q = ln K_f + 1/n ln C_eq

Langmuir
                  q = q_max b C_eq/ 1+ b C_eq

                  C_eq/q   =  1/q_max.b + C_eq/ q_max.

In the above equations, K_f and n are Freundlich constants, which affect the adsorption process, such as adsorption capacity and intensity of adsorption, respectively. The values of these constants, obtained by least squares fitting of the data on ln q and ln C_eq are 1.17 and 1.80. q_max (mgg^-1) and b are Langmuir constants related to monolayer adsorption capacity and energy of adsorption respectively. The values of the parameters, evaluated by the least square fitting of the data on C_eq versus C_eq/q are 27.73 and 0.008.

Effect of adsorbent dose

The dependence of Cr(VI) adsorption on the amount of tamarind pod shells is studied at room temperature and at pH2.0 by varying the adsorbent amount from 0.5 to 8g, while keeping the volume constant and varying the initial Cr(VI) concentration (Fig 3) It is apparent that the percent removal of Cr (VI) increases with increase in the dose of adsorbent due to the greater availability of the biosorption binding sites.

Fig 3: Effect of quantity of biomass on biosorption of Cr (VI) from solutions of different concentrations, pH 2.0 for contact time 120 min  at 120 rpm

Infrared spectroscopy

An un-reacted tamarind pod shell sample and pretreated with 100 mg/l Cr (VI) solution were analysed using FTIR, to determine the functional groups that play an important role in the biosorption of Cr(VI). The wavenumbers along with their corresponding functional groups are given in Table 1. The infrared spectroscopic studies show that several functional groups are available on the surface of the adsorbent for binding Cr (VI) ions.

Table: 1 IR absorption bands and the corresponding functional groups

 Desorption studies