Comments on “Removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal” by S.M. Nomanbhay and K. Palanisamy
In a recent article (Nomanbhay and Palanisamy, 2005), the authors presented in Figure 5 the data relevant to the change in the removal efficiency of chromium (E) with time. These data show that with an initial concentration of 20 mg/L chromium and agitation speed 200 rpm, the time needed to reach equilibrium conditions is 300 min for Acid Treated Oxidized Palm Shell Char (AOPSC). It can be also seen from the same figure that after 180 min of agitation, the E of AOPSC is 45%, while after continued agitation for 300 min the E to 70%. It is clear that after 180 min of agitation of chromium solution with AOPSC the system is still far from equilibrium. The above values of E correspond to an adsorbent dose of 40 g/L. It is expected that the rate of removal will be slower when using lower dosage of adsorbent (Raghuvanshi et al. 2004; Karthikeyan et al. 2005), hence the time needed to reach equilibrium conditions will be even more than 300 min.
The same concept applies to Chitosan Coated Beads (CCB) and Chitosan Coated Acid treated Beads (CCAB). The data presented in Figure 5 (Nomanbhay and Palanisamy, 2005) show that both CCB and CCAB just reach equilibrium after 180 min of agitation when the dosage of adsorbent is 40 g/L, so it is expected that the rate of removal will be slower when using lower dosage of adsorbent , hence the time needed to reach equilibrium conditions will be even more than 180 min.
In Figure 3 (Nomanbhay and Polanisamy, 2005) present their results regarding the effect on the adsorbent dose on E. The experiments in Figure 3 were carried out for a total agitation time of 180 min, and various adsorbent dosage up to only 30 g/L. Therefore, equilibrium conditions are not reached in all the experiments presented in Figure 3 for CCAB, CCB, and AOPSC.
Also, in their discussion of the effect of dosage the authors observed that after a certain dose E did not increase any more. They explained this by “This suggests that after a certain dose of adsorbent, the maximum adsorption sets in and hence the amount of ions remain constant even with further addition of the dose of adsorbent”. In my opinion this is not correct because the experiments did not allow enough agitation time to reach equilibrium conditions. Moreover, the equilibrium isotherm equations of Freundlich and Langmuir apply only to equilibrium conditions, so there is no point in fitting the experimental results of Figure 3 to isotherm equations, and consequently, the results in Table 2 of the article (Nomanbhay and Polanisamy, 2005) does not represent the correct values of isotherm equations.
NOMANBHAY, S.M. and POLANISAMY, K. Removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal. Electronic Journal of Biotechnology [online]. 15 April 2005, vol. 8, no. 1. Available from Internet: http://www.ejbiotechnology.info/content/vol8/issue1/full/7/index.html.
Note: Electronic Journal of Biotechnology is not responsible if on-line references cited on manuscripts are not available any more after the date of publication.
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