and 75% of their respective total sorption at equilibrium
in 60 min. The slower phase lasted for the next 30 min.
At the contact time of 60 min, the removal of copper(II),
cadmium(II) and zinc(II), respectively, was 1.93, 1.89 and
1.29 mg/g papaya wood, which corresponded to 97.3%
copper(II), 95.3% cadmium(II) and 66.6% zinc(II). Further
removal of the minor quantities of the metals remaining
in solution was not possible, as on the sorbate–sorbent
contact for 180 min, the metal sorption observed was 97.2%
copper(II), 95.4% cadmium(II) and 66.8% zinc(II), which
is virtually the same as was observed after 60 min contact
time. From these observations it was also noted that affinity
of papaya wood to biosorb the three metals was different,
which may be regarded as high for both copper(II) and
cadmium(II), and not as good for zinc(II). This difference in
the maximum uptake level of various metal ions by papaya
wood has been explained in terms of difference in the ionic
size of metals, the nature and distribution of active groups
on the biosorbent, and the mode of interaction between the
metal ions and the biosorbent [15]. The two-stage sorption,
the first rapid and quantitatively predominant and the second
slower and quantitatively insignificant, has been extensively
reported in literature [16]. The rapid stage, furthermore, may
last for several minutes to a few hours, while the slow one
continues for several hours to a day [17]. The rapid stage is
probably due to the abundant availability of active sites on
the biomass, and with the gradual occupancy of these sites,
the sorption becomes less efficient in the slower stage [18].
The fast metal uptake by the papaya wood may be attributed
to its highly porous and mesh structure, which provides
ready access and large surface area for the sorption of metals
on the binding sites. The porous structure of the papaya wood
eliminates the problem of diffusional limitation as would be
expected to occur for sorption on wood chips or blocks
and 75% of their respective total sorption at equilibrium
in 60 min. The slower phase lasted for the next 30 min.
At the contact time of 60 min, the removal of copper(II),
cadmium(II) and zinc(II), respectively, was 1.93, 1.89 and
1.29 mg/g papaya wood, which corresponded to 97.3%
copper(II), 95.3% cadmium(II) and 66.6% zinc(II). Further
removal of the minor quantities of the metals remaining
in solution was not possible, as on the sorbate–sorbent
contact for 180 min, the metal sorption observed was 97.2%
copper(II), 95.4% cadmium(II) and 66.8% zinc(II), which
is virtually the same as was observed after 60 min contact
time. From these observations it was also noted that affinity
of papaya wood to biosorb the three metals was different,
which may be regarded as high for both copper(II) and
cadmium(II), and not as good for zinc(II). This difference in
the maximum uptake level of various metal ions by papaya
wood has been explained in terms of difference in the ionic
size of metals, the nature and distribution of active groups
on the biosorbent, and the mode of interaction between the
metal ions and the biosorbent [15]. The two-stage sorption,
the first rapid and quantitatively predominant and the second
slower and quantitatively insignificant, has been extensively
reported in literature [16]. The rapid stage, furthermore, may
last for several minutes to a few hours, while the slow one
continues for several hours to a day [17]. The rapid stage is
probably due to the abundant availability of active sites on
the biomass, and with the gradual occupancy of these sites,
the sorption becomes less efficient in the slower stage [18].
The fast metal uptake by the papaya wood may be attributed
to its highly porous and mesh structure, which provides
ready access and large surface area for the sorption of metals
on the binding sites. The porous structure of the papaya wood
eliminates the problem of diffusional limitation as would be
expected to occur for sorption on wood chips or blocks
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