Iron-Based Magnetic Nanoparticles for Removal of Heavy Metals from Electroplating and Metal-Finishing Wastewater - Nanotechnologies for Water Environment Applications - page 242

the zero point of charge pH pzc , which is 6.3 for -Fe 2 O 3 . When solution pH is below the
pH pzc , the adsorbent surface is positively charged, and anion adsorption occurred by
simply electrostatic attraction. When solution pH is above the pH pzc , the adsorbent
surface is negatively charged, and cation adsorption occurred. With an increase in pH,
uptake of Cr(VI) ions decreased which is apparently due to the higher concentration of
OH - ions present in the mixture, that compete with Cr(VI) species for adsorption sites.
On the other hand, as the adsorption surface is negatively charged (pH > pH pzc ),
increasing electrostatic repulsion between negatively charged Cr(VI) species and
negatively charged nanoparticles would also result in a release of the adsorbed HCrO 4 -
and CrO 4 2- .
Another factor affecting this variation of adsorptive capacity in different pHs
may be due to the adsorption free energy of various chromium species (HCrO 4 - , H 2 CrO 4
and CrO 4 2- ) existing at different pHs (Manuel et al., 1995). At pH of 2.0-6.0, the
predominant Cr(VI) species mainly exists in the monovalent HCrO 4 - form, which is then
gradually converted to the divalent CrO 4 2- form as the pH increases. The adsorption free
energy of HCrO 4 - and CrO 4 2- is between -2.5 and -0.6 kcal/mol and between -2.1 and -
0.3 kcal/mol, respectively (Wang and Zhang, 1997). The adsorption free energy of
HCrO 4 - is lower than that of CrO 4 2- , and consequently, HCrO 4 - is more favorably
adsorbed than CrO 4 2- at the same concentration. The removal of Cr(VI) at a lower pH is
mainly due to the adsorption of HCrO 4 - , which is expected to be adsorbed in larger
quantities than CrO 4 2- under the same adsorption affinity. When the CrO 4 2- concentration
is much higher than HCrO 4 - at a higher pH, the adsorption free energy of CrO 4 2- is lower
than that of HCrO 4 - . Only under such circumstance can CrO 4 2- adsorption be more
favorable than HCrO 4 - .
As far as Cu and Ni are concerned, the increase in metal removal with pH is due
to a decline in competition between proton and metal species for surface sites, thereby
decreasing a positive surface charge and resulting in a lower coulombic repulsion of the
adsorbed metal. At pH lower than pH pzc , the adsorption of Cu and Ni should be reduced
to nearly zero; however, it is not the case for Cu. Even at pH lower than pH pzc , a large
amount of Cu ions were still adsorbed onto the -Fe 2 O 3 , which suggests that ion
exchange between Cu 2+ and H + may play a role in this pH range. This point will be
further examined in the following mechanism studies. Additionally, at the same pH and
initial metal concentration, a higher percentage removal was recorded for Cu(II) as
compared to Ni(II). In general, the preference of common hydrous solids for metals has
been related to the metal electronegativity. Electronegativity values for Cu(II) and Ni(II)
are 2.00 and 1.91, respectively (Seco et al., 1997); hence Cu(II) exhibited a stronger
attraction to -Fe 2 O 3 than Ni(II). Especially for transition metals like Cu and Ni, there
are additional complications. Transition meals by definition, differ in the number of d-
electrons in their valence shells. These different electronic configurations give rise to
something called ligand field effects. Ligand field effects are more important than ionic