pH of sample solution plays an important role on adsorption efficiency, and therefore is proved to be a primary parameter for SPE or MSPE. Moreover, a proper pH can bring down the interference
of the matrix and increase the selectivity of the method.The adsorption behavior of Cr(III) and Cr(VI) on Zincon–Si–MNPs was investigated based on the experimental procedure and the results are shown in Fig. 2. As can be seen that the recovery of Cr (III) increased with the increase of pH from 2.0 to 6.0, and then remained constant quantitatively, and finally decreased sharply
after pH49.5. The trend of the effect of pH on the recovery of Cr (VI) was similar with Cr(III), but the platform pH ranged from 6.0 to 7.0. According to the above results, we found that both Cr(III)
and Cr(VI) are quantitatively reserved on Zincon–Si–MNPs in the
pH range of 6.0–7.0, while only Cr(III) is retained on the absorbent
in the pH range of 9.0–9.5. So, speciation of Cr(III) and Cr(VI) can
be attained, and notably, no reduction and oxidation process is
needed prior to MSPE operation. For further study, pH 6.5 was
chosen for preconcentration of total Cr and pH 9.1 was selected for
speciation of Cr(III).
The different adsorption behavior of Cr(III) and Cr(VI) on
Zincon–Si–MNPs is due to the existence form of Cr(III) and Cr(VI)
in different pH conditions and surface nature of the absorbent. In
view of Cr(III), on one hand, it tends to be hydrolyzed in aqueous
solution at pH44 and forms several positively charged species
including Cr3+ and Cr(OH)2+ [32], and then can be chelated with N
atom of zincon, which leaded to increasing recovery from pH
4.0 to 6.0 and moreover quantitative recovery when pH was
bwteen 6.0 and 9.5. After pH49.5, however, the cations of Cr(III)
is prone to form precipitate, resulting in rapid decrease in the
recovery. Another reason for little recovery of Cr(III) below pH
4.0 is the strong acidc effect on chelation of zincon to Cr(III). With
respect to Cr(VI), it exists mainly in the form of HCrO4
− in the pH
range from 2 to 7, associated with small concentration of H2CrO4
and CrO4
2−
, as well as a portion of Cr2O7
2− formed by condensation
of two HCrO4
− ions at weak acidic solution. While CrO4
2− is the
predominant species of Cr(VI) when pH47, associated with small
and tiny concentration of HCrO4
− and H2CrO4, respectively [33]. In
acidic medium, the electrostatic interaction may occur between
the anions of Cr(VI) and protonated N-containg groups on the
surface of Zincon–Si–MNPs, and therefore, Cr(VI) anions were
reserved on the absorbent. Moreover, the recovery of Cr(VI)
increased as the pH increased from 2.0 to 6.0 and approched
100% when pH was 6.0–7.0. After pH47.0, however, the anions of
Cr(VI) cannot be absorbed because of the electrostatic repulsion of
dissociated phenolic hydroxyl group and the negatively charged
surface of Zincon–Si–MNPs, which caused significant decrease in
recovery of Cr(VI)