Oxidation of iodide by manganese oxide – An ATR-FTIR and
dissolution study
Wen-Hui Kuan*, Y. T. Lai,
Graduate Institute of Environmental and Resource Engineering, Ming Chi University of Technology, 84,
Gun-Juan Rd, Taishan 24301, New Taipei City, Taiwan, ROC
*Correspondence email: [email protected]; Fax: 886-2-29080346
Abstract: The interaction between manganese oxide (MnO2) and inorganic iodine species, iodide (I-)
and iodate (IO3-), were studied using batch reaction at various pH. The attenuated total reflectance
Fourier transform infrared spectroscopy (ATR-FTIR) in conjunction with bulk oxidation/adsorption
and dissolution measurements to probe the oxidation of iodide by manganese oxide surfaces. The
results indicated that at acidic condition, the iodide is partially oxidized into reactive iodine and iodate
by manganese oxide; and both iodide and iodate are adsorbed onto the solid oxide surface; at neutralcircum, iodide is also slightly oxidized to iodate but only the iodide could be adsorbed onto the oxide
surface; while the final concentration of iodide equal to the initial concentration of iodide in solution,
implying that there is no interaction occurred between manganese oxide and iodide at alkaline
condition. The ATR-FTIR results reveal that the major peaks 720 and 1068cm-1 of pyrolusite remained
in MnO2 with iodide but the minor peaks 2991 and 2914 cm-1 of pyrolusite are absent at acidic-circum.
This may imply that some of the surface functional groups attend the oxidation reaction of MnO2 and
are dissolved into solution leading to the disappearance of ATR-FTIR peaks. At acidic condition, the
concentration of dissolved Mn2+ from MnO2 with iodide markedly drops to a low value. Contrarily, the
dissolution of MnO2 with iodide increased at neutral and alkaline and 10. These phenomena could be
attributed to that the complexes form of adsorbed iodate may inhibit further dissolution of pyrolusite.
Keywords: oxidation, iodide, manganese oxide, ATR-FTIR
Introduction
Iodine, possessed multiple oxidation states and biophilic properties, exist in natural
environments with complex chemistry. The main inorganic species of iodine are
iodide (I-) and iodate (IO3-), however, the elemental iodine (I2) and hypoiodous acid
can also be present as metastable species at low concentrations (Allard et al. 2009).
Natural indoorganic compounds can be mainly produced by bacteria, alga or during
natural geochemical process (Keppler et al. 2000) and even generated during drinking
water treatment systems (Krasner et al. 2006). Recent literatures pointed out that these
organic iodinated compounds rise up severe environmental issues, such as of which
the disinfection by-products (DBPs) the iodidated ones are the most genotoxic but not
have been tested for carcinogenicity (Plewa et al. 2008, Richardson et al. 2008). As
one of the contributor to stratospheric ozone depletion, CH3I is formed as 10 fold to
CH3Cl and 7 fold to CH3Br during the methylation process of soil halogen content by
natural oxidation process at the same halogen quantity (Allard et al. 2010, Keppler et
al. 2000). Therefore, the fundamental science related to how the iodine-compounds
were born in natural watershed and a feasible technology to removal these compounds
from drinking water are indispensible.
Second to iron, manganese is an abundant transition metal in the Earth’s crust.
Manganese oxide mineral was believed to play the key role in a variety of natural
geochemical reactions both involving organic and inorganic compounds. These
reactions lead to the oxidation, reduction, and scavenging (sorption, precipitation) of
natural and anthropogenic target compounds (Gulley-Stahl et al. 2010). Lin et al.
(2008) verified that iodide may be oxidized by lead oxide in Pb-containing water
distribution systems. A layered MnO2, birnessite, have also been reported (Allard et
al. 2010, Fox et al. 2009) to serve as electron acceptors in the oxidation of iodide to
iodine and/or iodate. However, these studies focused on the analysis of iodine species
in solution, but not either monitored spectroscopically, evaluated as a function of pH,
or correlated to dissolution measurements. This study has utilized attenuated total
reflectance Fourier transform infrared spectroscopy (ATR-FTIR) in conjunction with
bulk oxidation/adsorption and dissolution measurements to probe the oxidation of
iodide by manganese oxide surfaces.
Material and Methods
The used MnO2 particles were purchased from the TOSOH Co. and proved to be a γMnO2 (pyrolusite) with 1x1 molecular sieve structure, by X-ray diffractometer (XRD,
Philips/ PANalytical X’pert PRO MPD). KI (purity 100%, Mallinckrodt Baker) and
KIO3 (purity 100%, J. T. Baker) was used without further purification before
experiments. All chemicals used in this study were of AnalaR grade and solutions
were prepared by with ultra pure water produced from a Milli-Q water purification
system (Milli-Q-Academic, Millpen RIOS16).
Before batch experiments, 11.76 mM MnO2 stock suspensions were aged at room
temperature under a N2 atmosphere for 24hr. The stock solution of KI and KIO3 were
prepared as 0.67 M right before the batch experiments in a dark glass container at 4oC.
The initial concentration of KI or KIO3 was 0.1 mM and the MnO2 load was 0.87 g/L
(10mM). Various concentrations of KOH and HCl solution were used to adjust
samples to desired pH. Batch experiments of KI or KIO3 reaction with MnO2 were
carried out in 50 mL darken glass vials fitted with Teflon-lined septa, maintained at
25oC by the water circulating temperature controller, and shaken at 100rpm in a tank
with a reciprocating motor (Firstek Model-B603DL). To avoid gas products, the
reaction vial was filled with reaction suspension leaving no headspace. After the
desired reaction period, a subsample from the suspension was centrifuged (Hermle,
Z36 HK) at 13,500 g for 20 min then filtered through a Millipore membrane filter
with pore size of 0.2 µm and the filtrate was preserved at 4oC for advanced analysis
and the remained solid in Teflon centrifugation tube was collected and freeze-dried
prior to ATR-FTIR (MIRzcleTM ATR, Pekin Elmer Spectrum One) analysis. All the
experiments were conducted in duplicate and all the data presented were the averages
of duplicate analysis.
The supernatant was analysed for Mn2+ dissolved from MnO2 solid into solution by
inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin Elmer,
Optima 2000DV). The measurement of iodide (I-) in solution was achieved by a high
performance liquid chromatography (HPLC, Dionex) equipped with IonPac AS11
analytical column and GS50 UV-Vis detector. Reactive iodine species, including I2
and HOI, in aqueous solution were quenched with phenol (Bichsel and von Gunten
2000) and analyzed as iodophenols by HPLC. Iodate (IO3-) in solution was analyzed
using Ionic chromatography (IC, Dionex) with AS4A-SC analytical column and
Dionex ICS-1500 conductivity detector. Infrared measurements were made with
Perkin-Elmer One, ATR-FTIR spectrometer equipped with a DTGS detector and a
horizontal ATR-FTIR (MIRzcleTM) attachment with a 45o ZnSe crystal.
Results and Discussion
Iodide could be oxidized by MnO2 to molecular iodine (I2) or iodate (IO3-), of which
molecular iodine species include I20(aq), I3-, HOI, I2OH, and OI-. According to the
speciation calculation of molecular iodine, the dominated species are I2 and I3-; and at
low iodine concentration and neutral to alkaline conditions is HOI. In this study the I2
and HOI are referred as reactive iodine. Figure 1 display The results indicated that at
acidic condition, the iodide is partially oxidized into iodate by manganese oxide and
both iodide and iodate are adsorbed onto the solid oxide surface; at neutral-circum,
iodide is also slightly oxidized to iodate but only the iodide could be adsorbed onto
the oxide surface; while final concentration of iodide equal to the initial concentration
Fig. 1 Product species percentage of iodide oxidation (10-4 M) by pyrolusite MnO2
(10-2 M) at pH 5 (represented as acidic condition), pH 7 (represented as neutral
condition), and pH 10 (represented as alkaline condition) for reaction time of 24 hr.
Solution species, I-, IO3-, and reactive I, were directly analyzed by instruments,
adsorbed IO3- was obtained from the IO3- adsorption experiments, and adsorbed I- was
calculated by the discrepancies between total iodine (10-4 M of iodide as initial
species) and the former two items (i.e. [I-]ads=[I-]initial-[I-]solu-[IO3-]solu-[reactive I]solu[IO3-]ads ).
of iodide in solution, implying that there is no interaction occurred between
manganese oxide and iodide at alkaline condition.
Although valuable aspects can be obtained from bulk oxidation and/or adsorption
measurements, only indirect conclusions regarding to the distribution of solution
speciation can be made. Spectroscopy investigation offers the possibility of more
direct insight on how the iodide and iodate reacting with oxide surfaces. Figure 2
show the iodide and iodate reacting on pyrolusite surfaces at pH5. Surface reaction
was determined by correlating spectra of iodide (or iodate) reacting with MnO2 to
spectra of standard iodine species reagent along with MnO2 solid. The spectrum of
Fig. 2(e) displays the iodide reacting with MnO2 at pH5. It reveals that the major peak
720 and 1068cm-1 of pyrolusite remained in MnO2 with iodide sample but the minor
peaks 2991 and 2914 cm-1 of pyrolusite are absent. In contrast, both major and minor
peaks still exist in the sample of MnO2 reacted with iodate. This may imply that some
of the surface functional groups attend the oxidation reaction of MnO2 and are
dissolved into solution leading to the disappearance of ATR-FTIR peaks. Because the
ATR-FTIR peaks of KI and KIO3 are significantly overlapped with that of MnO2, it is
difficult to distinguish the surface species on MnO2 reacted with iodide and iodate.
However, the 826 cm-1 peak of KI reagent was observed in the MnO2 reacted with
iodide, it appear to be the iodide also partially adsorbed onto MnO2 surface.
Fig. 2 ATR-FTIR spectra of (a) original MnO2, (b) KI, (c) KIO3, (d) I2, (e) iodide
reacting with MnO2 at pH 5 for 24 hr, (f) iodate adsorbed onto MnO2 at pH5 for 24 hr.
Dissolution profiles can provide insight into the interface reaction from the view of
oxide solid. The dissolution behaviour of MnO2 at pH 5, 7 and 10 in the presence and
absence of iodide are showed in Fig. 3. Dissolution of the MnO2 without iodide shows
relatively independence on pH of studied range. In absence of iodide, dissolution of
MnO2 at pH 5 is slightly lower than that at pH 7 and 10. In presence of iodide, MnO2
dissolution behaviour is intensified as a function of pH. At pH 5, the concentration of
dissolved Mn2+ from MnO2 with iodide markedly drops to a low value. Contrarily, the
dissolution of MnO2 with iodide increased at pH 7 and 10. According to bulk reaction
experiments, the iodide can be oxidized to iodate and both iodide and iodate were
adsorbed onto MnO2 surfaces at pH 4 but only slightly iodide was adsorbed onto
oxide surface without iodide oxidation at pH 7 and 10. Therefore, one explanation
could be that the complexes form of adsorbed iodate may inhibit further dissolution of
pyrolusite (Stumm 1997). The detailed experiments should be further performed.
Fig. 3 Dissolution of manganese oxide in the presence and absence of iodide as a
function of pH.
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