Iodine

Phototransformation in water

Link to relevant study record(s)

Reference

Endpoint:

phototransformation in water

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Study type:

direct photolysis

Qualifier:

no guideline followed

Principles of method if other than guideline:

This paper report an experimental study of the photochemical conversion of IO3−, in the presence of HA, to I− in the form of “free” aqueous ions (detected by UV−vis spectroscopy) and also fixed iodinated humic form inferred from the missing fraction of reduced iodate. Spectral fitting analysis was used to quantify the concentration changes in reactants and products over periods of 1−2 days. Spectroscopic analysis of the reactions of iodate solutions with a number of surrogate compounds with functional groups identified in humic acid structures was used to identify likely chemical routes and key species involved in the formation of fixed, nonvolatile iodine-containing species.

GLP compliance:

not specified

Specific details on test material used for the study:

Sigma-Aldrich, purity ≥99.5%

Radiolabelling:

no

Analytical method:

other: UV−vis spectroscopy

Light source:

Xenon lamp

Remarks:

1000-W xenon arc lamp (ozone-free Oriel “Solar Simulator”)

Details on light source:

The lamp beam was optically filtered using (1) a water filter to remove IR wavelengths and prevent heating of the solution, (2) a solar air mass filter (Oriel: AM 1.5) to “shape” the lamp spectrum in order to replicate the solar spectrum in the near UV−visible, and (3) a cutoff filter to remove any of the small output of the lamp at λ < 310 nm.

Transformation products:

yes

The UV panel shows a steadily increasing absorption between~210 and 240 nm, consistent with the contribution from the formation of I-, which absorbs strongly in this region, peaking at~225 nm (molar extinction coefficient ε ~ 1.4E+4 M-1 cm-1, compared with ε at 225 nm for IO3- of ~1000 M-1 cm-1). Absorption in the visible region is exclusively due to HA and shows a continually decreasing absorption at all wavelengths in this region with time. Other transient inorganic (nonionic) forms of iodine, i.e. HOI, I2 (and ICl in the presence of Cl-1) which may form in such aqueous systems exhibit weaker (ε < 1000 M-1 cm-1) absorption features in both the UV and visible regions (compared with I-, IO3-, and HA), and so are not spectroscopically detectable at the submicromolar levels likely to be present in our experiments. In addition, I2 and ICl have very low solubilities in water and so would readily evade to the gas phase: kH (298 K) for I2 is ~3 M/atm and for ICl is 110 M/atm.

To determine the change in concentration of reactants and products with time, a least-squares optimized spectral fitting analysis was performed using reference spectra taken of the initial iodate and HA solutions, and of a prepared 1E−4 M KI solution. The HA reference spectrum was first fitted to the visible part of the photolyzed solution spectra and then iodate and iodide reference spectra were simultaneously fitted at 225 nm. The absorption due to HA at 254 nm is ~0.1, which indicates a DOC of ~2 mg/L.

Both HA and iodate absorbances (concentrations) are seen to steadily decrease with time with a final [IO3−] after 18 h of 5.3 E−5 M, while the iodide concentration in solution increases to 1.3 E−5 M after 18 h, which represents ~22% of the iodine which has undergone conversion from the initial iodate form. This fraction is approximately constant with time, with an average of 0.19 as indicted by the dashed line in the bottom panel. The difference (average of ~80%) is assumed to be in the soluble form of iodinated HA, although there are no absorption features evident in the mixed solution spectra to verify this. The same detectable fraction was observed after prolonged (>10 h) irradiation of the HA−iodate solution to which chloride ions had been added at a concentration of 0.5 M, i.e. similar to that found in seawater.

 

The first-order loss rate of iodate during the periods of irradiation is (1.2 ± 0.3) E−5 s−1. Even though the solution was stored under dark conditions overnight (15 h), a slow dark reaction takes place between the two reactants at a rate which is about 20% of the photochemical rate. This dark reaction was further investigated by monitoring spectral changes from a HA solution mixed with a 1E-3 M solution of NaIO3, kept in the dark, and aliquots taken for spectroscopic analysis at the same time each day over a week. A continual change in HA absorbance from 400 to 600 nm was observed, with an~12% decrease after the first day and a total decrease of~62% after one week. However, even after one week, there was no evidence in these spectra for any additional absorbance due to products formed by the dark reaction.

Conclusions:

These experiments show that aerosol IO3- should be reduced photochemically in the lower atmosphere in the presence of HA on a time scale of about 2 days. The major product is soluble organic iodine, with about 80% yield.

Executive summary:

This paper describes a laboratory study of the photochemical reduction of IO3- in the presence of humic acid. These experiments show that aerosol IO3- should be reduced photochemically in the lower atmosphere in the presence of HA on a time scale of about 2 days. The major product is soluble organic iodine, with about 80% yield.

The first-order loss rate of iodate during the periods of irradiation is (1.2 ± 0.3) E−5 s−1. Even though the solution was stored under dark conditions overnight (15 h), a slow dark reaction takes place between the two reactants at a rate which is about 20% of the photochemical rate.

Additional information

Iodine is rapidly hydrolysed and forms several equilibria with different ionic species (I^-, OI^-, IO₃ ^-, I₃ ^-). Therefore the degradation route of phototransformation of iodine in water is regarded as negligible. However, Miyake and Tsunogai found indications that in marine surface waters photooxidation of iodide ions to iodine occur (Miyake, 1963).

Saunders, 2012 report an experimental study of the photochemical conversion of IO3−, in the presence of HA, to I− in the form of “free” aqueous ions (detected by UV−vis spectroscopy) and also fixed iodinated humic form inferred from the missing fraction of reduced iodate. These experiments show that aerosol IO3- should be reduced photochemically in the lower atmosphere in the presence of HA on a time scale of about 2 days. The major product is soluble organic iodine, with about 80% yield.