84
T.T.Y. Tan et al. / Journal of Molecular Catalysis A: Chemical 202 (2003) 73–85
acid, each stirred for 30 min and Method 3: simulta-
neous addition of both Se(VI) and formic acid and
pension. These experiments were performed based on
the knowledge that Se(VI) ions have a stronger affin-
ity to TiO2 as indicated from the adsorption isotherms
obtained earlier (Figs. 2 and 3). By subjecting TiO2 to
formic acid adsorption first, formate ions would have
a greater chance of adsorption onto TiO2 while Se(VI)
could still probably be adsorbed when introduced
later.
Both experiments of Sets C and D indicated that
adsorbing formic acid first resulted in a higher Se(VI)
photoreduction rate than by adsorbing Se(VI) first
(compare Method 1 with 2 in Sets C and D). However,
this was comparable to that of simultaneous adsorp-
tion (compare Method 1 with 3 in Sets C and D). This
showed that Se(VI) ions have greater affinity to TiO2.
The results from Set D also indicated that the fastest
rate also correlated well to stoichiometric adsorption
ratio of 3:1. Comparing Sets C and D, increasing the
initial formic acid concentration from 20 to 100 ppmC
would increase the reduction rates. This shows that
a high formic acid concentration was necessary to
maintain a favourable formate adsorption.
Acknowledgements
The author Timothy T.Y. Tan would like to ac-
knowledge the Australian Institute of Nuclear Science
and Engineering (AINSE) for financial support and
The University of New South Wales and the De-
partment of Education, Training and Youth Affairs
DETYA) for awarding the International Postgraduate
Research Scholarship. The authors would also like
to thank Dr. Myint Zaw from the Australian Nuclear
Science and Technology Organisation (ANSTO) for
his assistance in the various aspects of this project.
References
[1] Y. Zhang, J.N. Moore, Environ. Sci. Technol. 30 (1996) 2613.
[2] Y. Zhang, J.N. Moore, Appl. Geochem. 12 (1997) 685.
[3] Agency for Toxic Substances and Disease Registry (ATSDR)
[4] H.M. Ohlendorf, D.J. Hoffman, M.K. Saiki, T.W. Aidrich,
Sci. Total Environ. 52 (1986) 49–63.
[5] D. Wang, G. Alfthan, A. Aro, A. Makela, S. Knuuttila, T.
Hammar, Agric. Ecosys. Environ. 54 (1995) 137.
[6] G.M. Peters, W.A. Maher, D. Jolley, B.I. Carroll, V.G. Gomes,
A.V. Jenkinson, G.D. McOrist, Org. Geochem. 30 (1999a)
1287.
[7] G.M. Peters, W.A. Maher, F. Krikowa, A.C. Roach, H.K.
Jeswani, J.P. Barford, V.G. Gomes, D.D. Reible, Mar. Environ.
Res. 47 (1999) 491.
4. Conclusions
[8] P.H. Masscheleyn, R.D. Delaune, H.P. Patrick Jr., Environ.
Sci. Technol. 24 (1990) 91.
[9] P. Zhang, D.L. Sparks, Environ. Sci. Technol. 24 (1990) 1848.
[10] M.A.R. Abdel-Moati, Estuar. Coast. Shelf Sci. 46 (1998)
621–628.
[11] M.J. Jones, R. French, Local Government Engineering in
Australia, The Federation Press, Leichhardt, NSW, 1999,
p. 141.
[12] S. Sanuki, T. Kojima, K. Arai, S. Nagaoka, H. Majima,
Metall. Mater. Trans. B 30B (1999) 15.
The effects of pH and initial solute concentration
(formic acid and Se(VI)) on the UV/TiO2 reduction
process of Se(VI) ions were investigated. It was found
that the adsorption of both Se(VI) and formate ions
onto the TiO2 surface were essential for Se(VI) pho-
toreduction to elemental Se. The photoreduction rate
was depressed in the presence of oxygen. The ele-
mental Se was further reduced to hydrogen selenide
once the selenate ions were exhausted from the so-
lution. The optimum Se(VI) photoreduction rate was
found to be closely correlated to the molar adsorption
ratio of approximately 3:1 of formate-to-selenate on
the TiO2 surface. This was suggested to be due to the
limited surface sites as a result of competitive adsorp-
tion between Se(VI) and formate ions. It was possible
to adjust the amount of formate and Se(VI) ions ad-
sorbed onto the TiO2 surface to its optimum ratio by
changing the pH and initial solute concentration to
obtain optimum Se(VI) photoreduction rate.
[13] S. Sanuki, K. Shako, S. Nagaoka, H. Majima, Mater. Trans.,
JIM 41 (2000) 799.
[14] T.J. Sorg, G.S. Logsdon, J. Am. Water Works (1978) 379.
[15] A. Ramana, A. Sengupta, J. Environ. Eng. 118 (1992) 755.
[16] K.J. Gleason, in: R. Bartsch, J.D. Ways (Eds.), Chemical
Separations with Liquid Membranes, American Chemical
Society, Washington, DC, 1996, p. 342 (Chapter 24).
[17] Y.K. Kharaka, Appl. Geochem. 11 (1996) 797.
[18] M. Fujita, M. Lke, S. Nishimoto, K. Takahashi, M. Kashiwa,
J. Ferment. Bioeng. 83 (1997) 517.
[19] C. Garbisu, T. Ishii, T. Leighton, B.B. Buchanan, Chem. Geol.
132 (1996) 199.
[20] D.T. Maiers, P.L. Wichlacz, D.L. Thompson, D.F. Bruhn,
Appl. Environ. Microbiol. 54 (1988) 2591.