Near-UV photooxidation of As(III) by iron species in the presence of fulvic acid.

Photooxidation of As(III) in ternary As(III) - Fe(III) - Fulvic acid system at pH 4 was investigated by optical spectroscopy, steady-state photolysis (365 nm) and atomic-emission spectrometry with inductively coupled plasma techniques. It was found that at all values of [FA]/[Fe] ratio the main photoactive species is OH radical formed by photolysis of Fe(III) hydroxocomplexes. Addition of fulvic acid leads to mainly negative effect on As(III) photooxidation due to the following reasons: (i) slow dark reduction of photoactive Fe(III) species with formation of scattering particles and photoinert Fe(II) species; (ii) formation of photoreductive Fe(III)-FA complexes incapable to oxidize As(III), (iii) competition of both FA and Fe(III)-FA complexes for UVA quanta with FeOH complex and for OH radicals with As(III). Aging of ternary system is also very important parameter leading to one order decrease of quantum yields of both Fe(II) formation and As(III) photooxidation.