Photochemical liberation of allochthonous organically bound aluminum (Al) and iron (Fe) and in-lake hydroxide precipitation are important sources of these metals to lake sediments. Mass budgets of ionic Al and Fe (Ali, Fei), organically bound Al and Fe (Alo, Feo), and particulate Al and Fe (Alp, Fep) were measured for two western Czech Republic forest lakes (Plesne and Certovo) in the 2000-2003 hydrological years. The lakes were net sinks of Ali, Alo, and Feo and net sources of Alp and Fep. The average Alo and Feo inputs from terrestrial sources (66-110 and 12-17 mmol m(-2) yr(-1), respectively, on a lake area basis) were reduced 45% and 25% in the lakes. Mass budgets of dissolved organic carbon, particulate organic C, and Al species indicated that only a minor part of the observed in-lake retention of Alo could be explained by coagulation and sedimentation of organic matter, or from Ali hydrolysis and formation of Alp. Laboratory experiments with a short-time irradiation (approximately 300 nm, approximately 800 W m(-2)) of water from inlets to Plesne Lake showed the importance of photochemical processes in the liberation of Al and Fe from Alo and Feo. After 12 h of irradiation, Alo and Feo concentrations decreased 54 +/- 6% and 70 +/- 16%, respectively, compared to those of the dark controls. The photoliberated Alo and Feo increased the Ali and Fei concentrations reciprocally, on a 1:1 mass basis. The subsequent hydrolysis of Ali and Fe1 in lakes forms insoluble hydroxides, increasing the sediment concentrations of Al and Fe.

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Photochemical degradation of atrazine under different conditions was studied and compared, namely degradation via photocatalysis on TiO2, UV C photolysis, and homogeneous photocatalysis in the presence of added ferric ions. The reaction rate constants in heterogeneous photocatalytic reactions on TiO2 and of photolytic degradation by means of UV C light are similar, 0.018 min(-1) and 0.020 min(-1), respectively. The reaction rate constants in homogeneous photocatalytic reactions with Fe(III) added depend strongly on the Fe(III) concentration, 0.0017 min(-1) for 1.6 × 10(-6) mol l(-1) Fe(III) to 0.105 min(-1) for 3.3 × 10(-4) mol l(-1) Fe(III). In all types of reactions, dechlorination was observed; in homogeneous photocatalytic reactions and in UV C (250-300 nm) photolysis, dechlorination proceeds with a 1 : 1 stoichiometry to atrazine degradation, in photocatalytic reactions on TiO2, dechlorination measured as chloride ion release reaches only 1/5 of the substrate degradation. In photocatalytic reactions on TiO2, mineralisation of 40% carbon was observed.

Alteration of photochemical properties of humic substances (HS) by the addition of metals abundant in natural waters has been investigated. Excited HS states have been characterized by fluorescence spectra and by laser kinetic spectroscopy. The effect of Cu2+, Fe2+, and Mn2+ has been explained in terms of static quenching of the HS excited states. In agreement with this, the metal ions were found to inhibit HS sensitized photooxidative degradation of chlorophenols.