Calcium peroxide enhanced photo-formation of reactive species in oil sands process water: Implications for the photochemistry of pit lake

Naphthenic acids (NAs) are persistent, toxic, hydrocarbon-derived compounds formed during bitumen extraction and commonly found in oil sands process water (OSPW). Solar-activated calcium peroxide (CaO 2 ) shows potential for OSPW remediation, yet the photochemical mechanisms controlling hydrolysate and reactive species (HRS) formation remain unclear. Combined kinetic modeling with probe-based experiments was used to quantify steady-state HRS concentrations ([HRS] SS ) of hydrogen peroxide (H 2 O 2 , (0.4−2.5) × 10 −3 M (mol L –1 )), hydroxyl radicals ( • OH, (0−1.5) × 10 −15 M), singlet oxygen ( 1 O 2 , (9.6−6.8) × 10 −13 M), carbonate radicals (CO 3 •− , (1.7−2.6) × 10 −13 M), triplet-excited dissolved organic matter ( 3 DOM⁎, (1.6−2.0) × 10 −13 M), and superoxide (O 2 •− , (0.06−1.3)× 10 −10 M) under varying CaO 2 doses (0–0.3 g L –1 ). Corresponding apparent quantum yields ( [ Φ HRS a ] SS , mol Eins −1 ) across CaO 2 doses of 0–0.3 g L –1 were ~0.99 ( H 2 O 2 ), 0−0.29 ( • OH), 0.093−0.075 ( 1 O 2 ), (1.30−0.28) × 10 −4 (CO 3 •− ), 0.047−0.090 ( 3 DOM⁎), and 0.23−0.036 (O 2 •− ), respectively. The [ Φ HRS a ] SS for • OH and 1 O 2 decreased with increasing CaO₂ dose due to light-shielding effects, whereas 3 DOM⁎ and O 2 •− formation rates increased at higher doses. The optimized solar/CaO 2 (0.1 g L −1 ) achieved 68.9% removal of classical NAs (mainly •OH-mediated), 65.8% hydrocarbon abatement, and a 20% reduction in acute toxicity. It also improved biodegradability, increasing the BOD 5 /COD ratio from < 0.03 to 0.14, indicating partial transformation of recalcitrant organic matter into biodegradable intermediates. Synchronous fluorescence spectroscopy suggested the breakdown of multi-fused NAs into less toxic, mono-cyclic, and other more bioavailable intermediates. Additionally, total UV–vis absorbance and fluorescence intensity proved to be reliable surrogate parameters for tracking hydrocarbon removal kinetics in OSPW. These findings advance the mechanistic understanding of CaO 2 photochemistry in OSPW and support a sustainable strategy for pit lake remediation. • Solar-activated CaO 2 was effective in removing organic contaminants in OSPW. • Reactive species under varying CaO 2 doses were quantified. • Quantum yields of reactive species for solar/CaO 2 treatment of OSPW were determined. • UV-vis absorbance and fluorescence as reliable surrogates for OSPW remediation. • Solar/CaO 2 treatment reduces toxicity and boosts OSPW biodegradability.