Effect of salinity on the photolysis of chrysene adsorbed to a smectite clay

The rate of photooxidation (lambda, 300-800 nm) of the polycyclic aromatic hydrocarbon chrysene was significantly enhanced in aqueous suspensions of the smectite clay, Laponite RD, relative to its rate of photolysis in aqueous solution. The photodegradation of chrysene is reported at pH 8.30, at several different ionic strengths that correlate to freshwater and saltwater. The kinetics of chrysene loss are first order in chrysene. The photodegradation products 1,4-chrysenediol, 1,4-chrysenequinone, phthalic acid, and 2-formyl benzoic acid have been positively identified and quantified against analytical standards. The mechanism of chrysene degradation was probed by comparing the effects of added methanol, bicarbonate, diazabicyclooctane, chloride, bromide, and iodide on the rate of chrysene photooxidation. Chrysene photooxidation was suppressed by diazabicylooctane and halide ions but was unaffected by methanol or bicarbonate, implying that O-1(2) played a significant role in its removal. The overall rate of chrysene loss was governed by the salinity of the solution and the extent of surface coverage. The steady-state concentration of singlet oxygen was measured and varied positively with chrysene loading but was reduced dramatically at salinities comparable to the marine environment. The relative contributions of Cl-, Br-, and I- to the reduction in [O-1(2)](ss) are reported, with I- the most effective O-1(2) quencher. The implication of the research is that photoprocessing of particle-bound PAHs may fall off as rapidly as they are transported into marine environments. A predictive kinetic model for the photodegradation of adsorbed chrysene is presented.