With a photon flux of 3.6×1016 photons s−1, the linearization
of the LH equation gives kLH = 2.2mol min−1 and
KL ×kLH = 0.054 min. It gives an alternate value of the adsorption
constant: KL = 25,000 L mol−1, which is inconsistent with
the value of Langmuir adsorption constant found in the absence
of reaction (KL = 4100 L mol−1). This confirms that the rate
plateau occurs far before adsorption saturation. Values of KL
obtained at three different photon fluxes show that KL decreases
with light intensity, meaning that a phenomenon of naphthalene
photodesorption might occur.
This deviation from the ideal Langmuir–Hinshelwood
behaviour has been observed with many organic compounds
and well discussed by Emeline et al. [21]. In short, the reactant
concentration is less rate-determining when working with
high photon fluxes, since adsorption equilibrium cannot be
reached and photodesorption can happen. More generally, we
are aware that the introduction of the parameter “light” in the
heterogeneous photocatalytic system makes it differ from a well
understood heterogeneous catalytic system.
However the first order approximation is still correct at low
concentrations. For this reason, and because naphthalene is not
likely to be found at concentrations above 60mol L−1 in natural
or artificial effluents, we chose to work with concentrations
below 40mol L−1 for the rest of this study. In these conditions,
reaction is of first order with respect to naphthalene concentration
and the disappearance rate can be expressed as: