What's the mechanism for this isotopic substitution reaction?
To answer this question you should understand the kinetic isotope effect.

Since you deliver no real data about reaction rate constants I assume that the dissociation of $\ce{HF}$ is faster (has a larger $k_\text{dis}$) than the dissociation of $\ce{DF}$. I postulate the following mechanism:

$\ce{HF + D+ &lt;=&gt;[k_\text{dis}^\ce{HF}] H+ + F- + D+ &lt;=&gt;[][k_{\text{dis}}^\ce{DF}] DF + H+}$

Essentially it is a pretty simple reaction mechanism, but it tends to forming $\ce{DF}$ over $\ce{HF}$ because the deuterium dissociates more slowly.