Let $u = \dfrac{2z + i}{z - ki}$, $z = x + iy$ and $k > 0$. If the curve represented by $\operatorname{Re}(u) + \operatorname{Im}(u) = 1$ intersects the $y$-axis at points $P$ and $Q$ where $PQ = 5$, then the value of $k$ is: