Discretizing Multi-species Ion Exchange Equations by Finite Volume Method

Question

I'm solving a system of multispecies ion exchange equations (diffusion+drift fluxes) in 1-d spherical domain using finite volume method to obtain the ion concentrations at the next time step. After polishing the generic equation with so many assumptions, I ended up with this equation for species A \[\frac{{\partial {C_A}}}{{\partial t}} = D\frac{{{\partial ^2}{C_A}}}{{\partial {r^2}}} - {\mu _{_A}}{C_A}\frac{{{D_A}\frac{{{\partial ^2}{C_A}}}{{\partial {r^2}}} + {D_B}\frac{{{\partial ^2}{C_B}}}{{\partial {r^2}}} + 2{D_C}\frac{{{\partial ^2}{C_C}}}{{\partial {r^2}}}}}{{{\mu _A}{C_A} + {\mu _B}{C_B} + 4{\mu _D}{C_C}}} + \frac{2}{r}{D_A}\frac{{\partial {C_A}}}{{\partial r}} - \frac{2}{r}{\mu _A}{C_A}\frac{{{D_A}\frac{{\partial {C_A}}}{{\partial r}} + {D_B}\frac{{\partial {C_B}}}{{\partial r}} + 2{D_C}\frac{{\partial {C_C}}}{{\partial r}}}}{{{\mu _A}{C_A} + {\mu _B}{C_B} + 4{\mu _D}{C_C}}}\]

D is the diffusivity and mu is the ion mobility and they are all constants. There are two more similar equations for B and C. My question is: how can I discretize the second term in the RHS? should I assume that CA, CB and CD are known at initial time and discretize second order derivatives? knowing that the transient term in LHS can be solved using forward Euler's methods for time first order derivative (n is time index):

\[\frac{{\partial {C_A}}}{{\partial t}} = \frac{{{C_A}^{n + 1} - {C_A}^n}}{{\Delta t}}\]