where C and q are the concentrations of tetracycline in the bulk and
solid phases, respectively; C is the aqueous phase concentration at
the particle surface, in equilibrium with the corresponding concentration
in the solid phase q; Dp is the pore diffusion coefficient
within the sorbent; qp is the particle density; r is radius distance
measured from the center of particle; ap is the particle radius; kf
is the external mass transfer coefficient, and t is the time