I wanted to use a series capacitor for blocking but based on the low ac input frequency (5 Hz) it makes my capacitor very big. what is your suggestion for rejecting the VDC?
Strictly speaking you need large time constant, not necessarily large capacitor. You should talk about circuit impedance first. What's the load receiving the AC signal?
Now depends on the settling time you need. For settling time = 0.5 seconds, a 16nF cap and 10 Meg resistor. Do not push too much the settling time, otherwise you will attenuate your AC signal.
Since you are feeding into an opamp buffer config, then presumably the input impedance is >> 1Mohm even for a non-FET input opamp.
Here even a 5uF capacitor should provide you a decent DC block with less than 1% attenuation of your signal.
Of course if there are other elements in your circuit which you have not mentioned, then this result could be different.