General Hospital # 21, aka: Gwalthney Tobacco Factory Hospital.
Amongst its many uses during its operational period was indeed
used for treatment of Federal Prisoners. Of what is known those
that died there as well as most of the Medical facilities in the
eastern half of Richmond were initially buried at nearby Oakwood
Cemetery.

Post war with the establishment of the National Cemetery System
all Federal dead that could be located both in local cemeteries
and battlefield burials around Richmond were moved to one of these
new cemeteries. Those taken from Oakwood were moved to the Richmond
National Cemetery, just east of there. Unfortunately in the moves,
the individual identity of many of them were lost in the shuffle.
Even if the records record the death in the hospital, the burial
plot itself may or may not have still had a legible marker.
Oakwood cemetery records of that time period are fragmented at best.
Therefore commonly reinterred over at the National Cemetery as unknowns.
I referenced by files and did not find him listed as one of the identified.
If he died at Hosp-21 this is where he most likely rests today, although
his exact grave is not known.

Richmond at that time was one of the primary prisoner holding and exchange
points. This was before the shut down of the prisoner exchanges that
resulted in overcrowding and establishment of other prison camps further
south. I also noted another Zachariah Isbell that was listed as serving
in the 9th Alabama Regt Co.H (CS)

Respects
Frederick
(Richmond Va)