Pandas object with timestamps to string

Question

I have a pandas dataframe with a column iso in datetime64[ns] format. My goal is to first convert this column to the ISO 8601 format, and then to a plain string.

dtrain.shape
(10886, 12)

dtrain.dtypes
season                 int64
holiday                int64
workingday             int64
weather                int64
temp                 float64
atemp                float64
humidity               int64
windspeed            float64
casual                 int64
registered             int64
count                  int64
iso           datetime64[ns]

I have tried a few methods mentioned on StackOverflow, which throw errors:

dtrain['iso'].astype(datetime)
dtrain['iso'].tolist()/1e9

The following works, but converts the column to a Python object which contains both formats and cannot be converted to string with str() or as.type('str')

dtrain['iso'] =dtrain['iso'].dt.strftime('%Y-%m-%dT%H:%M%:%SZ')
dtrain['iso'].dtype
dtype('O')
dtrain['iso'] = dtrain['iso'].astype('str')
dtrain['iso']


datetime
2011-01-01 00:00:00    2011-01-01T00:00:00Z
2011-01-01 01:00:00    2011-01-01T01:00:00Z
2011-01-01 02:00:00    2011-01-01T02:00:00Z
2011-01-01 03:00:00    2011-01-01T03:00:00Z
2011-01-01 04:00:00    2011-01-01T04:00:00Z
                               ...         
2012-12-19 19:00:00    2012-12-19T19:00:00Z
2012-12-19 20:00:00    2012-12-19T20:00:00Z
2012-12-19 21:00:00    2012-12-19T21:00:00Z
2012-12-19 22:00:00    2012-12-19T22:00:00Z
2012-12-19 23:00:00    2012-12-19T23:00:00Z
Name: iso, Length: 10886, dtype: object

What is the workaround to convert a pandas dataframe object column to a string column (containing only the ISO 8601 format) in this case?