Add or append new column to csv file using shell script

I have a CSV file it includes some columns, in that I have the start time and end time. My requirement is to get the difference between the time and add the difference to a new column.

I'm getting the difference between time but not able to add it into new column properly for each line.

This is my sample csv.

4,ganesh-28,2019-09-26T16:56:40Z,closed,harshavardhanc,2019-09-26T16:57:02Z,1,1 3,ganesh-28,2019-09-26T16:54:25Z,closed,harshavardhanc,2019-09-26T16:54:55Z,1,1 2,ganesh-28,2019-09-26T16:52:59Z,closed,harshavardhanc,2019-09-26T16:55:19Z,1,1 1,ganesh-28,2019-09-26T16:46:52Z,closed,harshavardhanc,2019-09-26T16:47:25Z,1,1

This is the script.

    #!/bin/bash
    cat a.csv | while read line
       do
              created_at=$(date -d $(echo $line | awk -F ","'{print $3}') +%s)
              merged_at=$(date -d $(echo $line | awk -F ","'{print $6}') +%s)
              echo $created_at $merged_at
              diff=$(( $merged_at - $created_at ))
              h=`expr $diff / 3600`
              m=`expr $diff  % 3600 / 60`
              s=`expr $diff % 60`
              diff=$(printf "%02d:%02d:%02d\n" $h $m $s)
              echo $diff
              awk -v v1="$diff" -F","'BEGIN { OFS = "," } {$9=v1; print}' a.csv >> b.csv
done

I'm getting the output something like this.

4,ganesh-28,2019-09-26T16:56:40Z,closed,harshavardhanc,2019-09-26T16:57:02Z,1,1,00:00:22
3,ganesh-28,2019-09-26T16:54:25Z,closed,harshavardhanc,2019-09-26T16:54:55Z,1,1,00:00:22
2,ganesh-28,2019-09-26T16:52:59Z,closed,harshavardhanc,2019-09-26T16:55:19Z,1,1,00:00:22
1,ganesh-28,2019-09-26T16:46:52Z,closed,harshavardhanc,2019-09-26T16:47:25Z,1,1,00:00:22
4,ganesh-28,2019-09-26T16:56:40Z,closed,harshavardhanc,2019-09-26T16:57:02Z,1,1,00:00:30
3,ganesh-28,2019-09-26T16:54:25Z,closed,harshavardhanc,2019-09-26T16:54:55Z,1,1,00:00:30
2,ganesh-28,2019-09-26T16:52:59Z,closed,harshavardhanc,2019-09-26T16:55:19Z,1,1,00:00:30
1,ganesh-28,2019-09-26T16:46:52Z,closed,harshavardhanc,2019-09-26T16:47:25Z,1,1,00:00:30
4,ganesh-28,2019-09-26T16:56:40Z,closed,harshavardhanc,2019-09-26T16:57:02Z,1,1,00:02:20
3,ganesh-28,2019-09-26T16:54:25Z,closed,harshavardhanc,2019-09-26T16:54:55Z,1,1,00:02:20
2,ganesh-28,2019-09-26T16:52:59Z,closed,harshavardhanc,2019-09-26T16:55:19Z,1,1,00:02:20
1,ganesh-28,2019-09-26T16:46:52Z,closed,harshavardhanc,2019-09-26T16:47:25Z,1,1,00:02:20
4,ganesh-28,2019-09-26T16:56:40Z,closed,harshavardhanc,2019-09-26T16:57:02Z,1,1,00:00:33
3,ganesh-28,2019-09-26T16:54:25Z,closed,harshavardhanc,2019-09-26T16:54:55Z,1,1,00:00:33
2,ganesh-28,2019-09-26T16:52:59Z,closed,harshavardhanc,2019-09-26T16:55:19Z,1,1,00:00:33
1,ganesh-28,2019-09-26T16:46:52Z,closed,harshavardhanc,2019-09-26T16:47:25Z,1,1,00:00:33

Which is append the difference to all the line.

But my requirement to get the difference of time only for that line. The output should be like this.

4,ganesh-28,2019-09-26T16:56:40Z,closed,harshavardhanc,2019-09-26T16:57:02Z,1,1,00:00:22
3,ganesh-28,2019-09-26T16:54:25Z,closed,harshavardhanc,2019-09-26T16:54:55Z,1,1,00:00:30
2,ganesh-28,2019-09-26T16:52:59Z,closed,harshavardhanc,2019-09-26T16:55:19Z,1,1,00:02:20
1,ganesh-28,2019-09-26T16:46:52Z,closed,harshavardhanc,2019-09-26T16:47:25Z,1,1,00:00:33

Please someone help me to achieve this.