Unable to add elements to scala mutable list inside foreach loop

I am trying to add elements in a mutable scala list as below. I am reading the values from a dataframe row by row, extracting out values of column with name "_title" and adding it to list. But when for loop is complete, the list is still enmpty. this is the code:

import scala.collection.mutable.ListBuffer
val flatK = dfR.withColumn("UserValue", explode(col("UserValue")))
var colListA = new ListBuffer[String]()
//    var colSet : List[String] = List()
    for(i <- 0 until Integer.parseInt(dfR.count().toString)){
      flatK.filter($"columnIndex" === i).foreach{
        r=>
          val columnName = r.getAs[Row]("UserValue").getAs[String]("_title")
//          println(columnName)
          colListA.append(columnName)
      }
    }

println(columnName) actually prints the value I want to put inside my list. My dataframe dfR looks like this:

 +--------------------------------------------------------------+-----------+
|UserValue                                                     |columnIndex|
+--------------------------------------------------------------+-----------+
|[, last_mod_date, 2009-01-14T13:40:53]                        |0          |
|[, object_string, SOLIDS]                                     |0          |
|[, last_mod_date, 2009-01-13T22:58:30]                        |1          |
|[, object_string, TORSO]                                      |1          |

When I do

colListA += "elements"
colListA += "adds"

I can see elements added. But not inside that foreach loop. Can any one tell me what shall I try? Basically, I expect colList to be populated with last_mod_date and object_string.