Laravel : How to retrieve data from inside an nested array

I am trying to retrieve a specific column from a table in my database. Name of the column is verification_type_id and table is certificate_verification_type.

When I vardump() the variable which I am using to retrieve the column, here is what I get:

array(2)
{
    [14]=> array(2)
    {
        ["certificate_id"]=> int(9)
        ["verification_type_id"]=> int(2)
    }
    [15]=> array(2)
    {
        ["certificate_id"]=> int(9)
        ["verification_type_id"]=> int(3)
    }
}

Here is my laravel eloquent query:

$certVeriType = CertificateVerificationType::all()->where('certificate_id', $certType)->toArray();

How can I only retrieve the column 'verification_type_id' form the table. Here is the migration file of the table:

$table->integer('certificate_id');
$table->integer('verification_type_id');
$table->index('certificate_id');

Thanks in advance!

via Chebli Mohamed