Please see this Codex article for further guide, but in your case, you would use esc_attr() to escape the $second_col_class value which is being used in an HTML attribute, namely class:
<!-- bad -->
<div class="<?php echo $second_col_class; ?>">
<!-- good -->
<div class="<?php echo esc_attr( $second_col_class ); ?>">
<!-- good -->
<div class="<?php esc_attr_e( $second_col_class, 'text-domain' ); ?>">