How to add classes to post_class?

In the following code I would like to remove the div that wrap all the code and pass its class to article, but I do not know how to pass the variable $termString inside the post_class.

Can anybody help me?

<div class="<?php echo $termsString;?>">
    <article id="post-<?php the_ID(); ?>" <?php post_class('card'); ?>>
        <?php echo get_the_post_thumbnail($post_id, 'large', array('class' => 'img-fluid card-img-top')); ?>
        <div class="overlay"></div>
        <div class="card-body text-right">
            <h6 class="card-title"><?php the_title(); ?></h6>
            <p>Text description for this item</p>
        </div>
        <a class="card-link" href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"></a>  
    </article>
</div>

So, I have this now, and it is what I expected:

<article id="post-<?php the_ID(); ?>" <?php post_class('card grid-item wow fadeInUp ' . $termsString); ?>>

but I also need to add to these classes another class that comes from a custom field which name is “columns” and with value “col-12”.

This is what I am trying, but I think there’s some syntax error, the result I see from the Firefox inspector is “Array” instead of the value of “columns”:

<?php $custom_values = get_post_meta($post->ID, 'columns', true); ?>
                    <?php
                        $classes = array(
                            'card',
                            'grid-item',
                            'wow',
                            'fadeInUp',
                            $termsString,
                            $custom_values
                        );
                    ?>
                <article id="post-<?php the_ID(); ?>" <?php post_class( $classes ); ?>>

Edit: get_post_meta requires a third parameter, “false” returns the Array (default), “true” returns only the first result (NOT as an array). Now it is working! Thanks a lot.

$classes = [
    'card',
    'grid-item',
    'wow',
    'fadeInUp',
    $termsString
];

<div <?php post_class ( $classes ); ?>>
   ...
</div>