//$attachment_alt = get_post_meta($attachment_id, '_wp_attachment_image_alt', true );

$args = array( 'post_type' => 'attachment', 'orderby' => 'menu_order', 'order' => 'ASC', 'post_mime_type' => 'image' ,'post_status' => null, 'numberposts' => null );

$attachments = get_posts($args);
if ($attachments) {
    foreach ( $attachments as $attachment ) {
        $attachment_alt = 'alt="' . $attachment->post_title . '"';
         } }