This is a quick way to turn a simple comma-separated file with a list of urls in each column representing one sitemap into a valid XML Sitemap:

sitemap.column.xml.php

<?php
/**
 * User: artlung
 * Date: 2019-08-24
 * A variation of https://gist.github.com/artlung/210438 :
 *
 * "I have a list of URLs in 10 columns of a CSV file and I need a separate
 * sitemap for each column. Can we modify this code accordingly?"
 *
 * This is a quick way to turn a simple comma-separated file
 * with a list of urls in each column representing one sitemap (urls-in-columns.csv)
 * into a valid XML Sitemap:
 * http://en.wikipedia.org/wiki/Sitemaps
 * Put this file sitemap.column.xml.php and urls-in-columns.csv at
 * the webroot http://example.com/sitemap.column.xml.php
 * Then add the text in quotes below to your robots.txt file as new lines:
 * "Sitemap: http://example.com/sitemap.column.xml.php?n=0"
 * "Sitemap: http://example.com/sitemap.column.xml.php?n=1"
 * "Sitemap: http://example.com/sitemap.column.xml.php?n=2"
 * "Sitemap: http://example.com/sitemap.column.xml.php?n=3"
 * ...and so on.
 * Questions? email joe@artlung.com
 */
$filename = 'urls-in-columns.csv';
$filectime = filectime($filename);
$handle = fopen($filename, 'r');
$urls = fgetcsv($handle);
// First pass: how many columns in this csv?
$number_of_columns = count($urls);
$max_index = $number_of_columns - 1;
fclose($handle);
$requested_column_index = isset($_GET['n']) ? intval($_GET['n']) : 0;
if ($requested_column_index > $max_index) {
    $requested_column_index = 0;
}
// Second pass, read the csv for the data we need for the sitemap
$handle = fopen($filename, 'r');
while (($data = fgetcsv($handle)) !== FALSE) {
    $priority = '0.5';
    $url = trim($data[$requested_column_index]);
    if ($url) {
        $sitemap[] = array(
            'loc' => $url,
            'lastmod' => date('Y-m-d',$filectime),
            'changefreq' => 'weekly',
        'priority' => $priority,
        );
    }
}
fclose($handle);
header('Content-Type: text/xml');
echo '<?xml version=\'1.0\' encoding=\'UTF-8\'?>';
echo "\n";
echo '<!-- showing column index ' . $requested_column_index . " min n=0, max n={$max_index}; {$number_of_columns} columns". ' -->';
echo "\n";
echo '<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
			    http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">';
echo "\n";
foreach ($sitemap as $link) {
    echo "\t<url>\n";
    echo "\t\t<loc>" . htmlentities($link['loc']) . "</loc>\n";
    echo "\t\t<lastmod>{$link['lastmod']}</lastmod>\n";
    echo "\t\t<changefreq>{$link['changefreq']}</changefreq>\n";
    echo "\t\t<priority>{$link['priority']}</priority>\n";
    echo "\t</url>\n";
}
echo '</urlset>';