How to save multiple values with plugin select2 as one field in database

I've got the following js code to select multiple options.

{
                "label": "Bodemvocht:",
                "name": "bodemvocht",
                "type": "select2",
                "options": [
                     { "label": "", "value": "" },
                     { "label": "Droog", "value": "Droog" },
                     { "label": "Vochtig", "value": "Vochtig" },
                     { "label": "Nat", "value": "Nat" },
                     { "label": "Geen voorkeur", "value": "Geen voorkeur" }
                ],
                "opts": {
               "placeholder": "Selecteer vochtigheid",
               "multiple": "multiple",
               "separator": ";",
               "allowClear": true
                }

But when saving this I get an error (Array to string conversion) and only the text 'Array' is saved in the database. How to solve this properly?

Thanks for your help!