Most of the ones I see in jars come in water containing citric or lactic acid. These preservatives aren’t recommended, since I think they can be derived from yeasts, but rinsing under the tap might help. Refrigerated “fresh” ones tend to come in oil, which shouldn’t contain any preservatives. Anything containing lemon probably won’t have citric/lactic acid as a preservative either. Keep in mind that a lot of jarred black olives contain ferrous sulfate, which is basically iron. Excessive iron intake can cause all sorts of problems, but on the other hand it could be of benefit if you have a deficiency.