The most common mordants are Aluminium (Alum), Tin, Copper, Iron, and Chrome. You can probably see that these are all metals. Some of these are quite easy to dispose of and other are more difficult.
Alum (potassium aluminium sulfate) is commonly used as a soil acidifier for plants such as blueberries, hydrangeas, and other plants that enjoy acidic soils. Generally home dyers use pickling alum (ammonium aluminium sulfate) which can be found in your local grocery store along with other ingredients used in pickling. The same chemicals are generally recommended as soil conditioners for plants at a vastly higher concentration than the solution in your dyepot, so it is quite safe to pour out this sort of mordant under these types of plants.
Tin is another common mordant. It is usually found in tin food cans although in really small amounts. Copper is used in agricultural fungicides, and to control algae in ponds. Rita Buchanan, the author od "A Dyer's Garden" recommends pouring out tin and copper mordants on a gravel driveway or road where "they serve no good but do no harm".
Iron is naturally occuring in various soils. Dyers can simply simmer their fibers in cast iron pots to mordant the f so it is quite safe to pour it on the driveway, too.
Chrome is NOT SAFE to dicard at home. Prolonged exposure to chrome compounds has been shown to cause respiratory problems, allergies and even cancer. If you choose to use chrome handle it very carefully. Store any used chrome solution in a plastic jug that is clearly labeled until your municipality schedules a hazardous waste collection day.