There will likely be some sand or dirt caught in between the leaves. After halving the heads, immerse them in a large bowl of cold water and agitate them to loosen the dirt. Rinse and repeat, then place on towels or in a salad spinner to dry. Be warned, this might not reach all the dirt. You can quarter, instead of halve, the heads for better access, or cut the end of the bulb and separate all the leaves before rinsing. If you opt to separate the leaves, use a grill basket to prevent the bok choy from falling through the grates.