“I marinated shrimp in orange juice, olive oil, garlic, etc. I then took the shrimp out of the marinade and cooked it. Can I save the leftover marinade and freeze it to use again? It seems wasteful to throw it out.”
It is best to not re-use a marinade for both safety and flavor reasons. Once that marinade has been exposed to raw protein, it can grow harmful bacteria. You made the shrimp safe by cooking it as soon as it came out of the marinade. But the marinade may not freeze quickly enough after exposure to those microorganisms, and could develop many more. Think of freezing as putting them in deep sleep, but not killing them off. Going through another round of temperature changes and exposing the marinade to still more protein increases the likelihood of bacteria developing. From a culinary perspective, I believe your marinade would take on an overly fishy aroma and flavor.