Nitric acid are dangerous and mercury is poisonous. Another method uses cyanide to dissolve gold into a solution with a pH greater than 10. The resulting powder after throughly rinsing can be melted. The zinc will float on top of the gold and can be skimmed off. If the pH of the dissolution solution is not controlled, hydrogen cyanide can form if the pH becomes to low.
Hydrogen cyanide is what is used in gas chambers.
Finally, I have seen offers by companies claiming to provide environmentally safe recover methods. Unfortunately, the cost to run these systems may not be worth the amount of gold recovered.