What methods can be used to recover precious metals from wastewater?

The correct answer focuses on methods that are effective in recovering precious metals from wastewater. Reverse osmosis is a filtration process that can separate metals from liquids by utilizing a semi-permeable membrane that allows certain ions and molecules to pass while retaining others. This technique is particularly useful for concentrating precious metals, allowing for easier collection and processing.

Evaporation also plays a role by concentrating the wastewater and can lead to the precipitation of metals as the water evaporates. As the residual liquid volume decreases, the concentration of dissolved metals increases, potentially leading to their recovery. These methods are particularly relevant in wastewater treatment processes designed to minimize environmental impact while recovering economically valuable resources.

The other choices involve processes that may not be suitable or as effective for the specific goal of precious metal recovery from wastewater. For instance, combustion is typically not a recovery method for precious metals but rather a disposal technique. Similarly, freeze drying is generally used for the preservation of materials rather than for extracting metals.