Another important consideration for reducing these impacts of primary mining is to recycle gold . Printed circuit boards and other electronic components of discarded phones and computers can contain from 200 to more than 900 mg/kg of gold, which exceeds the gold concentrations in ore . The efficient recovery of this gold from printed circuit boards is therefore of potential economic benefit, which has prompted development of many pyrometallurgical and hydrometallurgical processing techniques . Many of these methods, however, produce emissions and byproducts that are hazardous to humans and the environment . As the prevalence of "urban mining" grows around the world, there is a need for more efficient, selective, and benign metal extraction techniques for electronic waste (e-waste) recycling .