Coal fly ash activated by NaOH roasting: Rare earth elements recovery and harmful trace elements migration

•Coal fly ash by NaOH roasting enhances the volatilization of mercury at lower temperatures.•More water-soluble arsenic and selenium compounds are generated by NaOH roasting.•More acid-soluble lead and cadmium compounds are obtained by NaOH roasting.•NaOH roasting can effectively improve the recovery of rare earth elements in coal fly ash.