Using pretreated waste oyster and clam shells and microwave hydrothermal treatment to recover boron from concentrated wastewater

► Pretreated oyster, hard clam and freshwater clam shells were reused as mineralizers. ► Mineralization is conducted with the microwave hydrothermal method to recover boron. ► Recovery efficiency could be affected by distinct solubilities of the precipitates. ► Pretreated oyster shell is an effective mineralizer for boron recovery.