Rapid boron removal from wastewater using low-crystalline magnesium oxide

Boron has applications in industries such as the glass and semiconductor industries. However, boron is toxic and has a significant effect on agriculture, and wastewater contaminated with boron should be treated before being discharged. Magnesium oxide (MgO) can remove boron but has not been used much in practical wastewater treatment systems because it requires between several hours and several days to immobilize boron. We have discovered a method to rapidly remove boron from wastewater by using highly reactive MgO. The MgO, in the form of low-crystalline magnesium oxide (LC-MgO), was obtained from basic magnesium carbonate (BMC) by low-temperature calcination at 500-700°C, while normal MgO is usually formed at over 1000°C. Solid analyses showed that LC-MgO had lower crystallinity and a higher specific surface area than normal MgO, both of which helped LC-MgO immobilize boron faster than normal MgO. Studies of the reaction kinetics showed that boron removal by MgO was a three-step process; (i) magnesium ions from MgO first dissolve into the aqueous phase, (ii) the dissolved magnesium ions form complexes with borate ions in the wastewater, (iii) these complexes precipitate out of solution as hydroxides when they react with free borate ions. The kinetic model also revealed that the higher surface area of LC-MgO enhanced the rate of dissolution and precipitation of magnesium ions, which increased the rate of boron removal.