Effective boron removal from synthetic wastewater by multi-stage calcium-based chemical oxo-precipitation process

Chemical oxo-precipitation (COP) is a facile method to remove boron by the precipitation of perborate salts with the aid of hydrogen peroxide. Despite that the COP using barium precipitant (Ba–COP) has been advanced to provide excellent boron removal, the production of hazardous sludge limits the implementation. This study aims to improve the COP using calcium precipitant (Ca–COP) to provide boron removal comparable to COP without the production of hazardous waste. In addition to the precipitation of calcium perborate, the formation of calcium carbonate and calcium peroxide played decisive roles in the Ca–COP. The instant precipitation of calcium perborates (CaPBs) at pH 11 could eliminate boron from 1000 to 30 mg-B/L in 15 min. The prominent uptake of atmospheric CO 2 induced the precipitation of calcium carbonate and consumed calcium ion, which resulted in the increment in boron level due to the incongruent dissolution of CaPBs. The formation of calcium carbonate and calcium perborate provided an additional route to remove boron through sorption. Accordingly, a multi-stage Ca–COP process was developed to remove precipitated CaPBs for preventing the incongruent dissolution and to dose calcium precipitant multiple times to induce the sorptive removal. The proposed process could eliminate boron from 1000 to 5 mg-B/L, which is comparable to the state-of-art Ba–COP process with 45% of its cost. • Dissolution of calcium perborate was triggered by the calcium depletion by CaCO 3 . • Formation of CaCO 3 and CaO 2 provided a sorptive route to remove boron. • Multi-step Ca–COP process attained 99.5% of boron removal from 1 g-B/L solution. • Ca–COP process was 45% cheaper than the Ba–COP process by avoiding hazardous sludge.