Order-of-magnitude enhancement in boron removal by membrane-free capacitive deionization

•Capacitive deionization enables membrane- and chemical-free boron removal.•Reversing discharge polarity enhances boron removal by capacitive deionization.•Order-of-magnitude enhancement of boron electrosorption per charge.•Single-pass separation process meeting WHO recommendations is demonstrated.•Low energy consumption of 0.2 kWh per m3.