Treating boron-containing wastewater by electrocoagulation-flotation (ECF) process with the stepwise addition of anionic surfactants

This study presents an innovative approach for boron removal from industrial wastewater, especially from coal-fired power plants and hydraulic fracturing processes, through an advanced electrocoagulation-flotation (ECF) method. This method, enhanced by the stepwise addition of anionic surfactants, effectively reduces the sludge-volume ratio (SVR) while ensuring efficient boron removal. Anionic surfactants, due to their compatibility with the positively charged boron‑aluminum hydroxide co-precipitates, enhance coagulation and solid-liquid separation. The research highlights the correlation between the hydrophobicity of the surfactant's carbon chain and reduced sludge volume and residual turbidity. Importantly, the stepwise addition of surfactants optimizes the dosage, maintaining low residual turbidity and SVR, even in high-conductivity (30.1 mS cm−1) wastewater from actual coal-fired power plants. Further, the study shows that longer reaction times and higher surfactant doses in a mono-polar electrode reactor can significantly improve boron removal and solid-liquid separation, marking a substantial advancement in wastewater treatment technologies. Boron is a common pollutant in wastewater of coal-fired power plants and the hydraulic fracturing process. It can be effectively removed by electrocoagulation (EC). However, the boron‑aluminum hydroxide particles after EC are too tiny to settle by sedimentation, and therefore, the sludge-volume ratio (SVR) is extremely high. By adding surfactants, the solid-liquid separation mechanism of EC can be shifted from settling to flotation (known as the electrocoagulation-flotation (ECF) process), effectively reducing the SVR without affecting boron removal. Due to the positively charged nature of boron‑aluminum hydroxide co-precipitates, anionic surfactants work better than cationic surfactants as coagulants. The higher the hydrophobicity of surfactant's carbon chain, the lower the sludge volume and residual turbidity. A stepwise addition method significantly reduced the surfactant dosage without causing an increase in the residual turbidity or SVR. The efficiencies of boron removal and solid-liquid separation by ECF in actual coal-fired power plant wastewater were significantly reduced due to the extremely high conductivity (30.1 mS cm−1). A stepwise addition method was also useful when treating real wastewater. The efficiencies of boron removal and solid-liquid separation were significantly be improved by a longer reaction time and a higher SDS dose by ECF with a mono-polar electrode reactor.