Electrocoagulation for removal of silica nano-particles from chemical-mechanical-planarization wastewater

Continuous-flow electrocoagulation process with vertical flow-channels was investigated as a method to treat synthetic chemical-mechanical-planarization (CMP) wastewater containing highly charged ultrafine silica particles (ζ = −55 mV, mean Rp = 45 nm at pH 9.5). The parallel-plate, monopolar electrochemical cells resembled a series of closed electrical circuits such that the electrical field strength was highly dependent of the current density and aqueous conductivity, but independent of the inter-electrode gap. The residual turbidity of the CMP wastewater decreased with the increases in either hydraulic retention time or applied current density, and removal efficiency as high as 95% was achieved for wastewater with both low (70 NTU) and high (400 NTU) initial turbidities. The charge loading linearly correlated with turbidity removal efficiency up to a level of 8 F m−3, presenting an appropriate design parameter. Further analysis indicated that turbidity removal was limited by the quantity of liberated ferrous ions at lower range of current density, but seemingly reached a critical level of current density beyond which the process performance gradually deteriorated. Comparisons between the effective particle retention time and the estimated electrophoretic migration time revealed that the electrocoagulation process was predominantly controlled by the rate of particle aggregation occurring near the anodic surfaces. Furthermore, this process generates lesser amount of dry sludge as compared to chemical coagulation with polyaluminum chloride, and does not require pH adjustment prior to treatment.