EDI For Removal And Recovery Of Cr(Ⅵ) From Wastewater

Hexavalent chromium species, Cr(VI), are highly toxic agents that act as carcinogens, mutagens, and teratogens in biological systems. Industrial wastewaters from chromium electroplating and finishing often contains a considerable amount of Cr(VI), which can pose a severe threat to public health and the environment if discharged directly. Therefore, effective treatment of the wastewater containing Cr(VI) is very important. In addition, since chromium is expensive, simultaneous removal and recovery of Cr(VI) from wastewater are highly desired in industry.The principal conventional methods for treatment of the wastewater containing Cr(VI) include reduction followed by precipitation, adsorption, reverse osmosis, electrodialysis, and ion exchange. The reduction followed by precipitation is effective under good control, but sludge is produced, which may cause secondary pollution. Adsorption is also effective in removing Cr(VI), but its operation is complex. On the other hand, although reverse osmosis and electrodialysis are superior in recovering Cr(VI), it is difficult to reduce Cr(VI) in the effluent to an acceptable level. Ion exchange is very effective in removing and recovering Cr(VI) from wastewater. However, large amounts of chemicals are needed for the regeneration of exhausted resin, which results in complex operation and thus high operational cost.This thesis systematically studied a novel method, namely electrodeionization (EDI), for the removal and recovery of Cr(VI) from wastewater. Major research works included three parts: batch EDI, continuous EDI, and coupled EDI and ED (EDI-ED). In the batch EDI process, new ion exchange resin in the dilute chamber was first used to remove Cr(VI) from wastewater. Then, the exhausted resin was regenerated electrically to recover Cr(VI). Finally, the regenerated resin was used again to treat wastewater containing Cr(VI). In the continuous EDI process, wastewater was continuously delivered under a electrical field, and Cr(VI) was simultaneously removed and recovered. The EDI-ED process is a two-stage concentrating system, in which EDI is used for separating Cr(VI) from wastewater whereas ED is used for concentrating the diluted chromic acid obtained in EDI.Experimental results showed that the weak-base resin used could be effectively regenerated electrically in the batch EDI process. About 93% capacity of the resin was restored. It was found that the weak-base resin regenerated electrically could remove Cr(VI) from wastewater as effectively as that regenerated chemically. Pure chromic acid was recovered in the anode chamber with a concentration of 5.03 g Cr(VI)/L. The energy consumption was calculated to be 5.33 kWh/mol Cr(VI). The success of batch EDI revealed the effective electromigration of Cr(VI) ions. This became the foundation for the subsequent study of continuous EDI.It was demonstrated that Cr(VI) could be removed and recovered from wastewater effectively in the continuous EDI process. After treatment, Cr(VI) concentration was reduced to 0.06-0.46 mg/L, and pure chromic acid solution containing Cr(VI) as high as 5010-6150 mg/L was obtained. The EDI configuration was optimized by proper selection of anion exchange resins and anion exchange membranes. Moreover, a new concept of charge-mass ratio (CMR) was proposed, which was a complex factor including current, flowrate, and influent Cr(VI) concentration. The experimental data verified the good correlation between the effluent Cr(VI) concentration and CMR. In addition, it was found that there was an optimal CMR value, at which high Cr(VI) removal efficiency and energy consumption minimization could be achieved.The EDI-ED process could run in a continuous and steady mode, with Cr(VI) removed and recovered from wastewater simultaneously. In a long term of EDI-ED run with an influent Cr(VI) concentration of 50 mg/L, the effluent Cr(VI) concentration was consistently below 0.5 mg/L, while the concentration of the chromic acid recovered finally in the ED anode chamber reached 1.41 mol/L, high enough for reuse in Cr-plating bath. The energy consumptions of EDI and ED were 4.70 and 1.45 kWh/mol Cr(VI), respectively. This indicates that the concentration of the recovered chromic acid solution is significantly enhanced to a reusable level, only at a cost of 31% increase in energy consumption.