Electrodialytic separation of phosphate from sewage sludge ash using electrospun ion exchange membranes

Abstract

• Electrodialysis in conjunction with leaching extraction was used to recover phosphate from sewage sludge ash. • Two novel bipolar membrane was synthesized to be used in the electrodialysis stack. • Polymer composites contain graphene was used in the interfacial layer of the bipolar membrane as a water dissociation catalyst. • At a lower flow-rate and voltage, the performance of electrospun membranes was better than the commercial membranes. Conventional electrodialysis (CED) and electrodialysis with a bipolar membrane (EDBM) in conjunction with leaching extraction were used to recover phosphate from sewage sludge ash (SSA). Two novel bipolar membranes (BPM-1 and BPM-2) were synthesized to be used in the EDBM process. Poly (2,6-dimethyl-1,4-phenylene oxide) was chosen as the polymeric backbone of the anion exchange layer (AEL), and the cationic agent was N, N, N, N-Tetramethylguanidine. As a fully charged polymer, sulfonated polystyrene was used to prepare the cation exchange layer (CEL). Graphene was used in the interfacial layer (IL) of BPM-2 as a water dissociation catalyst. Bipolar membranes (BPMs) were fabricated by electrospinning followed by heat treating. Experiments showed that electrospinning is an operative and well-controlled way to fabricate BPMs in which the AEL, CEL, as well as the IL, can be easily changed or added where desired. The electrospun BPMs had water uptake from 69.5 to 85.3% and linear expansion ratios of 7.1 to 8.9%. Current-voltage (I-V) tests and the acid and base production yield were examined to investigate the performance of BPMs. The improving of phosphoric acid production up to 45% in EDBM stack and decreasing of the potential drop value during the I-V test, showed the better performance of BPM-2 in water dissociation. Moreover, Phosphate recovery from SSA leachate solution was also investigated by CED in batch and continuous operation. Results showed that CED in the batch operation could effectively recover all the phosphate content in the solution. However, it was time consuming compared with a continuous operation. In CED continuous operation at a lower flow-rate and voltage, the performance of the electrospun membranes was better than the commercial membranes.