FO process for concentrating electroless nickel spent tank liquid: influence of pH on membrane fouling

ABSTRACT

Graph showing the experimental setup of forward osmosis, membrane fouling characterization, and the principle of pH regulation.

The control of pH is crucial for controlling inorganic membrane fouling during the forward osmosis (FO) process. In this study, two commercial FO membranes were employed to investigate the effect of pH of the pretreated feed solution (FS) on membrane fouling during the FO concentrating of electroless nickel spent tank liquid (ENSTL) pretreated by H2O2 oxidation-CaO precipitation process. The results showed that the cellulose triacetate (CTA) membrane had a higher water flux (up to 6.02 L/(m2 h)) than the aquaporin-based bionic membrane. The retention of total phosphorus, Ni and total organic carbon in both membranes reached up to 99.0%. The membrane fouling layer mainly consisted of a variety of inorganic calcium salt scaling materials (mainly Ca3(PO4)2) with the participation of elements such as P, S, Ni and Si. The AQP membrane with higher surface roughness favored the adsorption of contaminants. By adjusting the pH of FS from 10.0 to 6.0, the membrane fouling components, such as Ca3(PO4)2 and CaHPO4, which had low solubility, were substituted by Ca(H2PO4)2, which had high solubility and could be removed by simple physical cleaning. The pH regulation of the FS (pH = 6.0) improved membrane life and maintained stable FO use for a long period of time.