New insights into the organic fouling mechanism of an in situ Ca2+ modified thin film composite forward osmosis membrane

Abstract

In this study, the effect of organic substances on the fouling behavior of a thin film composite (TFC) membrane with in situ Ca²⁺ addition (TFC-Ca membrane) was evaluated. Bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) were used as surrogate foulants for protein, natural organic substances and polysaccharides, respectively, thus enabling the analysis of foulant-membrane interaction in the membrane fouling process. Fouling experiments were carried out and the fouling mechanism was investigated by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. SEM-EDX, ICP-OES and TOC analysis were applied to characterize the fouled TFC-Ca membrane. Results suggested that the interfacial free energies obtained from advanced contact angle measurements were correlated strongly with the rates of membrane fouling. In situ Ca²⁺ addition in the TFC membrane resulted in the decrease of the interfacial adhesion free energy (i.e., foulant-membrane interaction) and thus the mitigation of membrane fouling. The permeate flux of TFC-Ca FO membrane after organic fouling could be fully restored by simple physical cleaning. The antifouling mechanism of Ca²⁺ pre-binding carboxyl groups in the TFC-Ca FO membrane was demonstrated, which provides new insights into the development of antifouling TFC membranes in the future.

Fig. 1. Flux decline curves of TFC-control membrane and TFC-Ca membrane under different organic matters for (a) BSA was used as organic foulant, (b) HA used as organic foulant and (c) SA used as organic foulant. The concentration of organic foulant was 200 mg L⁻¹ in the feed solution containing 50 mM NaCl and 1.0 mM Ca²⁺.

Fig. 2. Cross-section observation of fouling TFC-control membranes (a–c) and TFC-Ca membranes (d–f) by SEM, (a and d): BSA as model organic substance, (b and e): HA as model organic substance, (c and f): SA as model organic substance.

Fig. 3. The amount of Ca²⁺ accumulated on the fouled TFC-control membranes and TFC-Ca membranes, (a) analyzed by EDX, (b) analyzed by ICP-OES.

Fig. 4. The accumulated organic foulants on the fouled TFC-control membrane and TFC-Ca membranes were analyzed by TOC after the fouling experiment.

Fig. 5. Normalized water fluxes of the TFC-control membranes (a) and the TFC-Ca membranes (b) after fouling and after physical cleaning.