Organic fouling in membrane distillation caused by biopolymers and humic substances: Role of cation types

Membrane fouling significantly limits the application of membrane distillation (MD) in wastewater treatment, and organic fouling is one important challenge, especially in the presence of inorganic salts. This study aimed at the impact of various concentrations of cations including Ca²⁺, Mg²⁺ and aluminum on organic fouling by sodium alginate (SA) and humic acid (HA). The results showed that the most severe SA fouling with Ca²⁺ was observed at 1 mmol/L in the range of 0.5–10 mmol/L, while the normalized flux decreased from 0.84 to 0.55 when Mg²⁺ increased from 0.5 to 10 mmol/L. The influence of aluminum concentration on SA fouling was similar to that of Ca²⁺, with the maximum flux decline rate of 52.3 % at 5.0 mmol/L aluminum. The addition of NaCl aggravated SA fouling with Ca²⁺ and aluminum, but alleviated that with Mg²⁺. Different SA fouling phenomena under Ca²⁺ and Mg²⁺ were attributed to the different mechanisms of forming transparent extracellular polymeric particles. Regardless of cation types, aggravated HA fouling was observed, and the higher the cation (Ca²⁺, Mg²⁺ or aluminum) concentrations, the more severe the HA fouling. These findings were further confirmed by Fourier infrared spectroscopy, the change of hydrophobicity, and scanning electron microscopy coupled with energy dispersive X-ray spectrometer.