Surface treatment and structure optimization of thin film composite membranes for enhanced removal of trace organic contaminants and improved antifouling performance

Abstract

Membrane separation processes used in wastewater reclamation encounter the critical challenges from (1) the trace organic contaminants (TrOCs) associated with water safety, and (2) membrane fouling linked to membrane performance and its life span. This thesis aims to explore effective modification approaches to tailor membrane surface properties for enhanced removal of TrOCs as well as improved antifouling performance. Further optimization of membrane structure to develop non-polyamide based thin film composite (TFC) membranes with high selectivity for the removal of TrOCs is also studied.

Using polydoamine (PDA) as a model hydrophilic coating layer, the PDA-coated NF90 membrane experienced up to 75% reduction in the passage of bisphenol A compared to the control (NF90 without coating). Meanwhile, we also observed systematic increase in the rejection of three hydrophobic parabens upon increased PDA coating time. Further sorption tests revealed that the hydrophilic PDA coating could effectively decrease EDCs sorption by the membrane, which is responsible for the improved rejection as predicted by the solution-diffusion theory. Based on the positive function of PDA coating on the rejection of EDCs, a highly selective surface coating combining PDA and further in situ immobilization of silver nanoparticles (AgNPs) was designed. The silver functionalization further improved EDCs rejection which can be attributed to a combination of enhanced size exclusion and suppressed hydrophobic interaction. A resistance-in-series analysis further reveals that the coating was highly permeable to water but highly resistant to EDCs, leading to an EDC selectivity that was an order of magnitude greater than those of the bare PDA coating and the base membrane NF90.

The PDA coating was also applied on a thin film composite (TFC) forward osmosis (FO) membrane to investigate its effects on FO mass transport and antifouling behavior. The coating significantly improved membrane surface hydrophilicity as well as reduced membrane surface roughness. The 0.5 h PDA coated membrane TFC-C0.5 achieved enhanced FO water flux and reduced reverse solute diffusion simultaneously. This reduction in reverse solute diffusion further reduced the internal concentration polarization inside the coated membrane, leading to an enhanced FO water flux. The coated membrane TFC-C0.5 also presented an improved antifouling performance compared to the control membrane using alginate as a model foulant.

Furthermore, a rapid, simple and green coating method using the coordination complex of tannic acid (TA) and ferric ion (Fe3+) was applied to enhance the removal of trace organic contaminants (TrOCs) by polyamide membranes. The TA-Fe coating could be formed in merely 10-20 seconds. Coating this TA-Fe thin film on a NF270 membrane reduced its effective pore size from 0.44 nm to 0.40 nm, resulting in the significantly increased rejection of both NaCl and trace organic contaminants. In comparison with the PDA coating (e.g., 0.5 h), the TA-Fe coating presented greater resistance to TrOCs (i.e., lower permeability of TrOCs). The advantages of fast coating process, greatly improved rejection performance, and use of green accessible materials make TA-Fe a highly promising coating material for large scale applications.

The TA-Fe coating was subsequently explored for direct use as a rejection layer to form a non-polyamide based TFC membrane. Transmission electron microscopy (TEM) confirmed the formation of a continuous and thin layer on the order of several tens nanometers on a porous substrate. This metal-organic based TFC presented comparable water permeability and salt rejection with current commercial NF membranes. More importantly, it showed greater rejections for EDCs than polyamide based NF membranes. The results suggested that traditional polyamide membranes optimized for salts rejection may not be adequate for the rejection of TrOCs.