Visualization of clogs developed from interaction between APDC and low-level radwaste relevant nuclides on RO membranes: A LA-ICP-MS study

• The surface and depth distribution of foulings were mapped using LA-ICP-MS.
• Surface mapping patterns strongly suggest the foulings containing APDC and studied metals.
• Depth profile analyses suggest APDC additive is crucial for the fouling formation.
• LA-ICP-MS is a powerful tool to uncover the fouling mechanism.

In this study, we used LA-ICP-MS to visualize the fouling induced by seeded filtration for efficient removing liquid low-level radwaste relevant nuclides using a reverse osmosis (RO) membrane. Evidenced by a significant permeate flux drop, these APDC additives pronouncedly increased the rejection by around 20% (from 68% to 84%) for cobalt, 30% for strontium (from 55% to 71%), and 70% for cesium (from 20% to 34%). Surface mapping using LA-ICP-MS showed that the hot zones of Co, Sr, and Cs in the APDC–Co, APDC–Sr and APDC–Cs systems were perfectly superimposed with the locations of the S moiety of APDC. This suggested that the improved rejection is closely related to the formation of APDC–metal complexes, and the possible coprecipitation of these elements with APDC–metal complexes. Further depth profile analyses revealed that the fouling mainly occurred in the first 2–6 μm of the RO membrane. Fitted with the Hermia model, it was evident that APDC additives facilitate the formation of complete blocking, intermediate blocking, and standard blocking, and as a consequence enhance Co rejection. By contrast, cake formation induced by APDC additives was mainly responsible for the better rejection of Sr and Cs. Importantly, our depth profile analyses undoubtedly suggested that a large particle/colloid is able to penetrate deeply into the membrane despite its pore size is much smaller than those particles/colloids.