A molecular-level mechanism analysis of PFS coagulation behaviors: Differences in natural organic matter and algal organic matter

Coagulation is generally considered as the key process to remove dissolved organic matters in water treatment plants, of which natural organic matter (NOM) and algal organic matter (AOM) are two commonly seen species. However, the removal efficiency of NOM and AOM by coagulation varies a lot. Here we unraveled the possible reasons for the different performance on NOM and AOM removal from molecule-level insights. With van Krevelen diagram, we observed that removed components in NOM and AOM were located in similar areas (high O/C and low H/C), indicating that polyferric sulfate (PFS) coagulation is prone to remove oxygen-containing and unsaturated components. Further, the results showed that NOM components are mainly composed of C, H, and O, with high O/C and low H/C, which therefore are easier to be removed (51%). However, similar to 50% AOM components are N-containing, which mainly located in areas of low O/C and high H/C, leading to the relatively poor coagulation performance (19%). With further data processing of the Fourier transform ion cyclotron resonance (FT-ICR) results, we found that molecules with higher oxidized state or more carboxyl groups were preferentially removed by PFS coagulation. This study reveals the PFS coagulation mechanism of NOM and AOM, and provides theoretical foundations for enhanced coagulation in practical applications.