Evaluation of fouling mechanisms for humic acid molecules in an activated biochar-ultrafiltration hybrid system

Activated biochar (AB), produced by incomplete biomass combustion of waste or byproducts, was found to be an effective adsorbent in an ultrafiltration (UF) hybrid system. Three different adsorbents, powdered activated carbon (PAC) and oxygen-/nitrogen-based AB (O-/N-AB), were characterized and quantified by elemental composition and aromatic carbon fractions. Based on the results, the economical O-/N-AB could be used as an alternative to coal-based commercially available PAC due to their enhanced inner pore site density. In particular, the adsorption capacity of N-AB (1.306 (mg/g)(mg/L)^1/n) for hydrophobic humic acid (HA) molecules was significantly higher than those of PAC (1.208 (mg/g)(mg/L)^1/n) and O-AB (1.230 (mg/g)(mg/L)^1/n). Although the UF hybrid system with all adsorbents showed a higher reversible fouling rate (11.1%) than a UF membrane system alone, due to the added adsorbents for HA filtration, the adsorbents adsorbed relatively small HA molecules, leading to a significantly lower feed-HA concentration in the membrane system, resulting in a decrease in the irreversible fouling rate (38.1%). Consequently, N-AB in the UF hybrid system showed better performance than PAC in terms of flux decline (12.9%) and HA rejection (4.1%), due to higher hydrophilicity and polar interactions with polar functional groups.